Cadmium toxicity symptoms and uptake mechanism in plants: a review

Sintomas de toxicidade de cádmio e mecanismo de absorção em plantas: uma revisão

N. E. M. Shaari M. T. F. M. Tajudin M. M. Khandaker A. Majrashi M. M. Alenazi U. A. Abdullahi K. S. Mohd About the authors

Abstract

Cadmium (Cd) is one of non-essential heavy metals which is released into environment naturally or anthropogenically. It is highly persistent toxic metals that are exceptionally distressing industrial and agriculture activities by contaminating soil, water and food. Its long-duration endurance in soil and water results in accumulation and uptake into plants, leading to the food chain. This becomes a serious global problem threatening humans and animals as food chain components. Living organisms, especially humans, are exposed to Cd through plants as one of the main vegetative food sources. This review paper is concentrated on the symptoms of the plants affected by Cd toxicity. The absorption of Cd triggers several seen and unseen symptoms by polluted plants such as stunted growth, chlorosis, necrosis and wilting. Apart from that, factors that affect the uptake and translocation of Cd in plants are elaborated to understand the mechanism that contributes to its accumulation. By insight of Cd accumulation, this review also discussed the phytoremediation techniques-phytoextraction, phytostimulation, phytostabilization, phytovolatization and rhizofiltration in bioremediating the Cd.

Keywords:
cadmium; toxicity; symptoms; translocation; bioavailability; phytoremediation

Resumo

O cádmio (Cd) é um dos metais pesados ​​não essenciais que é liberado no meio ambiente de forma natural ou antropogênica. São metais tóxicos altamente persistentes que prejudicam excepcionalmente as atividades industriais e agrícolas, contaminando o solo, a água e os alimentos. Sua resistência de longa duração no solo e na água resulta em acúmulo e absorção pelas plantas, levando à cadeia alimentar. Isso se torna um sério problema global que ameaça humanos e animais como componentes da cadeia alimentar. Os organismos vivos, principalmente os humanos, são expostos ao Cd através das plantas como uma das principais fontes de alimento vegetativo. Este artigo de revisão concentra-se nos sintomas das plantas afetadas pela toxicidade do Cd. A absorção de Cd desencadeia vários sintomas visíveis e invisíveis por plantas poluídas, como crescimento atrofiado, clorose, necrose e murcha. Além disso, são elaborados fatores que afetam a absorção e translocação de Cd nas plantas para entender o mecanismo que contribui para o seu acúmulo. A partir do conhecimento do acúmulo de Cd, esta revisão também discutiu as técnicas de fitorremediação - fitoextração, fitoestimulação, fitoestabilização, fitovolatização e rizofiltração na biorremediação do Cd.

Palavras-chave:
cádmio; toxicidade; sintomas; translocação; biodisponibilidade; fitorremediação

1. Introduction

Heavy metals are inevitably drawing global attention due to their well-known toxicity effects on human health and the environment. They are very persistent in our environment, including water, air and soil, instigating their bioaccumulation in the living organism. The non-essential and unknown roles of heavy metals, specifically mercury, cadmium, lead, and uranium, are harmful to plants with promising risks for children and women, notably in public (Chen et al., 2015CHEN, H., TENG, Y., LU, S., WANG, Y. and WANG, J., 2015. Contamination features and health risk of soil heavy metals in China. The Science of the Total Environment, vol. 512-513, pp. 143-153. http://dx.doi.org/10.1016/j.scitotenv.2015.01.025. PMid:25617996.
http://dx.doi.org/10.1016/j.scitotenv.20...
; Cárdenas-González et al., 2016CÁRDENAS-GONZÁLEZ, M., OSORIO-YÁÑEZ, C., GASPAR-RAMÍREZ, O., PAVKOVIĆ, M., OCHOA-MARTÍNEZ, A., LÓPEZ-VENTURA, D., MEDEIROS, M., BARBIER, O.C., PÉREZ-MALDONADO, I.N., SABBISETTI, V.S., BONVENTRE, J.V. and VAIDYA, V.S., 2016. Environmental exposure to Arsenic and Chromium in children is associated with kidney injury molecule-1. Environmental Research, vol. 150, pp. 653-662. http://dx.doi.org/10.1016/j.envres.2016.06.032. PMid:27431456.
http://dx.doi.org/10.1016/j.envres.2016....
). Cadmium (Cd) has, however, been described as the most troublesome heavy metal due to its more significant discharge from industries (Chien et al., 2003CHIEN, S.H., CARMONA, G., PROCHNOW, L.I. and AUSTIN, E.R., 2003. Cadmium avaibility from granulated and bulk-blended phosphate-potassium fertilizers. Journal of Environmental Quality, vol. 32, no. 5, pp. 1911-1914. http://dx.doi.org/10.2134/jeq2003.1911. PMid:14535337.
http://dx.doi.org/10.2134/jeq2003.1911...
). It has been articulated as unnecessary elements by living organisms for any physiological functions. Moreover, it has been identified as one of the most dangerous heavy metals besides lead (Pb) (Ghosh, 2010GHOSH, S., 2010. Wetland macrophytes as toxic metal accumulators. International Journal of Environmental Science, vol. 1, no. 4, pp. 523-528.). Cd enormous presence harms the environment by building up a serious threat to human health throughout the food chain (Rafiq et al., 2014RAFIQ, M.T., AZIZ, R., YANG, X., XIAO, W., RAFIQ, M.K., ALI, B. and LI, T., 2014. Cadmium phytoavailability to rice (Oryza sativa L.) grown in representative Chinese soils. A model to improve soil environmental quality guidelines for food safety. Ecotoxicology and Environmental Safety, vol. 103, pp. 101-107. http://dx.doi.org/10.1016/j.ecoenv.2013.10.016 PMid:24418797.
http://dx.doi.org/10.1016/j.ecoenv.2013....
). This problem cannot be disregarded, as it inevitably could lead to chronic health problems (Ali et al., 2015aALI, B., GILL, R.A., YANG, S., GILL, M.B., FAROOQ, M.A., LIU, D., DAUD, M.K., ALI, S. and ZHOU, W., 2015a. Regulation of cadmium-induced proteomic and metabolic changes by 5-aminolevulinic acid in leaves of Brassica napus L. PLoS One, vol. 10, no. 4, pp. e0123328. http://dx.doi.org/10.1371/journal.pone.0123328. PMid:25909456.
http://dx.doi.org/10.1371/journal.pone.0...
). It is worth known as precarious metals due to the diversity of health effects even being exposed at low concentrations.

Cadmium is manufactured and released into the environment as non-essential elements. It is released into the air by mines, metal smelters, and alloys, batteries, pigments, and plastics industries (Harrison, 2001HARRISON, N., 2001. Inorganic contaminants in food. In: D.H. WATSON, ed. Food chemical safety contaminants. Cambridge: Woodhead Publishing, pp. 148-168.). Notwithstanding this, the advanced agriculture industry has contributed to an anthropogenic means of aggravating Cd levels in agricultural soil (Leduc and Terry, 2005LEDUC, D.L. and TERRY, N., 2005. Phytoremediation of toxic trace elements in soil and water. Journal of Industrial Microbiology & Biotechnology, vol. 32, no. 11-12, pp. 514-520. http://dx.doi.org/10.1007/s10295-005-0227-0. PMid:15883830.
http://dx.doi.org/10.1007/s10295-005-022...
). Cadmium also has been detected in sewage sludge which is generally employed as plant fertilizer. Thus, Cd could be discovered in different vegetables and animals tissues (Kumar et al., 2007KUMAR, P., PRASAD, Y., PATRA, A.K. and SWARUP, D., 2007. Levels of cadmium and lead in tissues of freshwater fish (Clarias batrachus L.) and chicken in Western UP (India). Bulletin of Environmental Contamination and Toxicology, vol. 79, no. 4, pp. 396-400. http://dx.doi.org/10.1007/s00128-007-9263-y. PMid:17721732.
http://dx.doi.org/10.1007/s00128-007-926...
). Zhong et al. (2018)ZHONG, T., XUE, D., ZHAO, L. and ZHANG, X., 2018. Concentration of heavy metals in vegetables and potential health risk assessment in China. Environmental Geochemistry and Health, vol. 40, no. 1, pp. 313-322. PMid:28194624. emphasized that poor farming practices and untreated industrial utilization and municipal waste for irrigation have additionally contributed to agricultural soil contamination. Export of phosphate fertilizers, wastewater, Cd contaminated sewage sludge, and manure, metal processing, industrial traffic, and cement factories have also become the primary anthropogenic sources (Yang et al., 2004YANG, X.E., LONG, X.X., YE, H.B., HE, Z.L., CALVERT, D.V. and STOFFELLA, P.J., 2004. Cadmium tolerance and hyperaccumulation in a new Zn hyperaccumulating plant species (Sedum alfredii Hance). Plant and Soil, vol. 259, no. 1, pp. 181-189.). Only inorganic salts of cadmium are found in foods. Eventually, the intake of Cd occurs through the consumption of food sources from agricultural products that have been contaminated (UNEP, 2010UNITED NATIONS ENVIRONMENT PROGRAMME – UNEP, 2010. Chemicals Branch - DTIE: final review of scientific information on cadmium. Geneva: UNEP.). High levels of Cd are found in nuts and oilseeds. To minimize the uptake of Cd, most developed countries have set the maximum acceptable limit (5 μg dm3) in drinking water (UNEP, 2010UNITED NATIONS ENVIRONMENT PROGRAMME – UNEP, 2010. Chemicals Branch - DTIE: final review of scientific information on cadmium. Geneva: UNEP.). Apart from that, the breeding techniques have been applied to develop fewer Cd-uptake crops (Smolders and Mertens, 2013SMOLDERS, E. and MERTENS, J. 2013. Cadmium. In: B.J. ALLOWAY, ed. Heavy metals in soils: trace metals and metalloids in soils and their bioavailability. Dordrecht: Springer, pp. 283–311).

The plants readily absorb cadmium ions as opposed to lead and mercury ions. They are distributed equally over the plant. Cd is taken up to edible leaves, fruits, and seeds through the plant roots. Physiological mechanism was highlighted to be immensely helpful in carrying out tolerance response to stresses faced by most plant species (Siddiqui et al., 2012SIDDIQUI, M.H., AL-WHAIBI, M.H., SAKRAN, A.M., BASALAH, M.O. and ALI, H.M., 2012. Effect of calcium and potassium on antioxidant system of Vicia faba L. under cadmium stress. International Journal of Molecular Sciences, vol. 13, no. 6, pp. 6604-6619. http://dx.doi.org/10.3390/ijms13066604. PMid:22837652.
http://dx.doi.org/10.3390/ijms13066604...
). The abiotic stress initiated by Cd at phytotoxicity levels may introduce biochemical and molecular disturbances, subsequently causes oxidative stress. Oxidative stress unnecessarily affects the plant cells result in DNA, cell membrane and protein damage, genetic mutation, lipid peroxidation, growth and development reduction (Hossain et al., 2012HOSSAIN, M.A., PIYATIDA, P., SILVA, J.A.T. and FUJITA, M., 2012. Molecular mechanism of heavy metal toxicity and tolerance in plants: central role of glutathione in detoxification of reactive oxygen species and methylglyoxal and in heavy metal chelation. Le Journal de Botanique, vol. 2012, pp. 872875. http://dx.doi.org/10.1155/2012/872875.
http://dx.doi.org/10.1155/2012/872875...
). Instead of that, phytotoxicity of Cd triggers the alteration of mineral nutrients uptake and inhibition of stomatal opening by disrupting the water content of the plants (Hossain et al., 2010HOSSAIN, M.A., HASANUZZAMAN, M. and FUJITA, M., 2010. Up-regulation of antioxidant and glyoxalase systems by exogenous glycinebetaine and proline in mung bean confer tolerance to cadmium stress. Physiology and Molecular Biology of Plants, vol. 16, no. 3, pp. 259-272. http://dx.doi.org/10.1007/s12298-010-0028-4. PMid:23572976.
http://dx.doi.org/10.1007/s12298-010-002...
), photosynthesis, carbohydrate metabolism (Shi et al., 2009SHI, G.R., CAI, Q.S., LIU, Q.Q. and WU, L., 2009. Salicylic acid-mediated alleviation of cadmium toxicity in hemp plants in relation to cadmium uptake, photosynthesis, and antioxidant enzymes. Acta Physiologiae Plantarum, vol. 31, no. 5, pp. 969-977. http://dx.doi.org/10.1007/s11738-009-0312-5.
http://dx.doi.org/10.1007/s11738-009-031...
) and antioxidants metabolism (Khan et al., 2009KHAN, N.A., ANJUM, N.A., NAZAR, R. and IQBAL, N., 2009. Increased activity of ATP-sulfurylase and increased contents of cysteine and glutathione reduce high cadmium-induced oxidative stress in mustard cultivar with high photosynthetic potential. Russian Journal of Plant Physiology: a Comprehensive Russian Journal on Modern Phytophysiology, vol. 56, no. 5, pp. 670-677. http://dx.doi.org/10.1134/S1021443709050136.
http://dx.doi.org/10.1134/S1021443709050...
).

The most recent remediation strategies including physicochemical alteration and bioremediation are widely applied to alleviate Cd in soil. Application of physical methods and chemical approaches may be successful and efficient in their respective situations. However, they are universally perceived as being prohibitively expensive and failing to completely remove the element from the soil. The drawbacks of physicochemical strategies are restricting the amount of work that can be put into lowering Cd in the environment. Thus, bioremediation of Cd, particularly phytoremediation, has been extensively practised in which plants can absorb the metal and thereby lower its bioavailability (DalCorso et al., 2019DALCORSO, G., FASANI, E., MANARA, A., VISIOLI, G. and FURINI, A., 2019. Heavy metal pollutions: state of the art and innovation in phytoremediation. International Journal of Molecular Sciences, vol. 20, no. 14, pp. 3412. http://dx.doi.org/10.3390/ijms20143412. PMid:31336773.
http://dx.doi.org/10.3390/ijms20143412...
). This review seeks to provide an understanding of cadmium remediation alternatives in the context of phytoremediation approaches as well as factors of plant uptake, mode of absorption and bioaccumulation, and toxic effects of cadmium in plants.

2. Materials and Methods

This review study makes use of a collection of journals obtained from a journal database as well as a literature review of worldwide scientific journals written in English to conduct its research. This review examines studies and essential discoveries from years spanning from 2000 to 2021, and it offers a comparison of the research and noteworthy findings. A complete description and summary of prior investigations were provided in the form of tables and figures, which highlighted the findings.

3. Soil Properties and Cd Bioavailability

The environment is unearned affected by cadmium, and its toxicity possessions are ongoing distressing humans, plants, and animals. Cd shows high mobility of all other trace elements effortlessly entering the ecological food chain through soil. This exposure happens due to the contribution of intensive modern agricultural activities, the rapid rate of industrialization, and an increase in urbanization (Zhao et al., 2015ZHAO, F.J., MA, Y., ZHU, Y.G., TANG, Z. and MCGRATH, S.P., 2015. Soil contamination in china: current status and mitigation strategies. Environmental Science & Technology, vol. 49, no. 2, pp. 750-759. PMid:25514502.). The availability of Cd in soil influences the absorption of the element, however not all of the Cd presents in soil is available for plant uptake. Bioavailability and high mobility of Cd in soil increase the capability of the plant to accumulate the elements to other plants parts. However, other factors must be considered like soil pH (Yang et al., 2016YANG, Y., CHEN, W., WANG, M. and PENG, C., 2016. Regional accumulation characteristics of cadmium in vegetables: influencing factors, transfer model and indication of soil threshold content. Environmental Pollution, vol. 219, pp. 1036-1043. http://dx.doi.org/10.1016/j.envpol.2016.09.003. PMid:27613317.
http://dx.doi.org/10.1016/j.envpol.2016....
), temperature (Silber et al., 2012SILBER, A., BAR-YOSEF, B., SURYANO, S. and LEVKOVITCH, I., 2012. Zinc adsorption by perlite: effects of pH, ionic strength, temperature, and pre-use as growth substrate. Geoderma, vol. 170, pp. 159-167. http://dx.doi.org/10.1016/j.geoderma.2011.11.028.
http://dx.doi.org/10.1016/j.geoderma.201...
), reduction potential (Meng et al., 2019MENG, D., LI, J., LIU, T., LIU, Y., YAN, M., HU, J., LI, X., LIU, X., LIANG, Y., LIU, H. and YIN, H., 2019. Effects of redox potential on soil cadmium solubility: insight into microbial community. Journal of Environmental Sciences, vol. 75, pp. 224-232. http://dx.doi.org/10.1016/j.jes.2018.03.032 PMid:30473288.
http://dx.doi.org/10.1016/j.jes.2018.03....
), organic matter content (Mohamed et al., 2010MOHAMED, I., AHAMADOU, B., LI, M., GONG, C., CAI, P., LIANG, W. and HUANG, Q., 2010. Fractionation of copper and cadmium and their binding with soil organic matter in a contaminated soil amended with organic materials. Journal of Soils and Sediments, vol. 10, pp. 973-982.), capacity of cation exchange (Jiang et al. 2012JIANG, J., XU, R.K., JIANG, T.Y. and LI, Z., 2012. Immobilization of Cu(II), Pb(II) and Cd(II) by the addition of rice straw derived biochar to a simulated polluted Ultisol. Journal of Hazardous Materials, vol. 229–230, pp. 145-150. http://dx.doi.org/10.1016/j.jhazmat.2012.05.086. PMid:22704774.
http://dx.doi.org/10.1016/j.jhazmat.2012...
), and availability of other elements are noticeable substantial in the process (Helios-Rybicka and Wójcik, 2012HELIOS-RYBICKA, E. and WÓJCIK, R., 2012. Competitive sorption/desorption of Zn, Cd, Pb, Ni, Cu, and Cr by clay-bearing mining wastes. Applied Clay Science, vol. 65-66, pp. 6-13. http://dx.doi.org/10.1016/j.clay.2012.06.006.
http://dx.doi.org/10.1016/j.clay.2012.06...
).

3.1 The role of soil pH

The bioavailability of Cd in soil is apparently influenced by pH level of the soil. Acidic pH is frequently related to increase Cd bioavailability. Lower pH values resulted in a substantial increase of Cd bioavailability, therefore permitting the absorption activity of this element to other plant tissue (Yang et al., 2016YANG, Y., CHEN, W., WANG, M. and PENG, C., 2016. Regional accumulation characteristics of cadmium in vegetables: influencing factors, transfer model and indication of soil threshold content. Environmental Pollution, vol. 219, pp. 1036-1043. http://dx.doi.org/10.1016/j.envpol.2016.09.003. PMid:27613317.
http://dx.doi.org/10.1016/j.envpol.2016....
). As reported by Rafiq et al. (2014)RAFIQ, M.T., AZIZ, R., YANG, X., XIAO, W., RAFIQ, M.K., ALI, B. and LI, T., 2014. Cadmium phytoavailability to rice (Oryza sativa L.) grown in representative Chinese soils. A model to improve soil environmental quality guidelines for food safety. Ecotoxicology and Environmental Safety, vol. 103, pp. 101-107. http://dx.doi.org/10.1016/j.ecoenv.2013.10.016 PMid:24418797.
http://dx.doi.org/10.1016/j.ecoenv.2013....
, Cd concentrations in the grains of rice were found to be negatively and significantly correlated with soil pH. The acidic environment might offer higher chances of Cd transformation from immobile form into freely bioavailable form (Cd2+) (Li et al., 2014LI, L., WU, H., VAN GESTEL, C.A., PEIJNENBURG, W.J. and ALLEN, H.E., 2014. Soil acidification increases metal extractability and bioavailability in old orchard soils of Northeast Jiaodong Peninsula in China. Environmental Pollution, vol. 188, pp. 144-152. http://dx.doi.org/10.1016/j.envpol.2014.02.003. PMid:24583712.
http://dx.doi.org/10.1016/j.envpol.2014....
). In particular, acidic solution affords Cd2+ that will compete with H+ for binding sites of soil particles. Though, less competition from Cd2+ ions causing the H+ ions are more readily adsorbed on soil and organic matter. As a result, Cd desorption from soil particles into soil solution is enhanced, making Cd2+ ions more available for absorption by plant root cells (Zhai et al., 2018ZHAI, X., LI, Z., HUANG, B., LUO, N., HUANG, M., ZHANG, Q. and ZENG, G., 2018. Remediation of multiple heavy metal-contaminated soil through the combination of soil washing and in situ immobilization. The Science of the Total Environment, vol. 635, pp. 92-99. http://dx.doi.org/10.1016/j.scitotenv.2018.04.119. PMid:29660731.
http://dx.doi.org/10.1016/j.scitotenv.20...
).

Contrary to the basic environment, Cd is hydrolysed into hydroxy species like Cd(OH)+, enhancing the adsorption affinity to solid-phase exchange sites of soil. Thus, the mobilization and accumulation of Cd in plant will be substantially condensed (Shahid et al., 2017SHAHID, M., DUMAT, C., KHALID, S., NIAZI, N.K. and ANTUNES, P.M.C., 2017. Cadmium bioavailability, uptake, toxicity and detoxification in soil-plant system. Reviews of Environmental Contamination and Toxicology, vol. 241, pp. 73-137. PMid:27300014.). Furthermore, an increasing soil pH generates superfluous negatively charged adsorption sites on the soil colloid, resulting in increasing Cd2+ adsorption (Chen et al., 2019CHEN, G., SHAH, K.J., SHI, L., CHIANG, P.C. and YOU, Z., 2019. Red soil amelioration and heavy metal immobilization by a multi-element mineral amendment: performance and mechanisms. Environment and Pollution, vol. 254, no. Part A, pp. 112964. http://dx.doi.org/10.1016/j.envpol.2019.112964. PMid:31376602.
http://dx.doi.org/10.1016/j.envpol.2019....
). According to Rao et al. (2013)RAO, Z.X., HUANG, D.Y., ZHU, Q.H., LIU, S.L., LUO, Z.C., CAO, X.L., REN, X.F.,WANG, J.Y. and WANG, S., 2013. Effects of amendments on the availability of Cd in contaminated paddy soil: a three-year field experiment. Journal of Food Agriculture and Environment. vol. 11, pp. 2009-2014., the adsorption of Cd was enhanced by a factor of 0.35-0.37 with each pH unit decline in acidic paddy soils. Moreover, higher pH might also trigger the precipitation of Cd2+ into immobilized Cd(OH)2 form (Bolan et al., 2014BOLAN, N., KUNHIKRISHNAN, A., THANGARAJAN, R., KUMPIENE, J., PARK, J., MAKINO, T., KIRKHAM, M.B. and SCHECKEL, K., 2014. Remediation of heavy metal (loid) s contaminated soils–to mobilize or to immobilize? Journal of Hazardous Materials, vol. 266, pp. 141-166. http://dx.doi.org/10.1016/j.jhazmat.2013.12.018. PMid:24394669.
http://dx.doi.org/10.1016/j.jhazmat.2013...
).

3.2. The role of organic matter

Soil organic matter is the organic component of soil that is composed of three essential components: small plant remnants and small live soil organisms, decomposing organic matter, and stable organic matter or often known as humus. The breakdown of plants and animals as a part of organic matter components contributes significantly to Cd bioavailability through the formation of Cd and soil medium complex. The presence of organic matters in soil promotes the formation of stable organometallic complexes, which can limit the solubility of Cd in soil environment (Mohamed et al., 2010MOHAMED, I., AHAMADOU, B., LI, M., GONG, C., CAI, P., LIANG, W. and HUANG, Q., 2010. Fractionation of copper and cadmium and their binding with soil organic matter in a contaminated soil amended with organic materials. Journal of Soils and Sediments, vol. 10, pp. 973-982.). One of the recent findings reported that the addition of biochar enhanced the organic content of the soil, which improved its potential to stabilize Cd by transforming the element from labile fraction into less available form. Subsequently, the uptake of Cd by maize plants has been dramatically reduced (Alaboudi et al., 2019ALABOUDI, K.A., AHMED, B. and BRODIE, G., 2019. Effect of biochar on Pb, Cd and Cr availability and maize growth in artificial contaminated soil. Annals of Agricultural Science, vol. 64, no. 1, pp. 95-102. http://dx.doi.org/10.1016/j.aoas.2019.04.002.
http://dx.doi.org/10.1016/j.aoas.2019.04...
). Besides, the collaboration of organic matter and selenium has also proven to attenuate the Cd bioaccumulation in rice plantation system (Figure 1). According to the study, a range of 5.8% to 20.8% of Cd reduction could be attained efficaciously (Liu et al., 2020LIU, N., JIANG, Z., LI, X., LIU, H., LI, N. and WEI, S., 2020. Mitigation of rice cadmium (Cd) accumulation by joint application of organic amendments and selenium (Se) in high-Cd-contaminated soils. Chemosphere, vol. 241, pp. 125106. http://dx.doi.org/10.1016/j.chemosphere.2019.125106. PMid:31683428.
http://dx.doi.org/10.1016/j.chemosphere....
).

Figure 1
The effects of soil pH and organic matter in soil towards Cd bioavailability. The diagram shows the reduction of Cd uptake and Cd biovailability by manipulating soil pH and soil organic matter. Adopted from Liu et al. (2020)LIU, N., JIANG, Z., LI, X., LIU, H., LI, N. and WEI, S., 2020. Mitigation of rice cadmium (Cd) accumulation by joint application of organic amendments and selenium (Se) in high-Cd-contaminated soils. Chemosphere, vol. 241, pp. 125106. http://dx.doi.org/10.1016/j.chemosphere.2019.125106. PMid:31683428.
http://dx.doi.org/10.1016/j.chemosphere....
.

In another research, the introduction of biochar into Cd-contaminated soil also has reduced Cd bioavailability and increased the growth of oak forest seedlings. The amendment of biochar at rates of 1%, 3%, and 5% in 50 mg kg−1 of Cd-contaminated soil significantly enhanced the plant tolerance index by 40.9%, 56%, and 60.6%, respectively (Amirahmadi et al., 2020AMIRAHMADI, E., MOHAMMAD HOJJATI, S., KAMMANN, C., GHORBANI, M. and BIPARVA, P., 2020. The potential effectiveness of biochar application to reduce soil cd bioavailability and encourage oak seedling growth. Applied Sciences, vol. 10, no. 10, pp. 3410. http://dx.doi.org/10.3390/app10103410.
http://dx.doi.org/10.3390/app10103410...
). A reduction of total Cd can also be observed in switchgrass (Panicum virgatum) with incorporation of compost blend containing either beef cattle manure biochar or poultry litter biochar. Approximately 5% of both compost and beef cattle manure biochar lead to greater reduction in total Cd extracted from shoots and roots (Novak et al., 2019NOVAK, J.M., IPPOLITO, J.A., WATTS, D.W., SIGUA, G.C., DUCEY, T.F. and JOHNSON, M.G., 2019. Biochar compost blends facilitate switchgrass growth in mine soils by reducing Cd and Zn bioavailability. Biochar, vol. 1, no. 1, pp. 97-114. http://dx.doi.org/10.1007/s42773-019-00004-7.
http://dx.doi.org/10.1007/s42773-019-000...
). The combined application of peat and Fe(NO3)3 showed considerably increased effects on As and Cd immobility, hence restricting Cd bioavailability throughout the rice growth stages (Wang et al., 2019WANG, X.Y.U., LI, H.Y., LIU, F., WU, T., LIU, W. and LIU, C., 2019. Enhanced immobilization of arsenic and cadmium in a paddy soil by combined applications of woody peat and Fe(NO3)3: possible mechanisms and environmental implications. The Science of the Total Environment, vol. 649, pp. 535-543. http://dx.doi.org/10.1016/j.scitotenv.2018.08.387. PMid:30176464.
http://dx.doi.org/10.1016/j.scitotenv.20...
).

3.3. The role of reduction potential

The “redox potential” or “reduction potential” is used to describe the combination of reduction and oxidation reactions in the simplest terms. In other words, redox potential or reduction potential is the tendency of chemical substances to accept or donate electrons (DeLaune and Reddy, 2005DELAUNE, R.D. and REDDY, K.R. 2005. Redox potential. In: D. HILLEL and J.L. HATFIELD, eds. Encyclopedia of soils in the environment. Oxford: Elsevier, pp. 366-371. . http://dx.doi.org/10.1016/B0-12-348530-4/00212-5.
http://dx.doi.org/10.1016/B0-12-348530-4...
). In fact, redox potential is one of the critical factors manipulating Cd bioavailability by altering the solubility of Cd in soil, organic matter, minerals and rhizosphere microbes regulation (Meng et al., 2019MENG, D., LI, J., LIU, T., LIU, Y., YAN, M., HU, J., LI, X., LIU, X., LIANG, Y., LIU, H. and YIN, H., 2019. Effects of redox potential on soil cadmium solubility: insight into microbial community. Journal of Environmental Sciences, vol. 75, pp. 224-232. http://dx.doi.org/10.1016/j.jes.2018.03.032 PMid:30473288.
http://dx.doi.org/10.1016/j.jes.2018.03....
). Under oxidizing conditions (aerobic), Cd is frequently found as soluble salts and cationic ions (Cd2+). In contrast, it does exist as precipitates like CdS and CdCO3 under reducing conditions (anaerobic) (Sebastian and Prasad, 2013SEBASTIAN, A. and PRASAD, M.N.V., 2013. Cadmium minimization in rice: a review. Agronomy for Sustainable Development, vol. 34, no. 1, pp. 155-173. http://dx.doi.org/10.1007/s13593-013-0152-y.
http://dx.doi.org/10.1007/s13593-013-015...
). Typically, the change in redox potential condition would change the electron acceptors and the chelating capacity with Cd2+ (Nazar et al., 2012NAZAR, R., IQBAL, N., MASOOD, A., KHAN, M.I.R., SYEED, S. and KHAN, N.A., 2012. Cadmium toxicity in plants and role of mineral nutrients in its alleviation. American Journal of Plant Sciences, vol. 3, no. 10, pp. 1476-1489. http://dx.doi.org/10.4236/ajps.2012.310178.
http://dx.doi.org/10.4236/ajps.2012.3101...
).

A study has been conducted using azolla or known as mosquito fern to reduce the redox potential in soil. The incorporation of azolla resulted in an 80.3% reduction in Cd uptake and a 13.4% increase in plant yield, respectively (Liu et al., 2021LIU, C., GUO, B., LI, H., FU, Q., LI, N., LIN, Y. and XU, G., 2021. Azolla incorporation under flooding reduces grain cadmium accumulation by decreasing soil redox potential. Scientific Reports, vol. 11, no. 1, pp. 6325. http://dx.doi.org/10.1038/s41598-021-85648-x. PMid:33737581.
http://dx.doi.org/10.1038/s41598-021-856...
). The addition of periphyton has reduced potential reduction of rice cultivation soil by 40-120 mv led to higher Cd concentrations in soils (Lu et al., 2020LU, H., DONG, Y., FENG, Y., BAI, Y., TANG, X., LI, Y., YANG, L. and LIU, J., 2020. Paddy periphyton reduced cadmium accumulation in rice (Oryza sativa) by removing and immobilizing cadmium from the water-soil interface. Environmental Pollution, vol. 261, pp. 114103. http://dx.doi.org/10.1016/j.envpol.2020.114103 PMid:32066051.
http://dx.doi.org/10.1016/j.envpol.2020....
). In fact, the mobility of Cd in paddy soils was much affected by this reduction that leads to metals reductive dissolution including iron (Fe) and manganese (Mn) oxides (Wang et al., 2019WANG, X.Y.U., LI, H.Y., LIU, F., WU, T., LIU, W. and LIU, C., 2019. Enhanced immobilization of arsenic and cadmium in a paddy soil by combined applications of woody peat and Fe(NO3)3: possible mechanisms and environmental implications. The Science of the Total Environment, vol. 649, pp. 535-543. http://dx.doi.org/10.1016/j.scitotenv.2018.08.387. PMid:30176464.
http://dx.doi.org/10.1016/j.scitotenv.20...
).

4. Cadmium Toxicity Symptoms in Plant

The excess amount of Cd causes the toxicity of this chemical element and usually being measured by monitoring any abnormalities tackling by the plants. Apparent symptoms of Cd stress enable them to diagnose the severity of toxicity which is therefore useful in the sighting of stress effects. This will make it easier to coordinate successful approaches to strengthen the plant endurances to stress (Ahmad et al., 2003AHMAD, R., IKRAAM, M., EHSAN, U. and MAHMOOD, A., 2003. Influence of different fertilizer levels on the growth and productivity of three Mungbean (Vigna radiata L.) cultivars. International Journal of Agriculture and Biology, vol. 05, pp. 335-338.). The visual and non-visual symptoms of Cd toxicity have been observed by many researchers. The noticeable identified Cd toxicity symptoms entail growth stunting, chlorosis (leaf discolouration), necrosis, wilting, photosynthesis rate reduction and respiration inhibition (Navarro-León et al., 2019NAVARRO-LEÓN, E., OVIEDO-SILVA, J., RUIZ, J.M. and BLASCO, B., 2019. Possible role of HMA4a TILLING mutants of Brassica rapa in cadmium. Ecotoxicology and Environmental Safety, vol. 180, pp. 88-94. http://dx.doi.org/10.1016/j.ecoenv.2019.04.081 PMid:31078020.
http://dx.doi.org/10.1016/j.ecoenv.2019....
). At the same time, non-visual symptoms include biomass reduction and changes in mineral composition and symptoms at the sub-cellular level (Sanita di Toppi and Gabrielli, 1999SANITA DI TOPPI, L., and GABRIELLI, R., 1999. Response to cadmium in higher plants. Environmental and Experimental Botany. Environmental and Experimental Botany, vol. 41, no. 2, pp. 105–130. https://doi.org/10.1016/S0098-8472(98)00058-6.
https://doi.org/10.1016/S0098-8472(98)00...
). Cd severely reduces biomass production and even leads to integral plant death (Dias et al., 2012DIAS, M.C., MONTEIRO, C., MOUTINHO-PEREIRA, J., CORREIA, C., GONÇALVES, B. and SANTOS, C., 2012. Cadmium toxicity affects photosynthesis and plant growth at different levels. Acta Physiologiae Plantarum, vol. 35, no. 4, pp. 1281-1289. http://dx.doi.org/10.1007/s11738-012-1167-8.
http://dx.doi.org/10.1007/s11738-012-116...
). It is believed that Cd might cause growth retardation, photosynthesis inhibition, enzyme induction and inhibition, stomatal action alteration and free radicals generation in plants. However, the symptoms are emerged depending on the level of concentration of absorbed Cd.

4.1. Stunted growth

Stunting of plant can be described as one of the plant diseases because of dwarfing and loss of vigour. This symptom is mainly caused by infectious or non-infectious means. Bacteria, fungi, viruses, and nematodes may infect the plant and cause incurable stunting symptom. In contrast, the non-infectious means are caused by the physical environment, nutrition imbalance, and physical or chemicals injuries. These means can typically readily be cured. Cd is one of the non-infectious means capable of distressing the process of macro-elements absorption and transport, causing growth inhibition of root and aerial parts of the plant (Souza et al., 2008SOUZA, V.L., ALMEIDA, A.A., HORA JÚNIOR, B.T., GESTEIRA, A.S. and CASCARDO, J.C., 2008. Preliminary analysis of expressed sequences of genes in Genipa americana L. Plant root exposed to cadmium in nutrient solution. Genetics and Molecular Research, vol. 7, no. 4, pp. 1282-1288. http://dx.doi.org/10.4238/vol7-4gmr516. PMid:19065763.
http://dx.doi.org/10.4238/vol7-4gmr516...
).

The increasing concentration of Cd caused severely stunted growth of Helianthus annuus (Turgut et al., 2004TURGUT, C., PEPE, M.K. and CUTRIGHT, T.J., 2004. The effect of EDTA and citric acid on phytoremediation of Cd, Cr and Ni from soil using Helianthus annuus. Environmental Pollution, vol. 131, no. 1, pp. 147-154. http://dx.doi.org/10.1016/j.envpol.2004.01.017. PMid:15210283.
http://dx.doi.org/10.1016/j.envpol.2004....
). Hydrocotyle umbellate has been discovered to be affected by Cd. Its growth is stunted, and there is no production of new plantlets (Panyakhan et al., 2006PANYAKHAN, S., KRUATRACHUE, M., POKETHITIYOOK, P., SOONTHORNSARATHOON, V. and UPATHAM, S., 2006. Toxicity and accumulation of cadmium and zinc in hydrocotyle umbellata. Science Asia, vol. 32, no. 3, pp. 323-328. http://dx.doi.org/10.2306/scienceasia1513-1874.2006.32.323.
http://dx.doi.org/10.2306/scienceasia151...
). Treatment of Cd onto three Pisum sativum L. genotypes has also shown plant growth inhibition symptom (Rahman et al., 2017RAHMAN, M.F., GHOSAL, A., ALAM, M.F. and KABIR, A.H., 2017. Remediation of cadmium toxicity in field peas (Pisum sativum L.) through exogenous silicon. Ecotoxicology and Environmental Safety, vol. 135, pp. 165-172. http://dx.doi.org/10.1016/j.ecoenv.2016.09.019. PMid:27736676.
http://dx.doi.org/10.1016/j.ecoenv.2016....
). Wunder et al. (2009)WUNDER, D.A., DROSTE, A. and WINDISH, P.G., 2009. Megaspore germination and initial development of Regnellidium diphyllum Lindman (Pteridophyta, Marsileaceae) sporophytes in the presence of cadmium. Revista Brasileira de Botanica. Brazilian Journal of Botany, vol. 32, no. 1, pp. 177-181. http://dx.doi.org/10.1590/S0100-84042009000100017.
http://dx.doi.org/10.1590/S0100-84042009...
pointed out that the similar effect can be observed in Regnellidium diphyllum, which demonstrated a significant reduction in root and leaf growth with no formation of secondary leaf. Plant growth is inhibited by storing the excess cadmium ion in place of other essential nutrients to the plant growth like Zn and Fe as well as other micro and macronutrients. Furthermore, the Cd stress normally has a negative effect on plant growth because it causes a decrease in chlorophyll concentration, which leads to an obstruction of photosynthesis. This inhibition is believed referring to the Photosystem II (PSII) damage (Chu et al., 2018CHU, J., ZHU, F., CHEN, X., LIANG, H., WANG, R., WANG, X. and HUANG, X., 2018. Effects of cadmium on photosynthesis of Schima superba young plant detected by chlorophyll fluorescence. Environmental Science and Pollution Research, vol. 25, no. 11, pp. 10679-10687. http://dx.doi.org/10.1007/s11356-018-1294-x. PMid:29392606.
http://dx.doi.org/10.1007/s11356-018-129...
) which is one of the photosynthetic components. Lipid peroxidation is also one of the plant responses that caused plant growth inhibition. In general, lipid peroxidation will cause cell damage or membrane damage that blocks natural antioxidants in cells and cause water imbalance (Hossain et al., 2012HOSSAIN, M.A., PIYATIDA, P., SILVA, J.A.T. and FUJITA, M., 2012. Molecular mechanism of heavy metal toxicity and tolerance in plants: central role of glutathione in detoxification of reactive oxygen species and methylglyoxal and in heavy metal chelation. Le Journal de Botanique, vol. 2012, pp. 872875. http://dx.doi.org/10.1155/2012/872875.
http://dx.doi.org/10.1155/2012/872875...
).

4.2. Chlorosis

It is possible to describe chlorosis as the loss of natural green pigmentation of plant leaves. This is a state of plants become impotent to produce enough chlorophyll pigments. Chlorophyll is a pigment accountable for the green colour in leaves. Hence, insufficient chlorophyll in plant results in light greenish, yellowish, or white-yellowish leaves. Since the green colour of the leaves is responsible for chlorophyll, the chlorotic leaves are pale, yellow, or white yellow. Chlorosis may be caused by several factors like insufficient light exposure and deficiency of iron, phosphorus and manganese (Sivasankar et al., 2012SIVASANKAR, R., KALAIKANDHAN, R. and VIJAYARENGAN, P., 2012. Phytoremediating capability and nutrient status of four plant species under zinc stress. International Journal of Research in Plant Science, vol. 2, no. 1, pp. 8-15.) in soil. As a result of chlorosis, sugars or carbohydrates cannot be produced by plant through photosynthesis process. It became worse as the plant may die if being untreated. However, the plant will suffer from other plant disease known as rust though being treated. It was clearly shown by Arabidopsis thaliana mutant ppi2 after being treated with exogenic sucrose to top up the sugars content in the plant (Kubis et al., 2004KUBIS, S., PATEL, R., COMBE, J., BÉDARD, J., KOVACHEVA, S., LILLEY, K., BIEHL, A., LEISTER, D., RIÉOS, G., KONCZ, C. and JARVIS, P., 2004. Functional specialization amongst the Arabidopsis Toc159 family of chloroplast protein import receptors. The Plant Cell, vol. 16, no. 8, pp. 2059-2077. http://dx.doi.org/10.1105/tpc.104.023309. PMid:15273297.
http://dx.doi.org/10.1105/tpc.104.023309...
).

Bioavailability of cadmium in soil contributes to its hyperaccumulation movement in plants, distressing the uptake of important nutrients like calcium, phosphorus, potassium, and water. These agonies readily onset several symptoms, particularly leaf chlorosis and photosynthetic rate reduction. Moreover, the previous study had identified a comparable symptom of chlorosis caused by Cd was determined with a decrease in these essential nutrients (Epstein and Bloom, 2005EPSTEIN, E. and BLOOM, A.J. 2005. Mineral nutrition of plant principles and perspective. 2nd ed. Massachusetts: Sinauer Associates.). This symptom has also been identified in some plant species such as pea (Rahman et al., 2017RAHMAN, M.F., GHOSAL, A., ALAM, M.F. and KABIR, A.H., 2017. Remediation of cadmium toxicity in field peas (Pisum sativum L.) through exogenous silicon. Ecotoxicology and Environmental Safety, vol. 135, pp. 165-172. http://dx.doi.org/10.1016/j.ecoenv.2016.09.019. PMid:27736676.
http://dx.doi.org/10.1016/j.ecoenv.2016....
), oilseed rape (Ali et al., 2015aALI, B., GILL, R.A., YANG, S., GILL, M.B., FAROOQ, M.A., LIU, D., DAUD, M.K., ALI, S. and ZHOU, W., 2015a. Regulation of cadmium-induced proteomic and metabolic changes by 5-aminolevulinic acid in leaves of Brassica napus L. PLoS One, vol. 10, no. 4, pp. e0123328. http://dx.doi.org/10.1371/journal.pone.0123328. PMid:25909456.
http://dx.doi.org/10.1371/journal.pone.0...
) and rice grown in soil contaminated with Cd (Rafiq et al., 2014RAFIQ, M.T., AZIZ, R., YANG, X., XIAO, W., RAFIQ, M.K., ALI, B. and LI, T., 2014. Cadmium phytoavailability to rice (Oryza sativa L.) grown in representative Chinese soils. A model to improve soil environmental quality guidelines for food safety. Ecotoxicology and Environmental Safety, vol. 103, pp. 101-107. http://dx.doi.org/10.1016/j.ecoenv.2013.10.016 PMid:24418797.
http://dx.doi.org/10.1016/j.ecoenv.2013....
) and other plants species as shown in Table 1.

Table 1
Enlisted plant showing visual symptoms of Cd toxicity.

4.3. Necrosis

Necrosis is a condition where the tissues and plant cells decease or generally deteriorate in response to abiotic stress. It is a symptom that appeared due to plant disease or distress that the plant is experiencing. Even though the plant does not require a large intake of nutrients, the absence of nutrients like potassium, nitrogen, iron, and nickel will lead to necrosis. Nutrient deficiency might happen due to soil depletion, imbalance of soil pH or unsuitable fertilizer application. Other factors that lead to necrosis include pathogen invasion, nematode, and fungal problems (Khan, 2008KHAN, M.R., 2008. Plant nematodes: methodology, morphology, systematics, biology and ecology. Boca Raton: CRC Press.; van Doorn et al., 2011VAN DOORN, W.G., BEERS, E.P., DANGL, J.L., FRANKLIN-TONG, V.E., GALLOIS, P., HARA-NISHIMURA, I., JONES, A.M., KAWAI-YAMADA, M., LAM, E., MUNDY, J., MUR, L.A., PETERSEN, M., SMERTENKO, A., TALIANSKY, M., VAN BREUSEGEM, F., WOLPERT, T., WOLTERING, E., ZHIVOTOVSKY, B. and BOZHKOV, P.V., 2011. Morphological classification of plant cell deaths. Cell Death and Differentiation, vol. 18, no. 8, pp. 1241-1246. http://dx.doi.org/10.1038/cdd.2011.36. PMid:21494263.
http://dx.doi.org/10.1038/cdd.2011.36...
). In the case of Cd, the metal competes with plant nutrients, especially in the root region. Cd ions compete with several mineral nutrients that usually have the same chemical properties to secure a spot in plant absorption. Consequently, the competition between cations causes nutrient depletion of the plant (Barcelo and Poschenrieder, 1990BARCELO, J. and POSCHENRIEDER, C., 1990. Plant water relations as affected by heavy metal stress: a review. Journal of Plant Nutrition, vol. 13, no. 1, pp. 1-3. http://dx.doi.org/10.1080/01904169009364057.
http://dx.doi.org/10.1080/01904169009364...
). Necrosis symptoms can be identified with dark watery and dry papery spots on plant parts with possibly dark colour. However, not all spots will appear dark as some may be yellow or wilted, which is one of sign of plant cell death activation. Commonly, necrosis might be caused by a particular disease, but somehow might be due to weather conditions and the quality of water source.

4.4. Biomass reduction

The unintentional absorption of Cd by plant contributes to a necessary unfavourable impact on plant health, including biomass reduction and photosynthetic efficiency (Drava et al., 2012DRAVA, G., ROCCOTIELLO, E., MINGANTI, V., MANFREDI, A. and CORNARA, L., 2012. Effects of cadmium and arsenic on Pteris vittata under hydroponic conditions. Environmental Toxicology and Chemistry, vol. 31, pp. 1375–1380. http://dx.doi.org/doi:10.1002/etc.1835.
http://dx.doi.org/doi:10.1002/etc.1835...
). Reduction of biomass as a result of the Cd toxicity effect can be regarded as phytotoxicity symptoms. The biomass of rice root was drastically reduced when grown in Cd contaminated soil (Yixia et al., 2020YIXIA, C., SHIHAO, Z., KUNZHENG, C., FEI, H., BOGUI, P. and WEI, W., 2020. Cd accumulation, biomass and yield of rice are varied with silicon application at different growth phases under high concentration cadmium-contaminated soil. Chemosphere, vol. 242, 125128. https://doi.org/10.1016/j.chemosphere.2019.125128.
https://doi.org/10.1016/j.chemosphere.20...
). The biomass of canola and Indian mustard were also identified to be affected with more than 70% of reduction (Turan and Esringu, 2007TURAN, M. and ESRINGU, A., 2007. Phytoremediation based on canola (Brassica napus L.) and Indian mustard (Brassica juncea L) planted on spiked soil by aliquot amount of Cd, Cu, Pb and Zn. Plant Soil Environment, vol. 53, pp. 7-15.). The same symptom was observed in the hydroponic system plantation of Pistia stratoites. The plant has tolerated up to 20 mg/L of Cd but still resulted in plant biomass reduction with high Cd treatment (Das et al., 2014DAS, S., GOSWAMI, S. and TALUKDAR, A.D., 2014. A study on Cadmium phytoremediation potential of water lettuce, Pistia stratoites L. Bulletin of Environmental Contamination and Toxicology, vol. 92, no. 2, pp. 169-174. http://dx.doi.org/10.1007/s00128-013-1152-y. PMid:24220931.
http://dx.doi.org/10.1007/s00128-013-115...
). Inhibition of plant growth by Cd is the vital reason for plant biomass lessening.

4.5. Photosynthesis inhibition

Photosynthesis is the most vital activity carried out by plants in order to maintain the existence of humans, animals, and the plant itself. The conversion of water and carbon dioxide into energy-rich compound (which serves as food supply for living organisms) should not be interfered. Nevertheless, the release of Cd into environment has stressed the plants photosynthetic system by destructing the chloroplast structure, at the same time impairing chlorophyll production (Lysenko at al., 2015LYSENKO, E.A., KLAUS, A.A., PSHYBYTKO, N.L. and KUSNETSOV, V.V., 2015. Cadmium accumulation in chloroplasts and its impact on chloroplastic processes in barley and maize. Photosynthesis Research, vol. 125, no. 1-2, pp. 291-303. http://dx.doi.org/10.1007/s11120-014-0047-z. PMid:25315190.
http://dx.doi.org/10.1007/s11120-014-004...
). Cd also reduced the total chlorophyll and carotenoid content of Brassica napus (Ali et al., 2015bALI, E., MAODZEKA, A., HUSSAIN, N., SHAMSI, I.H. and JIANG, L., 2015b. The alleviation of cadmium toxicity in oilseed rape (Brassica napus) by the application of salicylic acid. Plant Growth Regulation, vol. 75, no. 3, pp. 641-655. http://dx.doi.org/10.1007/s10725-014-9966-0.
http://dx.doi.org/10.1007/s10725-014-996...
). The lower amount of chlorophyll pigments may cause the discoloration of leaves.

In wheat, Cd caused a substantial reduction in net photosynthetic rate, fluorescence efficiency and stomatal conductance but also led an increase of intercellular carbon dioxide (Shafi et al., 2011SHAFI, M., BAKHT, J., RAZUDDIN., HAYAT, Y. and ZHANG, G.P., 2011. Genotypic difference in the inhibition of photosynthesis and chlorophyll fluorescence by salinity and cadmium stresses in wheat. Journal of Plant Nutrition, vol. 34, no. 3, pp. 315-323. http://dx.doi.org/10.1080/01904167.2011.536874.
http://dx.doi.org/10.1080/01904167.2011....
). The failure of plant to undergo photosynthesis activities will totally inhibit the plant growth. The photosynthesis process of Schima superba has likewise been hindered by Cd, which has interfered with electron transport between PSII and PSI, energy distribution of PSI and CO2 assimilation (Chu et al., 2018CHU, J., ZHU, F., CHEN, X., LIANG, H., WANG, R., WANG, X. and HUANG, X., 2018. Effects of cadmium on photosynthesis of Schima superba young plant detected by chlorophyll fluorescence. Environmental Science and Pollution Research, vol. 25, no. 11, pp. 10679-10687. http://dx.doi.org/10.1007/s11356-018-1294-x. PMid:29392606.
http://dx.doi.org/10.1007/s11356-018-129...
). Since carbon fixation or CO2 assimilation is most probably inhibited by Cd, a build-up of high excitation has been accumulated which has the potential to harm the photosystem (Chu et al., 2018CHU, J., ZHU, F., CHEN, X., LIANG, H., WANG, R., WANG, X. and HUANG, X., 2018. Effects of cadmium on photosynthesis of Schima superba young plant detected by chlorophyll fluorescence. Environmental Science and Pollution Research, vol. 25, no. 11, pp. 10679-10687. http://dx.doi.org/10.1007/s11356-018-1294-x. PMid:29392606.
http://dx.doi.org/10.1007/s11356-018-129...
). However, it can be reduced if the excess energy is disbursed through heat dissipation pathway.

5. Cd Translocation and Accumulation in Plants

Cd uptake occurs through transporters of calcium, ferric, magnesium, copper and zinc (Clemens, 2006CLEMENS, S., 2006. Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants. Biochimie, vol. 88, no. 11, pp. 1707-1719. http://dx.doi.org/10.1016/j.biochi.2006.07.003. PMid:16914250.
http://dx.doi.org/10.1016/j.biochi.2006....
). Calcium (Ca) and Cd competes for the same Ca channels to transport. Suzuzki (2005) suggested that the low concentration of exogenous Ca in Hoagland media had improved the uptake of Cd, consequently resulting in Cd toxicity. Cd is highly mobile and water-soluble, make it merely reaches the xylem by entering the root’s cortical tissues through apoplastic and/or a symplastic pathway, as shown in Figure 2. An event of formation of complexes comprising organic acids and phytochelatin may occur afterwards (Lux et al., 2011LUX, A., MARTINKA, M., VACULIK, M. and WHITE, P.J., 2011. Root responses to cadmium in the rhizosphere: a review. Journal of Experimental Botany, vol. 62, no. 1, pp. 21-37. http://dx.doi.org/10.1093/jxb/erq281 PMid:20855455.
http://dx.doi.org/10.1093/jxb/erq281...
). Usually, Cd ions are higher in the roots since the restriction of this metal, thus limiting its translocation to shoots, attributed to a complex system involving adsorption, chelation and compartmentalization (Niaz et al., 2015NIAZ, A., SIAL, R.A., IQBAL, M., NAZ, T., GHAFOOR, A. and MURTAZA, G., 2015. Growth, tissue concentration and bioaccumulation of cadmium by different mungbean cultivars in a hydroponic study. Pakistan Journal of Agricultural Sciences, vol. 52, no. 4, pp. 931-936.). The apoplastic region of roots is negatively charged because of carboxylic groups; they act as the first line of defence which is critical for retaining cations such as Cd in roots (Gajdos et al., 2012GAJDOS, E., LÉVAI, L., VERES, S. and KOVÁCS, B., 2012. Effects of biofertilizers on maize and sunflower seedlings under cadmium stress. Communications in Soil Science and Plant Analysis, vol. 43, no. 1-2, pp. 272-279. http://dx.doi.org/10.1080/00103624.2011.638591.
http://dx.doi.org/10.1080/00103624.2011....
).

Figure 2
The diagram is showing the uptake and translocation of cadmium from root-to-shoot through apoplastic (red line) and symplastic (blue line) pathway. Adopted from Science and Plants for Schools (SAPS, 2014SCIENCE AND PLANT FOR SCHOOL – SAPS [online], 2014 [viewed 13 May 2021]. Available from: https://www.saps.org.uk/library/newsletter/1292
https://www.saps.org.uk/library/newslett...
) of the University of Cambridge (2014).

5.1. Cell wall binding

The immobilization of Cd can be prominent protection against Cd translocation from root to upper part of plant that happens mainly at the root stage. The roots cell wall is the first barrier structure that can be successfully infiltrated by metals ion. Being the first living structure as a target of heavy metals toxicity, cell walls may involve in tolerance. An earlier study demonstrated that the Cd tolerance of plants was raised as a result of the cell wall's ability to inhibit Cd absorption (Gutsch et al., 2018GUTSCH, A., ZOUAGHI, S., RENAUT, J., CUYPERS, A., HAUSMAN, J.F. and SERGEANT, K., 2018. Changes in the proteome of leaves in response to long-term cadmium exposure using a cell-wall targeted approach. International Journal of Molecular Sciences, vol. 19, no. 9, pp. 2498. http://dx.doi.org/10.3390/ijms19092498. PMid:30149497.
http://dx.doi.org/10.3390/ijms19092498...
). A remarkably lower metals adsorption onto cell wall is expected to induce tolerance. Nonetheless, a considerable accumulation of metal has occurred between cell wall and cell membrane, as described in previous research (Mehes-Smith et al., 2013MEHES-SMITH, M., NKONGOLO, K. and CHOLEW, E., 2013. Coping mechanisms of plants to metal contaminated soil. In: S. SILVERN and S. YOUNG, eds. Environmental change and sustainabilit. Rijeka: IntechOpen, pp. 53-90. http://dx.doi.org/10.5772/55124.
http://dx.doi.org/10.5772/55124...
). Heavy metals portray various degrees of affinity towards cell wall, which will primarily bind to polygalacturonic acids. Usually, metals cation naturally binds to plant with the presence of functional groups in cell wall. The carboxyls group is a main functional group of pectin components of cell wall that bind to divalent and trivalent metal ions under metal stress (Mehes-Smith et al., 2013MEHES-SMITH, M., NKONGOLO, K. and CHOLEW, E., 2013. Coping mechanisms of plants to metal contaminated soil. In: S. SILVERN and S. YOUNG, eds. Environmental change and sustainabilit. Rijeka: IntechOpen, pp. 53-90. http://dx.doi.org/10.5772/55124.
http://dx.doi.org/10.5772/55124...
; Gutsch et al., 2018GUTSCH, A., ZOUAGHI, S., RENAUT, J., CUYPERS, A., HAUSMAN, J.F. and SERGEANT, K., 2018. Changes in the proteome of leaves in response to long-term cadmium exposure using a cell-wall targeted approach. International Journal of Molecular Sciences, vol. 19, no. 9, pp. 2498. http://dx.doi.org/10.3390/ijms19092498. PMid:30149497.
http://dx.doi.org/10.3390/ijms19092498...
).

Boominathan and Doran (2003)BOOMINATHAN, R. and DORAN, P.M., 2003. Organic acid complexation, heavy metal distribution and the effect of ATPase inhibition in hairy roots of hyperaccumulator plant species. Journal of Biotechnology, vol. 101, no. 2, pp. 131-146. http://dx.doi.org/10.1016/S0168-1656(02)00320-6. PMid:12568742.
http://dx.doi.org/10.1016/S0168-1656(02)...
identified the hairy roots of Thlaspi caerulescens were able to hold most of the Cd in the cell wall. According to a recent study, boron can reduce Cd toxicity in Brassica napus by enhancing Cd chelation onto cell walls in the shoots and roots of the plant (Wu et al., 2020WU, X., SONG, H., GUAN, C. and ZHANG, Z., 2020. Boron alleviates cadmium toxicity in Brassica napus by promoting the chelation of cadmium onto the root cell wall components. The Science of the Total Environment, vol. 728, pp. 138833. http://dx.doi.org/10.1016/j.scitotenv.2020.138833. PMid:32339843.
http://dx.doi.org/10.1016/j.scitotenv.20...
). The study found that boron also increased the pectin content of root by decreasing the pectinase activity, hence increased the chelation capacity. Likewise, the amount of cellulose and hemicellulos were significantly reduced resulting in expression of particular genes (expansin, xyloglucan endotransglucosylase, and α-xylosidase) that improved the cell wall integrity. A study on Sedum plumbizincicola also highlighted the importance role of cell walls in Cd hyperaccumulation and detoxification (Peng et al., 2017PENG, J.S., WANG, Y., DING, G., MA, H., ZHANG, Y. and GONG, J., 2017. A pivotal role of cell wall in cadmium accumulation in the crassulaceae hyperaccumulator Sedum plumbizincicola. Molecular Plant, vol. 10, no. 5, pp. 771-774. http://dx.doi.org/10.1016/j.molp.2016.12.007 PMid:28025117.
http://dx.doi.org/10.1016/j.molp.2016.12...
).

5.2. Roots exudates

Almost half of the photosynthesis products are transported to roots and about 12%–40% are released in rhizosphere as exudates such as polysaccharides, amino acids and proteins (Hinsinger et al., 2006HINSINGER, P., PLASSARD, C. and JAILLARD, B., 2006. Rhizosphere: a new frontier for soil biogeochemistry. Journal of Geochemical Exploration, vol. 88, no. 1-3, pp. 210-213. http://dx.doi.org/10.1016/j.gexplo.2005.08.041.
http://dx.doi.org/10.1016/j.gexplo.2005....
). Additionally, exudates like inorganic ligands are also released into rhizosphere (Dong et al., 2007DONG, J., MAO, W.H., ZHANG, G.P., WU, F.B. and CAI, Y., 2007. Root excretion and plant tolerance to cadmium toxicity– a review. Plant, Soil and Environment, vol. 53, no. 5, pp. 193-200. http://dx.doi.org/10.17221/2205-PSE.
http://dx.doi.org/10.17221/2205-PSE...
). These exudates are being secreted as a function of energy sources for microorganisms and may also play a role as ligands responsible for chelating heavy metal in rhizosphere.

Chelation of heavy metals activity influences the changes of pH and Eh (redox measurement) condition in rhizhosphere resulting in metals mobilization in soil and metals accumulation in plants. The cadmium uptake is expected to be influenced by the changes in soluble root exudates in rhizosphere activities. Root exudates counter with metal ions causing the changes of metal solubility, mobility and phytoavailability. Echinochloa crusgalli secretes root exudates containing oxalic acid and citric acid, enhanced the heavy metals translocation from root to shoots (Kim at al., 2010KIM, S., LIM, H. and LEE, I., 2010. Enhanced heavy metal phytoextraction by Echinochloa crusgalli using root exudates. Journal of Bioscience and Bioengineering, vol. 109, no. 1, pp. 47-50. http://dx.doi.org/10.1016/j.jbiosc.2009.06.018. PMid:20129081.
http://dx.doi.org/10.1016/j.jbiosc.2009....
). Dicarboxylic acids, exudates have been discovered in durum wheat rhizosphere mobilized cadmium in various type of soil (Krishnamurti et al., 1997KRISHNAMURTI, G.S.R., CIESLINSKI, G., HUANG, P.M. and VAN REES, K.C.J., 1997. Kinetics of cadmium release from soils as influenced by organic acids-implication in cadmium availability. Journal of Environmental Quality, vol. 26, no. 1, pp. 271-277. http://dx.doi.org/10.2134/jeq1997.00472425002600010038x.
http://dx.doi.org/10.2134/jeq1997.004724...
). Unidentified exudates of Nicotiana tabacum L., Nicotiana rustica L. and Zea mays L. showed the capability to isolate cadmium from soil, depending on its bioavailability (Mench and Martin, 1991MENCH, M. and MARTIN, E., 1991. Mobilization of cadmium and other metals from two soils by root exudates of Zea mays L., Nicotiana tabacum L. and Nicotiana rustica L. Plant and Soil, vol. 132, no. 2, pp. 187-196. http://dx.doi.org/10.1007/BF00010399.
http://dx.doi.org/10.1007/BF00010399...
). Römheld (1987)RÖMHELD, V., 1987. Different strategies for iron acquisiton in higher plants. Physiologia Plantarum, vol. 70, no. 2, pp. 231-234. http://dx.doi.org/10.1111/j.1399-3054.1987.tb06137.x.
http://dx.doi.org/10.1111/j.1399-3054.19...
reported the production of phytosiderophores compound by the root’s exudates under iron stress. Phytosiderophore from barley roots was observed to mobilize heavy metals from soil and believed might involve cadmium as well (Treeby et al., 1989TREEBY, M., MARSCHNER, H. and RÖMHELD, V., 1989. Mobilization of iron and other micronutrient cations from a calcareous soil by plant-borne, microbial, and synthetic chelators. Plant and Soil, vol. 114, no. 2, pp. 217-226. http://dx.doi.org/10.1007/BF02220801.
http://dx.doi.org/10.1007/BF02220801...
). The compound might influence the accumulation of cadmium due to Zn deficiency. The presence of cadmium in soil has reduced phytosiderophore production, instigating transition metal enhancement (Fan et al., 2001FAN, T.W.M., LANE, A.N., SHENKER, M., BARTLEY, J.P., CROWLEY, D. and HIGASHI, R.M., 2001. Comprehensive chemical profiling of gramineous plant root exudates using high-resolution NMR and MS. Phytochemistry, vol. 57, no. 2, pp. 209-221. http://dx.doi.org/10.1016/S0031-9422(01)00007-3. PMid:11382236.
http://dx.doi.org/10.1016/S0031-9422(01)...
).

5.3. Mycorrhiza

Mycorrhizas are sometimes left out to be considered as extracellular strategy (Jentschke and Godbold, 2000JENTSCHKE, G. and GODBOLD, D.L., 2000. Metal toxicity and ectomycorrhizas. Physiologia Plantarum, vol. 109, no. 2, pp. 107-116. http://dx.doi.org/10.1034/j.1399-3054.2000.100201.x.
http://dx.doi.org/10.1034/j.1399-3054.20...
) in coping heavy metals contamination. However, it is qualified as an effective method in ameliorating the phytotoxicity effects of metals on host plant (Hall, 2002HALL, J.L., 2002. Cellular mechanisms for heavy metal detoxification and tolerance. Journal of Experimental Botany, vol. 53, no. 366, pp. 1-11. http://dx.doi.org/10.1093/jexbot/53.366.1. PMid:11741035.
http://dx.doi.org/10.1093/jexbot/53.366....
). It is believed to tolerate metals at the cellular level with strategies of metals binding to extracellular materials or sequestration in the vacuolar compartment. Mycorrhizae adopt few mechanisms like absorption, adsorption, or chelation, which offer an effective exclusion barrier that limits the entrance of metals into the plant (Hall, 2002HALL, J.L., 2002. Cellular mechanisms for heavy metal detoxification and tolerance. Journal of Experimental Botany, vol. 53, no. 366, pp. 1-11. http://dx.doi.org/10.1093/jexbot/53.366.1. PMid:11741035.
http://dx.doi.org/10.1093/jexbot/53.366....
). Galli et al. (1994)GALLI, U., SCHUEPP, H. and BRUNOLD, C., 1994. Heavy metal binding by mycorrhizal fungi. Physiologia Plantarum, vol. 92, no. 2, pp. 364-368. http://dx.doi.org/10.1111/j.1399-3054.1994.tb05349.x.
http://dx.doi.org/10.1111/j.1399-3054.19...
reported that mycorrhizal fungi infection positively increases the P and N availability. In addition, the toxic level of metals might be reduced or enhanced by the fungus infection depending on fungal isolates, metal elements and condition of experiments. The mycorrhizal association of plant fungi and plant root is the primary factor in the tolerance and accumulation of metal from contaminated soil. It has been acknowledged that ectomycorrhizas and arbuscular mycorrhizae are the most reliable fungal association effectively reduce the harmful effects of heavy metals, thus enhancing plant tolerance in metal-contaminated soil (Jentschke and Godbold, 2000JENTSCHKE, G. and GODBOLD, D.L., 2000. Metal toxicity and ectomycorrhizas. Physiologia Plantarum, vol. 109, no. 2, pp. 107-116. http://dx.doi.org/10.1034/j.1399-3054.2000.100201.x.
http://dx.doi.org/10.1034/j.1399-3054.20...
). Arbuscular mycorrhizal fungal application in Cd-contaminated soil has proficiently reduced the Cd uptake even at high concentration. Whereas the Cd uptake would be enhanced or reduced at lower concentration of exposure or availability (Galli et al., 1994GALLI, U., SCHUEPP, H. and BRUNOLD, C., 1994. Heavy metal binding by mycorrhizal fungi. Physiologia Plantarum, vol. 92, no. 2, pp. 364-368. http://dx.doi.org/10.1111/j.1399-3054.1994.tb05349.x.
http://dx.doi.org/10.1111/j.1399-3054.19...
).

Arbuscular mycorrhizae were found to release insoluble glycoprotein or glomalin that can attach with Cu, Cd, and Pb (Göhre and Paszkowski, 2006GÖHRE, V. and PASZKOWSKI, U., 2006. Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation. Planta, vol. 223, no. 6, pp. 1115-1122. http://dx.doi.org/10.1007/s00425-006-0225-0. PMid:16555102.
http://dx.doi.org/10.1007/s00425-006-022...
) where approximately 1g of glomalin could remove up to 4.3 mg copper, 0.08 mg cadmium, and 1.12 mg lead, respectively. Although it would seem achievable tolerance method, there are practical limits to large different response of metal toxicity by plant and fungal species (Hall, 2002HALL, J.L., 2002. Cellular mechanisms for heavy metal detoxification and tolerance. Journal of Experimental Botany, vol. 53, no. 366, pp. 1-11. http://dx.doi.org/10.1093/jexbot/53.366.1. PMid:11741035.
http://dx.doi.org/10.1093/jexbot/53.366....
). Fungus has been found to increase the host tolerance only if its tolerance is way better than the host plant.

5.4. Membrane transport

The toxicity effects of cadmium could be triggered by the activity of oxidation, cross-linking of protein thiols, membrane protein inhibition, and composition and fluidity of membrane lipids changes (Meharg, 1993MEHARG, A.A., 1993. The role of plasmalemma in metal tolerance in angiosperms. Physiologia Plantarum, vol. 88, no. 1, pp. 191-198. http://dx.doi.org/10.1111/j.1399-3054.1993.tb01777.x.
http://dx.doi.org/10.1111/j.1399-3054.19...
). Quartacci et al. (2001)QUARTACCI, M.F., COSI, E. and NAVARI-IZZO, F., 2001. Lipids and NADPH-dependent superoxide production in plasma membranes vesicles from roots of wheat grown under copper deficiency or excess. Journal of Experimental Botany, vol. 52, no. 354, pp. 77-84. PMid:11181715. reported the toxic effect of cadmium towards lipid composition of membranes. Additionally, cadmium absorption shows the reduction of ATPase activity of wheat plasma membrane and sunflower roots (Fodor et al., 1995FODOR, A., SZABÓ-NAGY, A. and ERDEI, L., 1995. The effects of cadmium on the fluidity and H+-ATPase activity of plasma membrane from sunflower and wheat roots. Journal of Plant Physiology, vol. 147, no. 1, pp. 787-792. http://dx.doi.org/10.1016/S0176-1617(11)81418-5.
http://dx.doi.org/10.1016/S0176-1617(11)...
). The movements of Cd into cells are aided by membrane transport through passive or active process. The passive process may happen when the concentration of extracellular Cd ions is outwardly high. In contrast, an energy-requiring process or active transport is required at low Cd concentration. Dissociation of H2CO3 at root plasma membrane occurs during root respiration producing intercellular HCO3 and H+. Free extracellular Cd2+ will be then absorbed by root epidermis cells surface after rapid shift with H+. This event may possibly happen through apoplastic pathway (Yamaguchi et al., 2011YAMAGUCHI, N., MORI, S., BABA, K., KABURAGI-YADA, S., ARAO, T., KITAJIMA, N., HOKURA, A. and TERADA, Y., 2011. Cadmium distribution in the root tissues of solanaceous plants with contrasting root-to-shoot Cd translocation efficiencies. Environmental and Experimental Botany, vol. 71, no. 2, pp. 198-206. http://dx.doi.org/10.1016/j.envexpbot.2010.12.002.
http://dx.doi.org/10.1016/j.envexpbot.20...
).

Additionally, the absorption of Cd is readily regulated as roots hairs offer extensive contact of the roots with greater surface area through root tissues via diffusion (Seregin and Ivanov, 2001SEREGIN, I.V. and IVANOV, V.B., 2001. Physiological aspects of cadmium and lead toxic effects on higher plants. Russian Journal of Plant Physiology: a Comprehensive Russian Journal on Modern Phytophysiology, vol. 48, no. 4, pp. 523-544. http://dx.doi.org/10.1023/A:1016719901147.
http://dx.doi.org/10.1023/A:101671990114...
). The enhancement of ion absorption by root hairs is needed as the ion diffusion in soil is terribly slow. Besides, this extracellular Cd2+ also carried up by non-selective cation channels, Zn/Fe-regulated transporters (Shahid et al., 2017SHAHID, M., DUMAT, C., KHALID, S., NIAZI, N.K. and ANTUNES, P.M.C., 2017. Cadmium bioavailability, uptake, toxicity and detoxification in soil-plant system. Reviews of Environmental Contamination and Toxicology, vol. 241, pp. 73-137. PMid:27300014.).

5.5. Root-to-shoot translocation

Roots are the first structure involve in cadmium allocation in various plant parts. The accumulation of Cd in shoot is much likely to hinge on the root factors. Shoot may accumulate Cd relying on the level of metals emanated by root. Roots might contain high amount of Cd and released a small portion of it towards shoot or vice versa. There are few intact transport processes that most likely intervene in the accumulation of Cd from root to shoot. The Cd will be (i) taken up by roots, followed by (ii) xylem loading aiding translocation to shoots and might further (iii) being translocated into seeds through phloem as illustrated in Figure 3 (Clemens et al., 2002CLEMENS, S., PALMGREN, M.G. and KRÄMER, U., 2002. A long way ahead: understanding and engineering plant metal accumulation. Trends in Plant Science, vol. 7, no. 7, pp. 309-315. http://dx.doi.org/10.1016/S1360-1385(02)02295-1. PMid:12119168.
http://dx.doi.org/10.1016/S1360-1385(02)...
). To make sure the accurate delivery of heavy metals to the target protein, plant cell uses a combination of compartmentalization, chelation and exclusion of metals (Clemens, 2001CLEMENS, S., 2001. Molecular mechanisms of plant metal tolerance and homeostasis. Planta, vol. 212, no. 4, pp. 475-486. http://dx.doi.org/10.1007/s004250000458. PMid:11525504.
http://dx.doi.org/10.1007/s004250000458...
) and transmembrane metal-transporting proteins are recognized as essential structure for the operation (Hall and Williams, 2003HALL, J.L. and WILLIAMS, L.E., 2003. Transition metal transporters in plants. Journal of Experimental Botany, vol. 54, no. 393, pp. 2601-2613. http://dx.doi.org/10.1093/jxb/erg303. PMid:14585824.
http://dx.doi.org/10.1093/jxb/erg303...
).

Figure 3
The main mechanism of metal ions translocation. (a) Rhizosphere acidification and availability of chelators mobilize the metal elements. (b) Metals are chelated and non-chelated metals will be sequestered into vacuole. (c) Root absorption greatly leads the transportation of metals through xylem loading. (d) Metals are delivered to the shoot exiting the xylem and move from cell-to-cell. (e) Metals uptake is driven by certain transporters that occupying endomembranes. Abbreviations and symbols: CW, cell wall; M, metal; filled circles, chelators; filled ovals, transporters; bean-shaped structures, metallochaperones. Adopted from Clemens et al. (2002)CLEMENS, S., PALMGREN, M.G. and KRÄMER, U., 2002. A long way ahead: understanding and engineering plant metal accumulation. Trends in Plant Science, vol. 7, no. 7, pp. 309-315. http://dx.doi.org/10.1016/S1360-1385(02)02295-1. PMid:12119168.
http://dx.doi.org/10.1016/S1360-1385(02)...
.

Plant roots are readily to take up dissolved nutrient mineral from soil solution. However, the chelated Cd2+ is likely not accessible for plant root absorption. During the respiration process, the carbonic acid is dissociated producing H+ and HCO3- at plasma membrane of root epidermal cells. Cd2+ is rapidly and non-energy driven adsorbed on root epidermal cells surface after swiftly being exchanged with H+ at plasma membrane. Subsequently, Cd2+ ions are ready to translocate or enter root epidermis layer through apoplast pathway (Yamaguchi et al., 2011YAMAGUCHI, N., MORI, S., BABA, K., KABURAGI-YADA, S., ARAO, T., KITAJIMA, N., HOKURA, A. and TERADA, Y., 2011. Cadmium distribution in the root tissues of solanaceous plants with contrasting root-to-shoot Cd translocation efficiencies. Environmental and Experimental Botany, vol. 71, no. 2, pp. 198-206. http://dx.doi.org/10.1016/j.envexpbot.2010.12.002.
http://dx.doi.org/10.1016/j.envexpbot.20...
). Apart from apoplast pathway, Cd2+ could pass through plant cells through simplast pathway by utilizing ion channels particular for Fe2+, Zn2+, and Ca2+ (Sadana et al., 2003SADANA, U.S., SAMAL, D. and CLAASSEN, N., 2003. Differences in manganese efficiency of wheat (Triticum aestivum L.) and raya (Brassica juncea L.) as related to root-shoot relations and manganese influx. Journal of Plant Nutrition and Soil Science, vol. 166, no. 3, pp. 385-389. http://dx.doi.org/10.1002/jpln.200390059.
http://dx.doi.org/10.1002/jpln.200390059...
). The combination of Cd2+ and Zn2+ and/or Ca2+ protein transport helps Cd2+ to infringing into root epidermis layer as a substitute of Zn2+ and Ca2+.

Cd passage through root surface crossing over root cortex and endodermis using apoplast and symplast pathway (Figure 2). Meanwhile, the divalent ions are directly driven by symplast pathway to stele and xylem (Akhter et al., 2014AKHTER, M.F., OMELON, C.R., GORDON, R.A., MOSER, D. and MACFIE, S.M., 2014. Localization and chemical speciation of cadmium in the roots of barley and lettuce. Environmental and Experimental Botany, vol. 100, pp. 10-19. http://dx.doi.org/10.1016/j.envexpbot.2013.12.005.
http://dx.doi.org/10.1016/j.envexpbot.20...
). Apparently, xylem is a component of apoplasmic region which regulate the Cd translocation into stem and leaves. The occurrence of xylem loading is controlled by sites of root cortex where the cells facilitate the loading of Cd into root xylem. Moreover, the retention and loading process in root xylem contribute to Cd translocation which intervened by Cd-chelating molecules for instance, phytochelatins, vacuolar sequestration and apoplastic barriers (Clemens, 2006CLEMENS, S., 2006. Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants. Biochimie, vol. 88, no. 11, pp. 1707-1719. http://dx.doi.org/10.1016/j.biochi.2006.07.003. PMid:16914250.
http://dx.doi.org/10.1016/j.biochi.2006....
; Szopiński et al., 2019SZOPIŃSKI, M., SITKO, K., GIEROŃ, Ż., RUSINOWSKI, S., CORSO, M., HERMANS, C., VERBRUGGEN, N. and MAŁKOWSKI, E., 2019. Toxic effects of Cd and Zn on the photosynthetic apparatus of the Arabidopsis halleri and Arabidopsis arenosa pseudo-metallophytes. Frontiers in Plant Science, vol. 10, pp. 748. http://dx.doi.org/10.3389/fpls.2019.00748. PMid:31244873.
http://dx.doi.org/10.3389/fpls.2019.0074...
). Majorly, the xylem loading is mediated by certain proteins, HMA P1B-type ATPases that contribute to Cd translocation (Belcastro et al., 2009BELCASTRO, M., MARINO, T., RUSSO, N. and TOSCANO, M., 2009. The role of glutathione in cadmium ion detoxification: coordination modes and binding properties: a density functional study. Journal of Inorganic Biochemistry, vol. 103, no. 1, pp. 50-57. http://dx.doi.org/10.1016/j.jinorgbio.2008.09.002. PMid:18951636.
http://dx.doi.org/10.1016/j.jinorgbio.20...
). P-type ATPase transporters are one of the transmembrane metal-transporting proteins that can be found in in a wide range of organism.

A broad range of evidence implies that rapid and effective heavy metals translocation from root-to-shoot is established with an increase of xylem load as a result of the constitutive genes coding overexpression for transmembrane metal-transporting proteins (Verbruggen et al., 2009VERBRUGGEN, N., HERMANS, C. and SCHAT, H., 2009. Molecular mechanisms of metal hyperaccumulation in plants. The New Phytologist, vol. 181, no. 4, pp. 759-776. http://dx.doi.org/10.1111/j.1469-8137.2008.02748.x. PMid:19192189.
http://dx.doi.org/10.1111/j.1469-8137.20...
). Arabidopsis thaliana has eight types of P-type ATPase transporter and AtHMA4 was determined to mediate the Cd xylem loading (Verret et al., 2004VERRET, F., GRAVOT, A., AUROY, P., LEONHARDT, N., DAVID, P., NUSSAUME, L., VAVASSEUR, A. and RICHAUD, P., 2004. Overexpression of AtHMA4 enhances root-to-shoot translocation of zinc and cadmium and plant metal tolerance. FEBS Letters, vol. 576, no. 3, pp. 306-312. http://dx.doi.org/10.1016/j.febslet.2004.09.023. PMid:15498553.
http://dx.doi.org/10.1016/j.febslet.2004...
). The homologues of this transporter also can be found in Cd-hyperaccumulator plants Thlaspi caerulescens and Arabidopsis halleri (Hanikenne et al., 2008HANIKENNE, M., TALKE, I.N., HAYDON, M.J., LANZ, C., NOLTE, A., MOTTE, P., KROYMANN, J., WEIGEL, D. and KRÄMER, U., 2008. Evolution of metal hyperaccumulation required cis-regulatory changes and triplication of HMA4. Nature, vol. 453, no. 7193, pp. 391-395. http://dx.doi.org/10.1038/nature06877. PMid:18425111.
http://dx.doi.org/10.1038/nature06877...
). The expression of HMA3 gene into specific transporter protein pump also helps in detoxification of heavy metal by sequestering Cd into the vacuole (Ueno et al., 2005UENO, D., MA, J.F., IWASHITA, T., ZHAO, F.J. and MCGRATH, S.P., 2005. Identification of the form of Cd in the leaves of a superior Cd-accumulating ecotype of Thlaspi caerulescens using 113Cd-NMR. Planta, vol. 221, no. 6, pp. 928-936. http://dx.doi.org/10.1007/s00425-005-1491-y. PMid:15883836.
http://dx.doi.org/10.1007/s00425-005-149...
). This expression has been detected in yeasts and plants (Morel et al., 2009MOREL, M., CROUZET, J., GRAVOT, A., AUROY, P., LEONHARDT, N., VAVASSEUR, A. and RICHAUD, P., 2009. AtHMA3, a P-1B-ATPase allowing Cd/Zn/Co/Pb vacuolar storage in Arabidopsis. Plant Physiology, vol. 149, no. 2, pp. 894-904. http://dx.doi.org/10.1104/pp.108.130294 PMid:19036834.
http://dx.doi.org/10.1104/pp.108.130294...
). The sequestration process improves in limitation of Cd translocation in Oriza sativa. However, a mutation of this transporter gene has prompted high Cd translocation from roots to shoots (Miyadate et al., 2011MIYADATE, H., ADACHI, S., HIRAIZUMI, A., TEZUKA, K., NAKAZAWA, N., KAWAMOTO, T., KATOU, K., KODAMA, I., SAKURAI, K., TAKAHASHI, H., SATOH-NAGASAWA, N., WATANABE, A., FUJIMURA, T. and AKAGI, H., 2011. OsHMA3, a P1B-type of ATPase affects root-to-shoot cadmium translocation in rice by mediating efflux into vacuoles. The New Phytologist, vol. 189, no. 1, pp. 190-199. http://dx.doi.org/10.1111/j.1469-8137.2010.03459.x PMid:20840506.
http://dx.doi.org/10.1111/j.1469-8137.20...
).

Other transporter like AtPDR8 is an ABC transporter which can be found mainly on the epidermis and root hairs membrane (Kim et al., 2007KIM, D.Y., BOVET, L., MAESHIMA, M., MARTINOIA, E. and LEE, Y., 2007. The ABC transporter AtPDR8 is a cadmium extrusion pump conferring heavy metal resistance. The Plant Journal, vol. 50, no. 2, pp. 207-218. http://dx.doi.org/10.1111/j.1365-313X.2007.03044.x. PMid:17355438.
http://dx.doi.org/10.1111/j.1365-313X.20...
). Cadmium will be taken up into the xylem and transported into other parts of the plant. Nevertheless, the loading of Cd from xylem parenchyma and vessels of xylem is influenced by the transporter’s activity.

6. Phytoremediation Techniques in Cd Remediation

Phytoremediation is a bioremediation technology which utilizes green plants to clean up pollutants from the environment. According to Ali et al. (2013)ALI, H., KHAN, E. and SAJAD, M.A., 2013. Phytoremediation of heavy metals – concepts and applications. Chemosphere, vol. 91, no. 7, pp. 869-881. http://dx.doi.org/10.1016/j.chemosphere.2013.01.075. PMid:23466085.
http://dx.doi.org/10.1016/j.chemosphere....
, phytoremediation can be best described as a green technology with cost effective, efficient, environmental eco-friendly technology. Another author defines phytoremediation as environmental remediation of contaminants using green plants to make them less toxic (Raskin et al., 1997RASKIN, I., SMITH, R.D. and SALT, D.E., 1997. Phytoremediation of metals: using plants to remove pollutants from the environment. Current Opinion in Biotechnology, vol. 8, no. 2, pp. 221-226. http://dx.doi.org/10.1016/S0958-1669(97)80106-1. PMid:9079727.
http://dx.doi.org/10.1016/S0958-1669(97)...
). Cd accessibility in soils, streams and rivers create the chances for Cd absorption and distribution by root to shoot and other plant parts. Different techniques are involved in phytoremediation of Cd, comprising phytoextraction, phytostabilization, phytotransformation, phytostimulation, phytovolatilization, and rhizofiltration as shown in Figure 4.

Figure 4
Phytoremediation of heavy metals and other organic pollutants in ecosystem. Modified from Bandara and Vithanage (2016)BANDARA, T. and VITHANAGE, M., 2016. Phytoremediation of shooting range soils. In: A.A. ANSARI, S.S. GILL, R. GILL, R. LANZA and L. NEWMAN, eds. Phytoremediation. Switzerland: Springer, pp. 469-488. http://dx.doi.org/10.1007/978-3-319-40148-5_17.
http://dx.doi.org/10.1007/978-3-319-4014...
.

6.1. Phytoextraction

Phytoextraction is a low-cost phytoremediation technique using plants eradicating heavy metals from the soil environment (Evangelou et al., 2007EVANGELOU, M.W., EBEL, M. and SCHAEFFER, A., 2007. Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents. Chemosphere, vol. 68, no. 6, pp. 989-1003. http://dx.doi.org/10.1016/j.chemosphere.2007.01.062. PMid:17349677.
http://dx.doi.org/10.1016/j.chemosphere....
; Shakoor et al., 2017SHAKOOR, A., ABDULLAH, M., SARFRAZ, R., ALTAF, M. and SANEYA, B., 2017. A comprehensive review on phytoremediation of cadmium (Cd) by mustard (Brassica juncea L.) and sunflower (Helianthus annuus L.). Journal of Biodiversity and Environmental Sciences, vol. 10, no. 3, pp. 88-98.) (Table 2). It is also termed as phytoabsorption or phytoaccumulation, which heavy metals are stored in shoot part after being extracted from plant root by up taking activities from water and soil environment (Rafati et al., 2011RAFATI, M., KHORASANI, N., MOATTAR, F., SHIRVANY, A., MORAGHEBI, F. and HOSSEINZADEH, F., 2011. Phytoremediation potential of Populus alba and Morus alba for cadmium, chromuim and nickel absorption from polluted soil. International Journal of Environmental of Research, vol. 5, no. 4, pp. 961-970.). Phytoextraction occurs when the plants absorb the metals and translocate them from roots to other plant parts. It is one of a reliable system to minimize the contamination without obliterating the soil composition and fertility by isolation process. It is extensively used due to the removal efficiency, however depending on few factors like soil properties, contaminant accessibility, plant species and chemical speciation of contaminants (Shabani and Sayadi, 2012SHABANI, N. and SAYADI, M.H., 2012. Evaluation of heavy metals accumulation by two emergent macrophytes from the polluted soil: an experimental study. The Environmentalist, vol. 32, no. 1, pp. 91-98.). The toxic metals or contaminants are absorbed, concentrated and precipitated from soils into biomass. It is ideally suited for diffusely polluted areas remediation at relatively low concentration and superficially.

Table 2
Implemented phytoremediation techniques in Cd bioremediation.

There are two types of mechanism involve in exclusion of heavy metals from soil. Hyperaccumulation is the first strategy of phytoextraction utilizing plants to remove the impurities from the soil and water body. The second strategy requires assistance of altered fluids containing chelating agent to enhance metal solubility in soil. This gives the chances for the plants to easily absorb the solubilize metals. Greger and Landberg (1999)GREGER, M. and LANDBERG, T., 1999. Use of willow in phytoextraction. International Journal of Phytoremediation, vol. 1, no. 2, pp. 115-123. http://dx.doi.org/10.1080/15226519908500010.
http://dx.doi.org/10.1080/15226519908500...
have identified the great phytoextraction potential of Willow (Salix viminalis) in extracting cadmium, zinc, and copper. In addition, it shows special features of high-energy uptake of heavy metals by root to shoot with a significant amount of biomass production. Phytoextraction is suitable for cadmium removal in agricultural soil as it can be easily implemented due to smooth removal of cadmium in contrast to other heavy metals (Robinson et al., 2000ROBINSON, B.H., MILLS, T.M., PETIT, D., FUNG, L.E., GREEN, S.R. and CLOTHIER, B.E., 2000. Natural and induced cadmium-accumulation in poplar and willow: implications for phytoremediation. Plant and Soil, vol. 227, no. 1/2, pp. 301-306. http://dx.doi.org/10.1023/A:1026515007319
http://dx.doi.org/10.1023/A:102651500731...
).

6.2. Phytostabilization

Phytostabilisation is a technique utilizing plant to immobilize the contaminants in soil reside near the roots through roots absorption, adsorption or precipitation in rhizosphere. It manipulates the stability of contaminants with long-term containment ensuing contaminants mobility and bioavailability constraint to enter the food chain (Wong, 2003WONG, M.H., 2003. Ecological restoration of mine degraded soils, with emphasis on metal contaminated soils. Chemosphere, vol. 50, no. 6, pp. 775-780. http://dx.doi.org/10.1016/S0045-6535(02)00232-1. PMid:12688490.
http://dx.doi.org/10.1016/S0045-6535(02)...
). In fact, it employs different type of plants to stabilize the pollutants from the soil sediment and sludges (Ali et al., 2013ALI, H., KHAN, E. and SAJAD, M.A., 2013. Phytoremediation of heavy metals – concepts and applications. Chemosphere, vol. 91, no. 7, pp. 869-881. http://dx.doi.org/10.1016/j.chemosphere.2013.01.075. PMid:23466085.
http://dx.doi.org/10.1016/j.chemosphere....
). In other view, phytostabilization is selected approaches deactivating the potential of toxic heavy metals from the soil environment contaminants (Vangronsveld et al., 2009VANGRONSVELD, J., HERZIG, R., WEYENS, N., BOULET, J., ADRIAENSEN, K., RUTTENS, A., THEWYS, T., VASSILEV, A., MEERS, E., NEHNEVAJOVA, E., VAN DER LELIE, D. and MENCH, M., 2009. Phytoremediation of contaminated soils and groundwater: lessons from the field. Environmental Science and Pollution Research International, vol. 16, no. 7, pp. 765-794. http://dx.doi.org/10.1007/s11356-009-0213-6. PMid:19557448.
http://dx.doi.org/10.1007/s11356-009-021...
) by turning the metallic toxic state into less toxic state. Therefore, the metals will become immobilize in the resource reducing the migration of metals to other sites (Eapen and D’Souza, 2005EAPEN, S. and D’SOUZA, S.F., 2005. Prospects of genetic engineering of plants for phytoremediation of toxic metals. Biotechnology Advances, vol. 23, no. 2, pp. 97-114. http://dx.doi.org/10.1016/j.biotechadv.2004.10.001. PMid:15694122.
http://dx.doi.org/10.1016/j.biotechadv.2...
). However, it depends on capability of roots in limiting the bioavailability of metals in the soil. The plant roots will create contamination zone and assist in metal immobilization through sorption, precipitation, complexation, or reduction of metal valence (Barcel and Poschenrieder, 2003BARCEL, J. and POSCHENRIEDER, C., 2003. Phytoremediation: principles and perspectives. Contributions in Science, vol. 2, pp. 333-344.).

The water erosion can be prevented by plant root system restraining the allocation of contaminants through adsorption or accumulation. Moreover, it provides an area around the roots that induce the precipitation of contaminants. This process being operated by different mechanisms. The leaching ability of contaminants through evapotranspiration and storage of water in the contaminated soil would be likely reduced by the help of vegetative cover. In addition, an increase of aggregation of soil could control the soil erosion when the plants add organic matters in the soil (Robinson et al., 2006ROBINSON, B., SCHULIN, R., NOWACK, B., ROULIER, S., MENON, M., CLOTHIER, B. and MILLS, T., 2006. Phytoremediation for the management of metal flux in contaminated sites. Forest Snow and Landscape Research, vol. 80, no. 2, pp. 221-224.). As a result, contaminants will become less biologically thus reducing its exposure towards animals and humans. Subsequently, the pollutants movement and intrusion into food chain and into ground water can be effectively prevented.

It does not require the soil removal and contaminated biomass disposal since it is discovered to be extremely efficient in fine soils and high organic matter content (Berti and Cunningham, 2000BERTI, W.R. and CUNNINGHAM, S.D., 2000. Phytostabilization of metals. In: I. RASKIN and B.D. ENSLEU, eds. Phytoremediation of toxic metals: using plants to cleanup the environment. New York: Willey. pp. 71–88.). Nevertheless, this technique perhaps can stop the translocation of heavy metals but not permanently eradicating them from the soil.

6.3. Phytovolatilisation

Phytovolatization is the contaminants uptake by root through absorption process accompanied by their transformation to volatile compounds and subsequent discharge into the atmosphere. It is an approach by which heavy metals are released into air after being removed from the soil or water (Figure 4). This possibly happens due to the phytotransformation process of heavy metals or toxic contaminants into more volatile and less toxic contaminants. These volatile contaminants will be released into the atmosphere along with transpiration process. Brassicae juncea naturally extracted selenium from the soil where it is frequently found as highly toxic selenocyanate. Part of selenocyanate will eventually be converted into a 500 to 700 time less toxic volatile dimethylselenide (Souza et al., 2002SOUZA, M.P., PICKERING, I.J., WALLA, M. and TERRY, N., 2002. Selenium assimilation and volatilisation from selenocyanate-treated indian mustard and muskgrass. Plant Physiology, vol. 128, no. 2, pp. 625-633. http://dx.doi.org/10.1104/pp.010686. PMid:11842165.
http://dx.doi.org/10.1104/pp.010686...
).

While this technique is thought to be more successful in remediating organic containments from the soil environment, compared to other techniques, it has more limitations. In fact, it does not permanently remove toxins from the soil, but rather draws them into atmosphere (Padmavathiamma and Li, 2007PADMAVATHIAMMA, P. and LI, L., 2007. Phytoremediation technology: hyperaccumulation metals in plants. Water, Air, and Soil Pollution, vol. 184, no. 1, pp. 105-126. http://dx.doi.org/10.1007/s11270-007-9401-5.
http://dx.doi.org/10.1007/s11270-007-940...
).

6.4. Phytostimulation

Phytostimulation is an enhancement of microbial activity in soil through stimulation of root compound exudation into rhizosphere. In other words, microbes or rhizobacteria receive adequate supply of nutrient known as exudates to enhance their growth. In return, this microbe will actively disintegrate pollutants in soil by degradation or biotransformation process. It has become a favourable technology of Cd and other organic pollutants remediation due to cost-effective and eco-friendly (Jia et al., 2016JIA, H., WANG, H., LU, H., JIANG, S., DAI, M., LIU, J. and YAN, C., 2016. Rhizodegradation potential and tolerance of Avicennia marina (Forsk.) Vierh in phenanthrene and pyrene contaminated sediments. Marine Pollution Bulletin, vol. 110, no. 1, pp. 112-118. http://dx.doi.org/10.1016/j.marpolbul.2016.06.075. PMid:27373941.
http://dx.doi.org/10.1016/j.marpolbul.20...
). In addition, it is considerably reliable technique for organic pollutant removal from the soil ecosystem.

6.5. Rhizofiltration

Rhizofiltration is one of phytoremediation techniques that involve the removal or filtration of heavy metals from water through the root biomass. Apparently, the contaminants are absorbed or adsorbed through roots surface from soil and water environment (Salt et al., 1995SALT, D.E., BLAYLOCK, M., KUMAR, N.P., DUSHENKOV, V., ENSLEY, B.D., CHET, I. and RASKIN, I., 1995. Phytoremediation: a novel strategy for the removal of toxic metals from the environment using plants. Nature Biotechnology, vol. 13, no. 5, pp. 468-474. http://dx.doi.org/10.1038/nbt0595-468. PMid:9634787.
http://dx.doi.org/10.1038/nbt0595-468...
) with the operation of certain chemical techniques such as reverse osmosis, ion exchange, and precipitation (Francis et al., 1999FRANCIS, C.W., TIMPSON, M.E. and WILSON, J.H., 1999. Bench-and pilot-scale studies relating to the removal of uranium from uranium-contaminated soils using carbonate and citrate lixiviants. Journal of Hazardous Materials, vol. 66, no. 1-2, pp. 67-87. http://dx.doi.org/10.1016/S0304-3894(98)00209-X. PMid:10379031.
http://dx.doi.org/10.1016/S0304-3894(98)...
). Unfortunately, these techniques are necessary to be much expensive and hardly operated. Thus, the application of rhizofiltration has been put into account as a promising phytoremediation technique. Various plant roots such as various grasses sunflower and mustard are utilized to get rid of toxic heavy metals like Cd, Ni, Cu, Zn and Pb (Lee and Yang, 2010LEE, M. and YANG, M., 2010. Rhizofiltration using sunflower (Helianthus annuus L.) and bean (Phaseolus vulgaris L. var. vulgaris) to remediate uranium contaminated groundwater. Journal of hazardous materials, vol. 173, no. 1, pp. 589-596.). The suitability of pants for rhizofiltration depends on root structure and root system. Generally, the plants that have longer and hairy root system providing substantial surface area are most suitable for rhizofiltration technique. This technique has been recognized to retain certain heavy metals within the roots, thus, partially treat industrial waste, and agricultural runoff (USEPA, 2000UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REPORTS – USEPA, 2000. Introduction to phytoremediation: EPA 600/R-99/107. Whashington, DC: EPA.). Indian mustard (Brassica juncea) and sunflower (Helianthus annuus) have been recognized as suitable plants for rhizofiltration in remediating most of toxic heavy metals (Turgut et al., 2004TURGUT, C., PEPE, M.K. and CUTRIGHT, T.J., 2004. The effect of EDTA and citric acid on phytoremediation of Cd, Cr and Ni from soil using Helianthus annuus. Environmental Pollution, vol. 131, no. 1, pp. 147-154. http://dx.doi.org/10.1016/j.envpol.2004.01.017. PMid:15210283.
http://dx.doi.org/10.1016/j.envpol.2004....
).

7. Conclusion

Cadmium is long last pollutant in environment that needed to be diminished as much as possible markedly from food chain to maintain the living’s health. Cd is non-essential heavy metals that become a concern because of its bioavailability and mobility in environment is incompetently high. The absorption of Cd by plants depend on the bioavailbility of the element in the soil which is regulated by soil conditions or characteristic of plantation medium. Low pH value and poor organic matter content in soils upsurge the Cd bioavailability, hence increases the absorption of the metal. A similar pattern can be observed in the redox potential of the soils. Consequently, the absorption of Cd by plants root followed by translocation to upper plant parts results in Cd bioaccumulation in plants. Due to the fact that this accumulation can not be hindered, the toxicity symptoms visibly appear. This comprehensive literature review has demonstrated the inexorable occurring detrimental toxic symptoms experienced by plants, particularly growth inhibition, leaf discoloration, necrosis, as well as pyhtosynthesis system alteration. Cd interferes the plant biological function by inhibiting the growth and reducing photosynthesis activity abruptly. The plant uptake and translocation of Cd are greatly assisted by multiple mechansism involving cell wall binding, roots exudates, mycorrhiza, membrane transport and root-to-shoot translocation. The cell wall composition provide a appropriate affinity for ionic Cd. Released exudates also have impact on plant uptake through manipulation of rhizosphre activities, soil pH and soil reduction potential. Though, mycorrhizae have the potential to be quite beneficial when it comes to metals tolerance. The accumulation of Cd from root to shoot occurs when the element is taken up by roots and subsequently translocated into shoot through xylem loading. This involves both apoplastic and symplastic pathways, which begin at the root surface. It is necessary to study on the symptoms of the phytotoxicity or toxicity of cadmium in order to provide basic knowledge that could lead to discovering better approaches to the remedy of Cd, heavy metals and other organic contaminants in plant. Apparently, it is also important to re-establish and remediate the ecosystem using a promising technique. Phytoremediation is one of biological technologies that has been employed to remediate pollutant from soil and water. The most often employed procedures consist of phytoextraction, phytostabilization, phytovolatization, phytostimulation and rhizofiltration. These various phytoremediation strategies often offer less expensive procedure with extra windfall of most environmental-friendly means.It is also reliable as offering many constructive and desirable results. A positive reintegration can be achieved by utilizing the prominent functional aspects portrayed by hyperaccumulators to extract, transform and stabilize heavy metals, especially Cd. For the time being, it is necessary to evaluate the efficiency of phytoremediation technologies to incorporate the available resources so that the optimum remediation results can be achieved.

Acknowledgements

We would like to thank all the researchers participation for the valuable work, which has been essential for contributing to the review substance, and for getting it into the current. The authors would like to thank the Ministry of Higher Education Malaysia and Universiti Sultan Zainal Abidin, Kuala Terengganu, Malaysia for supporting the Fundamental Research Grant Scheme (FRGS) project (FRGS/1/2019/WAB01/UNISZA/02/2). We also acknowledge the editing and partial publication support of Taif University researchers in supporting project number TURSP-2020/110 at Taif University, Taif, Saudi Arabia.

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Publication Dates

  • Publication in this collection
    07 Mar 2022
  • Date of issue
    2024

History

  • Received
    13 May 2021
  • Accepted
    28 Oct 2021
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