Hydrogel alleviates the stressful effect drought in Schinus terebinthifolia and helps with post-stress recovery

Hidrogel alivia o efeito estressante da seca em Schinus terebinthifolia e ajuda na recuperação pós-estresse

Abstract

The use of hydrogel has been a viable and promising management strategy for forest seedlings. We aimed to evaluate the effect of hydrogel on Schinus terebinthifolia Raddi seedlings subjected to water deficit and after normal water supply post-stress. The water management evaluated were: i) Control: plants irrigated daily, ii) Drought: water deficit (irrigation suspension), and iii) Drought + hydrogel: addition of the polymer when transplanting seedlings. Assessments were carried out in three periods: (a) P1 – photosynthesis (A) was monitored until plants in one of the drought water regimes presented values ​​close to 1.0 μmol CO₂ m⁻² s⁻¹, (b) Recovery (REC) – after P1, seedlings were subjected to resumption of irrigation similar to control, until plants previously subjected to drought without or with hydrogel showed to A ≥ 70% at control. In Post-Rec (c) – at end of REC, the seedlings received + 90 days of irrigation. The quantum photochemical potential efficiency in photosystem II and absorbed energy conversion remained higher with hydrogel in P1. The hydrogel alleviates the stressful effect drought on physiology of seedlings, mitigating the reduction of photosynthesis in P1 and contributes for recovery of growth characteristics, biomass and quality of seedlings in the Post-Rec. S. terebinthifolia seedlings showed phenotypic plasticity with potential of resilience for their recovery.

Keywords:
phenotypic plasticity; photosynthesis; photosystem II; water-retaining polymer; water deficit

Resumo

O uso do hidrogel tem sido uma estratégia de manejo viável e promissora para mudas florestais. Objetivamos avaliar o efeito do hidrogel em mudas de Schinus terebinthifolia Raddi submetidas a seca e a retomada da irrigação no pós-estresse. Os manejos hídricos avaliados foram: i) Controle: plantas irrigadas diariamente, ii) Seca: déficit hídrico (suspensão da irrigação), e iii) Seca + hidrogel: adição do polímero no transplantio das mudas. As avaliações foram realizadas em três períodos: (a) F1 – a fotossíntese (A) foi monitorada até que as plantas de algum dos regimes hídricos de seca apresentassem valores próximos a 1,0 μmol CO₂ m⁻² s⁻¹, (b) Recuperação (REC) – após a F1, as mudas foram submetidas a retomada da irrigação semelhante as controle, até que as plantas previamente submetidas a seca sem ou com hidrogel apresentassem valor de A ≥ 70% ao das controle. No Post-Rec (c) – ao final da REC as mudas receberam + 90 dias de irrigação. A eficiência potencial fotoquímica no fotossistema II e de conversão de energia absorvida mantiveram-se mais altas com o uso do hidrogel na F1. O hidrogel aliviou o efeito estressante da seca sobre A durante a F1 e auxiliou nas recuperações. A adição de hidrogel contribuiu no crescimento, biomassa e qualidade das mudas no Post-Rec. As mudas de S. terebinthifolia apresentam plasticidade fenotípica com potencial de resiliência na recuperação. O uso do hidrogel alivia o efeito estressante da seca sobre a fisiologia das mudas, e contribui na recuperação das características de crescimento de maneira mais eficiente.

Palavras-chave:
plasticidade fenotípica; fotossíntese; fotossistema II; polímero hidroretentor; déficit hídrico

1. Introduction

Schinus terebinthifolia Raddi (Anacardiaceae, Brazilian pink pepper), is a tree species, classified as a pioneer in the ecological succession (Gris et al., 2012; Pozzan et al., 2020; Viveiros et al., 2021), being indicated for recovery of degraded areas and integrated sustainable production systems (Siqueira et al., 2020; Gerber et al., 2023) owing to its ecosystemic functionality. Its leaves and fruits are recommended for the pharmaceutical industry as they contain antioxidant compounds (Governici et al., 2020; Vasconcelos et al., 2022). In addition, its fruits are appreciated and used in gastronomy and making drinks.

From the perspective of planting seedlings in forestry or reforestation areas, in front of global climate change scenario, water deficit is a worrying factor for the initial establishment of plants. This is because prolonged droughts and the absence of irrigation systems in these areas impair photochemical metabolism and photosynthesis (Santos et al., 2022a; Sousa et al., 2022), consequently affect the growth and quality of seedlings.

In this sense, the use of water-retaining polymer, known commercially as hydrogel, has been an economically viable and successful management strategy for seedlings of some forest species. The hydrogel is constituted of polyacrylamide, which when it comes into contact with water, has the ability to expand, increasing its volume and water retention capacity (Qureshi et al., 2020; Watanuki Filho et al., 2023). When in contact with the roots, they become surrounded by a gelatinous biofilm and absorb available water, maintaining turgidity and stabilizing metabolic processes even under water deficit (Beltramin et al., 2020; Santos et al., 2021), alleviating the stressful effect.

Considering that S. terebinthifolia is found in moist and well-drained areas, we hypothesized that although water deficit impairs the physiology and growth of seedlings, the addition of hydrogel alleviates the stressful effect and will contribute to more efficient post-stress recovery. We aimed to evaluate the effect of hydrogel on S. terebinthifolia seedlings subjected to water deficit and after normal water supply post-stress.

2. Material and Methods

2.1. Seed collection and processing

Ripe fruits of S. terebinthifolia were collected from mother plants located in Horto de Plantas Medicinais – HPM (22º11'43.7” S and 54º56'08.5” W, 452 m a.s.l.), at the Faculty of Agricultural Sciences, Universidade Federal da Grande Dourados – UFGD, Dourados – MS, Brazil. The fruits were processed manually, choosing the seeds according to their integrity and absence of damage, which were immersed in 2% sodium hypochlorite solution for 5 minutes for sanitization and washed in running water.

Subsequently, seeds were sown in 290 cm³ polyethylene tubes previously filled with Tropstrato^® substrate and maintained in nursery conditions with 50% shading, and daily irrigations until they reached an average height of 10 cm, which occurred 30 days after transplanting. The Tropstrato^® commercial substrate presented the following characteristics: pH CaCl₂ = 5.75; P = 65.70 mg dm^-3; K = 1.60 cmol_c dm^-3; Ca = 23.80 cmol_c dm^-3; Mg = 12.40 cmol_c dm^-3; Al = 0.00 cmol_c dm^-3; H + Al = 4.20 cmol_c dm^-3; sum of bases = 39.80 cmol_c dm^-3; cation exchange capacity = 42.10 cmol_c dm^-3; and base saturation (V%) = 64.80.

2.2. General conditions for experiment

The experiment was carried out under agricultural screen with upper and side coverage of black nylon screen brand Sombrite^® with 30% luminosity retention and additional upper protection of a 150 µm transparent plastic cover. Initially, plastic pots were previously filled with 10 kg substrate consisting of Oxisol + coarse sand (3:1, v/v).

The Forth Gel^® (hydrogel), consisting of polyacrylic potassium polyacrylamide (soil conditioner - class E; cationic exchange capacity= 53.22 cmol_c dm³), at a dose of 4 g L^-1 of water and it remained at rest for 30 minutes according to manufacture, until the product had a gel appearance. Subsequently, 100 mL of the product in gel was added at the time of seedling transplantation to each drought + hydrogel pot near the plant roots (Beltramin et al., 2020). The all plants were daily irrigations maintaining 70% of the water retention capacity (WRC) in the substrate for 30 days, characterized acclimatization period before the submission of plants to different water managements.

2.3. Water managements and evaluation periods

The water regimes included three water managements: i) Control: plants irrigated daily, 70% of the WRC was maintained in the substrate according to Souza et al. (2000), ii) Drought: water deficit, i.e., suspension of irrigation, and iii) Drought + hydrogel: water deficit + water retaining polymer, the procedures were similar to those in the previous water regime.

Assessments of non-destructive and destructive characteristics of plants were carried out in three periods: (i) P1 – photosynthetic rate (A) was monitored with IRGA (Infra Red Gas Analyzer) every two days until the plants in any of the water deficit water regimes presented values ​​close to 1.0 μmol CO₂ m⁻² s⁻¹, what happened after 20 days of water restriction. For the second period, (ii) recovery (REC) – after P1, all seedlings were subjected to normal water supply maintaining 70% WRC, until the plants previously subjected to water deficit without or with hydrogel presented an A value ≥ 70% compared to the control seedlings, what happened 35 days into the experiment (15 days irrigation resumption). In the third period, characterized as post recovery (iii) - Post-Rec – from 36 to 126 days of experiment (totaling 90 days) the seedlings continued to receive irrigation, maintaining 70% WRC.

The experimental design was completely randomized, in a split-plot scheme, the plots being made up of the 3 evaluation periods and in the subplots the 3 water management, totaling nine treatments (combination between periods with water management), with four replications. The experimental unit consisted of four pots with two plants in each; then, in each period, evaluations were carried out on two plants per replication.

2.4. Assessments

1. Photochemical activities: fully expanded leaves were subjected to dark conditions using leaf clips for 30 min, and subsequently, using a flash of 1,500 μmol m⁻² s⁻¹, with a portable fluorometer (OS-30p; Opti-Sciences Chlorophyll Fluorometer, Hudson, NY, USA) the potential photochemical efficiency of photosystem II (F_v/F_m) were evaluated. Using values of emission chlorophyll a fluorescence, the absorbed energy conversion efficiency (F_v/F₀), and the maximum basal yield of non-photochemical processes (F₀/F_m) were calculated.

2. Photosynthetic rate: using fully expanded sheets, the CO₂ assimilation (A, μmol CO₂ m⁻² s⁻¹) was quantified using a portable LCIPro-SD photosynthesis meter (IRGA - Infra Red Gas Analyzer) (Model ADC BioScientific Ltd.). Measurements were made between 8 and 10 a.m., with follows environmental conditions (Table 1).

   Table 1
   Photosynthetically active radiation (PAR), atmospheric CO₂ concentration (Ca) in grown environment and leaf temperature in Schinus terebinthifolia Raddi seedlings in different evaluation periods.
   

   Evaluation periods	PAR	Ca	Leaf temperature
   	(μmol photons m−2 s−1)	(ppm)	(º C)
   P1	904.68	423.52	23.79
   Recovery	823.00	427.22	26.16
   Post-Rec	1,111.92	404.11	36.24

3. Initial growth: seedlings were evaluated for height using a ruler graduated in cm, using as a criterion the distance from the collar to the inflection of the highest leaf, and the stem diameter was measured with a digital caliper inserted 1.0 cm above the level of the substrate. Subsequently, the plants were harvested and the length of the largest root was measured with a ruler graduated in cm. The fresh material was packed in Kraft^® paper bags, dried in an oven with forced air circulation at 60 º C ± 5 for 72 hours, weighing the total dry mass in a precision scale.

4. Seedling quality and phenotypic plasticity: using growth and biomass production data, the Dickson quality index was calculated – DQI (Dickson et al., 1960). The phenotypic plasticity index (PPI) was calculated for the A and F_v/F_m characteristics according to Valladares et al. (2006), with values ​​scale from 0.00 to 1.00.

2.5. Data analysis

All data were submitted to analysis of variance, and when significant by the F test (p ≤ 0.05), means were compared by the Scott-Knott test for treatments constituted by the combination of water management and evaluation periods at p ≤ 0.05 ± standard deviation (SD), using SISVAR software. The phenotypic plasticity index (PPI) was presented in a descriptive manner, without applying statistical analysis (Santos et al., 2023a).

3. Results

We observed that the photochemical potential quantum efficiency in photosystem II (F_v/F_m) was lower in seedlings grown under drought without hydrogel (0.570) compared to the other seedlings in P1. In REC and Post-Rec the values ​​did not differ statistically between water management (Figure 1A). The control seedlings had higher values ​​of absorbed energy conversion efficiency (F_v/F₀) in all periods, being that in P1 those under water deficit with hydrogel presented higher values ​​compared to those without hydrogel in the same period (Figure 1B). In Post-Rec, the lowest values ​​occurred in previously stressed seedlings, regardless of the use of the hydrogel. The maximum basal yield values ​​of non-photochemical processes (F₀/F_m) in P1 were higher in seedlings under water deficit, regardless of the use of the hydrogel, while in REC and Post-Rec the values ​​did not vary between water management (Figure 1C).

Figure 1
Potential photochemical efficiency of photosystem II – F_v/F_m (A), absorbed energy conversion efficiency – F_v/F₀ (B), and the maximum basal yield of non-photochemical processes – F₀/F_m (C) in Schinus terebinthifolia Raddi seedlings subjected to different water managements in three evaluation periods. Bars with different letters differ statistically from each other using the Scott-Knott test ± SD (p ≤ 0.05).

The photosynthetic rate (A) in S. terebinthifolia seedlings varied in function of water management and different evaluation periods (Figure 2). In P1, seedlings under water deficit without hydrogel showed lower value (1.11 μmol CO₂ m⁻² s⁻¹), while those in the same water condition, but with hydrogel maintained higher A (3.79 μmol CO₂ m⁻² s⁻¹). In REC and Post-Rec, seedlings previously subjected to water deficit without hydrogel had lower A values, with values ​​of 4.96 and 3.97 μmol CO₂ m⁻² s⁻¹, respectively, while those with hydrogel had values (≥ 5.58 μmol CO₂ m⁻² s⁻¹), that did not differ statistically from the control seedlings in these two periods.

Figure 2
Photosynthetic rate (A) in Schinus terebinthifolia Raddi seedlings subjected to different water management in three evaluation periods. Bars with different letters differ statistically from each other using the Scott-Knott test ± SD (p ≤ 0.05).

In P1, the seedlings presented height values ​​that did not differ statistically between water management systems. Conversely, in REC the seedlings previously stressed without hydrogel had a lower height (33.5 cm) compared to the others managements in this period. In Post-Rec, the seedlings previously under drought with hydrogel were larger (61.62 cm), differing from the others (Figure 3A). The lowest values ​​of stem diameter were observed in seedlings subjected to drought, regardless of the use of hydrogel, in P1 (Figure 3B). In REC, values ​​did not vary statistically between water management. In Post-Rec, the highest value occurred in control seedlings, followed by those previously subjected to drought with hydrogel, while those without hydrogel had the lowest value, showed averages of 7.22, 6.35 and 5.55 mm, respectively.

Figure 3
Plant height (A), stem diameter (B), root length (C), total dry mass (D), and Dickson quality index – DQI (E) of Schinus terebinthifolia Raddi seedlings subjected to different water managements in three evaluation periods. Bars with different letters differ statistically from each other using the Scott-Knott test ± SD (p ≤ 0.05).

We observed that root length was shorter in seedlings subjected to drought, regardless of the use of hydrogel, in P1 and REC (Figure 3C). In Post-Rec, all seedlings presented values ​​that did not differ statistically. In general, the lowest value of total dry mass (2.54 g) was observed in seedlings subjected to drought without hydrogel in P1, while seedlings with hydrogel and control did not differ statistically (Figure 3D). In REC, values ​​did not vary in function of water management, and in Post-Rec, control seedlings had the highest value, followed by those previously stressed with hydrogel, and those without hydrogel showed lower biomass production.

Regarding DQI, in P1, values ​​observed in seedlings under drought, regardless of the use of the hydrogel, did not differ statistically from the control, while in REC the controls were higher than the others (Figure 3E). In Post-Rec, the highest DQI values ​​occurred in seedlings grown under continuous irrigation, followed by seedlings previously under drought with hydrogel, while those without hydrogel were lower, with values ​​of 1.60, 1.17 and 0.80, respectively.

The phenotypic plasticity index (PPI) for A was higher than for F_v/F_m both without and with hydrogel in the three evaluation periods (Figure 4). In P1, the highest PPI values ​​for both characteristics occurred in seedlings without hydrogel compared to those with hydrogel. In REC and Post-Rec the PPI for A and F_v/F_m were reduced compared to observed in P1, while that with hydrogel reducing more markedly for A.

Figure 4
Phenotypic plasticity index (PPI) for photosynthesis (A) and potential photochemical efficiency in photosystem II (F_v/F_m) in Schinus terebinthifolia Raddi seedlings subjected to drought without and with hydrogel in three evaluation periods.

4. Discussion

S. terebinthifolia seedlings are responsive and sensitive to water deficit as they have markedly reduced photochemical activities in photosystem II in P1, especially F_v/F_m and values ​​of A, in addition to some growth characteristics such as root length and total dry mass of the plants. However, hydrogel alleviates the stressful effect during and after the period of water deficit, favoring the recovery of seedlings, reinforcing our initial hypothesis.

During P1, the decreased in photochemical activities in PS II owing to the fact that in the water deficit condition, instability occurs in the electron transfer process between the acceptors, decreasing the production of ATP and NADPH₂ (Sharma et al., 2020; Kang et al., 2023; Sperdouli et al., 2023), leading to lower values ​​of F_v/F₀ e F_v/F_m. Furthermore, some electrons not used in the photochemical processes of photosynthesis, here represented by F₀/F_m, they are directed to other molecules and form compounds that are harmful to the photosynthetic apparatus.

Under water deficit, electrons dissipate in the reaction centers when they unite with molecular oxygen and hydrogen, from the photolysis of water in photosystem II, form free radicals, including superoxide and hydrogen peroxide (Zhou et al., 2019; Lin et al., 2023), not determined in this study. Under conditions of oxidative stress, these reactive oxygen species compromise the integrity of membranes and cellular structures due to the electrolytes leakage, which negatively affects the functioning of the photosynthetic apparatus (Choudhury et al., 2022; Mishra et al., 2023; Santos et al., 2023b).

Conversely, S. terebinthifolia seedlings grown with hydrogel maintained the processes in PS II more stable under water deficit conditions, because in addition to providing water for photolysis, the water present in the water-retaining polymer, when absorbed by plant, possibly optimizes the amount of H⁺ in the chloroplast lumen and synchronization in the electron transport chain, contributing to the production of ATP and NADPH₂. Santos et al. (2021), studying Campomanesia xanthocarpa Cambess. seedlings under water deficit, observed that the hydrogel contributed positively to the light energy conversion efficiency and stability of F_v/F_m, result similar to that observed in our study with S. terebinthifolia. The increase in F₀/F_m in seedlings from all water management in Post-Rec can be explained by the increase in temperature in this evaluation period, but which did not result in losses to F_v/F_m.

Although the change in chlorophyll a fluorescence indicators suggests damage to the photochemical apparatus in the water deficit phase, similar to that described by Santos et al. (2022a), in Rec and Post-Rec it was observed that these damages were dynamic, i.e., reversible after normal water supply, especially due to the increase in F_v/F_m during these periods, regardless of the use of the hydrogel, indicating potential for adjustments due to physiological plasticity of the species, reinforced in the reduction of PPI values. Similarly, Dipteryx alata Vogel (Silva et al., 2022) and Eugenia myrcianthes Nied. (Santos et al., 2022b) also recovered post-stress.

The decreased in photosynthetic rate under drought owing to the fact that in conditions of low water availability in the soil, hormonal signaling occurs, in which abscisic acid induces stomatal closure (Sharma et al., 2023), limiting CO₂ entry, decreasing efficiency of carboxylation and synthesis of proteins associated to RuBisCO regeneration (Sherin et al., 2022; Mabizela et al., 2023). Furthermore, owing to lower efficiency of photochemical activities in PS II, energy substrate for the enzymatic phase is reduced, harming the maintenance of A values in the S. terebinthifolia seedlings. Another possible explanation for the lower A values ​​in previously stressed plants without hydrogel in the recovery periods may be associated with the higher PPI in P1, since possibly during this period plants require higher energy expenditure to adjust to this adverse condition, while those with hydrogel still had available water present in the water-retaining polymer for their metabolic functions.

The hydrogel helps maintain A values ​​by avoiding damage to photochemical phase during water deficit, because the polymer acts on hydraulic conductivity in the soil (Abdallah, 2019). The hydrogel is constituted of polyacrylamide, and when in the presence of water for hydration, undergoes swelling, gradually making water available for metabolic processes (Albalasmeh et al., 2022; Watanuki Filho et al., 2023). In addition, hydrogels as hydrophilic polymer networks are capable of swelling or de-swelling reversibly in water and retaining large volume of liquid in swollen state (Ahmed, 2015).

However, complementary studies are needed to better discuss the biodegradability potential of the hydro-retentive polymer studied, since according to Hosseinzadeh and Ahmadi (2023) they emphasize that hydrogels can be degraded in the soil according to several factors such as temperature, degree of stabilization and composition of the polymer chain. According to Xiong et al. (2018) the presence of polyacrylamide degradation influences the mobility of the molecule in the environment due to the more hydrophilic nature of polyacrylamide with a higher content of carboxylic groups after hydrolysis.

We emphasized that in addition to alleviates the stressful effect of drought in P1, the hydrogel helps with post-stress recovery more efficiently, since plants previously stressed without hydrogel had lower values. These results suggest that although S. terebinthifolia has plasticity, hydrogel accelerates recovery potential after normal water supply, presenting a hydroconditioning effect in response to better hydraulic conductivity in the soil. However, the answers regarding the effectiveness of the hydrogel in maintaining the photosynthetic rate are variable. For example, D. alata seedlings grown under water deficit with 50 mL of hydrogel did not maintain high A values (Silva et al., 2022), which may be a response in function on the dose used or specifically the adjustment and plasticity mechanisms of the species.

Regarding to plant height, the increase in values ​​over the evaluation periods indicates gradual growth throughout the cultivation cycle, which is an expected response. However, although there was no statistical difference in P1, the water deficit impaired the growth of plants without hydrogel. When water availability in the soil is lower than the water requirement of the species, there is a reduction in the potential cell turgor pressure (Seleiman et al., 2021), while, with hydrogel, the polymer chain favored the maintenance of turgidity. However, even resuming irrigation, the values ​​of these plants are lower than those observed in control and hydrogel seedlings, suggesting that the negative effect in the stress phase is maintained, impaired the growth recovery potential.

The reduction in stem diameter under drought and its impact on recovery negatively affects vessel elements, promoting cavitation, in which water entry into the root is lower than the rate of leaf transpiration, forming air columns, making it difficult to transport water, nutrients, photoassimilates and hormones between the different organs of the plant (Shao et al., 2008). This response reflects the lower biomass production in seedlings without hydrogel. Although previously stressed plants recover and increase diameter values, those with hydrogel presented higher values ​​in this period, reinforcing the beneficial and gradual effect of the hydrogel.

Generally, plants under water deficit presented higher root length ​​due to hormonal stimulated triggered by ABA (Teng et al., 2023); however, in S. terebinthifolia seedlings the results were atypical, similar to that observed for Inga vera Willd. as described by Santos et al. (2023b). Conversely, the lower values ​​in seedlings under drought with hydrogel in P1 is owing to the fact that the hydrogel was added close to the root at the time of transplanting, maintaining higher soil moisture in this condition of suspension of irrigation, not stimulating root expansion.

The lower values ​​of activities in PS II and photosynthesis reflected in lower production of photoassimilates under water deficit condition, especially in seedlings under drought without hydrogel due to the negative effects of low water availability as previously described. Consequently, changes in plant growth under water deficit had negative effects on quality standard of seedlings, especially without hydrogel, which even recovering in REC and Post-Rec were lower than the others. We reinforced that the hydrogel contributed positively to the morphophysiological recovery through better production and distribution of photoassimilates this cultivation condition.

Plants can adjust to different conditions depending on their plasticity potential. The PPI helps in understanding the morphophysiological adjustments that the plant makes for its survival, and the higher its value, the greater the plasticity of a given characteristic, as well as the intensity of stress is more accentuated, since the plant needs to adjust more to the adverse condition. In our study with S. terebinthifolia with hydrogel, PPI values ​​were lower, i.e., the values ​​of these plants were closer to those of the control, indicating less need for adjustments due to the benefits of the water-retaining polymer.

In a practical context, the use of water-retaining polymer is a promising management strategy in nursery farming as it reduces the frequency of irrigation and in afforestation or reforestation areas with S. terebinthifolia seedlings, which are subject to prolonged droughts, higher rates of water evaporation from the soil or lack of irrigation systems. In future perspectives, we suggest that studies be carried out evaluating the activity of the antioxidant protection metabolism of S. terebinthifolia during and after water deficit with the hydrogel, increasing information on the mechanisms of action of the water-retaining polymer.

5. Conclusions

S. terebinthifolia seedlings are sensitive to water deficit and present phenotypic plasticity with potential resilience for post-stress recovery. The use of hydrogel alleviates the stressful effect of drought on the physiology of seedlings, and contributes with post-stress recovery more efficiently.

Acknowledgements

The authors thank CAPES and CNPq, for granting the scholarships, and the FUNDECT, for financial support.

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Publication Dates

• Publication in this collection
  14 Feb 2025
• Date of issue
  2025

History

• Received
  23 Aug 2024
• Accepted
  29 Nov 2024