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Rhizobacteria in growth and quality of açaí seedlings

Rizobactérias no crescimento e qualidade de mudas de açaí

Abstract

The success of any plant development relies on healthy and vigorous seedlings, and the use of rhizobacteria is a sustainable alternative for the production of high-quality seedlings as they positively interfere in plant development. Thus, the objective of this study was to evaluate the effect of rhizobacteria on growth and quality of seedlings of açaí (Euterpe oleracea Mart.), a native palm of Brazil, which has significant ornamental value in addition to the ecological and economic role, mainly by providing sweet heart of palm and fruit pulp. The experimental design was entirely randomized. There were five treatments (Bacillus subtilis, Bacillus megaterium, Bacillus amyloliquefaciens and Azospirillum brasilense plus the absence of microorganisms - control); four replicates and ten plants per plot. The following characteristics were evaluated: shoot height (cm), root length (cm); stem diameter (mm); number of leaves; leaf area (cm2); shoot, and root as well as total dry matter (g). Shoot/root ratio was determined and Dickson Quality Index. The data were submitted to variance analysis and the means were compared using Tukey’s test at 5% probability. Pearson’s correlation matrix was also determined. The rhizobacterium Bacillus subtilis provided higher growth while Bacillus amyloliquefaciens provided lower growth and quality of açaí seedlings.

Keywords:
Azospirillum brasilense; Bacillus amyloliquefaciens; Bacillus megaterium; Bacillus subtillis; Euterpe oleracea

Resumo

O sucesso do desenvolvimento de qualquer planta é dependente de mudas sadias e vigorosas e o uso de rizobactérias é uma alternativa sustentável para a produção de mudas de alta qualidade pois interferem positivamente no desenvolvimento das plantas. Desta forma, o objetivo deste trabalho foi avaliar o efeito de rizobactérias no crescimento e na qualidade de mudas de açaí (Euterpe oleracea Mart.) palmeira nativa do Brasil, que apresenta expressivo valor ornamental além da importância ecológica e também econômica, principalmente pelo fornecimento de palmito doce e polpa dos frutos. O delineamento experimental foi o inteiramente casualizado. Foram cinco tratamentos (Bacillus subtilis, Bacillus megaterium, Bacillus amyloliquefaciens e Azospirillum brasilense mais a ausência de microrganismos - controle); quatro repetições e dez plantas por parcela. Foram avaliadas as características: comprimento da parte aérea (cm) e do sistema radicular (cm); diâmetro do coleto (mm); número de folhas; área foliar (cm2); massa seca da parte aérea, das raízes e total (g) e determinadas: razão parte aérea/raízes e o Índice de Qualidade de Dickson. Os dados foram submetidos à análise de variância e as médias comparadas pelo teste de Tukey, a 5% de probabilidade. Foi determinada, ainda, a matriz de correlação de Pearson. A rizobactéria Bacillus subtilis proporcionou maior e Bacillus amyloliquefaciens menor crescimento e qualidade das mudas de açaí.

Palavras-chave:
Azospirillum brasilense; Bacillus amyloliquefaciens; Bacillus megaterium; Bacillus subtillis; Euterpe oleracea

Introduction

Contemporary landscaping increasingly employs native species, adopting a sustainable approach (Araújo et al., 2022ARAÚJO, F.P.; KLEIN, P.A.; FERNANDES, M.; RENCK, M.V.K.; ROLIM, R.G. Se essa rua fosse minha eu mandava semear: plantas ornamentais nativas para manutenção de polinizadores em áreas urbanas nos campos de cima da serra, Rio Grande do Sul, Brasil. Pesquisas, Botânica, n.76, p.193-217, 2022.). The use of indigenous vegetation in landscaping is of great importance for the conservation of local native diversity, especially as an alternative in the substitution of exotic plants due to their adaptation characteristics to the environment, biological diversity and their major ecological role in landscaping (Prestes et al., 2020PRESTES, R.D.; DIEL, V.B.N.; GHELLAR, N.T. Potencial paisagístico de plantas nativas de Santo Ângelo-RS. Revista Interdisciplinar em Ciências da Saúde e Biológicas, v.4, p.27-39, 2020. https://doi.org/10.31512/ricsb.v4i2.280
https://doi.org/10.31512/ricsb.v4i2.280...
), in addition to enhancing the regional landscape identity and promoting the coexistence of fauna that depends on these plants (Araújo et al., 2022).

Therefore, it is important to promote the use of native palms in landscaping, including the species Euterpe oleracea Mart., popularly known as açaizeiro or açaí, which has significant ornamental value. The açaí palm is also economically important, with the heart of palm being the most traditional product and the fruit pulp being the one with the greatest economic interest (Silva, 2021SILVA, H. A economia do açaí em Belém-PA: vida urbana e biodiversidade em uma experiência singular de desenvolvimento econômico. Novos Cadernos NAEA, v.24, n.3, 2021. http://dx.doi.org/10.18542/ncn.v24i3.10540
http://dx.doi.org/10.18542/ncn.v24i3.105...
).

The açaizeiro is considered among the most promising species (D’arace et al., 2019D’ARACE, L.M.B.; PINHEIRO, K.A.O.; GOMES, J.M.; CARNEIRO, F.S; COSTA, N.S.L.; ROCHA, E.S.; SANTOS, M.L. Produção de açaí na região norte do Brasil. Revista Ibero-Americana de Ciências Ambientais, v.10, n.5, p.15-21, 2019. http://dx.doi.org/10.6008/CBPC2179-6858.2019.005.0002
http://dx.doi.org/10.6008/CBPC2179-6858....
) and, in parallel with its commercial expansion, there is a growing need for quality seedlings which involves the reduction of nursery time and its good performance in the field (Araújo et al., 2018ARAÚJO, C.S.; RUFINO, C.P.B.; BEZERRA, J.L.S.; ANDRADE NETO, R.C.; LUNZ, A.M.P. Crescimento de mudas de açaizeiro (Euterpe oleracea Mart.) submetidas a diferentes doses de fósforo. South American Journal of Basic Education, Technical and Technological, v.5, n.1, p.102-111, 2018.).

Consequently, the quality of açaí seedlings influences the survival and productivity of the plants after transplanting, and the employment of beneficial microorganisms in the seedling production process, which is directly related to the production of plant hormones, vitamins, or conversion of substances to a form that can be assimilated by the plant, aids in the adaptation of the seedlings (Pio-Gonçalves et al., 2022).

Among these microorganisms, rhizobacteria that promote plant growth stand out, being able to positively interfere in growth and development of plants in several ways, including producing phytohormones, alleviating drought stress, mitigating salinity stress, acting in the phytoextraction of heavy metals, nutrient supplementation and/or pathogen biocontrol (Dias and Santos, 2022DIAS, A. S.; SANTOS, C. C. Bactérias promotoras de crescimento de plantas: conceitos e potencial de uso. Nova Xavantina: Pantanal, 2022. 98p.).

The rhizobacterium Bacillus subtillis acts in disease biocontrol, stimulates plant growth, can solubilize phosphorus from soil, increase nitrogen fixation, produce siderophores that promotes its growth and suppress pathogens, and can also increase tolerance to stresses (Hashem et al., 2019HASHEM, A.; TABASSUM, B.; ALLAH, E.F.A. Bacillus subtilis: A plant-growth promoting rhizobacterium that also impacts biotic stress. Saudi Journal of Biological Sciences, v.26, p.1291-1297, 2019. ).

Bacillus megaterium is also related to the ability to solubilize inorganic phosphorus which increases the amount of available phosphorus and promotes plant growth (Huang et al., 2019HUANG, F.L.; ZHANG, Y.; ZHANG, L.P.; ANG, S.; FENG, Y.; RONG, N.H. Complete genome sequence of Bacillus megaterium JX285 isolated from Camellia oleifera rhizosphere. Computacional Biology Chemistry, v.79, p.1-5, 2019. https://doi.org/10.1016/j.compbiolchem.2018.12.024
https://doi.org/10.1016/j.compbiolchem.2...
). B. subtilis and B. megaterium have shown evident plant growth promotion effect for some species (Guimarães et al., 2021GUIMARÃES, V.F.; KLEIN, J.; SILVA, A.S.L.; KLEIN, D.K. Eficiência de inoculante contendo Bacillus megaterium (B119) e Bacillus subtilis (B2084) para a cultura do milho, associado à fertilização fosfatada. Research, Society and Development, v.10, n.12, e431101220920, 2021. http://dx.doi.org/10.33448/rsd-v10i12.20920
http://dx.doi.org/10.33448/rsd-v10i12.20...
; Santos et al., 2021SANTOS, A.F.; CORRÊA, B.O.; KLEIN, J.; BONO, J.A.M.; PEREIRA, L.C.; GUIMARÃES, V.F.; FERREIRA, M.B. Biometria e estado nutricional da cultura da aveia branca (Avena sativa L.) sob inoculação com Bacillus subtilis e B. megaterium. Research, Society and Development , v.10, n.5, e53410515270, 2021. http://dx.doi.org/10.33448/rsd-v10i5.15270
http://dx.doi.org/10.33448/rsd-v10i5.152...
; Silva et al., 2022SILVA, K.R.C.; SOUSA, L.A.M.; SILVA, F.L.S.; AZEVEDO, J.L.X.; SILVA, I.A.; PINTO JUNIOR, F.F.; SILVA, B.G.; ANDRADE, H.A.F.; DOIHARA, I.P.; SILVA-MATOS, R.R.S. Bacillus subtillis e Bacillus megaterium no crescimento inicial de melancia ‘Sugar Baby’. Research, Society and Development , v.11, n.13, e96111335034, 2022. http://dx.doi.org/10.33448/rsd-v11i13.35034
http://dx.doi.org/10.33448/rsd-v11i13.35...
).

Other rhizobacterium, such as Bacillus amyloliquefaciens, also shows good results in growth promotion of some plant species (Farzand et al., 2020FARZAND, Y.; MOOSA, A.; ZUBAIR, M.; KHAN, A.R.; AYAZ, M.; MASSAWE, V.C.; GAO, X. Transcriptional profiling of diffusible lipopeptides and fungal virulence genes during Bacillus amyloliquefaciens EZ1509-mediated suppression of Sclerotinia sclerotiorum. Phytopathology, v.110, n.2, p.317-326, 2020.; Wang et al., 2020WANG, C.J; WANG, Y.Z.; CHU, Z.H.; WANG, P.S.; LIU, B.Y.; LI, B.Y.; YU, X.L.; LUAN, B.H. Endophytic Bacillus amyloliquefaciens YTB1407 elicits resistance against two fungal pathogens in sweet potato (Ipomoea batatas (L.) Lam.). Journal of Plant Physiology, v.253, 153260, 2020. https://doi.org/10.1016/j.jplph.2020.153260
https://doi.org/10.1016/j.jplph.2020.153...
; Abreu et al., 2022ABREU, L.P.S.; MARTINAZZO, A.P.; TEODORO, C.E.S.; BERBERT, P.A. Alternativa sustentável de uso de Bacillus amyloliquefaciens no biocontrole de fungos fitopatogênicos: uma revisão. Revista de Ciências Ambientais, v.16, n.1, p.01-15, 2022. http://dx.doi.org/10.18316/rca.v16i1.8339
http://dx.doi.org/10.18316/rca.v16i1.833...
) promoting resistance against diseases (Ngalimat et al., 2021NGALIMAT, M.S.; YAHAYA, R.S.R.; BAHARUDIN, M.M.A.A.; YAMINUDIN, S.M.; KARIM, M.; AHMAD, S.A.; SABRI, S. A review on the biotechnological applications of the operational group Bacillus amyloliquefaciens. Microorganisms, v.9, n.3, p.614, 2021. http://dx.doi.org/10.3390/microorganisms9030614
http://dx.doi.org/10.3390/microorganisms...
). While Azospirillum brasilense assists plant growth mainly through the production of phytohormones, particularly indoleacetic acid, as well as by nitrogen fixation (Nguyen et al., 2019NGUYEN, M.L.; SPAEPEN, S.; JARDIN, P.; DELAPLACE, P. Biostimulant effects of rhizobacteria on wheat growth and nutrient uptake depend on nitrogen application and plant development. Archives of Agronomy and Soil Science, v.65, n.1, p.58-73, 2019. http://dx.doi.org/10.1080/03650340.2018.1485074
http://dx.doi.org/10.1080/03650340.2018....
).

The range of interferences in microbial life is wide. Abiotic factors, such as temperature, nutrients, pH, salinity, energy sources, and toxic elements; as well as biotic factors represented mainly by microbial genetics and the interaction between microorganisms have a restraining power on the survival and activity of microorganisms (Furtak and Galazka, 2019FURTAK, K.; GALAZKA, A. Edaphic factors and their influence on the microbiological biodiversity of the soil environment. Advancements of Microbiology, v.58, n.4, p.375-385, 2019. http://dx.doi.org/10.21307/PM-2019.58.4.375
http://dx.doi.org/10.21307/PM-2019.58.4....
; Cavalcante et al., 2022CAVALCANTE, F.G.; CHAVES, V.G.; SILVA, A.O.; MARTINS, C.M.; MARTINS, S.C.S. Actinobactérias benéficas do solo: potencialidades de uso como promotores de crescimento vegetal. Enciclopédia biosfera, v.19 n.40; p.15-35, 2022. http://dx.doi.org/10.18677/EnciBio_2022B2
http://dx.doi.org/10.18677/EnciBio_2022B...
).

Thus, the objective of this study was to evaluate the effects of rhizobacteria on the growth and quality of açaí (Euterpe oleracea Mart.) seedlings.

Material and Methods

The present study was carried out between November 2021 and March 2022, in a greenhouse located in the state of São Paulo under the coordinates 21°15’2” S, 48°16’47” W and 600 meters of altitude. The region climate is tropical savanna Aw type (with dry winter and rainy summer) (Andre and Garcia, 2015ANDRE, R.G.B.; GARCIA, A. Alguns aspectos climáticos do município de Jaboticabal - SP. Nucleus, v.12, n.2, p. 263-270, 2015. http://dx.doi.org/10.3738/1982.2278.1543.
http://dx.doi.org/10.3738/1982.2278.1543...
). The data with average, maximum, minimum temperatures and average relative humidity during the period of the experiment are shown in Figure 1 (UNESP, 2022).

Figure 1
Data of maximum (Tmax), minimum (Tmin) and average (Tmed) temperature, and average relative humidity (RH) obtained during the period of November 2021 to March 2022.

The design of the experiment was entirely randomized. There were five treatments (Bacillus subtilis, Bacillus megaterium, Bacillus amyloliquefaciens and Azospirillum brasilense, plus the absence of rhizobacteria - control); four repetitions and ten plants per plot.

Açaí seedlings, obtained from seeds at UNESP/FCAV, with lengths of 5 cm ± 1 cm, were planted in tubes with volumetric capacity of 280 cm³ containing Carolina Soil® as commercial substrate, composed of peat, vermiculite, roasted rice husk, calcined dolomite limestone, NPK 14-16-18 fertilizer and micronutrients (information obtained from the packaging), and then placed in polypropylene trays with capacity for 54 tubes.

The trays were placed suspended on metal mesh benches 70 cm from the ground in a covered greenhouse, with the sides protected with black screen that allows 50% of the light to pass through and also with clear plastic above the screen cover. The irrigation was performed by automatic micro sprinklers activated twice a day for 15 min each, with a flow rate of 30 L h-1.

The rhizobacteria used in this study are part of the collection of Soil Microbiology Laboratory of the Plant Production Department of UNESP-FCAV, Jaboticabal Campus, where they were grown separately, in nutrient broth medium, for seven days, in flasks kept in B.O.D. (Eletrolab, model 347 F, Brazil), at 25 °C. After the incubation period, the bacteria were centrifuged separately at 10,000 rpm for 10 min at 28 °C (Novatecnica, model MLW K24, Brazil). The inoculum concentration was standardized according to Barry and Thornsberry (1991BARRY A.L.; THORNSBERRY, C. Susceptibility tests: diffusion test procedures. In: BALOWS, A.; HAUSLER JÚNIOR, W.J.; HERRMANN, K.L.; ISENBERG, H.D. Manual of clinical microbiology. 5ed. Washington: American Society for Microbiology, 1991. 1384p.) and Sahm and Washington II (1991SAHM, D. F.; WASHINGTON, J. A. Antibacterial susceptibility tests: dilution methods. In: BALOWS, A.; HAUSLER JÚNIOR, W.J.; HERRMANN, K.L.; ISENBERG, H.D. Manual of clinical microbiology . 5th ed. Washington: American Society for Microbiology , 1991. 1384p.) at 1 x 107 CFU mL-1 using a spectrophotometer (Micronal, model B382, Brazil) at 695 nm absorbance. The microorganisms were inoculated twice, once at 30 days after the seedlings were planted and again at 60 days, by applying 1 mL of the solution directly into the substrate near the stem, using a mechanical micropipette (VF-1000, Digipet®). The seedlings belonging to the control treatment were not inoculated.

When the roots began to appear at the bottom of the tubes, the following characteristics were evaluated: shoot height (SH, cm), measured at the substrate level to the tip of the last leaf and root length (RL, cm), both using a ruler in centimeters; stem diameter (SD, mm), determined at the substrate level using a digital caliper accurate to 0.01 mm (Western® PRO DC-6); number of leaves (NL), verified by visual counting of fully expanded leaves; leaf area (LA, cm2), measured using an electronic leaf area meter (Li-3100C, LI-COR®, Lincoln, Nebraska, USA); shoot (SDM, g) and root dry matter (RDM, g), obtained after drying the shoots and roots in a forced air circulation oven at 70 °C, until reaching constant weight, and weighing them on a precision scale (0.001 g) (SHIMADZU®, model AY220); and the total dry matter (TDM, g), obtained by the sum of SDM and RDM.

From these measurements, the following seedling quality variables were determined: a) shoot/root ratio, obtained from the relation between SDM and RDM; b) Dickson’s quality index (DQI), obtained by the formula proposed by Dickson in 1960 and applied in several research studies as described by Souza et al. (2022SOUZA, A.M.B.; CHIODA, L.B.; FERREIRA, K.B.; VIEIRA, G.R.; CAMPOS, T.S.; PIVETTA, K.F.L. Initial growth of Syagrus romanzoffiana seedlings in biosolid-based substrate. Pesquisa Agropecuária Tropical, v.52, e70577, 2022. http://dx.doi.org/10.1590/1983-40632022v5270577
http://dx.doi.org/10.1590/1983-40632022v...
), where: DQI = [TDM (g)/[SH (cm)/SD (mm) + SDM (g) /RDM (g)].

The obtained data were submitted to analysis of variance, and the means were compared using Tukey’s test at 5% probability using the AgroEstat® estatistical software. Correlation analysis was also performed between the variables.

Results and Discussion

The evaluated characteristics in this study presented in table 1 are related to the growth and quality of açaí seedlings after inoculation or not (control treatment) of rhizobacteria. When considering that plant growth is defined as the irreversible increase in weight and volume of cells, tissues and organs, the quality of seedlings can be evaluated using growth indicators, such as shoot height, stem diameter and shoot, root and total dry matter; in addition to these, several metrics are used to evaluate the quality of forest seedlings such as shoot and root dry matter ratio and the Dickson Quality Index (Avelino et al., 2021AVELINO, N.R.; SCHILLING, A.C.; DALMOLIN, Â.C.; SANTOS, M.S.; MIELKE, M.S. Alocação de biomassa e indicadores de crescimento para a avaliação da qualidade de mudas de espécies florestais nativas. Ciência Florestal, v.31, n.4, p.1733-1750. 2021. http://dx.doi.org/10.5902/1980509843229
http://dx.doi.org/10.5902/1980509843229...
).

Table 1
Means of shoot height (SH, cm), stem diameter (SD, mm), number of leaves (NL), leaf area (LA, cm2), root length (RL, cm), shoot dry matter (SDM, g), root dry matter (RDM, g), total dry matter (TDM, g), shoot dry matter/root dry matter ratio (SDM/RDM) and Dickson’s quality index (DQI) in açaí (Euterpe oleracea) seedlings inoculated or not (control) with growth-promoting rhizobacteria.

In Table 1 it can be observed that Bacillus subtilis stood out with higher means in all studied characteristics, although it did not differ from the control and other bacteria for some characteristics. This bacterium, therefore, showed greater efficiency in producing phytohormones and enzymes, beneficial for seedling development (Mazzuchelli et al., 2014MAZZUCHELLI, R.D.C.L.; SOSSAI, B.F.; ARAUJO, F.F. Inoculação de Bacillus subtilis e Azospirillum brasilense na cultura do milho. Colloquium Agrariae, v.10, n.2, p.40-47, 2014. http://dx.doi.org/10.5747/ca.2014.v10.n2.a106
http://dx.doi.org/10.5747/ca.2014.v10.n2...
) that promoted both growth and quality of açaí seedlings. Castro et al. (2019CASTRO, G.L.S.; SILVA JÚNIOR, D.D.; VIANA, R.G.; RÊGO, M.C.F.; SILVA, G.B. Photosynthetic apparatus protection and drought effect mitigation in açaí palm seedlings by rhizobacteria. Acta Physiologiae Plantarum, v.41, n.9, p.1-12, 2019. http://dx.doi.org/10.1007/s11738-019-2952-4.
http://dx.doi.org/10.1007/s11738-019-295...
; 2020) also observed positive effect of B. subtilis in promoting growth of açaí seedlings.

The rhizobacterium Bacillus megaterium also showed satisfactory results for açaí seedlings; similarly, Guimarães et al. (2021GUIMARÃES, V.F.; KLEIN, J.; SILVA, A.S.L.; KLEIN, D.K. Eficiência de inoculante contendo Bacillus megaterium (B119) e Bacillus subtilis (B2084) para a cultura do milho, associado à fertilização fosfatada. Research, Society and Development, v.10, n.12, e431101220920, 2021. http://dx.doi.org/10.33448/rsd-v10i12.20920
http://dx.doi.org/10.33448/rsd-v10i12.20...
), Santos et al. (2021SANTOS, A.F.; CORRÊA, B.O.; KLEIN, J.; BONO, J.A.M.; PEREIRA, L.C.; GUIMARÃES, V.F.; FERREIRA, M.B. Biometria e estado nutricional da cultura da aveia branca (Avena sativa L.) sob inoculação com Bacillus subtilis e B. megaterium. Research, Society and Development , v.10, n.5, e53410515270, 2021. http://dx.doi.org/10.33448/rsd-v10i5.15270
http://dx.doi.org/10.33448/rsd-v10i5.152...
) and Silva et al. (2022SILVA, K.R.C.; SOUSA, L.A.M.; SILVA, F.L.S.; AZEVEDO, J.L.X.; SILVA, I.A.; PINTO JUNIOR, F.F.; SILVA, B.G.; ANDRADE, H.A.F.; DOIHARA, I.P.; SILVA-MATOS, R.R.S. Bacillus subtillis e Bacillus megaterium no crescimento inicial de melancia ‘Sugar Baby’. Research, Society and Development , v.11, n.13, e96111335034, 2022. http://dx.doi.org/10.33448/rsd-v11i13.35034
http://dx.doi.org/10.33448/rsd-v11i13.35...
), reported that employment of B. subtilis and B. megaterium has contributed to the increment of plant growth.

As for the inoculation with Bacillus amyloliquefaciens, the lowest means were obtained for most of the studied characteristics, although this species shows efficiency when associated with other plant species as observed by Chauhan et al. (2019CHAUHAN, P.S.; LATA, C.; TIWARI, S.; CHAUHAN, A.S.; MISHRA, S.K.; AGRAWAL, L.; NAUTIYAL, C.S. Transcriptional alterations reveal Bacillus amyloliquefaciens-rice cooperation under salt stress. Scientific reports, v.9, n.1, p.1-13, 2019. https://doi.org/10.1038/s41598-019-48309-8
https://doi.org/10.1038/s41598-019-48309...
) that by inoculating this bacterium in rice (Oryza sativa) seedlings noticed an increase in growth of shoots and roots.

This difference in response may be related to the cultivation method; many studies such as conducted by Chauhan et al. (2019CHAUHAN, P.S.; LATA, C.; TIWARI, S.; CHAUHAN, A.S.; MISHRA, S.K.; AGRAWAL, L.; NAUTIYAL, C.S. Transcriptional alterations reveal Bacillus amyloliquefaciens-rice cooperation under salt stress. Scientific reports, v.9, n.1, p.1-13, 2019. https://doi.org/10.1038/s41598-019-48309-8
https://doi.org/10.1038/s41598-019-48309...
) on rice, are conducted with crops grown into the ground, which is rich in microorganisms, and there is little information on the association of these microorganisms in plants raised in containers holding organic or inert substrates.

Sometimes, when the inoculation of rhizobacteria is performed directly on the soil, incompatibility of the introduced microorganisms with the native microbiota associated with the rhizosphere of the plant has been observed, which generates competition between them affecting the positive effects of the target microorganism (Kumari et al., 2019KUMARI, B.; MALLICK, M.A.; SOLANKI, M.K.; SOLANKI, A.C.; HORA, A.; GUO, W. Plant growth promoting rhizobacteria (PGPR): modern prospects for sustainable agriculture. In: ANSARI, R.; MAHMOOD, I. Plant health under biotic stress. Singapore: Springer, 2019. p.109-127.) and, consequently, inadequate development of the seedlings or, when there is compatibility, generating benefits. However, little is known about the establishment dynamics of microorganisms on substrates composed of organic and inert compounds.

As the development of the açaí seedlings took place in substrate, the main factor that may have influenced the bacteria was the environmental conditions. However, there was not much variation for temperature and relative humidity during the experiment (Figure 1). The unsatisfactory effect, especially for the rhizobacterium Bacillus amyloliquefaciens, which presents positive effects for other plant species, may then be due to the tube limitation, the temperature that may not have been ideal for this bacterium and mainly for not having good interaction with this studied plant species.

The superiority of seedlings inoculated with B. subtilis followed by B. megaterium and Azospirillum brasilense in Table 1, indicating that these seedlings have greater potential for success after planting when compared to those that did not receive rhizobacteria (control) or those where B. amyloliquefaciens was applied. Silva et al. (2020SILVA, O.M.C.; HERNÁNDEZ, M.M.; ARAÚJO, G.D.C.R.; CUNHA, F.L.; EVANGELISTA, D.V.D.P.; LELES, P.S.D.S.; MELO, L.A.D. Potencial uso da casca de café como constituinte de substrato para produção de mudas de espécies florestais. Ciência Florestal , v.30, n.4, p.1161-1175, 2020. https://doi.org/10.5902/1980509842500
https://doi.org/10.5902/1980509842500...
) when evaluating the stem diameter, they observed that larger diameters is an indicator for analyzing the survival and growth conditions of seedlings after planting.

Higher values of leaf area and number of leaves indicates better sunlight absorption, thereby better photosynthetic capacity of the seedlings allowing the development of other organs to be faster (Taiz et al., 2017TAIZ, L.; ZEIGER, E.; MOLLER, I.M.; MURPHY, A. Fisiologia e desenvolvimento vegetal. 6ed. Porto Alegre: Artmed Editora, 2017. 858p.). There was no difference between treatments for number of leaves (Table 1), however, there was superiority of seedlings inoculated with B. subtilis and B. megaterium for leaf area which reflected in other characteristics; this shows that these bacteria acted more efficiently in plant metabolism (Ngalimat et al., 2021NGALIMAT, M.S.; YAHAYA, R.S.R.; BAHARUDIN, M.M.A.A.; YAMINUDIN, S.M.; KARIM, M.; AHMAD, S.A.; SABRI, S. A review on the biotechnological applications of the operational group Bacillus amyloliquefaciens. Microorganisms, v.9, n.3, p.614, 2021. http://dx.doi.org/10.3390/microorganisms9030614
http://dx.doi.org/10.3390/microorganisms...
), resulting in higher quality seedlings.

There was no difference between treatments for shoot dry matter and root dry matter ratio (Table 1). This ratio is an efficient characteristic for evaluating forest seedling lot quality and is directly related to field establishment and competitiveness under environmental stress conditions such as drought stress (Grossnickle and Macdonald, 2018GROSSNICKLE, S.C.; MACDONALD, J.E. Why seedlings grow: influence of plant attributes. New forests, v.49, n.1, p.1-34, 2018. http://dx.doi.org/10.1007/s11056-017-9606-4
http://dx.doi.org/10.1007/s11056-017-960...
; Avelino et al., 2021AVELINO, N.R.; SCHILLING, A.C.; DALMOLIN, Â.C.; SANTOS, M.S.; MIELKE, M.S. Alocação de biomassa e indicadores de crescimento para a avaliação da qualidade de mudas de espécies florestais nativas. Ciência Florestal, v.31, n.4, p.1733-1750. 2021. http://dx.doi.org/10.5902/1980509843229
http://dx.doi.org/10.5902/1980509843229...
). However, even though there were no difference between treatments for this quality evaluation characteristic, other parameters such as root dry matter and Dickson Quality Index showed superiority for seedlings inoculated with B. subtilis (Table 1).

Root dry matter is acknowledged as one of the easiest indicators that best measures seedling establishment in the field because it directly influences water and nutrient uptake (Shen et al., 2019SHEN, Y.; UMAÑA, M.N.; LI, W.; FANG, M.; CHEN, Y.; LU, H.; YU, S. Coordination of leaf, stem and root traits in determining seedling mortality in a subtropical forest. Forest Ecology and Management, v.446, p.285-292, 2019. https://doi.org/10.1016/j.foreco.2019.05.032
https://doi.org/10.1016/j.foreco.2019.05...
; Avelino et al., 2021AVELINO, N.R.; SCHILLING, A.C.; DALMOLIN, Â.C.; SANTOS, M.S.; MIELKE, M.S. Alocação de biomassa e indicadores de crescimento para a avaliação da qualidade de mudas de espécies florestais nativas. Ciência Florestal, v.31, n.4, p.1733-1750. 2021. http://dx.doi.org/10.5902/1980509843229
http://dx.doi.org/10.5902/1980509843229...
). Consequently, seedlings with higher root dry matter will be more effectively established in field because of their ability to adapt after transplanting (Avelino et al., 2021).

The results for Dickson Quality Index strengthen the superiority of the seedlings inoculated with B. subtilis (Table 1). The Dickson Quality Index is based on the relationship between several growth indicators to determine the quality of the seedlings, i.e., shoot dry matter, root dry matter, total dry matter, shoot height and stem diameter. For this reason, the Dickson Quality Index formula highlights the balance between growth and survival potential of post-planting plants, and by taking into account several morphological characteristics, the potential errors that could be faced when using one or two indicators will be minimized. Thus, it is an excellent indicator of seedling quality since it includes in its calculation the sturdiness and biomass allocation balance (Mañas et al., 2009MAÑAS, P.; CASTRO, E.; DE LAS HERAS, J. Quality of maritime pine (Pinus pinaster Ait.) seedlings using waste materials as nursery growing media. New Forests, v.37, p.295-311, 2009. http://dx.doi.org/10.1007/s11056-008-9125-4
http://dx.doi.org/10.1007/s11056-008-912...
; Avelino et al., 2021AVELINO, N.R.; SCHILLING, A.C.; DALMOLIN, Â.C.; SANTOS, M.S.; MIELKE, M.S. Alocação de biomassa e indicadores de crescimento para a avaliação da qualidade de mudas de espécies florestais nativas. Ciência Florestal, v.31, n.4, p.1733-1750. 2021. http://dx.doi.org/10.5902/1980509843229
http://dx.doi.org/10.5902/1980509843229...
).

However, Dickson’s Quality Index provides different values influenced by several factors, but it has been used as the main indicator of the quality of forest seedlings as it has high correlation with post-planting survival in field (Avelino et al., 2021AVELINO, N.R.; SCHILLING, A.C.; DALMOLIN, Â.C.; SANTOS, M.S.; MIELKE, M.S. Alocação de biomassa e indicadores de crescimento para a avaliação da qualidade de mudas de espécies florestais nativas. Ciência Florestal, v.31, n.4, p.1733-1750. 2021. http://dx.doi.org/10.5902/1980509843229
http://dx.doi.org/10.5902/1980509843229...
).

Analyzing Pearson’s correlation matrix (Figure 2), significant positive and negative correlations were found among the characteristics. For growth and quality traits of seedlings the highest positive correlation values (above 0.80) were observed between number of leaves and Dickson Quality Index, stem diameter and shoot dry matter, shoot height and shoot dry matter, and shoot height and root dry matter.

Figure 2
Pearson’s correlation matrix between the analyzed variables of acai (Euterpe oleracea) seedlings inoculated or not (control) with growth-promoting rhizobacteria.

Most traits also showed strong and moderate correlation. Total dry matter showed moderate correlation with root dry matter and shoot height, and negative correlation with number of leaves, for other traits it was close to zero. The shoot dry matter/root dry matter ratio was negatively correlated with all traits except total dry matter, where there was no correlation.

The growth variables were positively related to the Dickson Quality Index except total dry matter. This result may indicate that the allocation of dry matter has little interference in the DQI values for this species.

Conclusions

The rhizobacterium B. subtilis promoted greater growth and quality of açaí seedlings while the species B. amyloliquefaciens showed the lowest values for the evaluated characteristics.

Acknowledgements

To the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for granting scholarships to the second author (Process nº 4199) and research productivity scholarship to the last author (Process nº 310500/2018-4 and nº 317010/2021-2).

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  • Editor: Petterson Baptista da Luz

Publication Dates

  • Publication in this collection
    31 July 2023
  • Date of issue
    Apr-Jun 2023

History

  • Received
    27 Jan 2023
  • Accepted
    10 Apr 2023
  • Published
    26 May 2023
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