Effects of different doses of a Bacillus-based inoculant on açaí palm seedlingsAbstract
The production of seedlings with adequate shoot and root development is essential for the success of commercial cultivation systems. Bacteria of the genus Bacillus, capable of phosphate solubilization, have been widely studied for their potential to increase nutrient availability to plants. This study aimed to evaluate the effects of applying different doses of the Bacillus-based commercial biofertilizer Biomaphos® on the growth of açaí seedlings (Euterpe precatoria). The experiment was conducted in a greenhouse under a completely randomized design, with ten replicates and four treatments: a control (without inoculation) and three doses of the Biomaphos® inoculant (4, 12, and 16 × 108 CFU mL−1). 150 days after inoculation, shoot and root system variables were evaluated. The application of the biofertilizer promoted increases in shoot variables, except for plant height, and enhanced root volume, surface area, and dry mass, with a linear reduction in root diameter. The results indicate that Biomaphos® promotes the growth of açaí seedlings under controlled conditions.
Keywords:
phosphate solubilization; Euterpe precatoria; Bacillus subtilis; Bacillus megaterium
Resumo
A produção de mudas com desenvolvimento aéreo e radicular adequado é fundamental para o sucesso dos sistemas de cultivo comercial. Bactérias do gênero Bacillus, com capacidade de solubilização de fosfato, têm sido amplamente estudadas pelo potencial de aumentar a disponibilidade de nutrientes para as plantas. Este estudo teve como objetivo avaliar os efeitos da aplicação de diferentes doses do biofertilizante comercial à base de Bacillus Biomaphos® no crescimento de mudas de açaí (Euterpe precatoria). O experimento foi conduzido em casa de vegetação, em delineamento inteiramente casualizado, com dez repetições e quatro tratamentos: controle (sem inoculação) e três doses do inoculante Biomaphos® (4, 12 e 16 × 108 UFC mL−1). Após 150 dias da inoculação, foram avaliadas variáveis da parte aérea e do sistema radicular. A aplicação do inoculante promoveu incremento nas variáveis da parte aérea, exceto na altura das plantas, e aumento no volume, área superficial e massa seca das raízes, com redução linear do diâmetro radicular. Os resultados indicam que o Biomaphos® favorece o crescimento de mudas de açaí em ambiente protegido.
Palavras-chave:
solubilização de fosfatos; Euterpe precatoria; Bacillus subtilis; Bacillus megaterium
1. Introduction
Açaí is widely recognized as a functional "superfood" due to its nutritional properties, which have continuously driven demand in domestic and international markets (Queiroga et al., 2023; Matos et al., 2023). This increasing demand has intensified pressure on natural populations of açaí palms, since part of the production still depends on extractivism. (Yamaguchi et al., 2015; Araújo et al., 2023). Furthermore, the majority of the supply originates from the cultivation of açaí-do-Pará (Euterpe oleracea Mart.), whose production exhibits a peak in the second half of the year (Lopes et al., 2022).
Seedling production represents a fundamental stage in the açaí production chain (Butzke et al., 2023). Increased commercial demand has resulted in a greater need for high-quality seedlings (Araújo et al., 2023). Despite the increase in production, limitations persist, including the slow initial growth of seedlings (Castro et al., 2020), which extends the nursery stage and increases production costs (Araújo et al., 2023).
Phosphorus (P) plays a fundamental role in plant development, as it is an essential macronutrient involved in various metabolic processes (Chouyia et al., 2022; Raghothama, 1999). This nutrient is of particular importance during the initial growth of plants (Malhotra et al., 2018). However, phosphorus supplied via mineral fertilizers, it can rapidly become unavailable, as it forms complexes with metal ions such as calcium, iron, and aluminum (Wendimu et al., 2023).
In this context, the utilization of plant growth-promoting microorganisms (PGPMs) has emerged as a promising approach to promote seedling growth by improving nutrient solubilization, stimulating phytohormone production, and promoting root development (Lopes et al., 2021). Among these microorganisms, bacteria of the genus Bacillus are particularly noteworthy, especially B. subtilis and B. megaterium, which are widely recognized for their beneficial effects in promoting plant growth in various crops (Garbin et al., 2022; Guimarães and Klein, 2023; Oliveira-Paiva et al., 2024).
The use of rhizobacteria to promote the growth of palm seedlings, including oil palm and coconut, has been extensively reported in the literature (Azlin et al., 2007; Om et al., 2009; George et al., 2013). In light of these findings, the present study evaluates the impact of inoculation with Bacillus-based biofertilizer on the growth of açaí palm seedlings (Euterpe precatoria Mart.) in a controlled environment.
2. Materials and Methods
2.1. Preparation of seedlings and substrate
Seeds were obtained from fruits of native açaí palms (E. precatoria) in the municipality of Iranduba, AM (3°22'54" S, 60°22'61" W). The preparation of the seeds was conducted in three stages, as outlined by Cesarin et al. (2020). Initially, the seeds underwent a washing process in running water. Subsequently, they were immersed in warm water (50°C) for a duration of 30 minutes. Finally, manual depulping was performed. After the depulping process, the seeds underwent thorough washing to ensure they were suitable for planting. Seed sowing was performed in plastic trays filled with the commercial substrate Vivatto Plus®, a peat-based substrate used for forest seedling production, characterized by high water retention capacity and adequate aeration. The seedlings remained in trays until the emergence and full expansion of the first leaf, after which they were transplanted to pots containing the experimental soil substrate.
The soil selected for use as a substrate was a Ferralsol, collected at a depth of 0 to 20 cm. A composite sample of the soil was utilized for chemical characterization (Table 1). Subsequently, the soil was subjected to homogenization, sieving, and sterilization in an autoclave (121°C at 1 atm, 1 h), repeated twice with a 24-hour interval (Mahmood et al., 2014). Consequently, 10 g kg−1 of Osmocote® slow-release fertilizer, fortified with micronutrients, was added. After five months, the fertilizer was reapplied. The fertilizer's composition is as follows: 15% nitrogen (N), 9% phosphorus pentoxide (P2O5), 12% potassium oxide (K2O), 1.3% magnesium (Mg), 6% sulfur (S), 0.05% copper (Cu), 0.46% iron (Fe), and 0.02% manganese (Mn).
2.2. Planting seedlings and inoculation
The seedlings were transferred from the trays to 2.5 L pots with 1.5 kg of soil substrate when the first leaf of E. precatoria was fully expanded and had reached a height of 15±1 cm. Soil moisture was maintained at approximately 75% of field capacity.
Inoculation was performed 30 days after transplanting the seedlings to the pots. Prior to application, the Biomaphos® inoculant was diluted in 9 mL of sterile saline solution [0.9% (w/v) NaCl], resulting in a concentration of 108 CFU (colony-forming units) mL−1. Subsequently, four doses corresponding to concentrations of 0, 4 × 108, 12 × 108, and 16 × 108 CFU mL−1 were applied to the substrate near the root region using a micropipette.
2.3. Experimental design
The experimental design was completely randomized, with ten replicates, in which each pot was considered an experimental unit. The treatments consisted of different concentrations of the Biomaphos® inoculant containing the strains Bacillus megaterium (BRM119) and B. subtilis (BRM2084): T0 (control), T1 (4 × 108 CFU mL−1), T2 (12 × 108 CFU mL−1), and T3 (16 × 108 CFU mL−1). All treatments received the same fertilization, varying only in the presence and concentration of the inoculant. The experiment was conducted in a greenhouse with aluminized screen (Aluminet® 50%) at the Embrapa experimental station located in Manaus, AM (2°53'38” S; 59°58'29” W).
2.4. Evaluation of shoot and root
The evaluations were conducted 180 days after transplanting the seedlings to the pots. The shoot variables evaluated were plant height (PH), stem diameter (SD), number of leaves (NL), specific leaf area (LA), and shoot dry weight (SDW).
Plant height (PH) was measured using a ruler, from the base of the stem to the tip of the last fully expanded leaf. Stem diameter (SD) was measured at the collar region, at the substrate surface, using a digital caliper with a precision of 0.01 mm. The number of leaves (NL) was determined by counting all fully expanded leaves. Specific leaf area (LA) was estimated using the method proposed by Benincasa (2003), based on the relationship between leaf disc area and dry weight. Shoot dry weight (SDW) was obtained after drying the shoot material in a forced-air oven at 65 °C until constant weight.
The root variables evaluated were root volume (RV), mean root diameter (RD), root surface area (RSA), and root dry weight (RDW). Root volume (RV), root surface area (RSA), and mean root diameter (RD) were determined from images of the root system analyzed using Safira v.1 software. Root dry weight (RDW) was obtained following the same drying procedure described for the shoot.
2.5. Statistical analysis
The results were subjected to regression analysis (Galton, 1886). The normality and homoscedasticity of the residuals were evaluated using the Shapiro-Wilk and Breusch-Pagan tests, respectively (Shapiro and Wilk, 1965; Breusch and Pagan, 1980). The evaluation of these results employed linear and polynomial regression models. To adjust the model, cross-validation was performed using the k-fold method. The optimal doses were ascertained by determining the vertex of the parabolas. The analyses were performed using RStudio 2023.12.1 software.
3. Results
The shoot variables - stem diameter (SD), number of leaves (NL), specific leaf area (LA), and shoot dry weight (SDW) - increased with inoculation of Bacillus-based biofertilizer. These variables showed decreasing parabolic growth with increasing inoculant dosage (Figures 1a, 1b, 1c, and 1d). Maximum stem diameter growth was obtained with a dose of 9.5 × 108 CFU mL-1 (Figure 1a) and highest number of leaves was obtained with a dose of 9.43 × 108 CFU mL-1. The largest specific leaf area was obtained at a dose of 10.52 × 108 CFU mL-1 (Figure 1c) and the largest shoot dry weight was obtained at a dose of 13.89 × 108 CFU mL-1 (Figura 1d).
Effects of different doses of Biomaphos® inoculant (B. subtilis and B. megaterium) on the aerial part (SD – Stem diameter; NL - number of leaves; LA - leaf area and SDW - shoot dry weight) of açaí seedlings 150 days after inoculation.
The effect of inoculant doses on root variables differed from that observed in the aboveground parts. Root diameter (RD) decreased linearly with increasing inoculant dose (Figure 2a). In contrast, root volume (RV), root surface area (RSA), and root dry weight (RDW) exhibited quadratic responses, characterized by an initial decrease followed by an increase at higher inoculant doses (Figures 2b, 2c, and 2d). The initial decrease reached estimated minimum values for RV, RSA, and RDW at doses of 10.72 × 108, 10.54 × 108, and 8.45 × 108 CFU/mL, respectively, followed by an increase as inoculant doses were raised.
Effects of different doses of Biomaphos® inoculant (B. subtilis and B. megaterium) on the root system (RD – root diameter; RV – root volume; RSA – root surface area and RDW – root dry weight) of açaí seedlings 150 days after inoculation.
4. Discussion
Inoculation with B. subtilis and B. megaterium resulted in increased of stem diameter (SD), number of leaves (NL), leaf area (LA), and shoot dry weight (SDW) of E. precatoria seedlings. An increase in stem diameter was observed in the inoculation of Bacillus in corn, beans, red pepper, and soybeans (Garbin et al., 2022; Zhao et al., 2022; Guimarães and Klein, 2023; Mendoza-Alatorre et al., 2024; Rocha Filho et al., 2025). An increase in the number of leaves was observed in rice crops inoculated with B. megaterium (Vaishnavi et al., 2024). In addition, an increase in specific leaf area was observed in açaí and avocado seedlings following the inoculation of Bacillus (Campos et al., 2023; Solórzano‐Acosta and Quispe, 2024). Finally, an increase in dry aboveground mass through Bacillus spp. inoculation was reported in oil palm, corn, and soybean crops (Lima et al., 2021; Caris et al., 2026; Silva et al., 2022; Guimarães and Klein, 2023).
The increase in aboveground variables is primarily associated with the ability of Bacillus strains to enhance P availability for plant metabolism through the solubilization of phosphate sources (Vey et al., 2025). This availability is achieved through various mechanisms, including the production of organic acids, enzymes, and siderophores, or by promoting growth through auxin production (Oliveira-Paiva et al., 2020; Oliveira-Paiva et al., 2024; Fumagalli et al., 2026). The increase in P in availability contributed to the increase in all variables, given that this nutrient acts in the synthesis of adenosine triphosphate (ATP), providing energy for metabolic processes such as cell division and the formation of growth tissues, such as the primary thickening meristem, where the diameter of Arecaceae limbs increases (Taiz et al., 2017), which can occur indirectly through the action of indole-3-acetic acid (IAA) produced by the strains (Lopes et al., 2021). Among other factors, the increase in specific leaf area occurs in response to the supply of energy in the form of ATP for the synthesis of macromolecules and division of meristematic cells at the base of the leaf and cell expansion in the more distal regions (Balemi and Negisho, 2012; Taiz et al., 2017). The influence of P on photosynthetic processes, tissue development, and leaf biosynthesis can indirectly favor the production of new leaves (Marschner, 2012; Araújo et al., 2021). Furthermore, the increased photosynthetic capacity of plants facilitates the accelerated development of other organs (Campos et al., 2023). The synthesis of IAA in conjunction with P by B. subtilis and B. megaterium has shown to exert a significant influence on the augmentation of DSW (Sousa et al., 2021). Additionally, the auxin-induced promotion of root growth may have facilitated better support and nutrition for the shoot (Jatan and Lata, 2021; Sousa et al., 2026).
Initial reductions were observed in root volume (RV), root surface area (RSA) and root dry weight (RDW). This reduction was the result of plant biomass being allocated to the aboveground parts. This occurred due to factors such as the availability of water, light, and nutrients. Consequently, the plants do not produce more roots under available nutrients conditions (Poorter and Nagel, 2000; Poorter et al., 2012; Ding et al., 2025). The P availability promotes more biomass allocation aboveground than belowground (Yang et al., 2025). Conversely, when P levels decrease occurred the roots augmentation (Xiao et al., 2024). Increased P uptake through solubilization and root modifications resulting from the inoculation of Bacillus megaterium and Bacillus subtilis has been observed in maize and tomato crops (Jensen et al., 2024; Oliveira-Paiva et al., 2024).
The quadratic response model for the variables RV, RSA and RDW showed that root growth decreased with an increasing microbial concentration of the inoculant. This effect is the result of microbial density influencing colonization and growth promotion by the microorganism in a non-linear manner (Bueno et al., 2022).
5. Conclusions
The inoculation of bacilli (B. subtilis and B. megaterium) has been demonstrated to promote the growth of açai seedlings in greenhouse conditions.
Acknowledgements
To “Instituto Nacional de Ciência e Tecnologia em Microrganismos Promotores de Crescimento de Plantas (INCT MicroAgro- 408267/2024-0)”, Project: PDPG – Pós Doutorado Estratégico/CAPES, proc. nº 88887.691688/2022-01, Project: Proap/CAPES, proc. nº 88881.344324/2026-01, CNPq 308426/2022-3 and “Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM): proc. nº 01.02.016301.04036/2022-10 and Edital nº 017/2024 – DIVULGA CT&I/FAPEAM” for financial support.
Data Availability Statement
All data analyzed in this study are available on the Open Science Framework (OSF) and can be accessed at: https://doi.org/10.17605/OSF.IO/4NHR6.
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