Development of maize under different pH values, humidity and presence of Azospirillum brasilense

Desenvolvimento do milho sob diferentes pH, umidade e presença de Azospirillum brasilense

Abstract

Abiotic factors play a crucial role in the productivity of agricultural systems, and stress resulting from these factors can significantly restrict crop yields. To address this challenge, the use of tools capable of mitigating adverse effects, such as growth-promoting bacteria, is essential. This study aimed to investigate the growth of maize plants inoculated or not with Azospirillum brasilense, subjected to variations in soil pH and moisture. Two experiments were conducted under controlled conditions, in which maize plants inoculated with A. brasilense were subjected to variations in (i) soil moisture (100, 75, 50, and 25% of field capacity) and (ii) hydrogenic potential (pH 4.5; 5.5; 5.0; 6.0; and 6.5). It was observed that low moisture favored root growth but limited the development of leaves and stems of maize plants at the V4 phenological stage. The increased root development provided by A. brasilense under conditions of water restriction resulted in a reduction in water use efficiency by the leaves. Additionally, more acidic and/or alkaline pH levels also reduced the dry mass of roots and aboveground parts, as well as the carboxylation efficiency of rubisco, but increased water use efficiency (At 25% FC water deficit, WUE increased by 9.5% in rhizobacterium-treated seeds and by 16% in the control). Inoculation with A. brasilense promoted greater development of maize plants, as evidenced by increased volume, root length, and leaf area. Although maize plants showed better initial development without water restriction and in soil with a pH of 5.9, A. brasilense allowed maize plant growth even at pH 5.1, without significant adverse impacts.

Keywords:
water deficit; root system; dry mass; phytohormones; diazotrophic bacteria

Resumo

Os fatores abióticos desempenham um papel crucial na produtividade dos sistemas agrícolas, e o estresse resultante desses fatores pode restringir significativamente o rendimento das culturas. Para enfrentar esse desafio, o uso de ferramentas capazes de mitigar os efeitos adversos, como as bactérias promotoras de crescimento, é essencial. Este estudo teve como objetivo investigar o crescimento de plantas de milho inoculadas ou não com Azospirillum brasilense, submetidas a variações no pH do solo e na umidade. Foram realizados dois experimentos sob condições controladas, nos quais plantas de milho inoculadas com A. brasilense foram submetidas a variações em (i) umidade do solo (100, 75, 50 e 25% da capacidade de campo) e (ii) potencial hidrogeniônico (pH 4,5; 5,5; 5,0; 6,0; e 6,5). Observou-se que a baixa umidade favoreceu o crescimento das raízes, mas limitou o desenvolvimento das folhas e caules das plantas de milho no estágio fenológico V4. O aumento do desenvolvimento das raízes proporcionado por A. brasilense sob condições de restrição hídrica resultou em uma redução na eficiência do uso da água pelas folhas. Além disso, níveis de pH mais ácidos e/ou alcalinos também reduziram a massa seca das raízes e das partes aéreas, bem como a eficiência da carboxilação da rubisco, mas aumentaram a eficiência do uso da água (com déficit hídrico de 25% da capacidade de campo, a eficiência do uso da água aumentou em 9,5% nas sementes tratadas com rizobactérias e em 16% no controle). A inoculação com A. brasilense promoveu maior desenvolvimento das plantas de milho, como evidenciado pelo aumento do volume, comprimento das raízes e área foliar. Embora as plantas de milho tenham mostrado melhor desenvolvimento inicial sem restrição hídrica e em solo com pH 5,9, A. brasilense permitiu o crescimento das plantas de milho até mesmo com pH 5,1, sem impactos adversos significativos.

Palavras-chave:
déficit hídrico; sistema radicular; massa seca; fitormônios; bactérias diazotróficas

1. Introduction

The maize (Zea mays L.) crop holds undeniable importance worldwide due to its extensive utilization in human and animal nutrition, as well as in industry. Its versatility as a food source, animal feed, raw material for biofuels, and other industrial applications significantly contributes to global food and economic security (Purwanto et al., 2015). Brazil is one of the few countries that can increase food production without using environmental preservation areas (Martin et al., 2023). Maize stands out as a prominent crop, playing a crucial role in agricultural and livestock production chains. However, despite its relevance, maize productivity faces a myriad of challenges, especially concerning soil management and water availability. Soil acidity is a common issue in many agricultural regions resulting in significant limitations for plant growth (Sousa et al., 2019). Low calcium availability and the presence of toxic aluminum in acidic soils represent serious obstacles to maize root development, directly affecting its ability to absorb water and essential nutrients (Kopittke et al., 2015). Studies have emphasized the importance of soil management strategies, such as liming and corrective applications, to mitigate the negative effects of soil acidity and enhance maize crop productivity (Sade et al., 2016).

Moreover, water scarcity is another critical factor limiting maize production in many parts of the world. In arid and semi-arid regions, where water availability is naturally limited, maize plants face additional challenges in obtaining sufficient water for proper growth and development (Kapoor et al., 2020). Water conservation strategies, such as the use of efficient irrigation systems and proper irrigation management, become essential to maximize maize crop yields under such conditions (Ingrao et al., 2023). In addition to challenges related to soil and water, proper nutrient management also plays a fundamental role in maize productivity. Balanced fertilization, with adequate application of nitrogen, phosphorus, potassium, and other essential nutrients, is essential to ensure healthy plant growth and maximize grain yields (Fageria et al., 2008).

In this context, the use of beneficial microorganisms, such as diazotrophic bacteria of the genus Azospirillum, has received increasing attention as a promising strategy to improve maize productivity (Schaefer et al., 2018; Lima et al., 2022). These bacteria are capable of atmospheric nitrogen fixation, providing an additional nitrogen source for plants and stimulating their growth and development. Recent studies have demonstrated the benefits of seed inoculation with Azospirillum in promoting root growth, improving nutrient absorption, and increasing tolerance to abiotic stress, including water scarcity and soil acidity (Zeffa et al., 2019).

Therefore, integrated strategies combining soil management practices, water management, and the use of beneficial microorganisms represent a promising approach to enhance the productivity and sustainability of maize crops worldwide. By better understanding the challenges faced by maize cultivation and exploring innovative solutions based on scientific evidence, we can advance towards more resilient, efficient, and sustainable agricultural systems. The symbiotic association between bacteria and plants stimulates growth through various mechanisms, including synthesis and secretion of phytohormones, increased nutrient availability, and biological nitrogen fixation (Ferreira et al., 2019).

In this research, the aim was to assess the growth of maize plants inoculated or not with Azospirillum brasilense subjected to variations in soil pH and moisture.

2. Materials and Methods

Two experiments were conducted at the Federal University of Santa Maria (UFSM), located in the central region of Rio Grande do Sul (29°43'02.93”S and 53°44'00.10”W) at an altitude of 119 meters. For the study of pH in the nutrient solution, a greenhouse with controlled temperature and humidity was utilized, while a conventional greenhouse was employed to measure the effect of soil moisture.

2.1. First experiment (soil moisture)

The first experiment assessed soil moisture levels, with and without seed inoculation with A. brasilense. The experimental design was completely randomized in a 4x2 factorial, with four replications, totaling 32 experimental units. The experiment was conducted from March to May 2016. Soil moisture levels consisted of 25%, 50%, 75%, and 100% of the soil moisture at field capacity (SMC). To determine the field capacity (SMC), air-dried soil was collected, sieved through a 5 mm mesh, and then placed in pots to saturate for a period of 24 hours. Subsequently, the pots were covered with plastic film to prevent evaporation and allowed to drain freely. The masses were measured at zero time, 16, 24, 48, 60, 84, and 120 hours. The experimental units corresponded to 8 L plastic pots placed on benches located at 0.70 m above the ground.

The soil used for cultivation consisted of a sandy Red Dystrophic Argisol (EMBRAPA, 2018), extracted from a layer of 0-0.20 m depth with the following chemical characteristics: pH 5.5 in H₂O; 87.76 mg dm^-3 of P (Mehlich-1 Extractor); 68.1 mg dm^-3 of K; 12.4 g kg^-1 of organic matter; 1.77 cmolc dm^-3 of Ca⁺²; 0.60 cmolc dm^-3 of Mg⁺²; and a CEC at pH 7.0 of 6.92 cmolc dm^-3. The base fertilization employed for sowing consisted of triple superphosphate (46% P₂O₅), potassium chloride (60% K₂O), and urea (45% N), as determined by the requirement to achieve an expected yield of 12 Mg ha^-1. This fertilization was adequately mixed and uniformly applied to the soil before filling the cultivation containers. Planting was performed on March 29, 2016, using the DEKALB 240 hybrid. Inoculation with A. brasilense strains occurred just before sowing. A liquid inoculant based on Azospirillum brasilense with a concentration of 2.0 x 10⁸ CFU ml^-1 was used at a dose of 300 ml for 60,000 seeds. Four seeds were sown per experimental unit, equally distributed in the pots at a depth of 2 cm. After emergence, only two plants per pot were retained.

2.2. Second experiment

The second experiment evaluated the pH in a nutrient solution and was installed on November 27, 2015. The experimental design was completely randomized, with ten treatments distributed in a bifactorial with three replications. The treatments consisted of five variations of pH levels in the nutrient solution (4.5, 5.0, 5.5, 6.0, and 6.5), with or without inoculation with A. brasilense.

Inoculation with Azospirillum brasilense in maize seeds (DEKALB 240 hybrid) used a dosage of 300 mL for 60,000 seeds, with a concentration of 2.0 x 10⁸ CFU ml^-1. The corn seedlings were grown on germination paper kept in a BOD chamber at a temperature of 25 °C. The germination stage was conducted to obtain seedlings with approximately 10 cm long roots for their preliminary fixation on a supporting structure, placed on the container containing the nutrient solution. The experimental units consisted of plastic boxes with a capacity for 17 liters of nutrient solution, measuring 30x23x17 cm, each accommodating twelve maize seedlings with healthy and uniform roots. These seedlings were attached to the styrofoam with the aid of a foam strip surrounding the root, allowing it to float on the solution while keeping the entire root system submerged. The nutrient solution used in the cultivation did not contain nitrogen in its formulation and was balanced at the following concentrations: 221.96 mg L^-1 of CaCl₂, 246.4 mg L^-1 of MgSO₄, and 544.4 mg L^-1 of KH₂PO₄. The micronutrient solution was composed of sodium molybdate 0.9 g L^-1, boric acid 1.5 g L^-1, copper sulfate 2.5 g L^-1, manganese sulfate 2.0 g L^-1, and zinc sulfate at a concentration of 1 g L^-1 of solution. During cultivation, the nutrient solution remained aerated (Air Compressor, 50 L/min, potency 35W, and the pH was adjusted daily according to the treatment, using 0.5 mol L^-1 HCl and NaOH for calibration, respectively. The nutrient solution was replaced every seven days.

2.3. Evaluated variables

Physiological parameters were assessed, encompassing the measurement of root system volume (RV), root length (RC) (the measurements were taken using WinRHIZO), shoot dry mass (DM) (including leaves and stem), root mass (RM), leaf area (LA), and leaf gas exchange for the experiment with variation in nutrient solution pH. Regarding soil moisture, RM, DM, gas exchange, and photosynthetic pigments (chlorophyll a, b, and total) were measured. Gas exchange measurements for both experiments followed the same methodology, conducted when plants reached the developmental stage with three fully expanded leaves (V3) (Ritchie et al., 1989). Gas exchange determination was carried out using the LI-COR LI-6400 XT portable Infra Red Gas Analyzer (IRGA) with a photosynthetic radiation of 1000 μmol m2 s⁻¹ and CO₂ concentration of 400 μmol mol^-1. For gas exchange determination, water use efficiency (WUE - mol CO₂ mol H2O^-1) obtained by the ratio of CO₂ fixed by photosynthesis to transpired water, and rubisco carboxylation efficiency (RCE - μmol m^-2 s^-1 Pa^-1) obtained by the ratio of CO₂ fixed by photosynthesis to internal CO₂ concentration were evaluated. These variables were measured between 10 am and 12 pm for all plants.

RV, RC, LA, and dry matter of maize plants cultivated in nutrient solution were determined at phenological stage V4, with three plants per experimental unit. The roots and leaves of the plants were scanned using an Epson 11000 XL scanner, placed in a small water blade to prevent overlap, especially of the roots. Subsequently, they were quantified using WinRhizo Pro Software to determine total root length, total root volume, and leaf area. After scanning, roots and leaves were placed in paper bags for drying in a forced air circulation oven at 65 °C for 72 hours until reaching constant mass. Weighing was conducted using a high precision balance (0.0001 g).

For RM and DM determination at different SMC levels, two plants at phenological stage V4 were collected, with their structures separated into roots, stems, and leaves, which were dried in a forced air circulation oven at 65 °C for 72 hours and then weighed using a high precision analytical digital balance with 0.0001 g accuracy. Photosynthetic pigments (chlorophyll a, b, and total) were determined in maize leaves at V3, at two median positions of the last fully expanded leaf according to (Argenta et al., 2001). Pigment content estimation was conducted using the Clorofilog CFL1030 chlorophyll meter, with two plants per pot, and expressed as chlorophyll index.

2.4. Statistical analysis

The parameters were subjected to testing of the assumptions of the mathematical model and subsequently to analysis of variance of the data using the F test. When significant, the means of the qualitative factors were subjected to the Scott-Knott test (5% probability), and for the means of the quantitative factors, subjected to polynomial regression analysis up to the third degree. For all graphically expressed results, a confidence interval was determined (P≤0.05). The SISVAR software (Ferreira, 2011) was used for the analyses.

3. Results

The dry mass of root and aerial parts of maize was influenced by the reduction in soil moisture and the presence of A. brasilense (Figure 1). When reducing the field capacity to 25% in the control treatment, there was a 4.6% increase in RM production (Figure 1A). Additionally, the dry matter of the aerial part (Figure 1B) at the V4 phenological stage also showed significant interaction for the different levels of field capacity moisture and the presence of the diazotrophic bacteria.

Figure 1
Dry matter of root (A) and aerial part (B) of maize plants at the V4 phenological stage subjected to soil moisture levels.

Soil water deficit reduced mass accumulation. Water use efficiency (WUE) interacted with soil moisture and the presence of A. brasilense (Figure 2A). The lowest efficiency occurred with 75% of FC (6.43 and 6.46 mol CO₂ mol H₂O^-1 for A. brasilense and control, respectively). In the 25% FC water deficit, there was an increase in WUE of 9.5% for seeds treated with the rhizobacterium and 16% for the control. This is related to the increase in the root system and decrease in leaf contribution at the initial stage of development (Figure 1). The instantaneous carboxylation efficiency of rubisco (RCE) was affected by soil moisture variation in maize plants. RCE was 13% higher in 25% FC moisture compared to 80% FC, where RCE was lower (Figure 2B).

Figure 2
Water use efficiency (A) and instantaneous carboxylation efficiency of rubisco (B) in maize leaf at V3 phenological stage subjected to different soil moisture levels.

These results are more associated with the reduction of internal carbon concentration (12.22% for A. brasilense and 7.80% for the control) than with the CO₂ assimilation rate in photosynthesis. Under water deficit, there is a lower intercellular concentration of carbon dioxide compared to the amount assimilated by the plant through photosynthesis. The chlorophyll index for chlorophyll a, b, and total (Table 1) at different soil moisture levels subjected to inoculation with A. brasilense statistically differs (P>0.05). Higher chlorophyll values were observed without water restriction (100% of FC), decreasing with increasing water deficit.

Table 1
Chlorophyll index (CI) of corn plants at V3 phenological stage for different soil moisture levels (FC, %) and seed inoculation presence in greenhouse.

Inoculation	Soil moisture
	25	50	75	100
	Chlorophyll a
A. brasilense	37.06 aD	37.67 aC	39.70 aB	40.62 bA
Control	37.14 aC	36.60 bD	38.81 bB	41.81 aA
Mean	38.68
CV%	0.59
	Chlorophyll b
A. brasilense	10.07 aC	8.17 bD	11.34 aB	12.39 aA
Control	9.85 aB	8.62 aC	11.25 aA	11.47 bA
Mean	10.39
CV%	2.1
	Chlorophyll total
A. brasilense	47.14 aC	45.80 aD	51.04 aB	53.01 aA
Control	46.99 aC	45.22 bD	50.06 bB	53.29 aA
Mean	49.07
CV%	0.66

• CV%: coefficient of variation, means followed by the same lowercase letter in the column and uppercase letter in the row do not show significance at 5% by the Scott-Knott test.

A mean increase of 1% in the chlorophyll index of total chlorophyll was observed at moisture levels of 75 and 50% of SMC (Table 1). However, for the moisture levels of 100 and 25% of SMC, there was no significant difference in total chlorophyll in plants that used the diazotrophic bacteria in seed treatment and the control (Table 1). The maximum technical efficiency for root dry matter production (RM) was achieved at pH 5.8, with 0.56 g accumulated until stage V4 of corn (Figure 1A). However, there was a higher accumulation of RM between pHs 4.9 to 6.3, with maximum efficiency at pH 5.6 with an addition of 0.032 g of higher RM.

The root volume (RV) of corn plants was also affected by pH variation, with an increase in root volume at all hydrogenic potential levels (Figure 3A), especially at pH 4.5. Even with the pH increasing up to 5.7, there was still a 5.6% increment in RV in inoculated plants compared to non-inoculated ones. Root length (RL) decreased by 74% at pH 4.5 compared to pH 5.9. The use of A. brasilense resulted in a 6.3% increase in root size in the pH 4.5 range, with positive increments of RL up to pH 5.9 (Figure 3B).

Figure 3
Root volume (A) and root length (B) of corn subjected to different hydrogenic potentials of nutrient solution and inoculation with A. brasilense in a greenhouse.

The maximum efficiency of aerial dry matter accumulation (DM) was achieved at pH 5.9 for the control and pH 5.5 for the presence of A. brasilense, with an increase of 4.4% in the latter case (Figure 4A). Leaf area (LA) and aerial dry matter (DM) of corn exhibit similar behavior, with higher LA at pH 5.6. Introduction of A. brasilense resulted in higher LA at pH 5.5, and there was still an increase in LA at pH 4.5, despite the lower accumulation (Figure 4B).

Figure 4
Dry mass of aerial part (A) and leaf area (B) of corn seedlings subjected to different pH levels of nutrient solution in a greenhouse.

The highest instantaneous carboxylation efficiency (RCE) was achieved at pH 5.5, being higher in the presence of the bacterium A. brasilense (Figure 5A). Although the response for RCE was quadratic, the acidification of the solution increased the difference in RCE compared to the control (Figure 5B). At pH 4.5, this efficiency was superior by 12.2%, while at pH 6.5, it was null.

Figure 5
Water use efficiency (A) and rubisco carboxylation efficiency index (B) in corn at V4 stage subjected to different pH levels of solution and corn seed inoculation.

4. Discussions

Under conditions of water restriction, plants adapt to the decrease in water availability in the soil; root elongation is a strategy to maximize water absorption (Cesari et al., 2016; Vejan et al., 2016). Maize plants inoculated with A. brasilense showed an increase in root mass in soils under water restriction, which was intensified with decreasing moisture levels. This increase was 14.1% at 25% soil field capacity moisture (Figure 1a). The results are linked to nitrogen supply through biological nitrogen fixation (FBN) and phytohormone synthesis (Zambonin et al., 2019). The efficacy of A. brasilense in root development is associated with its ability to promote morphological changes for adaptability, such as rapid modification in the structure of the cell membrane in response to different osmotic pressures (Cesari et al., 2016). This enables the bacteria to survive and colonize different rhizosphere environments, including within the roots (D’Angioli et al., 2017). Soil moisture availability up to 25% of field capacity resulted in a 46.33% reduction in accumulated aboveground biomass compared to soil without water restriction. This is linked to the mobilization of carbohydrates for root growth, due to the need for a more developed root system to meet water demand. Decreasing soil moisture from 80% to 50% of field capacity led to a 50% reduction in aboveground dry mass (Wu et al., 2011) (Figure 1B).

Seed inoculation in maize cultivation increased dry mass of the aerial part (DM) levels for soil moisture levels above 52% of field capacity, with the highest productivity at 100% of field capacity, 6.5% superior compared to without the bacteria. For soil moisture levels below 50% of field capacity, there was a reduction in leaf and stem production, with a 23% lower variation for DM at 25% of field capacity (Figure 1B). This agrees with the results of Schaefer et al. (2018), who observed that inoculation with A. brasilense resulted in a 13% increase in the corn DW for the 2014/15 harvest. This result is associated with the role of auxin, which, at high concentrations in plants under water stress, reduces endogenous cytokinin synthesis and increases cytokinin oxidase expression, thus reducing the final concentration of this phytohormone associated with aboveground plant growth and aerial apical meristems (Schaller et al., 2015). In comparing root mass (RM) and dry mass of the aerial part (DM) production, there is an inverse behavior in the early stages of crop growth. The greater accumulation of root mass corresponds to a lower accumulation of aboveground dry mass (Figure 1).

The water use efficiency (WUE) is linked to increased water absorption by the root system and processes of water loss through transpiration and entry of carbon dioxide for photosynthesis (Brodribb et al., 2015). The use of growth-promoting rhizobacteria reduced WUE by 0.48 mol CO₂mol H₂O-1 at 25% field capacity moisture. This is associated with increased root biomass formation, nitrogen increment by FBN, and phytohormone production, making maize plants capable of meeting their water demand in deficit environments. WUE relates the water lost in transpiration to the carbon fixed in photosynthesis. A 2.06% greater water efflux into the air was observed in the transpiration process when maize seeds were inoculated with A. brasilense under severe water restriction conditions (Jaimez et al., 2005). With soil moisture at 100% of field capacity, maize plants reduced WUE due to soil aeration limitation, affecting transpiration and the ability to meet transpirational demand. This reduced transpiration by 7.12% for A. brasilense and 11.14% for the control, impacting WUE at 100% field capacity. Plants under moderate water stress generally exhibit partial stomatal closure, limiting transpiration more than CO₂ diffusion, thereby increasing WUE (Pirasteh‐Anosheh et al., 2016).

Photosynthetic pigments interact with soil moisture. Higher chlorophyll values were observed without water restriction (100% field capacity), decreasing with increasing water deficit (Table 1). Evaluating the effect of contrasting acidity (pHs: 4.8 and 6.5) on the physiological and productive variables of wheat inoculated with A. brasilense, it was found that plants grown with a pH of 6.5 have higher levels of nitrate reductase and content of chlorophyll a, b, and total chlorophyll (Martin et al., 2024). This variation is linked to the instability of chlorophylls and the influence of water stress on their integrity (Kaur and Asthir, 2017). The average increase of 1% in the chlorophyll index of total chlorophyll at moisture levels of 75% and 50% of field capacity may be associated with cytokinin synthesis, a phytohormone produced by diazotrophic bacteria that slows down protein and chlorophyll degradation in plant leaves (Marcos et al., 2015).

For moisture levels of 100% and 25% of field capacity, there was no significant difference in total chlorophyll in plants utilizing diazotrophic bacteria. This indicates that in soils with excess or severe water deficit, rapid hormonal changes occur in plants altering the balance between hormone synthesis, degradation, and transport (Grzesiak et al., 2016).

Medium acidity (pH) is an important factor for plant development and directly affects soil solution, especially the rhizosphere (Nabati et al., 2023). Variation in hydrogen potential alters the solubility and availability of nutrients. Soil biota is influenced by pH, which can limit its development. The interaction of A. brasilense in maize cultivation at different hydrogen potentials shows influence for both acidic and alkaline pH levels on plant development and metabolism. Root development is affected by solution pH variation, with maximum technical efficiency observed between pH 5.5 and 6.0 (Figure 2). In this slightly acidic pH range, there is greater nutrient availability for the plant, favoring absorption by the root system. For pH slightly above 5.5, there is better development of maize seedlings in root biomass accumulation (Xiong et al., 2020)

Reducing the pH to 4.5 resulted in an 11.8% decrease in root mass (RM), associated with increased aluminum solubility, a toxic element that reduces the root system (Maia et al., 2018). Seed inoculation with A. brasilense influenced maize plants, resulting in a reduction of RM at both acidic and alkaline pH levels. The study of A. brasilense effect showed a 3.5% increase in maize RM at pH 5.5. This pH range favors maize development, increasing nutrient availability and positively influencing nitrogen biological fixation rates (Kunito et al., 2016)

There was an increase in root volume at all hydrogen potential levels (Figure 3B), especially at pH 4.5. This effect may be associated with greater development of secondary roots and root hairs (Zhang et al., 2018). Root length (RL) decreased by 74% at pH 4.5 compared to pH 5.9. The use of A. brasilense resulted in a 6.3% increase in root size in the pH 4.5 range, with positive RL increments up to pH 5.9 (Figure 3C). This reinforces the importance of maintaining pH between 5.5 and 6.0 for better root growth. Leaf and stem growth were affected by different hydrogen potentials, with less acidic pHs being ideal for greater leaf expansion and dry mass accumulation. In maize cultivation without bacteria, pH decrease drastically reduced dry matter accumulation, with a 19% loss for dry mass of the aerial part (DM) when varying pH from 5.9 to 4.5. This is associated with low nutrient availability in acidic soils and lesser root system development due to aluminum toxic action (Sade et al., 2016). The use of A. brasilense in lower pH soils is advantageous, as DM is important for production through light interception and photoassimilate production. Water Use Efficiency (WUE) showed better performance with seed inoculation with A. brasilense, being higher at pH 6.5 due to lower DM. With increased DM and root dry mass (RM), there was a decrease in WUE, being lower at pH 5.5 in the presence of the bacteria (Figure 5A). Increased water absorption capacity favors leaf development and greater water absorption by root development, highlighting the benefits of A. brasilense (Zeffa et al., 2019). This bacterium increases maize plant water absorption capacity, preventing stomatal closure and increasing internal CO₂ concentration, resulting in greater RUBISCO carboxylation activity (Chieb and Gachomo, 2023). Greater carboxylation efficiency in the presence of A. brasilense occurs at decreasing pH levels. This result may be associated with greater maize aerial part development and more efficient CO₂ absorption at acidic pHs, as soil acidity conditions stomatal closure and reduces CO₂ assimilation (Guo et al., 2024).

5. Conclusion

The use of Azospirillum brasilense expands the suitable pH range for maize development, allowing the plant to grow well within a pH range of 5.1 to 5.9 without impairing its growth. In more acidic environments, A. brasilense helps reduce losses in early growth. Under water-limited conditions, the presence enhances the plant's ability to absorb water, promoting a more robust development of the root system.

Acknowledgements

To the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq - Processes 312543/2023-9), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brasil - Finance code 001, and the Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS – Processes 22/2551-0001644-8) for granting scholarships to the authors. To the scholarship students and volunteers for helping in data collection.

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Publication Dates

• Publication in this collection
  31 Jan 2025
• Date of issue
  2024

History

• Received
  18 June 2024
• Accepted
  11 Nov 2024