1. Introduction
The water deficit affects not only the arid and semi-arid regions of the world but also areas located in countries with a humid tropical climate, such as Brazil [
1]. In these areas, prolonged water scarcity periods can reduce the productivity of important crops, such as corn [
2]. Water stress negatively affects plants’ physiological and biochemical processes that restrict growth, development, and productivity [
3,
4]. Drought causes a decrease in cell turgor, which is essential for proper cellular metabolisms, such as photosynthesis, enzyme activity, and nutrient uptake [
4,
5].
Corn (
Zea mays L.) is one of the most important crops in the world due to its multiplicity of uses, especially as raw material for the food industry. However, in areas where water scarcity is more pronounced, as in many developing countries and the semi-arid region of Brazil, the use of corn in human food as a main diet component is expressive [
6]. Thus, in these areas, water deficit is one factor that threatens food security. Therefore, it is necessary to find strategies to mitigate the adverse effects of water deficit in essential crops such as corn.
Previous research has demonstrated that beneficial microorganisms (BM) (also called plant growth-promoting bacteria) have the potential to attenuate environmental stresses in plants, such as water deficit [
7,
8,
9,
10]. BM in the soil can induce plants to produce osmoregulant substances such as organic acids, amino acids, and soluble sugars and, thus, act synergistically, contributing to drought tolerance [
11,
12]. These microorganisms can produce auxins such as indole acetic acid, increasing the length of plant roots and, thus, leading to the greater uptake of water and nutrients from the soil [
8]. In this sense, it has been observed that the inoculation of corn with
Bacillus amyloliquefaciens increased the nutrient uptake and promoted growth mechanisms in plants, increasing the concentration of amino acids such as tryptophan, isoleucine, alanine, valine, and tyrosine and sugars such as fructose and glucose [
11]. Likewise, Lima and collaborators [
13] observed that the inoculation of corn with
B. subtilis increased the leaf water content and stomatal regulation without impairing the photosynthetic rates. In addition to the effects on plants, BM can affect the biological activity of the soil, favor specific populations of microorganisms that act in key soil processes such as mineralization and the nitrification of soil nitrogen, and thus reduce the losses of this nutrient through leaching and volatilization processes [
14,
15].
The use of BM in agriculture is advantageous, because it is an environmentally friendly technology, since it can increase crop productivity and soil fertility without exerting any toxic effect on the environment [
16,
17]. The mechanisms of action of BM on plants such as nitrogen fixation, phosphate solubilization, the synthesis of phytohormones (especially IAA), and osmoregulant substances (such as amino sugars), especially in a controlled environment, have been demonstrated in some studies [
9,
15,
18]. However, it is necessary to expand these studies, especially under field conditions, for a better understanding of the effects of BM on the physiological aspects of plants and on soil microbiological activity, which could represent modes of action of BM as attenuating water deficit in plants. In this sense, we hypothesized that BM improves the gas exchange of corn under a water deficit, promotes an increase in soil biological activity, and thus attenuates the adverse effects of irrigation deficit, increasing growth and productivity.
This study aimed to evaluate the effect of beneficial bacteria on gas exchange, growth, production, and the protein content in corn grains and soil microbiological activity under a water deficit.
4. Discussion
In the present study, the possibility of BM attenuating the adverse effects of water deficit in corn was evaluated through the plant variables (growth, gas exchange, productivity, and protein content) and soil (mineral nitrogen and microbiological activity). The water deficit imposed by the 50% ETc water depth reduced corn’s growth and leaf area, as well as the nitrogen content in the leaves. Drought stress causes a decrease in cell turgor, which is essential for proper cellular metabolism, such as photosynthesis, enzymatic activity [
3,
4], and nutrient absorption [
5]. In addition, under water deficit, nitrogen contact with the roots, through diffusion and mainly by mass flow, can be reduced [
42], decreasing the uptake of this nutrient by the plant [
5]. On the other hand, regardless of the irrigation level, the inoculation of plants with
Bacillus subtilis promoted an increase of 13% in the leaf area index and 12% in the N content of the leaves compared to the treatment without the inoculation.
B. subtilis also increased the leaf area in sweet peppers [
43] and leaf N content in sugarcane [
44]. Aquino [
45] observed that five strains of
B. subtilis increased the foliar N content in corn crops compared to uninoculated plants. This indicates that
B. subtilis fixed atmospheric N and improved corn nitrogen nutrition [
13]. However, the effect of treatments containing BM on maize growth was not well defined, as observed in other studies [
7,
13].
The gas exchange measurement showed that corn plants closed their stomata (decreased stomatal conductance) under water deficit to reduce water loss. This result was accompanied by a decrease in the photosynthetic rate, an increase in the internal concentration of CO
2, and a decrease in the transpiration rate. In water restriction conditions, corn inoculation with
Azospirillum brasiliense increased the transpiration rate by 35% compared to the treatment without inoculation. Physiological changes with the use of BM have also been reported in previous works [
7,
46]. BM application in the soil can lead to stimulating the production of osmoregulatory substances by the plant and, thus, act synergistically, contributing to drought tolerance [
11,
12]. These organisms can produce auxins such as indole acetic acid, increasing the length of plant roots, thus leading to a greater absorption of water and nutrients from the soil [
8]. In this sense, it has been observed [
12] that the inoculation of corn with
B. amyloliquefaciens increased the nutrient absorption and promoted growth mechanisms in plants, such as an increased concentration of amino acids such as tryptophan, isoleucine, alanine, valine, and tyrosine and sugars such as fructose and glucose.
The negative effects of water restriction on maize growth and gas exchange were also reflected in a lower grain yield (38% reduction). Water scarcity negatively affects corn development at all phenological stages, promoting an increase in flowering days, maturation days, and anthesis interval and a decrease in leaf area, negatively affecting flowering and grain filling and seriously compromising corn production [
6]. At the irrigation level of 50% of ETc, the inoculation of plants with
B. amyloliquefaciens and with
A. brasiliense provided a productivity increase of 35% in relation to stressed plants without the inoculation. In previous works, it was observed that the inoculation of
B. amyloliquefaciens and
B. subtilis, associated or not with other BM, was efficient in attenuating water stress in several cultures, promoting an increase in growth and production [
9,
15,
18]. Water restriction increased the protein content (12% increase) in the grains. Other studies have also reported this effect [
16,
45]. Probably, the increase in protein concentration was due to the decrease in the thousand-grain weight (8% on average in the BM treatments) compared to the level of complete irrigation combined with a decrease in the rate of carbon assimilation and, consequently, the starch synthesis, increasing the proportion of proteins in the grains [
46]. Despite the increase in the protein content in the grains, the water deficit reduced the protein yield by 31% due to the severe decrease in grain yield. Although the water deficit caused this antagonistic effect between the protein content and the yield, this is a relevant aspect to be addressed in future research, because there is a possibility that there is a balancing point between the increase in the protein content in the grains caused by water stress and adequate corn grain yield through irrigation depth management during the phenological phases of the crop [
47,
48].
The concentration of mineral nitrogen (N), as ammonium (NH
4+) or NO
3−, increased due to the water deficit. In previous works, higher concentrations of mineral N were also observed in soil under water deficits [
5,
8,
49]. Water restriction possibly decreased the rate of N absorption and accumulation by corn, providing higher levels of mineral N in the soil and decreasing the levels of this nutrient in the leaves. The inoculation of plants with
B. amyloliquefaciens +
A. brasiliense consistently increased the NH
4+ content and respiration rate under water restriction but not under full irrigation. The role of BM in increasing mineral N in the soil needs to be better understood. Previous research demonstrated that
B. subtilis decreased N volatilization in the form of NH
3 after mineralization [
15]. In addition, an inoculation with
B. amyloliquefaciens +
A. brasiliense could provide an increase in the diversity of microorganisms involved in the mineralization of organic matter [
50] or even stimulate the decomposition of organic matter and N mineralization [
51] or stimulate root growth and increase the soil organic matter content [
52]. The increase in productivity in the T1 treatment is possibly due to the synergistic effect between
B. amyloliquefaciens and
A. brasiliense. In another work, with a corn crop, the authors observed that the solubilization of phosphorus bound to calcium and iron, and the mineralization of sodium phytate was greater when they were inoculated together in comparison to the separate inoculations, enhancing the release of organic acids. In the present work, the increase in microbial biomass carbon under the water deficit, mainly in the T2 treatment (
B. subtilis), was due to the higher microbiological activity. However, at this level of irrigation, the metabolic quotient was not altered. In this sense, Gebauer and collaborators [
17] observed that a water deficit favored certain groups of microorganisms that promoted plant growth in soil cultivated with wheat and barley, which was interpreted as an adaptive strategy of plants to water stress.
The multivariate principal component analysis (PCA) demonstrated, more emphatically, a clear separation of the average variables in the plants (except the internal CO2 and protein content) from the variables measured in the soil. Thus, according to PC1, the variables measured in the plants were closely related to the level of full irrigation (100% ETc). In comparison, most of the variables measured in the soil benefited from the irrigation deficit (50% ETc). The PCA also showed that the inoculation of corn with BM favored soil biological activity and promoted an increase in the grain yield, especially under the water deficit.