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Article

Foliar Application of γ-Polyglutamic Acid Enhances Growth, Yield, and Rhizosphere Microbiota of Summer Maize Under Varied Water Regimes

by
Yi Wang
1,2,
Zhenfeng Guo
1,
Shujie Zhang
1,
Fang Li
1 and
Yanlai Han
1,2,*
1
College of Resources and Environment, Henan Agricultural University, Zhengzhou 450046, China
2
State Key Laboratory of Wheat and Maize Crop Science, Zhengzhou 450046, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(3), 754; https://doi.org/10.3390/agronomy15030754
Submission received: 12 February 2025 / Revised: 12 March 2025 / Accepted: 18 March 2025 / Published: 20 March 2025
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Abstract

:
Amidst escalating global water scarcity and the urgent need for sustainable agricultural practices, the exploration of efficient and eco-friendly agricultural aids has become paramount. This field experiment investigated the effects of foliar γ-polyglutamic acid (γ-PGA) application on summer maize under two irrigation levels (0 m3·ha−1 and 900 m3·ha−1 at the bell-mouth stage) and three γ-PGA spraying treatments. The results demonstrated that both irrigation and γ-PGA spraying significantly influenced maize yield and various growth parameters, with notable interaction effects. Compared to the control, single and double applications of γ-PGA increased summer maize yield by 33.3% and 37.0%, respectively, while enhancing dry matter accumulation and nutrient uptake. Foliar γ-PGA application also altered the rhizosphere microbial community composition, increasing the abundances of Proteobacteria and Actinobacteria, as well as beneficial bacteria such as Arthrobacter, Sphingomonas, Streptomyces, and Altererythrobacter. Additionally, it significantly increased the population of Dung Saprotroph fungi, which are beneficial for crop growth. These findings suggest that foliar γ-PGA application can promote summer maize growth and holds significant potential for agricultural applications.

1. Introduction

Chemical fertilizers have been pivotal in enhancing agricultural productivity and crop quality [1]. However, their prolonged and improper use has led to environmental degradation, soil fertility decline, and increased non-point source pollution, severely impacting ecosystems [2]. The production of chemical fertilizers also consumes non-renewable resources, contributing to indirect environmental pollution. These challenges have spurred the search for sustainable agricultural practices to reduce reliance on chemical fertilizers [3,4].
The advent of bio-stimulants offers a promising solution to these issues [5]. These substances, even in small quantities, can stimulate plants or environmental microorganisms, enhancing nutrient availability, improving nutrient uptake and utilization efficiency, and bolstering plant tolerance to abiotic stresses, thereby increasing crop yields [5]. γ-Polyglutamic acid (γ-PGA), an anionic polymer composed of D- and L-glutamic acid units [6], possesses unique chemical properties, including strong water retention, fertilizer retention, and complexing capabilities, making it widely applicable in various fields, such as chemical engineering, food, and environmental science [7]. Numerous studies have reported that γ-PGA application can increase crop yields, improve product quality, and enhance resistance to abiotic stresses, like drought, cold, and salinity [8,9,10].
Maize, a globally crucial food crop, is vital for food security, livestock feed, and industrial raw materials [11]. However, in maize-growing regions across the globe, water scarcity poses a prevalent and acute challenge. During crucial growth phases of maize, particularly the large bell-mouth stage, inadequate water supply often arises, severely impeding the normal growth of the crop and substantially affecting global maize production. This issue leads to a yield loss ranging from 53% to 64% in maize [12]. Given that >60% global maize cultivation relies on rainfed systems [11], developing low-cost foliar bio-stimulants could directly benefit smallholder farmers in arid regions. Although previous studies have shown that γ-PGA can improve drought resistance in crops like wheat and maize by regulating physiological processes, such as abscisic acid (ABA) synthesis, and enhancing photosynthetic efficiency [13,14,15], most research has focused on soil application. This method requires high dosage (40–80 kg·ha−1) [16], resulting in increased costs. In contrast, studies on foliar application of γ-PGA as a bio-stimulant to enhance drought resistance are limited.
This study involved a field experiment with two irrigation levels and three PGA treatments to examine the impact of γ-PGA on maize growth and yield under varied water conditions, and its effect on soil microbial communities. Combining plant physiological measurements with soil microbial diversity analysis, the study clarified the compensatory effects and mechanisms of γ-PGA foliar application in water-deficient conditions. Results aim to advance the precise management of water and nutrients using bio-stimulants, offering technical and theoretical support for γ-PGA agricultural applications.

2. Materials and Methods

2.1. Experimental Design

The field experiment was conducted at the Wheat-Maize Rotation Nutrition and Fertilizer Application Experimental Station in North China from June to October 2019. The station is located within 112°42′ E–114°14′ E and 34°16′ N–34°58′ N. The experimental field, under a winter wheat/summer maize rotation system with straw return, featured loamy fluvo-aquic soil with the following properties in the 0–20 cm layer: total nitrogen 0.98 g·kg−1, available nitrogen 55.47 mg·kg−1, available phosphorus 20.05 mg·kg−1, available potassium 97.33 mg·kg−1, organic matter content 16.7 g·kg−1, and pH 7.25.
The experiment employed a 2×3 factorial design, combining two irrigation levels (W1: 0 m3·ha−1; W2: 900 m3·ha−1 at the bell-mouth stage) with three γ-PGA spraying treatments (S0: water control; S1: single γ-PGA spray at the jointing stage; S2: double γ-PGA spray at the jointing and bell-mouth stages), resulting in six treatments: W1S0, W1S1, W1S2, W2S0, W2S1, and W2S2. Each plot measured 4 m × 10 m, arranged in a split-plot design, with irrigation as the main plot and spraying as the sub-plot. Each treatment was replicated three times, totaling 18 plots. The maize variety ’Dedan 123’ was sown at a rate of 37.5 kg·ha−1, with thinning at the 4–5 leaf stage to a final density of 67,500 plants·ha−1. Fertilization was uniform across plots, with a base application of compound fertilizer (N-P2O5-K2O: 20-12-8) at 750 kg·ha−1 and a top-dressing of 90 kg·ha−1 nitrogen as urea at the bell-mouth stage. The overall fertilizer application rate was as follows: N, 192.3 kg·ha−1; P2O5, 90 kg·ha−1; K2O, 60 kg·ha−1. The γ-PGA used in the experiment was in liquid form, with γ-PGA as the main component. It was purified from the fermentation broth of Bacillus licheniformis strain by Henan Agricultural University, with a content of approximately 6 g·L−1, and the spraying amount per time was 50 g·ha−1.

2.2. Plant Sampling and Analysis

At the silking and maturity stages, five plants per plot were sampled for above-ground biomass measurement. Samples were dried and ground for total nitrogen, phosphorus, and potassium determination via H2SO4-H2O2 digestion, with nitrogen and phosphorus measured by colorimetry using a continuous-flow analyzer (SEAL AutoAnalyzer 3, SEAL Analytical Ltd., Fareham, UK) and potassium by flame photometry (FPMRC-640, Shanghai Yidian Analytical Instrument Co., Ltd., Shanghai, China).
Photosynthetic parameters of ear leaves at the silking stage were measured using a portable photosynthesis system (LI-6400; Li-Cor Inc., Lincoln, NE, USA), with five plants per replicate. Four parameters were analyzed: net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr). Leaf SPAD values were measured using a SPAD-502 m (Konica Minolta Sensing, Inc., Osaka, Japan).
At the filling stage, five plants per plot were measured for plant height, stem diameter (third internode), leaf length, width, and thickness.
At maturity, 20 representative ears per plot were harvested, dried, and threshed to determine grain number per ear and 100-grain weight. Theoretical yield (kg·ha−1) was calculated as: theoretical yield = 67,500 × grain number per ear × 100-grain weight (g) × 85%. Results of the theoretical yield were converted to kg·ha−1. Here, 67,500 represents the maize density of the experiment (plants per hectare), and 85% is the empirical adjustment coefficient for field losses, which accounts for factors such as harvesting inefficiency and barren stalks.

2.3. Soil Sampling and Microbial Analysis

During the filling stage, rhizosphere soil was collected from three maize plants per plot. Soil adhering to roots after gentle shaking was retained as rhizosphere soil, with samples from the same plot combined into one. Each treatment had three replicate rhizosphere soil samples.
Soil DNA was extracted using the FastDNA Spin Kit for Soil (MP Biomedicals, Irvine, CA, USA) following the manufacturer’s instructions. DNA quality and concentration were assessed using a NanoDrop ND-1000 (NanoDrop Technologies, Wilmington, DE, USA), with purified DNA stored at −80 °C for further analysis.

2.4. High-Throughput Sequencing and Bioinformatics

Bacterial 16S rRNA gene V4 regions were amplified using primers F515 (5′-GTGCCAGCMGCCGCGGTAA-3′) and R806 (5′-GGACTACVSGGGTATCTAAT-3′) [17], while fungal ITS1 regions were amplified using primers ITS1F (5′-CTTGGTCATTTAGAGGAAGTAA-3′) and ITS2 (5′-GCTGCGTTCTTCATCGATGC-3′) [18]. PCR products were purified and sequenced on the Illumina MiSeq platform. Sequencing data were processed using QIIME2 for quality control and denoising [19]. OTUs was clustered by UPARSE 7.0.1090 [20]. The specific process is as follows: 1, select high-quality sequences with consistent length; 2, identify the same sequences; 3, cluster based on 97% similarity to remove chimeras [21]; 4, use Vsearch statistical OTU abundance table [22]. OTU representative sequences were annotated using SpCLUST [23]. Unclassified sequences and archaeal sequences were removed. For ITS sequences, UNITE8.0 data was used for alignment annotation [24]. Bacterial function annotation was performed using PICRUSt [25]. Fungal function prediction was performed using the Funguid1.0 database [26]. SIMPER analysis was conducted using the simper() function from the vegan package in R v4.3.1. The circle visualization of the sample dominant species was generated on the Majorbio Cloud Platform (accessed on 8 November 2024, https://cloud.majorbio.com/page/tools.html).

2.5. Statistical Analysis

Statistical analysis was performed using SPSS (Version 26.0, IBM Corp.). A two-way ANOVA was performed to assess the main effects of irrigation and γ-PGA spraying, as well as their interaction, on measured parameters. Duncan’s multiple range test was used for significance testing.

3. Results

3.1. Effects on Biomass and Photosynthesis at Silking Stage

Irrigation and γ-PGA spraying significantly affected dry matter weight, Pn, and Gs. Interaction effects were significant for total dry matter accumulation, Gs, and SPAD (Figure 1).
Irrigation (W2) increased total dry matter accumulation by 40.7% compared to non-irrigation (W1). Single (S1) and double (S2) γ-PGA sprays increased dry matter accumulation by 8.3% and 5.2%, respectively, compared to the control (Figure 1). Under irrigation, Pn, Gs, Ci, and Tr increased by 75.1%, 221.0%, 18.2%, and 68.7%, respectively, compared to non-irrigation. Single and double γ-PGA sprays increased Pn by 19.6% and 50.2%, Gs by 70.2% and 131.7%, Ci by 7.2% and 14.0%, and Tr by 16.5% and 35.5%, respectively, compared to the control. Overall, double spraying is more effective in enhancing photosynthetic parameters than spraying once (Figure 1). Irrigation increased chlorophyll content by 20.0%, while single and double γ-PGA sprays increased this by 4.3% and 8.8%, respectively, with double spraying significantly higher than single spraying (Figure 1F).

3.2. Effects on Morphological Parameters at Filling Stage

Irrigation and γ-PGA spraying significantly affected plant height, stem diameter, and leaf length, with significant interaction effects on these parameters (Figure 2).
Irrigation increased plant height by 12.7%, stem diameter by 19.5%, leaf length by 10.9%, leaf width by 13.0%, and leaf thickness by 14.1%. Single and double γ-PGA sprays increased plant height by 2.0% and 5.9%, stem diameter by 5.3% and 11.6%, leaf length by 4.9% and 7.3%, leaf width by 3.4% and 8.0%, and leaf thickness by 7.0% and 11.6%, respectively, compared to the control. Under both W1 and W2, S2 treatments significantly improved plant height, stem diameter, leaf length, and thickness compared to the control (Figure 2).

3.3. Effects on Nutrient Accumulation

Both irrigation and γ-PGA spraying significantly increased the accumulation of nitrogen (N), phosphorus (P), and potassium (K) in summer maize at both the silking and maturity stages (Figure 3).
At the silking stage, W2 treatments increased the total accumulation of N, P, and K by 40.6%, 33.2%, and 52.8%, respectively, compared to W1. S1 and S2 treatments increased N accumulation by 18.3% and 27.3%, P by 12.6% and 24.0%, and K by 15.8% and 28.4%, respectively, compared to S0. These results indicate that γ-PGA enhances nutrient uptake and utilization, particularly under irrigated conditions (Figure 3A–C).
At the maturity stage, irrigation increased N, P, and K accumulation by 38.4%, 31.7%, and 49.2%, respectively, while γ-PGA spraying increased N accumulation by 16.8% and 25.6%, P by 11.9% and 22.7%, and K by 14.5% and 26.8%, respectively (Figure 3D–F). The interaction between irrigation and γ-PGA spraying was particularly significant for N accumulation, suggesting a synergistic effect on nutrient uptake. These findings highlight the potential of γ-PGA to improve nutrient efficiency and support maize growth under varying water regimes.

3.4. Effects on Yield and Yield Components

Irrigation and γ-PGA spraying significantly increased grain number per ear, 100-grain weight, and grain yield. Interaction effects were significant for 100-grain weight (Table 1). Irrigation increased grain number per ear by 47.6%, 100-grain weight by 16.9%, and yield by 72.6%. Single and double γ-PGA sprays increased grain number per ear by 14.4% and 18.5%, 100-grain weight by 14.9% (both), and yield by 33.3% and 37.0%, respectively, compared to the control. Under W1, single and double sprays increased yield by 30.7% and 42.7%, respectively, while under W2, they increased yield by 33.8% and 34.9%, respectively (Table 1).

3.5. Effects on Rhizosphere Microbiota

Analysis of rhizosphere microbial communities revealed that foliar γ-PGA application significantly altered the composition of both bacterial and fungal communities in the maize rhizosphere. At the phylum level, the dominant bacterial groups included Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, Bacteroidetes, and Gemmatimonadetes, while the dominant fungal phyla were Ascomycota, Mortierellomycota, and Basidiomycota (Figure 4). Compared to the control (W2S0), the W2S1 treatment (single γ-PGA spray) reduced the relative abundance of Acidobacteria but increased Actinobacteria and Proteobacteria. This trend was more pronounced in the W2S2 treatment (double γ-PGA spray), suggesting that γ-PGA application selectively enriches bacterial taxa beneficial for plant growth and soil health (Figure 4). At the genus level, γ-PGA application increased the relative abundances of beneficial bacteria, such as Arthrobacter (nitrogen fixation), Sphingomonas (soil remediation), Streptomyces (disease suppression), and Altererythrobacter (Soil purification) (Table 2).
Fungal community analysis revealed that γ-PGA application significantly increased the relative abundance of Mortierellomycota, particularly in the W2S2 treatment, while reducing Ascomycota. The enrichment of Mortierellomycota, which includes saprotrophic fungi, may enhance organic matter decomposition and nutrient release. Additionally, γ-PGA application increased the abundance of Dung Saprotroph fungi, which degrade recalcitrant organic matter, contributing to soil fertility and nutrient availability (Figure 4 and Figure 5). FUNGuild-based functional analysis indicated that γ-PGA application promoted saprotrophic fungi, such as Dung Saprotroph–Plant Saprotroph and Endophyte–Litter Saprotroph–Soil Saprotroph, further supporting its role in enhancing organic matter decomposition and nutrient cycling (Figure 5).
Notably, γ-PGA application increased the abundances of Chaetomium grande (biocontrol) and Mortierella alpina (growth promotion), while potentially pathogenic Fusarium was reduced in the double-spray treatment (Table 3). Compared to the control, the W2S1 treatment reduced the abundance of Mortierella elongata, while the W2S2 treatment increased it. Additionally, γ-PGA application led to a decline in the abundance of Pleosporales. These findings highlight the potential of γ-PGA to modulate fungal communities in ways that enhance soil fertility and nutrient availability.

4. Discussion

4.1. γ-PGA Enhances Maize Growth and Yield

The results of this study demonstrate that foliar application of γ-PGA significantly enhances the growth and yield of summer maize under both irrigated and non-irrigated conditions. While this observed 33.3–37.0% yield enhancement aligns with previous γ-PGA studies, the magnitude of the effect shows notable variation across different crop species and environmental conditions. For instance, soil application of γ-PGA at 40–80 kg·ha−1 in maize increased yields by 4.9–21.8% [16], whereas our foliar application at 50 g·ha−1 achieved higher gains, likely due to the direct leaf-targeted delivery. Similarly, pot studies with winter wheat reported yield increases of 29.3–34.7% [27], constrasting with field trials showing 7.17% [28] or 7.62–14.67% [29] improvements. Guo et al. [30] observed 9.00–26.31% yield increases in winter wheat using γ-PGA super absorbent polymer (γ-PGA SAP) under different irrigation during soil application, further highlighting the method-dependent efficacy.
The positive effects of γ-PGA on maize growth and yield can be attributed to its role in regulating key metabolic processes, including brassinosteroid biosynthesis, alpha-linolenic acid metabolism, phenylpropanoid biosynthesis, and nitrogen metabolism [31]. RNA-seq data suggest that γ-PGA enhances photosynthesis by upregulating the expression of photosynthesis-related genes [10]. Research on tobacco shows that it can regulate carbon-nitrogen metabolism in leaves [32], and in Chinese cabbage it enhances nitrogen metabolism related enzymes and nutrient contents in leaves [33]. γ-PGA pretreatment activates Ca2+ signaling in plants, leading to H2O2 production and increased biosynthesis of stress hormones, like brassinolide and jasmonic acid [34]. This promotes proline accumulation and enhances antioxidant capacity, improving stress tolerance and growth.
Research indicates that the application of γ-PGA significantly influences signal transduction in plants, thereby exerting a cascading regulatory effect on plant metabolic pathways. This regulation enhances plant growth and development, nutrient absorption, and pathways related to stress resistance [9,10,35,36]. Under drought conditions, γ-PGA has been shown to upregulate genes related to ABA biosynthesis and signal transduction, increasing ABA content in maize and enhancing drought resistance [37]. This is particularly relevant given that drought stress can cause a 53–64% reduction in maize yield, making it a critical factor in global maize production [22,36]. By improving photosynthetic efficiency, root development [36], and nutrient uptake, γ-PGA helps mitigate the negative effects of water scarcity on maize growth and yield.

4.2. γ-PGA Modulates Rhizosphere Microbiota

The application of γ-PGA not only enhances plant growth but also significantly influences the composition and function of rhizosphere microbial communities. This study found that γ-PGA application increased the abundances of beneficial bacterial taxa, such as Arthrobacter, Sphingomonas, Streptomyces, and Altererythrobacter (Table 2). These bacteria play crucial roles in soil nutrient cycling, plant growth promotion, and stress tolerance. For example, Arthrobacter is known for its ability to accumulate nitrite and promote plant growth, while Sphingomonas has strong bioremediation capabilities and can improve soil structure and water retention [38,39]. Streptomyces, a key group of soil actinomycetes, has been shown to promote root growth and enhance plant stress tolerance [40]. Altererythrobacter plays a role in soil purification and nutrient cycling, further supporting plant growth [41].
In addition to bacterial shifts, γ-PGA application also altered fungal communities in the maize rhizosphere. The increase in Mortierellomycota and Dung Saprotroph fungi suggests that γ-PGA promotes the decomposition of organic matter and nutrient release in the soil, thereby enhancing soil fertility and plant nutrient uptake [42]. Functional analysis using FUNGuild indicated that γ-PGA application increased the abundance of fungi with saprotrophic functions, such as Dung Saprotroph-Plant Saprotroph and Endophyte-litter Saprotroph-Soil Saprotroph (Figure 5). These fungi play a crucial role in organic matter decomposition and nutrient cycling, further supporting plant growth and stress tolerance. The coordinated shifts in bacterial and fungal communities under γ-PGA treatment suggest a synergistic enhancement of nutrient cycling and stress resilience in the rhizosphere.
Foliar application of γ-PGA likely influences rhizosphere microbial communities indirectly through altered root exudation profiles. Previous studies showed that γ-PGA enhances root biomass and nutrient uptake [36], which increased the quantity and quality of maize root exudates (e.g., lipids, organic compounds, organic acids and derivatives) under drought stress [13]. These exudates serve as microbial substrates, shaping community composition by favoring taxa with specific metabolic capabilities [43]. For example, γ-PGA-treated maize in our study exhibited enrichment of nitrogen-fixing Arthrobacter [44] and phosphate-solubilizing Sphingomonas [45] (Table 2), which may be attributed to increased root exudation of organic acids that solubilize nutrients. Additionally, γ-PGA-induced drought tolerance [46] could reduce stress-related exudates (e.g., phenolic compounds) [47], creating a more favorable rhizosphere environment for beneficial microbes [13]. This aligns with our SIMPER analysis showing Chaetomium (a drought-tolerant fungus) [48] as a key discriminator in γ-PGA-treated soils (Table 3). In addition to root exudation-mediated effects, a portion of the foliar-applied γ-PGA may directly reach the soil through spray droplets [49], altering soil physicochemical properties and microbial communities [9,24]. These microbial changes contribute to improved soil fertility, nutrient cycling, and plant stress tolerance, ultimately enhancing maize growth and yield.

5. Conclusions

The present study reveals a significant interactive effect between irrigation conditions and foliar application of γ-PGA on maize growth. The application of γ-PGA significantly increased maize yields by 33.3% to 37.0%, with the W2S2 processing the highest grain yield. This indicated that in corn production areas with limited water supply or insufficient irrigation infrastructure, foliar spraying of γ-PGA can improve crop water use efficiency and effectively alleviate the inhibitory effect of drought stress on grain formation. These results highlighted the potential of γ-PGA as a bio-stimulant in agricultural applications, and also demonstrate its positive role in crop response to environmental stresses.

Author Contributions

Y.H. designed the research; Y.W., Z.G. and S.Z. conducted the experiment and data analysis; Y.W. wrote the manuscript; F.L. and Y.H. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 32372811), the National Key R&D Program of China (Project No. 2018YFD0300706), and the Key Scientific and Technological Program in Henan Province (Project No. 232102111028).

Data Availability Statement

Data are contained within this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effects of different treatments on biomass (A), leaf photosynthetic indices (B–E) and leaf SPAD value (F) of summer maize at the silking stage. Data are shown as mean ± standard error of three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05). W: Irrigation treatment; W1: 0 m3·ha−1 irrigation at the large bell-mouth stage; W2: 900 m3·ha−1 irrigation at the large bell-mouth stage; S: γ-polyglutamic acid (γ-PGA) spraying treatment; S0: Water spraying (once at the jointing stage and once at the large bell-mouth stage); S1: 50 g·ha−1 γ-PGA spraying at the jointing stage and water at the large bell-mouth stage; S2: 50 g·ha−1 γ-PGA spraying at both the jointing and large bell-mouth stages. *, p < 0.05; **, p < 0.01; ns, no significant difference.
Figure 1. Effects of different treatments on biomass (A), leaf photosynthetic indices (B–E) and leaf SPAD value (F) of summer maize at the silking stage. Data are shown as mean ± standard error of three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05). W: Irrigation treatment; W1: 0 m3·ha−1 irrigation at the large bell-mouth stage; W2: 900 m3·ha−1 irrigation at the large bell-mouth stage; S: γ-polyglutamic acid (γ-PGA) spraying treatment; S0: Water spraying (once at the jointing stage and once at the large bell-mouth stage); S1: 50 g·ha−1 γ-PGA spraying at the jointing stage and water at the large bell-mouth stage; S2: 50 g·ha−1 γ-PGA spraying at both the jointing and large bell-mouth stages. *, p < 0.05; **, p < 0.01; ns, no significant difference.
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Figure 2. Effects of different treatments on plant height (A), stem diameter (B), leaf length (C), leaf width (D) and leaf thickness (E) of summer maize plants at the filling stage. Data are shown as mean ± standard error for three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05). * p < 0.05; ** p < 0.01; ns, no significant difference.
Figure 2. Effects of different treatments on plant height (A), stem diameter (B), leaf length (C), leaf width (D) and leaf thickness (E) of summer maize plants at the filling stage. Data are shown as mean ± standard error for three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05). * p < 0.05; ** p < 0.01; ns, no significant difference.
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Figure 3. Effect of different treatments on the total accumulation of nitrogen (A,D), phosphorus (B,E), and potassium (C,F) in summer maize at silking stage (A–C) and maturity stage (D–F). Data are shown as mean ± standard error for three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05); ** p < 0.01; ns, no significant difference.
Figure 3. Effect of different treatments on the total accumulation of nitrogen (A,D), phosphorus (B,E), and potassium (C,F) in summer maize at silking stage (A–C) and maturity stage (D–F). Data are shown as mean ± standard error for three replicates. Different lowercase letters indicate significant differences among treatments (Duncan’s test, p < 0.05); ** p < 0.01; ns, no significant difference.
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Figure 4. The influence of γ-PGA on the relative abundances of bacterial (A,C) and fungal (B,D) communities at the phylum level in the rhizosphere soil of maize.
Figure 4. The influence of γ-PGA on the relative abundances of bacterial (A,C) and fungal (B,D) communities at the phylum level in the rhizosphere soil of maize.
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Figure 5. Prediction of fungal community function.
Figure 5. Prediction of fungal community function.
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Table 1. Effects of different treatments on yield and composition of summer maize.
Table 1. Effects of different treatments on yield and composition of summer maize.
TreatmentGrain Number per Ear100-Grain Weight (g)Yield (kg·ha−1)
W1W2AverageW1W2AverageW1W2Average
S0403 ± 61678 ± 42541 ± 157 b22.65 ± 0.8723.67 ± 0.5223.16 ± 0.85 b5471 ± 10089564 ± 6897518 ± 2371 b
S1515 ± 19724 ± 29619 ± 117 a23.34 ± 2.0829.89 ± 0.9726.61 ± 3.87 a7149 ± 36812,794 ± 87310,025 ± 3165 a
S2536 ± 59747 ± 68641 ± 128 a24.46 ± 1.8228.78 ± 0.9026.62 ± 2.69 a7809 ± 84512,901 ± 30810,301 ± 2837 a
Average485 ± 75 b716 ± 52 a600 ± 13523.48 ± 1.65 b27.45 ± 2.96 a25.47 ± 3.096809 ± 1247 b11,753 ± 1540 a9281 ± 2937
W0.002 **0.023 *0.003 **
S0.041 *0.000 **0.001 **
W×S0.573 ns0.001 **0.255 ns
Data are shown as mean ± standard error of three replicates. Lowercase letters indicate significant differences among different treatments (Duncan’s test, p < 0.05). *, p < 0.05; **, p < 0.01; ns, no significant difference.
Table 2. Top 10 bacterial community variation based on SIMPER analysis.
Table 2. Top 10 bacterial community variation based on SIMPER analysis.
OTUClassificationContribution RateUp/Down Function
W2S1 vs. W2S0OTU207p_Acidobacteria; c_Subgroup 61.832%downUnknown
OTU1153g_Arthrobacter1.71%upSoil purification, nitrogen fixation
OTU3937p_Acidobacteria; c_Subgroup 61.267%downUnknown
OTU615f_Pyrinomonadaceae; g_RB411.034%downUnknown
OTU528p_Acidobacteria; c_Subgroup 60.818%downUnknown
OTU2789g_Streptomyces0.755%upSuppress soil-borne diseases
OTU2610g_Sphingomonas0.659%upSoil remediation, rhizosphere promotion
OTU174g_Altererythrobacter0.622%upSoil purification
OTU475f_Blastocatellaceae0.604%downCarbon cycle
OTU3485p_Chloroflexi; c_KD4-960.596%upCarbon sequestration
W2S2 vs. W2S0OTU207p_Acidobacteria; c_Subgroup 62.003%downUnknown
OTU1153g__Arthrobacter1.880%downSoil purification
OTU3937p_Acidobacteria; c_Subgroup 61.602%downUnknown
OTU528p_Acidobacteria; c_Subgroup 61.032%downUnknown
OTU615f_Pyrinomonadaceae; g_RB410.969%downUnknown
OTU475f_Blastocatellaceae0.822%upCarbon cycle
OTU2789g_Streptomyces0.766%upSuppress soil-borne diseases
OTU2610g_Sphingomonas0.520%upSoil remediation, rhizosphere promotion
OTU2650f_Blastocatellaceae0.481%downCarbon cycl
OTU1886g_Blastococcus0.474%upnitrogen fixation
Note: p, phylum; c, class; f, family; g, genus.
Table 3. Top 10 fungal community variation based on SIMPER analysis.
Table 3. Top 10 fungal community variation based on SIMPER analysis.
OTUClassificationContribution RateUp/Down Function
W2S1 vs. W2S0OTU379s_Chaetomium grande10.31%upSuppress soil-borne diseases
OTU22s_Mortierella alpina5.71%upPromote growth
OTU77o_Pleosporales3.55%downUnknown
OTU819s_Mortierella alpina3.44%downPromote growth
OTU72s_Mortierella elongata3.03%downPromote growth
OTU365f_Chaetomiaceae3.01%upBiological control
OTU377g_Fusarium2.49%upPathogenic bacteria
OTU739p_unclassified_k_Fungi2.17%downUnknown
OTU350c_Sordariomycetes1.97%downSoil purification
OTU317s_Preussia terricola1.89%downSuppress soil-borne diseases
W2S2 vs. W2S0OTU379s_Chaetomium grande11.37%upSuppress soil-borne diseases
OTU22s_Mortierella alpina7.08%upPromote growth
OTU819s_Mortierella alpina4.01%upPromote growth
OTU72s_Mortierella elongata3.55%upPromote growth
OTU77o_Pleosporales3.48%downUnknown
OTU331f_Chaetomiaceae2.60%upDegradation of organic matter
OTU739p_unclassified_k__Fungi2.20%downUnknown
OTU317s_Preussia terricola1.88%downUnknown
OTU350c_Sordariomycetes1.86%downDung fungi
OTU377g_Fusarium1.85%downPathogenic bacteria
Note: p, phylum; c, class; o, order; f, family; g, genus; s, species.
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Wang, Y.; Guo, Z.; Zhang, S.; Li, F.; Han, Y. Foliar Application of γ-Polyglutamic Acid Enhances Growth, Yield, and Rhizosphere Microbiota of Summer Maize Under Varied Water Regimes. Agronomy 2025, 15, 754. https://doi.org/10.3390/agronomy15030754

AMA Style

Wang Y, Guo Z, Zhang S, Li F, Han Y. Foliar Application of γ-Polyglutamic Acid Enhances Growth, Yield, and Rhizosphere Microbiota of Summer Maize Under Varied Water Regimes. Agronomy. 2025; 15(3):754. https://doi.org/10.3390/agronomy15030754

Chicago/Turabian Style

Wang, Yi, Zhenfeng Guo, Shujie Zhang, Fang Li, and Yanlai Han. 2025. "Foliar Application of γ-Polyglutamic Acid Enhances Growth, Yield, and Rhizosphere Microbiota of Summer Maize Under Varied Water Regimes" Agronomy 15, no. 3: 754. https://doi.org/10.3390/agronomy15030754

APA Style

Wang, Y., Guo, Z., Zhang, S., Li, F., & Han, Y. (2025). Foliar Application of γ-Polyglutamic Acid Enhances Growth, Yield, and Rhizosphere Microbiota of Summer Maize Under Varied Water Regimes. Agronomy, 15(3), 754. https://doi.org/10.3390/agronomy15030754

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