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Article

Effect of Bacillus amyloliquefaciens QST713 on Inter-Root Substrate Environment of Cucumber under Low-Calcium Stress

by
Bin Li
1,*,†,
Li Zhang
1,2,†,
Lincao Wei
1,
Yujie Yang
1,
Zhexuan Wang
1,
Bo Qiao
1 and
Lingjuan Han
3
1
College of Horticulture, Shanxi Agricultural University, Jinzhong 030801, China
2
College of Horticulture and Plant Protection, Inner Mongolia Agricultural University, Hohhot 010010, China
3
College of Grassland Science, Shanxi Agricultural University, Jinzhong 030801, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2024, 14(3), 542; https://doi.org/10.3390/agronomy14030542
Submission received: 31 January 2024 / Revised: 1 March 2024 / Accepted: 5 March 2024 / Published: 7 March 2024
(This article belongs to the Special Issue Plant Nutrition Enhancing Through Microbial Processes in Rhizosphere)
Browse Figures
Versions Notes

Abstract

:
(1) Background: Low-calcium stress can have adverse effects on the rhizosphere environment of cucumber, subsequently impacting cucumber growth. However, plant-growth-promoting rhizobacteria can directly or indirectly enhance plant growth and induce plant tolerance, thereby mitigating the detrimental effects of low-calcium stress on cucumber growth. This study aims to elucidate the role of Bacillus amyloliquefaciens QST713 in the rhizosphere environment of cucumber under low-calcium stress, providing a theoretical basis for the application and promotion of Bacillus amyloliquefaciens. (2) Methods: This study used the ‘JinYou NO.4′ cucumber variety as test material, setting four treatments of CK, CK+Q, LCa, LCa+Q. We conducted measurements of plant height and stem diameter for four groups of cucumber plants: before treatment (0 d), and at 10 d, 20 d, 30 d, and 60 d after treatment. Additionally, we determined the biomass of cucumber plants under different treatments during the peak fruiting period. Inter-root matrix samples of cucumber were collected during the fruiting late period, and the physical and chemical properties and nutrient contents of the inter-root matrix of cucumber were determined, and bacterial microbial diversity and bacterial microbial communities were analysed using Illumina-MiSeq high-throughput sequencing technology. (3) Results: Low-calcium stress significantly inhibits the growth of cucumber plants. However, the application of Bacillus amyloliquefaciens QST713 effectively mitigates the inhibitory effects of low-calcium stress on cucumber growth. The application of Bacillus amyloliquefaciens QST713 was able to improve the physicochemical environment of the matrix and enhanced the absorption and utilisation of matrix nutrients in cucumber. The high-throughput sequencing analysis showed that the richness and diversity of bacterial communities and the number of bacteria decreased significantly under low-calcium stress, and increased significantly after the application of Bacillus amyloliquefaciens QST713. The composition of the dominant bacterial groups of the inter-root matrix of cucumber was basically the same among the four treatments, and the main difference was in the abundance of bacteria. The application of Bacillus amyloliquefaciens QST713 increased the relative abundance of bacteria that decreased under low-calcium stress, and decreased the relative abundance of bacteria that increased under low-calcium stress. (4) Conclusions: The results of this study elucidated the positive effects of Bacillus amyloliquefaciens QST713 on the growth and inter-root environment of cucumber under low-calcium stress, and provided a theoretical basis for in-depth research on the resistance of Bacillus amyloliquefaciens and its popularised application.

1. Introduction

Cucumber (Cucumis sativus L.) belongs to the cucurbitaceae family of annual trailing or climbing herbaceous plants, which is widely appreciated by modern people for its unique flavour. In recent years, the cucumber industry has been experiencing rapid growth, with its planting area and production consistently ranking first in the world [1]. However, in facility cucumber cultivation, an excessive application of nitrogen and potassium fertilizers can lead to an increase in NH4+ and K+ content, resulting in soil acidification and a weakened ability of cucumber plants to absorb calcium. This ultimately leads to physiological calcium deficiency [2]. The main reason for this deficiency is that calcium transport in the plant is driven by transpiration, with xylem serving as the transport channel. However, transpiration is lower in young and tender parts as well as in the fruits, making it difficult for calcium to be redistributed to these areas [3]. Consequently, calcium deficiency is more likely to occur [3]. Currently, the prevention and control of calcium deficiency mainly rely on single calcium supplementation techniques. Therefore, it is of great significance to study the physiological diseases related to calcium deficiency in cucumbers and propose comprehensive control techniques, considering that cucumbers are a key vegetable in modern agriculture.
In recent times, there has been a growing interest in finding substitutes for chemical fertilisers in order to address their adverse impact [4,5]. Microbial fertilisers have emerged as a promising alternative for organic and sustainable agriculture, garnering significant attention in fertiliser research [6,7,8]. Numerous studies have demonstrated that the use of microbial fertilisers can enhance soil organic carbon content, promote soil enzyme activity, increase fertiliser utilisation, stimulate crop growth, and impact the structural and functional diversity of soil microbial communities [9,10,11,12]. Soil microorganisms are a diverse and species-rich group that play a crucial role in transforming substances in the soil, directly impacting the structure and function of soil ecosystems [13]. Research has identified that soil physicochemical properties, climate, and other factors drive soil microbial population size and functional diversity [14,15,16,17]. Meanwhile, human activities, such as fertilisation, can directly impact the structure of soil microbial communities by perturbing the physicochemical properties of the soil [18,19,20,21].
The plant rhizosphere denotes the specific microenvironment encompassing the root system, comprised of the soil microzone in close proximity to the root system and the microorganisms and protists inhabiting this zone [22]. Plant-growth-promoting rhizobacteria (PGPR) represent a cluster of microorganisms present within the root system or inter-root domain, capable of stimulating plant growth by either direct or indirect means. Inter-root microorganisms can enhance energy conversion, material cycling, and plant growth through direct means, including the decomposition of organic matter, sequestration of nutrients, and secretion of plant growth regulators [23,24]. Additionally, they can indirectly promote biocontrol by competing for ecological niches between roots. The production of secondary metabolites, modification of the microbial community’s composition, and induction of systemic resistance in plants [25,26] can regulate plant morphology, growth, and development. Furthermore, this process enhances the quality of the inter-root microenvironment, which ultimately promotes plant growth and development. Improving the quality of the inter-root microenvironment and regulating the growth of harmful microorganisms are essential in achieving the ultimate aim of augmenting agricultural production for enhanced quality. A well-balanced inter-root nutrient supply can be enhanced with reasonable inter-root microbial population composition, rich microbial diversity, and high microbial vigour, which will ensure the stability and sustainability of inter-root ecology, as well as improve crop yield and quality [27].
Bacillus amyloliquefaciens is a common plant inter-root growth-promoting bacterium. The physiological effects of this strain on plants are mainly growth promotion and the induction of disease resistance. Numerous studies have shown that plant inter-root-promoting bacteria can secrete substances such as ferrophiles, IAA, and lysophosphatase to promote plant growth. There are more studies on the antimicrobial ability, bacteriolytic ability, ability to induce disease resistance, and promotion of plant growth of Bacillus amyloliquefaciens, but there are relatively few studies on the resistance of Bacillus amyloliquefaciens to adversity, and there have been some studies that found that Bacillus amyloliquefaciens has the ability to resist drought, salt tolerance, and resistance to high temperature, and in the previous study, it was found that Bacillus amyloliquefaciens QST713 was able to alleviate the low-calcium stress in cucumber. On the basis of this study, we used ‘Jinyou NO.4’ cucumber as the test material, and further investigated the effects of Bacillus amyloliquefaciens QST713 on the cucumber growth, rhizosphere physicochemical environment, substrate enzyme activity, and bacterial diversity under low-calcium stress. The aim was to elucidate the effect of Bacillus amyloliquefaciens QST713 on the growth and inter-root environment of cucumber under low-calcium stress, which is of great significance to improve the development and application of Bacillus amyloliquefaciens, and provide a theoretical basis for the popularisation of the application of Bacillus amyloliquefaciens.

2. Materials and Methods

2.1. Experimental Materials and Experimental Design

The experiment was conducted in Greenhouse No.2 at the Horticultural Experiment Station of Shanxi Agricultural University, spanning from April 2021 to July 2021. The cucumber variety employed in the study was ‘Jin You NO.4’, purchased from Tianjin Kerun Agricultural Science and Technology Co., Ltd., located in Tianjin, China;The Bacillus amyloliquefaciens QST713 isolated from the commercial formulation of Zhuorun®, originated from North America, purchased from Bayer CropScience (China) Co., Ltd., located in Hangzhou, Zhejiang. Uniformly sized cucumber seeds with a full shape were selected and soaked for 6 h. Sterile water-moistened gauze was used to rinse the soaked cucumber seeds several times. Subsequently, the seeds were carefully placed on the gauze for germination in a constant temperature incubator set at 28 °C. Once the seeds germinated and displayed whitening, they were sown in 50-hole trays filled with sterilized coco-coir. When the seedlings reached the two-leaf one-heart stage and exhibited similar growth potential, they were transplanted into black cultivation bags measuring 30 cm × 30 cm × 30 cm, filled with an equal amount of sterilized coco-coir. After a resting period of 20 days, the experimental treatments were initiated.
A Hoagland nutrient solution was used to simulate low-calcium stress by reducing the concentration of Ca(NO3)2 in the nutrient solution, and the corresponding molar mass of NaNO3 was used to make up for the nitrogen deficiency due to the reduction in Ca(NO3)2. A total of four experimental treatments were set up: CK (normal calcium concentration: 4 mmol/L of Ca(NO3)2); CK+Q (4 mmol/L of Ca(NO3)2 + Bacillus amyloliquefaciens QST713); LCa (low calcium concentration: 0.4 mmol/L of Ca(NO3)2); LCa+Q (0.4 mmol/L of Ca(NO3)2 + Bacillus amyloliquefaciens QST713). The concentration of the bacterial solution was 3.0 × 108 CFU/mL, and the bacterial solution was applied three times throughout the whole growing period, and the treatments of bacterial application were applied at the time of planting, at the period of the flowering and fruiting (40 d after planting), and at the fruiting peak period (60 d after planting). Each treatment involved the application of 0.5 mL of the bacterial solution directly to the roots, diluted 100 times with sterile water. The control group received an equivalent volume of sterile water. Each treatment was replicated in 20 pots. Starting from the initiation of the treatment, every two days, each pot was irrigated with 2 L of a corresponding nutrient solution. Sampling was conducted during the fruiting late stage (85 days after planting).

2.2. Determination of Experimental Indexes

Five cucumber plants exhibiting consistent growth were chosen and labelled before the initiation of the treatment. The plant height and stem diameter of these selected cucumber plants were measured before treatment (0 d), and at 10 d, 20 d, 30 d, and 60 d after treatment. During the peak fruiting period, five cucumber plants from each treatment group were selected. These plants were divided into their above-ground and below-ground parts. After thorough washing with clean water, they were further rinsed with distilled water. The fresh weight of the plant parts was measured and recorded. Subsequently, the plant parts were placed inside clean envelope bags and subjected to blanching in an oven at 105 °C for a duration of 30 min. Following this, the plant parts were dried at 75 °C until a constant weight was achieved. Finally, the dry weight of both the above-ground and below-ground parts was measured and recorded.
At the fruiting late stage, three cucumber plants from different treatments were carefully selected. The roots of the plants were gently shaken to remove any loosely adhered coir material [28]. Subsequently, the coir that tightly adhered to the roots was collected as the inter-root matrix. Any gravel and crop roots were removed using sterilized forceps. The inter-root matrix from the cucumber plants was then placed in sterile sealing bags and securely sealed. The collected inter-root matrix was immediately transferred to a portable icebox for transportation back to the laboratory. Upon arrival at the laboratory, the inter-root matrix was sieved through a 2 mm sieve to remove any impurities. The matrix was divided into two portions for a further analysis. One portion was placed in a shaded area to undergo air-drying, ensuring the removal of excess moisture and impurities. This dried portion was later utilized for determining the physicochemical properties and enzymatic activity indicators of the matrix. The other portion was stored at −80 °C for the purpose of a microbial diversity analysis [29].
Physicochemical properties of the matrix: The pH value of the matrix was determined using a portable pH meter(pH-100A, Shanghai Lichen Instrument Technology Co., Ltd, is located in Shanghai, China), while the electrical conductivity (EC) value was measured using a conductivity meter(CT-2, Shanghai Lichen Instrument Technology Co., Ltd, is located in Shanghai, China). The bulk density of the matrix was determined using the ring knife method, and the specific gravity of the matrix was obtained using the pycnometer method. The porosity of the matrix was calculated using the following formula:
Total porosity (%) = (1 − bulk density/specific gravity) × 100.
The total nitrogen content of the matrix was determined using Automatic Kjeldahl Apparatus(KN520, Jinan Alva Instrument Co., Ltd. is located in Jinan City, Shandong Province, China), while the total phosphorus content of the matrix was measured using the phosphomolybdate colourimetric method [30]. The total potassium content of the matrix was determined using flame atomic absorption spectroscopy [31]. The organic matter content of the matrix was determined using the potassium dichromate volumetric method [30]. Each treatment was replicated three times to ensure statistical validity.
The enzyme activities of the inter-root matrix were determined by the method of Bo Zhu [32], urease activity was determined by the phenol–sodium hypochlorite method, sucrase activity was determined by the 3,5-dinitrosalicylic acid method, phosphatase activity was determined by the disodium benzene phosphate method, matrix catalase activity was determined by the KMnO4 titration method, dehydrogenase activity was determined by a TTC colourimetric assay, polyphenol oxidase and peroxidase activity were determined by an o-phenyltrienol colourimetric assay, and polyphenol oxidase and peroxidase were determined by an indanedione colourimetric assay. Matrix protease was determined by ninhydrin colourimetry. To ensure statistical significance, each treatment was replicated three times, allowing for a robust data analysis and reliable interpretation of the enzyme activity results.
The inter-root matrix samples, which were stored at −80 °C, were transported to Meiji Bioscience for Illumina MiSeq sequencing, is located in Shanghai, China.

2.3. Data Analysis

In the data analysis, Microsoft Excel 2010 and Graphpad Prism 8.0.2 were utilized for data processing and graphing, while SPSS 25.0 was employed for a statistical analysis. The significance of differences was examined using the LSD test with a significance level of p < 0.05. Moreover, the analysis of bacterial diversity data was performed on the Meiji BioCloud platform, which is provided by Shanghai Meiji BioPharmaceutical Co, is located in Shanghai, China.

3. Results

3.1. Effect of Bacillus amyloliquefaciens QST713 on Cucumber Growth under Low-Calcium Stress

According to Figure 1, prior to the treatment, there were no significant differences observed in cucumber plant height and stem diameter among the different treatments. However, starting from 10 days after the treatment, low-calcium stress exerted a significant inhibitory effect on cucumber plant height and stem diameter. As the treatment duration progressed, the inhibitory effect became more pronounced. At the end of the 60-day treatment period, the cucumber plants subjected to low-calcium stress (LCa) exhibited a significant reduction in plant height and stem diameter of 14.08% and 6.41%, respectively, compared to the control group (CK). The application of Bacillus amyloliquefaciens QST713 significantly improved cucumber plant height and stem diameter. With the treatment of time, the influence of Bacillus amyloliquefaciens QST713 on plant height and stem diameter became stronger. At the 60-day mark, the treatment with Bacillus amyloliquefaciens QST713 resulted in a significant increase in cucumber plant height and stem diameter of 4.92% and 6.67%, respectively, compared to the control group (CK) under normal cultivation conditions. Under low-calcium stress, the application of Bacillus amyloliquefaciens QST713 also enhanced cucumber plant height and stem diameter, mitigating the stress-induced growth inhibition. Specifically, the LCa+Q treatment exhibited a significant increase in plant height and stem diameter of 8.04% and 3.49%, respectively, compared to the LCa treatment alone.
According to Table 1, during the fruiting stage, cucumber plant biomass significantly decreased under low-calcium stress. Compared to the control group (CK) under normal cultivation conditions, the cucumber plants in the LCa group exhibited a significant reduction in shoot fresh weight and dry weight by 17.59% and 17.18%, respectively. The root fresh weight and dry weight also significantly decreased by 16.55% and 16.34%, respectively. The total fresh weight and dry weight showed significant reductions of 17.47% and 17.10%, respectively. Bacillus amyloliquefaciens QST713, as a rhizosphere-promoting bacterium, can promote the growth of cucumber plants. Under normal cultivation conditions, the application of Bacillus amyloliquefaciens QST713 significantly increased the total fresh weight and dry weight of cucumber plants by 9.57% and 12.98%, respectively. Furthermore, under low-calcium stress, the application of Bacillus amyloliquefaciens QST713 also enhanced the biomass of cucumber plants, alleviating the effects of stress. Specifically, compared to the LCa group, the LCa+Q treatment group showed significant increases in shoot fresh weight and dry weight by 10.29% and 15.53%, respectively. The root fresh weight and dry weight significantly increased by 17.34% and 12.61%, respectively. The total fresh weight and dry weight exhibited significant increases of 11.06% and 15.24%, respectively.

3.2. Effect of Bacillus amyloliquefaciens QST713 on the Physicochemical Properties of Cucumber Inter-Root Matrix under Low-Calcium Stress

As shown in Table 2, during the fruiting late stage, the physicochemical properties of the cucumber rhizosphere matrix were significantly affected under low-calcium stress. The pH, EC value, and bulk density of the rhizosphere matrix exhibited significant increases compared to the control group (CK), with increases of 4.73%, 16.52%, and 6.67%, respectively. On the other hand, the porosity decreased significantly by 7.61% compared to the CK.
The application of Bacillus amyloliquefaciens QST713 significantly reduced the pH, EC value, and bulk density of the cucumber rhizosphere matrix, while significantly increasing the porosity. In the CK+Q treatment, compared to the CK, the pH, EC value, and bulk density decreased significantly by 4.57%, 5.00%, and 20.00%, respectively, while the porosity increased significantly by 5.10%. In the LCa+Q treatment, compared to the LCa treatment alone, the pH, EC value, and bulk density decreased significantly by 3.31%, 9.35%, and 18.75%, respectively, while the porosity increased significantly by 5.87%. These results indicate that the application of Bacillus amyloliquefaciens QST713 can improve the physicochemical properties and ventilation environment of the rhizosphere matrix under low-calcium stress, thereby alleviating the negative effects of low-calcium stress on cucumber growth and development.

3.3. Effect of Bacillus amyloliquefaciens QST713 on Nutrient Content in Cucumber Inter-Root Matrix under Low-Calcium Stress

As shown in Table 3, during the fruiting late stage, low-calcium stress significantly impacts the nutrient content in the cucumber rhizosphere matrix, indicating a lower nutrient utilization efficiency. Compared to the control group (CK), the levels of organic matter, nitrogen, phosphorus, and potassium in the cucumber rhizosphere matrix exhibit significant increases, with respective increments of 16.38%, 12.55%, 19.67%, and 22.67%.
The application of Bacillus amyloliquefaciens QST713 facilitates the decomposition of organic matter in the rhizosphere matrix, thereby promoting nutrient absorption and utilization. In comparison to the CK group, the CK+Q treatment results in significant reductions in organic matter, nitrogen, phosphorus, and potassium content, with respective reductions of 15.35%, 22.05%, 15.41%, and 14.30%. When compared to the low-calcium stress treatment alone (LCa), the LCa+Q treatment exhibits increased organic matter consumption and enhanced utilization rates of nitrogen, phosphorus, and potassium, leading to significant decreases of 12.98%, 17.23%, 13.50%, and 12.55%, respectively. These findings demonstrate that low-calcium stress significantly affects the absorption and utilization of organic matter, nitrogen, phosphorus, and potassium in the cucumber rhizosphere matrix. However, the application of Bacillus amyloliquefaciens QST713 promotes nutrient absorption and utilization, mitigating the detrimental impact of low-calcium stress on rhizosphere nutrient uptake and utilization. Consequently, it enhances the tolerance of cucumber plants to low-calcium stress.

3.4. Effect of Bacillus amyloliquefaciens QST713 on Inter-Root Matrix Enzyme Activities of Cucumber under Low-Calcium Stress

In Figure 2, the inter-root matrix activities of urease, sucrase, acid phosphatase, catalase, dehydrogenase, peroxidase, polyphenol oxidase, and protease in cucumber are represented by labels A to H, respectively. Under different treatments, low-calcium stress notably inhibits and reduces the inter-root enzyme activities in cucumber. However, the application of Bacillus amyloliquefaciens QST713 effectively enhances the inter-root matrix enzyme activities in cucumber. Comparing the CK+Q treatment to the CK treatment, there are varying degrees of increased inter-root enzyme activities in the cucumber. Additionally, when the LCa+Q treatment is compared to low-calcium stress alone, the inter-root stromal enzyme activities significantly increase. These findings indicate that the application of Bacillus amyloliquefaciens QST713 mitigates the negative impact of low-calcium stress on inter-root stromal enzyme activities, thereby alleviating the detrimental effects of low-calcium stress on cucumber growth.

3.5. Effect of Bacillus amyloliquefaciens QST713 on the Inter-Root Bacterial Community of Cucumber under Low-Calcium Stress

3.5.1. Evaluation of Illumina Sequencing Results of Cucumber Inter-Root Substrates with Different Treatments

As shown in Table 4, Illumina Miseq high-throughput sequencing was conducted on 12 samples of the cucumber rhizosphere matrix. The data underwent filtering and optimization, resulting in a total of 743,994 valid sequences. On average, each matrix sample contained approximately 62,000 ± 3542 bacterial sequences. The majority of high-quality sequences had a length distribution centred around 410 base pairs (bp), with an average sequence length of 412.39 bp. The length of the bacterial 16S rRNA V3–V4 region was approximately 400 bp, which closely matched the distribution of sequence lengths. The analysis results revealed that the library coverage of all 12 samples exceeded 98%, indicating that the majority of bacterial populations could be detected. The sequencing depth met the requirements and accurately represented the true microbial composition of the samples. Thus, the sequencing results provided a reliable depiction of the microbial communities within the samples.
As shown in Figure 3, the rarefaction curves of cucumber inter-root matrix bacteria under different treatments are displayed. The horizontal axis represents the sequencing number of the sample, and the position of the curve’s plateau indicates sequencing saturation. A flat curve suggests that increasing the sequencing data would not result in the discovery of additional Operational Taxonomic Units (OTUs). Conversely, a non-flat curve indicates sequencing insufficiency, where more data would uncover more OTUs. The OTUs were classified at a 97% similarity level, and rarefaction curves were generated for each sample. From the figure, it can be observed that the curves for each sample are relatively flat. This indicates that further sampling would only yield a small number of new OTUs, implying that the sample size is sufficient, and the sampling depth meets the analysis requirements.

3.5.2. Analysis of Alpha Diversity Index of Cucumber Inter-Root Substrate Bacterial Community in Different Treatments

As illustrated in Table 5, a diversity analysis was conducted at a 97% similarity level for validated sequences. The Sobs values represent the actual observed species, which were reduced under low-calcium stress compared to the control CK, while they significantly increased in all treatments where Bacillus amyloliquefaciens QST713 was applied.
The Chao1 and ACE indices are diversity estimation indices used to predict the number of microbial species (OTUs) in the samples based on the measured number of tags, OTUs, and their relative proportions. These indices reflect the richness of bacterial species, as shown in the table. Compared to the control under low-calcium stress (CK), the Chao1 and ACE indices decreased to varying degrees. However, both indices increased in the treatment with Bacillus amyloliquefaciens QST713, indicating that the application of this bacterium enhanced the bacterial richness in the inter-root matrix of cucumber. Conversely, low-calcium stress affected the abundance of substrate microorganisms, resulting in a lower number of microorganisms.
The Shannon index, which combines OTU abundance and uniformity, is a diversity index. A higher Shannon index indicates greater diversity in the bacterial population. On the other hand, the Simpson index approaches zero as species richness in the sample increases. From the table, it can be observed that the Shannon index increased with the application of Bacillus amyloliquefaciens QST713, while the Simpson index is relatively small. These findings suggest that the bacterial flora in the inter-root matrix of cucumber exhibited increased diversity due to the presence of Bacillus amyloliquefaciens QST713.

3.5.3. Distribution and Comparison of OTUs in Cucumber Inter-Root Matrix Communities of Different Treatments’ Venn Diagram

Through an OTU analysis and bioinformatics analysis, a total of 3719 OTUs were identified in the inter-root matrix of cucumber during the fruiting late period. These OTUs corresponded to 33 phyla, 98 classes, 249 orders, 409 families, 723 genera, and 1415 bacterial species.
The Venn diagram visually represents the shared and unique OTUs across different samples, illustrating the overlap between them. The OTUs were classified at a 97% similarity level. Figure 4a displayed the number of OTUs in the low-calcium stress treatment group compared to the control (CK). There are 2182 shared OTUs, with 585 unique OTUs in the control group and 494 unique OTUs in the LCa treatment group. This indicated a reduction in the number of inter-root bacterial OTUs in cucumber under low-calcium stress compared to the control; Figure 4b depicts the number of OTUs in the treatment group with Bacillus amyloliquefaciens QST713 applied (CK+Q) in comparison to the control (CK). There are 2334 shared OTUs, with 433 unique OTUs in the control group and 589 unique OTUs in the CK+Q treatment group. This highlights that the application of Bacillus amyloliquefaciens QST713 enriched the microbial community in the inter-root matrix and increased the number of OTUs; Figure 4c showcases the comparison between the treatment groups receiving Bacillus amyloliquefaciens QST713 combined with low-calcium stress and the treatment group subjected to low-calcium stress alone. There are 2203 shared OTUs, with 473 unique OTUs in the LCa treatment group and 490 unique OTUs in the combined treatment group. Therefore, the application of Bacillus amyloliquefaciens QST713 increased the number of inter-root matrix bacterial communities and enhanced the diversity of microorganisms in the inter-root matrix under low-calcium stress.

3.5.4. Principal Coordinate Analysis (PCoA) of Cucumber Inter-Root Matrix Communities in Different Treatments

As shown in Figure 5, the principal coordinate analysis (PCoA) based on the Bray–Curtis algorithm clearly demonstrates the variation in microbial community structure within the inter-root matrix under different treatments. All treatments exhibit distinct separation from each other, indicating significant differences between samples from different treatments (R2 = 0.9537, p = 0.001). Within the total variance of the dataset, the first two principal components collectively account for 60.90% of the total bacterial population. Specifically, the first principal component (PC1) plays a crucial role by explaining 44.61% of the overall variance in the bacterial community. This highlights the substantial importance of PC1 in shaping the overall microbial composition within the inter-root matrix.

3.5.5. Diversity Analysis of Dominant Bacterial Communities in the Inter-Root Matrix Based on Different Taxonomic Levels

The top 10 most abundant bacterial phyla were selected to investigate changes in the inter-root microbial community across different treatment samples (Figure 6). Among all samples, the six most abundant bacterial phyla, in descending order, were Proteobacteria, Actinobacteriota, Chloroflexi, Patescibacteria, Bacteroidota, and Acidobacteriota, collectively accounting for over 80% of the bacterial sequences.
Proteobacteria exhibited the highest relative abundance in both the CK and CK+Q treatments, while Actinobacteriota dominated in both the LCa and LCa+Q experimental groups. This suggests that low-calcium stress significantly increased the relative abundance of Actinobacteriota within the cucumber inter-root matrix. Consequently, low-calcium stress led to an increase in Actinobacteriota and resulted in a decrease in the relative abundance of the dominant phylum, Proteobacteria. Compared to the control group (CK), under low-calcium stress, the relative abundance of Proteobacteria, Chloroflexi, and Patescibacteria in the cucumber rhizosphere substrate was lower, while the relative abundance of Actinobacteriota was higher. In the CK+Q treatment, the relative abundances of Proteobacteria, Chloroflexi, and Patescibacteria increased, whereas that of Actinobacteriota was lower compared to the control CK. In the LCa+Q treatment, the relative abundances of Chloroflexi and Acidobacteriota increased, while the relative abundance of the phylum Actinobacteriota was comparatively lower, when compared to low-calcium stress alone.
These findings indicate that the application of Bacillus amyloliquefaciens QST713 can enhance the relative abundances of Proteobacteria and Chloroflexi in the inter-root matrix while effectively reducing the relative abundance of Actinobacteriota. Furthermore, in the CK+Q treatment, the relative abundance of Acidobacteriota was comparatively lower compared to the control CK, whereas in the LCa+Q treatment, it increased compared to low-calcium stress alone. This suggests that the application of Bacillus amyloliquefaciens can modify the structural composition of the microbial community under low-calcium stress, thereby alleviating its effects.
As shown in Figure 7, which presents the top 20 taxonomic genera of cucumber inter-root matrix bacterial communities ranked by relative abundance at the genus level, the overall structure of the cucumber inter-root microflora in the four treatments exhibited similarities, with the main differences lying in the abundance of different bacterial genera. Notably, the inter-root microflora of the LCa and LCa+Q treatments displayed a higher structural similarity and clustered together, forming a distinct branch. In contrast, the inter-root microorganisms of the CK and CK+Q treatments clustered together in a separate branch, indicating that low-calcium stress exerted a more pronounced impact on the cucumber inter-root microorganisms.
Among the top 20 taxonomic genera of bacteria, 10 species were clearly classified. These included Actinplanes, Nocardioides, Streptomyces, Rhodoplanes, TM7a, Devosia, Bauldia, Pseudolabrys, Hyphomicrobium, and Actinplanes being the dominant genus across all four treatments. Compared to the control group (CK), the relative abundance of TM7a, Devosia, Acidibacter, and Hyphomicrobium genera is lower in the root-associated matrix of cucumber under low-calcium stress. On the other hand, the relative abundance of Actinoplanes, Nocardioides, Streptomyces, and Pseudolabrys genera is higher in the same conditions. When comparing the treatment with the application of Bacillus amyloliquefaciens QST713 to the treatment without bacterial application, Actinoplanes, Nocardioides, Streptomyces, Devosia, Bauldia, and Acidibacter genera exhibit relatively lower abundance, while TM7a, Pseudolabrys, and Hyphomicrobium genera show relatively higher abundance. This suggests that the application of Bacillus amyloliquefaciens was capable of increasing the abundance of genera that were reduced under low-calcium stress and decreasing the abundance of genera that were elevated under low-calcium stress, thereby modulating the microbial community in the inter-root matrix.
As shown in Figure 8, a differential species analysis was performed on the root-associated bacterial genera. The top 15 abundant species with a significance level of p < 0.01 were selected. Among the top 15 differentially abundant bacterial genera, 6 of them have been classified at the genus level. These include Streptomyces, Pseudolabrys, Acidibacter, Marmoricola, Soilrubrobacter, and Terrimonas. Streptomyces, Pseudolabrys, and Acidibacter are among the top 20 genera in terms of relative abundance. Streptomyces and Acidibacter show significant differences across all four treatments (p < 0.001), with a notable decrease when Bacillus amyloliquefaciens QST713 is applied. Pseudolabrys reaches significant differences at the p < 0.01 level for all four treatments, with a significant increase observed when Bacillus amyloliquefaciens QST713 is applied.

4. Discussion

The inter-root matrix of plants encompasses the shallow matrix present on the surface of the root system, which is influenced by root secretions. Plant-growth-promoting rhizobacteria play a pivotal role in supplying nutrients to plants through the life activities of microorganisms, thereby promoting crop growth. Moreover, these PGPR facilitate the efficient utilization of nutrients by plants, enhancing crop resistance against diseases and pests. This, in turn, leads to improvements in crop quality and increased crop yield [29].
In this experiment, cucumber plants under low-calcium stress showed significant inhibition in plant height, stem diameter, and biomass. As the stress duration increased, plant height, stem diameter, and biomass were significantly reduced. However, the application of Bacillus amyloliquefaciens QST713 alleviated the damage caused by low-calcium stress on cucumber plants. Plant height, stem diameter, and biomass significantly increased with the application of Bacillus amyloliquefaciens QST713. The effects of Bacillus amyloliquefaciens QST713 on the cucumber substrate under low-calcium stress were analysed from three aspects: physicochemical properties of the root-associated matrix, enzyme activity, and bacterial community diversity. These analyses provided preliminary insights into the mechanisms by which Bacillus amyloliquefaciens QST713 mitigates low-calcium stress in cucumber, shedding light on its potential benefits for alleviating the negative impacts of low-calcium stress.
Soil physicochemical properties and nutrient indicators serve as valuable indicators of soil fertility, providing insights into the effectiveness of microbial agents in soil improvement. Mao [33] demonstrated a close correlation between conductivity and soil microbial activity, as well as other soil components. In this experiment, the physicochemical properties of the inter-root matrix in cucumber under different treatments were assessed. The pH of the matrix plays a crucial role not only in crop growth and development but also in shaping the microbial populations within the matrix, subsequently influencing crop growth [34]. Under low-calcium stress, a significant increase in pH and EC values was observed in the inter-root matrix of cucumber. This increase can be attributed to the impact of low-calcium stress on nutrient absorption and utilization, leading to salt accumulation and the subsequent elevation in pH and EC values. Interestingly, the application of Bacillus amyloliquefaciens QST713 was found to reduce the pH and EC values of the cucumber inter-root matrix. This reduction may be attributed to the metabolic activity of Bacillus amyloliquefaciens QST713, which produces organic acids that exert a buffering effect on the matrix pH. These findings align with the results of a study by Chu [35], where the application of microbial agents demonstrated a similar reduction in soil conductivity within a certain range.
Soil bulk density and soil porosity serve as indicators of soil compaction [36]. Within a certain range, lower bulk density values and higher porosity indicate a looser soil structure, which is beneficial for plant root growth. Li [37] conducted a study on the effects of organic microbial fertilizers with different ratios on soil bulk density and porosity in cucumber fields and found that it enhanced soil aeration. In this experiment, under low-calcium stress, the cucumber root matrix exhibited an increase in bulk density and a decrease in porosity. However, the application of Bacillus amyloliquefaciens QST713 resulted in a reduction in bulk density and an increase in porosity of the cucumber root matrix. This suggests that Bacillus amyloliquefaciens QST713 has the ability to loosen the matrix, thereby improving its ventilation conditions.
Soil organic matter plays a vital role in fertilization and soil modification. Although it cannot be directly utilized as plant-absorbable nutrients, it serves as a nutrient reservoir for plants. During the processes of soil accumulation and decomposition, soil organic matter supplies and regulates the essential nutrients required for plant growth. It contains a diverse range of elements necessary for plants and exhibits a high capacity for substitution and absorption, enabling it to retain a substantial amount of nutrients [38]. Furthermore, soil organic matter contributes to the improvement in soil structure and physical properties, enhancing soil water storage and fertilizer retention capacity. The findings of this experiment revealed that the inter-root substrate of cucumber under low-calcium stress exhibited a high organic matter content. However, the application of Bacillus amyloliquefaciens QST713 reduced the organic matter content in the matrix. This can be attributed to the fact that the organic matter in the experimental substrate primarily originated from sterilized coir and cucumber fibrous roots during growth and development. Under low-calcium stress, cucumber growth is significantly inhibited compared to normal cultivation conditions (CK). The substrate becomes compacted, and microbial activity is weakened, resulting in a reduced capacity for organic matter decomposition. As a consequence, there is a decrease in the absorption and utilization of organic matter by cucumber plants. Therefore, during the fruiting late stage, the organic matter in the cucumber rhizosphere substrate tends to accumulate. Conversely, the application of Bacillus amyloliquefaciens QST713 effectively decomposed the organic matter, thereby supplying the necessary nutrients for plant growth. Several experimental studies have demonstrated that microbial fertilizers can enhance nutrient levels in the soil, consequently improving soil fertility [39]. The experiment used equal amounts of sterilized coconut coir and a quantified nutrient solution for the pot cultivation of cucumbers. The higher nitrogen, phosphorus, and potassium content in the inter-root matrix of cucumber under low-calcium stress can be attributed to the reduced nutrient uptake by cucumber plants in such conditions, resulting in a greater residual content of nitrogen, phosphorus, and potassium in the matrix. However, the application of Bacillus amyloliquefaciens QST713 significantly reduced the total nitrogen, total phosphorus, and total potassium content in the inter-root matrix of cucumber. It indicates that more nutrients were available to plants, indicating that Bacillus amyloliquefaciens QST713 promoted the absorption and utilization of nutrients. Bacillus amyloliquefaciens QST713 is a rhizosphere-promoting bacterium with functions such as phosphorus solubilization, potassium solubilization, and nitrogen fixation. It enhances the absorption of nitrogen, phosphorus, and potassium by cucumber plants, thereby promoting their growth and development. Research has indicated that microbial agents can facilitate the uptake of elements by the root system, as well as the transport and accumulation of these elements in stems, leaves, and fruits [40].
Soil enzymes are primarily secreted by microorganisms, living plants, and animals, and they are released through the decomposition of plant and animal residues and remains in the soil [41]. The activities of soil enzymes are closely related to soil physicochemical properties, soil biomass, and biodiversity [42]. Studies have shown that the inter-root inoculation of plants with microbial agents can enhance soil enzyme activities such as nitrate enzymes, sucrase, urease, and protease. However, the effects vary at different stages of plant growth, and substrate enzyme activity depends on the microbial population and duration of action [43]. The soil enzyme system represents the most physiologically active component of the soil, and the intensity of enzyme activity varies during different reproductive periods of crops [44].The results of this experimental study demonstrated that the application of Bacillus amyloliquefaciens QST713 significantly increased the activities of inter-root substrate enzymes, including urease, sucrase, phosphatase, catalase, dehydrogenase, catalase, and polyphenol oxidase in cucumber, compared to normal cultivation conditions. This indicates that under normal cultivation conditions, Bacillus amyloliquefaciens QST713 improved the inter-root environmental conditions and accelerated the transformation of inter-root substances, thereby promoting better plant growth. Low-calcium stress significantly inhibited the activity of inter-root matrix enzymes in cucumber, while the application of Bacillus amyloliquefaciens QST713 significantly increased the activity of inter-root matrix enzymes. Bacillus amyloliquefaciens QST713 has been shown to improve the microbial environment of the substrate under low-calcium stress and enhance the activity of substrate enzymes. Additionally, Bacillus amyloliquefaciens QST713 provides nutrients to the substrate, thereby increasing microbial activity and alleviating the effects of low-calcium stress on the substrate.
The matrix microbial community structure plays a crucial role in the overall ecosystem, as it is closely associated with ecosystem function and reflects substrate quality to some extent. In this experiment, the Illumina MiSeq high-throughput sequencing platform was employed to sequence the 16S rDNA of cucumber inter-root bacteria. Under low-calcium stress, the bacterial richness index and diversity index of the inter-root matrix decreased, accompanied by a reduction in the bacterial number. However, with the application of Bacillus amyloliquefaciens QST713, the bacterial richness index and diversity index of the inter-root matrix of cucumber increased, along with an increase in the bacterial number. The introduction of Bacillus amyloliquefaciens QST713, an inter-root growth-promoting bacterium and a biocontrol bacterium, could lead to variations in bacterial numbers and alterations in community structure. Consequently, these alterations can have an impact on the evenness and richness of species, ultimately leading to changes in the bacterial diversity index of substrate microorganisms.
In this experiment, we examined the differences in the relative abundance of cucumber inter-root matrix communities at the phylum and genus levels under different treatments. The results revealed that the six most abundant bacterial phyla were Proteobacteria, Actinobacteriota, Chloroflexi, Patescibacteria, Bacteroidota, and Acidobacteriota, collectively accounting for over 80% of all bacterial sequences. Among them, Proteobacteria, Actinobacteriota, and Acidobacteriota are widely distributed bacteria [45]. Proteobacteria play significant roles in global carbon, nitrogen, and sulphur cycles, and they are found in various environments, including farmland. In this study, it was observed that under low-calcium stress, the relative abundance of Proteobacteria was lower in the inter-root substrate of cucumber. The application of Bacillus amyloliquefaciens QST713 under low-calcium stress had little effect on Proteobacteria. However, in the CK+Q treatment, the relative abundance of Proteobacteria increased. This suggests that Bacillus amyloliquefaciens can promote the abundance of Proteobacteria in the normal cultivation environment. This finding is consistent with previous studies [46,47], which also observed a positive correlation between the presence of Bacillus amyloliquefaciens and the abundance of Proteobacteria. The ranking of the top twenty bacterial genera in different cucumber inter-root substrate bacterial communities was consistent, with variations observed in the abundance of these bacterial groups. Actinoplanes emerged as the most dominant genus across all four treatments. Under low-calcium stress, certain bacterial genera decreased in abundance, while others increased in the inter-root matrix of cucumber. The application of Bacillus amyloliquefaciens QST713 was able to enhance the relative abundance of bacteria that declined under low-calcium stress and reduce the relative abundance of bacteria that increased under such conditions. This suggests that Bacillus amyloliquefaciens QST713 can modify the composition of the inter-root bacterial community in the presence of low-calcium stress, leading to variations in the abundance of dominant bacterial groups. The variations in the biological and ecological functions of different bacterial communities can have a direct impact on the ecological functions of the inter-root soil microdomains. Consequently, this can influence the uptake and utilization of inter-root nutrients by cucumber plants.

5. Conclusions

In conclusion, the growth of cucumber plants was significantly impeded under low-calcium stress, resulting in notable reductions in plant height, stem diameter and biomass accumulation. Low-calcium stress also had significant effects on the physicochemical properties of the substrate, nutrient absorption and utilization, enzyme activity in the inter-root substrate, bacterial abundance, diversity indices, and overall bacterial population. The application of Bacillus amyloliquefaciens QST713 proved to be beneficial by improving substrate aeration, enhancing substrate decomposition, and promoting the absorption and utilization of nitrogen, phosphorus, and potassium by cucumber plants. This resulted in increased richness and diversity indices, as well as a shift in the bacterial community structure within the inter-root matrix. Moreover, the application of Bacillus amyloliquefaciens QST713 effectively mitigated the detrimental effects of low-calcium stress on cucumber plants, ultimately promoting their growth. The findings of this study provide valuable insights into the influence of Bacillus amyloliquefaciens QST713 on the inter-root environment of cucumber plants under low-calcium stress. These results serve as a basis and technology for further investigations into the resistance capabilities of Bacillus amyloliquefaciens QST713. Additionally, this research enriches our understanding of the functional role of Bacillus amyloliquefaciens and offers a certain foundation and technical support for studying its resistance abilities.

Author Contributions

Conceptualization, B.L. and L.Z.; Data curation, L.Z.; Formal analysis, L.W., Y.Y. and B.Q.; Investigation, L.Z.; Methodology, B.L. and L.Z.; Project administration, B.L.; Resources, B.L.; Writing—original draft, L.Z. and Z.W.; Writing—review and editing, B.L. and L.H. All authors have read and agreed to the published version of the manuscript.

Funding

The Applied Fundamental Research Program of Shanxi Province: 20210302123401, 20210302124137; Shanxi Province Key R&D Plan: 202302010101003, 202102140601013-03; Modern Agro-industry Technology Research System in Shanxi Province: 2024; the Project of Quwo Fruit and Vegetable Research Institute of Shanxi Agricultural University: 2021QWGS-1.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. Data not readily available for public consumption due to privacy and other issues.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effects of Bacillus amyloliquefaciens QST713 on plant growth of cucumber plants under low-calcium stress. (a) The plant height of cucumber. (b) The stem diameter of cucumber. Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncanʹs test with p < 0.05.
Figure 1. Effects of Bacillus amyloliquefaciens QST713 on plant growth of cucumber plants under low-calcium stress. (a) The plant height of cucumber. (b) The stem diameter of cucumber. Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncanʹs test with p < 0.05.
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Figure 2. Effect of Bacillus amyloliquefaciens QST713 on inter-root enzyme activities of cucumber under low-calcium stress. (a) The activity of matrix urease. (b) The activity of matrix sucrase. (c) The activity of matrix acid phosphatase. (d) The activity of matrix catalase. (e) The activity of matrix dehydrogenase. (f) The activity of matrix peroxidase. (g) The activity of matrix polyphenol oxidase. (h) The activity of matrix proteases. Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncanʹs test with p < 0.05.
Figure 2. Effect of Bacillus amyloliquefaciens QST713 on inter-root enzyme activities of cucumber under low-calcium stress. (a) The activity of matrix urease. (b) The activity of matrix sucrase. (c) The activity of matrix acid phosphatase. (d) The activity of matrix catalase. (e) The activity of matrix dehydrogenase. (f) The activity of matrix peroxidase. (g) The activity of matrix polyphenol oxidase. (h) The activity of matrix proteases. Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncanʹs test with p < 0.05.
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Figure 3. Sample rarefaction curves for different treatment matrices.
Figure 3. Sample rarefaction curves for different treatment matrices.
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Figure 4. Venn diagram of bacterial communities in matrix samples of different comparison groups. Each circle in the figure represents a treatment group, and the numbers in the overlapping part of the circles represent the number of bacterial communities shared between the comparison groups, while the numbers without overlapping parts represent the number of bacterial communities specific to the comparison groups. (a) CK group vs. LCa group. (b) CK group vs. CK+Q group. (c) LCa group vs. LCa+Q group.
Figure 4. Venn diagram of bacterial communities in matrix samples of different comparison groups. Each circle in the figure represents a treatment group, and the numbers in the overlapping part of the circles represent the number of bacterial communities shared between the comparison groups, while the numbers without overlapping parts represent the number of bacterial communities specific to the comparison groups. (a) CK group vs. LCa group. (b) CK group vs. CK+Q group. (c) LCa group vs. LCa+Q group.
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Figure 5. PCoA plot of microbial communities in inter-root samples of different treatments at OTU level.
Figure 5. PCoA plot of microbial communities in inter-root samples of different treatments at OTU level.
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Figure 6. Abundance maps of microbial composition of inter-root substrates at the phylum-based taxonomic level for different treatments.
Figure 6. Abundance maps of microbial composition of inter-root substrates at the phylum-based taxonomic level for different treatments.
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Figure 7. Heatmap of the abundance of microbial composition of the inter-root matrix at genus level.
Figure 7. Heatmap of the abundance of microbial composition of the inter-root matrix at genus level.
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Figure 8. Differential species analysis of the inter-root matrix microorganisms at genus level for different treatments. “***” indicates a significant difference at the p < 0.001 level, and “**” indicates a significant difference at the p < 0.01 level.
Figure 8. Differential species analysis of the inter-root matrix microorganisms at genus level for different treatments. “***” indicates a significant difference at the p < 0.001 level, and “**” indicates a significant difference at the p < 0.01 level.
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Table 1. Effects of Bacillus amyloliquefaciens QST713 on cucumber plant biomass under low-calcium stress.
Table 1. Effects of Bacillus amyloliquefaciens QST713 on cucumber plant biomass under low-calcium stress.
TreatmentShoot Fresh
Weight
g		Shoot Dry
Weight
g		Root Fresh
Weight
g		Root Dry
Weight
g		Total Fresh
Weight
g		Total Dry
Weight
g
CK555.31 ± 5.43 b52.32 ± 1.06 b67.93 ± 4.03 b5.69 ± 0.25 b623.24 ± 9.40 b58.01 ± 1.29 b
CK+Q604.80 ± 4.02 a58.72 ± 1.86 a78.09 ± 0.99 a6.82 ± 0.18 a682.89 ± 4.99 a65.54 ± 2.03 a
LCa457.63 ± 5.58 d43.33 ± 1.23 c56.69 ± 2.73 c4.76 ± 0.04 c514.33 ± 8.16 d48.09 ± 1.19 c
LCa+Q504.72 ± 0.44 c50.06 ± 0.60 b66.52 ± 1.38 b5.36 ± 0.02 b571.23 ± 1.64 c55.42 ± 0.60 b
Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncan’s test with p < 0.05.
Table 2. Effect of Bacillus amyloliquefaciens QST713 on physicochemical properties of cucumber inter-root matrix under low-calcium stress.
Table 2. Effect of Bacillus amyloliquefaciens QST713 on physicochemical properties of cucumber inter-root matrix under low-calcium stress.
TreatmentpHEC
(us·cm−1)		Bulk Density
(g·cm−3)		Porosity
(%)
CK6.34 ± 0.12 b140.00 ± 2.14 d0.15 ± 0.00 b88.29 ± 0.87 b
CK+Q6.05 ± 0.05 c133.00 ± 2.52 c0.12 ± 0.00 d92.79 ± 0.58 a
LCa6.64 ± 0.04 a163.13 ± 1.42 a0.16 ± 0.00 a81.57 ± 1.94 c
LCa+Q6.42 ± 0.03 ab147.87 ± 1.25 b0.13 ± 0.00 c86.06 ± 0.31 b
Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncan’s test with p < 0.05.
Table 3. Effect of Bacillus amyloliquefaciens QST713 on inter-root nutrient content of cucumber under low-calcium stress.
Table 3. Effect of Bacillus amyloliquefaciens QST713 on inter-root nutrient content of cucumber under low-calcium stress.
TreatmentOrganic Matter
(g·kg−1)		Nitrogen
(%)		Phosphorus
(μg·g−1 DW)		Potassium
(mg·g−1 DW)
CK305.93 ± 5.17 b2.63 ± 0.02 b144.95 ± 0.30 b15.45 ± 0.21 c
CK+Q258.96 ± 7.47 c2.05 ± 0.05 d122.61 ± 2.73 c13.24 ± 0.28 d
LCa356.04 ± 12.11 a2.96 ± 0.04 a173.46 ± 2.85 a18.89 ± 0.06 a
LCa+Q309.81 ± 4.50 b2.45 ± 0.03 c150.05 ± 3.65 b16.52 ± 0.17 b
Three replicates were used for determination, and each value is presented as “mean ± standard deviation (SD)”. Note: Lowercase letters represent Duncan’s test with p < 0.05.
Table 4. Effect of Bacillus amyloliquefaciens QST713 on inter-root nutrient content of cucumber under low-calcium stress.
Table 4. Effect of Bacillus amyloliquefaciens QST713 on inter-root nutrient content of cucumber under low-calcium stress.
TreatmentSeq Num.Base Num.Mean LengthMin LengthMax LengthCoverage/%
CK-167,50327,816,132412.0720349098.49%
CK-262,62225,829,676412.4721749598.37%
CK-365,15926,901,175412.8524948698.59%
CK+Q-160,16424,797,631412.1723249898.40%
CK+Q-261,57525,388,341412.3224649298.35%
CK+Q-363,45526,174,644412.4920348698.36%
LCa-160,81925,084,857412.4520150798.56%
LCa-256,33823,242,520412.5524046898.53%
LCa-361,60925,428,312412.7420348298.50%
LCa+Q-160,38624,884,589412.0920453598.35%
LCa+Q-256,91723,477,478412.4924650798.49%
LCa+Q-367,44727,784,810411.9524546798.45%
Mean62,00025,567,514412.3922449398.45%
Table 5. Alpha diversity index of cucumber inter-root substrates in different treatments.
Table 5. Alpha diversity index of cucumber inter-root substrates in different treatments.
TreatmentAlpha Diversity Index
SobsACEChao1ShannonSimpson
CK2039 bc2649.04 bc2641.54 b6.04 ab0.008 a
CK+Q2186 a2833.40 a2843.38 a6.22 a0.006 a
LCa1960 c2545.82 c2573.43 b5.90 b0.011 a
LCa+Q2119 ab2733.28 ab2748.50 ab6.15 a0.007 a
Note: Lowercase letters represent Duncan’s test with p < 0.05.
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MDPI and ACS Style

Li, B.; Zhang, L.; Wei, L.; Yang, Y.; Wang, Z.; Qiao, B.; Han, L. Effect of Bacillus amyloliquefaciens QST713 on Inter-Root Substrate Environment of Cucumber under Low-Calcium Stress. Agronomy 2024, 14, 542. https://doi.org/10.3390/agronomy14030542

AMA Style

Li B, Zhang L, Wei L, Yang Y, Wang Z, Qiao B, Han L. Effect of Bacillus amyloliquefaciens QST713 on Inter-Root Substrate Environment of Cucumber under Low-Calcium Stress. Agronomy. 2024; 14(3):542. https://doi.org/10.3390/agronomy14030542

Chicago/Turabian Style

Li, Bin, Li Zhang, Lincao Wei, Yujie Yang, Zhexuan Wang, Bo Qiao, and Lingjuan Han. 2024. "Effect of Bacillus amyloliquefaciens QST713 on Inter-Root Substrate Environment of Cucumber under Low-Calcium Stress" Agronomy 14, no. 3: 542. https://doi.org/10.3390/agronomy14030542

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