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Article

Growth Promotion of Guava “Pear” (Psidium guajava cv.) by Sinorhizobium mexicanum in Southern Mexican Agricultural Fields

by
Clara Ivette Rincón-Molina
1,
Esperanza Martínez-Romero
2,
Luis Alberto Manzano-Gómez
3 and
Reiner Rincón-Rosales
1,*
1
Laboratorio de Ecología Genómica, Tecnológico Nacional de México, Instituto Tecnológico de Tuxtla Gutiérrez, Tuxtla Gutiérrez 29050, Chiapas, Mexico
2
Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca 62210, Morelos, Mexico
3
Departamento de Investigación y Desarrollo, 3R Biotec SA de CV, Tuxtla Gutiérrez 29000, Chiapas, Mexico
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(19), 12391; https://0-doi-org.brum.beds.ac.uk/10.3390/su141912391
Submission received: 23 August 2022 / Revised: 19 September 2022 / Accepted: 27 September 2022 / Published: 29 September 2022
(This article belongs to the Special Issue Plant-Microbe Interactions and Soil Fertility Status for Enhancing Sustainable Agriculture)
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Abstract

:
Biofertilizers formulated with nitrogen-fixing bacteria represent an alternative to chemical fertilizers because they increase soil fertility and protect the environment. Therefore, the objective of this study was to analyze the effects on the growth of guava “pear” (Psidium guajava cv.) after inoculation with a nitrogen fixing bacterium Sinorhizobium mexicanum ITTG-R7T. The study was carried out in an agricultural rural area of Chiapas, Mexico, where farmers do not have programs of regenerative agriculture. First, the agricultural soil was subjected to physicochemical and metagenomic analysis in order to determine the soil quality and its bacterial community composition. Likewise, multifunctional biochemical tests and plant inoculation assays were evaluated to determine the potential of S. mexicanum ITTG-R7T as plant-growth-promoting bacteria (PGPB). The site was rain fed and had silty clay loam soil with abundant Bacillaceae. S. mexicanum ITTG-R7T showed different properties as PGPB such as the production of indole compounds, synthesis of extracellular enzymes, phosphate solubilization, synthesis of siderophores, ACC (1-aminocyclopropane-1-carboxylate) deaminase, and nitrogenase activity (ARA). When the strain ITTG-R7 T was combined with chemical nutrients, it had the highest positive effect on the growth and development of guava plants. Guava biofertilization with ITTG-R7T had a significant influence (p < 0.05) mainly on the total plant height (368.83 cm), number of flowers (36.0) and the amount of chlorophyll (2.81 mg mL−1) in comparison with the other treatments evaluated. ITTG-R7T is a promising strain for improving the guava crop yield.

Graphical Abstract

1. Introduction

The use of chemical fertilizers and genetic plant selection have resulted in increased crop yields in agriculture. However, during the 40 years of the Green Revolution [1], the excessive application of fertilizers affected the environment and the soil biodiversity, which is the key for crop productivity [2,3,4,5,6,7,8]. Microbial diversity studies (by DNA sequencing) have shown a novel view of the diversity and structure of the microbial communities in soil [9], which may promote plant development by providing essential elements such as nitrogen, phosphorus, potassium, and iron [10,11,12]. Additionally, plant growth stimulation by microorganisms can be due to the production of phytohormones such as indole acetic acid (IAA), siderophores, and the biocontrol of phytopathogens [13]. Some microorganisms can fix atmospheric nitrogen (N2); a process known as biological nitrogen fixation (BNF) [14,15], which is an alternative to reduce the negative impact of nitrogen chemical fertilizers. Nitrogen limits crop production and its demand is continuously growing.
The phyla α-Proteobacteria and β-Proteobacteria harbor different bacterial genera capable of forming nodules in legumes including the Sinorhizobium genus. Sinorhizobium have the ability to fix nitrogen in symbiosis with leguminous plants [16,17,18]. Among these, Sinorhizobium mexicanum ITTG-R7T, isolated from nodules of the legume Acaciella angustissima stimulated the growth of different plant species [13,19,20,21]. Sinorhizobium mexicanum ITTG-R7T stands out for being the best strain to promote plant growth and to compete in interstrain nodule competition assays [20]. Additionally, the complete genome sequencing of the strain S. mexicanum ITTG-R7T has allowed for the identification of a cluster of genes related to the ability to fix N2, solubilize phosphate, auxin synthesis, and siderophore production [22]. These genomic qualities detected in the strain ITTG-R7T led us to biochemically determine the PGPB potential of this sinorhizobian bacterium.
In Southern Mexico, fruit growing is one of the most preponderant activities because from it emanates the regional and national food security. Thereby, it is a priority to attend to field producers to guarantee production by improving yields on cultivated fields, thus resulting in economic benefits for various social groups. Guava “pear” is grown mainly in the metropolitan region of Chiapas, Mexico, where agriculture is the most important economic activity. Local guava farmers apply large amounts of chemical fertilizers and other toxic agricultural inputs in order to increase the crop yield. However, the desired results are not obtained. In fact, this agricultural practice causes deterioration of the soil and contamination of the environment. The PGPB inoculants have proven to be an efficient, easy to apply, and a safe alternative that can replace the indiscriminate use of toxic agricultural compounds. Some studies focused on the application of biofertilizers in guava (P. guajava) crops have shown several benefits. Devi et al. [23] evaluated the effect of inoculation with Azospirillum and Azotobacter in guava trees. In this experiment, a positive effect on fruit quality and an increase in yield were reported. In the same way, Shukla et al. [24] reported that the yield and guava fruit quality boosters improved with the application of biofertilizers and organic inputs. The present study evaluated the growth of guava (P. guajava) plants after field inoculation with S. mexicanum ITTG-R7T in a rural area of Chiapas, Mexico.

2. Materials and Methods

2.1. Study Area

The biofertilization experiment was carried out in an active agricultural field in the most important guava-producing zone called “Río Grande” at Ribera de Monte Rico (Nacamucuyi) in Chiapas, Mexico (16°71′04″ N and 93°03′12″ W), at an average height of 400 m.a.s.l (Figure 1). An area of 2100 m2 was used for the sampling and inoculation experiments. Soil samples were obtained from five randomly located points from 15 cm from the top layer.
Additionally, the solar radiation interception fraction of the experimental plot was derived from the normalized differential vegetation index (NDVI) measurements from Sentinel-2 images (10 m resolution). The NDVI served as an indicator of the overall tree performance by quantifying the plant’s biomass, vigor, and density through satellite images. The NDVI values were obtained at the initial and final times of the experiment with Auravant® (www.auravant.com) (accessed on 21 June 2021).

2.2. Bacterial Strain

The nitrogen-fixing bacterium Sinorhizobium mexicanum ITTG-R7T (DQ411930) was used in this study. The strain was isolated from the shrubby legume Acaciella angustissima and identified through 16S rRNA gene sequencing [19]. In a previous work, the complete genome sequencing of ITTG-R7T was carried out in order to detect genes related to the nitrogen fixing ability [22].

2.3. Soil Physicochemical Analyses

Soil samples from the agricultural crop field were analyzed. Soil texture was determined by the hydrometer method [25]. The pH and electric conductivity (EC) were measured by using a digital pH meter, a Mettler Toledo in a 1:10 (weight/volume) aqueous solution. Cation exchange capacity (CEC) was determined according to the Official Mexican Standard NOM-021-SEMARNAT-2000. The determination of total nitrogen and total carbon was carried out by using a FLASH 2000® auto-analyzer. The determination of total phosphorus was conducted with the solubilization method of HNO3/HClO4 [13]. All determinations were conducted in triplicate.

2.4. Assessment of the Bacterial Community in Guava Agricultural Soil

Total DNA was extracted from agricultural soil samples using the ZymoBIOMICS® Commercial Kit. The DNA sample was sent to Macrogen Inc. (DNA Sequencing Service, Seoul, Korea) for amplification of the V3–V4 bacterial 16S rRNA variable region using the primers Bakt_341F (CCTACGGGNGGCWGCAG) and Bakt_805R (GACTACHVGGGTATCTAATCC). Sequencing was performed using the paired-end Illumina Miseq 2 × 300. Analyses from the raw sequencing data were carried out using QIIME 2.0 software [26]. Operational taxonomic units (OTUs) were determined with a similarity level of 97% using the UCLUST algorithm [27]. Python Nearest Alignment Space Termination (PyNAST) was used for sequence alignments against the Greengenes core set (available from http://greengenes.lbl.gov/) (accessed on 10 December 2021) and the representative sequences of each OTU, filtered at a threshold of 75% [28]. Taxonomic classification was performed using the naïve Bayesian rRNA classifier (http://rdp.cme.msu.edu/classifier/classifier.jsp) (accessed on 10 December 2021) at a confidence threshold of 80% [29]. Stacked bars were constructed with the pheatmap package in R [30]. The sequence data were deposited at the NCBI database under accession number PRJNA845003.

2.5. Sinorhizobium mexicanum ITTG-R7T as PGPB

To test phosphate solubilization, ITTG-R7T was inoculated in the National Botanical Research Institute’s phosphate growth medium (NBRIP) and pH adjusted to 7.0 [31]. The phosphate solubilization index (PSI) was calculated when a clear halo surrounding the bacterial colony after 5 days of incubation at 30 °C was recognized [32]. The method described by Brick et al. [33] was followed to detect the ITTG-R7T production of indole substances. Indole-3-acetic acid (IAA) was identified as indicated by Rincón-Molina et al. [13]. The acetylene reduction assay (ARA) was carried out to assess the nitrogenase activity. The culture and detection conditions for the acetylene reduction activity measures were specified according to the methodology described by Rizo et al. [34]. ACC (1-aminocyclopropane-1-carboxylate) deaminase activity was tested with 0.03% of ACC as the sole nitrogen source in a culture medium [35]. Bacterial siderophore production was tested in aa CAS-agar medium [chromeazurol-S (CAS), iron(III), and hexadecyl trimethyl ammonium bromide (HDTMA)] at 28–30 °C for 5 days [36]. Protease activity was determined following the method developed by Mahanta et al. [37]. Amylase activity was assayed in a soluble starch–yeast extract medium [38]. Lipase activity was measured in culture medium with Tween 80 [39]. Cellulase activity was determined with carboxymethylcellulose [40]. The same tests were performed for the bacterium Azospirillum brasilense CD, which was used as a reference PGPB strain. All of the assays were performed in triplicate.

2.6. Experimental Design for Field Inoculation

The present study was conducted at the experimental plot on 4-year-old guava plants, which were uniform in size and vigor, and spaced 7.0 m between the rows and 7.0 m between plants. The experimental unit consisted of one guava tree. A completely randomized design was used for the experiment with six replications of each treatment [41]. The completely randomized design was chosen as it is a statistical technique that reduces and controls the variance of the experimental error to achieve greater precision in the response variables. In this experiment, the effects of five treatments on the growth of guava plants were evaluated. Treatments consisted of: T1: [S. mexicanum ITTG-R7T], T2: [S. mexicanum ITTG-R7T + Triple 17 (17% N, 17% P, and 17% K) + Diammonium phosphate (18% N, 20% P) + Nitrabor® (15.4% N + 25.9% CaO + 0.3% B)], T3: [Azospirillum brasilense CD] as a PGPB reference, T4: [(Triple 17 (17% N, 17% P, and 17% K) + Diammonium phosphate (18% N, 20% P) + Nitrabor® (15.4% N + 25.9% CaO + 0.3% B)], and T5: [non-inoculated and non-chemically fertilized plants] as the negative controls. Mineral N fertilizer in the form of urea (46.5% N) was added. Superphosphate fertilizer (15.5% P2O5) was applied at a rate of 300 kg ha−1, and potassium fertilizer at a rate of 125 kg ha−1 in the form of potassium sulfate (48% K2O). Plants treated with chemical fertilizers served as the positive controls. The strain A. brasilense CD which is commercially available, was applied in a separate treatment (T3) as a nitrogen-fixing bacterium commercially used.

2.7. Bacterial Inoculation in Guava Crops

For bacterial inoculation, S. mexicanum ITTG-R7T and Azospirillum brasilense CD strains were grown in nutrient broth at 28 °C and diluted to a final concentration of 106 CFU mL−1 [42]. Plants were inoculated with 100 mL of bacterial suspension, which was applied directly to the plant base near the root zone. The treatments were applied by the ring method 70 cm away from tree trunk. Every 3 months, the plants were inoculated over an experimental period of 9 months. In the same way, triple 17 fertilizer, diammonium phosphate, and nitrabor were applied to the plants using a fertigation system and according to the phytotechnical procedure used by the farmer. At the end of the experiment, the tree heights were recorded. The tree basal perimeter was measured and the coverage of each tree was registered by measuring the length between the point to which the branches had grown in the east–west and north–south directions, obtaining an average. The number of flowers was counted by selecting four branches in each direction of the tree; then, the average was calculated and expressed as the number of flowers per branch. The total chlorophyll contents were determined according to the procedure followed by Rincón-Molina et al. [13]. The total number of fruits per tree was counted at the end of the experiment.

2.8. Statistical Analysis

Data obtained from the inoculation test were analyzed by ANOVA at a significance level of alpha = 0.05 by using the statistical software Statgraphics Centurion XV.2 (The Plains, VA, USA). The comparison of means was carried out by the Tukey test (p < 0.05).

3. Results

3.1. Characteristics of the Agricultural Field

The measurements of the normalized differential vegetation index (NDVI) by Auravant® indicated values ranging between 0.3380 ± (0.0573) and 0.6088 ± (0.0816) prior to the application of the treatments in the crops (Figure 2A). After the application of treatments and before the end of the experiment, NDVI values between 0.4127 ± (0.0263) and 0.7052 ± (0.0350) were registered (Figure 2B).

3.2. Physicochemical Soil Characteristics

Different physicochemical parameters were evaluated in the guava crop soil sample, which was silty clay loam. The pH was near-neutral [6.4 ± (0.012)] and the electrical conductivity (EC) was 0.91 ± (0.015) dSm−1. The cation exchange capacity (CEC) value was 21.51 ± (0.018) Cmol kg−1. In relation to the fertility parameters N, C, and P, it was found that the amount of total N was 0.16% ± (0.020), 0.87% ± (0.015) for organic C content, and 43.04 ± (0.990) mg kg−1 of available P. The C:N ratio is considered as an important parameter related to soil fertility. In this study, the soil of the guava crop had a low value of the C:N ratio (5.4 ± 0.15) according to the Official Mexican Standard NOM-021-SEMARNAT-2000, indicating a rapid mineralization and release of N, which is available for plant uptake.

3.3. Bacterial Community Structure

The bacterial community structure of the guava crop (P. guajava cv) rhizospheric soil is shown in Figure 3. The taxonomic classification by 16S rRNA sequence analysis showed that Actinobacteria was the most abundant phylum (>20% relative abundance) followed by Proteobacteria, Acidobacteria, and Firmicutes (Figure 3A). In contrast, Bacillaceae was the most dominant family in the agricultural soil sample, followed by Vicinamibacteraceae, Sphingomonadaceae, Rubrobacteriaceae, Nitrosomonadaceae, Micromonosporaceae, Gemmatimonadaceae, and Gaiellaceae (Figure 3B). Gaiella (~12% relative abundance) and Bacillus (>4% relative abundance) were the most abundant genera (Figure 3C).

3.4. PGPB Efficiency by Sinorhizobium ITTG-R7T Strain

S. mexicanum ITTG-R7T and Azospirillum brasilense CD (used as the reference PGPB) showed biochemical characteristics associated with plant growth promotion (Table 1). Bacterial strains had the capacity to solubilize inorganic phosphate. These strains formed clear zones (solubilization halos) around the colonies (Figure S1). The phosphate solubilization index was higher in S. mexicanum ITTG-R7T (2.98 ± 0.10). Strains had the ability to synthesize indole-3-acetic acid (IAA). Additionally, the strain ITTG-R7T exhibited a higher nitrogenase activity (ARA) around 3.21 nmol C2H4 per culture h−1 in comparison with the strain A. brasilense CD. The two strains showed ACC deaminase activity and an abundant production of siderophores. In relation to the extracellular enzymes, ITTG-R7T produced protease, amylase, lipase, and cellulases. With respect to the strain A. brasilense CD, it only had the capacity to produce amulase and cellulase.

3.5. Effect of Bacterial Inoculation in Guava Development

In this experiment, the application of biofertilizers formulated with the strain S. mexicanum ITTG-R7T had a positive effect on the growth of guava plants (Table 2). The plants inoculated with ITTG-R7T strain [Treatment T1] had a higher height compared to the other treatments. The application of the ITTG-R7T strain alone or mixed with the chemical fertilizer (Triple 17 + diammonium phosphate + Nitrabor) had a significant effect (p < 0.05) on the foliar cover of plants. The basal diameter of the stem increased in those plants that were treated with both the fertilizer (Treatment T2) as well as with the biofertilizer [Treatment T1]. A significant effect (p < 0.05) was observed in the plants treated with the ITTG-R7T strain in relation to the number of flowers. Trees inoculated with S. mexicanum ITTG-R7T strain as well as those chemically fertilized registered a significant increase in the number of fruits compared to the control plants (without fertilization/uninoculated). The amount of total chlorophyll was higher in the plants inoculated with S. mexicanum ITTG-R7.

4. Discussion

Legume inoculation with rhizobia is a common agricultural practice, especially for soybean in the USA, Brazil, and Argentina [43]. Commercial production of rhizobial inoculants occurs worldwide and in Brazil, it has been estimated that soybean Bradyrhizobium inoculants allow for savings of billions of dollars when used instead of chemical nitrogen fertilizers [44]. Inoculation is successful in promoting crop yields mainly in regions where the specific legume rhizobia are not present. We found that rhizobia are scarce in soil in Chiapas where guava is grown, and this would allow the effects of sinorhizobial inoculation to be clearly observed, as reported here. While rhizobial inoculation is common in legumes, it is not usually tested in non-legumes, which are more commonly inoculated with Azospirillum, which is a well-known plant growth promoting rhizobacteria (PGPR) producing auxins. However, rhizobial inoculation has been found to increase tomato [45] and cereal growth [46,47]. The beneficial effects of the S. mexicanum ITTG-R7T strain could be due to auxin production, as occurs in Azospirillum or ACC deaminase, as found in Burkholderia and other PGPRs. The positive effect on growth in guava (P. guajava cv.) was observed with the inoculation of ITTG-R7T mixed with chemical nutrients (Triple 17 + diammonium phosphate + Nitrabor). Similar effects have been found in fruit crops inoculated with Azospirillum brasilense. The auxins produced by the bacteria stimulate the increase in root mass and this improves the efficient use of chemical fertilizers [24,48]. For fruit crops, it has previously been reported that combinations of biofertilizers with nitrogen and other nutrients provide the best effects on plant development, and thus, on yields [49,50]. Bacteria as biofertilizers with plant growth potential when combined with inorganic and organic inputs help with crop nutrient uptake; thus the results suggest that S. mexicanum ITTG-R7T plays a key role in nutritional improvement. Plants inoculated only with the ITTG-R7T strain had the highest positive effect and were statistically similar to those obtained with chemical fertilizers, but higher than the results obtained with the application of the commercial strain A. brasilense CD. The superior effect obtained by S. mexicanum ITTG-R7T compared with A. brasilence CD can be attributed to the fact that ITTG-R7T is a native strain, adapted to the environmental conditions of the area [19,21]. These findings are important because they ensure the use of the strain ITTG-R7T as an efficient inoculant, which can be compared with commercial products. Similarly, it suggests a gradual reduction in the application of chemical fertilizers in guava crop management.
The long search of PGPRs in different research groups has revealed many bacterial candidates that are capable of promoting growth in distinct plant species. Among the many explored bacteria, rhizobia are considered among the favorite because they have been used for more than hundred years in agriculture and they are not pathogens. In addition, rhizobia may help to solubilize phosphate, which would also contribute to plant growth. Considering the PGPB multifunctional qualities observed in the S. mexicanum ITTG-R7T strain (such as N2 fixation, phosphate solubilization, auxin synthesis, among others) and based on the results obtained from the inoculation test, the use of this bacterium can represent an efficient, simple and low-cost agrobiotechnological alternative for fruit producers in areas of low income and poorly developed agriculture. Sinorhizobium mexicanum ITTG-R7T was isolated from the legume Acaciella angustissima and described as a new species of Sinorhizobium [19]. This bacterium is characterized by its high capacity to fix nitrogen, it solubilizes phosphate, and synthesizes auxins. We observed that ITTG-R7T was better than Azospirillum brasilense CD (used commercially as a biofertilizer) in relation to the ability to solubilize phosphate and fix N2. ITTG-R7T possesses in its genome genes related to these PGPB qualities such as nifH genes that are involved in the biosynthesis of the iron–molybdenum cofactor (FeMo–Co) required for nitrogenase [22]. They also contain three clusters of phn genes that confer the ability to degrade phosphonates (organophosphorus compounds). ITTG-R7T may contribute to the mineralization of phosphorus to bioavailable chemical forms for plants.
The measurements of the NDVI obtained through satellite images indirectly serve to assess the crop health from the analysis of observable biomass in the crop field (plant leaves). NDVI values between 0.2 and 0.4 correspond to areas with scarce vegetation; moderate vegetation tends to vary between 0.4 and 0.6; indices above 0.6 indicate the highest possible density of green leaves [51,52]. Through the satellite images, it was also possible to determine the initial status of the agricultural field; for example, if there was waterlogging or if any area of the land suffered from drought. A color change in the plots was observed during the assay, which might indicate leaf loss or plant wilting. Lack of water was detected before the rainy season. Some authors have reported that through technological innovations such as satellite information, data can be generated in real-time, which would allow farmers to make the best decisions for the sustainable management of their crops [53,54].
The soil sample was moderately acid (pH 6.4). A moderate CEC might indicate the retention and mobility of nutrients, the presence of weatherable primary minerals, and the accumulation of minerals. Similarly, the EC determined in the soil sample with values less than 1.0 dSm−1 indicated negligible salinity effects that could be due to chemical fertilizers. A low percentage of organic C content (0.87%) was determined, which may be the product of slow plant material decomposition. The amounts of total N and available P were found in high quantities. The P content in the rhizosphere samples are high, indicating that a mineralization process is taking place due to microbial activity. Regarding the C/N ratio, the physicochemical analysis showed a low value. These parameters are related to fertility and influence soil functionality.
The 16S rRNA gene sequence analysis showed that the Bacillaceae family dominated in soil samples. Gaiellaceae and Vicinamibacteraceae, which were also found, are related to carbon transformation in soil. Bacteria within the Sphingomonadaceae family are associated with soil bioremediation (the degradation of phenols and other aromatic compounds) [55,56]. They exhibit antagonistic activity against pathogens, and some genera within this group induce plant growth promotion, although they can also be considered as pathogenic for plants and humans. Additionally, members of Gemmatimonadaceae are generally present in sewage/sludge, they exhibit phosphate removal activity, and are drought-tolerant (sandy soils) [57,58]. In this study, we proposed to complement the physicochemical analyses with assessments of the structure and diversity of the bacterial communities present in agricultural soils. We consider that both perspectives represent a broad vision to determine the health and quality of the soil; additionally, we could ascertain the microorganisms with which the biofertilizers will have to compete for nutrients. There are currently no reports regarding the bacterial diversity in guava crop soils in this area of Mexico; in addition, this analysis enables future projects to infer the possible biochemical functionalities that the different types of bacteria may be performing.

5. Conclusions

Bacteria associated with fruit plants with PGPB properties represent an alternative to enhance the management of agricultural crops in rural vulnerable zones in Mexico. In this study, the native strain S. mexicanum ITTG-R7T emerged as a bacterial species with various biological qualities that promote plant growth. It is highlighted that the use of this strain as a biofertilizer can contribute to the growth of guava plants and to improve the quality and health of the soil. Likewise, the combination of biofertilizers formulated with S. mexicanum and nitrogenous fertilizers showed important effects on plant growth, which is interesting for the formulation of nutrients with greater efficiency for this type of fruit crops. This study is part of a current research in Chiapas, Mexico that promotes the application of safe microbial inoculants in fruit growing, the societal appropriation of biotechnologies, and their access to information.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/su141912391/s1, Figure S1: Phosphate solubilization by Sinorhizobium mexicanum ITTG-R7T.

Author Contributions

R.R.-R. and C.I.R.-M. designed the study. C.I.R.-M. and L.A.M.-G. performed the laboratory experiments and data analysis. L.A.M.-G. contributed the new reagents and analytical tools. C.I.R.-M. and R.R.-R. performed the data analysis. C.I.R.-M., E.M.-R., and R.R.-R. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support was received from Tecnológico Nacional de México Nos. 13529.22-P and 14094.22-P.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors declare that all relevant data supporting the findings of this study are included in the article.

Acknowledgments

We thank CONACYT for granting a postdoctoral scholarship to Clara I. Rincón. We especially thank Julio Martínez for reviewing the manuscript. We thank Manuel Jimenez from the Rio Grande Ranch for allowing us to carry out the experiments at the cultivation field.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The experimental site “Río Grande” at Ribera de Monte Rico (Nacamucuyi), Chiapas, Mexico.
Figure 1. The experimental site “Río Grande” at Ribera de Monte Rico (Nacamucuyi), Chiapas, Mexico.
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Figure 2. Images of the guava crops for the normalized differential vegetation index (NDVI) measures. (A) NDVI values before the application of treatments; (B) NDVI values after the application of treatments.
Figure 2. Images of the guava crops for the normalized differential vegetation index (NDVI) measures. (A) NDVI values before the application of treatments; (B) NDVI values after the application of treatments.
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Figure 3. The bacterial community structure of the rhizospheric soil of the guava crop (P. guajava cv). (A) Relative abundance at phyla level. (B) Relative abundance at the family level. (C) Relative abundance at genera level.
Figure 3. The bacterial community structure of the rhizospheric soil of the guava crop (P. guajava cv). (A) Relative abundance at phyla level. (B) Relative abundance at the family level. (C) Relative abundance at genera level.
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Table
Table 1. The plant growth promotion activities of S. mexicanum ITTG-R7T.
Table 1. The plant growth promotion activities of S. mexicanum ITTG-R7T.
StrainP Solubilization
Index		IAAARA ¥ACC
Deaminase		 Extracellular Enzymes
SiderophoreProteaseAmylaseLipaseCellulase
S. mexicanum
ITTG-R7T		2.98 ± (0.10) ≠+3.21 ± (0.16)++++++
A. brasilense
CD		2.42 ± (0.52)+2.92 ± (0.23)++−+−+
+: Positive activity; −: Negative activity; ≠ Mean values of three replicates. The values in parenthesis are standard deviations; ¥ ARA, acetylene reduction assay (nmol C2H4 per culture fresh weigh h−1).
Table
Table 2. The growth parameters for guava (P. guajava cv.) plants inoculated with S. mexicanum ITTG-R7T.
Table 2. The growth parameters for guava (P. guajava cv.) plants inoculated with S. mexicanum ITTG-R7T.
TreatmentTotal
Plant
Height
(cm)		Foliar
Cover
(cm)		Basal
Diameter
(cm)		Number
of Flowers		Number of FruitsTotal
Chlorophyll (mg mL−1)
T1: [S. mexicanum ITTG-R7T]368.83 A *553.16 A97.83 AB36.0 A63.33 A2.81 A
T2: [S. mexicanum ITTG-R7T + Triple 17 + Diammonium phosphate + Nitrabor]327.5 B516.83 AB107.16 A29.16 B52.66 AB3.70 B
T3: [A. brasilense CD]312.67 B462.33 B64.0 C21.66 C34.83 BC2.65 B
T4: [Triple 17 + Diammonium phosphate + Nitrabor]336.33 B475.5 B86.16 B23.5 BC50.33 AB2.70 B
T5: [Control (non-inoculated, non-fertilized)]267.5 C375.0 C57.0 C19.0 C24.83 C2.04 C
p-value0.00000.00000.00000.00360.00010.0000
HSD £ (p <0.05)24.727558.489612.09036.729920.07750.3114
* Mean values of six replicates. Means followed by the same letter are non-significant (Tukey test, p < 0.05). £ HSD = honest significant difference.
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Rincón-Molina, C.I.; Martínez-Romero, E.; Manzano-Gómez, L.A.; Rincón-Rosales, R. Growth Promotion of Guava “Pear” (Psidium guajava cv.) by Sinorhizobium mexicanum in Southern Mexican Agricultural Fields. Sustainability 2022, 14, 12391. https://0-doi-org.brum.beds.ac.uk/10.3390/su141912391

AMA Style

Rincón-Molina CI, Martínez-Romero E, Manzano-Gómez LA, Rincón-Rosales R. Growth Promotion of Guava “Pear” (Psidium guajava cv.) by Sinorhizobium mexicanum in Southern Mexican Agricultural Fields. Sustainability. 2022; 14(19):12391. https://0-doi-org.brum.beds.ac.uk/10.3390/su141912391

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Rincón-Molina, Clara Ivette, Esperanza Martínez-Romero, Luis Alberto Manzano-Gómez, and Reiner Rincón-Rosales. 2022. "Growth Promotion of Guava “Pear” (Psidium guajava cv.) by Sinorhizobium mexicanum in Southern Mexican Agricultural Fields" Sustainability 14, no. 19: 12391. https://0-doi-org.brum.beds.ac.uk/10.3390/su141912391

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