Document Type : Research Paper
Authors
1 Department of Soil Science, Faculty of Agriculture, Shahid Bahonar University of Kerman, Kerman, Iran.
2 Department of Soil Science, Faculty of Agriculture, Vali-e-asr University of Rafsanjan, Kerman, Iran.
Abstract
Introduction: Plants are usually exposed to a wide variety of abiotic stresses which can seriously inhibit plant growth and development. To address this, numerous strategies have been proposed and used by researchers, including increased irrigation rounds, cultivation of salinity- and drought-resistant genetically modified crops (GMO crops), and application of plant-growth-promoting rhizobacteria (PGPRs). PGPRs can act as an efficacious, long-lasting, and crucial option to ameliorate the negative impacts of abiotic stresses in crops. Plant growth promoting rhizobacteria can serve as key in sustainable agriculture by improving soil fertility, plant tolerance, crop productivity, and maintaining a balanced nutrient cycling. Consequently, search for new strains of PGPR for biofertilizer and development of microbial diversity map for any region is helpful. Hence, the present study investigates the effect of native salinity-resistant PGPRs on the physiological and biochemical characteristics and productivity of alfalfa plants in soil under salinity stress.
Materials and methods: The present study was based on a completely randomized factorial design. The experiments were conducted on alfalfa plants (Bami variety) in four repetitions and under two levels of salinity (control and 200 mM sodium chloride and calcium chloride) with five strains of PGPRs from Sinorhizobium meliloti sp., two positive controls (i.e., 20 mg/kg of phosphorus and 70 mg/kg of nitrogen fertilizers), two negative controls (i.e., no fertilizer and no bacteria), and two treatments (including positive control and negative control). Growth parameters (dry weight of aerial parts, roots, and nodules), osmolytes (reducing sugars, soluble proteins, and proline), uptake of K+ and K+/Na+ ratio, and concentration of malondialdehyde (MDA) in alfalfa plants in non-saline and saline soils were measured at the end of 60-day experiments.
Results and discussion: The analysis of variance (ANOVA) results revealed a significant effect of salinity on the dry weight of aerial parts and roots, the weight and number of nodes in each pot, the K+/Na+ ratio in roots and aerial parts, and the concentration of reducing sugars, proline, MDA, and soluble proteins in the aerial parts. The effect of PGPRs was also found to be significant on all the above traits. Under no salinity stress and compared to negative control plants, the dry weight of aerial parts in plants inoculated with superior PGPRs (SM89, SM16, and SM65) was raised by 2.3, 1.9, and 1.8 folds, while this increase in plants inoculated with mild (SM73) and weak (SM21) PGPRs was 1.4 and 1.2 folds, respectively. Under salinity stress (200 mM NaCl and CaCl2) and compared to negative control plants, the increase in dry weight of aerial parts of plants inoculated with superior PGPRs (SM89, SM16, and SM65) was increased by 4.2, 4, and 2.1 folds, while this increase in plants inoculated with mild (SM73) and weak (SM21) PGPRs was raised by 1.7 and 1.2 folds, respectively. Despite a drop in the growth of aerial parts in plants under salinity, salinity-resistant PGPRs were able to significantly enhance the growth of aerial parts of plants in saline conditions compared to the controls (receiving no fertilizer and PGPRs). Salinity stress reduced other growth parameters, the rate of K+ uptake, and the K+/Na+ ratio, while it contrarily increased the concentration of reducing sugars, soluble proteins, proline, Na+, and MDA in plants. Inoculation of alfalfa plants with two superior PGPRs (SM89 and SM16) was found to significantly improve growth parameters, uptake of K+, osmolytes, and K+/Na+ ratio in alfalfa plants under non-saline conditions and salt stress, compared to control plants (not inoculated with PGPRs and receiving no fertilizer). Ultimately, each inoculation of plants with all three superior PGPRs reduced the concentration of MDA and Na+ in alfalfa plants.
Conclusion: Experiments on the biochemical and physiological plant–PGPR interactions revealed that plant responses to stresses are largely controlled by microbial communication. PGPRs can trigger systemic resistance in plants through their metabolites, which function as extracellular signals, thereby enabling plants to survive under abiotic stresses. In the present study, microbial inoculation was found to significantly improve the physiological functioning of the plants. The results revealed that adding native salinity-resistant PGPRs to the soil can diminish the negative effects of salinity stress on alfalfa plants. Likewise, inoculation and enrichment of the plant's rhizosphere with beneficial and resistant microbiomes were efficient for sustaining the growth of plants under abiotic stresses such as salinity.
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