Silicon Improves Plant Growth-Promoting Effect of Nodule Non-Rhizobial Bacterium on Nitrogen Concentration of Alfalfa Under Salinity Stress

Salinity stress is one of the important factors reducing the growth and yield of crops such as legumes in many parts of the world, especially in arid and semi-arid regions. The combined use of silicon (Si) and plant-associated beneficial bacteria in mitigating environmental stresses including salinity has gained importance in recent years. However, there is limited information on the plant growth-promoting (PGP) potential of the non-rhizobial bacteria that colonize within root nodules of legumes alone or in the combination with Si to improve the tolerance of legumes to salinity stress. In this study, the PGP potential of the non-rhizobial bacteria of root nodules alone or in the combination with Si to alleviate salinity stress in alfalfa plant was investigated. Based on the results of a series of evaluations, among 60 bacterial isolates, isolated from the root nodules of alfalfa plants irrigated with saline water (EC, 8.60 dS m−1), two bacterial isolates, 40 K and N44, were selected as effective non-rhizobial strain and rhizobial one, respectively, for further study. A pot-experiment in a completely randomized design with factorial arrangement in three repetitions under greenhouse conditions was adopted to assay the effect of non-rhizobial strain 40 K alone or in combination with Si on the growth of alfalfa plant inoculated with its rhizobial symbiosis, N44 strain, under salinity stress. Experimental treatments included: Si factor at three levels of 0, 1, and 4 mM Si from potassium silicate source, nodule non-rhizobial strain factor at two levels of B0 (control), and non-rhizobial strain 40 K, and salinity (saline water) factor at three levels of 0, 5, and 15 dS m−1. Salinity stress reduced plant biomass, chlorophyll content, and the concentration of potassium and nitrogen of plant and increased the concentration of sodium in shoot tissue and the activity of antioxidant enzymes (catalase and superoxide dismutase) in alfalfa plant. However, the use of non-rhizobial strain and Si improved the morphological and physiological characteristics of the plant, reduced the plant sodium concentration, and increased the potassium concentration of alfalfa plants. The highest improvement in plant growth indices (shoot N concentration) was recorded in plants inoculated with non-rhizobial strain in the presence of Si at a concentration of 4 mM. Based on 16S rRNA sequence analysis, 40 K and N44 isolates were identified as Advenella incenata and Ensifer meliloti, respectively. The results of this study showed that Si fertilization can be suggested as a sustainable strategy for improving the plant growth promoting effects of non-rhizobial bacteria on the growth of alfalfa (increased shoot N concentration) and encouraging ecological adaptability of this plant under saline water irrigation.