Effects of Inoculation with Different Plant Growth-Promoting Rhizobacteria on the Eco-Physiological and Stomatal Characteristics of Walnut Seedlings under Drought Stress

Plant growth-promoting rhizobacteria (PGPR) can promote plant growth and induce systemic resistance to biological and abiotic stresses. However, do all PGPR have significant effects in arid environments, and which PGPR have the most optimal effects? This study used a pot experiment to investigate the effects of inoculation with two different PGPR on the physiological and ecological characteristics of walnut (Juglans regia) seedlings under drought stress: Bacillus subtilis GE1, which secretes protease only, and Pseudomonas brassicacearum X123, which secretes protease and indoleacetic acid (IAA). The leaves inoculated with X123 under drought stress had higher net photosynthetic rate (P-n), transpiration rate (T-r), and stomatal conductance (g(s)), especially stomatal length and stomatal width, compared to GE1 inoculation under drought stress. Moreover, inoculation with X123 significantly increased the leaf superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities by 20.84% and 12.41%, respectively, and the gibberellin (GA) and zeatin (ZT) contents by 72.07% and 19.17%, respectively, whereas the leaf soluble sugar and soluble protein contents significantly decreased compared with GE1 inoculation. These results indicated that the effects of GE1 inoculation on the physiological and ecological characteristics of walnut seedling leaves were significantly weaker in comparison with X123 inoculation as a result of its functional characteristics. The application of different PGPR on the drought tolerance of J. regia showed significant differences. Therefore, the selection of appropriate PGPR is key to achieving positive treatment effects under drought conditions.