Effects of Plant Growth-Promoting Rhizobacteria on the Physioecological Characteristics and Growth of Walnut Seedlings under Drought Stress

Drought is one of the most brutal environmental factors limiting the productivity of fruit trees. The search for new and efficient microorganisms from unexplored environments that can be used to mitigate the negative effects of water stress is an interesting alternative to alleviate the drought stress experienced by plants. This study aimed to determine the effects of PGPR inoculation on the growth and physioecological characteristics of walnut (Juglans regia) seedlings under drought stress. A pot experiment was conducted using J. regia seedlings with controlled water supplies at different levels (light, moderate, and severe drought stress and control) and with or without inoculation with Bacillus cereus L90, a type of PGPR that produces high levels of cytokinins and indoleacetic acid (IAA). Under well-watered conditions, there was no obvious effect of PGPR inoculation on the antioxidant enzyme activities, osmotic adjustment levels, and photosynthetic characteristics of J. regia. As the stress intensity increased, B. cereus inoculation increased the antioxidant enzyme activities in walnut seedlings and changed their photosynthetic characteristics. However, levels of osmotic adjustment substances were decreased as a result of PGPR inoculation. Regardless of water status, B. cereus inoculation induced a significant increase in IAA, gibberellins, and zeatin contents in J. regia. Under well-watered and light stress conditions, the abscisic acid content of walnut was significantly increased by B. cereus inoculation. Additionally, B. cereus inoculation significantly promoted the growth of plants in terms of ground diameter and plant height. As a result, PGPR inoculation could improve the drought resistance of J. regia and improve its photosynthetic characteristics and growth, suggesting that it is a useful supplementary measure for use in afforestation in arid and semiarid environments.