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.

Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that survive within the range of plant rhizosphere and can promote plant growth. The effects of PGPR in promoting plant growth, activating soil nutrients, reducing fertilizer application, and improving the resistance of plant inducible system have been widely investigated. However, few studies have investigated PGPR as elicitors of tolerance to abiotic stresses, especially drought stress. In this study, the effects of Acinetobacter calcoaceticus X128 on the photosynthetic rate (P_n), stomatal conductance (G_s), intracellular CO₂ concentration (C_i), and total chlorophyll content [Chl(a+b)] of Sambucus williamsii Hance seedling leaves under moderate drought stress and drought-rewatering conditions were determined. Compared with those of uninoculated seedlings, the average P_n values during the entire drought stress of inoculated seedlings increased by 12.99%. As the drought duration was lengthened, C_i of uninoculated leaves continued to increase after rapidly declining, whereas G_s continuously decreased. Furthermore, their photosynthetic properties were simultaneously restricted by stomatal and non-stomatal factors. After X128 inoculation, C_i and G_s of S. williamsii Hance leaves continued to decrease, and their photosynthetic properties were mainly restricted by stomatal factors. At the end of the drought stress, water stress reduced [Chl(a + b)] of S. williamsii Hance leaves by 13.49%. However, X128 inoculation decreased this deficit to only 7.39%. After water supply was recovered, P_n, G_s, and [Chl(a+b)] in uninoculated leaves were reduced by 14.23%, 12.02%, and 5.86%, respectively, relative to those under well-watered conditions. However, C_i increased by 6.48%. Compared with those of uninoculated seedlings, P_n, G_s, and [Chl(a+b)] in X128-inoculated seedlings were increased by 9.83%, 9.30%, and 6.85%, respectively. Therefore, the inoculation of X128 under arid environments can mitigate the reduction of chlorophyll, delay the restriction caused by non-stomatal factors to P_n in plant leaves under water stress, and can be more conducive to the recovery of photosynthetic functions of leaves after water supply is recovered.

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.

This study aimed to determine the effects of root pruning on the physicochemical characteristics and microbial activities of poplar rhizosphere soil. The root systems of 5-year-old poplar (Populus×euramericana cv. ‘Neva’) trees were manually pruned at 6, 8, or 10 times diameter at breast height (DBH) from the trunk (severe, moderate, and light, respectively) along both inter-row sides. Moderate root pruning significantly increased the concentrations of amino acids, organic acids, and total sugars in the root exudates and decreased the pH of rhizosphere soil. This treatment also increased the contents of available nitrogen, phosphorus, potassium, and total organic carbon as well as high-, medium-, and low-activity organic carbon in rhizosphere soil. Moreover, moderate pruning increased the contents of microbial biomass carbon and nitrogen, and enhanced basal respiration, in addition to decreasing the metabolic quotients in rhizosphere soil by 8.9%, 5.0%, and 11.4% compared with control, light, and severe root pruning treatments, respectively. Moderate pruning increased the growth rates of DBH, tree height, and volume to the highest levels. Furthermore, these indices were not significantly different between the light root pruning and control groups, but varied significantly between severe and moderate root-pruning treatments. Thus, root pruning, depending on the distance from the trunk, significantly influences the physicochemical properties and microbial activities in poplar rhizosphere soil.