Effects of Bacillus amyloliquefaciens QST713 on Mineral Nutrient Utilization of Alfalfa under Drought Stress

Drought stress is one of the major impediments to plant growth. Plant growth-promoting rhizobacteria (PGPR) can mitigate moisture stress in plants by increasing the ability of plant nutrient uptake and transport. In this study, we investigated the root phenotype, mineral nutrients (in leaves, roots, and soil), soil pH, water saturation deficit (WSD), free water content (FWC), and bound water content (BWC) of leaves of two alfalfa varieties, ‘Galalxie Max’ (drought-tolerant) and ‘Saidi 7’ (drought-sensitive), in the presence or absence of Bacillus amyloliquefaciens QST713 under drought stress conditions. The results showed that water stress negatively affected both cultivar root morphology (total root length, average diameter, total surface area, and volume) and the contents of K and Fe in leaves, roots, and soil. It also reduced the Mn and Zn contents in the soil while increasing the content of Na in the leaves and soil. Additionally, alfalfa plants under drought stress exhibited higher levels of soil pH, WSD, and BWC but lower contents of FWC and ratios of BWC/FWC in the leaves of both cultivars. However, QST713 application significantly enhanced the total root length, average root diameter, and the contents of K and Fe in alfalfa leaves, roots, and soil, as well as the BWC/FWC ratio in leaves under drought stress conditions. A significant reduction in the Na content was detected in QST713-treated alfalfa leaves and soil under drought stress. Furthermore, QST713 application noticeably decreased soil pH and WSD. The current findings showed that QST713 enhanced the water stress tolerance of alfalfa plants by ameliorating root morphology, reducing soil pH, and improving the BWC/FWC ratio, consequently promoting the accumulation of mineral nutrients (mainly K and Fe). Overall, Bacillus amyloliquefaciens QST713 can serve as a potential green fertilizer in sustainable agriculture to improve soil nutrients and enhance plant production under increasing drought conditions.