Synergistic effect of extra potassium application and AM fungi on drought tolerance of Lycium barbarum

Drought stress, one of the major abiotic stresses, affects plants growth by changing root morphology, impacting leaf water status and photosynthesis, and interrupting the reactive oxygen species (ROS) balance in plants. Arbuscular mycorrhizal (AM) fungi and extra potassium can promote plants growth under drought stress. However, it is not clear whether AM fungi and extra potassium application would affect the photosynthesis and antioxidant system, which help Lycium barbarum to alleviate drought stress. In this study, the synergistic effects of AM fungus (Rhizophagus irregularis) and extra potassium (0 mM, 3 mM, and 10 mM K+) on growth, photosynthesis, leaf water status, root morphology, activities of antioxidant enzymes, concentrations of antioxidant substances, and expression of genes of L. barbarum under drought stress (30% field capacity) were evaluated. Under drought stress, inoculation of R. irregularis and application of potassium significantly increased the photosynthesis of L. barbarum and the expression of LbrbcL and LbrbcS, which might be due to the significant increase of the root growth and leaf relative water content. Moreover, the activities of antioxidant enzymes, concentrations of antioxidant substances, and the expression of three genes encoding superoxide dismutase (SOD) were increased. The concentration of proline and expression of LbP5CS were significantly up-regulated by potassium application and down-regulated by R. irregularis inoculation. The concentrations of malondialdehyde (MDA) and hydrogen peroxide (H2O2) were significantly decreased. The results suggested that R. irregularis and extra potassium have a synergistic effect on the enhancement of L. barbarum against drought tolerance through reduction of ROS accumulation and oxidative damage by improving photosynthetic and antioxidant capacity.