Arbuscular Mycorrhizal Fungus and Exogenous Potassium Application Improved Lycium barbarum Salt Tolerance

Salt stress is one of the major abiotic stress, impedes plant photosynthetic processes, changes root architecture to impact leaf water status, and reduces potassium uptake and K+/Na+ ratio. Arbuscular mycorrhizal (AM) fungus and extra potassium promote plants tolerance of salt stress, respectively. However, little is known about the combined influence of AM fungus and extra potassium under salt stress. In current study, we analyzed the effects of AM fungus (Rhizophagus irregularis), potassium application (0, 1.6, and 6.4 mM K+), and salt stress (0 and 100 mM NaCl) on photosynthesis, leaf water status, root architecture, concentrations of Na+ and K+, shoot/root Na+, K+/Na+ homeostasis, and the relative expression of genes related to K+ uptake and transport (LbHAK, LbKT1, and LbSKOR) of Lycium barbarum. Under salt stress, inoculation of R. irregularis and application of potassium increased the net photosynthetic rate and stomatal conductance and reduced the intercellular CO2 concentration to improve photosynthesis. Inoculation of R. irregularis and application of potassium increased leaf relative water content and reduced water saturation deficit. Inoculation of R. irregularis and potassium application also modified root architecture, particularly in terms of root elongation and SRL reduction. Moreover, they increased K+ concentration, but evidently reduced Na+ transport to shoot. Regardless of salinity, AM plants had a significant decrease in shoot/root Na+ ratio compared with NM plants under each potassium condition. Additionally, R. irregularis and extra potassium upregulated the relative expressions of LbHAK, LbKT1, and LbSKOR, which are involved in K+/Na+ homeostasis. This study suggests that the beneficial effects of R. irregularis and extra potassium on photosynthetic capacity, root architecture, and K+/Na+ homeostasis improved the growth and salt tolerance of L. barbarum under salt stress.