Exogenous Salicylic Acid Optimizes Photosynthesis, Antioxidant Metabolism, and Gene Expression in Perennial Ryegrass Subjected to Salt Stress

Salicylic acid (SA) is a plant growth regulator that can enhance the abiotic stress tolerance of plants; however, the physiological mechanisms are not yet fully understood. The objective of this study was to examine whether exogenous SA could enhance the salt tolerance of perennial ryegrass (Loliumperenne L.; PRG) and investigate the physiological underlying mechanisms. SA was applied to the foliage of PRG at five concentrations (0, 0.25, 0.5, and 1 mM). The SA-treated grass was grown under either control (0 mM NaCl) or salt stress (250 mM NaCl) conditions for 24 d. The SA treatments reduced the leaf electrolyte leakage (EL), malonaldehyde (MDA), and hydrogen peroxide (H₂O₂) content by 36%, 41%, and 40%, respectively, relative to the control under salt stress as measured at 24 d. The SA treatments also alleviated the decline in the leaf photosynthetic rate (Pn), stomatal conductance (gs), nitrate activity (NR), turfgrass quality (TQ) ratings, and chlorophyll (Chl) content under salt stress. In addition, exogenous SA increased the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), and peroxidase (POD) as well as the expression levels of the Cyt Cu/ZnSOD, FeSOD, APX, CAT, and POD genes under salt stress. The results of this study suggested that the foliar application of SA at 0.25 and 0.5 mM may enhance photosynthesis and antioxidant defense systems and thus improve tolerance to salt stress in perennial ryegrass.