Effects of salicylic acid on heat tolerance associated with antioxidant metabolism in Kentucky bluegrass

Turf quality of Kentucky bluegrass (Poa pratensis L.) often declines during summer when temperatures exceed its optimum range. This study was designed to determine whether application of salicylic acid (SA) to the shoots and soil could improve heat tolerance of Kentucky bluegrass, and to investigate whether SA-induced heat tolerance is related to changes in antioxidant activities. Effects of SA at different concentrations (0, 0.1, 0.25, 0.5,1, and 1.5 mmol) on heat tolerance were examined in Kentucky bluegrass exposed to 46°C for 72 h in a growth chamber. Influences of SA on the production of active oxygen species (AOS), superoxide anion (O₂^.-), and hydrogen peroxide (H₂O₂), and activities of antioxidant enzymes, superoxide dismutase (SOD), and catalase (CAT), were examined. Among SA concentrations, 0.25 mmol was most effective in enhancing heat tolerance in Kentucky bluegrass, which was manifested by improved re-growth potential following heat stress of 72 h and maintenance of leaf water content at 77% during the 12-h stress period similar to that under normal temperature conditions. The O₂^.- generating rate increased significantly at 6 h of heat stress, and SOD activity increased significantly at 2 h but decreased to the control level at 6 h of heat stress in SA-nntreated plants. The SA application suppressed the increase of O₂^.- generating rate and enhanced SOD activity significantly at 2 and 6 h of heat stress, respectively. The SA application decreased H₂O₂ level significantly at 2 and 12 h of heat stress, and increased CAT activity significantly within 12 h of heat stress. The results suggest that SA application enhanced heat tolerance in Kentucky bluegrass and SA could be involved in the scavenging of AOS by increasing SOD and CAT activities under heat stress.