Effects of melatonin and alginic acid on antioxidant enzyme system of Rhododendron lapponicum under high temperature stress

【Objective】 The study aimed to explore the effects of spraying exogenous melatonin（MT）and alginic acid （AA）on the antioxidant enzyme system of Rhododendron lapponicum under high temperature stress，providing direction for research on heat-resistant agents for R. lapponicum. 【Method】 Using the heat-sensitive R. lapponicum variety Prince Lavender as the experimental material，the plants were subjected to treatments of a control group（water，CK）and concentration groups（MT at 100，200 and 300 μmol/L and AA at 200，300 and 400 mg/L）. Following the treatments，high temperature stress（day/night stress temperature was 40 ℃/28 ℃）was applied to study the effects of exogenous MT and AA on the antioxidant enzyme activity and antioxidant content of R. lapponicum under high temperature stress. 【Result】 Under high temperature stress，the content of malondialdehyde（MDA）of R. lapponicum in all treatments showed an overall increasing trend with the onset and duration of the stress. The activity of superoxide dismutase（SOD）initially increased，then decreased，and subsequently increased again. The activities of peroxidase（POD）and catalase（CAT）both exhibited a trend of first increasing and then decreasing，while the content of glutathione（GSH）also showed an initial increase followed by a decrease. Meanwhile，as high temperature stress began and continued，the activity of plasma membrane H⁺-ATPase in all treatments gradually decreased，whereas the activity of plasma membrane Ca²⁺-ATPase exhibited a trend of first increasing and then decreasing. After 96 h of high temperature stress，compared to the CK during the same period，the contents of MDA in the 6 concentration groups was significantly decreased by 6.36%-33.68%（P<0.05，the same below），while SOD activity was significantly increased by 21.68%-73.96%，POD activity was significantly increased by 12.92%-49.02%，CAT activity increased by 1.25%-43.10%，GSH content increased by 7.70%-49.79%，the activity of plasma membrane H⁺-ATPase increased significantly by 11.61%-53.00%，and the activity of plasma membrane Ca²⁺-ATPase significantly increased by 9.36%-50.89%. The results of the correlation analysis indicated that，except for the lack of significant correlation between CAT activity and SOD activity，all other indicators showed extremely significant correlations（P<0.01）. 【Conclusion】 Spraying appropriate concentrations of exogenous MT and AA on the leaves can reduce the MDA content of R. lapponicum under high temperature stress，increase the activities of SOD，POD，CAT and the content of GSH，and simultaneously enhance the activities of plasma membranes H⁺-ATPase and Ca²⁺-ATPase，alleviating the adverse effects of high temperature stress on the growth of R. lapponicum. During the same period，the indexes of exogenous MT treatment are generally better than those of AA，with the best overall effect achieved by spraying 100 μmol/L MT，while 200 mg/L AA also shows fine effect.