[Alleviation effects of melatonin and Ca²⁺ on melon seedlings under salt stress]

To assess the role of exogenous melatonin (MT) and Ca²⁺ in melon under salt stress, the content of mineral elements (Cl^-, Na⁺, K⁺, Mg²⁺, Ca²⁺), the values of Na⁺/K⁺, Na⁺/Ca²⁺, Na⁺/Mg²⁺, the activity of H⁺-ATP, the accumulation of osmotic substances and membrane lipid peroxidation in melon under salt stress were investigated in the environmental conditions (day/night 25/18 ℃) controlled by artificial climate chamber. The results showed that salt stress significantly inhibited growth of the melon seedlings with the increased contents of Cl^- and Na⁺ in roots and lea-ves, and the decreased contents of K⁺, Mg²⁺ and Ca²⁺, compared with the control. Under salt stress, exogenous application of MT or Ca²⁺ remarkably reduced the contents of Cl^- and Na⁺ in roots and leaves, increased the contents of K⁺, Mg²⁺ and Ca²⁺, and decreased values of Na⁺/K⁺, Na⁺/Ca²⁺ and Na⁺/Mg²⁺. Additionally, exogenous melatonin or Ca²⁺ increased H⁺-ATP activity and osmotic adjustments, and further alleviated cell membrane injuries imposed by salt stress, displaying lower MDA content and relative conductivity. Collectively, this work suggested that single or combined applications of exogenous MT and Ca²⁺ effectively reduced the content of Cl^- and Na⁺, improved ion balance by enhancing H⁺-ATP activity, and increased the content of osmotic adjustment substances for ameliorating membrane lipid peroxidation, thereby enhancing plant adaptation to salt stress, especially combined applications of exogenous MT and Ca²⁺. Our results further showed that the combined application of exogenous MT and Ca²⁺ resulted in a synergistic effect on increasing salt tolerance in melon seedlings.