Abstract: Taking cucumber cultivar ‘Jinchun 4’ as test material, and by the method of foliar spraying, this paper studied the effects of exogenous melatonin (MT) on the ascorbic acid (AsA) metabolism system in cucumber seedlings under high temperature stress. Under the stress of high temperature, the leaf hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents of the cucumber seedlings increased obviously, ascorbic acid (AsA) and glutathione (GSH) contents had a persistent decrease while dehydroascorbate (DHA) and oxidized-glutathione (GSSG) contents had a gradual increase, and AsA/DHA and GSH/GSSG decreased greatly. The activities of ascorbate peroxidase (APx), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) in the seedling leaves under high temperature stress increased markedly, with the maximum after 12 h stress. Foliar spraying MT could effectively restrain the accumulation of H₂O₂ and MDA in seedling leaves, increase the leaf AsA and GSH contents and the activities of ascorbic acid metabolizing enzymes APx, MDHAR, GR and DHAR, and accordingly, enhance the H₂O₂-scavenging ability, inhibit the production of active O₂, maintain the stability of cell membrane, reduce the damage of high temperature to the plants, and improve the ability of cucumber seedlings against high temperature stress.

Key words: melatonin, cucumber, high temperature stress, ascorbate metabolism, light quality,  , light-emitting diode (LED), tobacco, metabolome, secondary metabolism.