Foliar Application of Arginine, Proline, Dopamine, and Melatonin Enhances Growth, Yield and Fruit Biochemical Traits of Greenhouse Cucumber (Cucumis sativus L.) under Sub-Optimal Temperature Conditions

Low temperature is a major environmental factor that adversely affects the growth and productivity of cold-sensitive plants such as cucumber. This study evaluated the effects of foliar application of proline, arginine, dopamine, and melatonin on the growth, yield, and biochemical properties of greenhouse cucumber (Cucumis sativus L. cv. ‘Talisya’) under cold stress. Foliar spray treatments were proline (2.5, 5, and 10 mM Pro), arginine (1 and 2 mM Arg), dopamine (0.1 and 0.2 mM DOP), melatonin (0.1 mM MEL), and a distilled water as control. Results indicated that foliar application of amino acids significantly enhanced plant height, fruit number, total yield, chlorophyll content, fruit firmness, total soluble solids, total acid, phenolic compounds, and antioxidant enzyme activity. Additionally, amino acids reduced malondialdehyde (MDA) and hydrogen peroxide (H2O2) accumulation in cucumber fruits under suboptimal temperature conditions. The highest plant height, fruit number, and yield were observed with 5 mM Pro and 2 mM Arg. Maximum chlorophyll content, phenolic compounds and phenylalanine ammonia-lyase activity were recorded in fruits of plants treated with Arg and 2.5 and 5 mM Pro. The highest catalase and superoxide dismutase activity and antioxidant capacity were associated with 5 and 2.5 mM Pro. The lowest MDA and H2O2 accumulation were observed in fruits treated with 2 mM Arg and 0.1 mM DOP. Overall, the application of 5 mM proline, 2 mM arginine, and 0.1 mM dopamine was recommended to enhance the yield and quality of greenhouse cucumber cv. ‘Talisya’ under suboptimal temperature conditions.