Dopamine Alleviates Chilling Stress in Watermelon Seedlings via Modulation of Proline Content, Antioxidant Enzyme Activity, and Polyamine Metabolism

Dopamine acts as a strong water-soluble antioxidant that mediates many physiological processes in plants. In the present study, we utilized two cultivars to investigate the effect of dopamine in mitigating chilling stress in watermelon (Citrullus lanatus) seedlings. Under chilling stress, exogenous application of dopamine (100 μM) increased seedling health index by 23.32% (cv. Xi Nong NO. 8) and 35.19% (cv. Jing Xin NO. 1) compared to chilling-stressed seedlings alone. As the stress was prolonged, chilling treatment caused a significant reduction in net rates of photosynthesis and chlorophyll contents. Dopamine supplementation significantly mitigated these adverse effects caused by chilling stress. After 8 days of chilling stress, supplementation of dopamine to chilling treatment seedlings further increases in levels of proline in cv. Xi Nong NO. 8 and cv. Jing Xin NO. 1 by 45.24% and 58.28%, respectively. Our results indicated that application of dopamine further enhanced antioxidant enzymes, which contributed to alleviate oxidative stress-induced chilling treatment. In addition, application of dopamine significantly improved polyamine levels by stimulating enzymes involved in the polyamine metabolism. From these outcomes, it is clear that supplementation of dopamine mitigates the harmful effects of chilling stress in watermelon seedlings through modulation of osmolytes, antioxidant enzymes, and polyamine metabolism.