Effects of Dopamine on Antioxidation, Mineral Nutrients, and Fruit Quality in Cucumber Under Nitrate Stress

In greenhouses, growth of plants is affected by accumulation of nitrate in the soil due to extended durations of nitrogen fertilizer application. Dopamine has been shown to play an important role in plant growth and stress resistance. We studied effects of dopamine on antioxidation, mineral nutrients, and fruit quality in cucumber (Cucumis sativus L. “Jin You No. 1”) under nitrate stress. Excessive nitrate inhibited root growth, resulting in reduced root length, volume, and activity. The content of malondialdehyde and superoxide anion increased significantly, and the activities of antioxidant enzymes such as superoxide dismutase, catalase, and ascorbate peroxidase increased, whereas peroxidase decreased under high-nitrate conditions. We also found that the length, cross-sectional diameter, and total yield of fruits were significantly decreased under nitrate stress. Moreover, fruit quality, such as reducing sugar, soluble sugar, soluble protein, and vitamin C, was significantly reduced under high-nitrate conditions. Excessive nitrate caused accumulation of nitrogen and potassium in the roots, stems, and leaves of plants, but reduced accumulation of phosphorus. Conversely, dopamine promoted root growth and root activity, reduced malondialdehyde and superoxide anion content, increased activities of antioxidant enzymes, improved yield and quality, and balanced ion absorption. Our results show that dopamine plays an overall positive role for cucumbers under nitrate stress.