4-Hydroxymelatonin alleviates nickel stress, improves physiochemical traits of Solanum melongena: Regulation of polyamine metabolism and antioxidative enzyme

•Nickel stress reduced growth of Solanum melongena seedlings.•4-OHMT elevated the activity of enzymatic antioxidants in treated plants.•Plants treated with 4-OHMT exhibited higher level of no-enzymatic antioxidants.•Application of 4-OHMT modulated the amount of polyamines.•Modulation in antioxidants and polyamine contents alleviated Ni stress in plants. The core objective of current research was to reveal the potential of 4-hydroxymelatonin (4-OHMT) in alleviation of nickel (Ni) toxicity in Solanum melongena L. Nickel toxicity reduced growth, leaf relative water content (LRWC), chlorophyll (Chl) content, intercellular carbon dioxide concentration, stomatal conductivity and transpiration rate in S. melongena seedlings. However, exogenous application of 4-OHMT enhanced growth, net photosynthesis rate and gas exchange characteristics in treated seedlings. Moreover, 4-OHMT amplified the activity of superoxide dismutase, catalase and ascorbate peroxidase. Additionally, 4-OHMT applied plants exhibited higher level of glutathione (GSH). The stress alleviation capability of 4-OHMT in S. melongena seedlings is attributed to the reduced level of hydrogen peroxide, malondialdehyde and electrolyte leakage. 4-Hydroxymelatonin reduced Ni content in root and shoot of S. melongena seedlings. Further research with reference to potential of 4-OHMT for stress mitigation in other crops will assist in increasing the crop productivity.