Abstract
Melatonin has been identified as a signal molecule that regulates plant responses to different abiotic and biotic stresses. Melatonin (MT) and its precursor tryptophan (Try) have a major role in improving plant stress tolerance to different environmental stresses such as water deficiency. The rapid increase in the Egyptian population caused insufficient protein sources, especially those of animal origin, in their diet. The possible solution is to augment the diet with legumes such as white lupine which are relatively rich in protein. Thus, the current experimental work was carried out to find changes in growth, biochemical aspects and yield quantity and quality of white lupine plant with spraying of both MT and Try at different concentrations on plant shoot under water deficit stress conditions. Results showed that water deficit (75 or 50% of water irrigation requirements; WIR) caused significant reduction in growth, photosynthetic pigments, indole acetic acid and yield compared with those received 100% WIR. Seed yield significantly decreased (p < 0.05) by 26.98 and 41.64% by decreasing WIR to 75 and 50%. The decrease was accompanied by significant increase in phenolic contents, hydrogen peroxide, lipid peroxidation and some antioxidant enzymes, while nitrate reductase enzyme was decreased. However, external application of either MT or Try significantly alleviated the adverse effects of water deficit (growth suppression), since MT or Try-treated plants recovered more quickly than untreated plants. Moreover, MT or Try-treated plants had higher photosynthetic pigments, indole acetic acid, phenolic, as well as yield quantity and quality under the three WIR as compared with untreated plants. Melatonin treatment at 100 µM and Tryptophan at 200 µM increased weight of seeds/plant by 78.29 and 52.19%, 71.49 and 43.78% and 41.21 and 13.07% in plants irrigated with 100, 75 and 50% WIR, respectively. Exogenous MT and Try significantly reduced hydrogen peroxide and malondialdehyde content, while markedly increased the activities of antioxidant enzymes and nitrate reductase under different WIR. Finally, the current study concluded that MT and Try treatments alleviated the detrimental effects of water deficiency and accelerated the recovery mainly via improving white lupine plants tolerance in forms of enhancing photosynthetic pigments, indole acetic acid, phenolic and antioxidant capacity.