Long-term exogenous application of melatonin improves nutrient uptake fluxes in apple plants under moderate drought stress

To examine the potential roles of melatonin in nutrient uptake, we investigated the influence of its long-term exogenous application on 'Naganofuji No.2′ apple (Malus domestica Borkh.) under moderate drought conditions. Both growth and the uptake of macro- and micronutrients were generally decreased in stressed plants. However, the application of exogenous melatonin significantly mitigated this growth reduction and enabled plants to maintain uptake fluxes. This addition of melatonin also markedly alleviated the inhibitory effects of drought on photosynthesis, stomatal apertures, chlorophyll levels, and relative water content, and it controlled the burst of relative electrolyte leakage and hydrogen peroxide. Our investigation with stable isotopes further verified that exogenous melatonin was associated with significant increases in the uptake, utilization, and accumulation of δ15N under drought conditions. Stress sharply reduced the activity of enzymes involved in nitrogen metabolism (NR, NiR, GS, and GOGAT), but the application of melatonin substantially reversed that response. We also examined whether melatonin might control the expression of genes for N-metabolism and transport. Here, the transcript levels of metabolic genes (NR, NiR, GS, Fd-GOGAT, and NADH-GOGAT) and uptake genes (AMT1;2, AMT1;5, AMT1;6, AMT2;1, NRT1;1, NRT2;4, NRT2;5, and NRT2;7) were greatly up-regulated in the leaves. Exogenous melatonin treatment also significantly increased the concentration of endogenous melatonin. Thus, understanding the role of melatonin in nutrient uptake introduces new possibilities to use this compound for agriculture purposes and provides a valuable foundation for enhancing plant tolerance and adaptability to future drought stress.