Exogenous Melatonin Achieves Drought Tolerance by Improving Photosynthesis in Maize Seedlings Leaves

The beneficial roles of melatonin in plants have been widely reported, but its effect on photosynthesis under drought stress is not clear. In the study, correlated tests were performed to discover melatonin part on photosynthesis in maize (Zea mays L.) seedlings leaves during drought stress. The results showed that melatonin application can mitigate the drought stress-induced damage by protecting chloroplast membrane structure and physiological function. Under drought stress condition, melatonin-treated plants maintained higher photosynthetic rate (P-n), stomatal conductance (g(s)), intercellular CO2 concentration (C-i) and transpiration rate (T-r). The photosystem I (PSI) reaction central proteins-encoding genes (psaA/psaB), cytochrome b6/f complex subunits-encoding genes (petA/petB), photosystem II (PSII) reaction central proteins-encoding genes (psbA/psbB), PSII antenna proteins-encoding genes (psbC/psbD) and photosynthetic enzymes (phosphoenolpyruvate carboxylase (PEPC), pyruvate phosphate dikinase (PPDK), NADP-malic enzyme (NADP-ME) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)) activity were enhanced. These evidences suggested that exogenous melatonin significantly increased the effective quantum yield and electron transport rate on PSII and PSI, but also decreased energy dissipation and donor and acceptor side impairment on PSI. To sum up, our study provides new insights to further investigate melatonin regulation mechanism on enhancement maize seedings drought tolerance.