Exogenous Melatonin Improves Drought Tolerance in Maize Seedlings by Regulating Photosynthesis and the Ascorbate–Glutathione Cycle

Abstract

Melatonin is known to exert protective effects in maize against drought stress, but the knowledge regarding interaction among the melatonin, photosynthetic efficiency, ascorbate–glutathione cycle still remains elusive. Two contrasting maize (Zea mays L.) genotypes, SD609 (drought-tolerant) and SD902 (drought-sensitive), were subjected to moderate and severe drought stress along with 100 μM melatonin treatment. Exogenous melatonin increased fluorescence curve levels for both genotypes under drought stress, and higher in SD609 than SD902. In both genotypes, melatonin application also significantly increased the photochemical efficiency of photosystem II (PSII), the effective quantum yield of PSII and photosystem I (PSI), and the electron transport rate between PSII and PSI under drought stress, but it decreased the quantum yield of energy dissipation and the donor side and acceptor side impairment of PSI under drought stress, regardless of stress severity. Exogenous melatonin also contributed to the reduction of membrane damage under drought stress, as reflected by significantly decreased levels of MDA, superoxide anions and H2O2 of both genotypes exposed to drought. Furthermore, melatonin treatment increased enzyme activity in AsA–GSH cycle, and upregulated the genes expression of related enzymes in AsA–GSH cycle. The effects of melatonin were more pronounced in SD609 than SD902. Collectively, these results indicated that exogenous melatonin application increased photosynthetic electron transport rate and accelerated the AsA–GSH cycle, which are factors that play an important role in drought tolerance in maize. The beneficial effect of melatonin on the drought-tolerance SD609 was more obvious.