Melatonin Alleviates Drought-Induced Damage of Photosynthetic Apparatus in Maize Seedlings

Melatonin plays an important role in the enhancement of plant tolerance to drought stress. The underlying mechanisms of this melatonin-induced protection of the photosynthetic apparatus against drought stress are still not fully understood in maize (Zea mays L.). This study investigated the effects of 100 and 200 μM exogenous melatonin additions on the photosynthetic electron transport process in maize seedlings under drought stress. The chlorophyll a fluorescence transient (OJIP) and modulated 820 nm reflection (MR) analysis showed that drought stress damaged the donor and acceptor side of photosystem II (PSII). Furthermore, the function of PSII reaction centers and the quantum yields of PSI and PSII were significant reduced. These fluorescence parameters were restored in melatonin-treated plants. Energy conversion analysis indicated that melatonin-treated plants maintained a stronger ability to ensure high photosynthetic capacity, as evidenced by higher PSI photochemical quantum yield Y(I) and PSII photochemical quantum yield Y(II). Melatonin treatment decreased the regulated energy dissipation Y(NPQ) and non-regulated energy dissipation Y(NO) in PSII. Moreover, melatonin treatment mitigated the limitations of both the acceptor-side and donor-side of PSI under drought stress, as indicated by lower PSI acceptor side limitation Y(NA) and PSI donor side limitation Y(ND). In addition, 100 μM melatonin was the more effective concentration to alleviate the damage to the photosynthetic apparatus. Thus, exogenous melatonin alleviated the drought stress-induced damage of the photosynthetic apparatus in maize by improving the photosynthetic electron transport efficiency between both photosystems and by enhancing the quantum yields of PSI and PSII photochemistry.