Effects of Melatonin on the Photosynthetic Characteristics of Zanthoxylum armatum under Waterlogging Stress

Zanthoxylum armatum DC. is an economic tree species sensitive to waterlogging. Melatonin can improve the waterlogging tolerance of plants. In this study, we investigated the effects of different waterlogging times (including 0, 1, 2 and 3 d) and melatonin concentration (including 0, 400 and 800 mu mol/L) on photosynthetic pigments, photosynthetic capabilities and chlorophyll fluorescence parameters of Z. armatum seedlings under waterlogging stress. Our results showed that waterlogging stress decreased the chlorophyll contents, photosynthetic rate (P-n), stomatal conductance (G(s)), maximum efficiency of PSII photochemistry (F-v/F-m), quantum yield of PSII photochemistry (Y(II)), effective photochemical quantum yield of PSII (F '(v)/F '(m)), as well as efficiency of electron transfer (ETR) with the increase of waterlogging time. On the contrary, the intercellular CO2 concentration (C-i), photochemical quenching (q(P)) and nonphotochemical quenching coefficient (NPQ) increased under waterlogging stress. However, melatonin could mitigate waterlogging-induced reductions in photosynthetic capacity. Melatonin improved photosynthesis by enhancing the chlorophyll and carotenoid content and photosynthesis rate. In addition, melatonin promotes stomatal function, electron transport in PSII and photosynthesis efficiency. Melatonin had a concentration-dependent effect on Z. armatum seedlings under waterlogging stress, and the optimum concentration is 400 mu mol/L. It indicated that exogenous melatonin application could effectively ameliorate the repression of photosynthesis caused by waterlogging, and thus enhance the seedling tolerance to waterlogging stress.