Effects of Exogenous Melatonin on Chrysanthemum Physiological Characteristics and Photosynthesis under Drought Stress

Melatonin has been confirmed to relieve drought stress in various plant species. However, the mechanism in garden chrysanthemums remains to be elucidated. Here, we examined the putative role of exogenous melatonin application in improving drought-stress tolerance in Chrysanthemum seedlings. Treatment with melatonin (100 µM) significantly mitigated the negative effects of drought stress on the seedling photosynthetic parameters and physiological and biochemical indexes. The increased tolerance conferred by exogenous melatonin was attributable to increased photosynthetic activity, which alleviates drought-induced oxidative damage by reducing malondialdehyde accumulation. In other words, the combined application of MT and PEG was better than their individual applications, as it prevented heat-induced membrane damages by declining the hydrogen peroxide (28%), malondialdehyde contents (38%), and relative electrical conductivity (19%) as compared to PEG. Interestingly, the melatonin application stimulated the activity of major antioxidant enzymes such as superoxide dismutase (21%), catalase (26%) and polyphenol oxidase (35%). In addition, exogenous melatonin promoted photosynthesis. Compared with the PEG drought-stress treatment, the melatonin treatment significantly increased the net photosynthetic rate (39%), stomatal conductance (27%), and transpiration rates (18%), while decreasing the intercellular CO2 concentration (10%). The photochemical quenching coefficient (46%), photosystem II (PSII) maximum photochemical quantum yield (22%), apparent electron transfer rate (37%), and PSII potential photochemical efficiency (18%) showed an upward trend. In summary, exogenous spraying of MT can effectively improve drought tolerance and mitigate the damage caused by drought stress in the “Xuanqiu ninghong” Chrysanthemum.