Melatonin delays leaf senescence and improves cucumber yield by modulating chlorophyll degradation and photoinhibition of PSII and PSI

Decreases in chlorophyll and photosynthetic efficiency co-occur during aging and leaf senescence. Here, we report the positive functions of melatonin (MT) in delaying leaf senescence in cucumber plants. The results showed that natural leaf senescence induced the accumulation of endogenous MT and that spraying MT delayed leaf senescence in a concentration-dependent manner. Moreover, application of MT and transient overexpression of CsASMT, the key gene in MT synthesis, both suppressed leaf senescence simulated by darkness. This was evidenced by a decrease in expression of the senescence-related gene SAG12 and the cell death-related gene PDCD, an increase in chlorophyll content in terms of downregulation of chlorophyll-degradation genes and normal chloroplast structure, and higher activity of PSI and PSII reaction centers, electron transfer efficiency and photosynthesis-associated protein levels, compared with these measures in the H2O-treated seedlings. In addition, the data from field spraying experiments demonstrated that MT indeed alleviated the decline in chlorophyll content and increased photosynthetic efficiency in both functional and senescent leaves, which ultimately promoted the growth and yield of cucumber in the solar greenhouse. Together, these results indicate that MT acts as a positive regulator to downregulate SAG12 and PDCD mRNA abundances and alleviate damage to the photosynthetic apparatus by suppressing chloroplast disintegration and photosynthesis-associated protein degradation during leaf senescence.