Mitigative Effect Of Exogenous Melatonin On Senescence Of Cucumber Leaves

In order to study the effect of exogenous melatonin（N-acetyl-5-methoxy,melatonin,MT）on the senescence of cucumber leaves,cucumber（Cucumis sativus L.）"Shuofeng 210"was used as material,the contents of endogenous MT and the expression of the key genes for MT biosynthesis in cucumber leaves at different leaf ages（7d,14d,21d,28d,35d and 42d）were studied to explore the relationship between MT and leaf senescence,the effects of 100μmol·L^-1 MT on chlorophyll metabolism,photosynthetic function,antioxidant system,growth and yield of cucumber at different leaf ages were studied by water treatment as control（CK）,the physiological mechanism of exogenous MT on cucumber leaf senescence was revealed.The main findings are as follows:1.With the increase of leaf age,the m RNA expressions of key endogenous MT synthetases tryptophan decarboxylic enzyme（TDC）,N-acetyl-5-hydroxytryptamine methyltransferase（ASMT）and tryptamine 5-hydroxylase（T5H）and the content of endogenous MT were significantly up-regulated,suggesting that endogenous MT was involved in the regulation of leaf natural senescence.2.The content of malondialdehyde（MDA）and electrolyte leakage rate（EL）in cucumber leaves increased with the increase of leaf age,but the content of MDA and EL in MT treatment were always lower than those in clean water control.At 7～28d leaf age,the m RNA relative expression levels of aging-related genes（SAG20）and programmed cell death related genes（PDCD）change slightly.At 35d leaf age,the m RNA relative expression levels of SAG20 and PDCD were significantly upregulated,but compared with the water control,the m RNA relative expression levels of both are significantly decreased under MT treatment.These results indicated that MT could participate in the regulation of cucumber leaf senescence.3.During the aging process of cucumber leaves,chlorophyll a and b contents showed a trend of first increasing and then decreasing,and reached the maximum value at 28d leaf age.The activities of plant magnesium ion chelatase（Mg CH）and ferrous ion chelatase（Fe CH）,the activities of PPH and PAO,the key enzymes of chlorophyll degradation,and their relative expression levels were similar to those of chlorophyll content.Compared with H₂O treatment,MT treatment significantly increased Mg CH and Fe CH activities,decreased PPH and PAO activities and their m RNA expression,and increased chlorophyll content in aging leaves4.MT treatment could significantly increase photosynthetic rate（P_n）,maximum carboxylation rate of Rubisco(V_c,max),maximum regeneration rate of ribulose-1,5-diphosphate（Jmax）of cucumber functional and senescent leaves.And the actual photochemical efficiency(Φ_{PS II}),the maximum photochemical efficiency（F_v/F_m）,the photochemical quenching coefficient（q P）and the electron transfer efficiency（ETR）of the photosystem II（ΦPS II）significantly reduced the non-photochemical quenching coefficient（NPQ）.In addition,the relative variable fluorescence（V_J）at point J（30ms）and relative variable fluorescence（V_I）at point I（30ms）in OJIP curve tended to decrease with the increase of leaf age before 35d.After 35d,leaves began to senescence,and V_J and V_I increased significantly.The number of active reaction centers per unit area（Rc/Cm）,quantum yield for electron transport(φ_Eo),activity of photosystem I（PSI）reaction center（ΔI/I₀）and activity and relative expression of Rubisco activase（RCA）decreased with the aging of leaves.MT treatment significantly reduced V_J and V_I,and significantly increased Rc/Cm,ΔI/I₀ and RCA activities and m RNA abundance,suggesting that MT can delay the damage of PS I andΦPS II reaction centers to senescence,and improve the photosynthetic efficiency of functional and senescent leaves.Further measurement showed that MT treatment could significantly increase cucumber growth and yield.5.With the increase of leaf age,the contents of hydrogen peroxide（H₂O₂）and superoxide anion（O₂.^-）in leaves gradually increased.At the same time,SOD,CAT and APX activities and their m RNA relative expression levels,reduced ascorbic acid（ASA）,reduced glutathione（GSH）contents,reduced/oxidized ascorbic acid（ASA/DHA）,reduced/oxidized glutathione（GSH/GSS）contents was also significantly increased,but compared with CK,the antioxidant enzyme activity and redox species substance content of aging leaves under MT treatment were significantly higher than CK,and H₂O₂ and O₂.^-contents were significantly lower than CK.Therefore,MT up-regulated the antioxidant capacity of leaves,reduced the accumulation of reactive oxygen species in leaves,and then delayed leaf senescence.In addition,MT pretreatment significantly up-regulated the expression of ARF1,a key transcription factor of IAA signal,and down-regulated ABI5,a key transcription factor of abscisic acid signal in senile leaves.