Effect Of Exogenous Melatonin On PS Ⅱ Photocheistry And Antioxsidant Systems Of Watermelon Seedlings Under Salt And PEG Stress

Salt and PEG stress inhibited growth of watermelon seedlings by destroyed photosynthesis and antioxidant system.Melatonin is the first line of defence existing widely in plant tissues which plays an important to increase crop resistance to salt and drought.This paper preliminarily explained that exogenous melatonin alleviated salt and PEG damage of watermelon by affected its photosynthesis,PS Ⅱ photochemistry and antioxidant system.The major results showed as follows:（1）The results showed that salt stress remarkably suppressed total chlorophylls（chl a + b）and the actual photochemical efficiency of PSⅡ（ΦPS Ⅱ）in watermelon plants,but increased non-photochemical quenching（NPQ）value,The maximal photochemical efficiency of PSⅡ（Fv/Fm）did not changed under salt stress.The application of melatonin on watermelon seedlings could alleviate salt stress-induced photooxidation because it has been proved that it played an important role in the dissipation of excess excitation energy by participating in NPQ and improving the actual photochemical efficiency of photosystem Ⅱ.（2）Salinity caused a great increase in lipid peroxidation（MDA）,relative electrolyte leakage（REL）,hydrogen peroxide（H₂O₂）concentrations and the contents of dehydroascorbate（DHA）and oxidized-glutathione（GSSG）.Meanwhile,NaCl markedly reduced the activities of some key antioxidant enzymes,i.e.superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,acorbate peroxidase（APX）,monodehydroascorbate reductase（MDHAR）and dehydroascorbate reductase（DHAR）,and causing a decrease in the ratio of AsA/DHA and GSH/GSSG,as well as the contents of ascorbic acid（AsA）and glutathione（GSH）to different degerees in the leaves.Melatonin mitigated this adverse situation which demonstrated melatonin could enhance the salinity tolerance through reducing oxidative damages and increase enzyme activity in leaves of watermelon（3）The stress of PEG for 3 days caused a significant decrease in net photosynthetic rate（Pn）and stomatal conductance（Gs）.The experiment demonstrated that Fv/Fm and ΦPSⅡ were reduced by PEG stress.Moreover PEG stress markedly reduced the photochemical qP and significantly increased and NPQ,However,stressed plants supplied with exogenous melatonin reverted this situation,which indicated that melatonin can alleviate the detrimental effects on photosynthesis and interference of PSⅡ photosynthetic electron transport caused byPEG stress through enhancing the regulated energy dissipation.（4）Moreover application of melatonin decreased PEG lipid peroxidation and modulated differently ascorbic acid,glutathione and the total antioxidant power of tissues as well as the activities of APX,CAT,MDHAR,DHAR compared to PEG alone treatment which showed melatonin alleviated the damaging effects of drought on watermelon seedlings...