EFFECT OF EXOGENOUS MELATONIN ON PHOTOSYNTHESIS AND CHLOROPHYLL FLUORESCENCE PARAMETERS IN LEAVES OF CUCUMBER SEEDLINGS UNDER HIGH TEMPERATURE STRESS

Effects of Melatonin(MT) on photosynthetic and fluorescence parameters in leaves of cucumber seedling with spraying exogenous were investigated,using cucumber variety of ‘Jinchun 4’under high temperature stress.Results showed that net photosynthetic rate(Pn) declined sharply,but stomatal conductance(Gs),intercellular CO2 concentration(Ci) and transpiration rate(Tr) increased after treated for 24h at high temperature of 40℃ ± 1℃.The Pn,Gs,and Tr increased at first then decreased rapidly after treated with exogenous MT,while Ci was the opposite,which suggested that Pn reduction was mainly due to non-stomatal factors.The decrease in primary conversional efficiency of light energy(Fv/Fm),electron transport rate(ETR),PSII quantum yield of electron transport(ΦPSII) and chlorophyll contents accounted for the decline of Pn through further analysis on chlorophyll fluorescence parameters.The seedlings treated with MT had higher contents of chlorophyll,Fv/Fm,ETR and ΦPSII.It was illustrated that melatonin can alleviate the heat stress on photosynthetic organ damage and improving photosynthetic capacity of cucumber seedlings.