Effects of high temperature stress on chlorophyll fluorescence characteristics of different-type cucumber seedlings

Seedlings of 5 different-type cucumber inbred lines under high temperature stress(day 38℃/night 28℃and day 42℃/night 32℃)were investigated in terms of heat injury index and chlorophyll fluorescence parameters.The results indicated that under high temperature stress,the maximum fluorescence(Fm),maximum photochemical efficiency(Fv/Fm)of photosystemⅡ(PSⅡ),actual photochemical yield(Yield),photochemical fluorescence quenching coefficient(qP),actual photochemical efficiency(ΦPSⅡ)of PSⅡ,alterable fluorescence drop ratio(Rfd)and partition coefficient(α)of excited energy of PSⅠwere decreased with the temperature rising.The initial fluorescence(Fo),excited energy pressure(1-qP)of PSⅡ,partition coefficient(β)of excited energy of PSⅡ,and partition imbalance(β/α-1)of excited energy between PSⅠandⅡwere increased with the temperature rising. The apparent electron transport rate(ETR)was rapidly increased after 38℃treatment but sharply decreased after 42℃treatment.The correlation analysis showed that the Yield and ETR had a significant negative correlation with the heat injury index and could be used as reliable indexes to determine the heat tolerance of cucumber at seedling stage.