Research on Heat-resistant Capability of Photosystem II in Pumpkin

The fast chlorophyll a fluorescence transient and its parameters were measured by a PEA luminoscope(plant efficiency analyzer from Hansatech) in two varieties of pumpkin(Cucurbita pepo L.),Mingzhutezao with lower heat-resistant capability and Lǜbo with higher one.The parameters of chlorophyll fluorescence did not change sharply from 36℃ to 40℃.As the temperature increasing from 44℃,Fo went up slowly,but Fm,Fv and Fv/Fm came down in wide range,and their variations of Mingzhutezao were larger than those of Lǜbo.After treated for 15 minutes at 44℃,the K-phase of Mingzhutezao appeared at about 0.8 ms on the fluorescence kinetics curve,so the regular O-J-I-P transient was transformed to an O-K-J-I-P one.Meanwhile,the K-phase of Lǜbo appeared when the treatment time was up to 25 minutes at 44℃.When the temperature was 48℃,the K-phase of Mingzhutezao and Lǜbo appeared after treated for 2 minutes and 6 minutes respectively.As the temperature of heat stress increased from 36℃ to 50℃,RC/CSo reduced gradually,while Fvi/Fv rose up.