| Fast chlorophyll fluorescence parameters were measured by a PEA,Plant Efficiency Analyzer from Hansatech,in two varieties of autumn cucumbers(Cucumis sativus L.)with short-fruit-type or long-fruit-type,to go along the analyses on the chlorophyll a fluorescence kinetics,the thermodynamics of photosystemⅡ(PSⅡ)heat shock stability and genetic analysis,with two contrast varieties of XINTAIMICI with lower heat-resistant temperature capability and JINCHUNⅣwith higher one.Along with the temperature increasing(5 min at 30℃～57℃),The construction and functions of PSⅡshow more sensitive to heat shock stress as maximal photochemical efficiency of PSⅡ(Fv/Fm),density of reaction centers(RC/CSo)and oxygen-evolving-complex activity(pk)show "S" descend;on the contrary,dissipated energy flux per RC(DIo/RC)shows "S" ascend.The generalized analysis reflects the ternary mechanism under the heat shock,i.e.,the reversible deactivation of PSⅡreaction centers,the passivation of oxygen-evolving-complex(OEC)and the energy dissipation, play a vital role in protect PSⅡto prevent photoprohibition.In excess of 30℃of the temperature of heat shock stress,pk begins descending;over 40℃,RC/CSo begins descending;over 44℃,DIo/RC represents increasing;over 51℃,for XINTAIMICI,high temperature aggravates the burden for protection of dissipative framework in PSⅡ.Denaturalization midpoint temperature(Tm)calculated from Gibbs free energy(△GD) indicates that the heat stability of protein complex excel reaction center complex and OEC. Each of Tm on Fv/Fm,RC/CSo or pk shows that the thermotolerance of JINCHUNⅣis the best among the four cultivars,autumn cucumbers with short or long fruits take second place,and XINTAIMICI is the worst.Variation degree of Fv/Fm is most effected by heredity,little by environment, whereas environment is the chief factor of traits inherit of pk4s/30and variation potential of RC/CSo54/30is lower.Compared with other parameters,pk48/30on 48℃and the heritability RC/CSo54/30on 54℃are both lower.Along with the temperature increasing,the electrolyte leakage shows upward trend,which describe that the damage of cell membrane by high temperature is becoming worse.The trend of 07-17,07-25 is flattening that shows cell membrane damage is light.On low heat stress temperature,the differences of cell membrane thermal stability is not obviously.On high temperature,cell membrane thermal stability of 07-37 is the worst,next is 07-38,and cell membrane thermal stability of 07-17 is the best,which show the heat-resistant capacity of 07-17 and 07-25 is stronger.The cluster analysis result of Fv/Fm,RC/CSo,pk shows autumn cucumber in eastern of He Bei Province can be classified four kinds judging by the threshold of 4.30.RC/CSo has poor stability,with a wide range of options on 48℃and 54℃.Its selective changes is greatly impacted by the changes of external environment(temperature). Changes in trends of pk48/30describe that OEC is impacted by environment easily,so less transport through genetic.Compared with other chlorophyll fluorescence parameters,the heritability of Fv/Fm48/30and pk54/30is higher.The genetic advance of RC/CSo48/30is higher on 48℃;and the absolute genetic advance of pk54/30is higher on 54℃,it has no greater differences on relative genetic advance among three chlorophyll fluorescence parameters.And on 48℃,Fv/Fm48/30and RC/CSo48/30shows negative correlation,Fv/Fm48/30and pk48/30,RC/CSo48/30and pk48/30show positive correlation,the correlation coefficients of Fv/Fm48/30and pk48/30is higher than others,but on 54℃the parameters show positive correlation to each other.The correlation coefficient of Fv/Fm54/30and pk54/30whereas the correlation coefficient of RC/CSo54/30与pk54/30is lower.
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