Abstract: The effect of heat shock stress on the photochemical activity and chlorophyll fluorescence spectra of tomato fruit surface was studied by a MINI-IMAGING-PAM system and QE65000 spectrometer. The results showed that the maximal photochemical efficiency（Fv/Fm）decreased steadliy under lower temperatue of heat shock stress（36–43 ℃），which indicated that the PSⅡ could be ihhibited partly under temperature stress，and at this point，the increased Y（NPQ）dissipated the surplus energy so as to alleviate further damage to photosynthetic apparatus. When the temperature exceeded 43 ℃，the quantum yield of nonregulated energy dissipation Y（NO）increased significantly，Fv/Fm，the open PSⅡ reaction centers（Fv′/Fm′）and the apparent electron transport rate（ETR）decreased sharply，and Y（NPQ）began to decrease，indicating that antenna pigment dissipation mechanism of PSⅡ reaction center might have been 　　 damaged，and consequently the self-adjustment function of high-temperature stress began to decrease. Meantime，PSⅡ reaction center turned inactive and the photoinhibition was aggravated. When the temperature exceeded the enaturalization midpoint temperature（T m（Fv/Fm），51.4 ℃），the relative deviation（β/α–1）from full balance between two photosystems increased，while the fluorescence decline ratio（Rfd）decreased sharply，showing that the excitation energy distribution was seriously imbalanced and the assimilation capacity of CO2 was extremely weak. T m（Fv/Fm） calculated from Gibbs free energ（△GD）was higher than T m[Y(II)]，indicating that the heat resistance of PSⅡ was a little stronger than that of the whole photosynthesis.

Key words: tomato, fruit, heat shock stress, chlorophyll fluorescence, fluorescence exicition spectra, Gibbs free energy