Abstract: To study the photochemical mechanism of leaves in maize seedlings responded to low temperature stress,the leaf chlorophyll fluorescence was examined under ambient temperature (25℃) and low temperature (2℃) based on chlorophyll fluorescence kinetics.Then,the rapid fluorescence transient (OJIP curve) was obtained,and analyzed with JIP-test.The maximal fluorescence intensity (F_m) and potential photochemical efficiency of photosystem Ⅱ (PSⅡ) (F_V/F_O) decreased significantly by 55.3% and 65.9% under low temperature as compared to ambient temperature.With normalized fluorescence transients,W_OJ and W_JI increased under low temperature,but W_IP decreased.The relative variable fluorescence intensity at the J-step and I-step (V_j and V_i) exhibited an opposite trend under low temperature,i.e.V_j increased by 19.7% while V_i decreased by 16.4%.Relative variable fluorescence at 300 μs of the chlorophyll fluorescence transient (W_K) representing activities of the donor side increased by 44.7% under low temperature.Simultaneously,the fraction of oxygen-evolving complexes (OEC) decreased by 13.1%.The normalized total complementary area above the OJIP transient (S_m),approximated initial slope of the fluorescence transient (M_O) and frequency of Q_A reduction (N) which reflect the change of electron transport activities of the acceptor side increased by 210%,49.7%,294%.Maximum quantum yield for PSⅡ primary photochemistry (φP_o),probability that a trapped exciton moves an electron into the electron transport chain beyond Q_A^- (ψ_O) and quantum yield for PSⅡ electron transport (φE_O),decreased by 24.9%,6.82%,29.7% under low temperature.But quantum yield for dissipation (φD_O) increased significantly by 141%.The above parameters represent quantum efficiency or flux ratios.Absorption flux per reaction center (RC) (ABS/RC),electron transport flux per RC (ET_O/RC),trapped energy flux per RC (TR_O/RC) and dissipated energy flux per RC (DI_O/RC) increased significantly by 70.4%,24.7%,16.1% and 328% under low temperature,while the density of RCs (RC/CS_O) decreased by 37.4%.Two performance indexes on an absorption basis (PI_abs,PI_CSm) decreased by 81.6% and 90.6% under low temperature.Collectively,these results indicate that photosynthesis is a sensitive biological process in maize seedlings under low temperature.The protective mechanism underlying response of photosynthesis under low temperature stress is that the damaged oxygen-evolving complex due to low temperature leads to the decline in the electron-supplying capacity.Moreover,the inactivation of RC is severe,but the efficiency of single RC is enhanced.The redundant energy is consumed as heat coupled with the reduction of reactive oxygen species (ROS).Moreover,the donor side damage of PSⅡ due to low temperature stress results in the blocking of electron transport,thereby influencing photosynthesis capacity.