SummaryTomato (Lycopersicon esculentum Mill.) leaves were subjected to a low night temperature (LNT; 9°C) for 3, 6, or 9 d, followed by recovery for 9 d at 15°C. Tomato leaves subjected to a night temperature of 15°C were used as controls. Changes in photosynthesis, chlorophyll fluorescence, and the allocation of absorbed light energy were studied. LNT treatment for 3, 6, or 9 d resulted in the reversible inhibition of the net photosynthetic rate (Pn). The potential activity of photosystem II (PSII; Fv/Fo) was clearly reduced and showed greater sensitivity to LNT treatment than the maximum quantum efficiency of PSII (Fv/Fm). The decline in the actual photochemical efficiency of PSII reaction centres ( psii) was attributed to a decrease in the photochemical quenching coefficient (qP) and the efficiency of excitation energy capture by open PSII centres (Fv'/Fm'). Meanwhile, the rate of electron transport (ETR) decreased relative to control tomato leaves, and the allocation of light energy absorbed by PSII antennae into the photochemical reaction centres (P) decreased, whereas the allocations to thermal dissipation (D) and excessive energy (E) increased. The effect of LNT treatment at 9ºC for up to 9 d on PSII was reversible.