Abstract: Taking  chlorophyll a fluorescence transient OJIP as the probe, this paper studied the effects of foliar spraying 10 mmol·L^-1 of CaCl₂ on the functions of photosystem Ⅱ (PSⅡ) in tobacco seedling leaves under drought stress. Drought stress decreased the conversion efficiency of PSⅡ primary light energy (F_v/F_m) and electron transport rate (ETR), and inhibited the primal process of photosynthesis, resulting in an obvious photoinhibition. After the spraying of CaCl₂, the decrement of quantum yield for electron transport (φE_o) under drought stress was obviously lower, and the increment of electron transport efficiency of per reaction center (ET_o/RC) was obviously higher, compared with those after the spraying of water. Foliar spraying CaCl₂ increased the amount of PSⅡ-captured light energy used for photosynthesis electron transport, the efficiency of surplus active reaction center, as well as the energy transport in electron transport chain, making the PSⅡ remain relatively high activity under drought stress, and accordingly, the drought resistance of the tobacco seedlings was improved.

Key words: flue-cured tobacco, exogenous calcium, drought stress, photosystem Ⅱ, photoinhibition