The adaption of photosynthesis,being a key metabolic process,plays an important role in plant resistance to air drought.In this study,the Siberian crabapple (Malus baccata L.Borkh.) in the forest-steppe zone of Transbaikalia region,Russia,was subjected to air drought stress and its photosynthesis characteristics were analyzed.The results show that air drought and sufficient soil moisture supply lead to the decrease in the total chlorophyll (Chl) content,while the ratio of Chls to carotenoids is constant in the Siberian crabapple tree.The function of photosystem Ⅱ (PS-Ⅱ) in the crabapple trees is characterized by a decrease in the fraction of absorbed light energy spent on the photochemical work and an increase in the proportion of non-photosynthetic thermal quenching.These changes indicate the photosynthetic down-regulation that acts as a universal photoprotective mechanism.During the midday hours,the combination of high air temperature and low air humidity leads to the decrease in the maximum photochemical quantum yield of photosystem Ⅱ (Fv/Fm) and the efficiency of photosynthesis (PABs).The parameters of leaf gas exchange show the significant differences in these values between the control and experimental variants.During the ming hours,the Siberian crabapple,growing in the Irkutsk City,assimilates carbon dioxide more intensively.Due to the higher air humidity,the stomata are kept open and the necessary amount of carbon dioxide entries the sites of carboxylation.The low air humidity combined with wind in the experimental variants leads to the unreasonably high water loss in the crabapple leaves by more than 27％ as compared to the control variant (Irkutsk City).However,water use efficiency in the ming hours increases during plant photosynthetic processes,i.e.,42％ higher than that of control.This,apparently,is a reflection of the adaptation processes of the Siberian crabapple to the air drought and parching wind.