| New-released oat (Avena nuda) cultivar ‘Dingyou No. 6’ seedlings were exposed to Hoagland nutrient solution with five NaCl or NaHCO3 levels (0, 50, 100, 150, 200 mmol·L-1) for 4 days, and the effects of NaCl and NaHCO3 on photosynthetic pigment content, gas exchange and chlorophyll fluorescence parameters, and xanthophyll cycle in oat seedling leaves were studied by a sandy culture. The results showed: (1) Under NaCl stress, the chlorophyll contents, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), PSⅡ maximum photochemical efficiency (Fv/Fm), actual photochemical efficiency (ΦPSⅡ), photochemical quenching (qP), and Hill reaction activity were decreased significantly with increasing concentration of NaCl, whereas the non-photochemical quenching (NPQ), regulated energy dissipation Y（NPQ）, non-regulated energy dissipation Y（NO）, the relative deviation（β/α-1）from full balance between two photosystems, and the de-epoxidation extent of xanthophyll cycle (A+Z)/(V+A+Z) were increased significantly, as well as the carotenoid content and stomatal limited value (Ls) exhibited a humped-curve, the intercellular CO2 concentration (Ci) decreased and then increased. (2) When oat seedling were treated with NaHCO3, the chlorophyll contents, Pn，Gs，Tr，Ls，Fv/Fm，ΦPSⅡ，qP and Hill reaction activity significantly decreased, and the Ci，NPQ，Y（NPQ），Y（NO），（β/α-1），（A+Z）/（V+A+Z）increased obviously except an humped-curve of carotenoid content with the increasing of NaHCO3 concentration. (3) At the same concentration of Na+, the degree of decrease in chlorophyll content, Pn，Gs，Tr，Fv/Fm，ΦPSⅡ，qP and Hill reaction activity and increase in NPQ，Y（NPQ），Y（NO），（β/α-1），（A+Z）/（V+A+Z） were higher in NaHCO3 tress than in NaCl stress. These findings indicate that the injury by alkali stress to photosynthetic system of oat seedlings was greater than that by salt stress. |