The seedlings of wheat were treated by salt-stress (SS, molar ratio of NaCl : Na 2 SO 4 = 1 : 1) and alkali-stress (AS, molar ratio of NaHCO 3 : Na 2 CO 3 = 1 : 1). Relative growth rate (RGR), leaf area, and water content decreased with increasing salinity, and the extents of the reduction under AS were greater than those under SS. The contents of photosynthetic pigments did not decrease under SS, but increased at low salinity. On the contrary, the contents of photosynthetic pigments decreased sharply under AS with increasing salinity. Under SS, the changes of net photosynthetic rate (P N ), stomatal conductance (g s ), and transpiration rate (E) were similar and all varied in a single-peak curve with increasing salinity, and they were lower than those of control only at salinity over 150 mM. Under AS, P N , g s , and E decreased sharply with rising salinity. The decrease of g s might cause the obvious decreases of E and intercellular CO 2 concentration, and the increase of water use efficiency under both stresses. The Na + content and Na + /K + ratio in shoot increased and the K + content in shoot decreased under both stresses, and the changing extents under AS were greater than those under SS. Thus SS and AS are two distinctive stresses with different characters; the destructive effects of AS on the growth and photosynthesis of wheat are more severe than those under SS. High pH is the key feature of the AS that is different from SS. The buffer capacity is essentially the measure of high pH action on plant. The deposition of mineral elements and the intracellular unbalance of Na + and K + caused by the high pH at AS might be the reason of the decrease of P N and g s and of the destruction of photosynthetic pigments.

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