We compared the effects of salt-stresses (SS, 1 : 1 molar ratio of NaCl to Na 2 SO 4 ) and alkali-stresses (AS, 1 : 1 molar ratio of NaHCO 3 to Na 2 CO 3 ) on the growth, photosynthesis, solute accumulation, and ion balance of barley seedlings, to elucidate the mechanism of AS (high-pH) damage to plants and the physiological adaptive mechanism of plants to AS. The effects of SS on the water content, root system activity, membrane permeability, and the content of photosynthetic pigments were much less than those of AS. However, AS damaged root function, photosynthetic pigments, and the membrane system, led to the severe reductions in water content, root system activity, content of photosynthetic pigments, and net photosynthetic rate, and a sharp increase in electrolyte leakage rate. Moreover, with salinity higher than 60 mM, Na + content increased slowly under SS and sharply under AS. This indicates that high-pH caused by AS might interfere with control of Na + uptake in roots and increase intracellular Na + to a toxic level, which may be the main cause of some damage emerging under higher AS. Under SS, barley accumulated organic acids, Cl − , SO 4 2− , and NO 3 − to balance the massive influx of cations, the contribution of inorganic ions to ion balance was greater than that of organic acids. However, AS might inhibit absorptions of NO 3 -and Cl -, enhance organic acid synthesis, and SO 4 2− absorption to maintain intracellular ion balance and stable pH.
