Germination, growth, osmotic adjustment and ionic balance of wheat in response to saline and alkaline stresses

The effects of saline and alkaline stresses (from 1:1 molar ratios of NaCl : Na2SO4 and NaHCO3 : Na2CO3, respectively) were compared on the germination, growth, osmotic adjustment and ionic balance of wheat seedlings to elucidate the mechanism of alkaline stress (high pH) damage to plants and their physiological adaptive mechanism to alkaline stress. The effects of salines on the activity and free concentrations of various ions in nutrient solutions were analyzed with the program GEOCHEM-PC. This analysis showed that alkaline stress caused a heavy precipitate of phosphate and metal ions, excluding Na+ and K+, which caused a sharp decrease in ionic activity and in the free concentrations of various ions. The inhibitory effects of saline stress on the growth and germination of wheat seeds were reduced compared with the effects of alkaline stress. Alkaline stress damaged root function, photosynthetic pigments and the membrane system and led to severe reductions in root system activity and in the content of photosynthetic pigments, and to a sharp increase in electrolyte leakage. Massive Na+ influx may be the main cause of damage from alkaline stress. The effect of alkaline stress on the accumulation of inorganic ions was stronger than that of saline stress. Under alkaline stress, Na+ sharply increased and NO3- and H2PO4- decreased in shoots, which caused a severe deficit in negative charge. Wheat might enhance organic acid synthesis to remedy the shortage of inorganic anions and maintain a stable intracellular pH, and accumulate betaine and soluble sugars to cope with the osmotic stress from the high Na+ concentration in the vacuoles.