Rice growth and yield responses to saline water irrigation are related to Na+/K+ ratio in plants.

Rice growth and yield response to salinity can be influenced by the duration and the timing of salt stress. The present study tested the effects of saline water irrigation from vegetative growth to maturity on rice growth and yield and ion concentrations in the straw and root and related them to changes in soil salinity and soil solute potential. The treatments consisted of five levels of saline water irrigation (electrical conductivity ~0.25 (control), 4, 6, 8, and 10 dS m-1) with two rice cultivars (BRRI dhan67 and BRRI dhan99) grown in pots in a rain shelter. Grain weight per pot, dry straw weight, and root weight were significantly reduced with increasing water salinity, but BRRI dhan99 was less affected. With prolonged saline water irrigation, salt concentration increased in the soil and lowered the soil solute potential. Increased saline water induced higher concentrations of Na+ in the straw (527-1200 mmol kg-1 at 4-10 dS m-1) relative to the root. By contrast, higher Cl- concentrations accumulated in the root than in the straw. The decrease of K+ in the straw and root for increasing salinity was inconsistent, but the Na+/K+ ratio sharply increased in the straw with higher water salinity. The increased Na+/K+ explained most grain weight loss due to higher salinity (R2 = 0.93) followed by Na+ (R2 = 0.87) and Cl-1 (R2 = 0.53). We conclude that the prolonged saline water irrigation has a cumulative effect on root zone salinity and solute potential that depresses grain yield in rice by increasing the Na+/K+ ratio in plants.