Solution culture experiments were conducted using the Zn-inefficient cultivar IR26 and the Zn-efficient cultivar IR8192-31-2 to study on physiological and biochemical mechanisms of rice plant adaptation to Zn deficiency with relation to bicarbonate tolerance. The results showed Bicarbonate strongly inhibited root growth of Zn-inefficient cultivar especially at low Zn concentration. In contrast, it had little effect on the root growth of Zn-fficient culrivar. Bicarbonate increased the concentrations of malate and citrate in the roots of both cultivars, but to greater extent for the Zn-inefficient than for the Zn-efficient cultivars. The results implied that the inhibitory effect of bicarbonate on root growth of the Zn-inefficient cultivar might result from high accumulation of organic acids in the root cells. PEP-case is a key enzyme for the synthesis of organic acid, so it was concluded that increase of organic acid by bicarbonate resulted from increase of PEP-case activity.