Effect of bicarbonate on elongation and distribution of organic acids in root and root zone of Zn-efficient and Zn-inefficient rice (Oryza sativa L.) genotypes

Bicarbonate content in growth medium has been considered to be one of the major factors in inducing zinc (Zn) deficiency in lowland rice. In this study, the effects of bicarbonate on root elongation, distribution and exudation of organic acids were examined in the Zn-efficient and Zn-inefficient rice genotypes. Bicarbonate significantly increased the growth of fine roots in the Zn-efficient genotype but inhibited their growth in the Zn-inefficient genotype. In fine root zones of 2-8 mm from the tip, of plants grown with bicarbonate, malate, citrate, and fumarate accumulation was greater in the Zn-inefficient genotype, but not the Zn-efficient genotype, than the control. Enhanced exudation of malate at fine root zones of 1-2 cm from the tip by bicarbonate was noted in the Zn-efficient genotype, but not in the Zn-inefficient genotype. Bicarbonate induced greater exudation of citrate at fine root zones of 1-2 cm from the tip in the Zn-efficient than in the Zn-inefficient genotypes. The increase in the concentrations of malate and citrate in xylem sap by bicarbonate were found to be greater for the Zn-efficient genotype than that for the Zn-inefficient genotype. The results indicated that bicarbonate-mediated inhibition of fine root elongation in the Zn-inefficient genotype might result from excessive accumulation of organic acids, particularly malate, in root elongation zones. In the Zn-inefficient rice genotype the increased accumulation of malate and citrate in fine roots by bicarbonate appeared to result from less exudation of them to the rhizosphere and transport up to the shoots.