Department of Plant Sciences

It has been frequently suggested that root exudates play a role in Zn mobilization. We studied root exudation of low-molecular weight organic anions (LMWOAs) in three aerobic rice genotypes in relation to their Zn uptake efficiency. Plants were grown in a rhizotron filled with a Zn deficient clay soil added with or without 5 mg Zn kg-1 soil as ZnSO4. Soil solution was sampled when symptoms of Zn deficiency for –Zn plants appeared. We measured LWMOAs in the soil solution and found that plants responded to Zn deficiency with increased (64%) root exudation of malate. The efficient genotype exuded higher amounts of malate than the inefficient genotype. The genotype with medium Zn uptake efficiency, however, exuded less malate than the inefficient genotype. No significant correlation between malate concentrations in rhizosphere and plant Zn uptake efficiency was found. These results suggest that Zn mobilization by soil-grown rice genotypes cannot be explained by increased malate exudation and the variation in plant Zn uptake is most likely not be explained by their capacity to exude LMWOAs as a response to Zn deficiency.