Variations between Two Rice Genotypes in Root Secretion of Organic Acids and Plant Pb Uptake

To attempt to understand certain mechanisms causing the variations between rice genotypes with regard to Pb uptake and accumulation, pot soil experiments were conducted with two rice genotypes at different levels of Pb, i.e., 34.05 (the control), 800, 2000 mg Pb kg^-1 soil. The two rice genotypes differ significantly with regard to Pb uptake and accumulation. Root secretions of low-molecular-weight organic acids (LMWOA) for each treatment were measured with ion chromatograph. The results showed that LMWOA concentrations in the soil planted with the genotype of indica (cv. Shan you 63, a higher Pb accumulator) were all higher than those in the soil planted with the genotype of japonica (cv. Wu yun jing 7, a lower Pb accumulator) at different soil Pb levels, although the magnitudes of the differences varied for individual LMWOA and depend on soil Pb concentrations. For all six LMWOA, there were significant differences at P < 0.05 or < 0.01 levels for soils treated with 800 and 2000 mg kg^-1 Pb. The magnitude of the differences was greater under soil Pb treatments, especially at relatively low level (for example, 800 mg Pb kg^-1 soil), than in the control. Acetic acid and formic acid constituted more than 97% of the total concentration of the six LMWOA, while citric acid constituted only about 0.1%. The rice genotype with higher concentrations of LMWOA in soil accumulated more Pb in the plants. The results indicate that LMWOA secretion by rice root, especially in Pb-contaminated soils, is likely to be one of the mechanisms determining the plant Pb uptake properties of rice genotypes.