Cadmium distribution in maize inbred lines: effects of pH and level of Cd supply

In order to investigate the physiological basis of the differential Cd distribution and the degree of variation of this Cd distribution among maize inbred lines, six inbreds designated earlier as 'shoot Cd excluders' (B73, H99, and H96) and 'non-shoot Cd excluders' (B37, H98, and N28) were grown in nutrient solution culture at different external Cd levels or at different pH. The characterization of the inbreds according to their shoot/root partitioning of Cd was consistent, independent of pH or level of Cd supply. The Cd concentrations in the plants were highest at the highest pH of the solution cultures. Generally, there was a positive correlation between the Cd concentrations in shoots and xylem exudates. It was shown that the Cd concentration in the roots is particularly important in the Cd distribution process. Above a 'critical' internal Cd concentration in the roots, specific for each inbred, the ability to retain Cd is strongly diminished. It is concluded that structural and/or physiological characteristics of the roots are involved in Cd partitioning.