Availability of zinc and cadmium to different plant species

A pot study was conducted to investigate whether differences in the amounts of Zn and Cd taken up by a number of plant species (canola, capeweed, subterranean clover, lettuce, swiss chard, ryegrass, and wheat) growing in a single soil could be explained as the result of plants having access to different soil pools of Zn and Cd. Prior to potting, the soil was spiked with carrier-free Zn-65 and Cd-109. Total shoot concentrations of both Zn and Cd varied greatly across the plant species studied. The concentration of Cd in the shoots ranged from 0.075 mg/kg in wheat cv. RAC to 2.27 mg/kg in capeweed, while the concentration of Zn in the shoots ranged from 33 mg/kg in wheat cv; RAC to 259 mg/kg in swiss chard. However, the specific activity of Zn in the shoots was found to be the same in all cases. Hence the experiments showed that all plant species were accessing the same pool of Zn in the soil, despite differences in the total amounts of Zn taken up. In contrast, the specific activity of Cd in canola was found to be nearly double the specific activity of Cd in the other plants, suggesting that canola was unable to access a pool of soil Cd that was available for uptake by the other species. The percentage of total soil Zn and Cd that aas in bioavailable pools was calculated to be approximately 12 and 36 (or 20% for canola), respectively, implying that significant percentages of these metals exist in non plant available forms in this soil.