AVAILABILITY OF ZINC AND CADMIUM TO DIFFERENT PLANT-SPECIES

A pot study was conducted to investigate whether differences in the am ounts of Zn and Cd taken up by a number of plant species (canola, cape weed, subterranean clover, lettuce, swiss chard, ryegrass, and wheat) growing in a single soil could be explained as the result of plants ha ving access to different soil pools of Zn and Cd. Prior to potting, th e soil was spiked with carrier-free Zn-65 and Cd-109. Total shoot conc entrations of both Zn and Cd varied greatly across the plant species s tudied. 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 i n 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 a ll plant species were accessing the same pool of Zn in the soil, despi te differences in the total amounts of Zn taken up. In contrast, the s pecific activity of Cd in canola was found to be nearly double the spe cific activity of Cd in the other plants, suggesting that canola was u nable 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 bio available pools was calculated to be approximately 12 and 36 (or 20% f or canola), respectively, implying that significant percentages of the se metals exist in non plant available forms in this soil.