Comparative studies of nickel, cobalt, and copper uptake by some nickel hyperaccumulators of the genus Alyssum

The uptake of Ni, Co, and Cu by the nickel hyperaccumulator Alyssum troodii Boiss and the non-accumulator Aurinia saxatilis (L.) Desv. were studied in pot trials using artificial rooting media with varying concentrations of the metals added as soluble salts, singly and in combination. The ability of five other Ni hyperaccumulating species of Alyssum to hyperaccumulate Co was also investigated. Leaves and stems of A. troodii accumulated Ni to almost the same extent (8000–10 000 μg g-1). In roots, the highest Ni concentration was 2000 μg g-1. In leaves of Au. saxatilis, the maximum Ni concentration was only 380 μg g-1 and the level in roots was even lower. In media containing Co, the maximum concentration of this element in A. troodii (2325 μg g-1) was ten times higher than in the non-accumulator species. Slightly less Co was found in stems and roots of both species. Among the other Ni hyperaccumulators, the maximum concentration of Co in leaves ranged from about 1000–8000 μg g-1. Copper concentrations were the same in all organs of both species when they were grown in copper-rich media and were in the range 40–80 μg g-1, showing that neither plant was capable of taking up Cu at levels comparable to those of Ni and Co. When both plants were grown in media containing equal amounts of both Co and Ni, the Co concentrations in plant organs were the same as for specimens grown in media containing Co only. However, the Ni levels were lower in both species. Uptake of Co therefore appeared to suppress Ni uptake. Pot trials showed that the order of tolerance was Ni>Cu>Co for A. troodii and Ni>Co≈Cu for Au. saxatilis, whereas the seedling tests showed the order to be Co>Ni>Cu. At metal concentrations ≥10 000 μg g-1, the overall tolerance of A. troodii was greater than that of Au. saxatilis which exhibited equally low tolerance to Ni and Cu. We conclude that in A. troodii, A. corsicum Duby, A. heldreichii Hausskn., A. murale Waldstein & Kitaibel, A. pintodasilvae T.R. Dudley, and A. tenium Halacsy, Ni tolerance and hyperaccumulation conveys the same character towards Co. This behaviour should be investigated in other hyperaccumulators of Ni and/or Co.