RESUMEN
Concentrations of cadmium in the grain of durum wheat (Triticum turgidum L. var durum) are often above the internationally acceptable limit of 0.2 mg kg(-1). Cultivars that vary in concentrations of cadmium in the grain have been identified but the physiology behind differential accumulation has not been determined. Three pairs of near-isogenic lines (isolines) of durum wheat that vary in aboveground cadmium accumulation (8982-TL 'high' and 'low', W9260-BC 'high' and 'low', and W9261-BG 'high' and 'low') were used to test the hypothesis that the greater amounts of cadmium in shoots of the 'high' isolines are correlated with greater volumes of water transpired. In general, cadmium content was positively correlated with transpiration only in the 'low' isolines. Although shoots of the 'high' isolines of W9260-BC and W9261-BG contained higher concentrations of cadmium than did their corresponding 'low' isolines, they did not transpire larger volumes of water. In addition, isolines of 8982-TL transpired less water than did the other pairs of isolines yet both 'high' and 'low' isolines of 8982-TL contained higher amounts of cadmium than did the other pairs. The difference between 'high' and 'low' isolines appears to be related to the relative contribution of transpiration to cadmium translocation to the shoot. Increased transpiration was associated with increased cadmium content in the 'low' isolines but in the 'high' isolines increased cadmium in the shoot occurred independently of the volume of water transpired.
RESUMEN
The cell walls of plants, including those of algae, have the capacity to bind metal ions in negatively charged sites. The authors had already shown that the wild type (walled) strain of the unicellular green alga Chlamydomonas reinhardtii Dangeard was more tolerant to Cd, Co, Cu, and Ni than a wall-less mutant of the same species. The objective of the present study was to determine if the tolerance to metals was associated with an increased adsorption of the same metals to the cell wall. Adsorbed metal was defined as that fraction that could be removed with a solution containing Na(2)EDTA and CaCl(2). The fraction that remained after the EDTA/CaCl(2) wash was considered to be strongly bound in the cell. When exposed to metals, singly, in solution for 24 h, cells of both strains accumulated the metals. The original hypothesis was supported by the results for Cd, Co, and Ni insofar as significantly higher concentrations of these metals were in the loosely bound fraction of the walled strain in comparison with the wall-less strain. However, there are three reasons why the potentially protective effect of the cell wall did not explain differential tolerance of the two strains. After 24 h of exposure (1) less Cd was accumulated internally by the wall-less strain than by the walled strain, (2) very little of the accumulated Cu was in the loosely bound fraction of the walled strain, and (3) the two strains accumulated comparable and relatively high amounts of internal Cu. Unexpectedly, significant amounts of Cd and Cu were also removable from the surface of the wall-less cells. One possible explanation for these apparently externally bound metals in the wall-less strain is that the cells exuded metal-chelating molecules that decreased the ability of metal ions to penetrate the plasma membrane. It was concluded that metal tolerance in this alga must involve a complex of mechanisms involving both internal and external detoxification of metal ions.