Differential accumulation of cadmium in near-isogenic lines of durum wheat: no role for phytochelatins

Certain cultivars of some crops, including durum wheat (Triticum durum Desf.), have a propensity to accumulate cadmium in the grain. In the 1980s, a Canadian wheat breeding program generated five pairs of near-isogenic lines of durum wheat that vary in cadmium-accumulation. Within each pair, one member accumulates twofold to threefold higher concentrations of cadmium in the shoot and grain. However, the physiological explanation for the high-low phenotype is unknown. We studied correlations between concentrations of cadmium and non-protein thiols, including phytochelatins, in these five pairs of near-isogenic lines to test the hypothesis that differential retention of cadmium-binding complexes in the root would explain the phenotype. The expected high-low pattern of cadmium accumulation was found in three of the pairs. In one pair, cadmium was positively correlated with cysteine and glutathione in the roots and with phytochelatins 2 and 4 in the shoots but in another pair cadmium was strongly negatively correlated with phytochelatins 2 and 4 in the shoots and unrelated to cysteine or glutathione. No correlations between concentrations of cadmium and the non-protein thiols were found in the third pair or in the remaining two pairs. The production of phytochelatins is a well-described response to cadmium but the lack of consistent correlation between cadmium and non-protein thiols in these five near-isogenic lines indicates that complexation with non-protein thiols does not explain differential translocation of cadmium in durum wheat.