Remobilization of cadmium in maturing shoots of near isogenic lines of durum wheat that differ in grain cadmium accumulation

Cadmium accumulation, in grain of durum wheat (Triticum turgidum L. var. durum) represents a concern to consumers. In an effort to understand the regulation of Cd accumulation in maturing grain, the remobilization of Cd-109 applied to stem and flag leaves was examined in two near-isogenic lines that differ in grain Cd accumulation. Absorbed Cd-109 was primarily retained in the labelling flap (50-54 % and 65-80 % for stem and flag leaves, respectively). Cadmium exported from the stem flap initially (3 d) accumulated in the stem in a declining gradient towards the head. Subsequent remobilization of Cd deposited in the stem was associated with Cd accumulation in the grain. Cadmium exported from the flag leaf flap was primarily directed to the grain. Little (< 1 %) Cd accumulated in the glumes or rachis, and transport of Cd to shoot tissues below the flag leaf node was low (< 1 %). On average, 9 % and 17 % of absorbed 109Cd accumulated in the grain 14 d after labelling the stem and flag leaf, respectively. Irrespective of labelling position, the low Cd-accumulating isoline averaged 1.5-2-fold lower Cd accumulation per grain and Cd concentration in the grain than the high Cd-accumulating isoline. Cadmium accumulation in the grain was inversely correlated with Cd retention in the stem (stem labelled) and labelling flap (flag leaf labelled) for both isolines. Cadmium translocation to the grain was not inhibited by Zn when both were applied simultaneously (50 pM Cd-109; 0.5 muM Zn-65) to the flag leaf. These results show that elevated remobilization of Cd from the leaves and stem to the maturing grain may be partially responsible for the high accumulation of Cd in durum wheat grain.