Characterization of rapid intervascular transport of cadmium in rice stem by radioisotope imaging

Participation of the intervascular transport system within the rice stem during cadmium (Cd) partitioning was investigated by characterizing Cd-109 behaviour in the shoot. In addition, Ca-45, P-32, and S-35 partitioning patterns were analysed for comparison with that of Cd-109. Each tracer was applied to the seedling roots for 15min, and the shoots were harvested either at 15min (i.e. immediately after tracer application) or at 48h. Distribution patterns of each element at 15min were studied to identify the primary transport pathway before remobilization was initiated. P-32 was preferentially transported to completely expanded leaf blades having the highest transpiration rate. The newest leaf received minimal amounts of P-32. In contrast, the amount of S-35 transported to the newest leaf was similar to that transported to the other mature leaf blades. Preferential movement towards the newest leaf was evident for Cd-109 and Ca-45. These results directly indicate that elemental transport is differentially regulated in the vegetative stem as early as 15min before the elements are transported to leaves. Cd behaviour in the stem was investigated in detail by obtaining serial section images from the bottom part of shoots after Cd-109 was applied to a single crown root. At 30min, the maximum amount of Cd-109 was distributed in the peripheral cylinder of the longitudinal vascular bundles (PV) and, interestingly, some amount of Cd-109 was transported downwards along the PV. This transport manner of Cd-109 provides evidence that Cd can be loaded on the phloem at the stem immediately after Cd is transported from the root.