Quantitative estimation of the contribution of the phloem in cadmium transport to grains in rice plants (Oryza sativa L.)

The contribution of the phloem in translocation of cadmium (Cd) to grains of rice plants (Oryza sativa L. cv. Kantou) was estimated. We measured Cd concentrations in phloem sap collected from the uppermost internode of rice plants at the grain-filling stage. Cadmium concentration was 17.8 +/- 4.5 mu mol L-1 in plants treated with a nutrient solution containing 10 mu mol L-1 Cd for 2 days. Subsequently, distribution ratios of Cd-109 between the grains and the glumes were determined after feeding of Cd-109 to the cut ends of the uppermost internodes or onto the surface of the flag leaves or to the roots. The results suggested that 91-100% of the Cd in the grains was deposited from the phloem. To our knowledge, this is the first determination of Cd concentrations in phloem sap transported to grains, and the first estimation of the contribution of the phloem in Cd transport to rice grains.