Genotypic variations in cadmium levels of rice grain

To identify rice grain genotypes with a lower cadmium content, 49 cultivars of rice were tested under upland conditions, using two types of soils contaminated with cadmium (A: alluvial and B: volcanic ash soil) in 1999 and 2000. Results of ranking of grain cadmium concentrations were similar between these 2 years. NIPPONBARE, KOSHIHIKARI, and HU-LO-TAO belonged to the rice grain group with the lowest cadmium contents. KASALATH could be categorized as a variety with a medium cadmium content, hereafter referred to as medium cadmium variety, and MILYANG23 and PEH-KUH-TSAO-TU as varieties with high, cadmium contents, hereafter referred to as high cadmium varieties. These five varieties except for HU-LO-TAO from the above six typical varieties were grown in submerged pots (paddy conditions), and the ranking of the grain cadmium content was almost the same as that in the upland condition pots. These results imply that it may be possible to select paddy rice varieties with a low cadmium content even when growth occurred under upland conditions. The possibility of screening for lower cadmium content in rice grain in solution culture was examined. The shoot cadmium concentrations in solution culture were not significantly lower in NIPPONBARE and KOSHIHIKARI than those of MILYANG23 and PEHKUH-TSAO-TU after 4d of cadmium addition. Only PEH-KUH-TSAO-TU showed a higher shoot cadmium concentration than the other varieties after 27d of cadmium addition. Thus, as a preliminary evaluation, a screening method using solution culture would not be suitable for detecting genotypes with a low cadmium content in grain within 1 month after cadmium addition. Our results suggest that the status of available cadmium in soil differs from that of available cadmium in solution culture. Cadmium distribution for grain (i.e., grain cadmium content ratio to total uptake in the terrestrial part of plants) was evaluated among these 5 varieties. KASALATH absorbed a relatively high amount of cadmium, although cadmium distribution to grain in KASALATH was the lowest among the 5 varieties. On the other hand, in MILYANG23, a much higher amount of cadmium was translocated to the grain than in the other 4 varieties, suggesting the existence of genetic variability in cadmium translocation from shoots to grains.