Differences in cadmium accumulation between indica and japonica rice cultivars in the reproductive stage

Excessive cadmium (Cd) in rice grains is of great concern worldwide, particularly in southern China where heavy metal pollution in the soil is widespread. Much work has been done regarding the key genes responsible for Cd absorption, transport, and accumulation in rice, but little is known about the differences of Cd accumulation between indica and japonica rice cultivars during the reproductive stage. Furthermore, physiological parameters, such as nonstructural carbohydrate content, involved in Cd accumulation have not been fully elucidated. We studied several indica and japonica cultivars under three different Cd treatment levels and harvested them at different periods after heading. Differences in Cd accumulation between subspecies mainly were generated during the reproductive stage. An increase in the Cd pollution level caused the average absorption rate of Cd in the aerial parts of the indica cultivars in the reproductive stage to be 6.17, 4.52, and 3.89 times greater than that of the japonica cultivars across the three Cd treatments. The contribution of Cd absorption by shoots to Cd accumulation at the pre- or postheading stages was 33.8% and 66.2% in indica, and 44.9% and 55.1% in japonica. We found a significant negative correlation between Cd content in the rice grains and the content of nonstructural carbohydrates in the sheath (P < 0.05). Cd translocation from sheath to grain occurred along with sugar transfer in the indica cultivars. The Cd content of the indica cultivar grain was 1.84-4.14 times higher than that of the japonica cultivars (P < 0.05). The japonica cultivars thus met the cereal Cd limits of China (0.2 mg kg^-1) under low and moderate soil Cd pollution. These findings are helpful for the selection of proper cultivars and field management practices to alleviate Cd exposure risk in rice production.