Calcium Decreases Cadmium Concentration in Root but Facilitates Cadmium Translocation from Root to Shoot in Rice

Understanding cadmium (Cd) accumulation in rice is important for the development of plant-based strategies for crop safety. Cd and calcium (Ca) are antagonistic ions competing for uptake in plants when they co-occur in the environment. However, how Ca effect Cd uptake by root and translocation from root to shoot in rice plants is still unclear. Rice seedlings were exposed to different combinations of 1 mg/L of Cd and different Ca (0, 40, 80, 160 and 320 mg/L) levels for 7 days in a hydroponic system. Application of Ca improved the dry weight of the rice seedlings treated with Cd and reduced Cd concentration in root by 12.6 ~ 27.2%, possibly by protecting the integrity of the cell wall and plasma membrane besides competing for absorption sites. Cd concentration in rice root reduced most in the application of 160 mg/L of Ca. Inversely, Ca addition facilitated Cd translocation from roots to shoots, with evidence that translocation factor and the expression of genes that were involved in Cd transport (OsNRAMP5 and OsHMA2) were both significantly higher (P < 0.01). This results also provide new insight that decreasing Cd concentration in rice cannot simply by adjusting Ca concentrations.