Increasing phosphorus inhibits the retention and prevention of cadmium by iron plaque and promotes cadmium accumulation in rice plants

Iron (Fe) plaque is an important component of rice roots because it influences the uptake and transport of cadmium (Cd) in rice. In this study, a hydroponic experiment was developed to investigate the influence of phosphorus (P) on the formation of iron plaque on the root surface as well as the influence on Cd uptake in rice plants. Three important results were obtained, as follows. (1) During the formation of iron plaque induced by exogenous Fe, P supply was beneficial for the iron plaque formation, but it restrained the Cd retention capability, resulting in a decrease in Cd in iron plaque by 35.48-61.93%, and leading to an increase in Cd in rice roots from 72.13 mg kg^-1 to 112.78 mg kg^-1 (2) After the iron plaque induction, the formation of iron plaque was inhibited by P supply, resulting in the amount of iron plaque decreasing by 18.46-54.57%, and the Cd in iron plaque decreased by 3.93-31.78%. Then, the Cd retention capability and the prevention effect simultaneously decreased, and as a result, the Cd in rice roots increased from 100.83 mg kg^-1 to 146.03 mg kg^-1 (3) Without exogenous Fe induction, P sufficiency continued to increase the amount of iron plaque and decrease the Cd in iron plaque, and increase the Cd in rice plants. These results suggested that P supply increases the amount of iron plaque, which is ineffective for Cd retention, such as non-reddish-brown iron plaque (NIP), and then decreases the capacity of iron plaque to retain Cd. Additionally, the P supply decreased the amount of formed iron plaque, causing the decreasing prevention effect. Therefore, excessive application of P fertilizer should be avoided in Cd-contaminated paddy fields.