Visualizing Root Plaque Formation and Arsenic Sequestration on Rice Roots

Rice accumulates more arsenic than other crops, presenting a risk to consumers. Much of the arsenic in the porewater can be sorbed by iron-rich root plaques that form when rice is grown under flooded conditions. However, these plaques have also been implicated as a source of arsenic due to potential reductive dissolution and arsenic release during grain filling. Here, we obtained root cross sections and the associated iron plaques in soil thin sections taken from flooded rice paddies. Synchrotron x-ray fluorescence imaging revealed strong co-localization of arsenic, iron, and manganese. Multiple energy mapping showed a mixture of arsenite and arsenate present in iron plaques, with differences in the localization of these species in different roots. We also used vinyl films to precipitate iron plaques adjacent to rice roots. These films, when examined by synchrotron x-ray fluorescence showed a similar distribution of elements as intact soil thin sections. These techniques provide new insights into the speciation of arsenic in rice root plaques.