The role of root anatomy, organic acids and iron plaque on mercury accumulation in rice

Background and aims: Rice contaminated by mercury [Hg, especially methylmercury (MeHg)] has given rise to great concern in recent years. This study investigated variations in ecophysiological features (anatomy, organic acid secretions, Fe plaque formation) of rice roots and their effects on the uptake and accumulation of total mercury (THg) and MeHg by rice plants. Methods: The development of apoplastic barriers in roots of four rice cultivars was observed by a hydroponic experiment while the concentrations of five organic acids, Fe and THg in Fe plaque were determined using a rhizobag trial with different Hg treatments. Results: Cultivars with low Hg accumulation tended to develop strong apoplastic barriers in endodermis, secrete less organic acids and form more Fe plaque on root surfaces and in rhizosphere. Fe concentrations were positively correlated with THg concentrations in rhizosphere’s Fe plaque (R² = 0.60, P < 0.01), whereas the latter was negatively correlated with bioavailable Hg concentrations in rhizosphere (R² = 0.40, P < 0.01). Conclusions: Organic acids and Fe plaque formation of rice roots play important roles in Hg uptake and accumulation. The development of apoplastic barriers in root restricts Hg uptake but the significance of suberin deposition on Hg uptake needs further investigations.