Rice uptake of cadmium and arsenic: evaluation, uptake mechanism and mitigation

Abstract

Arsenic (As) and cadmium (Cd) are two trace elements that can be taken up and accumulated in rice (Oryza sativa L.). Rice is a globally important staple food, and As or Cd uptake could potentially pose health risks for rice consumers because both elements are categorized as a human carcinogen. In order to estimate and control the health risk posed by As and Cd in rice grain, it is important to evaluate rice grain As and Cd concentrations, understand how soil chemical parameters and water managements affect As and Cd uptake, and understand the uptake and translocation mechanism of Cd in rice. In this dissertation, rice As and/or Cd concentrations were evaluated from three rice production regions, where rice As and/or Cd levels were not previously reported. In addition, the impact of rice paddy soil chemical parameters and water managements on rice As and Cd uptake were investigated, and the impact of Mn availability on Cd uptake and translocation in rice were investigated at both chemical and the plant molecular level. Rice grain grown in smallscale rice farms in the northeastern United States and in Everglades Agricultural Area (EAA) in southern Florida were found to pose little As and Cd health risks for rice consumers. In contrast, rice grain produced in Cambodia may pose Cd health risks for rice consumers. Soil As or Cd concentrations, water management, and soil pH were found to be important factors influencing rice As and/or Cd uptake. Higher Mn activity in hydroponic rice study was found to compete with Cd for uptake transporter OsNramp5 into rice plant. However, the decreased Cd concentration or mass accumulation in rice grain was not as significant as in rice root under higher Mn activity. The data suggest that grain Cd accumulation was also complicated by Cd root to shoot and shoot to grain translocation, which likely differs from Mn translocation pathways within rice plant but involved with Cd transporter OsCCX2. These findings have implications for the role of Mn as a Cd mitigation strategy and should be further explored.