Growing rice grain with controlled cadmium concentrations

Two solution studies were conducted a) to investigate the uptake of zinc (Zn) and cadmium (Cd) by rice plants (Oryza sativa L.) and interaction between these elements, and b) to determine experimental conditions for growing rice grain with desired Cd concentration for an animal feeding study. In both studies, free metal activities of cadmium and cationic microelements were buffered by an excess of chelating agents.-The first study was a factorial design with two Zn levels (1.0 and 3.89 muM) and four Cd levels (0.81, 1.44, 2.56 and 4.55 muM) in the solution. In the second study, rice was grown in two solutions of different micro- and macro-element compositions and three Cd levels (0.0, 0.5, and 2.0 muM). In the first study, solution Zn concentration of 3.89 muM and corresponding free metal activity (pZn(2+)) of 6.00 was toxic to young rice plants. With time, Zn concentrations in rice plants decreased while Cd concentrations increased. Toxic concentration of Cd in roots (about 100 mg kg(-1)) associated with a 20% reduction in the root dry matter occurred at the free Cd2+ activities in the solution (pCd(2+)) in the range of 10.25-9.75. Sufficient Zn level in plants slightly stimulated Cd transfer from roots to shoots as opposed to barely sufficient or slightly deficient Zn concentration in shoots. However, the better Zn status in plants clearly diminished severity of Cd toxicity symptoms in shoots. The use of nutrient solutions adapted for rice growth allowed the rice grown in the second experiment to produce grain under controlled conditions. Cadmium in the brown rice grain I was 0.1 to 0.8 mg kg(-1), covering the range needed for feeding experiments relevant to rice Cd risk to humans. Composition of the nutrient solutions, in addition to solution Cd level, had a significant effect on Cd concentration in grain. Correlation of grain Cd concentration with solution Cd2+ activity was much stronger than with total solution Cd. Results of both experiments supported hypothesis that Cd uptake and transport within rice plants is an active process.