Submitted to: Journal of Plant Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2001
Publication Date: 7/1/2002
Citation: Interpretive Summary: Rice has been found to transfer soil Cd to grain in high enough amounts to cause human renal tubular dysfunction, while garden foods and other grains do not. Other research indicated that levels of Fe, Zn and Ca in grain of rice caused malnutrition of humans, which increased Cd absorption and retention in kidney. Recent studies have shown that supplementation of rice based diets with Fe and Zn strongly reduced kidney Cd of rats. Thus improved understanding of the bioavailability of Cd in rice grain is required in order to develop limits to protect consumers. To obtain intrinsically labeled 109Cd levels in rice grain which are relevant for feeding tests of rice Cd bioavailability, one must be able to grow rice with appropriate levels of Cd and normal levels of nutrients which may interact with Cd bioavailability. This paper reports our attempt to grow rice grain with desired levels of Cd for feeding tests, with varied levels of Cd and Zn. Subsequently, we adapted published methods to grow rice to grain in nutrient solutions to control of Cd and Zn phytoavailability, and using these methods, were able to produce rice grain with levels of Cd relevant to feeding tests. In these experiments, lower solution Zn allowed the higher Cd levels in the test to cause toxicity. Also, increasing Zn supply somewhat increased Cd transport from roots to shoots. Between controlled nutrient supply and genetic control by the plants, grain levels of elements were within the range found for field produced crops. Thus the goal of producing rice grain with needed levels of Cd, Zn and Fe for feeding animals to test rice Cd bioavailability was met, and these methods can be used to produce labeled grain for feeding experiments.
Technical Abstract: Two solution studies were conducted a) to investigate the uptake of Zn and Cd by rice plants 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 sof chelating agents. The first study was a factorial design with two Zn levels (1.0 and 3.89 mM) and four Cd levels (0.81, 1.44, 2.56 and 4.55 mM) 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 mM). In the first study, solution Zn concentration of 3.89 mM and corresponding free metal activity (pZn2+) 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 (pCd2+) 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 levels 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 grain production under controlled conditions. Cd in the brown rice grain 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.