Effect of Zinc and Cadmium Concentrations on the Rates of Their Absorption

by Rice and on Some Growth Characteristics of the Plant (Oriza sativa L)[1]

Part 2: Yield and composition

A. Charati and M. J. Malakouti [2]

18^th World Congress of Soil Science, Philadelphia, Pennsylvania, July 9-15,2006, USA

Abstract: Twenty soil samples with the widest range of physicochemical properties, especially with respect to the levels of available phosphorus, zinc and cadmium, were selected out of some 40 surface soils (0-30 cm) collected in rice paddies of East Mazandaran to be used for evaluating the effect of soil zinc and cadmium levels on the growth and chemical composition of rice. A randomized complete block factorial experiment with three levels of zinc treatments (0,5, and 10 μg, Zn/g of soil as ZnSO₄.7H₂O) two levels of cadmium (0 and 30 ~g Cd/g of soil as CdSO₄.8H₂O) and three replications was carried out on 20 soils during 2002 in a greenhouse using plastic containers filled with 6 Kg of soil samples. Nitrogen as urea was used at a rate of 200 μg/g on each soil in several applications. The amounts of other nutrients including phosphorus, potassium, and copper were calculated on the basis of soil tests and applied as solutions along with other fertilizers. At the end of the experiment, three remaining four month old rice plants that had produced grain were cut off at the crown from each pot. The plants were oven dried after counting the number of tillers per plant. The grain and the straw were then separated and weighed before measuring the concentrations of zinc by atomic absorption and cadmium contents by ICP-AES instrument with detection sensitivity in the range of ppb. The results showed that zinc application did not affect grain yield significantly even though it decreased the weight of unhusked grain in 20 soils by 1.37 and 2.3 per cent on the average, in response to 10 μg Zn/g soil treatments, respectively. As a whole, the application of zinc during rice plants seed formation stage had insignificant effect on the weight of rice grain and even decreased it to some extent. This negative effect could have been caused by the space limitations of the experimental container. However, zinc application increased the concentration of this nutrient in the grain. The average concentrations of zinc in the unhusked rice grain increased significantly from 61.8 μg/g in the control samples to 64.3 and 64.6 μg/g in the samples that were treated with 5 and 10 μg Zn/g of soil, respectively. The total uptake of zinc by rice varied from 1498 μg/pot for 20 soils on the average in the control samples to 1507 and 1467 μg/g for zinc treatments of 5 and 10 μg/g of soil, respectively, but not significantly different in statistical terms. The negligible increases in total zinc absorptions can be explained in terms of small decreases in the weight of rice grains. Cadmium applications did not affect the weight of rice grains, or their cadmium concentrations appreciably, so that cadmium in the control samples was 22 μg/kg of grain dry matter and 97 in the samples that received 3 μg Cd/g of soil both in the p.p.b. range. Effect of Zn-Cd interactions on the weight of rice grain was shown to be insignificant statistically, negligible and without any particular trend. Whereas 10 μg Zn-3 μg Cd treatment produced 23.16 g of grain per pot the control Zn-Cd treatment produced 24.05 g of grain on the average. The Cd-Zn interaction negatively affected the concentrations of Zn in the grain; in other words, whereas increasing levels of soil zinc increased grain's zinc concentrations, cadmium additions decreased the grain's zinc concentrations at every level of soil zinc. Accordingly the lowest grain zinc concentration of 62.5 μg/g of dry weight was obtained at 0 μgZn-3 μgCd per g. of soil and the highest such concentration of 64.8 μg/g obtained at 10 μg Zn-O μg Cd per g of soil, not significantly different in statistical terms. The effect of Zn-Cd interactions on the concentrations of cadmium in the grains was also seen to be insignificant and without any trends.

Keywords: Rice; Zinc; Cadmium; Rice paddies.

[1] This article was extracted from a Ph. D. Thesis in Soils, Science and Research Division, Azad Islamic University.

[1] Ph.D. student in soils; and professor at Tarbiat Modaress University, respectively.