Effect of Zinc and Cadmium Concentrations on the Rates

of their Absorption by Rice (Oriza sativa L.)

Part 1: Vegetative Growth[1]

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

18th 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 concentrations of available phosphorus, zinc and cadmium, were selected from among some 40 surface (0-30 cm) samples that were collected in rice paddies of East Mazandaran during 2001 to study the effect of zinc and cadmium concentrations applied during the vegetative stage of rice on some growth characteristics and chemical composition of rice crop. A randomized complete block factorial experiment including three zinc treatments, two cadmium treatments and three replications (2x3x20) was carried out in a greenhouse during 2002 using 20 soils. ZnSO₄.7H₂O was applied at the rates of 0, 5, and 10 µg/g, and cadmium as CdSO₄.8H₂O at the rates of O and 3 µg/g on the 20 soils contained in plastic buckets. Nitrogen in the form of urea was added at the rate of 100µg/g in several split applications; other nutrients such as phosphorus, potassium and copper were also applied to 6 kg soils in the buckets as solutions at rates calculated on the basis of soil tests along with zinc and cadmium fertilizers. Eight weeks after planting the germinated seeds onto flooded soils, rice plants were cut off just above the crown for the determination of dry matter production, as well as tissue zinc concentrations by atomic absorption and tissue cadmium concentrations by ICP-AES instruments with a detection sensitivity in the range of ppb dry matter production increased from 12.24 g/pot to 14.08 and 13.95 g/pot on the average as a result of zinc treatments. The rates of increase of dry matter production in response to 5, and 10 µg zinc per g soil were calculated to be 15 and 14 percent, respectively, for the 20 different soils. The concentrations of zinc in rice shoots were measured to be 50µg/g for the control, 50 and 54µg/g for the rice tissues that received 5 and 10 µg zinc per g of soils, respectively. Total absorption of zinc by rice plants also increased from 611 µg/pot to 720 and 764 µg/pot in response to the addition of 5 and 10 µg Zn per g of soils, respectively. Therefore, even though the concentration of zinc in rice shoots were not appreciably affected by zinc additions, but total absorption of this nutrient however, increased with increasing rates of its application.  This apparent discrepancy can be explained in terms of increases in dry matter production in response to the increasing rates of zinc fertilizer. The addition of 3 µg Cd per g of soils did not affect the rate of growth of rice plants. On the other hand, the total absorption and the tissue concentrations of cadmium did not exceed the ppb range despite the application of 3 µg Cd per g of soils, so that the concentrations of Cd in rice tissues were measured to increase from 45 µg/kg of tissues in control samples to 64 µg/kg in the tissues of plants receiving 3 µg Cd per g of soils. Effects of interaction between zinc and cadmium on the growth rate of rice plant and its zinc and cadmium contents were quite negligible and statistically insignificant. However, there seemed to be a negative correlation between soil Zn-Cd concentrations and growth parameters. Cadmium application decreased in dry matter production as well as the concentrations of Zn in plant tissue at every level of zinc treatment, and similarly, zinc application resulted in lower levels of Cd in plant tissue at each level of Cd treatment.

Keywords: Rice; Zinc; Cadmium; Rice paddies.

1 This article was extracted from a Ph.D. thesis in soils, Science and Research Division of Azad Islamic University.

2 Researcher, Mazandaran Agriculture and Natural Resources Research Center and Professor, Trarbiat Modarres University, respectively.