DIFFERENCES IN THE CADMIUM CONTENT OF SOME COMMON WESTERN-AUSTRALIAN PASTURE PLANTS GROWN IN A SOIL AMENDED WITH CADMIUM - DESCRIBING THE EFFECTS OF LEVEL OF CADMIUM SUPPLY

The uptake of cadmium (Cd) by capeweed (Arctotheca calendula), subterranean clover (Trifolium subterraneum), santiago medic (Medicago santiago), wimmera ryegrass (Lolium rigidum) and kikuyu grass (Pennisetum clandestunum) was measured over an eight week period following seedling emergence from a loamy sand amended with nine concentrations of Cd (0-50 mug g-1). The uptake of Cd from soil amended with either 0 or 1 mug Cd g-1 was also measured at 7 day intervals over the eight week growing period. With the exception of wimmera ryegrass, yields were reduced by addition of Cd, and this reduction could be described by simple linear or quadratic equations. Addition of Cd increased the concentration of Cd in plants and the increase could be described using a rescaled Mitscherlich function. However, the accumulation of Cd at high levels of addition was depressed due to the effect of Cd supply on yield and a modified function was used to describe this effect. The concentration of Cd in tops (mug g-1) did not vary markedly with plant age. For Cd additions corresponding to typical levels of plant-available Cd in Western Australian (WA) pasture soils, the concentration of Cd in tops harvested six or eight weeks after emergence was about four times greater in capeweed than in subterranean clover or kikuyu, and about eight times greater than in wimmera ryegrass or santiago medic. However, because of differences in the moisture content of tops, there was only a threefold difference in the potential contribution to the Cd burden of grazing sheep between capeweed or subterranean clover at typical levels of soil Cd. For most plants, Cd concentrations in roots were about ten times greater than in tops, except in capeweed which translocated more of the Cd taken up to tops. A reduction in the Cd burden of grazing animals in WA would most likely be achieved by the production of pastures that are low in capeweed and dominated by species which can survive the drier periods of the grazing season.