HEAVY METALS IN AUSTRALIAN SUGARCANE

Abstract

In eastern Australia, 12 sugar cane (Saccharum officinarum L.) varieties and their different parts were screened for plant-mobile heavy metals to assess whether genetic differences were of greater significance than the soil/environment for uptake and within plant distribution. The purpose was to better understand factors controlling the cycling of heavy metals in sugar cane production systems. Soil pH (1:5 soil/water) ranged from 4.5 to 6.4 and all breeding-trial sites contained relatively low levels of extractable Cd, Hg, and Pb and variable levels of Cu and Zn. Internal concentrations of Cd and Zn were more influenced by soil/environment than by variety, while the distribution of metals in plant parts was quite consistent. About 77% of the Cd and 56% of the Zn were contained in the stem, which relocates to the mill following harvest. There was little Hg in all plant parts (concentrations<0.05 mg kg−1 d.w.). From a predictive viewpoint, correlations between extractable heavy metals such as Cd in soils and corresponding plant concentrations were inconsistent, with the narrow range in soil concentrations seen as a contributing factor. On present evidence, the uptake of heavy metals by sugar cane can be adequately managed by manipulating soil properties rather than by varietial selection. It is also clear that for each 100 tonnes of fresh, mature cane, about 0.2 g of Cd and 110 g of Zn will relocate to the soil surface with the trash. Corresponding quantities moved to the mill are 0.54 g of Cd and 143 g of Zn, with amounts expected to be higher for cane grown in strongly acidic soils with above average levels of heavy metals.