Cadmium effects on vegetables : production, physiology and biochemistry

Cadmium (Cd) is absorbed through the lungs and the digestive tract; however, for most human non-smokers, the major route of Cd entry into the body is by ingestion. Perhaps 5% of ingested Cd is absorbed and once absorbed, Cd accumulates mostly in the liver and kidneys, where it can cause a variety of health problems. This applies equally to grazing stock. The major entry point of Cd into the food chain is therefore uptakes of traces of Cd by crop plants. This thesis focuses on Cd in vegetables. The study was conducted in an Australian context, where geogenic Cd contributions to soils are typically low. Most of the Cd in soils on Australian vegetable farms originates from materials added to boost crop production. Phosphate fertilisers were and remain the dominant Cd source. Most Australian soils are P-deficient and high rates of P fertiliser are essential for successful vegetable cropping. The P fertilisers used throughout Australia were formerly made from guano deposits mined on Pacific Islands. These contained high percentages of Cd. Current rock sources contain lower Cd concentrations. Studies showed that all the vegetable samples which contained excessive Cd concentrations were leafy vegetables, including cabbage, lettuce, silverbeet, parsley and bok choy. Cadmium impaired photosynthesis; reduced dry weight of the whole plant; slowed leaf elongation rate; decreased the length of fully expanded leaves; slowed the rate at which new leaves appeared; and altered foliar concentrations of Zn, Mn, Cu, Ca and S. These profound changes affected all parts of the plant; consequently, Cd did not affect the proportion of dry weight partitioned to the roots, stems and leaves