Effects of increasing cadmium (Cd) application rates on shoot and root growth, uptake and accumulation of Cd, and activity of antioxidative defense enzymes were studied in two durum wheat (Triticum durum) cultivars (cvs. Balcah-2000 and Balcah-85) differing in Cd tolerance. These durum wheat cultivars have been selected from a screening study including 10 bread and 6 durum wheat cultivars. The selected cultivars were grown in nutrient solution under controlled environmental conditions and subjected to increasing Cd concentrations (e.g., 0, 0.5, 2, 10, and 30 uM Cd). Genotypic variation in tolerance to increasing Cd stress was observed based on the development of necrotic patches on the base of the oldest leaves and reduction in dry matter production. Based on these parameters Balcah-85 was ranked as the Cd-tolerant and Balcah-2000 the Cd-sensitive genotype. The results of the root uptake and accumulation of Cd in root and shoot showed that the distinct genotypic difference in tolerance to Cd toxicity between two durum wheat cultivars was very closely related to the differential partitioning of Cd between roots and shoots. Both cultivars responded in a very similar way in total uptake of Cd by roots, but differed greatly in root accumulation and root-to-shoot transport of Cd. Compared to Balcah-2000, Balcah-85 had higher capacity to retain Cd in roots and reduce Cd transport into shoots. Consequently, in Balcah-2000 the shoot concentration and content of Cd were nearly 2-fold higher than that of Balcah-85, indicating a possible detoxification mechanism existing in Balcah 85 to retain of Cd in roots and prevent photosynthetic tissues from Cd toxicity. Genotypic variation was also studied in terms of antioxidative enzymes including ascorbate peroxidase (AP), glutathione reductase (GR), superoxide dismutase (SOD) and catalase (CAT). The results obtained did not show any consistent relationship between Cd tolerance and activities of the antioxidative enzymes. At the highest level of Cd applications there was a clear decrease in 02-(superoxide)-generating NADPH oxidase activity in both cultivars, possibly due to inactivation of the enzyme by binding of Cd to NADPH. The results obtained in the study clearly demonstrated that differential tolerance to Cd toxicity between durum wheat cultivars is related very closely to detoxification mechanism of the absorbed Cd in roots (e.g. binding to cell wall and/or compartmentalization in vacuole)
