-

Growing plants on soils containing toxic levels of heavy metals results in growth reduction and transfer of heavy metals to food chain. Copper and Mn are among the important metal contaminations of agricultural and pasture soils respectively. The physicochemical properties and thereby the tolerance mechanisms of plants against toxic levels of Cu and Mn are very different. In this research three important crop species including rice, maize and sunflower were used to study the physiological responses of plants to toxic levels of these heavy metals. Results showed that, all tested species were more sensitive to Cu than Mn at equimolar concentrations in the medium. Rice was the most tolerant plant to Cu and the most susceptible to Mn toxicity. In contrast, sunflower was the susceptible to Cu and tolerant to Mn. There was any positive relation between the rate of metal accumulation and growth reduction in all of the studied species, so that, the most tolerant species for Cu and Mn (rice and sunflower, respectively) were also the most accumulating species. Comparison of uptake and translocation of two heavy metals demonstrated that in all three tested plants the uptake values of Cu was higher than that of Mn, and in contrast, the highest translocation values were obtained for Mn comparing to Cu. Therefore, roots were the main accumulating organs for Cu, and shoots the most accumulating parts for Mn. Retranslocation experiment showed that, loaded Cu at toxic levels in the first and second leaves, was remobilized near completely to younger leaves. In contrast, any reduction of Mn content in the old leaves was shown throughout the growth period, in the exception of that in rice. A higher remobilization of Mn in the latter species was likely the cause of a higher susceptibility to Mn toxicity and the cause of a poor apoplasmic compartmentation.