Osmoprotectants and physiological responses of three Eucalyptus species to toxic concentrations of lead

One of the serious problems across the world is heavy metal pollution in water bodies that caused by Lead (Pb). This study was designed to find out the effect of Pb toxicity on physiological and biochemical changes in three Eucalyptus species (Eucalyptus camaldulensis Dehnh., E. microtheca F. Muell and E. occidentalis Endl.).Seeds were grown in marble chips and irrigated with nutrient solution. The seedling (42 days old) were exposed to Pb(NO3)2 treatments (Pb:50, 100, 200 mM) for ten months. After this period, leaf, stem and root tissues were harvested. Lead content were determined by ICP-OES and some physiological and morphological characters were determined. The order of Pb accumulation in three species were E. occidentalis> E. camaldulensis> E. microtheca. The concentrations of lead in root tissue were higher than leaf and stem tissue and stem concentration was lower than the concentration of leaf (root Cu>leaf Cu>stem Cu). Tissue concentration increased as Pb increased in three species. The content of prolin, soluble sugar raised by increasing metal concentrations, but the content of pigments, relative water content and specific leaf area decreased. These results suggest that eucalypts have efficient mechanism to tolerate Pb toxicity, as evidenced by accumulating of osmoprotectants. Comparison of three Eucalyptus species revealed that E. occidentalis had the highest concentrations of Pb in the leaves and stems. Accumulation of Pb in the E. occidentalis leaves was 2.38 and 1.82 times more than E. microtheca and E.camaldulensis respectively. Although the least amount of stress tolerance index (0.76) was observed in E. occidentalis. In general, the results showed that Eucalypts could be considered as a tolerant species for hyperaccumulation of lead and phytoremediation of contaminated soil by lead.