Individual And Interactive Toxicity Of Cadmium And Copper In Relation To Metal Accumulation,Subcellular Distribution And Chemical Form In Brassica Napus

Oilseed rape (Brassica napus L.) is an important source of edible oil worldwide, which also possesses attributes to withstand a certain level of metals toxicity. In the prospect of making progress in applied technologies so as to exploit its full potential against metal stress, it is important to understand the metabolism of metal in this species, which could vary with respect to metals properties. Cadmium (Cd) and copper (Cu) are two potentially toxic metals showing different chemical and biological properties; hence they are suggested to have some specificity in their toxicological pathways in plant. Furthermore, due to their frequent occurrence in common sources of soil contamination, the two metals are expected to coexist in contaminated soils and impose a mixture toxicity to plants. The present study was designed to identify some specificity in the toxicity responses of B. napus to Cd and Cu stress, and to assess the implications of their interactive effects. The pattern of metal accumulation, subcellular distribution and chemical form in plants and the associated physio-chemical changes were investigated to that end. Several experiments were carried out and the main findings are as follows:(1) Single and combined toxicity of Cd and Cu were assessed regarding metal accumulation and related physio-chemical changes in five-week-old B. napus seedlings hydroponically exposed to different levels of metal stress (0,50 and 200 μM). Observed results showed a higher uptake and translocation rate for Cd compared to Cu, though both metals primarily accumulated in plants roots. Despite its moderate bio-accumulation however, Cu caused higher oxidative stress, greater chlorophyll reduction and severe growth inhibition, which denoted a limited potential of B. napus for remediation of Cu contaminated soils. In plants equally growth-inhibited by these metals, Cu proved more deleterious to K and Mn nutrition, to chlorophyll content and to the overall pathway of reduced glutathione (GSH), while the Cd effect was more prominent on Zn content. Plant growth inhibition and oxidative stress were more pronounced in cultivar Zheda 622 than cultivar ZS 758, but alterations extent in mineral composition appeared similar in the two cultivars. In mixture treatments, Cd and Cu reciprocally affected their accumulation in plants and brought about phytotoxic effects different from predictions. Cd had a stimulatory effect on Cu bioaccumulation, which was suggested to drive metals synergistic effects on plant growth and oxidative stress, and a greater synergetic effect was observed in the sensitive cultivar (Zheda 622) as compared to the tolerant cultivar (ZS 758). By contrast, no metals synergetic effects were noted on nutrient contents reduction. Contrary to Cd effect, addition of Cu to the medium restricted the uptake of Cd. Besides, the two metals didn’t affect each other’s translocation factor, suggesting that they scarcely interacted within plant.(2) Possible specificity in the toxicological pathways of Cd and Cu in B. napus were also verified in relation to the pattern of metal distribution at cellular level and their chemical form in plant. Results delineated that cell wall and vacuole stored the largest part of both metals; however, Cd was less retained at cell wall compared to Cu, which partly explained its higher location in the vacuole-containing fraction. Despite its high exchange capacity, notable amount of Cu was also found in the vacuole-containing fraction, suggesting relatively low charge density at the cell wall of studied cultivars. Nonetheless, the tolerant cultivar ZS 758 retained more Cd and Cu at cell wall than the sensitive one Zheda 622; although no genotypic differences were detected as to metals concentrations in the soluble fraction. Across sensitive organelles, Cu preferentially accumulated in chloroplast, while Cd was almost equally distributed in chloroplast and mitochondrion; the two metals intruded nucleus at relatively lesser extent. A differential behavior between Cd and Cu was also noted with regard to their chemical form. The larger toxic fraction of Cd was represented by the water extractable form, while ethanol extractable form was the predominant toxic form of Cu. Further, the two metals differentially interacted with various cellular ligands, which might indicate their varied detoxification paths in B. napus. Cu was primarily complexed by phosphates ligands and additionally by organo-ligands and oxalates, but the role of phosphate and oxalates ligands appeared secondary in Cd inactivation, for which the action of organo-ligands predominated.