Colza (Brassica napus, v. Jaguar) Responses to Low Level of Metal Inputs Through Sewage Sludge Application: Induction of phytochelatin synthesis (10 pp)

Background, Aim and Scope   Most studies of sewage sludge disposal effects on plants have focused on high metal loadings. Less attention has been paid to plant responses to trace metal loadings below the recommended limit values. Materials and Methods: Here, a lysimetric experiment was conducted to assess the uptake, distribution and binding of trace metals by metal-induced, sulfhydryl-rich peptides (phytochelatins) in colza (Brassica napus, v. Jaguar) grown on a clayey, silty soil amended with a sewage sludge compost containing trace metal contents far below the recommended limit values established by French legislation. Chemical fractionation of unamended and sludge-amended soils was performed using a sequential extraction technique. Results: Copper concentrations in plant tissues were not affected by compost disposal. Its application at a single rate equivalent to 30 t/ha stimulated the growth of plants. Lead was not detectable in the plant material (< 1 mg g-1 dry wt.). Plants grown on the amended soil accumulated significantly more zinc than control plants. These phytochelatin complexes detected in leaves had a lower molecular weight than those extracted from roots. Those extracted from roots were composed of one type of phytochelatins (PCs) such as in leaves or a mixture of glutathione, PC2 and PC4. In comparison with control plants, sewage sludge compost application caused the synthesis of longer chain PCs in roots and in leaves. Furthermore, in comparison with control roots, glutathione and phytochelatin mixtures of higher molecular weight were detected in roots produced on the amended soil, whereas no significant increase in \total\ Cu and Zn content was observed in these organs after sludge application. Discussion: Compost application induces a significant increase in the proportion of the most labile forms of zinc and especially its pH 4.7 acid-soluble forms and, as a consequence, a higher accumulation of zinc in plants. Effects of copper are limited due to its strong affinity for humic substances and lead does not seem to be transported in any organ of plants. The presence of phytochelatins, even in plants grown on the unamended soil, proved the ability of colza to synthesize them in the presence of zinc and copper. Conclusions: These primary results seem to prove, on one hand, ability of colza (Brassica napus, v. Jaguar) to synthesize phytochelatins as well as in roots, in leaves and, on the other hand, the sensitivity of the PC induction as suggested by their identification in plants grown on the control soil. Synthesis of longer chain PCs in roots and in leaves, and formation of glutathione and phytochelatin mixtures in roots, are plant responses to sewage sludge compost application. Recommendations and Perspectives: Phytochelatin analysis is thus supposedly able to be one of the bioindicators that may be used as an ecotoxicological risk assessment of wastes. Due to its ability to synthesize phytochelatins, colza could be chosen as a plant test. Phytochelatin analysis could also be limited to roots (more sensitive than leaves). However, further experiments are needed. Quantitative analysis of phytochelatins had not been carried out due to insufficient amounts of pure phytochelatin standards, that had allow us to better study relationships between trace metal amounts to vegetal response.