From green to clean: a promising and sustainable approach to remove toxic metals from contaminated soils

Phytoextraction is an in situ decontamination technique using metal accumulating plants and appropriate soil amendments to transport and concentrate metals from the soil into the aboveground parts of plants, which are harvested with conventional agriculture methods. Since most of the metal hyperaccumulating plants only produce very low biomass, and most of plants producing high biomass accumulate only moderate amounts of metals, the current research is mainly focused on the overcoming of this deficiency to optimise metal phytoextraction. The main goal of our study aimed at improvement of phytoextraction through improved metal accumulation in sunflowers producing a high biomass. The potential use of their oil and biomass for technical purpose (biodiesel, biogas and energy) allows to produce an added value and to improve the economical balance of phytoextraction. Chemical mutagenesis (non-GMO approach) and appropriate fertilization treatments stimulating metal bioavailability in the soil were used as an alternative to genetic engineering to enhance both metal accumulation and extraction efficiency of oil crops. The effect of chemical mutagenesis on metal accumulation and extraction potential of two sunflower cultivars was directly assessed on a metal contaminated field. Theoretical calculations for phytoextraction potential of new variants show that the best sunflower mutants of the 2nd generation can produce up to 26 t dry matter yield per ha and remove 13.3 kg Zn per ha and year at the sewage sludge contaminated site; that is a gain factor of 9 compared to Zn removal of sunflower controls. Results of field experiments on the same metal contaminated site confirmed the improved yield, metal accumulation and metal extraction efficiency by new sunflower mutant lines in the 3rd and 4th generation. Sunflowers still showed a yield improvement by a factor of 4-6 and metal extraction by a factor of 3-4 for Cd, 5 for Zn and 5-6 for Pb.