Sunflower mutants with improved growth and metal accumulation traits show a potential for soil decontamination

Over the last two decades, the use of plants has been proposed as an alternative technique to remove toxic metals from contaminated soils. This technique, called phytoextraction, can use either hyperaccumulating species, able to accumulate and tolerate high amounts of metal, but producing low biomass, or high yielding crops compensating moderate metal accumulation by a high biomass. Both types of plants can be considered for metal removal, but soil decontamination still takes quite a long time. Therefore plants used for metal removal need to be improved. This paper summarizes our previous and present work aimed at the improvement of sunflowers for phytoextraction by chemical mutagenesis. Improved yield and metal accumulation in sunflower mutants were already observed in the M2 mutant generation, where three new sunflower phenotypes were found: mutants with a significantly enhanced biomass production and no changed metal accumulation; mutants with a slightly improved biomass production and an enhanced metal accumulation in shoots; and mutants with reduced metal uptake. The same alterations in growth and metal accumulation were observed in the following generation. The best M3 sunflower mutants showed a 3-5 times higher Cd, a 4-5 times higher Zn, and a 3-5 times higher Pb extraction, as compared to the control inbred line. The stability of improved traits, yield and metal uptake, was confirmed also in the 4th generation, where mutant lines still provided a significantly enhanced metal extraction. Metal translocation from root to shoot and distribution within the shoot (stem, leaves and flower) of mutant lines and control sunflowers grown on a metal contaminated soil was studied in detail in the 5th generation under greenhouse conditions. Sunflower mutant seedlings show after three months of cultivation on contaminated soils a very good metal translocation capacity; thus the metals were primarily accumulated by sunflower leaves. (author)