Studio degli effetti del Cr(III) su piante di salice: confronto fra differenti specie e condizioni sperimentali

The increasing numbers of chromium contaminated sites in Italy requires the adoption of low-cost and large scale clean-up technologies. As an alternative to the traditional methods, phytoremediation could be exploited, which uses herbaceous and woody plants in association with the rizosphere microorganisms to remove and/or detoxify soils, waters and sediments from environmental pollutants. Trees have been suggested as suitable plants for the phytoremediation technologies, being high yielding biomass and genetically variable organisms. Particularly, species of the genus Salix are fast growing plants, excellent biomass producers, and their roots can explore large zones in deep. Therefore, due to the need to identify plant species characterized by high phytoremediation potential, in the present study Salix spp. have been sceened for tolerance to and accumulation of trivalent chromium (CrIII). At first, seven Salix species were considered: S. alba L. sub. typica, S. daphnoides Vill., S. elaeagnos Scop., S. fragilis L., S. purpurea L., S. matsudana Marsh. var. tortuosa e S. nigra Koidz. The responses of plants to CrIII were evaluated in terms of duration of treatment (since 4 hours to 1 year), growing environment (climatic chamber or green-house), growing substrate (hydroponics, soil, perlite), and metal concentration (50, 100 e 300 ?M di CrCl3 for hydroponic and agriperlite experiments; 50 ?moli Cr Kg-1 for soil trial). Four Salix species were selected in perlite experiments (50 and 100 ?M di CrCl3) on the basis of cross-results relative to high CrIII tolerance and accumulation: Salix alba sub. typica; S. elaeagnos, S. fragilis e S. matsudana var. tortuosa. The remaining three species displayed low Cr accumulation and/or tolerance, and were not used for further experiments. Selected plant species were in part grown in perlite with 100 and 300 ?M di CrCl3, and in part inside pots containing soil with no Cr (control) or CrIII 50 ?moli Cr Kg-1. Plants grown in perlite showed a slight decrease of biomass production and reduced Fe and S contents in response to Cr treatment. Also, Cr accumulation in roots accounted for 1426 ?g Cr g-1 on a dry weight basis (d. wt.) after 28 days of treatment. Conversely, plants grown in CrIII contaminated soil produced more biomass and chlorophyll than the controls, while containing up to 700 ?g Cr g-1 d.wt. in roots after 45 days in plus Cr. Among the four species, S. fragilis e S. matsudana were those accumulating high Cr amounts in both experiments. Therefore, the two species were further used in hydroponic experiments in the presence of 100 or 300 ?M CrIII for 15 days. Both plant species showed toxic effects due to CrIII exposure, such as decrease of leaf gas exchange and photosynthetic activity, reduction of growth, structural and ultrastructural alterations of root tips and leaves. Moreover, the increased activity of such enzymes as catalase and guaiacol peroxidase, involved in scavenging reactive oxygen species (ROS), was observed. Chromium accumulation in roots was significant at both short- (up to 4426 ?g Cr g-1 d.wt. after 4 hours of treatment) and long- (up to 6833 ?g Cr g-1 d.wt. after 15 days of treatment) exposure of plants to CrIII. In conclusion, the Salix species used in this study have shown high variability in root Cr accumulation, while the translocation rate of the metal to the shoot was generally limited. The effects of chromium on plants relied on the Cr concentration applied and on the plant growing substrate. Indeed, Cr was more available in hydroponic solution than in soil and perlite and, as a consequence, it was more promptly absorbed and caused more visible phytotoxic symptoms. Our results indicate that the employment of Salix spp., particularly of S. fragilis, may be useful in the phytoremediation of CrIII contaminated sites. In fact, S. fragilis was the species that accumulated high levels of Cr in all the experimental conditions tested. Therefore, the capacity of Cr accumulation seems to be an intrinsic species-related feature, whereas the growth medium would affect the bioavailability of Cr to plants, and therefore Cr accumulation in plant tissues. In soil contaminated with CrIII the presence of Salix plants allowed the removal of 26% more than that measured in soil subjected to the loss of Cr for water leaching