CADMIUM TOLERANCE, ACCUMULATION AND PARTITIONING IN POPLAR AND WILLOW CLONES: RESULTS OF AN HYDROPONIC SELECTION

The pollution of soil and water by heavy metal due to human activities represents a serious environmental problem at global scale. In particular, cadmium is considered one of the most dangerous heavy metals, having toxic effects on plants and animals. Cadmium enters the environment from industrial processes, heating systems, urban traffic, phosphate fertilizers and mineralisation of rocks. Plants exposed to toxic cadmium concentration undergo a stress condition, revealed by damage symptoms such as chlorosis, growth inhibition, reductions in water and nutrient uptake, alteration of enzyme activity and photosynthesis impairment. To remove cadmium and other pollutants from contaminated areas, unconventional techniques that utilise biological processes have been successfully applied. In particular, plants can be used for removing or degrading pollutant from soil and, for that regards heavy metal, accumulate them in the harvestable parts. This technology, called phytoremediation, is less expensive and environmental disruptive than conventional remediation systems that consist mainly in the excavation and incineration of soil. In the last years forest trees have been studied for assessing the potentiality to remediate contaminated sites. Most of the studied conducted on trees, especially in Salicaceae plants, evidenced that tolerance to heavy metals depends on their compartmentalisation in the roots and low translocation to the leaves. This is probably the major constraint to overcome for a more efficient utilization of these species to clean up soils from pollutant contamination. In this context, screening of clones characterised by different biomass production for heavy metal tolerance, accumulation and distribution among plant parts could be very effective to select plant material to be utilised for phytoremediation strategies. In the present work, rooted cuttings of some poplar and salix clones were exposed for three weeks to 50 µM cadmium sulphate in hydroponics in a growth chamber and morpho-physiological parameters and cadmium content distribution among plant parts were evaluated. Total leaf area and rooting characteristics were affected by cadmium treatment differently among clones and species. Poplar clones evidenced a remarkable variability in cadmium accumulation and distribution while salix clones showed more homogeneity for this trait. Tolerance index (Ti), translocation factor (Tf) and bio-concentration factor (BCF) were calculated. Mean values for all clones of the two salicaceae species showed a remarkable higher ability of salix to tolerate cadmium respect to poplar, as expressed by tolerance index (Ti) calculated on dry weight of plants root and shoots. For that regards mean values of Tf, Salix evidenced a more than twice capability respect to poplar, while the opposite behaviour was observed for the total BCF.