Willows (Salix spp.) are suitable for heavy metal phytoextraction due to their high element accumulation, high metal transport from roots to the shoots and high biomass production. Many species and hybrids of Salix spp. are known to colonise contaminated soils. For example, S. alba, S. viminalis, S. cinerea and S. caprea naturally invade polluted dredged sediment disposal sites. Therefore, there is an opportunity for genotype selection by checking survival, regeneration, and potentially also by testing metals accumulating in tissues which could then be harvested with remediating contaminated soils. Willow species and clones have different tolerance to particular metals, and significant differences were found for the uptake and accumulation of metals among willow varieties and clones. Soil contamination by cadmium due to human and natural activities is one of the most serious environmental problems in the world. The aim of this study is to investigate the cadmium uptake potential of two cultivated varieties (‘Weishanhu’ and ‘Yizhibi’) of Salix integra, of which the variety ‘Yizhibi’ has stronger tolerance to cadmium stress than ‘Weishanhu’ according to previous studies, and to discuss the possible mechanisms which involved in root adaptation under hydroponically cadmium treatment.
The results showed the highest content of cadmium was observed in root tissues for both varieties of S. integra, and the cadmium content in phloem tissues was lower than that in roots. The cadmium content in leaves showed the lowest level among roots, phloem and xylem. The cadmium content in the aboveground (including phloem, xylem and leaf) tissues of two varieties of S.integra increased significantly compared to the control with the increase of cadmium treatment in the range of 10-70 μmol/L, and it came to the highest cadmium accumulation at the concentration of 70 μmol/L cadmium, while when exposed to 90 μmol/L cadmium solution, the cadmium content declined in the aboveground parts. The same accumulation patterns were found in roots for two varieties of S. integra in the range of 10-70 μmol/L cadmium treatment. However, when the roots were exposed to 90 μmol/L cadmium treatment, the cadmium content in ‘Weishanhu’ declined sharply, but still rose for that in ‘Yizhibi’. This suggests the roots of ‘Yizhibi’ have greater potentential in uptake of cadmium than the variety ‘Weishanhu’ when exposed to higher concentration of cadmium.
The higher uptake and accumulate potential of ‘Yizhibi’ depends on the relatively unchangeable root morphology and fractal dimension under cadmium stress. Comparing to the root images of ‘Weishanhu’ and ‘Yizhibi’ under cadmium treatment, we found the total length, total surface area and volume of roots of ‘Yizhibi’ changed a little compared to the control, while that of ‘Weishanhu’ decreased under cadmium treatment. The fractal dimension of two varieties of S. integra had minor decline, which may suggest both ‘Weishanhu’ and ‘Yizhibi’ prefer to simple root configuration under cadmium treatment. We also found the nutrient uptake in leaves of two varieties of S. integra changed in different cadmium concentration. The Ca and Mn content in leaves decreased significantly in the concentration of 50 and 70 μmol/L cadmium, and no obvious change was observed for Fe content in leaves. It is interesting that the Cu content in leaves of ‘Weishanhu’ increased significantly in the concentration of 50 μmol/L cadmium.