Acclimation of Salix to metal stress

Abstract

Nine different clones of six species of Salix (Salix cordata Muhlenb. non Michaux, S. fragilis L., S. caprea L., S. cinerea L., S. burjatica Nazarov. and S. viminalis L.) and one hybrid (S.×calodendron Wimm.) were exposed to heavy metals in solution culture in an attempt to increase innate metal resistance. Resistance was estimated using comparative root measurements, and metal uptake was also studied. The first experiment entailed pretreatments with background nutrient solution, or 0·25 and 0·50 mg Cu l⁻¹ amendments, and re-exposure to each of the same concentrations. In a second experiment clones were exposed to sub-toxic concentrations of single metals (0·15 mg Cu l⁻¹, 0·15 mg Cd l⁻¹ or 2·5 mg Zn l⁻¹) and dual-combination treatments (0·075 mg Cu l⁻¹+0·075 mg Cd l⁻¹, 0·075 mg Cu l⁻¹+1·25 mg Zn l⁻¹ or 0·075 mg Cd l⁻¹+1·25 mg Zn l⁻¹)¹ with concentrations gradually raised 10-fold over 128 d. Plants tested in the first experiment, following pre-exposure to Cu, were no more resistant to subsequent exposure to this metal. In the second experiment, gradual cumulative doses resulted in reduced phytotoxicity and increased resistance, most notably to Cd. There appeared to be an inverse relationship between metal uptake and resistance. Copper uptake was restricted to the roots, whereas Cd and Zn were more evenly distributed throughout the plant. Exposure to dual combinations of metals resulted in several interaction effects on uptake: increased root-bound Cu in all combinations, and the increase in uptake of both Cd and Zn into the root tissues when supplied with Cu. The implications of these results for the use of willows in phytoremediation programmes are discussed.