ACCLIMATION OF SALIX TO METAL STRESS

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. x calodendron Wimm.) were exposed to heavy metals in solution culture in an attempt to inc rease innate metal resistance. Resistance was estimated using comparat ive root measurements, and metal uptake was also studied. The first ex periment entailed pretreatments with background nutrient solution, or 0.25 and 0.50 mg Cu l(-1) amendments, and re-exposure to each of the s ame 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 0.07 5 mg Cd l(-1) + 1.25 mg Zn l(-1))(1) with concentrations gradually rai sed 10-fold over 128 d. Plants tested in the first experiment, followi ng pre-exposure to Cu, were no more resistant to subsequent exposure t o this metal. In the second experiment, gradual cumulative doses resul ted 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 o n uptake: increased root-bound Cu in all combinations, and the increas e 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 phyto remediation programmes are discussed.