Effects of the growth stage on the heavy metal tolerance of alfalfa plants

Alfalfa plants grown in soil at different growth stages were exposed to separate batches of Cr(VI) at 100 mg/l, and Cd(II), Cu(II), Ni(II), or Zn(II) at 500 mg/l. Four days after germination, all metals, except Zn(II), had lethal effects on the alfalfa seedlings. Furthermore, when applied 16 days after germination, Cr(VI) and Ni(II) still had lethal effects on the seedlings and Cd(II) and Cu(II) destroyed more than 50% of the plant population. While approximately 90% of the plants exposed to Cd(II), Cu(II) and Zn(II) were able to grow without apparent negative effects 20 days after germination, Cr(VI) and Ni(11) still showed lethal effects. The concentration of heavy metals in shoot dry tissues was 1209 mg/kg for Cd, 887 mg/kg for Cu and 645 mg/kg for Zn. These results demonstrated that the tolerance of alfalfa plants to Cd, Cu and Zn was positively correlated with the age of the plants. Also, these results opened the possibility of using alfalfa plants, via transplant, to clean up soils where the concentration of Cd, Cu or Zn is high enough to avoid alfalfa seed germination. (C) 2003 Elsevier Science Ltd. All rights reserved.