Differential tolerance to copper and zinc of micropropagated birches tested in hydroponics

Copper and zinc tolerances of 10 micropropagated birch (Betula pendula and B. pubescens) clones were studied in hydroponic culture. Tolerance indices were determined, based on the mean growth rate of the longest root in 1 wk. A seed-derived clone (142A), from a lead/Zn-contaminated site showed more tolerance to Cu and Zn than bud-derived clones (HA02 and HA18) from a Cu/nickel-contaminated site or an ozone-tolerant clone (KL-2-M) from an uncontaminated area. For Cu, the EC50 values were 30, 14, 8 and 11 mu M in clones 142A, HA02, HA18 and KL-2-M, respectively. For Zn, the EC(50)s were 4000 and 350 mu M in clones 142A and KL-2-M, respectively. The relative Cu and Zn tolerances of the other clones were estimated by growing the plants in 30 mu M CuSO4, and in 2000 or 350 mu M ZnSO4, respectively. It is of interest that the Zn-tolerant clone 142A was tolerant to Cu, although this metal was present at a very low concentration in the soil where the parent tree grows. Another clone (142B), from another seed of the same parent tree, was tolerant neither to Zn nor Cu. Compared with their own EC(50)s for root growth for Cu, 142A look up more Cu than KL-2-M, suggesting that the higher tolerance of the former clone is not explained by reduced uptake of Cu. The Zn uptake in clones 142A and KL-2-M was studied at 4000 mu M and 800 mu M Zn, respectively. Interestingly, the roots of both clones contained the same amount of Zn, even though clone 142A was exposed to a fivefold concentration of Zn.