A possible explanation for such behavior could be the availability of
other metals, since plants exposed to stress activate the Cu–Zn superoxide
dismutase enzyme to protect themselves. This mechanism can be
activated only if there is Zn or Cu available (Goldstein et al., 2006).
Fig. 2 shows Cu and Zn concentration in topsoils at sampling sites
from 1 to 6, where Pb concentration was elevated, and the ANOVA
comparison among sites for each metal. Sampling site 5 presented
significantly lower Zn concentration than sites 1, 2 and 6; and lower
Cu concentration than sites 1, 6 and 4. These results could indicate
that Cu–Zn superoxide dismutase system was not activated at site 5
due to lack of those elements. Plants, devoid of such protection, could
not stop the entry of Pb, and it resulted to accumulation.