Sensitivity of Mediterranean woody seedlings to copper, nickel and zinc

The restoration of heavy metal contaminated areas requires information on the response of native plant species to these contaminants. The sensitivity of most Mediterranean woody species to heavy metals has not been established, and little is known about phytotoxic thresholds and environmental risks. We have evaluated the response of four plant species commonly used in ecological restoration, Pinus halepensis, Pistacia lentiscus, Juniperus oxycedrus, and Rhamnus alaternus, grown in nutrient solutions containing a range of copper, nickel and zinc concentrations. Seedlings of these species were exposed to 0.048, 1 and 4 μM of Cu; 0, 25 and 50 μM of Ni; and 0.073, 25 and 100 μM of Zn in a hydroponic silica sand culture for 12 weeks. For all four species, the heavy metal concentration increased in plants as the solution concentration increased and was always higher in roots than in shoots. Pinus halepensis and P. lentiscus showed a higher capacity to accumulate metals in roots than J. oxycedrus and R. alaternus, while the allocation to shoots was considerably higher in the latter two. Intermediate heavy-metal doses enhanced biomass accumulation, whereas the highest doses resulted in reductions in biomass. Decreases in shoot biomass occurred at internal concentrations ranging from 25 to 128 μg g−1 of Zn, and 1.7 to 4.1 μg g−1 of Cu. Nickel phytoxicity could not be established within the range of doses used. Rhamnus alaternus and J. oxycedrus showed higher sensitivity to Cu and Zn than P. halepensis and, especially, P. lentiscus. Contrasted responses to heavy metals must be taken into account when using Mediterranean woody species for the restoration of heavy metal contaminated sites.