ARS | Publication request: TOXICITY OF ZINC AND COPPER TO BRASSICA SPECIES: IMPLICATIONS FOR PHYTOREMEDIATION

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 2, 1996
Publication Date: N/A

Interpretive Summary: Heavy metal (Pb, Zn, Cd, Cu, Cr and Ni) contamination of some soils poses serious problems to both human health and agriculture in the superfund sites in the US as well as aboard. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage at land- fills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. As over 70% of the metal contaminated sites in the U.S. involve 2 or more metals, the possibility of synergistic effects of multiple metal toxicities may be important for the remediation of these sites. Thus, in this study, we investigated the individual and combined toxicities of zinc and copper to Brassica species that might be used to remediate heavy metal contaminated sites. We found that copper was more toxic than zinc, and exposure to both heavy metals induced micronutrient (iron) deficiency in the plants, as well as causing a significant inhibition of root growth and a decrease in the accumulation of each metal in the shoots. These findings indicate that when remediating sites contaminated with these two metals, it may be necessary to use leaf applications of iron to promote better plant health and shoot biomass production, as well as apply organic materials to the soil to tie up the copper and minimize its toxic effects.

Technical Abstract: The toxicity of Zn and Cu in three species from the genus Brassica was examined to determine if these plants showed sufficient tolerance and metal accumulation to be used to phytoremediate a site contaminated with these two heavy metals. Hydroponically-grown twelve day old plants of Brassica juncea, B. rapa, and B. napus were grown hydroponically for an additional two weeks in the presence of either elevated Zn (6.5 mg/L), Cu (0.32 mg/L) or Zn+Cu in order to quantify the toxic effects of these metals on several different growth parameters. With few exceptions, both root and shoot dry weight for all three species decreased significantly in the presence of heavy metals. Cu inhibited lateral root elongation in B. rapa, B. napus, and, to a lesser extent, B. juncea, while Zn tended to only decrease lateral root diameter. Both metals reduced shoot Fe and Mn concentrations in all three Brassica species to levels associated with Fe and Mn deficiencies. These deficiencies, however, did not correlate with observe patterns of leaf chlorosis. Nonetheless, heavy metal-induced inhibition of Fe and Mn accumulation may have been a significant factor in reducing plant growth. In terms of heavy metal removal, the Brassica species were more effective at removing Zn from the nutrient solution than Cu. Some accessions removed the equivalent of 10 to 12% of the total Zn from the nutrient solution. The extent of heavy metal removal was reduced when both metals were present. The implications of these results for phytoremediation are discussed.