Effect of heavy metals on peppermint and cornmint

Heavy metal pollution of agricultural soils and air is one of the most severe ecological problems on a world scale and in Bulgaria in particular. The biggest sources of pollution in Bulgaria are some non-ferrous metals smelters, such as the Non-Ferrous Metals Combine (NFMC) near Plovdiv, situated on very fertile soils. Vegetable, arable and animal production in this area results in contaminated produce with excessive amounts of Cd, Pb, Cu, Mn and Zn.In order to discover some crops which could be grown on these areas without contamination of the end product, we conducted (in 1991–1993) field experiments in the vicinities of NFMC near Plovdiv. As experimental material we used Mentha piperita L. (cv Tundza and Clone No 1) and Mentha arvensis var piperascens Malinv. (cv Mentolna-14). Plants have been grown on three Plots: Plot No 1-at a distance of 400 m from the source of pollution; Plot No 2-at 3 km from the source of pollution and on a control Plot-in the experimental gardens of University of Agriculture in Plovdiv, at 10 km from the source of pollution. It was established that heavy metal pollution of soil and air at a distance of 400 m from the source of pollution decreased the yields of fresh herbage by 9–16% and the yield of essential oil by up to 14% compared to the control, but did not negatively affect the essential oil content and its quality.Oils obtained from Plot 1 at a distance of 400 m from the source of pollution have not been contaminated with heavy metals.Cultivar response to heavy metal pollution was established. A positive correlation between Pb concentration in leaves and in essential oil was found.Heavy metal concentration in the plant parts was found to be in order: for Cd roots > leaves > rhizomes > stems; Pb roots = leaves > rhizomes = stems; Cu roots > rhizomes = stems = leaves; Mn roots > leaves > stems = rhizomes; Zn leaves > roots > stems = rhizomes.The tested cultivars of peppermint and cornmint could be successfully grown in highly heavy metal polluted areas, as in the area around NFMC near Plovdiv, without contamination of the end product-the essential oils.Despite of the yield reduction (up to 14%), due to heavy metal contamination, mint still remained a very profitable crop and it could be used as substitute for the other highly contaminated crops.