Plants as Biomonitors of trace elements. A comparative study between lichens and higher plants

Biological monitoring is an effective method to estimate the levels of air pollutant and their impact on biological receptors. The aim of this work is to test the different capability to accumulate heavy metals of two kind of vegetables, a Spermatophyta and a Tallophyta, The plant species are: Parmelia caperata (L.) Ach. and Parmotrema chinense (Osbeck) Hale & Ahti, two foliose epiphytic lichens, and Nerium oleander L., an urban ornamental shrubs diffusely utilized in parks, gardens and along the roads. The lichen samples with their substrate bark were collected in the Forestal reserve of the Park of Vesuvio (“unpolluted site”); pot grown Nerium oleander plants came from the Botanical garden of the Facoltà of Agraria in Portici (Naples). The samples were exposed in four sites localized in the urban area of Portici, a densely populated town near Naples with very heavy road traffic. The first sampling was carried out after 15 days from exposition, the other 3 sampling were carried out at 1 month intervals. The samples were air-dried in the laboratory, ground to a fine powder, digested in mixed acid solution and analysed by ICP for determination of the total concentration of Fe, Zn, Cu and Pb. Our data suggest that: 1) Starting from the first sampling, the lichen species show a remarkable increase in the measured concentration of all tested pollutants. This confirm previous reports by other authors. The heavy metal concentrations in the last sampling were three to five times higher than the starting values. 2) The leaves of oleander show a lower increase in heavy metals laid down upon their surface; however, during the time of exposition to pollutant, this amount increased proportionally to what are measured in the lichens. Both lichens and leaves, show a direct correspondance between road traffic level in the different sites and heavy metal concentration. Our data confirm the higher capability of the lichens to accumulate pollutants for their morphological and physiological characteristics, but they put forward the possibility to consider oleander as a bioindicator too.