Heavy Metal Content of Some Savanna Plant Species in Relation to Air Pollution

The Sb, Cr, Ni, As, Pb and Cu contents in leaves of Dichrostachys cinerea, Ficus platyphylla, Isoberlinia doka and Securiniga virosa growing around the Kaduna Refinery and Petrochemical Complex (KRPC) were compared with heavy metal contents of the same species growing in a relatively “clean” environment, during the rainy and dry seasons of 2001. Concentrations of Cr, Ni, As and Pb in the leaves of the plant species monitored were significantly lower during the dry season than during the rainy season, particularly in the polluted site (P < 0.05). The concentrations Sb and Cu were lower only in F. platyphylla and S. virosa. Patterns of accumulation of individual heavy metals were variable. All four species of plants accumulated Pb to a higher degree (P < 0.05), than Cr and Ni, which had accumulation patterns similar, and higher than that of As. Arsenic was the least (P < 0.05) accumulated. Accumulation of Cu was different to other metals, being higher than Pb, Cr, Ni, and As (P < 0.05), and second only to Sb, which was hyper-accumulated in all four plants species. Heavy metal contents of the four species varied between rainy and dry seasons. The differences in metal contents observed between seasons, could be linked to relative availability of cations to plants as influenced by soil moisture content. Variations in pattern of accumulation of individual metals, could be related to the genetically induced capacity of plants to alleviate the toxic effects of heavy metals through mechanisms, which include binding metals to peptides and organic acids, formation of phytochelatins, cellular adaptations, efflux pumping systems and other mechanisms.