OCCURRENCE OF HEAVY-METALS IN AIR AND THEIR ACCUMULATION BY TROPICAL PLANTS GROWING AROUND AN INDUSTRIAL-AREA

This study deals with the chemical nature of airborne particles and their accumulation in certain tropical plants growing around an industrial area. Particles of > 10.9, 1.6-5.4 and < 0.7 mum were significantly more abundant than those of 0.7-1.6 and 5.4-10.9 mum at all the polluted sites. The concentrations of chromium, copper, iron and lead were high in particles of < 10.9 and < 0.7 mum; while manganese and mercury were abundant in 5.4-10.9 and 0.7-1.6 mum-sized particles; and nickel was high in < 0.7 and 0.7-1.6 mum particles. The epidermal traits of five different plants (Cassia siamea, Calotropis procera, Ipmoea fistulosa, Zizyphus mauritiana and Mangifera indica) indicate unaltered stomatal indices, trichome length and trichome frequency in C. siamea, while stomatal index and trichome length decreased, and trichome frequency increased in all other plants. Changes in epidermal traits were found to be greatest in M. indica followed by I. fistulosa, C procera, Z. mauritiana and C siamea at all the polluted sites when compared to that of the reference site. A direct relationship was evident between the modified trichrome frequency and dust collecting efficiency of plants which was greatest in M. indica and lowest in C. siamea. Inspite of a low dust collecting efficiency, C. siamea accumulated heavy metals in greater amounts which indicates no relationship between the metal content of the deposited dust and accumulated metal content. This study, based on the differential accumulation of metals in different plants with relation to metal content of deposited dust and modified epidermal traits, lists the species studied in the following order of tolerance (scavenging potential): C. siamea > Z. mauritiana > C. procera > I. fistulosa > M. indica.