Effects of road traffic on photosynthetic pigments and heavy metal accumulation in tree species of Kumasi Metropolis, Ghana

Abstract

This study was undertaken to examine changes in the content of pigments and accumulation of metals from vehicular pollution in selected species of roadside trees under vehicular pollution. A major arterial road with heavy vehicle emissions in the Kumasi Metropolis was designated as the polluted site, while Kwame Nkrumah University of Science and Technology Campus was designated as the control site. Four tree species (Terminalia catappa, Mangifera indica, Ficus platyphylla and Polyalthia longifolia) selected for the study were well distributed and abundant in the polluted and control sites. Photosynthetic pigments and levels of heavy metals (Pb, Cu, Cd and zinc) were assessed in their leaves. Chlorophyll and carotenoid contents were determined by absorption spectrometry, while the metal accumulation index (MAI) was used to determine the total metal accumulation capacity of the tree species. We observed a reduction in photosynthetic pigments in the leaf samples from the polluted site. Ficus platyphylla had the maximum reduction in total chlorophyll (49.34%), whereas Terminalia catappa recorded the lowest reduction (33.88%). Similarly, the largest decrease (31.58%) of carotenoid content was found in Terminalia catappa trees and the lowest in Polyalthia longifolia (16.67%). The Polyalthia longifolia, Ficus platyphylla and Terminalia catappa leaf samples collected at the polluted site recorded a higher ratio of chlorophyll a/b. Heavy metal (Cu, Pb, Zn and Cd) accumulation in leaf samples was higher in the polluted site than in the control, as expected. The highest metal MAI value was recorded in Mangifera indica (5.35) followed by Polyalthia longifolia with 4.30. The findings from this study specifically demonstrate that air contamination induced by vehicles decreases the level of photosynthetic pigments in trees subjected to roadside emissions. It is clear that both chlorophyll a/b and chlorophyll/carotenoid ratios will act as very useful stress-level markers. Elevated heavy metal levels in the tree species along arterial roadsides indicate that they serve as heavy metals sink. The change in MAI resulting from different pollution burden is an indication that the removal capabilities of the tree species differ from each other. We therefore suggest M. indica and P. longifolia as potential species to be used in air pollution reduction plans in the city.