Photosynthetic pigments and heavy metal accumulation in urban tree species as bioindicators of vehicular pollution in Winneba Municipality, Ghana

This study investigated the impact of vehicular pollution on photosynthetic pigments and heavy metal accumulation in four dominant roadside tree species, Albizia lebbeck (L.) Benth., Azadirachta indica A. Juss, Khaya senegalensis (Desr.) A. Juss, and Senna siamea (Lam.) H.S. Irwin & Barneby. Tree species were selected based on their abundance and distribution along a major arterial road with high traffic volume (Winneba junction to WindyBay Avenue) and compared to a low traffic control road at the University of Education, South campus. Leaf samples were collected and analysed for photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids) using spectrophotometry. At the same time, heavy metal concentrations (Cr, Cu, Fe, Mn, and Ni) were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES), and the Metal Accumulation Index (MAI) was calculated to evaluate metal uptake capacity of the tree species. Statistical analysis, including independent t tests and Pearson’s correlation analysis (p < 0.05 and 0.001), was applied using SPSS version 26. The results indicated significant reductions in photosynthetic pigments in leaves under pollution, with A. lebbeck showing the highest reduction in total chlorophyll (91.95%), while S. siamea exhibited minimal reductions (4.73%), indicating species-specific differences in pollution tolerance. Heavy metal concentrations were significantly higher in leaves from the polluted road, with K. senegalensis showing the highest chromium uptake (85.71%). Correlation analysis revealed negative associations between heavy metal concentrations and photosynthetic pigments in most species, suggesting oxidative stress-induced pigment degradation. The MAI identified K. senegalensis and A. indica as effective Cr and Cu pollution bioindicators, respectively. These findings emphasize the potential of urban trees as bioindicators and phytoremediators in managing vehicular pollution. The study highlights the importance of selecting pollution-tolerant species for urban greening to enhance air quality and ecosystem resilience in rapidly urbanizing regions like Winneba.