Photosynthetic Pigments and Heavy Metal Accumulation in Urban Tree Species as Bioindicators of Vehicular Pollution

This study investigated the impact of vehicular pollution on photosynthetic pigments and heavy metal accumulation in four dominant roadside tree species, Albizia lebbeck , Azadirachta indica , Khaya senegalensis , and Senna siamea . Leaf samples were collected along a major arterial road with heavy traffic and compared with those from a low-traffic control road. Photosynthetic pigments (chlorophyll a , chlorophyll b , and carotenoids) were quantified using spectrophotometry, while heavy metal concentrations (Cr, Cu, Fe, Mn, and Ni) were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES). 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 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 Metal Accumulation Index (MAI) identified K. senegalensis and A. indica as effective bioindicators for Chromium and Copper pollution, respectively. These findings emphasize the role of urban trees in mitigating vehicular pollution by acting as bioindicators and sinks for heavy metals. The study highlights the importance of selecting pollution-tolerant species for urban greening and phytoremediation efforts, particularly in rapidly urbanizing regions.