Biomonitoring and phytoremediation potential of Conocarpus erectus (Buttonwood) for mitigating air pollution from highway traffic

Abstract

This study investigates the impact of vehicular-released heavy metals (Zn, Fe, Pb, Cd, and Ni) on foliar physiological and biochemical parameters of Conocarpus erectus and its relationship with the Air Pollution Tolerance Index (APTI). Leaf samples were collected from six sites along a busy road (Bahawalpur to Rahim Yar Khan, KLP Road) in Punjab, Pakistan, during the spring season, with control samples taken 50 m away from the roadside. Heavy metal concentrations were analyzed using an Atomic Absorption Spectrophotometer (AAS), revealing significantly higher levels of Pb, Cd, Ni, and Cu in leaf dust at polluted sites, particularly in Ahmadpur. Exposure to heavy metal pollution led to notable reductions in shoot length, leaf area, and biomass, with Khanbella exhibiting the most severe declines. Gas exchange parameters were altered, with increased CO₂ assimilation and stomatal conductance at Taranda, while transpiration rates decreased across polluted sites. Biochemical responses, including elevated total soluble proteins, free amino acids, and ascorbic acid, indicated adaptive metabolic shifts. Photosynthetic pigments, particularly chlorophyll b and carotenoids, were significantly reduced under pollution stress, whereas antioxidant activity remained stable. Anatomical modifications, such as increased vascular bundle and cortical thickness at Taranda and Iqbalabad, reflected structural resilience. Stomatal density and area were significantly lower at polluted sites, suggesting regulatory adjustments to minimize water loss. APTI values (12.21–9.98) were significantly higher at roadside locations compared to controls, highlighting the strong resilience of C. erectus to pollution. The species high APTI values and capacity to tolerate heavy metal contamination suggest its suitability for urban roadside plantations, where it can mitigate the adverse effects of vehicular emissions and contribute to environmental sustainability.