Effect of automobile related metal pollution on plants: A case study along two roads (Faisalabad to Sargodha and Pindi Bhattian to Lillah) in the Punjab, Pakistan

Abstract

Automobiles release a number of toxic metals into the surrounding environment. They enter human body through food chain and cause many toxic effects. Plants prove good indicators of their existence. In this study five herbaceous plant species (Calotropis procera, Datura alba, Ricinus communis, Parthenium hysterophorus and Cenchrus ciliaris) commonly growing along two roads i.e. Motorway (M-2) and Faisalabad-Sargodha road (FSR) in the Punjab, Pakistan, were collected. Plant and soil samples were collected in all the four seasons (2013-2014) from roadsides. Samples taken 100 m away from roads were designated as control. Lead (Pb), cadmium (Cd), nickel (Ni) and zinc (Zn) metals were analyzed in all the plants and soil samples by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrophotometer). Carbon (C) and nitrogen (N) contents in plants and soil samples were also measured. Relative plant attributes i.e. photosynthetic pigments (chlorophyll a, b, total chlorophyll contents and carotenoids), gas exchange characters (photosynthetic rate (A), transpiration rate (E), stomatal conductance (gs), internal CO2 concentration (Ci) and water use efficiency (WUE)), total soluble proteins, total free amino acids and total antioxidant activity were studied. Significantly higher concentrations of all the metals were found along roadsides in plants and soils as compared to controls and they clearly showed spatial and temporal variations. In both plants and soil samples, higher contamination of metals was recorded during summer season, while, the least contamination was noticed during winter season. The metals concentrations were obtained in the order Zn > Ni > Pb > Cd. Higher metals, C and N concentrations were recorded along FSR road as compared to M-2. Pull-111 was proved to be the most polluted site. A general reduction in photosynthetic pigments, photosynthetic rate, transpiration rate, stomatal conductance and total soluble proteins were recorded, whereas, increase in internal CO2 concentration, water use efficiency, total free amino acids and total antioxidant activity was observed under metal toxicity. Among plants, Calotropis procera leaves accumulated the highest level of Pb, Cd and Ni, while, Ricinus communis showed tendency to accumulate high quantities of Zn, thus, these plant species can be used as good biomonitors / phytoremediators. The metal contents in plants at most of the sites showed significant positive correlation with traffic density. High level of metals was also found in fuel (petrol and diesel) and soot samples. So, control measures are required to overcome transport sector related pollution which may become severe in forthcoming days.