Heavy Metal Pollution from Vehicular Emissions and its Phytomonitoring Along Two Roads i.e. Pindi Bhattian to Kala Shah Kaku and Lahore to Gujranwala

Abstract

The increase in heavy metal pollution along the roadside environment has become a major global concern recently. Therefore, current project was planned to monitor the level of heavy metals along the roads. For this study, five wild plant species (Calotropis procera, Cenchrus ciliaris, Cynodon dactylon, Nerium oleander and Parthenium hysterophorus) generally growing near the two roads [a segment of Motorway (M-2) from Pindi Bhattian to Kala Shah Kaku and a segment of Grand Trunk road (G.T. road) from Lahore to Gujranwala] in Punjab, Pakistan, were collected. The plant leaves and soil samples were collected from five sites along each road. The control samples of leaves and soil were collected at a distance of 50 meter away from roadside. The collected samples were analyzed during the four seasons of the year (2015-2016). Metals such as lead (Pb), cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) were examined in all the plant leaves and soil samples using Atomic Absorption Spectrophotometer (AAS). Some plant physiological parameters such as gas exchange characteristics [photosynthetic rate (A), stomatal conductance (gs), transpiration rate (E), sub-stomatal CO2 concentration (Ci) and water use efficiency (WUE)] were evaluated. Plant biochemical attributes such as the contents of photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll and carotenoids), total free amino acids, total soluble proteins and total antioxidant activity were also determined. Significantly higher contents of the studied metals were detected in soil and plants along the roadside as compared to their control samples and they indicated clear spatio-temporal variations. The highest contamination of metals in both plants and soil samples was noted during summer, whereas, minimum was observed during winter. The contents of metals were found in the order: Cd < Pb < Ni < Cu < Zn. The higher contents of all the metals were noted along G.T. road as compared to M-2. However, Kala Shah Kaku site along M-2 and Muridke site along G.T. road appeared as the more polluted sites. The correlation between metal contents in soil/plants and traffic density was significantly positive for almost all the sites during all the seasons. Significant positive correlation between metal contents in soil and metal content in plants was also observed. The high contents of metals were also obtained in petrol, diesel, soot and used motor oil samples. Physiological parameters such as A, E and gs were significantly lower while Ci and WUE were higher in all the plant species along the roadsides. The contents of photosynthetic pigments and total soluble proteins were significantly lower whereas total antioxidant activity and total free amino acids were significantly higher in roadside plant species under metal stress. Among plant species, C. procera accumulated the maximum contents of Cd, Ni and Pb whereas, N. oleander had potential to accumulate high concentrations of Cu and Zn, hence, these plants can be suggested as the best choice as phytomonitors and/or phytoremediators of the metal pollution.