Physio-biochemical responses of apple-of-sodom [Calotropis procera (Aiton) W. T. Aiton] to vehicular pollution

Abstract

Vehicular traffic has become a primary source of pollution along the roadsides which adversely affects plants and other biota. This study was conducted to assess the effects of vehicular pollutants on Calotropis procera growing at different sites along two major roads [Pindi Bhattian to Kala Shah Kaku (M-2) and Lahore to Gujranwala (G.T. road)] in the Punjab province of Pakistan. Control samples of Calotropis procera were collected at a distance of 50 m away from the roads. The photosynthetic rate (A), stomatal conductance (gs), transpiration rate (E), water use efficiency (WUE), contents of photosynthetic pigments, total free amino acids and antioxidant activity of C. procera were determined. The results showed significantly lower contents of chlorophyll a, chlorophyll b, total chlorophyll and carotenoids in C. procera along the roadside as compared to control plants. Similarly, stomatal conductance was also significantly lower, as was the case with photosynthetic and transpiration rates in C. procera growing along the roadside. The overall decrease in photosynthetic rate of C. procera was 34.90% and 44.79% along M-2 and G.T. road, respectively as compared to control. Reduced photosynthesis in roadside plants resulted in higher levels of water use efficiency. The higher contents of free amino acids and antioxidant activity were recorded and might be ascribed to the activation of the defense system of plants to cope with the detrimental effects of vehicle-related air pollutants. The correlation of different attributes of C. procera with traffic density was significant that suggested the direct impact of vehicular emissions on this species. Hence, C. procera can be used as a biomonitor of vehicular pollution.