Response of tropical trees to elevated Ozone: a Free Air Ozone Enrichmentstudy.

Abstract

Tropospheric ozone (O3) has become one of the main urban air pollutants. In the present study, we assessed impact of ambient and future ground-level O3 on nine commonly growing urban tree species under Free Air Ozone Enrichment (FAOE)condition. During the study period, meanambient and elevated ozone (EO3) concentrations were 48.59 and 69.62ppb, respectively. Under EO3 treatment,stomatal density (SD) significantly decreased and guard cell length (GCL) increased in Azadirachta indica, Bougainvillea spectabilis, Plumeria rubra, Saraca asoca and Tabernaemontana divaricata, while SD increased and GCL decreased in Ficus benghalensis and Terminalia arjuna. Proline levels increased in all the nine plant species under EO3 condition. EO3 significantly reduced photosynthetic rate, stomatal conductance (gs), and transpiration rates (E). Only A. indica and N. indicum showed higher gs and E under EO3 treatment. Water use efficiency (WUE) significantlyincreased in F. benghalensis anddecreased in A. indica and T. divaricata. Air Pollution Tolerance Index (APTI) significantly increased in Ficus religiosa and S. asoca whereas it decreased in B. spectabilis and A. indica. Of all the plant species B. spectabilis and A. indica were the mostsensitive to EO3 (high gs and less ascorbic acid content) whileS. asoca and F. religiosa were the mosttolerant (lowgs and more ascorbic acid content). The sensitivity of urban tree species to EO3 is a cause ofconcern and should be considered for future urban forestry programmes. Our study should guidemore such studies toidentify tolerant trees for urban air pollution abatement.