Response of tropical trees to elevated Ozone: a Free Air Ozone Enrichment study

Tropospheric ozone (O 3 ) has become one of the main urban air pollutants. In the present study, we assessed impact of ambient and future ground-level O 3 on nine commonly growing urban tree species under Free Air Ozone Enrichment (FAOE) condition. During the study period, mean ambient and elevated ozone (EO 3 ) concentrations were 48.59 and 69.62 ppb, respectively. Under EO 3 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 EO 3 condition. EO 3 significantly reduced photosynthetic rate, stomatal conductance (gs), and transpiration rates (E). Only A. indica and N. indicum showed higher gs and E under EO 3 treatment. Water use efficiency (WUE) significantly increased in F. benghalensis and decreased 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 most sensitive to EO 3 (high g s and less ascorbic acid content) while S. asoca and F. religiosa were the most tolerant (low g s and more ascorbic acid content). The sensitivity of urban tree species to EO 3 is a cause of concern and should be considered for future urban forestry programmes. Our study should guide more such studies to identify tolerant trees for urban air pollution abatement.