Effects of Elevated Ozone Concentrations on Evergreen Shrubs in Delhi, India

Tropospheric ozone (O₃), a potent greenhouse gas and secondary air pollutant, stands out as a major photochemical pollutant, posing a considerable threat to plant life. Prolonged exposure to this gas triggers adverse changes in various plant parameters, hindering growth, and hastening senescence. This makes increasing ambient O₃ concentrations an urgent environmental concern. An essential aspect of addressing this issue lies in comprehending the mechanisms underlying O₃ resistance in plants, particularly for fostering sustainable urban greening in polluted environments. Certain indicative plant parameters, including morphological characteristics, and biochemical and antioxidant capacity, play pivotal roles in regulating the variation in O₃ resistance. However, the specific contributions of each trait remain somewhat elusive and understudied. Moreover, O₃ resistance exhibits significant variability within and across plant species, influenced by factors such as the timing of O₃ exposure and the plant's developmental stage. The present study delves into the nuances of O₃ sensitivity among four evergreen shrub species—Bougainvillea glabra, Buxus sempervirens, Duranta goldiana, and Ficus panda—that are commonly chosen for roadside plantations in Delhi. The selected species were exposed to elevated ozone concentrations, in a controlled fashion, in Open-Top Chambers (OTCs) for an entire growing season. The study examined changes in photosynthetic pigments (total chlorophyll, carotenoid), antioxidant capacity (ascorbic acid), and lipid peroxidation. The findings revealed significant discernible impacts of ozone fumigation alongside ambient air concentration. Compared to control samples (ambient air exposure conditions), under elevated ozone treatment, chlorophyll content and total carotenoid levels decreased substantially, while the ascorbic acid content and lipid peroxidation levels increased. Species that underwent the highest decrease in chlorophyll (C) content are the species that experienced the lowest increase in ascorbic acid (AA) levels, and vice versa (i.e., B. sempervirens [C-34.27%, AA+20.83%], D. goldiana [C-25%, AA+35.95%], F. panda [C-23.85%, AA+54.12%], and B. glabra [C-18%, AA+55.27%]). Based on the parametric changes observed in the considered species, it can be concluded that B. glabra is the most appropriate species (followed by F. panda) for systematic urban plantation in areas with high O₃ concentrations as it exhibited the highest tolerance. This research contributes valuable insights for selecting suitable plant species in ozone-polluted areas, facilitating environmentally conscious decision-making for the establishment of resilient and sustainable urban green spaces.