Contrasting physiological responses of ozone-tolerant Phaseolus vulgaris and Nicotiana tobaccum varieties to ozone and nitric acid

Abstract

Ozone (O₃) and nitric acid (HNO₃) are synthesized by the same atmospheric photochemical processes and are almost always co-pollutants. Effects of O₃ on plants have been well-elucidated, yet less is known about the effects of HNO₃ on plants. We investigated the physiological effects of experimental O₃ and HNO₃ fumigation on Phaseolus vulgaris (snap bean) and Nicotiana tobaccum (tobacco) varieties with known sensitivity to O₃, but unknown responses to HNO₃. Responses were measured as leaf absorptance, aboveground plant biomass, and photosynthetic CO₂-response curve parameters. Our results demonstrate that O₃ reduced absorptance, stomatal conductance and plant biomass in both species, and maximum photosynthetic rate in P. vulgaris, whereas the main effect of HNO₃ was an increase in mesophyll conductance. Overall, the results suggest that HNO₃ affects mesophyll conductance through increased nitrogen absorbed by leaves during HNO₃ deposition which in turn increases photosynthetic demand for CO₂, or that damage to epicuticular waxes on leaves increased diffusion of CO₂ to sites of carboxylation.