Stomata are less responsive to environmental stimuli in high background ozone in Dactylis glomerata and Ranunculus acris

Two mesotrophic grassland species, Ranunculus acris and Dactylis glomerata were exposed to a range of ozone treatments (16.2 to 89.5 ppb) and two watering regimes under naturally fluctuating photosynthetically active radiation (PAR), vapour pressure deficit (VPD) and temperature. Stomatal conductance was measured throughout the experiments, and the combined data set (>1000 measurements) was analysed for effects of low and high ozone on responses to environmental stimuli. We show that when D. glomerata and R.acris were grown in high background ozone the stomata consistently lose the ability to respond, or have reduced response, to PAR, VPD, temperature, and soil moisture content in comparison to their response in low background ozone. The maximum stomatal conductance (gmax) was also significantly higher in the high background treatment for D. glomerata. We discuss the hypotheses for the reduced sensitivity of stomatal closure to a changing environment and the associated implications for ozone flux modelling.