Integrating O3 influences on terrestrial processes: Photosynthetic and stomatal response data available for large-scale modeling

Background/Question/Methods

Plants have a strong influence on climate by controlling the transfer of carbon dioxide and water, two of the most powerful greenhouse gases, between the biosphere and atmosphere through the processes of photosynthesis and transpiration.  Chronic exposure to surface ozone (O₃) differentially affects photosynthesis and transpiration because it damages stomatal conductance, the common link that controls both processes, in addition to biochemical aspects of photosynthesis.  Because of the integral role of O₃ in altering plant interactions with the atmosphere, there is a strong motivation to incorporate the influence of O₃ into large-scale models.  However, there are currently no existing analyses documenting both photosynthesis and stomatal conductance responses to O₃ exposure through time using a common O₃ parameter.  Using data from peer-review manuscripts, we have compiled photosynthetic and stomatal responses to chronic O₃ exposure as a standardized function of cumulative O₃ uptake (CUO), which integrates O₃ flux into leaves through time.

Results/Conclusions

Crops, temperate deciduous and temperate evergreen trees make up 90% of available literature data, yet these plant coverage types are only 25% of Earth’s land surface.  Tropical trees account for 30% of global NPP and grasslands cover 30% of Earth’s surface, but only 6 studies included information to calculate CUO.  Results demonstrated that there was no overall correlation of photosynthesis with CUO and a positive correlation of conductance with CUO.  Plant sensitivity was estimated using a ratio of photosynthesis to stomatal conductance in control treatments, assuming that higher conductance allowed higher O₃ flux rates and that internal defense was a function of photosynthesis.  While plant types responded similarly to CUO, plant sensitivity plays a strong role in determining responses to O₃.  The lack of correlation between photosynthesis and CUO suggests that predicted decreases in background O₃ concentrations would not result in an increase of carbon fixation by plants and increases in O₃ in polluted regions will not exacerbate the detrimental effects.  Predicting global responses of the biosphere to chronic O₃ exposure will be improved if responses of photosynthesis and stomatal conductance to CUO are measured in tropical tree and grassland species.