Does living in elevated CO2 ameliorate tree response to ozone? A review on stomatal responses

Short-term elevated O3 reduces photosynthesis, which reduces stomatal conductance (gs) in response to increased substomatal CO2 concentration (Ci). Further exposure causes stomata to become sluggish in response to environmental stimuli. Exposure to elevated CO2 stimulates rapid stomata closure in response to increased Ci. This reduction in gs may not be sustained over time as photosynthesis down-regulates and with it, gs. The relationship between gs and photosynthesis may not be constant because stomata respond more slowly to environmental changes than photosynthesis, and because elevated CO2 may alter guard cell sensitivity to other signals. Also, reduced stomatal density (and gs) in response to long-term CO2 enrichment suggests sustained reduction in gs. Elevated CO2 is believed to ameliorate the deleterious O3 effects by reducing gs and thus the potential O3 flux into leaves. Confirmation that gs acclimation to CO2 enrichment does not lessen over time is critical for developing meaningful O3 flux scenarios. Understanding stomatal behavior under concurrent exposure to elevated CO2 and O3 is essential for predictive modeling of tree water balance.