Environmental factors that induce spatial heterogeneity of stomatal conductance, gs, called stomatal patchiness, also reduce the photochemical capacity of CO2 fixation, yet current methods cannot distinguish between the relative effect of stomatal patchiness and biochemical limitations on photosynthetic capacity. We evaluate effects of stomatal patchiness and the biochemical capacity of CO2 fixation on the sensitivity of net photosynthetic rate (PN) to stomatal conductance (gs), θ (θ = δP N/gs). A qualitative model shows that stomatal patchiness increases the sensitivity θ while reduced biochemical capacity of CO2 fixation lowers θ. We used this feature to distinguish between stomatal patchiness and mesophyll impairments in the photochemistry of CO2 fixation. We compared gas exchange of sunflower (Helianthus annuus L.) plants grown in a growth chamber and fed abscisic acid, ABA (10-5 M), for 10 d with control plants (-ABA). PN and gs oscillated more frequently in ABA-treated than in control plants when the leaves were placed into the leaf chamber and exposed to a dry atmosphere. When compared with the initial CO2 response measured at the beginning of the treatment (day zero), both ABA and control leaves showed reduced PN at particular sub-stomatal CO2 concentration (ci) during the oscillations. A lower reduction of P N at particular g s indicated overestimation of ci due to stomatal patchiness and/or omitted cuticular conductance, gc. The initial period of damp oscillation was characterised by inhibition of chloroplast processes while stomatal patchiness prevailed at the steady state of gas exchange. The sensitivity θ remained at the original pre-treatment values at high gs in both ABA and control plants. At low gs, θ decreased in ABA-treated plants indicating an ABA-induced impairment of chloroplast processes. In control plants, gc neglected in the calculation of gs was the likely reason for apparent depression of photosynthesis at low gs. and J. Šantrůček ... [et al.].