Tropospheric ozone (O3) is the air pollutant of most concern to vegetation at present. Ozone impacts on stomata are still controversial, as both decreased stomatal conductance and slow stomatal responses to environmental stimuli (namely, stomatal sluggishness) have been shown. We postulated that the light environment affects stomatal sluggishness. To concurrently manipulate O3 and light conditions and measure gas exchange at leaf level, we developed an innovative O3 exposure system by modifying a commercially available gas exchange system. We exposed the first trifoliate leaf of the O3-sensitive genotype S156 of snapbean (Phaseolus vulgaris) to a 1-h O3 exposure (150 ppb) under 1000 μmol m−2 s−1 photosynthetic photon flux density, so that stomata were fully open and O3 uptake was maximized. Then, leaves were subjected to different light intensities (200, 1000, or 1500 μmol m−2 s−1) until a steady state was reached. As a metric of sluggishness, we quantified the stomatal responses to a sharp water stress generated by cutting the petiole at steady state. The results showed that O3 exposure induced stomatal sluggishness only under high light (stomata needed 53 % more time to half stomatal conductance relative to steady state) and did not when the plants were under lower light intensities. We conclude that O3-induced stomatal sluggishness may occur only in fully irradiated leaves, and suggest it is a minor response when entire crowns and canopies are assessed and a major reason of the higher O3 sensitivity of sun leaves than of shade leaves. © 2016, Springer International Publishing Switzerland.

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