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We tested whether short-term exposure to realistic ozone pollution (≤150ppb, 1h) and soil water stress (soil water content ≤15%) slow stomatal dynamics in an ozone-sensitive cultivar of snapbean. Both ozone exposure and water stress caused stomata to be sluggish in the degree of closure after leaf severing, while ozone also delayed the time the closing signal was perceived. Ozone-induced aberrations lasted up to the night and caused incomplete closure of stomata. No synergic effect was observed in the dynamic measurements. In contrast, at steady-state, water stress protected the plants from the negative ozone effects on stomatal conductance. Ambient ozone peaks may thus cause sluggish stomatal response and increase leaf water loss both under well watered and drought conditions.