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To follow stomatal responses to ozone (O3) in different Arabidopsis lines, we constructed a rapid-response O3 exposure/gas-exchange measurement device consisting of eight through-flow whole-rosette cuvettes. To separate rosette from roots and growth substrate, plant is grown through an agar-filled hole in a polished glass plate fixed on the pot. Following insertion of the plant, the plate forms air-tight bottom surface of the cuvette; thus the rosette is enclosed without touching it during any phase of the insertion of the plant to the cuvette. The device allows monitoring rapid responses in the stomatal function. For example, an acute exposure to 150 ppb O3 decreased stomatal conductance to 60–70% of its initial value within 9–12 min. Thereafter, the conductance regained its preexposure value within further 30–40 min in spite of the continuing O3 exposure. The transient decrease was absent in the abscisic acid-insensitive mutant abi2 defective in a class 2C protein phosphatase. This provides an in vivo confirmation that the early transient decrease in stomatal conductance is not a result of physical damage by the reactive oxygen species (ROS) formed from O3 breakdown but reflects the biological action of ROS, transduced through a signalling cascade. Thus, the apparatus will be helpful in specifying complex molecular and genetic interactions in rapid responses in guard cells in vivo.