For all experiments, epidermal peels from the two or three youngest fully expanded leaves from 3- to 4-week-old, unbolted Arabidopsis plants were used. Unless otherwise stated, bioassays were performed in Col-0 cultivar. To measure promotion of stomatal closure, epidermal peels were floated in 10:10 buffer under light (under the same conditions as used previously for plant growth) for at least 2 h. Then ABA, LPS, and bacterial suspensions were added to the incubation medium, and peels were further incubated as indicated. For the inhibition of opening experiments, peels were floated in the dark in 10:0 buffer (10 mm MES-KOH, pH 6.15) for 2 h to promote stomatal closure. Peels were then transferred to 10:10 buffer containing ABA for a further 2 h and were subsequently placed on a microscope slide, where apertures of 40 stomata from each experiment were measured in a Carl Zeiss microscope (400×) with the aid of an eyepiece micrometer. Data are presented as the average from 80 to 120 aperture measurements per treatment, collected from two or three independent experiments. For V. faba stomatal bioassays, peels were obtained from mature leaves of 2- to 3-week-old plants. Assays were performed as described for Arabidopsis, except that CO₂-free 10:10 buffer was used.