The effects of ABA on stomatal closure in guard cells of wild-type, mek1 null, and over-expressing Arabidopsis plants. Isolated epidermis of Arabidopsis was incubated in MES–KCl for 3 h under conditions promoting stomatal opening and were then transferred to fresh MES-KCl containing no ABA (open circles) and 1 μM ABA (filled circles) in the wild type; no ABA (open triangles) and 1 μM ABA (filled triangles) in mutants mek1; no ABA (open squares) and 1 μM ABA (filled squares) in over-expressing plants for only another 4 h. Stomatal apertures were determined with a project microscope at 1 h intervals during the 4 h incubation. Each assay was repeated three times. The data were presented as means of 150 measurements ±SE.