SA acts upstream of ABA in stomatal closure response. Data shown here are stomatal apertures of Col-0, eds5-1, sid2, and nahG leaf peels treated with buffer or ABA (10 μ  m; A); Col-0, eds5-1, sid2, and aba2-1 leaf peels treated with buffer or SA (20 μ  m; B); Col-0 and npr1-1 leaf peels treated with buffer or SA (20 μ  m; C); and Col-0, npr1-1, and fls2 leaf peels treated with buffer or ABA (10 μ  m; D). All results are displayed as means of 30 to 60 stomata, with ses indicated. Statistical differences between chemical treatments and buffer controls are detected with two-tailed t test (***, P < 0.001).