Effects of ABA, sodium nitroprusside (SNP), NONOate, and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) treatments on the stomatal aperture of the wild type and hy1-100, abi4, and hy1-100/abi4 mutants. Arabidopsis leaves of each ecotype were treated with or without ABA (10 μm) or SNP (50 μm), NONOate (50 μm), Old SNP (50 μm), or Old NONOate (50 μm) in MES-KCl buffer for 2 h after 0.5 h of pretreatment with cPTIO (400 μm). Photographs were immediately taken with a microscope. Stomatal apertures were then measured (n = 50 from three independent experiments). Seedlings without chemical treatments were regarded as controls (Con). Data are means ± se from at least three independent experiments. Differences among treatments were analyzed by one-way ANOVA, taking P < 0.05 as significant according to Tukey’s multiple range test.