Nitrite and ABA do not induce stomatal closure and NO synthesis in the nia1, nia2 NR-deficient mutant. (a) Nitrite-induced stomatal closure. Epidermal peels from leaves of either wild type (white bars) or nia1, nia2 (black bars) were treated with various concentrations of sodium nitrite, and stomatal apertures were measured after 3 h. (b) ABA-induced stomatal closure. Epidermal peels from leaves of either wild type (white bars) or nia1, nia2 (black bars) were treated with various concentrations of ABA, and stomatal apertures were measured after 3 h. Data for a and b are from three independent experiments (n = 60 stomata per treatment ± SE). (c) ABA- and nitrite-induced NO synthesis. DAF-2DA fluorescence was determined (30 min) from control, nitrite-treated (1 mM), or ABA-treated (50 μM) guard cells of either wild type (white bars) or nia1, nia2 (black bars; n = 44–124 ± SE).