4. Nitric oxide synthase (NOS) inhibitor reduces ariginine inhibition of stomatal opening. Epidermal strips were pre-incubated for 2 h in the opening medium in the dark (dark), and were then transferred to the opening medium (light) or the medium containing pharmacological reagents (1 mM arginine, 100 µM cPTIO and 25 µM L-NAME). After incubation in the light for 2 h, the stomatal apertures were measured. Bars represent standard errors of means.