Stomatal Aperture Measurements. Epidermal peels were prepared from abaxial epidermis as described (10) and incubated in 30 or 10 mM KCl and 10 mM Mes-iminodiacetic acid (pH 6.5) at 20°C, unless otherwise noted. To standardize the initial state, the epidermal strips were kept in the incubation solution for 30 min in darkness. Then, they were exposed to treatments inducing stomatal aperture: white light (300 μE·m⁻²·s⁻¹, 30 mM KCl), blue light (30 μE·m⁻²·s⁻¹, 30 mM KCl), red light (80 μE·m⁻²·s⁻¹, 30 mM KCl), fusicoccin (10 μM, 10 mM KCl), or CO₂-free air (30 mM KCl). Effects of Cs⁺ or Na⁺ on stomatal behavior were investigated either in 30 mM CsNO₃ and 10 mM KCl, using fusicoccin (10 μM) to trigger stomatal opening, or in 20 mM NaCl and 10 mM KCl, using white light (300 μE·m⁻²·s⁻¹) to trigger stomatal opening. Stomatal apertures were measured (pore width; at least 40 measurements per epidermis for each experimental point) with an optical microscope (Optiphot; Nikon) fitted with a camera lucida and a digitizing table (Houston Instruments) linked to a personal computer.