Recent studies have shown that blue light-specific stomatal opening is reversed by green light and that far-red light can be used to probe phytochrome-dependent stomatal movements. Here, blue-green reversibility and far-red light were used to probe the stomatal responses of the npq1 mutant and the phot1 phot2 double mutant of Arabidopsis. In plants grown at 50 mumol m(-2) s(-1), red light (photosynthetic)-mediated opening in isolated stomata from wild type (WT) and both mutants saturated at 100 mumol m(-2) s(-1). Higher fluence rates caused stomatal closing, most likely due to photo-inhibition. Blue light-specific opening, probed by adding blue light (10 mumol m(-2) s(-1)) to a 100 mumol m(-2) s(-1) red background, was found in WT, but not in npq1 or phot1 phot2 double mutant stomata. Under 50 mumol m(-2) s(-1) red light, 10 mumol m(-2) s(-1) blue light opened stomata in both WT and npql mutant stomata but not in the phot1 phot2 double mutant. In npql, blue light-stimulated opening was reversed by far-red but not green light, indicating that npql has a phytochrome-mediated response and lacks a blue light-specific response. Stomata of the phot1 phot2 double mutant opened in response to 20 to 50 mumol m(-2) s(-1) blue light. This opening was green light reversible and far-red light insensitive, indicating that stomata of the phot1 phot2 double mutant have a detectable blue light-specific response.

Reversal by green light of blue-light-stimulated stomatal opening was found across a number of plant species, including leguminous and nonleguminous dicots and grass and nongrass monocots. Simultaneous exposure to equal fluence rates of blue and green light resulted in ∼50% reversal of normal blue light opening. Complete reversal occurred when the fluence rate of green light was approximately twice that of blue light. These results suggest that blue-green reversibility of stomatal opening is a basic photobiological property of guard cells. The blue-green reversibility of stomatal opening has been hypothesized to ensue from the cycling of two interconvertible, isomeric forms of the blue-light photoreceptor, zeaxanthin. Testing of blue-green reversibility could provide a valuable diagnostic tool for zeaxanthin-mediated blue-light photoperception.