Reversal of blue light-stimulated stomatal opening by green light

Blue light-stimulated stomatal opening in detached epidermis of Vicia faba is reversed by green light. A 30 s green light pulse eliminated the transie nt opening stimulated by an immediately preceding blue light pulse. Opening was restored by a subsequent blue light pulse. An initial green light puls e did not alter the response to a subsequent blue light pulse. Reversal als o occurred under continuous illumination, with or without a saturating red light background. The magnitude of the green light reversal depended on flu ence rate, with full reversal observed at a green light fluence rate twice that of the blue light. Continuous green light given alone stimulated a sli ght stomatal opening, and had no effect on red light-stimulated opening. An action spectrum for the green light effect showed a maximum at 540 nm and minor peaks at 490 and 580 nm. This spectrum is similar to the action spect rum for blue light-stimulated stomatal opening, red-shifted by about 90 nm. The carotenoid zeaxanthin has been implicated as a photoreceptor for the s tomatal blue light response. Blue/green reversibility might be explained by a pair of interconvertible zeaxanthin isomers, one absorbing in the blue a nd the other in the green, with the green absorbing form being the physiolo gically active one.