The ultraviolet action spectrum for stomatal opening in broad bean

The ultraviolet action spectrum for stomatal opening was measured using epi dermal peels from leaves of broad bean (Vicia faba). The spectrum was calcu lated from hyperbolic fluence response curves using 11 wavelengths ranging from 275 to 459 nm. The action spectrum exhibits a major peak at approximat ely 280 nm and a minor peak at approximately 360 nm. The response at 280 nm is about three times greater than the response at 459 nm. Under the condit ions utilized (i.e. the absence of saturating red light), stomatal opening saturated at extremely low fluence rates: <0.2 mu mol m(-2) s(-1) at 280 nm , and approximately 1.0 mu mol m(-2) s(-1) at 459 nm. The threshold for blu e-light-induced stomatal opening was approximately 0.02 mu mol m(-2) s(-1). In light-mixing experiments, the addition of 280 nm light to saturating 65 0 nm (red) light caused additional stomatal opening, which is indicative of separate photoreceptors. In contrast, adding 280 nm of light to saturating 459 nm (blue) light did not increase stomatal opening, suggesting that the y both excite the same receptor. The results with white light were similar to those with blue light. We infer that ultraviolet light acts via the blue light photoreceptor rather than through photosynthesis. The additional abs orbance peak at 360 nm suggests that the chromophore is either a flavin or a cis-carotenoid, both of which exhibit peaks in this region. It is propose d that the chromophore can be excited either directly by blue light or by e nergy transferred from the protein portion of the protein-pigment complex a fter it absorbs 280 nm light.