Plant Physiol, January 2000, Vol. 122, pp. 99-106

The Ultraviolet Action Spectrum for Stomatal Opening in Broad Bean¹

Biology Department, Santa Clara University, Santa Clara, California 95053 (W.E.); and Chemistry Department (T.E.S., R.A.B.) and Biology Department (L.T.), University of California, Santa Cruz, California 95064.

The ultraviolet action spectrum for stomatal opening was measured using epidermal peels from leaves of broad bean (Vicia faba). The spectrum was calculated from hyperbolic fluence response curves using 11 wavelengths ranging from 275 to 459 nm. The action spectrum exhibits a major peak at approximately 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 conditions utilized (i.e. the absence of saturating red light), stomatal opening saturated at extremely low fluence rates: <0.2 µmol m² s¹ at 280 nm, and approximately 1.0 µmol m² s¹ at 459 nm. The threshold for blue-light-induced stomatal opening was approximately 0.02 µmol m² s¹. In light-mixing experiments, the addition of 280 nm light to saturating 650 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 they 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 absorbance 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 proposed that the chromophore can be excited either directly by blue light or by energy transferred from the protein portion of the protein-pigment complex after it absorbs 280 nm light.