NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 160

involved. The B response is driven by a B absorbing photoreceptor (BAP) with action
maxima at 420, 450 and 470 nm 57 , and the R response is driven by photosynthesis
alone 58-60 .
Relatively little attention has been paid to the UV waveband with respect to
stomatal opening. It has been reported that UV-A radiation causes stomatal opening in
broad bean 61 and Teramura et al 62 reported reduced stomatal resistance in cucumber in
response to UV-B radiation in long-term experiments. Generally, the action of UV-B
radiation has been to cause stomatal closure 63-66 ; however, these responses are probably
caused by the relatively high fluence rates of UV-B used and the consequent damaging
effect on membrane permeability 67 . Nogués et al 68 demonstrated decreasing stomatal
conductance with increasing UV-B irradiance in three different species.
There appears to have been only one true action spectrum carried out to study the
relative effectiveness of the UV wavelengths in inducing stomatal opening. In a study of
broad bean epidermal strips, Eisinger et al 69 found a major peak near 280 nm (284 nm)
and subsidiary peaks near 350 and 450 nm (Fig. 4). It is important to note that low
fluences were used by Eisinger et al 69 ; relatively high fluence rates of UV-B radiation
(ca. 2 µmol m -2 s -1 over one hour) tend to result in stomatal closure 64 . Eisinger et al
found that adding low fluence rate 280 nm radiation to saturating R light caused
additional stomatal opening whereas addition of 280 nm radiation to saturating B light
did not. From this observation, and evidence in the literature, the authors conclude that
rather than a specific UV-B photoreceptor being involved, the UV-B and B
photoreceptors are synonymous and that the 280 nm response results from energy being
transferred to the chromophore of the BAP from the protein moiety of the
protein-pigment complex.
Figure 4 . Action spectrum for the induction of stomatal opening in broad bean epidermal strips.
Samples were exposed to monochromatic radiation for one hour. Redrawn after Eisinger et al 69 .