Ultraviolet-B radiation induces complex alterations in stomatal behaviour

Both visible and UV wavelengths play an important role in controlling stomatal aperture. We have analysed effects of UV-B radiation on stomatal aperture in Vicia faba, and found them to be complex. Depending on the metabolic state of the guard cell, high fluences of UV-B either stimulate stomatal opening or stomatal closing. Neither of these responses is readily reversed, i.e. once stomata have been exposed to UV-B, they are unable to re-adjust their aperture in response to environmental stimuli like changes in light, humidity or ABA. This lack of responsiveness is unlikely to be due to widespread cellular damage, as UV-induced stomatal closure is largely reverted in response to the H+-ATPase activator fusicoccin. It is speculated that UV-B impacts upstream from the plasmalemma based enzyme complexes which facilitate the solute fluxes leading to stomatal opening. Our data may help accommodate seemingly contradictory reports on the effects of UV-B on stomatal aperture and/or conductance.