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.