Both visible and UV wavelengths play an important role inABA. This lack of responsiveness is unlikely to be due to controlling stomatal aperture. We have analysed effects of widespread cellular damage, as UV-induced stomatal closure is largely reverted in response to the H + -ATPase activator UV-B radiation on stomatal aperture in Vicia faba, and found them to be complex. Depending on the metabolic state of the fusicoccin. It is speculated that UV-B impacts upstream from the plasmalemma based enzyme complexes which facilitate the guard cell, high fluences of UV-B either stimulate stomatal opening or stomatal closing. Neither of these responses is solute fluxes leading to stomatal opening. Our data may help readily reversed, i.e. once stomata have been exposed to accommodate seemingly contradictory reports on the effects UV-B, they are unable to re-adjust their aperture in response of UV-B on stomatal aperture and/or conductance. to environmental stimuli like changes in light, humidity or , Giannini et al. 1996, Zeuthen et al. 1997, Nogués et al. 1998).The adjustment of stomatal conductance is an integrated response to a large number of environmental signals including CO 2 , humidity, radiation, temperature and water supply. The mechanism underlying the adjustment of stomatal conductance is the tightly controlled osmotic solute flux, notably K + , from guard cells and the resultant changes in guard cell turgor and stomatal aperture (Blatt 1993, MacRobbie 1998. Both visible and UV radiation affect stomatal behaviour in a wavelength dependent manner. UV-A stimulates opening of stomata (Negash et al. 1987). In contrast, germicidal UV-C induces stomatal closure (Wright and Murphy 1982, Negash et al. 1987) and this might well reflect widespread cellular destruction resulting from direct radiation damage to DNA, proteins and lipids. The UV-C effect is most effectively triggered by (non-physiological) wavelengths between 255 and 285 nm, with longer wavelengths being less effective (Negash and Bjö rn 1986). Environmentally relevant UV-B radiation also impacts on stomata. A number of groups focusing on plant performance and photosynthesis under supplemental UV-B, observed UV-induced decreases in stomatal conductance and/or aperture under growth chamber (Teramura et al.