Characterization of stomatal closure caused by ultraviolet-B radiation

The effects of ultraviolet-B (UV-B) radiation on stomatal conductance (g(s) ) in pea (Pisum sativum L.), commelina (Commelina communis L.), and oilseed rape (Brassica napus L.) plants were investigated. Plants were grown in a greenhouse either with three different high ratios of UV-B to photosyntheti cally active radiation or with no UV-B radiation. Pea plants grown in the h ighest UV-B radiation (0.63 W m(-2)) exhibited a substantial decrease of ad axial and abaxial g(s) (approximately 80% and 40%, respectively). With grow th in 0.30 W m(-2) of UV-B adaxial g(s) was decreased by 23%, with no effec t on abaxial g(s), and lower UV-B irradiance of 0.21 W m(-2) had no effect on either surface. Although abaxial g(s) increased when leaves were turned over in control plants, it did not in plants grown with the highest UV-B. A daxial g(s) in commelina and oilseed rape also decreased on exposure to hig h UV-B (0.63 W m(-2)). For previously unexposed pea plants the time course of the effect of UV-E on g(s) was slow, with a lag of approximately 4 h, an d a time constant of approximately 3 h. We conclude that there is a direct effect of UV-B on stomata in addition to that caused by changes in mesophyl l photosynthesis.