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 photosynthetically active radiation or with no UV-B radiation. Pea plants grown in the highest UV-B radiation (0.63 W m(−2)) exhibited a substantial decrease of adaxial and abaxial g(s) (approximately 80% and 40%, respectively). With growth in 0.30 W m(−2) of UV-B adaxial g(s) was decreased by 23%, with no effect 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. Adaxial g(s) in commelina and oilseed rape also decreased on exposure to high UV-B (0.63 W m(−2)). For previously unexposed pea plants the time course of the effect of UV-B on g(s) was slow, with a lag of approximately 4 h, and 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 mesophyll photosynthesis.