Characterization of Stomatal Closure Caused by Ultraviolet-B Radiation

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Abstract
The effects of ultraviolet-B (UV-B) radiation on stomatal conductance (g s) in pea (Pisum sativumL.), 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 adaxialg 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.