RESUMEN
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.
RESUMEN
The effects of ultraviolet-B (UV-B) radiation on water relations, leaf development, and gas-exchange characteristics in pea (Pisum sativum L. cv Meteor) plants subjected to drought were investigated. Plants grown throughout their development under a high irradiance of UV-B radiation (0.63 W m-2) were compared with those grown without UV-B radiation, and after 12 d one-half of the plants were subjected to 24 d of drought that resulted in mild water stress. UV-B radiation resulted in a decrease of adaxial stomatal conductance by approximately 65%, increasing stomatal limitation of CO2 uptake by 10 to 15%. However, there was no loss of mesophyll light-saturated photosynthetic activity. Growth in UV-B radiation resulted in large reductions of leaf area and plant biomass, which were associated with a decline in leaf cell numbers and cell division. UV-B radiation also inhibited epidermal cell expansion of the exposed surface of leaves. There was an interaction between UV-B radiation and drought treatments: UV-B radiation both delayed and reduced the severity of drought stress through reductions in plant water-loss rates, stomatal conductance, and leaf area.