ULTRAVIOLET-B RADIATION EFFECTS ON WATER RELATIONS, LEAF DEVELOPMENT,AND PHOTOSYNTHESIS IN DROUGHTED PEA-PLANTS

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 gro wn throughout their development under a high irradiance of UV-B radiat ion (0.63 W m(-2)) were compared with those grown without UV-B radiati on, and after 12 d one-half of the plants were subjected to 24 d of dr ought that resulted in mild water stress. UV-B radiation resulted in a decrease of adaxial stomatal conductance by approximately 65%, increa sing stomatal limitation of CO2 uptake by 10 to 15%. However, there wa s 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 an d cell division. UV-B radiation also inhibited epidermal cell expansio n of the exposed surface of leaves. There was an interaction between U V-B radiation and drought treatments: UV-B radiation both delayed and reduced the severity of drought stress through reductions in plant wat er-toss rates, stomatal conductance, and leaf area.