GROWTH AND NITROGEN-UTILIZATION IN SEEDLINGS OF MOUNTAIN BIRCH (BETULA-PUBESCENS SSP. TORTUOSA) AS AFFECTED BY ULTRAVIOLET-RADIATION (UV-A AND UV-B) UNDER LABORATORY AND OUTDOOR CONDITIONS

Growth patterns and nitrogen economy were studied in pot-grown seedlin gs of mountain birch subjected to different ultraviolet radiation unde r both laboratory and outdoor conditions at Abisko in northern Sweden. In the laboratory, nutrient supply, temperature, humidity, ultraviole t radiation-A (UV-A, 320-400 nm) and B (UV-B, 280-320 nm) were control led, while photosynthetically active radiation (PAR, 400-700 nm) and p hotoperiod varied naturally. Under outdoor conditions nutrient supply was controlled, and the irradiation treatments were ambient and above- ambient W-B using additional fluorescent lamps. Mountain birch nitroge n economy was affected by increased ultraviolet radiation, as reflecte d by a changed relationship between plant growth and plant nitrogen bo th in the laboratory and outdoors. In the laboratory enhanced UV-A dec reased leaf area per unit plant biomass (leaf area ratio) but increase d biomass productivity, both per unit leaf area (leaf area productivit y) and per unit leaf nitrogen (leaf nitrogen productivity). Low levels of W-B affected growth patterns and nitrogen economy in a similar way to enhanced W-A. High levels of W-B clearly decreased relative growth rate and nitrogen productivity, as leaf area ratio, leaf area product ivity and leaf nitrogen productivity were all decreased. Under outdoor conditions above-ambient levels of W-B did;not alter growth or biomas s allocation traits of the seedlings, whilst nitrogen productivity was increased. Mountain birch seedlings originating from different mother trees varied significantly in their responses to different ultraviole t radiation.