The influence of present-day levels of ultraviolet-B radiation on seedlings of two Southern Hemisphere temperate tree species

Seedlings of two Southern Hemisphere temperate trees species (mountain beec h: Nothofagus solandri var. cliffortioides (Hook. f.) Poole and broadleaf: Griselinia littoralis Raoul) were grown in the field to determine the effec ts of present-day levels of ultraviolet-B radiation (UV-B) on growth, bioma ss, UV-B absorbing compounds, leaf optical properties and photoinhibition. Plants were covered with either UV-B transmitting or UV-B absorbing filters . After 125 days of typical summer weather, total biomass of both species w as not affected by the UV-B treatments. Without UV-B, height increased (23% ) and the number of leaves produced decreased (-21%) in beech, but broadlea f was unaffected. The effect of UV-B on beech height and leaf number was ma nifest during a second flush of leaves suggesting differences in response t o UV-B of leaves initiated in different seasons and UV-B radiation regimes. Leaves of both species were essentially opaque to the transmission of UV-B . In the absence of UV-B the transmission of photosynthetically active radi ation through leaves of both species increased, foliar nitrogen concentrati ons increased and levels of UV-B absorbing compounds decreased. In the youn gest leaves of beech but not of broadleaf, removal of UV-B reduced midday p hotoinhibition, and did not alter the complete recovery of the fluorescence ratio F-V/F-M in the evening to predawn levels. As leaves of both species aged, midday photoinhibition decreased, with the result that UV-B had no ef fect on photoinhibition in mature leaves. Results of this experiment show t hat even under present-day UV-B levels, UV-B radiation modifies the physiol ogy, optical properties and secondary compounds of leaves of both beech and broadleaf seedlings.