carotenoids; chlorophyll a fluorescence; photosystem 2; xanthophyll cycle pigments

Cloned saplings of beech (7-y-old) were exposed to enhanced UV-B irradiation (+25%) continuously over three growing seasons (1999-2001). Analysis of CO2 assimilation, variable chlorophyll (Chl) a fluorescence, and pigment composition was performed in late summer of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. This influence was responsible for the stimulation of the net assimilation rate (Pn) over a range of irradiances. The increase in Pn was partially connected to increase of the area leaf mass, and thus to the increased leaf thickness. Even a higher degree of UV-B induced stimulation was observed at the level of photosystem 2 photochemistry as judged from the irradiance response of electron transport rate and photochemical quenching of Chl a.

Cloned saplings of beech (7-y-old) were exposed to enhanced UV-B irradiation (+25%) continuously over three growing seasons (1999-2001). Analysis of CO2 assimilation, variable chlorophyll (Chl) a fluorescence, and pigment composition was performed in late summer of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. This influence was responsible for the stimulation of the net assimilation rate (Pn) over a range of irradiances. The increase in Pn was partially connected to increase of the area leaf mass, and thus to the increased leaf thickness. Even a higher degree of UV-B induced stimulation was observed at the level of photosystem 2 photochemistry as judged from the irradiance response of electron transport rate and photochemical quenching of Chl a.