Seasonal changes of photosynthetic assimilation of Norway spruce under theimpact of enhanced UV-B radiation

The cloned saplings of Norway spruce (7 years old) were exposed to enhanced UV-B irradiation (+ 25%) continuously over three growing seasons. An analy sis of CO2 assimilation and pigment composition was performed in the beginn ing (June) and the end (September) of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. The UV-B was administ ered at levels that were, at any moment, at 125% of the ambient UV-B radiat ion. The results of the long-term experiment supports an idea on cumulative exposure UV-B effect. Four months under the influence of enhanced UV-B rad iation was responsible for the depression of A(N) over the whole interval o f investigated PPFD/C-i. A significant decrease in A(Nmax) (up to 9%) and a lpha (up to 22%), and increase in Gamma(i) (up to 57%) and of R-D (up to 19 %) was found. The long-term effect of enhanced UV-B radiation caused a sign ificant decrease in A(Nsat) (up to 60%) and tau (up to 8%), and a significa nt increase in Gamma(C) (up to 27%), compared to controls. The chlorophyll a+b content was significantly decreased in the UV-B exposed variant. This c hange was mainly caused by a decrease in the chlorophyll a content (up to 2 0%). Thus, the negative effect of the long-term 21% enhancement UV-B radiat ion on photosynthetic gas exchange and photosynthetic pigment is concluded. Investigations of the gas-exchange parameters showed that the main limitat ion of assimilation can be identified at the level of RuBP regeneration rel ated to electron transport. This finding is supported by the analysis of ph otosynthetic pigment content that indicates a simultaneous decrease of the chlorophyll a content and the antenna size. Thus, the potential increased s ensitivity to the photoinhibition of photosynthesis resulting from the long -term influence of enhanced UV-B radiation is suggested. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.