Seasonal changes of photosynthetic assimilation of Norway spruce under the impact 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 analysis of CO2 assimilation and pigment composition was performed in the beginning (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 administered at levels that were, at any moment, at 125% of the ambient UV-B radiation. 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 radiation was responsible for the depression of A(N) over the whole interval of investigated PPFD/C-i. A significant decrease in A(Nmax) (up to 9%) and alpha (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 significant decrease in A(Nsat) (up to 60%) and tau (up to 8%), and a significant 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 change was mainly caused by a decrease in the chlorophyll a content (up to 20%). Thus, the negative effect of the long-term 21% enhancement UV-B radiation on photosynthetic gas exchange and photosynthetic pigment is concluded. Investigations of the gas-exchange parameters showed that the main limitation of assimilation can be identified at the level of RuBP regeneration related to electron transport. This finding is supported by the analysis of photosynthetic pigment content that indicates a simultaneous decrease of the chlorophyll a content and the antenna size. Thus, the potential increased sensitivity 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.