Effect of enhanced UV-B radiation on chlorophyll a fluorescence parametersin Norway spruce needles

Cloned saplings of Norway spruce (7 years old) were continuously exposed to enhanced UV-B irradiation (+25%) over three growing seasons. An analysis o f variable chlorophyll a fluorescence was performed at the beginning (June) and the end (September) of the third growing season in order to evaluate t he longterm effects of elevated W-B irradiation on the primary photosynthet ic reactions. A time-dependent response to elevated UV-B radiation was obse rved. The saturated PPFD electron transport rate (ETR) in the control (C-va riant) was 1.92 times that of the exposed plants (E-variant). The midday de pression of the maximal photochemical yield of PSII (F-V/F-M) was deeper in the E-variants, and the extent of this depression, as compared with the C- variants, increased at the end of the growing season. A light-induced decline in the F-V/F-M ratio was followed by a bi-phasic re covery phase upon return to shade conditions. The rapid recovery phase, pos sibly related to epoxidation of zeaxanthin, showed a sharper slope in the U V-B exposed E-variants. Thus, the long-term exposure to the elevated UV-B r adiation was related to induction of the interconversion between violaxanth in and zeaxanthin. The slow phase of recovery of the F-V/F-M ratio possibly represents the reactivation of PSII by means of D1 protein turnover. The s lope of the slow phase of the recovery in E-variants was 0.58 time that of the C-variants at the end of the growing season (i.e. 16 weeks of UV-B illu mination). This indicates impairment of the D1 protein turnover as a result of the long-term influence of UV-B radiation enhancement. The results show that in Norway spruce a long-term exposure to enhanced UV- B radiation under field conditions can be a reason for negative changes at the level of primary photosynthetic reactions.