Effect of Enhanced UV-B Radiation on Chlorophyll a Fluorescence Parameters in Norway Spruce Needles

Abstract

Cloned saplings of Norway spruce (7 years old) were continuously exposed to enhanced UV-B irradiation (+ 25 %) over three growing seasons. An analysis of variable chlorophyll a fluorescence was performed at the beginning Gune) and the end (September) of the third growing season in order to evaluate the longterm effects of elevated UV-B irradiation on the primary photosynthetic reactions. A time-dependent response to elevated UV-B radiation was observed. The saturated PPFD electron transport rate (ETR) in the control (C-variant) was 1.92 times that of the exposed plants (E-variant). The midday depression of the maximal photochemical yield of PSII (FV/FM) 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 FV/FM ratio was followed by a bi-phasic recovery phase upon return to shade conditions. The rapid recovery phase, possibly related to epoxidation of zeaxanthin, showed a sharper slope in the UV-B exposed E-variants. Thus, the long-term exposure to the elevated UV-B radiation was related to induction of the interconversion between violaxanthin and zeaxanthin. The slow phase of recovery of the Fy/FMratio possibly represents the reactivation of PSII by means of 0 1 protein turnover. The slope 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 illumination). This indicates impairment of the 01 protein turnover as a result of the long-term influence ofUV-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.