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, analysis of variable chlorophyll a fluorescence and pigment composition was performed in the end (September) of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. 16 weeks under the influence of enhanced UV-B radiation was responsible for the depression of AN over the whole interval of investigated PPFD/Ci. Significant decrease of ANmax (up to 9%) and α (up to 22%), and increase of Γi (up to 57%) and of RD (up to 19%) was found. The long-term effect of enhanced UV-B radiation caused the significant decrease of ANsal (up to 60%) and τ (up to 8%), and significant increase of Γc (up to 27%), compared to control. Investigations of the gas-exchange parameters discovered that the main limitation of assimilation can be identified at the level of RuBP regeneration related to the electron transport. The chlorophyll a+b content was significantly decreased in UV-B exposed variant. UV-B enhancement resulted in over-reduction of PS II at lower values of irradiance. The midday depression of the maximal photochemical yield (FV/FM) was deeper in E-variant. A light-induced decline in the Fv/FM ratio was followed by biphasic recovery phase upon return to shade conditions. The rapid recovery phase, related to epoxidation of zeaxanthin, showed sharper slope in UV-B exposed variant. The slow phase of recovery of the FV/FM ratio represents reactivation of PSII by means of D1 protein turnover. The kinetics of the slow phase of recovery in E-variant was 0.58 time that of the C-variant. This indicates impairment of the D1 protein turnover as a result of long-term influence of UV-B radiation enhancement. Presented results show that in Norway spruce long-term exposure to enhanced UV-B radiation under field conditions can be reason for negative changes at the level of primary photosynthetic reactions and increased sensitivity to the photoinhibition of photosynthesis.