An investigation of functional differentiation of assimilation activity of sun versus shade foliage was carried out in a Norway spruce monoculture stand (age: 15 years). The PPFD saturated rate of CO2 uptake (ANmax) was 42% higher, the initial slope of AN/PPFD relation (Alpha) was 13% lower for exposed shoots (E) compared to the shaded needles (S). The CO2 and PPFD saturated rate (ANsat) and the initial slope of AN/Ci relation (Tau) were 12 and 39% higher in E- compared to S-needles. The CO2 saturated rate of Rubisco carboxylation (VCsat)in the S-needles amounted to 89% in the E-needles. Thus, the Rubisco carboxylation was identified as the main limitation of assimilation in the S-needles. The exposure of E-layer needles to higher photosystem PSII excitation pressure than for S-layer increased capacity of non-radiative dissipation (NPQ) by 30% what provides more efficient protection against over-reduction of PS II reaction centres. The steeper dependence of Fo quenching on NPQ (by 53%) in E-than in S-needles supports that non-radiative dissipation localized in LHC's was enhanced for E-needles.
