Response of sun versus shade foliage photosynthesis to radiation in Norwayspruce

An investigation of functional differentiation of assimilation activity of sun versus shade foliage was carried out in a Norway spruce monoculture sta nd (age: 15 years). The PPFD saturated rate of CO2 uptake (A(Nmax)) was 42% higher, the initial slope of A(N)/PPFD relation (alpha) was 13% lower for exposed shoots (E) compared to the shaded needles (S). The CO2 and PPFD sat urated rate (A(NSat)) and the initial slope of A(N)/C-i relation (tau) were 12 and 39% higher in E- compared to S-needles. The CO2 saturated rate of R ubisco carboxylation (V-Csat) in the S-needles amounted to 89% in the E-nee dles. Thus, the Rubisco carboxylation was identified as the main limitation of assimilation in the S-needles. The exposure of E-layer needles to highe r photosystem PSTI excitation pressure than for S-layer increased capacity of non-radiative dissipation (NPQ) by 30% what provides more efficient prot ection against over-reduction of PS II reaction centres. The steeper depend ence of F-0 quenching on NPQ (by 53%) in E- than in S- needles supports tha t non-radiative dissipation localized in LHC's was enhanced for E-needles.