Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1.

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (YI) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810 nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4 · O2 evolution) which arrived at the PSI donor side after a delay of 2 ms. The intrinsic quantum yield of PSI (yI, electrons per photon absorbed by PSI) was measured at 712 nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (aI) and PSII (aII) in leaves. For comparison, pigment–protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (yI = 0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quantawere absorbed by PSII, but processedwith the low intrinsic yield yII = 0.63. In PSII, the quantumyield of charge separation was 0.89 as detected by variable fluorescence Fv/Fm, but 29% of separated charges recombined (Laisk A, Eichelmann Hand Oja V, Photosynth. Res. 113, 145–155). Atwavelengths less than 580 nmabout 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment–protein complexes.