A study of Photosystem II fluorescence emission in terms of the antenna chlorophyll-protein complexes

Abstract

Fluorescence emission spectra for the seven chlorophyll-protein complexes comprising the antenna system of Photosystem II (PS II) have been measured. All four outer antenna complexes (LHC II, CP24, CP26, CP29) have relatively greater emission near 648 nm and 680 nm with respect to the inner antenna complexes (CP43, CP47, D1/D2/cyt b-559). The emission spectra for both outer and inner antenna were calculated from the measured emission spectra of the single chlorophyll-protein complexes, using as weighting factors the excited state population at thermodynamic equilibrium in the various chlorophyll-protein complexes suggested by Jennings et al. (Jennings, R.C., Bassi, R., Garlaschi, F.M., Dainese, P. and Zucchelli, G. (1993) Biochemistry 32, 3203–3210). Subsequently, the overall emission spectra for the total PS II antenna (i.e., outer plus inner antenna) were calculated for situations in which varying excited state levels were assumed for the inner and outer antenna. In an attempt to determine the steady-state distribution of excited states between outer and inner antenna these calculated fluorescence spectra were compared with those measured for (a), PS II particles prepared from maize and (b), chloroplasts of wild-type barley and the chlorina F2 mutant. From this comparison it is concluded that at steady-state fluorescence emission, between 28% and 38% of the excited states in PS II are associated with the inner antenna and between 62% and 72% with the outer antenna. These results suggest that the PS II antenna is organised as a very shallow energy funnel. This antenna organisation is discussed in terms of the generation of non-photochemical quenching mechanisms which are designed to protect PS II from high light stress.