Action spectra and functional antenna sizes of Photosystems I and II in relation to the thylakoid membrane organization and pigment composition

The functional organization of competent photosynthetic units in developing thylakoids from intermittent-light grown pea as well as in the unstacked, stacked and phosphorylated stacked thylakoids from its mature chloroplasts was characterized by polarographic measurements of action spectra, reaction centre contents and optical cross-sections for PS I-mediated O-2 uptake and PS II-mediated O-2 evolution. The minimum antenna sizes of 60 and 37 chlorophyll cr molecules for PS I and PS II, respectively, were determined in developing thylakoids with a ratio of Chl a/Chl b>50. In mature chloroplasts, the embedded light-harvesting chlorophyll a/b-binding (LHC) protein complexes increased the PS I and PS II effective antenna sizes by 3-6 times depending on the thylakoid membrane organization. In unstacked thylakoids, a randomization of PS I, PS II and LHC II led to the most uniform spectral distribution of light harvesting between the two photosystems but caused the maximal difference of their antenna sizes to be 370 and 100 Chls for the competent PS I and PS II units, respectively. Following the Mg2+-induced stacking of thylakoids, opposite complementary changes of the action spectra, antenna sizes and CN a/Chl b ratios indicated a redistribution of a LHC II pool of similar to 100 Chi (a + b) molecules from PS I to PS II. Unlike to the stroma-exposed PS II beta in unstacked thylakoids, the granal PS II alpha units of similar to 200 Chls demonstrated an additional 2-fold increase of the effective antenna size due to energy transfer within PS II dimers under strong background illumination, which closed >90% of reaction centres. Protein phosphorylation of the stacked thylakoids induced a significant inactivation of the O-2-evolving PS II centres but did nor cause complementary changes of the action spectra and antenna sizes of the competent PS I and PS II. In this case, light harvesting parameters of the O-2-evolving PS II units were nearly unaffected, whereas the obvious relative increase of the PS I activity at 650 nm and its decrease at >700 nm both in the action spectrum and optical cross-section measurements might suggest a substitution of PS I beta units in the O-2-reducing fraction by another distinct fraction of alpha-type which in turn is not the same to PS I alpha units in unstacked thylakoids.