Organization of the marginal regions of pea granal thylakoids

Thylakoids of pea chloroplasts isolated from plants grown during various ti me intervals from June to August were subjected to fragmentation. Using a m odified procedure, a fraction of larger particles was separated from those previously considered as fragments of intergranal thylakoids. The particles of the fraction isolated were identified as fragments of marginal regions of granal thylakoids (margins). The relative yield of these fragments depen ded on the time interval of plant growth. Two types of low-temperature fluo rescence spectra corresponding to a high and low yield of the fraction were detected. The characteristics of the first one were a high fluorescence in tensity in the short-wave region and the presence of bands with maxima at 6 87 and 696 nm emitted by photosystem II (PSII). The ratio of PSII to PSI co mplexes (PSII/PSI) in the fractions characterized by a low and high yield v aried from 1 to 5. The analysis of excitation spectra of long-wave fluoresc ence of PSI showed that PSI complexes in the margin fragments obtained at a low fraction yield were depleted in chlorophyll forms with a 682-nm absorp tion maximum and enriched in those with a 668-nm maximum. Since an increase in the yield of the margin-fragment fraction is due to an increased unstac king of granal thylakoids, the differences in the characteristics of fragme nts obtained with a low and a high yield reflect the changes in the composi tion of granal thylakoids in the direction from the margin to the centrum, that is, a decrease in the relative content of PSI complexes and alteration s in the composition and size of its light-harvesting antenna. The consiste ncy between the data obtained and the present view concerning the different functions of PSI located in different thylakoid regions is discussed.