Organization of the Marginal Regions of Pea Granal Thylakoids

Thylakoids of pea chloroplasts isolated from plants grown during various time intervals from June to August were subjected to fragmentation. Using a modified 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 depended on the time interval of plant growth. Two types of low-temperature fluorescence spectra corresponding to a high and low yield of the fraction were detected. The characteristics of the first one were a high fluorescence intensity in the short-wave region and the presence of bands with maxima at 687 and 696 nm emitted by photosystem II (PSII). The ratio of PSII to PSI complexes (PSII/PSI) in the fractions characterized by a low and high yield varied from 1 to 5. The analysis of excitation spectra of long-wave fluorescence 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 absorption 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 unstacking of granal thylakoids, the differences in the characteristics of fragments obtained with a low and a high yield reflect the changes in the composition of granal thylakoids in the direction from the margin to the centrum, that is, a decrease in the relative content of PSI complexes and alterations in the composition and size of its light-harvesting antenna. The consistency between the data obtained and the present view concerning the different functions of PSI located in different thylakoid regions is discussed.