SURFACE-CHARGES, THE HETEROGENEOUS LATERAL DISTRIBUTION OF THE 2 PHOTOSYSTEMS, AND THYLAKOID STACKING

Spinach grown in light which preferentially excites Photosystem II (PSII-light) had increased amounts of PS I reaction centres on a chlorophyll (Chl) basis, a higher Chl a/Chl b ratio but less extensive thylakoid stacking, when compared with growth in light preferentially favouring excitation of PS I (PSI-light). Surprisingly, despite the marked difference in the extent of thylakoid stacking, the overall surface charge densities of thylakoid membranes were similar for plants grown in PSII- and PSI-light. The amounts of PS II reaction centres, cytochrome f and adenosine triphosphate synthase were also similar on a Chl basis for the two growth light qualities. Thylakoids of the Chl-b-less barley mutant had a smaller magnitude of surface charge density, although the extent of thylakoid stacking (Goodchild, D.J., Highkin, H.R. and Boardman, N.K. (1966) Exp. Cell Res. 43, 684-688) was much smaller, compared with wild-type barley. The results are interpreted in terms of the possible mechanisms governing the heterogeneous lateral distribution of the two photosystems. They suggest that PS I is laterally segregated to non-appressed thylakoid membrane domains because of the excess negative charges it carries on the outer membrane surface, whereas PS II is mainly sequestered into the appressed regions because of an association with its Chl a/b-proteins which appear to contribute more to van der Waals attraction than hitherto recognized.