INFLUENCE OF BOUNDARY LAYER CATION CHARGE ON CHLOROPLAST SELECTIVE SCATTER SPECTRUM AND ABSORPTION

Abstract

— Chloroplast optical properties, e.g. the absorption intensity and scatter profiles, change as experimental variables cause departures from ‘ideal’ responses of isolated, moderate-intensity bands. Chloroplast contraction induced by cation charge, protons, or increased osmolar strength causes a decrease in optical cross-section, lower absorption, and peak height flattening. Refractivity and scatter are increased, and associated (multiple) band overlap gives a red-shift to the selective scatter profile. Interfacial cation charge density at the membrane surface of swollen granal stack vesicles is a determinant factor. Mutual repulsion of membrane fixed negative charges to extend the thylakoids and screening by cations of the boundary layer causing contraction would account for this. Inverting a part-filled cuvette to mix the contents caused reversible swelling, as though cations were temporarily displaced from the boundary layer. Gradient-prepared unswollen chloroplasts show a scatter red-shift and absorption decrease with H+-ionophores and membrane disruptive agents, consistent with H+ efflux across the thylakoids. Warming ice-cold samples to room temperature has the same effect. Therefore, in intact chloroplasts protons trapped within the thylakoid system exert a pressure to escape, but in swollen granal stacks this is lost. This illustrates the evidence on chloroplast inner environment optical methods can reveal that is not available in other approaches.