The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem 11 (PSII) was PSll,(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSIIcenters (about 37% of PSII) contained 135 ± 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII. The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyllprotein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.

Progressive solubilization of spinach chloroplast thylakoids by Triton X-100 was employed to investigate the domain organization of the electron transport complexes in the thylakoid membrane. Triton/chlorophyll ratios of 1 : 1 were sufficient to disrupt fully the continuity of the thylakoid membrane network, but not sufficient to solubilize either photosystem I (PSI), photosystem 11 (PSII) or the cytochrome b,-f(Cyt b6-f) Complex. Progressive with the Triton concentration increase (Triton/Chl > 1 : l), a differential solubilization of the three electron transport complexes was observed. (a) Solubilization of the Cyt b6-f complex from the thylakoid membrane preceded that of PSI and apparently occurred early in the solubilization of stroma-exposed segments of the chloroplast lamellae. (b) The initial removal of chlorophyll (up to 40% of the total) occurred upon solubilization of PSI from the stroma-exposed lamella regions in which PSI is localized. (c) The tightly appressed membrane of the grana partition regions was markedly resistant to solubilization by Triton X-100. Thus, solubilization of PSII from this membrane region was initiated only after all Cyt b6-fand PSI complexes were removed from the chloroplast lamellae. The results support the notion of extreme lateral heterogeneity in the organization of the electron transport complexes in higher plant chloroplasts and suggest a Cyt b6-f localization in the membrane of the narrow fret regions which serve as a continuum between the grana and stroma lamellae.The thylakoid membrane of higher plant chloroplasts shows distinct differentiation into grana and stroma-exposed lamellae [l]. In the grana, disc-shaped thylakoids are pressed against each other at the 'partition region'. This thylakoid stacking is enabled upon screening of surface charges by divalent Mgz+ ions [2]. Stroma-exposed lamellae are interconnected with the grana via the narrow membrane in the 'fret' region [3] thus helping to form an extensive and continuous thylakoid network.The functional significance of this membrane differentiation is clearly the localization of different photosystems in grana and stroma-exposed regions [4 -71. It is now believed that most of PSII is segregated in the thylakoid membrane of the grana partition regions [6-91 whereas PSI is found exclusively in stroma-exposed thylakoids [7]. This lateral separation of PSII from PSI in the thylakoid membrane implies long-distance electron transport from the membrane of the grana partition regions to the stroma-exposed regions of chloroplasts. The average lateral distance of PSII from PSI is of the order of 0.3-0.5 pm. As discussed by other authors [9 -111, the nature of the electron shuttle between grana and stroma lamellae (plastoquinone or plastocyanin) largely depends on the localization of the intermediate cytochrome (Cyt) b6-f complex in the thylakoid membrane.

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