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Besides the non-cyclic electron transport driven by the two photosystems (PSII and PSI), the cyclic electron transport pathways around PSI are also essential for efficient photosynthesis. As one of these pathways, the NAD(P)H dehydrogenase complex (NDH complex) mediated cyclic electron transport has been well studied. Along with the identification of the plastid terminal oxydase (PTOX), the functions of NDH-mediated cyclic and chlororepiratory electron transport in energy supply for photosynthesis as well as in the resistance to photooxidative stress have increasingly been brought to the researchers' attention. In the present paper, the structural characteristics of NDH complex, the regulatory mechanism, and the physiological significance of NDH mediated cyclic electron transport and chlororespiration are reviewed.

State transition of the photosynthetic apparatus in plants is a short-term adaptation mediated mainly by the reversible phosphorylation of the main light-harvesting complex protein (LHCII) and its migration between photosystem I (PSI) and photosystem II (PSII). In higher plants and Chlamydomonas, LHCII phosphorylation is mainly controlled by the redox state of plastoquinone pool and cytochrome b(6)f complex, while salt could induce a redox-independent LHCII phosphorylation via transient changes in ion concentrations in Dunaliella. State transition can balance the distribution of excitation energy between PSII and PSI by changes in light absorption cross section and excitation energy spillover between the two photosystems. The preliminary results got in the studies of green algae reveal that state transition can also balance the ATP supply and demand.

The changes in trans-thylakoid membrane proton motive force caused by red light and caused by far-red light in the halotolerant green alga, Dunaliella salina are investigated. Irradiation with red light decreased the intensity of the fast phase of millisecond delayed light emission (ms-DLE) in D. salina, and far-red light led to the opposite effects. Under low temperature conditions (4 degrees C), red light still decreased ms-DLE fast phase intensity, however, far-red light did not enhance the ms-DLE fast phase intensity as it did at room temperature. In the presence of the uncoupler, nigericin, which eliminates the proton gradient across the thylakoid membrane, there was still a decrease in ms-DLE after red light irradiation, while far-red light had no stimulatory effects anymore. The far-red light-induced increase in ms-DLE fast phase is thus suggested to be due to the proton gradient formed by water oxidation in photosystem II. Previous studies with higher plants revealed that far red light increased ms-DLE fast phase intensity slightly, while red light caused a transient increase in ms-DLE fast phase intensity followed by a gradual decrease. Taken together, green algae differ from higher plants with respect to red light- and far red light-induced changes in ms-DLE. The possible reason is discussed.