PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Probing electron fluxes through Photosystem I in cucumber leaf discs by far-red light. Wah Soon Chow^*,1, Alexander Hope², ¹ Research School of Biological Sciences, Canberra, Australian Capital Territory, Australia² School of Biological Sciences, Adelaide, South Australia, Australia

ABSTRACT- Upon illumination, P700 (a special chlorophyll pair in Photosystem I) is oxidised. While the electron is transferred onwards along a chain of carriers, the hole is filled by an electron ultimately originating from water molecules split in PS II during illumination or from elsewhere. Far-red light, mainly absorbed by PS I, was used to photo-oxidise P700 to a steady-state, in which about 90% of P700 was in the form P700+. On cessation of far-red illumination, P700+ was re-reduced with multi-phasic kinetics, characterised by three exponential decays with rate coefficients k1 = 10 per s, k2 = 1 per s and k3 = 0.1 per s, approximately. The initial rate of re-reduction of P700+ at the instant of turning off far-red light, contributed by each phase, is given by the product of the rate coefficient and the amplitude for that phase; these rates are assumed equal to the electron fluxes to P700+ at steady state. The three fluxes are interpreted as reflecting three pathways of electron flow to P700+. The fast phase is attributed to electron flow from PS II to PS I, the intermediate phase to electron donation from reduced ferredoxin to the intersystem pool, and the slow phase to electron donation from stromal reductants catalyzed by NAD(P)H dehydrogenase or to ascorbate that has diffused across the thylakoid membrane to the lumen.

KEY WORDS: far-red light, cyclic electron transport, P700, photosystem I