Institution: | Department of Biology, University of California, San Diego, P.O. Box 109, La Jolla, Calif. 92093, U.S.A. |
Abstract: | The parameters listed in the title were determined within the context of a model for the photochemical apparatus of photosynthesis. The fluorescence of variable yield at 750 nm at ?196 °C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at ?196 °C at the minimum, FO, level and the maximum, FM, level of the emission at 750 nm. The difference spectrum, FM–FO, which represents the excitation spectrum for FV is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex. Fluorescence at the FO level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, , which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of FO/FV measured at 692 nm and the extent of FV at 750 nm. According to this procedure the excitation spectrum of Photosystem I at ?196 °C was determined by subtracting 1/3 of the excitation spectrum of FV at 750 nm from the excitation spectrum of FO at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex. The wavelength dependence of was determined from fluorescence measurements at 692 and 750 nm at ?196 °C. is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus. |