Modulation of the PSI Unit Cross Section as Monitored by the Low-Temperature Fluorescence Ratio F685/F730 at 77°K of the Pigment-Protein Complex CPIa

  • G. Akoyunoglou
  • J. H. Argyroudi-Akoyunoglou


For optimization of excitation energy utilization, two types of mechanisms have been proposed to operate in photosynthetic organisms: that of spillover of excitation energy from the antennae of PSII to the reaction center of PSI; and that of modulation of the absorption cross section of the photosynthetic units (1–4). Even though distinctly different, the two mechanisms do not exclude each other. The constant quantum yield of photosynthesis observed at wavelengths between 600 and 680 nm, where only the PSII absorbs, has been explained as the result of transfer of excitation energy from PSII to PSI (5). Similarly, the increase in the Chla fluorescence yield of low-salt chloroplasts and in the 77°K fluorescence ratio F685/F730, upon cation addition, have been considered to monitor the control of spillover by Mg++. (In the absence of cations spillover is enhanced, and the PSI Chl fluoresces more). The suppression of the F685/F730 ratio at 77°K in algae preilluminated with PSII light, has been also attributed to spillover (1,2). On the contrary, the state I/II transitions (high/low fluorescence state) observed in algae by manipulation of the light quality (PSI light vs PSII light) have been attributed to changes in the cross section of the PS units (3). The variation in the cross section of the PS units was calculated to be in the order of 10% (3).


Fluorescence Change Stroma Lamella Optical Cross Section Cation Addition PSII Light 
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Copyright information

© Plenum Press, New York 1986

Authors and Affiliations

  • G. Akoyunoglou
    • 1
  • J. H. Argyroudi-Akoyunoglou
    • 1
  1. 1.Nuclear Research Center “Demokritos”AthensGreece

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