Changes in the photosynthetic apparatus of red algae induced by spectral alteration of the light field. II. Further characterization of the light-dependent regulation of the apparent quantum yield of PS I

Abstract

Several red algae show a light-dependent regulation of the apparent quantum yield of PS I (Rehm, A.M., Gülzow, M. and Ried, A. (1989) Biochim. Biophys. Acta 973, 131–137). The induction of a reduced quantum yield of PS I by light 1 (L1) (exciting preferably chlorophyll) and its revision either in L2 (exciting the phycobiliproteins) or in darkness show different characteristics in Porphyra yezoensis, Rhodella violacea and Porphyridium purpureum. The process of induction, however, is always faster than its reversion. The ability to regulate the quantum yield of PS I is strongly influenced by temperature, not by NH 4CI. The 77 K fluorescence spectra of PS I in L1 (452 nm) do not show differences after pretreatment in L1, L2 or darkness which could be correlated with an altered quantum yield of PS I. Also, the wavelength of the actinic L1 (694–720 nm) has no influence on the observable differences in P-700 and cytochrome f oxidation after preillumination with L1 or L2. These data are interpreted as evidence against a possible decrease of the apparent quantum yield of PS I as a consequence of a functional decoupling of antenna and reaction center of PS I. The fact that in thylakoids the addition of ascorbate (2–10 mM) largely protects from a reduction of the apparent quantum yield of PS I by L1 preillumination and the clear dependence of the PS I activity upon the redox potential of the medium indicates a control of the quantum yield of PS I by a thylakoidal redox component accessible to external redox components. The redox midpoint potential of this component is + 140 mV at pH 7.8. This redox center does not participate in linear electron transport between the photosystems and is located nearby PS I.