The effect of
dehydration on the reaction pattern of
photosystem II (PS II) has been studied by measuring and analyzing spectral changes induced by continuous wavelength illumination in films of untreated and
hydroxylamine-washed PS II membrane fragments dehydrated to different levels. The obtained data revealed (i) the extent of light-induced formation of about one Q(A)(-*)per 230
chlorophylls (Chl) remains virtually invariant to
dehydration down to the lowest values of relative humidity (6-8% RH); (ii) a decrease of the RH to 30% leads to severe blockage of the electron transfer from Q(A)(-*) to Q(B) and the progressive replacement of water oxidation by photooxidation of high potential (HP)
cytochrome (Cyt) b559 in untreated PS II samples or accessory Chl and
carotenoid (Car) molecules in samples with preoxidized Cyt b559; (iii) photooxidation of Cyt b559 is followed by its photoreduction, concomitant with photooxidation of Chl and Car; (iv) in dry samples with preoxidized Cyt b559, not more than a half of total Cyt b559 can be photochemically reduced, independent of the extent of Cyt b559 in the HP form; (v) at low RH values, Cyt b559 photoreduction in samples with preoxidized
heme groups and photoaccumulation of Q(A)(-*) take place with biphasic kinetics with similar rate constants for both processes; (vi) Cyt b559 photoreduction in dry samples is
DCMU insensitive, while the dark rereduction of photooxidized Cyt b559 is inhibited by
DCMU; (vii) fast and slow kinetic phases of Cyt b559 photoreduction dramatically differ in their dependencies on the intensity of CW illumination and are associated with electron donation to Cyt b559 from Q(A)(-*) and
pheophytin(-*), respectively. The pathways of light-induced electron transfer in PS II involving Cyt b559 are discussed.