2 FUNDAMENTALLY DIFFERENT TYPES OF VARIABLE CHLOROPHYLL FLUORESCENCE IN-VIVO

Upon onset of saturating continuous light only the first part of the o bserved polyphasic fluorescence rise follows Q(A) reduction (photochem ical phase), whereas the remaining part (thermal phases) is kineticall y limited by relatively slow reactions with light saturated half-times in the order of 10-50 ms. A simple hypothesis is presented for the in terpretation of these fundamentally different types of variable fluore scence. The hypothesis, which is based on the reversible radical pair model of PSII, assumes stimulation of both prompt and recombination fl uorescence upon Q(A) reduction, with only recombination fluorescence b eing in competition with nonradiative energy loss processes at the rea ction centers. It is proposed that changes in the rate constants of th ese processes modulate the yield of recombination fluorescence in clos ed centers, thus causing large variations in the maximal fluorescence yield and also giving rise to the 'thermal phases'. This hypothesis ca n reconcile numerous experimental findings which so far have seemed di fficult to interpret.