The numerical analysis of the oxygen flash yield Yn sequences, alone, does not allow to choose between two models: equal S state misses with non negligible double hits or unequal misses with nearly no double hits. Nevertheless, the comparison of the sequences in different conditions shows that the equal miss model is unrealistic: in very different experimental conditions (non saturating flash, different batch of Chlorella or chloroplasts), a parallel variation of the homogeneous miss and double hit factors is observed. This correlation seems strange within the equal miss model: misses come from incomplete reaction (i.e. for exemple insufficient light) and double hits i.e. double advancement come, in principle, from excessive light or too long flash; for these reasons, opposite variation of misses and double hits as a function of light intensity are expected. Within the equal miss model the inverse is exactly observed: at low flash light intensity (11%) which increases the misses, 16% of double hits are needed, which is quite unrealistic. In contrast, the unequal miss model explains such result quite naturally by a m athem atical property: any theoretical sequence with only a unique S state miss and no double hit can be fitted with homogeneous misses and double hits, which increase in parallel as a function of the damping. Evidence for unequal misses in oxygen flash yield sequence is provided by the heterogeneous properties of the light saturation curves (M. J. Delrieu, Biochim. Biophys. Acta 592, 478-494 (1980)). At high flash intensity, all, excepted the transition S'2 → S3, are saturated; the transition S'2-→S3 is far from saturation and its very large saturating light intensity is actually not known. A comparative study, in the same chloroplast batch, of the oxygen yield patterns with attenuated flashes and of the experimental saturation curves of S states shows that only photochemical misses (due to non saturation) exist. At high intensity, there is only a unique miss for the transition S2→S3 i.e. the probability for this transition is low. A model involving a second acceptor could explain the slow increase of transition probability of S'2→S3 at high flash intensity
In the model of Forbush et al. (1971) the observed damping of the flash yield sequence of photosynthetic O2 evolution was related to a certain percentage of 'misses' (a; i.e. centers not converted). The possibility of a miss was supposed to be equal for all statesWe propose a new model and a new recurrence law that gives better quantitative agreement with the O2 yield oscillations observed in Chlorella during a sequence of flashes. We find a better fit with all experimental results by assuming very unequal misses; the misses occur nearly exclusively on S2 (and also sometimes on S,).In the simpler case of only one miss on one state, half of S2 exists as an inactive form S+ because it is in apparent equilibrium with pool A. The active form of S, is converted to S, in a flash and the unchanged inactive form S+ explains the miss:hv hu SI -s+e s2-s, (S+ is a transition state between S , to S , associated with Q-).In the dark, the apparent equilibrium constant K , between pool A and Q (i.e. So, S, in the dark) is very large; this explains why there is no miss on these states. In light, the experimental value of K,, between pool A and Q (i.e. S,, S, in the light) is 1, and this explains why the misses are large for states S,, S,: i.e., S;+/S2= 1 and sometimes S;+/S,-O.l.This new model predicts that the total number of active states ZS, = So + SI + S, + S, is an oscillating function of the flash number. This sum FSI is also the nuhber of trapping centers for excitons. As fluorescence is proportional to excitons that are not trapped, our model explains why the fluorescence oscillates as a function of the flash number. We find also that the initial rates of 0, evolution after ( n -1) flashes vs the 0, yield of the nrh flash are not exactly on a straight line, which also favors our model.
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