A model for the primary mechanism of light utilization by chlorophyll in photosynthesis, involving singlet-triplet annihilation, has recently been proposed (Fong, 1974). In that model it is asserted that the photosystem II fluorescence yield doubling observed on strong illumination is predicted exactly.
In this paper the model is critically examined. It is shown that a number of fundamental errors occur in the above model, such that the doubling prediction is incorrect. It is shown that such doubling can be accounted for only if a reaction center chlorophyll in its first excited singlet state interacts with an antenna chlorophyll in its lowest triplet state, and that this doubling arises independently of chemical activity. In view of this reversal of the physical situation from the Fong model, there is no longer justification for a highly symmetric reaction center dimer adduct configuration. The consequences of the corrected treatment are examined. Within the upconversion framework a model is proposed, which is consistent with the photosystem II fluorescence yield doubling on strong illumination, as well as with the absence of such doubling in photosystem I.