Fluorescence induction in green plants revisited origin of variabilities in sigmoidicities on different time scales of irradiation

The yield of the variable fluorescence in green plants is known to increase sigmoidally as a function of the fraction of closed photosystem II reaction centers under continuous illumination. However, in double-pulse, pump-probe experiments, the shapes of the fluorescence induction curves tend to be exponential rather than sigmoidal for actinic flashes less than a few microseconds in duration, but become more sigmoidally shaped for actinic pulse lengths of 50 μs, or longer. It is shown here that the shapes of the induction curves cannot be due to artefacts arising from the pump-probe double-flash measuring technique. The near-exponentially of the induction curves is not due to singlet-triplet annihilation effects either, which are known to shorten the lifetimes of the singlet excitation, and thus could decrease the effective connectivities between adjacent photosynthetic units. The dependence of the shapes of the induction effects can be accounted for in terms of an integrated lake-model version of the sequential double-hit model proposed earlier (L. Valkunas, N.E. Geacintov, L. France and J. Breton, Biophys. J. 59 (1991) 397). Finally, some new experiments are proposed to further verify the validity of the sequential hit model.