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 inc rease 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 inductio n curves tend to be exponential rather than sigmoidal for actinic flas hes less than a few microseconds in duration, but become more sigmoida lly shaped for actinic pulse lengths of 50-mu-s, or longer. It is show n here that the shapes of the induction curves cannot be due to artefa cts arising from the pump-probe double-flash measuring technique. The near-exponentiality of the induction curves is not due to singlet-trip let annihilation effects either, which are known to shorten the lifeti mes of the singlet excitation, and thus could decrease the effective c onnectivities 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 pr oposed 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.