THEORY OF FLUORESCENCE INDUCTION IN PHOTOSYSTEM-II - DERIVATION OF ANALYTICAL EXPRESSIONS IN A MODEL INCLUDING EXCITON-RADICAL-PAIR EQUILIBRIUM AND RESTRICTED ENERGY-TRANSFER BETWEEN PHOTOSYNTHETIC UNITS

The theoretical relationships between the fluorescence and photochemic al yields of PS II and the fraction of open reaction centers are exami ned in a general model endowed with the following features: i) a homog eneous, infinite PS II domain; ii) exciton-radical-pair equilibrium; a nd iii) different rates of exciton transfer between core and periphera l antenna beds. Simple analytical relations are derived for the yields and their time courses in induction experiments. The introduction of the exciton-radical-pair equilibrium, for both the open and closed sta tes of the trap, is shown to be equivalent to an irreversible trapping scheme with modified parameters. Variation of the interunit transfer rate allows continuous modulation from the case of separated units to the pure lake model. Broadly used relations for estimating the relativ e amount of reaction centers from the complementary area of the fluore scence kinetics or the photochemical yield from fluorescence levels ar e examined in this framework. Their dependence on parameters controlli ng exciton decay is discussed, allowing assessment of their range of a pplicability, An experimental induction curve is analyzed, with a disc ussion of its decomposition into alpha and beta contributions. The sig moidicity of the induction kinetics is characterized by a single param eter J related to Joliot's rho, which is shown to depend on both the c onnectivity of the photosynthetic units and reaction center parameters . On the other hand, the relation between J and the extreme fluorescen ce levels (or the deviation from the linear Stern-Volmer dependence of 1/Phi(f) on the fraction of open traps) is controlled only by antenna connectivity. Experimental data are consistent with a model of connec ted units for PS IIalpha, intermediate between the pure lake model of unrestricted exciton transfer and the isolated units model.