In presence of DCMU, the sigmoidicity of Chl a fluorescence induction curve is correlated to the energetic connectivity (grouping) between photosynthetic units PS II. We are using the experimental data of the fluorescence transients without DCMU (i.e. the O–J–I–P fast fluorescence induction curves) to calculate the overall probability ‘p’ of PS II connectivity of the samples (e.g. leaves or chloroplasts). The values obtained using the same method on DCMU treated samples give very close results. Beside the practical advantage of the quantitative determination of the grouping probability on plants from O–J–I–P fluorescence transients, we also present a possible role of the regulation of the connectivity process in photosynthesis as a response to external perturbations. A theoretical discussion based on numerical simulation and fitting of the experimental data, shows three different possible approaches of the fluorescence signal. All are deduced from the same reaction scheme of the acceptor part of PS II centre, involving the primary acceptor pheophytine (Ph), the two bound quinones (Q_A and Q_B), and the plastoquinone pool. The fraction of closed reaction centres B(t), responsible for the fluorescence transient, is calculated using different definitions as distinct combinations of the redox components for every case. The fluorescence rise of two models is based on the conventional assumption, that the presence of Q_A⁻ determines the high fluorescence state, and one model is based solely on the photochemical charge separation in the reaction centre complex. The transformation of a fluorescence induction curve into a kinetic of the fraction of closed reaction centres is a precondition for the simulation and fitting of biochemical models with experimental data.