Numerical simulation of the fluorescence induction in plants. - The variable fluorescence in the fast phase ot chlorophyll a fluorescence induction phenomenon is closely related to the electron in photosystem II (PS II) of plants, algae and cyanobactena We present here two theoretical models that simulate successfully the experimental data on the O-J-I-P fluorescence transient using PS II reactions. The "core" ot both these proposed theoretical models are chaige stabilization and the two-electron-gate process on the acceptor side of PS II. The assumptions include: (1) PS IIs are homogeneous; (2) PS lIs are unconnected, and (1) the redox reactions of PS II obey a first order kinetics. The two models tested are (1) individual (in which the electron carriers Q_A and Q_B have been considered to function as individual entities), and (2) complex (in which the electron carriers Q_A and Q_B have been considered to function in pairs in the PS II reaction center complexes). Data lot dynamic analysis of both models weie obtained through numerical integration of the Ordinary Differential Equations (ODE). Livermore Solver of Ordinary Ditfeiential Equations with Method Switching (LSODA) procedure was included in a specialized simulation software Gepasi We have considered as input parameters the initial concentrations of reactants and the rate constants for redox reactions. We have also verified oui theoretical models by simulating fluorescence transients under three different experimental conditions (I) vanation of the light intensity; (2) DCMU treatment; and (3) re-exposure of the samples to actinic light after a defined period of darkness (double-hit experiments). The theoretical curves obtained converge quite well toward the experimental ones, proving that the redox reactions considered in our models are adequate tor describing the fluorescence induction phenomenon, and that the numerical simulation by using highly developed programs is a powerful tool to investigate the dynamics ot the pnmary reactions ot photosynthesis