Lebedeva G.V., Belyaeva N.E., Riznichenko G.Yu., Demin O.V.1
Moscow State University, Bioplogical Faculty, Department of Biophysics, 119899, Vorobjovy Gory, Moscow, Russia, e-mail: lebed@biophys.msu.ru;
1A.N.Belozersky Institute of Physico-Chemical Biology, Moscow State University, 119899, Vorobjovy Gory, Moscow, Russia
One of the most well-studied induction effects of photosynthesis is the fast (in the interval of 1 s) rise of fluorescence yield after the beginning of illumination of the plant sample. The origin of nonmonotone behavior of the fast fluorescence kinetics is usually explained according to two basic hypothesis about the role of photosystem I and heterogeneity of PS II units.
We present a model of primary events of photosynthesis, which includes detailed consideration of the light-driven electron transfer processes and takes into account the conversion of photosynthetic energy. We focus on the investigation of the effects of transmembrane electric potential formation and decreasing lumenal pH on the parameters of fluorescence kinetics.
Three different chloroplast compartments were considered: the thylakoid membranes, intrathylakoid space (lumen) and stroma of the chloroplast. The light-driven electron transfer processes taking place in the complexes of photosystem II, cytochrome b6/f and photosystem I were considered in details. The corresponding system of differential equations was reduced according to the hierarchy of time scales. These processes generate H on the thylakoid membrane by transmembrane electrone transfer and coupled uptake of protons into the thylakoid lumen. In the model the dependence of certain electron transfer stages on electric potential was introduced. We also considered a number of processes, describing H consumption:
ATP synthesis and ion fluxes of H+, K+ and Cl- through the thylakoid membrane.
The fluorescence was assumed to be proportional to the total concentration of photosystem II excited states multiplied by the corresponding rate constants of their inactivation by fluorescence emission.
The model allows to give a realistic description of the time course of chlorophyll fluorescence after the beginning of illumination: nonmonotone behavior of the fast fluorescence rise and the subsequent slow decrease in fluorescence yield. The process of the transmembrane electric potential formation as well as initiated ion fluxes were shown to play a significant role in the determination of the fast fluorescence rise pattern. The model needs further refinements to give a proper description of pH formation and the slow phase of fluorescence induction.
The work was supported by the Russian Foundation for Basic Research grant N 98-04-48868