Published in IET Systems Biology Received on 5th August 2008 Revised on 9th April 2009 doi: 10.1049/iet-syb.2008.0157 ISSN 1751-8849 Observation of plastoquinone kinetics in photosystem II from delayed fluorescence measurements Y. Guo B. Wirth J. Tan Department of Biological Engineering, University of Missouri, Columbia, MO 65211, USA E-mail: tanj@missouri.edu Abstract: Attempts to account for the variations in photosystem II (PSII) under general conditions result in non-linear and cumbersome models that are difficult to validate and render few insights about the system kinetics. In this research, the authors experimentally show that under certain conditions, linear-system techniques could be applied to advantage for probing some basic kinetic characteristics of the plastoquinones (PQs). The PQ redox states of the reaction centres were represented in a conditionally linear model structure with delayed fluorescence (DF) as a measurable output. DF data were acquired for different plant samples and conditions. After least-squares parameter optimisation, not only could the model closely describe the measured DF, but more significantly, the estimated parameters correctly reflected the expected changes induced by drought or [3-(3,4-dichlorophenyl)-1,1- dimethylurea] (DCMU) stress. Analysis showed that for short-pulse illumination, the PQ kinetic states of the reaction centres in an initially dark-adapted plant leaf can be represented as a time-invariant bilinear system in a five-dimensional state space.The system becomes linear for constant illuminations, but the system matrix and the kinetic behaviour are illumination dependent. In particular, the system behaves differently between lights-on and lights-off conditions. The simplicity of the model structure, nonetheless, permits observation and analysis of the PQ kinetics of PSII reaction centres from DF measurements by using linear-system techniques. 1 Introduction Photosynthesis starts from light-induced separation of charge pairs and transport of electrons by various electron carriers [1, 2]. In the early stages of the electron transport process, the major carriers are plastoquinone (PQ ) molecules in the reaction centres of photosystem II (PSII). An electron generated by a pair of chlorophyll a molecules capturing a photon is transferred, via a pheophytin molecule, to a tightly bound quinone molecule commonly referred to as quinone A or Q A and then to a loosely bound quinone referred to as quinone B or Q B . A double-reduced Q B will take up two protons from the chloroplast stroma to form a plastoquinol (QH 2 ), which diffuses through the thylakoid membrane to another protein complex. QH 2 is then oxidised, passing two electrons downstream, shedding (or pumping) two protons to the thylakoid lumen and returning itself to the oxidised form of quinone (Q B ), which is subsequently reused. The PQs not only form a critical link of the electron transport chain in the light-dependent part of photosynthesis, their states can also be associated with the optical properties of photosynthetic plants or organisms because they are adjacent to the photon–electron energy conversion process performed by the chlorophyll molecules. In particular, they are related to emission of delayed fluorescence (DF) [3]. Experiments have shown that following illumination, photosynthetic systems emit weak luminescence that can last for seconds or minutes. This emission is different from the more commonly measured prompt fluorescence (PF) that has a lifetime in the femtoseconds or picoseconds range. The common view is that DF arises from back reactions of the electron transport process [4–7]. DF was first discovered by Strehler and Arnold [8] and subsequent investigations have yielded a significant amount of information regarding its features and connections to the photosynthetic mechanism [9]. DF 90 IET Syst. Biol., 2010, Vol. 4, Iss. 1, pp. 90–98 & The Institution of Engineering and Technology 2009 doi: 10.1049/iet-syb.2008.0157 www.ietdl.org