title: Chlorophyll a fluorescence induction kinetics in leaves predicted from a model describing each discrete step of excitation energy and electron transfer associated with Photosystem II.
abstract: Chlorophyll a fluorescence induction (FI) is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises from an initial level O to a maximum P through transient steps, termed J and I. FI kinetics reflect the overall performance of photosystem II (PSII). Although FI kinetics are commonly and easily measured, there is a lack of consensus as to what controls the characteristic series of transients, partially because most of the current models of FI focus on subsets of reactions of PSII, but not the whole. Here we present a model of fluorescence induction, which includes all discrete energy and electron transfer steps in and around PSII, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence induction including O J I P transients. The fluorescence emission in this model was calculated directly from the amount of excited singlet-state chlorophyll in the core and peripheral antennae of PSII. Electron and energy transfer were simulated by a series of linked differential equations. A variable step numerical integration procedure (ode15s) from MATLAB provided a computationally efficient method of solving these linked equations. This in silico representation of the complete molecular system provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the O J I P kinetics and fluorescence emission at these points. Simulations based on this model showed that J corresponds to the peak concentrations of Q(-)AQB (QA and QB are the first and second quinone electron acceptor of PSII respectively) and Q(-)AQ(-)B and I to the first shoulder in the increase in concentration of Q(-)AQ(2-)B. The P peak coincides with maximum concentrations of both Q(-)AQ(2-)B and PQH2. In addition, simulations using this model suggest that different ratios of the peripheral antenna and core antenna lead to differences in fluorescence emission at O without affecting fluorescence emission at J, I and P. An increase in the concentration of QB-nonreducing PSII centers leads to higher fluorescence emission at O and correspondingly decreases the variable to maximum fluorescence ratio (F v/F m).
authors.affiliation: Department of Plant Biology and Crop Sciences, University of Illinois at Urbana-Champaign, 379 Madigan Laboratory, 1201 W. Gregory Drive, Urbana, IL 61801, USA.
authors.forename: Xin-Guang
authors.initials: XG
authors.lastname: Zhu
authors.lastname: Govindjee
authors.forename: Neil R
authors.initials: NR
authors.lastname: Baker
authors.forename: Eric
authors.initials: E
authors.lastname: deSturler
authors.forename: Donald O
authors.initials: DO
authors.lastname: Ort
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Long
mesh_terms: D002734:Chlorophyll / Q000737:chemistry / Q000502:physiology*; D000077194:Chlorophyll A; D003198:Computer Simulation; D004579:Electron Transport; D005453:Fluorescence; D007700:Kinetics; D008954:Models, Biological; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000502:physiology*; D018515:Plant Leaves / Q000737:chemistry / Q000502:physiology
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D000077194:Chlorophyll A