|De Sturler, Eric|
Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2005
Publication Date: 11/1/2005
Citation: Zhu, X., Govindjee, Baker, N., De Sturler, E., Ort, D.R., Long, S.P. 2005. 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. Planta. 223:114-133. Interpretive Summary: The measurement of fluorescence light emission from leaves of intact crop plants has emerged over the past decade as the single most important technique for investigating the effects of environmental stress and global change on photosynthetic performance. The technique is nondestructive, highly sensitive and very rapid. While a great deal is understood about the cause and meaning of changes in the amount and timing of this fluorescence emission there has never been attempted a holistic quantitative rationalization of the component reactions that comprise the overall process. In this work, we present and valid such a holistic model for the description and interpretation of leaf level chlorophyll fluorescence. The importance of this work is that it identifies the component photosynthetic process that are contributing to each phase of the chlorophyll fluorescence profile. This will in turn allow for more in depth and holistic interpretation of these data and which will be of value to the wide range of crop physiologists that use this technique in the analysis of crop plant performance.
Technical Abstract: Induction of chlorophyll a fluorescence is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises to a maximum through a series of transients, termed O J I and P fluorescence induction curve (FI). 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 inductions, which includes all discrete energy and electron transfer steps in and around photosystem II, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence inductions 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 nad fluorescence emission at these points. Simulations based on this model showed that J corresponded to the peak concentrations of QA-QB (QA: the first quinone electron acceptor of photosystem II; QB: the second quinone electron acceptor of photosystem II) and QA-QB- and I to the first shoulder in the increase in concentration of QA-QB2-. The P peak coincided with maximum concentrations of both QA-QB2- 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. Increase inactive PSH center increase fluorescence emission at O phase and correspondingly decrease Fv/Fm.