A Model of the Generalized Stoichiometry of Electron Transport Limited C3 Photosynthesis: Development and Applications

Abstract

We describe an extended Farquhar, Von Caemmerer and Berry (FvCB) model for the RuBP regeneration-limited or electron transport-limited steady-state C(3) photosynthesis. Analytical algorithms are presented to account for (i) the effects of Photosystem (PS) I and II photochemical efficiencies and of cyclic electron transport around PS I (CET) on the photosynthetic quantum yields and related interphotosystem excitation partitioning, and (ii) CET and pseudocyclic electron transport (PET) that may act in concert with linear electron transport (LET, with or without the Q-cycle) to permit flexibility in the ratio of NADPH and ATP synthesis to meet the variable demands of the carbon reduction cycle and photorespiration. The two widely used forms of the original FvCB model represent the most and least efficient electron transport stoichiometry, respectively, of special cases covered by the extended model. The generalized model integrates most basic elements of C(3) photosynthesis. The model implies that even within the electron transport-limited range the relationship between quantum yields of CO(2) assimilation and PS II photochemical efficiency is linear only if the latter varies in proportion with PS I photochemical efficiency. The model can be used (i) to assess any occurrence of alternative electron transport and to answer `what-if' questions with respect to uncertain or unmeasured parameters, and (ii) to estimate photosynthetic parameters by curve-fitting to combined gas exchange and biophysical measurements (e. g. chlorophyll fluorescence) under various irradiance and CO(2) levels. As long as current biophysical measurements were accurate, our analyses support (i) the possible in vivo occurrence of CET and basal PET even under limiting irradiance, (ii) CET as a `brake' for LET to accommodate the balance between quantum yields of electron transport and CO(2) assimilation, and (iii) the mode of a variable Q-cycle to obtain a correct NADPH/ATP ratio with varying light and CO(2) levels if no ATP from chloroplast is used for processes other than carbon reduction and photorespiration. Our model provides a tool to facilitate understanding the stoichiometries, bioenergetics and regulation of photosynthesis under different environmental conditions.