Influence of the photoperiod on redox regulation and stress responses in Arabidopsis thaliana L. (Heynh.) plants under long- and short-day conditions

Becker B, Holtgrefe S, Jung S, et al. Influence of the photoperiod on redox regulation and stress responses in Arabidopsis thaliana L. (Heynh.) plants under long- and short-day conditions. PLANTA. 2006;224(2):380-393.Arabidopsis thaliana L. (Heynh.) plants were grown in low light (150 mu mol photons m(-2) s(-1) and 20 degrees C) either in short days (7.5 h photoperiod) or long days ( 16 h photoperiod), and then transferred into high light and low temperature (350-800 mu mol photons m(-2) s(-1) at 12 degrees C). Plants grown in short days responded with a rapid increase in NADP-malate dehydrogenase (EC 1.1.1.82) activation state. However, persisting overreduction revealed a new level of regulation of the malate valve. Activity measurements and Northern-blot analyses indicated that NADP-malate dehydrogenase transcript and protein levels increased within a few hours. Using macroarrays, additional changes in gene expression were identified. Transcript levels for several enzymes of glutathione metabolism and of some photosynthetic genes increased. The cellular glutathione level increased, but its redox state remained unchanged. A different situation was observed in plants grown in long-day conditions. Neither NADP-malate dehydrogenase nor glutathione content changed, but the expression of several antioxidative enzymes increased strongly. We conclude that the endogenous systems that measure day length interact with redox regulation, and override the interpretation of the signals, i.e. they redirect redox-mediated acclimation signals to allow for more efficient light usage and redox poising in short days to systems for the prevention of oxidative damages when grown under long-day conditions

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

We describe an extended Farquhar, Von Caemmerer and Berry (FvCB) model for the RuBP regeneration-limited or electron transport-limited steady-state C3 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 interphotosys-tem 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 C3 photosynthesis. The model implies that even within the electron transport-limited range the relationship between quantum yields of CO2 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 CO2 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 CO2 assimilation, and (iii) the mode of a variable Q-cycle to obtain a correct NADPH/ATP ratio with varying light and CO2 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