Photosynthesis is the central process in which energy is taken up into the biosphere and only major pathway for uptake of CO 2 from the atmosphere. It is thus critical to understand the processes that limit and regulate the productivity of photosynthesis. The photosynthetic apparatus must balance and regulate energy flow to meet biochemical demands while preventing over-excitation of the reaction centers, which in turn can lead to photoinhibition. Thus, at least some of the properties of the

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... ht reactions are expected to be regulated in response to environmental challenges and consequently are of great importance for understanding the ability (or lack of ability) of plants to acclimate to environmental extremes. Our work developed a unique set of in vivo spectroscopic tools that have allowed us to probe the importance of 1) The effects of storage of proton motive force (pmf ) in the form of both electric field (Δψ) and pH difference (ΔpH); 2) alteration in the stoichiometry of proton pumping to electron transfer at key steps; 3) the influence of changes in the conductivity for proton efflux from the thylakoid of the ATP synthase; 4) the mechanisms of steps of the electron transfer process that pump protons; and 5) the mechanisms by which reactive O 2 is generated as a side reaction to photosynthesis, and how these processes are minimized. The central intermediate in both ATP synthesis and regulation of the photosynthetic apparatus if the transthylakoid proton motive force (pmf). Proton motive force established across the thylakoid membrane by photosynthetic electron transfer functions both to drive the synthesis of ATP and initiate processes that down-regulate photosynthesis. At the same time, excessively low lumen pH can lead to the destruction of some lumenal components and sensitization of the photosynthetic apparatus to photoinhibition (see review in . Therefore, in order to understand the energy budget of photosynthesis, its regulation and responses to environmental stresses, it is essential to know the magnitude of pmf, its distribution between ΔpH and the electric field (Δψ) as well as the relationships between these parameters and ΔG ATP , and down-regulatory and inhibitory processes. The focus of our research is on understanding the relationships among electron transfer, proton pumping and the regulation and control of photosynthesis. We have capitalized on new instrumentation and techniques developed in our laboratory to observe, for the first time, the fluxes of electrons and protons through individual components of the photosynthetic apparatus in the steady-state. Such measurements are allowing us to resolve key issues about the overall energy budget of photosynthesis, its regulation and response to environmental changes. The role of lumen pH in regulating photosynthesis. Based on our own data, as well as a review of past literature, we have proposed a model wherein, under permissive conditions, photosynthesis is regulated so that lumen pH remains moderate (between 5.8 and 6.5). In this range, lumen pH modulates the activity of the violaxanthin deepoxidase, but does not significantly restrict the turnover of the cytochrome b 6 f complex, nor does it destabilize the oxygen evolving complex. Only under stressed conditions, where light input exceeds the capacity of both photosynthesis and down-regulatory processes, does lumen pH decrease below 5, possibly contributing to photoinhibition. A value of n=4 for the stoichiometry of protons pumped through the ATP synthase per ATP synthesized will, in large part, allow moderate lumen pH to sustain the observed levels of ΔG ATP [1]. However, to fully account for observed levels of ΔG ATP , a significant contribution from Δψ to pmf is needed. The extents of transthylakoid ΔpH and Δψ and implications for the synthesis of ATP by the CF1-CF0 ATP synthase. It is almost universally believed that the CF O -CF 1 -ATP synthase (ATP synthase) is driven by either a difference in proton concentrations (ΔpH) or an electric