Determining the limitations and regulation of photosynthetic energy transduction in leaves

Abstract

The light-dependent production of ATP and reductants by the photosynthetic apparatus in vivo involves a series of electron and proton transfers. Consideration is given as to how electron fluxes through photosystem I (PSI), using absorption spectroscopy, and through photosystem II (PSII), using chlorophyll fluorescence analyses, can be estimated in vivo. Measurements of light-induced electrochromic shifts using absorption spectroscopy provide a means of analyzing the proton fluxes across the thylakoid membranes in vivo. Regulation of these electron and proton fluxes is required for the thylakoids to meet the fluctuating metabolic demands of the cell. Chloroplasts exhibit a wide and flexible range of mechanisms to regulate electron and proton fluxes that enable chloroplasts to match light use for ATP and reductant production with the prevailing metabolic requirements. Non-invasive probing of electron fluxes through PSI and PSII, and proton fluxes across the thylakoid membranes can provide insights into the operation of such regulatory processes in vivo.

The Roles of ATP Synthase and the Cytochrome b 6/f Complexes in Limiting Chloroplast Electron Transport and Determining Photosynthetic Capacity

“…The parameter of 12qL, which reflects the plastoquinone reduction state (Kramer et al, 2004;Baker et al, 2007), was lower at 40°C than at 25°C. NPQ was lower at 40°C than at 25°C (Fig.…”

“…Photosynthetic CO 2 assimilation rate can be viewed as being limited either by the capacity of Rubisco to consume ribulose 1,5-bisP (RuBP; at lower CO 2 ) or by the capacity of the chloroplast electron transport to generate ATP and NADPH for RuBP regeneration (at higher CO 2 ; Farquhar et al, 1980). However, within this framework of limitations, significant uncertainties remain in our understanding of how electron transport and ATP synthesis are coordinated and affect electron transport capacity and photosynthesis (Baker et al, 2007).…”

“…However, there are few studies that have considered the role of chloroplast ATP synthase as a limiting factor for the thylakoid reactions. Recent studies have documented that the in vivo activity of ATP synthase is modulated, especially at low or high CO 2 concentration where CO 2 assimilation is restricted either by CO 2 concentration or end product limitation (Kanazawa and Kramer, 2002;Kramer et al, 2004;Baker et al, 2007). The conductivity of proton efflux from the lumen (gH + ) through the ATP synthase could be modulated to regulate the thylakoid proton motive force (pmf; Kramer et al, 2004), providing flexibility in the ratio of ATP production per H + .…”

Hacking the thylakoid proton motive force for improved photosynthesis: modulating ion flux rates that control proton motive force partitioning into Δ ψ and ΔpH

“…The electron transfer steps are tightly coupled to proton transfer reactions into the thylakoid lumen, storing potential energy in the pmf to drive the synthesis of ATP (8,9). Both NADPH and ATP, in the correct ratios, are required for driving the assimilation of CO 2 and other cellular processes (10). The electron and proton transfer processes can be highly efficient, but when energy capture outpaces the capacity of photosynthesis, a situation that can occur at high light and/or under adverse environmental conditions, reactive intermediates can accumulate in the photosynthetic apparatus, leading to generation of reactive oxygen species (ROS), mainly O 2 -• in PSI and mainly 1O2 in PSII, and these are responsible for oxidative photodamage.…”

Chloroplastic iron-sulfur scaffold protein NFU3 is essential to overall plant fitness

“…A low F v value is indicative of an inability of PSII to perform primary photochemistry. 9 A high F o value could be the result of an over-accumulation of reduced Q A (bound primary plastoquinone electron acceptor of PSII) in the dark. 10 To identify the causal parameter(s) for reduced F v /F m in the nfu3 mutants, we determined the minimal, maximal, and variable fluorescence of dark-adapted wild-type and nfu3 plants (Fig.…”