The electrical potential as a gauge of photosynthetic performance in plant chloroplasts : a patch-clamp study

T. van Voorthuysen

Research output: Thesisinternal PhD, WU

Abstract

<p>The earliest events in the energisation of the photosynthetic membrane upon light capture are the formation of a transmembrane electrical potential (AV) and a transmembrane proton gradient (ΔpH). In this thesis ΔΨis employed for the study of the bioenergetics of chloroplast photosynthesis and its regulation by ΔpH in the shade adapted plant peperomia ( <em>Peperomia metallic</em> a) and the high-light adapted plant spinach ( <em>Spinacia oleracea</em> ). Electrochromism (P515) was used and a patch-clamp method was developed yielding two complementing tools for the detection of ΔΨ. The patch-clamp method enables the detection of relatively large light- induced currents (photocurrents) or potentials (photopotentials) of a single <em>P. metallica</em> chloroplast. An electrical equivalent scheme is introduced incorporating amongst others the thylakoid membrane resistance and capacitance and an access resistance which, at least partly, is supposed to be associated with low (lateral) conductance phases of thylakoid lamellae. The light-induced electrical responses reflect the operation of the photosynthetic current-generators and the way generated current is distributed throughout the chloroplast conductance network. Simultaneous measurements of light- and current-induced responses allow the separation of electrogenic events from changes in chloroplast conductances. A kinetically well defined slow secondary phase (R1 /Q) could be distilled from the flash- induced photocurrent/-potential which is related to the turnover of the cyt. <em>b <sub>6</sub> f</em> complex (Q-cycle). Generally, the rise of R1/Q was sigmoidal. This biphasic rise is modelled by a consecutive reaction scheme with two relaxation times of 13 and 28 ms which likely reflect the oxidation of plastoquinol and reduction of plastoquinone at the lumen and stroma membrane/water interface of the <em>b <sub>6</sub> f</em> complex, respectively. A P515 fraction (Rl /RC <sub>f</sub> ) of about 20 % is inadequately stabilised in dark-adapted spinach chloroplasts and decays rapidly with a relaxation time of 1 - 2 ms. A fast dissipation of ΔΨas generated by photosystern (PS) II is suggested to cause R1/RC <sub>f</sub> . It is hypothesised that adequate charge stabilisation depends on efficient energy coupling between PS II and the cyt. <em>b <sub>6</sub> f</em> complex which is only guaranteed in superclusters composed of both protein complexes. Energisation causes a suppression of about 50 % of PS II-dependent charge separation which is dark reversible with a relaxation time of about 20 s and is likely induced by the low lumenal pH created by lightdriven proton pumping. The results are best explained by a reaction center quenching model in which a fraction of PS II centers exhibits a rapid charge recombination. Flash-induced changes in chloroplast conductances are first demonstrated. The seal conductance decreases transiently upon a brief flash with a minimum of 0.3 - 5 % at 50 - 200 ms after the flash and a slow relaxation in 1 - 10 s. It is proposed that an important part of the conductance changes is intimately associated with changes in the lateral conductances of thylakoids, in particular those of the narrow spaced grana thylakoids.<br/>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Vredenberg, W.J., Promotor
  • Snel, J.H.F., Promotor, External person
Award date2 Jun 1997
Place of PublicationS.l.
Publisher
Print ISBNs9789054856696
Publication statusPublished - 1997

Keywords

  • photosynthesis
  • chloroplasts
  • electromagnetic field
  • electrodynamics
  • electromagnetism

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