Journal Article

Photosystem II Cycle and Alternative Electron Flow in Leaves

Agu Laisk, Hillar Eichelmann, Vello Oja, Bakhtier Rasulov and Heikko Rämma

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 47, issue 7, pages 972-983
Published in print July 2006 | ISSN: 0032-0781
Published online July 2006 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcj070
Photosystem II Cycle and Alternative Electron Flow in Leaves

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  • Biochemistry
  • Molecular and Cell Biology
  • Plant Sciences and Forestry

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Sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.) were grown in the laboratory and leaves were taken from field-grown birch trees (Betula pendula Roth). Chlorophyll fluorescence, CO2 uptake and O2 evolution were measured and electron transport rates were calculated, JC from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, JO from the O2 evolution rate, and JF from Chl fluorescence parameters. Mesophyll diffusion resistance, rmd, used for the calculation of JC, was determined such that the in vivo Rubisco kinetic curve with respect to the carboxylation site CO2 concentration became a rectangular hyperbola with Km(CO2) of 10 μM at 22.5°C. In sunflower, in the absence of external O2, JO = 1.07JC when absorbed photon flux density (PAD) was varied, showing that the O2-independent components of the alternative electron flow to acceptors other than CO2 made up 7% of JC. Under saturating light, JF, however, was 20–30% faster than JC, and JFJC depended little on CO2 and O2 concentrations. The inter-relationship between JFJC and non-photochemical quenching (NPQ) was variable, dependent on the CO2 concentration. We conclude that the relatively fast electron flow JFJC appearing at light saturation of photosynthesis contains a minor component coupled with proton translocation, serving for nitrite, oxaloacetate and oxygen reduction, and a major component that is mostly cyclic electron transport around PSII. The rate of the PSII cycle is sufficient to release the excess excitation pressure on PSII significantly. Although the O2-dependent Mehler-type alternative electron flow appeared to be under the detection threshold, its importance is discussed considering the documented enhancement of photosynthesis by oxygen.

Keywords: Alternative; Electron transport; Leaf; Photosystem II cycle

Journal Article.  7663 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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