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Photosystem II Cycle and Alternative Electron Flow in Leaves

Laisk, Agu, Eichelmann, Hillar, Oja, Vello, Rasulov, Bakhtier, Rämma, Heikko
Plant & cell physiology 2006 v.47 no.7 pp. 972-983
Betula pendula, Helianthus annuus, Nicotiana tabacum, carbon dioxide, carboxylation, cell physiology, chlorophyll, detection limit, electron transfer, fluorescence, mesophyll, nitrites, oxaloacetic acid, oxygen, photons, photosystem II, ribulose-bisphosphate carboxylase, tobacco
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 JF - JC depended little on CO₂ and O₂ concentrations. The inter-relationship between JF - JC and non-photochemical quenching (NPQ) was variable, dependent on the CO₂ concentration. We conclude that the relatively fast electron flow JF - JC 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.