Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut

Tartu, Estonia

Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut

Tartu, Estonia
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Oja V.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut | Eichelmann H.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut | Anijalg A.,Tartu Ulikooli Fuusika Instituut | Ramma H.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut | Laisk A.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Photosynthesis Research | Year: 2010

Oxidation of photosystem I (PSI) donors under far-red light (FRL), slow re-reduction by stromal reductants and fast re-reduction in the dark subsequent to illumination by white light (WL) were recorded in leaves of several C3 plants at 810 and 950 nm. During the re-reduction from stromal reductants the mutual interdependence of the two signals followed the theoretical relationship calculated assuming redox equilibrium between plastocyanin (PC) and P700, with the equilibrium constant of 40 ± 10 (ΔEm = 86-99 mV) in most of the measured 24 leaves of nine plant species. The presence of non-oxidizable PC of up to 13% of the whole pool, indicating partial control of electron transport by PC diffusion, was transiently detected during a saturation pulse of white light superimposed on FRL or on low WL. Nevertheless, non-oxidizable PC was absent in the steady state during fast light-saturated photosynthesis. It is concluded that in leaves during steady state photosynthesis the electron transport rate is not critically limited by PC diffusion, but the high-potential electron carriers PC and P700 remain close to the redox equilibrium. © Springer Science+Business Media B.V. 2010.


Laisk A.,Tartu Ulikooli Molekulaar Ja Rakubioloogia Instituut | Oja V.,Tartu Ulikooli Molekulaar Ja Rakubioloogia Instituut | Eichelmann H.,Tartu Ulikooli Molekulaar Ja Rakubioloogia Instituut | Dall'Osto L.,University of Verona
Biochimica et Biophysica Acta - Bioenergetics | Year: 2014

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (YI) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810 nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4 · O 2 evolution) which arrived at the PSI donor side after a delay of 2 ms. The intrinsic quantum yield of PSI (yI, electrons per photon absorbed by PSI) was measured at 712 nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (aI) and PSII (aII) in leaves. For comparison, pigment-protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (yI = 0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quanta were absorbed by PSII, but processed with the low intrinsic yield yII = 0.63. In PSII, the quantum yield of charge separation was 0.89 as detected by variable fluorescence Fv/Fm, but 29% of separated charges recombined (Laisk A, Eichelmann H and Oja V, Photosynth. Res. 113, 145-155). At wavelengths less than 580 nm about 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment-protein complexes. © 2013 Elsevier B.V.


PubMed | University of Verona and Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Type: Journal Article | Journal: Biochimica et biophysica acta | Year: 2014

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (YI) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4O2 evolution) which arrived at the PSI donor side after a delay of 2ms. The intrinsic quantum yield of PSI (yI, electrons per photon absorbed by PSI) was measured at 712nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (aI) and PSII (aII) in leaves. For comparison, pigment-protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (yI=0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quanta were absorbed by PSII, but processed with the low intrinsic yield yII=0.63. In PSII, the quantum yield of charge separation was 0.89 as detected by variable fluorescence Fv/Fm, but 29% of separated charges recombined (Laisk A, Eichelmann H and Oja V, Photosynth. Res. 113, 145-155). At wavelengths less than 580nm about 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment-protein complexes.


Laisk A.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut | Oja V.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut | Eichelmann H.,Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Photosynthesis Research | Year: 2012

Oxygen evolution and Chl fluorescence induction were measured during multiple turnover light pulses (MTP) of 630-nm wavelength, intensities from 250 to 8,000 μmol quanta m-2 s-1 and duration from 0.3 to 200 ms in sunflower leaves at 22 °C. The ambient O2 concentration was 10-30 ppm and MTP were applied after pre-illumination under far-red light (FRL), which oxidized plastoquinone (PQ) and randomized S-states because of the partial excitation of PSII. Electron (e - ) flow was calculated as 4·O2 evolution. Illumination with MTP of increasing length resulted in increasing O2 evolution per pulse, which was differentiated against pulse length to find the time course of O2 evolution rate with sub-millisecond resolution. Comparison of the quantum yields, Y IIO = e - /hν from O2 evolution and Y IIF = (F m - F)/F m from Chl fluorescence, detected significant losses not accompanied by fluorescence emission. These quantum losses are discussed to be caused by charge recombination between Q A - and oxidized TyrZ at a rate of about 1,000 s-1, either directly or via the donor side equilibrium complex QA → P D1 + ↔ TyrZ ox, or because of cycling facilitated by Cyt b 559. Predicted from the suggested mechanism, charge recombination is enhanced by damage to the water-oxidizing complex and by restricted PSII acceptor side oxidation. The rate of PSII charge recombination/cycling is fast enough for being important in photoprotection. © 2012 Springer Science+Business Media B.V.


PubMed | Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Type: Journal Article | Journal: Photosynthesis research | Year: 2011

Oxygen evolution per single-turnover flash (STF) or multiple-turnover pulse (MTP) was measured with a zirconium O(2) analyzer from sunflower leaves at 22 C. STF were generated by Xe arc lamp, MTP by red LED light of up to 18000 mol quanta m(-2) s(-1). Ambient O(2) concentration was 10-30 ppm, STF and MTP were superimposed on far-red background light in order to oxidize plastoquinone (PQ) and randomize S-states. Electron (e(-)) flow was calculated as 4 times O(2) evolution. Q (A) Q (B) electron transport was investigated firing double STF with a delay of 0 to 2 ms between the two. Total O(2) evolution per two flashes equaled to that from a single flash when the delay was zero and doubled when the delay exceeded 2 ms. This trend was fitted with two exponentials with time constants of 0.25 and 0.95 ms, equal amplitudes. Illumination with MTP of increasing length resulted in increasing O(2) evolution per pulse, which was differentiated with an aim to find the time course of O(2) evolution with sub-millisecond resolution. At the highest pulse intensity of 2.9 photons ms(-1) per PSII, 3 e(-) initially accumulated inside PSII and the catalytic rate of PQ reduction was determined from the throughput rate of the fourth and fifth e(-). A light response curve for the reduction of completely oxidized PQ was a rectangular hyperbola with the initial slope of 1.2 PSII quanta per e(-) and V (m) of 0.6 e(-) ms(-1) per PSII. When PQ was gradually reduced during longer MTP, V (m) decreased proportionally with the fraction of oxidized PQ. It is suggested that the linear kinetics with respect to PQ are apparent, caused by strong product inhibition due to about equal binding constants of PQ and PQH(2) to the Q (B) site. The strong product inhibition is an appropriate mechanism for down-regulation of PSII electron transport in accordance with rate of PQH(2) oxidation by cytochrome b(6)f.


PubMed | Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Type: Journal Article | Journal: Photosynthesis research | Year: 2012

Oxygen evolution and Chl fluorescence induction were measured during multiple turnover light pulses (MTP) of 630-nm wavelength, intensities from 250 to 8,000mol quanta m(-2)s(-1) and duration from 0.3 to 200ms in sunflower leaves at 22C. The ambient O(2) concentration was 10-30ppm and MTP were applied after pre-illumination under far-red light (FRL), which oxidized plastoquinone (PQ) and randomized S-states because of the partial excitation of PSII. Electron (e ( - )) flow was calculated as 4O(2) evolution. Illumination with MTP of increasing length resulted in increasing O(2) evolution per pulse, which was differentiated against pulse length to find the time course of O(2) evolution rate with sub-millisecond resolution. Comparison of the quantum yields, Y (IIO)=e ( - )/h from O(2) evolution and Y (IIF)=(F (m)-F)/F (m) from Chl fluorescence, detected significant losses not accompanied by fluorescence emission. These quantum losses are discussed to be caused by charge recombination between Q (A) (-) and oxidized TyrZ at a rate of about 1,000s(-1), either directly or via the donor side equilibrium complex Q(A)P (D1) (+) TyrZ(ox), or because of cycling facilitated by Cyt b (559). Predicted from the suggested mechanism, charge recombination is enhanced by damage to the water-oxidizing complex and by restricted PSII acceptor side oxidation. The rate of PSII charge recombination/cycling is fast enough for being important in photoprotection.


PubMed | Tartu Ulikooli Molekulaar ja Rakubioloogia Instituut
Type: Journal Article | Journal: Photosynthesis research | Year: 2010

Oxidation of photosystem I (PSI) donors under far-red light (FRL), slow re-reduction by stromal reductants and fast re-reduction in the dark subsequent to illumination by white light (WL) were recorded in leaves of several C(3) plants at 810 and 950 nm. During the re-reduction from stromal reductants the mutual interdependence of the two signals followed the theoretical relationship calculated assuming redox equilibrium between plastocyanin (PC) and P700, with the equilibrium constant of 40 +/- 10 (Delta E (m) = 86-99 mV) in most of the measured 24 leaves of nine plant species. The presence of non-oxidizable PC of up to 13% of the whole pool, indicating partial control of electron transport by PC diffusion, was transiently detected during a saturation pulse of white light superimposed on FRL or on low WL. Nevertheless, non-oxidizable PC was absent in the steady state during fast light-saturated photosynthesis. It is concluded that in leaves during steady state photosynthesis the electron transport rate is not critically limited by PC diffusion, but the high-potential electron carriers PC and P700 remain close to the redox equilibrium.

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