Oxygen evolution from single- and multiple-turnover light pulses: temporal kinetics of electron transport through PSII in sunflower leaves

Abstract

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 μmol 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.

Oxidation of plastohydroquinone by photosystem II and by dioxygen in leaves

“…It would be difficult to explain such a linearity on the basis of occasional overlapping of several nonlinearities having opposite direction. Rather we tend to believe that during the decay from F r to F 0 fluorescence proportionally reflected the PQH 2 level, consistently with PQH 2 being a strongly binding competitive product inhibitor for PSII [51]. Thus, we interpret our experiments showing that the kinetics of PQH 2 oxidation are of the first order with respect to PQH 2 concentration over its full redox range; with respect to O 2 the kinetics saturate with an apparent K m (O 2 ) of 60 μM (4.6%).…”

“…As in the PSI cycle [58], there is a fast component in the PSII cycle that evidently is not proton-coupled, but facilitates charge recombination within PSII at high light intensities [33]. Parallel measurements revealed that the higher the light intensity, the larger became the gap between the rate of PSII charge separation as detected from Chl fluorescence and the rate of LEF through PSII as detected from O 2 evolution [34,51]. The difference was explained by charge recombination on the donor side of PSII, from excited P680* to oxidized tyrosine Z [35].…”

“…With AL + FRL the steady-state Chl fluorescence yield F decreased, approaching F 0 at PAD b 500 μmol m − 2 s − 1 , indicating oxidation of PQH 2 due to the high PSI activity supported by FRL. At low actinic PADs this pattern of F significantly changed while FRL was turned off for 40 s: F increased since PSII was slightly overexcited compared with PSI at 630 nm, causing accumulation of PQH 2 and related increase in Q A reduction due to the reversible binding of PQH 2 to the Q B site [51].…”

“…In order to evaluate the rate of PQH 2 oxidation by O 2 in comparison with the PQH 2 oxidizing PSII cycle we proceed from the total pool size of 3 PQH 2 per PSII in sunflower [51] and PSII density of 1.6 μmol m −2 , resulting in 9.6 μmol e − m −2 in the fully reduced PQH 2 pool. Using the fluorescence decay rate constant of 0.054 s −1 at 21% O 2 (Fig.…”

Action spectrum of the redox state of the plastoquinone pool defines its function in plant acclimation

“…Orange light of 630 nm that acted as PSI light when applied at very low intensity in F 0 rise experiments ( Figure 2) caused an intermediate redox state at the moderate intensity used in PQ measurements (Figure 3). These results indicate that the photosystem preferences of the wavelengths exert large effects on the redox state of the PQ pool under monochromatic light of moderate intensity, although the absorption differences between the photosystems are modest ( Figure 2); see also Canaani and Malkin (1984), Veeranjaneyulu and Leblanc (1994) Photosynthetic electron transfer reflects the state of the PQ pool A reduced PQ pool promotes high Chl a fluorescence by slowing down reoxidation of Q A (De Wijn and van Gorkom, 2001;Oja et al, 2011) and, indeed, leaves illuminated with wavelengths favoring PSII had a higher Chl a fluorescence yield than leaves illuminated with PSI wavelengths (Figure 4). Accordingly, the combined absorbance signal of the oxidized primary donor of PSI (P 700 + ) and PC (Schreiber et al, 1988) was weaker in PSII than in PSI light (Figure 4).…”