Possible photoacoustic detection of cyclic electron transport around Photosystem II in photoinhibited thylakoid preparations

Stabilization of oxygen evolution and primary electron transport reactions in photosystem II against heat stress with glycinebetaine and sucrose

The protective action of co-solutes, such as sucrose and glycinebetaine, against the thermal inactivation of photosystem II function was studied in untreated and Mn-depleted photosystem II preparations. It was shown that, in addition to the reactions that depend on the oxygen evolving activity of the photosystem, those that implicate more intimately the reaction center itself are protected by high concentrations of osmolytes. However, the temperature required to inhibit oxygen evolution totally in the presence of osmolytes is lower than that required to eliminate reactions, such as P680 (primary electron donor in photosystem II) photo-oxidation and pheophytin photo reduetion, which only involve charge separation and primary electron transport processes. The energy storage measured from the thermal dissipation yield during photoacoustic experiments and the yield of variable fluorescence are also protected to a significant degree (up to 30%) at temperatures at which oxygen evolution is totally inhibited. It is suggested that a cyclic electron transport reaction around photosystem II may be preserved under these conditions and may be responsible for the energy storage measured at relatively high temperatures. This interpretation is also supported by thermoluminescence data involving the recombination between reduced electron acceptors and oxidized electron donors at - 30 and - 55 °C. The data also imply that a high concentration of osmolyte allows the stabilization of the photosystem core complex together with the oxygen-evolving complex. The stabilization effect is understood in terms of the minimization of protein-water interactions as proposed by the theory of Arakawa and Timasheff (Biophys. J., 47 (1985) 411--414).

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (YO2) and photochemical yield of PS II (ΦPS II) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, YO2 and ΦPS II followed each other, suggesting a close coupling between the oxidation of water and QA reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432-443). Following a light-to-dark transition, however, the relationship between QA reduction and the fraction of PS II reaction centers capable of evolving O2 became temporarily uncoupled. ΦPS II recovered to the preillumination levels within 5-10 s, while the YO2 required up to 60 s to recover under aerobic conditions. The recovery of YO2 was independent of the redox state of QA, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (σPS II). The hysteresis between YO2 and the reduction of QA during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.

The donor side of Photosystem II as the copper-inhibitory binding site : Fluorescence and polarografic studies

We have measured, under Cu (II) toxicity conditions, the oxygen-evolving capacity of spinach PS II particles in the Hill reactions H2O→SiMo (in the presence and absence of DCMU) and H2O→PPBQ, as well as the fluorescence induction curve of Tris-washed spinach PS II particles. Cu (II) inhibits both Hill reactions and, in the first case, the DCMU-insensitive H2O → SiMo activity. In addition, the variable fluorescence is lowered by Cu (II). We have interpreted our results in terms of a donor side inhibition close to the reaction center. The same polarographic and fluorescence measurements carried out at different pHs indicate that Cu (II) could bind to amino acid residues that can be protonated and deprotonated. In order to reverse the Cu (II) inhibition by a posterior EDTA treatment, in experiments of preincubation of PS II particles with Cu (II) in light we have demonstrated that light is essential for the damage due to Cu (II) and that this furthermore is irreversible.

Relationship between quenching of variable fluorescence and thermal dissipation in isolated thylakoid membranes: similar terminology and mathematical treatments may be used

Simultaneous measurements of chlorophyll fluorescence and thermal emission using photoacoustic spectroscopy have been done in isolated thylakoid membranes to study the relationship between the photochemical quenching of fluorescence (qPF) and energy storage measured in photoacoustic experiments. It is shown that energy storage can be interpreted as the photochemical quenching of a variable component of thermal dissipation termed qPH. The parameters qPF were similarly sensitive to light intensity as demonstrated by their half-saturation light intensity. However, the nonvariable part of thermal dissipation (Ho) represented a greater proportion of the maximal thermal dissipation yield in comparison with the corresponding non-variable component of fluorescence (Fo) as a result of the thermal energy losses occurring during electron transport. A residual qPH found when qPF was removed indicated the participation of cyclic photosystem I or photosystem II in the measured qPH. The participation of cyclic photosystem I was also suggested by a low constant K, representing the quasi equilibria between (re)oxidized and reduced photosystem II quinone acceptors as determined from the logarithmic plots of the hyperbolic relationship obtained between qPH and light intensity. It is finally concluded that the terminology and mathematical treatments used for fluorescence measurements can be applied to thermal dissipation.

The use of photothermal radiometry in assessing leaf photosynthesis: II. Correlation of energy storage to Photosystem II fluorescence parameters

Following the first part of this work (Malkin et al. (1991) Photosynth Res 29: 87-96), where modulated photothermal radiometry (PTR) was used to measure energy storage (ES) in intact leaves as a function of P700 redox state, we report here on simultaneous ES and fluorescence measurements, which characterize the state of PS II. PTR monitors the conversion of modulated light into heat by measuring the modulated infra-red radiation emitted from the sample. The ratio [PTR+-PTR-]/PTR+, where PTR indicates the PTR signal and the subscripts +,- indicate the presence or absence of saturating background light, is used to quantitate ES. We searched carefully for the right conditions where the background light does not introduce a significant rise in the leaf temperature, which influences the PTR signal as such, otherwise the above ratio deviates from the true ES. Under such conditions, ES and the fluorescence parameters, F (momentary fluorescence level) Fm' (fluorescence of fully reduced PS II reaction centers) were measured during the induction phase of photosynthesis and in the steady state. ES and the parameter γ=(Fm'-F)/Fm', considered by Genty et al. ((1989) Biochim Biophys Acta 990: 87-92) to reflect the yield of PS II, had similar kinetics during the induction phase. Both reached a final maximum plateau after about 4-5 min. of illumination. In different experiments, where the measuring light intensities varied, γ was approximately linearly related to ES. This linear relationship was found in the same way also in steady-state measurements, where these parameters varied by using different background light intensities. Extrapolation to an ES value of zero indicates a finite non-zero value of γ. A possible explanation for this may be found in the existence an electron transport cycle around PS II which does not store energy in the range corresponding to the modulation frequency used (ca. 3.6 Hz).