Effect of Membrane Fluidity on Photosynthetic Oxygen Production Reactions

The effect of changes of membrane fluidity on the oxygen evolving capability of isolated thylakoids was investigated. Alteration of the lipid phase fluidity was achieved by incorporation of the plant sterol stigmasterol. Incorporation of stigmasterol in the lipid bilayer of thylakoid membranes results in rigidization of the hydrophobic phase of thylakoid membranes and decreases the degree of packing of the lipid head groups. These changes of lipid order are accompanied by a reduction of oxygen evolution, measured with 1,4-benzoquinone as an electron acceptor, and by a more pronounced inhibition of PSI-mediated electron transport. By analysis of the parameters of oxygen flash yields and oxygen burst under continuous illumination it was shown that after treatment with stigmasterol: 1.) the number of active oxygen-evolving centres decreased; 2.) the remaining active oxygen-evolving centres were not affected in respect to the oscillation pattern; 3.) the contribution of the slow oxygenevolving centres in oxygen burst yield was increased. The effect of stigmasterol was compared with the well-studied effect of cholesterol. Results were discussed in terms of determining the role of lipid order for the organization and functioning of the photosynthetic machinery.

Photoacclimation involves modulation of the photosynthetic oxygen-evolving reactions in Dunaliella tertiolecta and Phaeodactylum tricornutum

Net energy accumulation by marine microalgae at very low photon fluxes involves modulation of several attributes related to both the growth and photosynthetic physiology of these organisms. Here we studied flash-induced oscillatory patterns in oxygen evolution by previously dark-adapted cells of the green alga Dunaliella tertiolecta (Butcher) and the diatom Phaeodactylum tricornutum (Bohlin). The activity of the oxygen-evolving complex was found to be species-specific and influenced by photoacclimation. Results from measurements of oxygen flash yield obtained for these organisms grown under light-saturating conditions are directly comparable to those previously reported in the literature for other microalgae and higher plants. However, similar measurements on cells grown in low-light and/or light-starved conditions indicate an increased level of backward transitions (double misses) leading to the formation of super-reduced states (i.e. S–1 and S–2). Thus, in this communication, we present the first evidence that super-reduced states can be generated in vivo and speculate, on how they may be physiologically important.

Photosystem II activity and triazine resistance in weeds

The oxygen-evolving properties of broken chloroplasts isolated from biotypes of Chemopodium album and Amaranthus retroflexus that were either sensitive or resistant to s-triazine herbicides were compared. The pattern of oxygen flash yields produced by herbicide-sensitive, dark-adapted chloroplasts of either species was reminiscent of that found with spinach chloroplasts. In contrast, dark-adapted chloroplasts isolated from the herbicide-resistant biotypes exhibited a highly damped oxygen flash pattern in which there was significant oxygen released after the first light flash. Analysis of these results with the Kok model of photosystem II (Kok, B., Forbush, B., and McGloin, M. 1970. Photochem. Photobiol. 11: 457–475) suggested that the unusual properties of the resistant organelles were due to the survival of significant amounts of S3 and S2 states during dark adaptation and to a higher proportion of inactive photosystem II reaction centres during each light flash. Deactivation experiments verified the suggestion that the S3 and S2 states more readily survive a 10-min dark period in resistant organelles. Information about electron transport on the oxidizing side of photosystem II was obtained with a modulated oxygen electrode and suggested that there was no difference between the two biotypes in the value of the rate constant of the reaction that limits the rate of electron transport between the water-splitting step and photosystem II.

Evidence for Unequal Misses in Oxygen Flash Yield Sequence in Photosynthesis

The numerical analysis of the oxygen flash yield Yn sequences, alone, does not allow to choose between two models: equal S state misses with non negligible double hits or unequal misses with nearly no double hits. Nevertheless, the comparison of the sequences in different conditions shows that the equal miss model is unrealistic: in very different experimental conditions (non saturating flash, different batch of Chlorella or chloroplasts), a parallel variation of the homogeneous miss and double hit factors is observed. This correlation seems strange within the equal miss model: misses come from incomplete reaction (i.e. for exemple insufficient light) and double hits i.e. double advancement come, in principle, from excessive light or too long flash; for these reasons, opposite variation of misses and double hits as a function of light intensity are expected. Within the equal miss model the inverse is exactly observed: at low flash light intensity (11%) which increases the misses, 16% of double hits are needed, which is quite unrealistic. In contrast, the unequal miss model explains such result quite naturally by a m athem atical property: any theoretical sequence with only a unique S state miss and no double hit can be fitted with homogeneous misses and double hits, which increase in parallel as a function of the damping. Evidence for unequal misses in oxygen flash yield sequence is provided by the heterogeneous properties of the light saturation curves (M. J. Delrieu, Biochim. Biophys. Acta 592, 478-494 (1980)). At high flash intensity, all, excepted the transition S'2 → S3, are saturated; the transition S'2-→S3 is far from saturation and its very large saturating light intensity is actually not known. A comparative study, in the same chloroplast batch, of the oxygen yield patterns with attenuated flashes and of the experimental saturation curves of S states shows that only photochemical misses (due to non saturation) exist. At high intensity, there is only a unique miss for the transition S2→S3 i.e. the probability for this transition is low. A model involving a second acceptor could explain the slow increase of transition probability of S'2→S3 at high flash intensity