The primary biochemical defect in the genetically well characterized dgd1 mutant of Arabidopsis thaliana causes a 90% reduction in the relative amount of the galactolipid digalactosyldiacylglycerol (DGDG). To study the effect of this DGDG deficiency on photosystem II (PS II), time-resolved transients of laser-flash-induced changes of the relative fluorescence quantum yield Fvar,rel(t) were measured in whole leaves from wild-type and the dgd1 mutant. The results obtained reveal (i) in untreated leaves the decay kinetics of Fvar, rel(t) reflecting QA.- reoxidation by endogenous plastoquinone are very similar in wild-type and the dgd1 mutant at room temperature, (ii) the Arrhenius plot of the temperature dependence of electron transfer from QA.- to QB exhibits a break point at about 19 degrees C in wild-type and about 12 degrees C in the dgd1 mutant, (iii) in leaves treated with DCMU the slow reoxidation of QA.- by the PS II donor side is blocked to a much higher extent in the dgd1 mutant (about 50%) compared to wild-type (about 10%), and iv) the normalized amplitude of Fvar,rel(t = 1 micros) reflecting the percentage of fast P680.+ reduction by YZ exhibits a characteristic period four oscillation in wild-type while this feature is strongly damped in the dgd1 mutant. Presumably, the severe DGDG deficiency is causing the thermal down shift of a lipid phase transition that affects the QA.- reoxidation by QB. Most strikingly, the properties of the WOC are modified as a result of reduced DGDG content. Thus, the lipid DGDG appears to be of structural relevance for the WOC.

The characteristic period four oscillation patterns of oxygen evolution induced by a train of single-turnover flashes were measured as a function of temperature in dark-adapted photosystem II (PS II) membrane fragments that were reconstituted with native plastoquinone-9 (PQ-9) by a recently developed procedure [Kurreck, J., Seeliger, A. G., Reifarth, F., Karge, M., & Renger, G. (1995) Biochemistry 34, 15721-15731]. The following results were obtained: (a) within the range 0-35 degrees C, the probabilities of misses (alpha) and double-hits (beta) and the dark population of redox state S1 exhibit similar dependencies on the temperature; (b) below a characteristic temperature theta(c) these parameters remain virtually independent of temperature, above theta(c) (theta(c) = 20 degrees C for alpha and beta; theta(c) = 30 degrees C for S1) the values of alpha and beta increase whereas S1 decreases; and (c) the dark decay of S2 and S3 via fast and slow kinetics owing to reduction of the water oxidase by Y(D) and other endogenous electron donor(s), respectively, exhibits comparatively strong temperature dependencies with the following activation energies: E(A)(S2(fast)) = 60 +/- 10 kJ/mol, EA(S3(fast)) = 55 +/- 10 kJ/mol, E(A)(S2(slow)) = 80 +/- 5 kJ/mol, and E(A)(S3(slow)) = 75 +/- 5 kJ/mol. These values of PQ-9 reconstituted PS II membrane fragments are very similar to those that were previously reported for thylakoids [Messinger, J., Schröder, W. P., & Renger, G. (1993) Biochemistry 32, 7658-7668]. These findings reveal that the reaction coordinates of feeding electrons by endogenous electron donors into the water oxidizing complex (WOC) that attains the redox states S2 and S3 is virtually invariant to Triton X-100 treatment used in the isolation procedure of PS II membrane fragments from thylakoids. Implications of these findings are discussed.