Investigations on the reaction pattern of photosystem II in leaves from Arabidopsis thaliana wild type plants and mutants with genetically modified lipid content.

The role of digalactosyldiacylglycerol (DGDG) for the functional competence of photosystem II (PS II) has been analyzed in leaves of Arabidopsis thaliana plants where the lipid composition was selectively modified by genetic mutations. Measurements with a newly developed laser flash fluorometer and data evaluation within the framework of an extended "3-quencher" model lead to the following results: (i) the normalized fluorescence transients F(t)/F(0) induced by an actinic laser flash in dark adapted leaves are virtually the same in wild type (WT) and mutants with diminished (about 50%) monogalactosyldiacylglycerol (MGDG) content (mgd1 mutant); (ii) significant changes of the F(t)/F(0) curves are observed in mutants with a severely reduced DGDG content; (iii) in mutants dgd1 and dgd1 dgd2-1 with DGDG contents of 1/15 of the control and below the detection limit, respectively, the probability of the dissipative recombination reaction between P680(+)(*) and Q(A)(-) increases by factors of about two and four, respectively; (iv) the acceptor side reactions are only slightly affected; (v) excitation with actinic laser flash energies above the saturation level of photosynthesis gives rise to elevated carotenoid triplet formation in mutants dgd1 and dgd1 dgd2-1; and (vi) the relationship between DGDG content and functional effect(s) on PS II is strikingly nonlinear. A small fraction of DGDG molecules of the total pool is inferred to be specifically bound to PS II as an essential constituent for its functional competence.