title: Evidence for a fluorescence yield change driven by a light-induced conformational change within photosystem II during the fast chlorophyll a fluorescence rise.
abstract: Experiments were carried out to identify a process co-determining with Q(A) the fluorescence rise between F(0) and F(M). With 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU), the fluorescence rise is sigmoidal, in its absence it is not. Lowering the temperature to -10°C the sigmoidicity is lost. It is shown that the sigmoidicity is due to the kinetic overlap between the reduction kinetics of Q(A) and a second process; an overlap that disappears at low temperature because the temperature dependences of the two processes differ. This second process can still relax at -60°C where recombination between Q(A)(-) and the donor side of photosystem (PS) II is blocked. This suggests that it is not a redox reaction but a conformational change can explain the data. Without DCMU, a reduced photosynthetic electron transport chain (ETC) is a pre-condition for reaching the F(M). About 40% of the variable fluorescence relaxes in 100ms. Re-induction while the ETC is still reduced takes a few ms and this is a photochemical process. The fact that the process can relax and be re-induced in the absence of changes in the redox state of the plastoquinone (PQ) pool implies that it is unrelated to the Q(B)-occupancy state and PQ-pool quenching. In both +/-DCMU the process studied represents ~30% of the fluorescence rise. The presented observations are best described within a conformational protein relaxation concept. In untreated leaves we assume that conformational changes are only induced when Q(A) is reduced and relax rapidly on re-oxidation. This would explain the relationship between the fluorescence rise and the ETC-reduction.
authors.affiliation: Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, H-6701 Szeged, Hungary. gert.schansker@gmail.com
authors.forename: Gert
authors.initials: G
authors.lastname: Schansker
authors.forename: Szilvia Z
authors.initials: SZ
authors.lastname: Tóth
authors.forename: László
authors.initials: L
authors.lastname: Kovács
authors.forename: Alfred R
authors.initials: AR
authors.lastname: Holzwarth
authors.forename: Gyözö
authors.initials: G
authors.lastname: Garab
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D005453:Fluorescence; D008027:Light; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011487:Protein Conformation; D013696:Temperature
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D000077194:Chlorophyll A