Herbicide-Induced Changes in Charge Recombination and Redox Potential of Q_Ain the T4 Mutant ofBlastochloris viridis

Abstract

To gain new insights into the function of photosystem II (PSII) herbicides DCMU (a urea herbicide) and bromoxynil (a phenolic herbicide), we have studied their effects in a better understood system, the bacterial photosynthetic reaction center of the terbutryn-resistant mutant T4 of Blastochloris (Bl.) viridis. This mutant is uniquely sensitive to these herbicides. We have used redox potentiometry and time-resolved absorption spectroscopy in the nanosecond and microsecond time scale. At room temperature the P(+)(*)Q(A)(-)(*) charge recombination in the presence of bromoxynil was faster than in the presence of DCMU. Two phases of P(+)(*)Q(A)(-)(*) recombination were observed. In accordance with the literature, the two phases were attributed to two different populations of reaction centers. Although the herbicides did induce small differences in the activation barriers of the charge recombination reactions, these did not explain the large herbicide-induced differences in the kinetics at ambient temperature. Instead, these were attributed to a change in the relative amplitude of the phases, with the fast:slow ratio being approximately 3:1 with bromoxynil and approximately 1:2 with DCMU at 300 K. Redox titrations of Q(A) were performed with and without herbicides at pH 6.5. The E(m) was shifted by approximately -75 mV by bromoxynil and by approximately +55 mV by DCMU. As the titrations were done over a time range that is assumed to be much longer than that for the transition between the two different populations, the potentials measured are considered to be a weighted average of two potentials for Q(A). The influence of the herbicides can thus be considered to be on the equilibrium of the two reaction center forms. This may also be the case in photosystem II.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

“…the conformation in which Q A /Q A Ϫ has a lower potential. Such a relationship between the redox potential and the ratio of conformers has been described before in the bacterial reaction center (65). Equally the influence of the herbicides on the E m of Q A was interpreted with a shift in the equilibrium between two conformational forms of the bacterial reaction center (65).…”

Primary Radical Pair P+H- Lifetime in Rhodobacter sphaeroides with Blocked Electron Transfer to QA. Effect of o-Phenanthroline

“…32 These two conformers are characterized by two different rates of P + Q A -charge recombination, and the equilibrium between them is affected by herbicides. In effect, different herbicides change the P + Q Adecay kinetics.…”

Photosystem II: The machinery of photosynthetic water splitting

“…The electron transfer from Q À A to Q B becomes completely blocked at temperatures below 200 K in thylakoids and PS II membrane fragments Joliot 1973, Renger G et al 1993). Surprisingly, the temperature that is required for 50% activity is markedly higher for ET from Q À A to Q B than to Q À B in samples from higher plants and in PS II core complexes from thermophilic cyanobacteria (Fufezan et al 2005). The threshold temperatures for the onset of electron transfer from Q À A to Q B and of increased protein flexibility are virtually the same as shown by Mößbauer spectroscopy using 57 Fe labelled nonheme iron (NHFe) centers as a probe (Garbers et al 1998) and by quasielastic neutron scattering (QENS) studies (Pieper et al 2007).…”