Photosystem II electron

In photosystem I, absorption of a photon leads to an excited state that functions as a reducing agent. The electrons are passed from one species to another with several intermediate species that include ferrodoxin (a protein containing iron and sulfur) before finally reducing C02. In photosystem II, electrons are transferred to a series of intermediates, of which a cytochrome bf complex is one entity. Ultimately, the transfer of electrons leads to the reaction... 

Metribuzin is a member of the substituted as-triazinone group of herbicides. Activity is due to interference with photosystem II electron transport in plant chloroplasts (Dodge, 1983). The metabolism of metribuzin in plants has been the subject of many short-term and long-term studies dating back to the early 1970s. 

Mattoo, A.K., U. Pick, H. Hoffmann-Falk, and M. Edelman (1981). The rapidly metabolized 32000-dalton polypeptide of the chloro-plast is the proteinaceous shield regulating photosystem II electron transport and mediating diuron herbicide sensitivity. Proc. Natl. Acad. Scl, 78 1572-1576. 

Vermass, W.F.J. and C.J. Arntzen (1984). Synthetic quinones influencing herbicide binding and photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes. Biochim. Biophys. Acta., 725 483 -91. 

Note Kinetics of the inhibition of photosystem II electron flow in these macrophytes by 50 pg/L linuron and the subsequent recovery of inhibition by washing with uncontaminated well water are expressed as half-life times (f1/2). 

Bowes, J.M., Horton, P. and Bendall, D.S. 1983a. Characterization of photosystem II electron acceptors in Phormidium laminosum. Arch. Biochem. Biophys., 225, 353-359. 

Fig. 1. Difference spectra of the S1-+S2 transition measured by two different research groups. Figure (A) also includes the difference spectra of Mn(IV)-minus-Mn(lll) [dashed curve] and Mn(lll)-minus-Mn(ll) [dotted curve] for model manganese compounds. Figure source (A) Dekker, van Gorkom, Brok and Ouwehand (1984) Optical characterization of photosystem II electron donors. Biochim Biophys Acta 764 308 (B) Lavergne (1991) Improved UV-visible spectra of the S-state transitions in the photosynthetic oxygen-evolving system. Biochim Biophys Acta 1060 185 (C) van Leeuwen, Heimann and van Gorkom (1993) Absorbance difference spectra of the S-state transitions in photosystem II core particles. Photosynthesis Res 38 328.

To facilitate the following discussion on the origin of the major thermoluminescence bands, a framework consisting of components in the photosystem-II electron-transport chain is shown in Fig. 3, along with the charge-recombination mechanisms currently assigned to the major thermoluminescence bands ... 

R1. Y Inoue (1996) Photosynthetic luminescence as a simple probe of photosystem II electron transport. In ... 

In photosystem II electrons that lost part of their energy during photoreaction I take up energy again, the energy being used for the reduction of NADP (nicotinamide adenine dinucleotide diphosphate) in photoreaction II. NADPHj formed is a stable transport metabolite of hydrogen. 

Fig. 6. The relationship between the quantum efficiency for CO2 assi mi lation (Oj-02) and the quantum efficiency for Photosystem II electron transport (p3 ) measured at each irradiance simultaneously with CO2 fixation.

Fig. 8. (a) The irradiance dependency of qg (O) and the quantum efficiency of electron transport by open Photosystem II reaction centres (excl ) (b) the irradiance dependency of the quantum efficiencies for de-excitation by the basal, constitutive pathway and the inducible (NPQ or non-photochemical quenching) pathway at each irradiance the sum of these de-excitation pathways and the quantum yield for Photosystem II electron transport is one. 

Fig. 12. (a) The relationship between the quantum efficiency of Photosystem I electron transport (Opj,) and the quantum efficiencies for Photosystem II electron transport d>psu) and the quantum efficiencies for open Photosystera II reaction centers These data were obtained from leaves of JuanuUoa aurantiaca ( , O) and Begonia luzonensis ( , ) under conditions of increasing irradiance and a leaf of JuanuUoa aurantiaca kept at constant irradiance, but subjected to decreasing CO2 concentrations (350-35 ppm) in a non-photorespiratory atmosphere (A, A), (b) The irradiance response of the rate constants for non-cyclic electron transport. 

Compounds of non-adenine structure were sought based on the considerations of steric similarities between A -substituted adenine and phenylurea and also between phenylurea, A -arylcarbamates and s-triazine herbicides. Many of the compounds found are known as inhibitors of photosystem II electron flow and some also inhibit the cytokinin-stimulated callus growth [36,37], It has been concluded that phenylureas, s-triazines and /V-ary 1 carbamates share the same place of action, which is the cytokinin receptor localized in chloroplasts [28]. 

Mohanty N, Vass I, Demeter S. Copper toxicity affects photosystem II electron transport at the secondary quinone acceptor, Qg. Plant Physiol 1989 90 175-179. 

In higher plants altered photosynthetic electron transport in the triazine-resistant mutants (2, 3) has been correlated with slower growth and lower yield (jT). Experiments with Chlamydomonas, a unicellular alga, suggest that trlazine resistance is not necessarily associated with an alteration of the photosystem II electron transport kinetics ( 5). Selection of mutants resistant to other classes of photosystem II inhibiting herbicides, (e.g. dluron, bromacll) should also be feasible based on successful Isolation of such mutants in Chlamydomonas (16). 

Gonzalez, V.M. et al. (1997) Inhibition of a photosystem II electron transfer reaction by the natural product sorgoleone. J. Agric. Food Chem. 45, 1415-1421... 

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