Hill reagents

Mode of Action of the Herbicide DCMU When chloroplasts are treated with 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU, or diuron), a potent herbicide, 02 evolution and photophosphorylation cease. Oxygen evolution, but not photophosphorylation, can be restored by addition of an external electron acceptor, or Hill reagent. How does DCMU act as a weed killer Suggest a location for the inhibitory action of this herbicide in the scheme shown in Figure 19-49. Explain. 

Electron rlVans for to a Hill Reagent Isolated spinach chloroplasts evolve 02 when illuminated in the presence of potassium ferricyanide (a Hill reagent), according to the equation... 

A close relationship exists (correlation coefficient = 0.98) between light-induced proton translocation coupled to noncyclic electron flow, with either NADP1 or ferricyanide as Hill reagent, and the percentage of cytochrome b-559 in its HP form. 

Answer DCMU must inhibit the electron-transfer system linking photosystem II and photosystem I at a position ahead of the first site of ATP production. DCMU competes with PQB for electrons from PQA (Table 19-4). Addition of a Hill reagent allows H20 to be split and 02 to be evolved, but electrons are pulled out of the system before the point of ATP synthesis and before the production of NADPH. DCMU kills plants by inhibiting ATP production. 

Mehler AH (1951) Studies on reactions of illuminated chloroplasts. I. Mechanisms of the reduction of oxygen and other Hill reagents. Arch Biochem Biophys 33 65-77 Mehler AH and Brown AH (1952) Studies on reactions of illuminated chloroplasts. HI. Simultaneous photoproduction and consumption of oxygen studied with oxygen isotopes. Arch Biochem Biophys 38 365-370 Melis A(1996) Excitation energy transfer functional anddynamic aspects of Lhc (cab) proteins. In Ort DR and Yocum CF (eds) Oxygenic Photosynthesis, pp 523-538. Kluwer Academic Publishers, Dordrecht... 

Hill reaction (see). Electrons are withdrawn from water or OH ions, then transported to NADF via an electron transport chain this results in the production of molecular oxygen with the formation of NADPH -I- H. Since Hill reagents oxidize water by withdrawal of electrons, NADP is the natural Hill reagent. 

Other oxidants (Hill reagents) utilized for the study of the photochemical reaction include ferricyanide (77) and oxidation-reduction... 

OxiDATION-RliDUCnON POTENTIALS OF HiLL REAGENTS AND OtHEB StBTEMS OF... 

The reduction of coenzymes by chloroplast fragments appears to be a highly specific process, rather than an indiscriminate reduction of fortuitously present Hill reagents. Jagendorf (190) has carried out experiments with carefully prepared chloroplasts which cytologically... 

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