Dedicated to Professor Karl Heinz Biichel on the occasion of his 60th birthdayPhotosynthesis and herbicide research have a long common history. Soon after the introduction of a new group of highly effective herbicides in 1956, the substituted aryl ureas, their mode of action was recognized to be the inhibition of light-driven electron flow and photosynthetic oxygen evolution. Many subsequent highly successful commercial herbicides, including those from other chemical classes, act by this mechanism. The study of their interaction with the photosynthetic system at the molecular level is a prime example of how rapidly advances in unraveling the mechanism of photosynthesis can be translated into practical uses. At the same time the herbicides continued to be valuable and efficient tools in photosynthesis research. In the past herbicides were instrumental in establishing many principal features of the biochemistry and biophysics of photosynthesis, in particular the role of plastoquinone, which is displaced from its binding pocket in one of the protein subunits of the photosystem 11. Their further use in elucidating the molecular biology of photosynthesis is illustrated by the recent importance of herbicide-tolerant mutants for determining structural aspects of photosystem 11. We will review the past and present state of the interaction of herbicide and photosynthesis research and will provide a model for the orientation of herbicides within the threedimensional structure of their target, the D1 protein of photosystem 11.

A halogenated benzoquinone has been found to inhibit the photosynthetic electron transport system in isolated chloroplasts. 2·10-6ᴍ of dibromo-thymoquinone inhibit the Hill- reaction with NADP, methylviologen or anthraquinone to 100%, but do not effect the photoreduction of NADP at the expense of an artificial electron donor. The Hill - reaction with ferricyanide is inhibited even at the high concentration of 2·10-5ᴍ of dibromo-thymoquinone to only 60%. The remaining reduction in the presence of the inhibitor reflects the rate of ferricyanide reduction by photosystem II. It is concluded that the inhibition of electron transport by the quinone occurs between photosystem I and II and close to or at the functional site of plastoquinone.

The folding through the thylakoid membrane of the D-1 herbicide binding polypeptide and of the homologous D-2 subunit of photosystem II is predicted from comparison of amino acid sequences and hydropathy index plots with the folding of the subunits L and M of a bacterial photosystem. As the functional amino acids involved in Q and Fe binding in the bacterial photosystem of R. viridis, as indicated by the X-ray structure, are conserved in the homologous D-1 and D-2 subunits of photosystem II, a detailed topology of the binding niche of QB and of herbicides on photosystem II is proposed. The model is supported by the observed amino acid changes in herbicide tolerant plants and algae. These changes are all in the binding domain on the matrix side of the D-1 polypeptide, and turn out to be of functional significance in the QB binding.New inhibitors of QB function are described. Their chemical structure, i.e. pyridones, quinolones, chromones and benzodiones, contains the features of the phenolic type herbicides. Their essential elements, π-charges at particular atoms, QSAR and steric requirements for optimal inhibitory potency are discussed and compared with the "classical" herbicides of the urea/triazine type.

The O -dinitrophenyl derivatives of 2 -bromo-and 2-iodo-4-nitrothymol are inhibitors of photo­ synthetic electron flow from water to NADP or methylviologen, yielding 50% inhibition at 0.5 μм. Photoreductions by either photosystem I or photosystem II alone are not inhibited. The inhibition site is bypassed by TM PD. The inhibition pattern identical to the one of dibromothymoquinone. It is reduction of plastoquinone.

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