Introduction

Weed control using organic chemicals commenced just over half a century ago, when in 1932, 4,6-dinitro-o-cresol (DNOC) was first used as a weed-controlling agent. The phenoxyacetic acids such as 2,4-D followed in the 1940s. Chemical weed control was widely accepted when the ureas (1951), the triazines (1955) and the bipyridiniums (1960) became available. The latter three groups of herbicides act via the photosynthetic process. The ureas and triazines effectively block photosynthetic electron transport at the level of the Photosystem II acceptor site. Many reviews are now available on the effects of herbicides on Photosystem II, for example: Van Rensen (1982), Pfister & Urbach (1983), Sandmann & Böger (1986) and Renger (1986).

Research on the action of herbicides inhibiting photosynthesis has yielded much detailed information about their mechanisms of action. Furthermore, our understanding of the photosynthetic process has been greatly enhanced by the use of these chemicals as specific inhibitors. This chapter highlights important events of the research on Photosystem II herbicides and surveys some recent developments.

The photosynthetic electron transport pathway

The light energy conversion processes of photosynthesis are located in the grana of the chloroplasts, while the reduction of carbon dioxide occurs within the stroma. Grana consist of stacks of thylakoids, i.e. vesicle-like structures having an internal space surrounded by a membrane. The grana are interconnected by unappressed stroma thylakoids. The thylakoid membranes contain the electron and proton translocating components (Fig. 1).