Since the dawn of agriculture,man has waged war against weeds. Weeds compete with crop species for water, nutrients, and light and ultimately reduce crop yields.

Traditionalmethodsofweedcontrol,suchascroprotation, manual hoeing, or tractor-drawn cultivators were largely replaced in the 1940s by labor-saving chemical weed control. Modern agriculture is almost completely dependent upon the intensive use of herbicides.

A wide spectrum of herbicides is now available that interfere with a variety of cell functions. Many of the commercially more important herbicides, however, act by interfering with photosynthetic electron transport.

Two major classes of such herbicides are derivatives of urea, such as monuron and diuron, and the triazine herbicides, triazine and simazine

Both the urea and triazine herbicides are taken up by the roots and transported to the leaves. There they bind to the QB binding site of the D1 protein in PSII (also known as the herbicide-bindingprotein).

The herbicide interferes with the binding of plastoquinone to the same site and thus blocks the transfer of electrons to plastoquinone. Because of its action in blocking electron transport at this point, DCMU is commonly used in laboratory experiments where the investigator wishes to block electron transport between PSII and PSI.

The triazine herbicides are used extensively to control weeds in corn fields, since corn roots contain an enzyme that degrades the herbicide to an inactive form.

Other plants are also resistant. Some, such as cotton, sequester the herbicide in special glands while others avoid taking it up by way of root systems that penetrate deep below the application zones.

The availability of herbicide-resistant genes together with recombinant DNA technology has stimulated considerable interest in the prospects for developing additional herbicide-resistant crop plants. It is possible, for example, to transfer the gene for the altered D1 protein into crop species and confer resistance to triazine herbicides. This approach will be successful, however, only if weed species do not continue to acquire resistance to the same herbicides through natural evolutionary change.

FIGURE: The chemical structures of some common herbicides that act by interfering with photosynthesis.