Photosynthesis Inhibitors

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WSSA Group Numbers

5, 6, and 7

Where Used

Most often used in cropland, though they are also used in residential or roadside areas as soil sterilants (bare ground herbicides).

Common Active Ingredients

Group 5: atrazine, simazine, and metribuzin; Group 6: bromoxynil; Group 7: tebuthiron.

Effect on Plant

This mode of action includes herbicides that inhibit one of several processes of photosynthesis. Affected plants cannot make sugars in the chloroplast, and the disruption of this process produces reactive compounds that destroy cell membranes. Herbicides in this group may be either systemic or contact herbicides.

Injury Symptoms

Groups 5 and 7: Soil applications cause yellowing either along the veins or between the veins (Figure 18). Browning starts along the leaf edges and progresses towards the center of the leaf (Figure 19). Grasses show symptoms of yellowing and/or browning starting from leaf tips and moving towards leaf bases. Applications to leaves cause leaf browning that may occur within hours or days.

Group 6: These herbicides are used as foliar applied contact herbicides.Applications to leaves result in necrotic spots or blotches where the herbicide contacts the plant. Symptoms appear within one or two days.