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Figure 1 Enzymes--Enzymes function as steps in biological
processes. Enzymes are also extremely specialized in their function. As a
result, many different enzymes are involved with the many different biological
processes that occur within a plant. Some herbicides can stop specific enzymes
from functioning, resulting in a disruption of specific plant processes; this
often leads to the death of the plant. This herbicide-enzyme relationship is
very specific and any chemical modification of the herbicide or enzyme can
eliminate herbicidal activity.
Back to previous document. Figure 2 Photosynthetic Inhibitors--The photosynthetic process occurs within a plant cell's chloroplasts. Certain herbicides can inhibit photosynthesis by binding to specific sites within the chloroplast. The relationship of a herbicide to the chloroplast binding site is very specific and any modification of the herbicide or binding site can eliminate herbicidal activity. 
Back to previous document. . Figure 3 Selection for herbicide resistance begins when a herbicide resistant biotype survives a particular herbicide application. The resistant biotype survives, matures, and sets seed. If the same herbicide continues to be applied and the resistant weeds reproduce, eventually the majority of the weeds will be resistant to the herbicide. 
Back to previous document. Figure 4 Simulated progression of resistant kochia exposed to repeated annual applications of Glean. This simulation assumes an initial seed population of 100,000/m2 with a 1 in 10,000,000 occurrence of resistant biotypes in the initial population and 90% weed control. 
Back to previous document. Figure 5 Development of resistance to single site of action herbicides is more likely than to multiple site of action herbicides because a change (mutation) in only one gene may be enough to affect a herbicide's binding potential to a single action site. It is less likely that the appropriate mutations will occur at multiple action sites. 
Back to previous document. Figure 6. Changes (mutations) in a site of action may or may not result in resistance to other herbicides in the same family or that interact at the same site of action. The outcome is dependent upon the herbicide site of action relationship. For example, herbicides A and B may share part of a binding site on a particular enzyme, whereas herbicide C may bind at an entirely different site on the enzyme. Therefore, a genetic change that affects the enzyme may have different effects on any particular herbicide-enzyme relationship and any corresponding effects on crop injury or weed control. 
Back to previous document. 
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