Introduction

Most herbicides on entering the target plant undergo some metabolic transformation. This generally results in a loss of biological activity but there are a few instances where the parent molecule becomes activated in the plant.

Although most proherbicides or those which require bioactivation were almost certainly discovered by chance, it may be interesting to consider whether this feature has any significance or is just a scientific curiosity. Possible advantages may be summarised:

1. the proherbicide may have superior physical properties from the point of view of penetration or stability.

2. the bioactivation mechanism may alter the selectivity of the compound in a useful manner.

3. the reactivity of compounds may be toned down by the presence of protecting groups, allowing better distribution, especially if the activating mechanism is located near the site of herbicidal action. An extreme case of this might be a suicide inhibitor of an enzyme, although no herbicides have been reported to have this mechanis.

4. delayed release of the active herbicide may be a desirable feature allowing better distribution and a more prolonged effect.

These are some of the possible advantages of proherbicides, but the knowledge that bioactivation is occurring is essential if the mode of action and structure: activity relationships of an herbicide are to be clearly understood. For example, one explanation for the situation with the imidazolinones where activity against the target enzyme is up to a thousand fold less than that against intact cells could involve some form of bioactivation.