Introduction

All biological organisms have defense mechanisms to protect them from the negative effects of small quantities of foreign compounds (xenobiotics). These foreign compounds in plants include pesticides. Herbicides are a type of pesticide toxic to plants specifically, because they inhibit metabolic pathways unique to plants (e.g. photosynthesis). Herbicides are used successfully in weed management systems because they selectively harm sensitive weeds while leaving crops undamaged. Many factors contribute to successful herbicide action (e.g. successful placement or absorption and movement to target sites); however, the major reason herbicides are selective against weeds in crops is because crop plants are able to metabolize the herbicide to a non-toxic form. The basis for herbicide selectivity relies on enzymatic systems used in the plant’s normal metabolic processes.

The relative rates of herbicide absorption, translocation, and metabolism usually determine whether or not a herbicide will elicit a phytotoxic response. To better understand how these processes may each influence herbicidal action, they should be considered separately. Absorption has primary control over translocation, metabolism and phytotoxic action because the total amount of herbicide available for these processes is determined by the amount of herbicide absorbed by the plant. Metabolism influences both herbicide absorption and phytotoxic action. Metabolism generally converts the herbicide to a form with reduced phytotoxicity, thus, increasing the concentration gradient of the parent herbicide, so more is absorbed. Herbicide metabolism also influences phytotoxic action by either rendering the herbicide less or more active.

GENERAL VIEW OF THE MAJOR PHASES IN HERBICIDE METABOLISM -

Plants use a three-phase process to convert xenobiotics, such as herbicides or insecticides, into intermediates with reduced phytotoxicity. The ultimate result of these metabolic conversions is often movement of the altered xenobiotic into the vacuole of the plant cell or incorporation into cells walls.

These three phases are:

• Phase I - initial reactions such as oxidation, reduction or hydrolysis,
• Phase II - primary conjugation with endogenous substrates such as sugars, amino acids, or glutathione,
• Phase III - secondary conjugation, formation of insoluble residues or sequestration in the vacuole.

View the metabolism animation overall and see how a generic herbicide is altered through these three phases of metabolism.

How does herbicide metabolism protect crops but not weeds from injury or death?

Answer: Crops are able to biochemically convert herbicides to non-toxic molecules so the herbicide will no longer harm the crop. Many weeds cannot do this and so they die.

Rarely can plants mineralize a xenobiotic or completely oxidize it to CO₂ like microorganisms can. On the other hand, animals and insects use only the first two phases, but and metabolism is usually faster and localized in an organ such as the liver or fat body; the metabolite is then eliminated from the body by excretion. Plants must alter these chemicals for long-term storage as non-toxic compounds because they are unable to excrete waste products.

Table 1 has detailed examples of metabolic conversions for each of the Phases. Click on the highlighted words to see specific examples of these metabolic processes. Written descriptions of processes are provided below.