Active Absorption — General Concepts

Up until now, we have discussed how herbicides can be passively absorbed into plant cells, where their absorption is driven by passive diffusion, and occurs down a concentration gradient. Most herbicides are absorbed in this manner. However, a few are absorbed via mechanisms of active transport, such as 2,4-D, glyphosate and paraquat.

Active transport moves a molecule across a membrane against its concentration gradient. This type of transport usually involves a protein carrier located on the membrane which uses energy to move the herbicide from one side of the membrane to the other. The protein can carry the molecule into the plant cell, increasing its concentration within the cell to levels well above those outside the cell.

We know that charged, hydrophilic herbicides move into plants cells via active transport based on experimental evidence which has shown the following:

• absorption occurs in two phases over time, with a rapid initial phase, followed by a slower rate,
• the herbicide accumulates against a concentration gradient to reach concentrations in the cell higher than outside the cell,
• energy is required for herbicide absorption as evidenced by temperature coefficients (Q₁₀) of absorption greater than 2,
• saturation kinetics are observed at increasing external herbicide concentrations,
• absorption is reduced in the presence of metabolic inhibitors because energy is required for active transport, and
• compounds of similar structure will compete with the herbicide for the membrane carriers which slow absorption of the herbicide

Interestingly, out of all the different herbicide chemistries currently available, evidence for active transport of herbicides across plant membranes so far has been shown for only 2,4-D, glyphosate, and paraquat. You can see active absorption proccess animated in Herbicide Uptake by Leaves and Cells. Details for each of these three herbicides are described in the next topic.