Herbicide Characteristics Important for Phloem Movement

One of the basic concepts about what makes a foliar-applied herbicide a good candidate for translocation or phloem mobility was based on the observation that many herbicides that appear to have good phloem mobility were also “weak acids” (see animation on weak acids). The first organic herbicide developed, 2,4-D, is a weak acid along with most other herbicides that share the same mode of action. This group of herbicides is called the auxinic herbicides and they appear to mimic many of the actions of the naturally occurring auxin, indole-3-acetic acid or IAA. IAA is also a weak acid. Weak acids are compounds containing a functional group, usually a carboxylic acid, that gains or loses a hydrogen ion depending on the pH of the surrounding solution (Figure 6). The pH at which the herbicide is present in a 50:50 ratio of ionized (hydrophilic) to non-ionized (lipophilic) forms is called the pKa. For this to be important in phloem mobility, the pH at which this transition occurs needs to be close to physiological pH. If the herbicide’s pKa is between 3.5 and 5.5 then cellular absorption is enhanced by a process called ion trapping (see animation).

*This animation has no audio.*

This means that when the herbicide is in the more acidic environment of the cell wall a higher percentage of the herbicide molecules will be non-ionized and lipophilic, allowing them to cross the cell membrane more easily. The alkaline environment of the cytoplasm causes the herbicide to revert to the ionized, hydrophilic form, trapping it inside the cell.

The problem with this concept, often called the weak acid theory, is that many herbicides that have phloem mobility are either not weak acids or have functional groups with pKa’s that are outside the physiological range. For example, picloram, which is often used for perennial weed control, is phloem mobile but has a pKa of 2.3 (Table 3). Historically, the herbicide most often cited as evidence against the weak acid theory is amitrole. Amitrole is not a weak acid, is highly water-soluble, is only soluble in the most polar organic solvent (ethanol), and translocates in both the xylem and phloem.

Information collected over the years on herbicide characteristics and phloem mobility has indicated that if a compound is intermediate in its membrane permeability it will have some phloem mobility. Membrane permeability is estimated by log K_ow values. Herbicides with log K_ow values between –1 and 1 should have phloem mobility following foliar applications. This does not mean that weak acid functional groups are not important. If an herbicide has intermediate membrane permeability and a weak acid group with a pKa in the range 3.5 to 5.5, then phloem mobility will be enhanced.

Imazapyr (Arsenal) represents an herbicide with all the characteristics necessary for good phloem mobility (Table 3). Imazapyr has intermediate permeability coupled with a weak acid functional group and it has good phloem mobility. Atrazine, on the other hand, has very high membrane permeability based on a log Kow value of 2.7. Atrazine is lipophilic so it crosses membranes very easily. This means that atrazine moves readily between the xylem and phloem and since the xylem is moving at approximately twice the rate of the phloem, the net direction of movement appears to be in the xylem. The term ’psuedo-apoplastic’ movement is often used to describe atrazine translocation in plants and it also explains why very little atrazine moves beyond the treated leaves. The psuedo-apoplastic movement of atrazine also explains why herbicides symptoms tend to develop at leaf margins. The leaf margin is where the vascular bundles terminate and this area would tend to accumulate more atrazine.

Table 3. Relationship between herbicide characteristics and translocation in the plant.