Foliar Absorption of Herbicides

The primary barrier to foliar absorption of herbicides (or any pesticide) is the cuticle. The cuticle is not a simple wax layer covering the leaf surface. The cuticle is a complex matrix of materials that vary in water solubility and include waxes, cutin and pectin (Figure 1). The most lipophilic (fat loving) components of the cuticle are the surface epicuticular and cuticular waxes. These layers restrict water movement into and out of the plant and are significant barriers to herbicide absorption. Below the surface epicuticular and cuticular wax layers, a less lipophilic layer of cutin occurs interlaced with strands of pectin and embedded wax. Pectin strands are actually hydrophilic (water loving) and are believed to provide an avenue for the absorption of hydrophilic herbicides like glyphosate (Roundup and many generics). The cuticle ends at the cell wall of the underlying epidermis. Depending on their lipophilic or hydrophilic properties, there are several possible routes that herbicides might follow as they move through the cuticle.

Lipophilic herbicides are readily absorbed into cuticular waxes, moving easily through embedded waxes to the cutin and pectin layers. At this point, the cuticle becomes more hydrophilic and movement of lipophilic herbicides slows. Initial absorption into cuticular waxes can be very high as a percentage of the total amount of herbicide intercepted by the leaf. Lipophilic herbicides may have difficulty moving out of the cuticle and into the underlying epidermal cells. A certain amount of these lipophilic herbicides can also be retained in the plasma membrane of epidermal cells. The plasma membrane is the lipid bilayer that separates the cytoplasm from the cell wall.

Some characteristics of lipophilic herbicides are illustrated in Table 1. Note that these herbicides have low water solubility and log K_owvalues greater than 1. Log K_ow is defined as the log of the octanol-water partition coefficent. This unitless number is the ratio of the amount of herbicide found in octanol (organic solvent) compared to the amount of herbicide found in water and expressed on a log scale (See Basics of Cellular Absorption lesson for more details). In general terms, it is a good indicator of the lipophilic or hydrophilic nature of a pesticide. Two herbicides listed in Table 1, Aim and Fusilade, are formulated as ethyl or butyl esters. In this form they are rapidly absorbed into the cuticle; however, once inside the plant the ester linkage is cleaved, producing the free acid which is the active form of the herbicide. Formulating herbicides in this way takes advantage of the fact that lipophilic herbicides are rapidly absorbed by plants.  

Hydrophilic compounds, on the other hand, are slow to move into cuticular waxes and absorption is often enhanced with surfactants and liquid fertilizer additives that help to dissolve surface waxes and slow the drying time of spray droplets. Hydrophilic herbicides move more readily through cutin rather than embedded waxes. Herbicide movement becomes easier as these herbicides approach the pectin and cell wall layers. Hydrophilic herbicides may have a difficult time crossing the lipophilic plasma membrane of target cells. Herbicides that have certain functional groups (primarily carboxylic acid) will change from hydrophilic to lipophilic depending on the pH of the surrounding environment. The importance of this process, known as “ion trapping” or 'acid trapping' for herbicide absorption and translocation will be discussed later in this lesson.

Characteristics of some common foliar-applied, hydrophilic herbicides are illustrated in Table 2. Notice that these herbicides have many contrasting characteristics to those listed in Table 1. These herbicides are highly water soluble. The most commonly used of these herbicides is Roundup or glyphosate. Even though Roundup is very water soluble, foliar absorption can be as high as 80%. How is this possible? One idea is that there are hydrophilic or aqueous routes through the cuticle and by combining these herbicides with the appropriate surfactant, foliar absorption is achieved. Environmental conditions can have a much greater impact on foliar absorption of hydrophilic herbicides compared to lipopilic herbicides. The water content of the cuticle is lower under conditions of low relative humidity or drought stress pulling wax units in the cuticle closer together. This results in more restricted aqueous routes through the cuticle under drought conditions. Remember, foliar absorption is passive and driven by the process of diffusion (see Basics of Cellular lesson).