Postemergence herbicides dominate the soybean market and are an important component of weed management systems in corn. One advantage of postemergence herbicides over soil-applied products is that the chemicals are applied directly to the target, thus avoiding interactions between the herbicide and soil. Direct application to the target somewhat reduces variability in herbicide performance; however, there are still several factors which influence movement of the herbicide into the target. These factors are responsible for many of the fluctuations in effectiveness of postemergence herbicides. An understanding of these factors can help persons involved in weed management design more effective herbicide programs, respond to fluctuations in environmental conditions, and determine the cause of performance problems in the field. In this first article, plant and chemical characteristics that influence herbicide absorption will be discussed. In later articles I will address herbicide translocation and herbicide additives.
In order to be effective a postemergence herbicide must move from the leaf surface and reach the target site. The target sites for all herbicides are located within the cytoplasm of plant cells. The cytoplasm is the substrate within the cell membrane and cell wall that contains the organelles (nucleus, chloroplasts, mitochondria, etc.) essential for life. The process that drives herbicide absorption is simple diffusion. When the spray droplet lands on the leaf surface it places a high concentration of the chemical on the leaf surface and no herbicide inside the leaf. Chemicals move from areas of high concentration to low concentration, thus as soon as the spray droplet contacts the leaf absorption of the chemical begins. Figure 1 is a hypothetical diagram of a herbicide (yellow) diffusing into a leaf (green).
Unfortunately several factors make absorption a much more complex process than illustrated in this simple figure. Rather than being a homogenous substrate as indicated in Figure 1, the leaf is a complex structure that imposes several barriers to herbicide movement. In order to reach the cytoplasm, a herbicide must move first through the cuticle, then the cell wall and finally the cell membrane. Differences in solubility characteristics of these different leaf components present the primary obstacles to herbicide absorption. Some components are lipophilic (oil-loving or non-polar) whereas others are hydrophilic (water-loving or polar). The differences in solubility of these leaf parts creates a situation similar to trying to mix water and oil in terms of herbicide absorption.
A cross-section of a typical leaf is illustrated in Figure 2. Leaves of all plants are protected by the cuticle. The cuticle consists of a surface coating of epicuticular wax underlayed by a mixed substrate of cutin and wax. The wax portion of the cuticle is lipophilic and helps reduce water loss from the leaf. Cutin is a hydrophilic substance providing the foundation for the cuticle. Under the cuticle rests the cell wall, a mixture of cellulose, hemicellulose, and other hydrophilic substances. The final barrier is the cell membrane (plasma membrane), a lipophilic structure that controls movement of materials in and out of plant cells. In Figure 2 the lipophilic components of the leaves are indicated in green. Regardless of the route a herbicide takes, at some point of time it must move from lipophilic to hydrophilic components. This is important since a water soluble herbicide would associate with hydrophilic portions of the leaf and not enter the lipophilic regions, whereas oil soluble herbicides would preferentially associate with lipophilic regions.
So how do herbicides overcome this hurdle? Most postemergence herbicides have a common characteristic that facilitates movement through plant parts having completely different chemical characteristics. Postemergence herbicides typically are a type of chemical known as a weak acid. These chemicals have the ability to accept or donate hydrogen ions depending upon the pH of the solution they are in. Glyphosate is an example of a weak acid, and like most postemergence herbicides is formulated as a salt of the parent acid in order to produce a product that is convenient to handle and mix with other products.
When weak acids are in neutral or alkaline solutions they donate a hydrogen ion to the solution, in doing so the herbicide takes on a negative charge. In this state the herbicide is hydrophilic and is well suited for transport in the aqueous parts of the plant (cytoplasm, cutin). If a weak acid herbicide is placed in an acidic solution, it picks up hydrogen ions from the solution, therefore neutralizing the charge and increasing the lipophilic nature of the herbicide. In the lipophilic state the herbicide is well suited for transport through the epicuticular wax or the cell membrane. Herbicide absorption is facilitated by pH gradients within the plant which are established to drive numerous physiological processes. Plants have specialized carrier proteins that pump hydrogen ions from the cytoplasm across the cell membrane into the intercellular spaces. This raises the pH (alkaline) of the cytoplasm whereas it lowers the pH (acid) of the solution outside of the cell. Therefore, herbicides outside of the cell pick up hydrogen ions and become lipophilic, increasing their ability to move through the cell membrane. Once across the membrane the herbicide enters the alkaline cytoplasm and loses the hydrogen ion, increasing its water solubility and facilitating movement in the cytoplasm to the target site.
This simple explanation fails to consider the large differences among plants in the structure and composition of leaves of different plant species. The upper leaf surface of velvetleaf and common lambsquarter are shown in Figure 3. At the lower magnification the globules that give lambsquarter leaves their mealy appearance are visible, whereas the 'star-shaped' leaf hairs can be seen on velvetleaf. At higher magnification it can be seen that the surface of the lambsquarter cuticle is covered with waxy platelets.
The characteristics of the leaf surface greatly influence how spray droplets and herbicides interact with the leaf. The epicutilar wax on the surface of leaves repels water, the carrier used for most herbicide applications. The wettability of common lambsquarter and velvetleaf leaf surfaces is illustrated in Table 4. The crystalline wax platelets on common lambsquarter leaves greatly reduce contact between the spray droplet and leaf, even with the addition of surfactants. There was greater contact between the water droplet and velvetleaf leaf surface without a surfactant than on lambsquarter when a surfactant was added to the water. Application parameters (droplet size, spray additives, etc.) impact herbicide performance more on weeds with difficult to wet leaves than on species with leaf surfaces that are easily wetted.
Not only are there major differences in leaf surface characteristics among species, but the leaf surface is very responsive to environmental conditions. The amount and types of epicuticular wax present on the cuticle vary with leaf position on the plant and environmental conditions. Plants exude more wax onto the leaf surface when they are under water stress - this is a defensive response to limit water loss through evapotranspiration. These changes in cuticular characteristics contribute to the variability observed in herbicide performance.
Unfortunately we don't understand how plants respond to weather sufficiently to make precise recommendations on adjusting herbicide application parameters. Attempts have been made to develop simple equations to guide management decisions, such as adding together relative humidity and temperature. The types of spray additives used or herbicide rates are adjusted according to the sum of these factors. While this type of system is based on sound physiological principles, actual plant responses to the environment are much more complex than represented by this type of equation. Thus, little is gained by using this type of system.
In summary, herbicide absorption in leaves is driven by the concentration gradient between the leaf surface and leaf interior. In order to be absorbed into the leaf, the herbicide must be able to move through both lipophilic and hydrophilic substances. The ability of weak acids to change their polarity depending upon pH is a critical factor in the absorption process. The leaf surfaces of weeds vary widely and influence both spray retention and herbicide penetration. Spray additives are critical for enhancing herbicide movement through the cuticle, and will be discussed in a later article.