Root Absorption of Herbicides

Compared to foliar absorption, root absorption is a simpler process. Roots do not have cuticles like leaves; although, mature roots may be covered by a suberized layer. This means that there are few barriers to herbicide absorption by plant roots. Since roots are essentially lipophilic, lipophilic herbicides will be readily absorbed. In fact, herbicide log K_ow values are good predictors of root absorption and xylem translocation. Theoretically, absorption could occur anywhere the root system comes in contact with the herbicide; however, there is evidence to suggest that most herbicide absorption occurs in the area a few millimeters behind the root tip. This is the area where most water and nutrient absorption occurs and is characterized by a profusion of root hairs intended to increase the surface area of the root. The Casparian strip is also less developed in this area. If we assume that herbicide absorption is primarily due to mass flow of the soil solution and diffusion in response to concentration gradients, then this area of the root is the likely location of most herbicide absorption.

Herbicides can be divided into four categories relative to the site of absorption.

1. Herbicides that are absorbed by roots only and have activity at root meristems. Herbicides in this group do not translocate to the shoot because they are very lipophilic and tend to partition into cell membranes and other lipophilic areas. Pendimethalin (Prowl®) is an example of an herbicide with these characteristics. Pendimethalin controls certain small-seeded grasses and broadleaf weeds by being rapidly absorbed by primary roots and stopping their development (Table 1, Figure 1). Pendimethalin’s selectivity is often due to placement selectivity. These herbicides move so little in the soil that weeds must germinate in the layer of treated soil in order to be controlled. Crops planted below the layer of treated soil avoid injury because there is no translocation of absorbed herbicide to root or shoot meristems as shoot tissue passes through the layer of herbicide treated soil.  
2. Herbicides that are absorbed by both roots and shoots with some movement in developing seedling. These herbicides do not stop germination, but kill seedlings before emergence. Metolachlor (Dual Magnum or Dual II Magnum) is an excellent example of this herbicide group (Table 1, Figure 1). These herbicides are absorbed by roots and below-ground shoot tissue and have some ability to translocate within the seedling before emergence.  
3. Herbicides that are absorbed primarily by the shoot in the vapor phase and have some movement into developing seedlings. EPTC (Eptam) is a good example of this herbicide group. EPTC has very low water solubility and a very high vapor pressure, meaning that it readily moves from the soil solution to becomes a gas or vapor that accumulates in the soil free space (air filled spaces, sometimes called the pore volume). Since the plant shoot is very lipophilic herbicides like EPTC can be absorbed by the shoot as a gas.  Other herbicides in this chemical family, the carbamothioates, have similar characteristics (Table 1, Figure 1).
4. Herbicides that are absorbed by roots, but do not have herbicidal activity in the root system. These herbicides require translocation to the developing above-ground shoot to have activity. Sulfentrazone (Spartan/Authority) and atrazine are good examples of this type of herbicide (Table 1, Figure 1). Their mode of action involves light energy and therefore requires movement to above-ground plant parts.

Figure 1. Chemical structures for Table 1

The influence of soil pH on water solubility of an herbicide is another important factor influencing root absorption. Some herbicides will be neutral or uncharged regardless of soil pH. Pendimethalin and metolachlor are examples of this type of herbicide (Table 1). Rate recommendations for these herbicides are made strictly on the basis of soil texture and % organic matter. For other herbicides, water solubility changes depending on the pH of the soil solution. These herbicides contain a functional group that changes from neutral to a negative charge depending on soil pH. The neutral form of the herbicide is less water soluble and more lipophilic. This means that adsorption to organic matter will be higher. The negatively charged form of the herbicide is more water-soluble and is less likely to be adsorbed to soil organic matter. This increase in water-solubility means that there is more herbicide available in the soil solution, so more herbicide is available for root absorption. Sulfentrazone (Spartan) is a good example of an herbicide where rate recommendations include provisions for soil pH. Less herbicide is required for good weed control at soil pH above 7.5. Sulfentrazone has a functional group that significantly changes the herbicide’s water solubility in the range of pH 6 to 7. The pH at which sulfentrazone occurs as a 50:50 mixture of  the neutral (lipophilic) and charged (hydrophilic) form is pH 6.56. This is called the pKa. 

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