Herbicide Translocation to the Shoot
The xylem is the primary route for herbicide movement from root to shoot; however, herbicides that are absorbed by the shoot as it comes in contact with treated soil do not need to move significant distances in the xylem to reach their site of action. In fact, they may not move in the xylem at all. Herbicides like metolachlor (Dual Magnum) and EPTC (Eptam) are examples of herbicide that would not move significant distances in the xylem, primarily because these herbicides kill weed seedling prior to emergence. On the other hand, sulfentrazone (Spartan), clomazone (Command), mesiotrione (Callisto), and atrazine are examples of herbicides that translocate some distance in the xylem following root absorption. These herbicides will not kill plants until after emergence.
The xylem is the major water-conducting tissue in plants and contains no cytoplasm or plasma membrane. It can be classified as part of the free space. Once herbicides have navigated around the Casparian strip, movement into the xylem can occur without crossing any cell membranes. Herbicides translocating to the shoot using the xylem tend to accumulate in mature leaves because these leaves are transpiring the most water. The greatest accumulation within the mature leaf occurs at the leaf margin because that is the termination point for the xylem.
Figure 3. Dry bean injury due to root absorption of atrazine. Notice that older leaves are showing significant symptoms compared to younger leaves. (photo by R. Wilson, UNL)
The terms ’xylem mobile’ or ’phloem mobile’ gives the impression that herbicide translocation is restricted to one system or the other. In reality, herbicide translocation is a complex process that may involve both xylem and phloem movement. Atrazine is a good example of an herbicide that has been difficult to classify in terms of translocation method (Figure 3). The term pseudo-apoplastic movement is often used to describe atrazine translocation. Atrazine has characteristics that allow rapid movement across plasma membranes, giving approximately equal access to the xylem and phloem (remember that the xylem and phloem are separated by a single cell layer). Since the xylem is moving at approximately twice the rate of the phloem, the net movement appears to be in the apoplast or xylem.
We tend to think of auxinic herbicides like 2,4-D, MCPA, picloram, or dicamba for example, as herbicides that move primarily in the phloem and translocate to areas of rapid growth (meristems). Soil-applied auxinic herbicides produce symptoms at the apical meristems of susceptible plants. How is this possible given the fact that phloem movement in the root system would result in herbicide accumulation at the root tip? This situation is another example of herbicide movement in both the xylem and phloem. These herbicides travel in the xylem to margins older exporting leaves and then move into the phloem (which also terminates at the leaf margins) to translocate with sugars to the apical meristem (Figure 4).
Figure 4. Example of auxinic herbicide being absorbed from the soil, but accumulating at the growing point. The herbicide has moved from xylem to phloem, probably in the unifoliolate leaves (photo by R. Wilson, UNL)
The maximum herbicide concentration in the shoot is achieved when herbicides are not highly lipophilic or hydrophilic. Herbicides that are highly lipophilic tend to partition into cell membranes and lipid bodies of the root, while herbicides that are very hydrophilic will have problem navigating around the Casparian strip.
In addition to herbicide characteristics, environment can have a significant impact on herbicide translocation to the shoot. The major driving force of water movement is the evaporation of water from the surface of mesophyll cells, into the intercellular space and eventually out through the stomata. Drought conditions that cause the stomata to close will limit water movement to the shoot and reduce herbicide translocation.