Predicting Root Absorption and Translocation Based on Herbicide Characteristics

Plant scientists realized years ago that root absorption and subsequent translocation of various pesticides appeared to be related to a compound’s lipophilicity. In this lesson we have discussed log Kow values as easily measured parameters that readily characterize an herbicide’s lipophilic or hydrophilic nature. An herbicide’s log Kow value can also be useful in predicting root absorption and translocation.

Root absorption can be conveniently described by a compound’s Root Concentration Factor or RCF. RCF values can be easily measured (especially if radio-labeled herbicide is available) by simply exposing intact or macerated root tissue to a known herbicide concentration (in aqueous solution) and measuring the amount of herbicide in the root system following equilibrium. The RCF value defined by Shone and Wood (1974) is fairly simple:

RCF = (Concentration in roots)/(Concentration in external solution)

The RCF is independent of herbicide concentration in the external solution. Measuring the RCF values at increasing herbicide concentrations in the external solution will increase the herbicide concentration in the root, but since the RCF value is a ratio of internal to external concentration the RCF value will remain constant.

Herbicides with very high log Kow values will concentrate in the root tissues by partitioning into lipophilic constituents like cell and vacuolar membranes. This is the basic “like dissolves like” principle with lipophilic herbicides being more soluble in lipophilic plant parts. Figure 5 clearly demonstrates the ability of log K_ow values to predict root absorption as characterized by the RCF values. At log K_ow values between -1 and 1 the RCF values are flat. This means that hydrophilic herbicides are simply equilibrating with the water in the root cells and free spaces resulting in RCF values around 0.90. The root is approximately 90% water. Starting somewhere between log K_ow values of 1 and 2 there is a fairly linear increase in RCF values with increasing log K_ow values. These data were generated with analogs of carbamate insecticides and phenylurea herbicides so the relationship appears to apply across a wide range of non-ionized pesticides. The influence of an ionizable functional group on root absorption and translocation will be discussed later.

The efficiency with which an herbicide moves from the root and translocates to the shoot is described by the compounds Transpiration Stream Concentration Factor or TSCF. The TSCF is estimated by determining the amount of the compound accumulating in the shoot per volume of water translocated.

TSCF= (Concentration in transpiration stream)/(Concentration in external solution)

This appears to be a passive process since even the most xylem mobile compounds have TSCF values between 0.6 and 0.8. This means that the herbicide concentration in the xylem never reaches complete equilibrium with the external herbicide concentration.  The bell shaped curve shown if Figure 6 illustrates that there is an optimum log K_ow value for xylem translocation.  Data used in Figure 6 were derived from experiments conducted in aqueous solution, eliminating the impact of the soil.  When the impact of soil (i.e., clay and organic matter) is incorporated the optimum log K_ow values shift from 2.5 to 3.5 to a log Kow between 0 and 1 (Hsu et al. 1990).  The reason for this shift in optimum log K_ow values is due to the fact that herbicides with log K_ow values between 2.5 and 3.5 would be strongly adsorbed to clay and organic matter.  This would significantly limit the accumulation of these compounds into root tissue and subsequently reduce xylem loading because of reduced bioavailability.

Up to this point the concepts presented were limited to herbicides without ionizable functional groups. What effect does an ionizable functional group have an herbicide’s RCF and TSCF? In general terms, an ionizable functional group will have little affect on these parameters unless the pKa of that functional group occurs within a pH range of 3.5 to 7.5. The water solubility and subsequent bioavailability of an herbicide can be greatly affected by soil pH if the herbicide has a functional group with a pKa of say 6.5.  Sulfentrazone has a pKa of 6.5 and the water solubility can vary from 110 ppm to over 1600 ppm depending on the soil pH (Table 1).  This change in water solubility means less herbicide is available for plant absorption at soil pH below 6.5, while a much greater amount is available at soil pH above 6.5. This means a lower TSCF value at soil pH values below 6.5. In practical terms, this translates into higher herbicide rates being required to produce the same level of control. 

Herbicides with pKa values 3.5 to 4.5 will have an easier time crossing the Casparian strip due to a process called acid trapping.  This concept is discussed in much greater detail in the Foliar Absorption and Phloem Translocation elesson. Briefly, this means that since the cell wall has a pH of 3.5-4.0, the herbicide will be in the non-ionized form in this part of the apoplast and this decrease in water solubility will make it easier to cross into the symplast, providing a route around the Casparian strip.

Literature Cited

Briggs, G.G., R.H. Bromilow and A.A Evans. 1982.  Relationships between lipophilicity and root uptake and translocation of non-ionised chemicals by barley. Pestcide Sci. 13:495-504.

Hsu ,F.C., R.L. Marxmiller and A.Y.S. Yang.  1990.  Study of root uptake and xylem translocation of cinmethylin and related compounds in detopped soybean roots using a pressure chamber technique.  Plant Physiol. 93:1573-1578.

Shone, M.G.T. and A.V. Wood. 1974.  A comparison of the uptake and translocation of some organic herbicides and a systemic fungicide by barley I. Absorption in relation to physico-chemical properties.  J Exp Bot 25:390-400.