Xylem herbicide transport

Dichlobenil is applied to soil and is incorporated or watered in to reduce losses by evaporation. It is a neutral compound of intermediate lipophilicity (log Xow 2.9) and is translocated slowly in xylem this transport may not be important in its action for it kills germinating weeds before or soon after they emerge. The herbicidal activity of chlorthiamid, a thioamide, depends upon its decomposition to dichlobenil. 

Dichlorophenyl-dimethyl urea, DCMU, Diuron, 3-(3,4-dichlorophenyl)-l,l-dimethylurea, N -(3,4-dichlorophenyl)-N,N-dimethyluTaK a systemic herbicide which blocks electron transport from photosystem II to photosystem I (see Photosynthesis). It is absorbed principally by the roots then translocated ac-ropetally in the xylem. 

Movement of herbicides in the vapor phase can be important for transport from soil to plant (Section 9.3.2), and also some compounds transported to leaves from roots via the xylem may be then lost from leaves by volatilization. Worthing and Ashton and Crafts give the vapor pressures for most herbicides. 

The physicochemical properties of compounds affect not only the transport of compounds in plants but also their penetration through the leaf cuticle. This topic is discussed in Chapter 8, and here only brief comment is made on the implications of penetration for subsequent transport. The influence of physicochemical properties on the movement of compounds across cuticles seems to be rather similar to that observed for membranes, in that compounds of intermediate lipophilicity (log Ko values of 1 to 3) appear to penetrate the most rapidly. Uptake of more polar and more lipophilic compounds can, however, be greatly increased by the use of appropriate surfactants. Acidic compounds are taken up relatively slowly, presumably because of poor penetration of the anions in consequence, they are often applied as esters which enter the plant rapidly and are then metabolized to the active acid. Thus, herbicides of a wide range of structures and physical properties are applied to plant foliage the use of surfactants and adjuvants increases the uptake of chemical into the leaf and hence its availability for transport via xylem and phloem but does not otherwise influence transport patterns. 

Bentazon, mefluidide, and perfluidone are of intermediate lipophilicity with pKa = 3.5, 4.6, and 2.5, respectively. Mefluidide is phloem translocated, but the contact herbicide bentazon is not moved symplastically perfluidone moves in the xylem following uptake by roots, but experiments testing its symplastic transport have not been reported. 

All compounds with log less than 4 exhibit at least some movement in the xylem, and nonionized compounds (and probably cations also) of log less than 0 together with weak acids additionally move in phloem. While the accumulation of weak acids in plant compartments such as phloem is an energy-dependent process, there is no good evidence that specific carriers are used in the long-distance transport of current herbicides. 

---