Figure 7.14 The use of autoradiography to study the translocation patterns of 1 4C-clopyralid in Galium sparine (cleavers). (A-C) Photographs of treated plants showing position of 1 4C-clopyralid application (arrowed). (D-F) Autoradiographs of plants A, B and C, respectively, showing patterns of 14C-translocation from sites of application to regions of active growth (from Thompson, 1989).

Figure 7.15 Pathways of metabolism of (A) MCPA and (B) 2,4-D.

Further sugar conjugates that are more polar than either the glucose esters or glycosides mentioned earlier can also be formed. Monocotyledonous species in particular appear able to form glycosides with two or more sugar residues, although their possible contribution to selectivity is uncertain (Pillmoor and Gaunt, 1981). In addition, studies using radiolabelled herbicides have shown that some phenoxyacetic acids and their metabolites can also become bound to insoluble fractions in monocotyledons. Structural polymers in the cell wall are often implicated and lignin, pectin and cellulose have all been suggested to bind auxin metabolites. Further studies are now needed to identify the ligands and characterise these binding phenomena. Such information is clearly needed since these bound residues are seldom found in dicotyledonous plants. Indeed, resistant monocots generally contain very low levels of free auxin-type herbicide in contrast to susceptible dicots, and this may be an important feature of selectivity to these herbicides. In most instances the products of metabolism are more hydrophilic, non-phytotoxic and polar than the parent herbicide, and can be stored, sequestered in the vacuole, or become bound to structural polymers. Each factor will contribute to the lowering of the cytoplasmic pool of free herbicide, reducing the level of auxin-receptor occupancy in sensitive tissues.

These observations on metabolism are not confined to the phenoxyalkanoic acids. In a study of triclopyr selectivity in wheat, barley and chickweed, Lewer and Owen (1990) were able to correlate rates of metabolism with species selectivity. They found that resistant wheat plants rapidly metabolised triclopyr to a glucose ester within 12h, but susceptible chickweed slowly converted the herbicide to triclopyr-aspartate over a 48-h period. In addition, levels of free herbicide remained higher in the weed than in the crop plants.

Metabolism forms the basis of selectivity of the phenoxybutyric acid herbicides, MCPB and 2, 4-DB. MCPA and 2, 4-D cannot be used in legume crops because they kill both legumes and weeds, but their butyric acid derivatives are selective in these crops. Selectivity is achieved by the conversation of the inactive phenoxybutyric acid derivative to an active phenoxyacetic acid only in broadleaf weeds by the process of P-oxidation, which successfully removes two CH2 residues from the side-chain so that an active auxin-herbicide is only produced when an odd number of CH2 residues is originally present (Figures 7.16 and 7.17).

In this way the phenoxycaproic acids (n = 5) may also have theoretical use as selective herbicides in legume crops, although only the phenoxybutyric acid derivatives (n = 3) have been commercially developed.

The possibility that selectivity is due to differential receptor sensitivity has already been raised in Section 7.4 t It is tempting to suggest that the auxin receptor in dicots is more accessible to auxin-type herbicides than the receptor in monocots. However, no direct supporting evidence is currently available. Alternatively, it may be argued that monocots are less sensitive to auxin-type herbicides because their leaves intercept and retain less herbicide, and that since their mature vascular tissues lack a layer of cambium (cells capable of cell division), they may not possess sensitive cells capable of auxin reception.

och2ch2ch2cooh och2cooh

MCPB (inactive)

in weeds only p-oxidation ch3

MCPB (inactive)

M cpa (active)

Figure 7.16 Bioactivation of MCPB in susceptible broadleaf weeds.


Figure 7.17 The effect of p-oxidation on phenoxyacids containing an odd number and an even number of CH2 residues. Only the former gives rise to active auxin-herbicides.


phenoxyacetic acid (active)

phenoxypropionic acid (inactive)

Figure 7.17 The effect of p-oxidation on phenoxyacids containing an odd number and an even number of CH2 residues. Only the former gives rise to active auxin-herbicides.

In conclusion, the selectivity of the auxin- type herbicides is clearly a complex topic, dependent on many interacting aspects of herbicide behaviour and plant physiology.


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