Herbicide uptake

Soon after the discovery of triazine-resistant common groundsel, another equally important discovery was made. Radosevich and DeVilliers (1976) found that the mechanism of resistance in this weed was due to insensitive chloro-plasts that were capable of photosynthesis, even in the presence of simazine or atrazine. This was surprising because earlier research had confirmed that there were no differences in plant selectivity or susceptibility due to the origin of chloroplasts. Moreland (1969) had reported that isolated chloroplasts were equally inhibited to simazine whether they came from tolerant com or susceptible spinach. Radosevich and Appleby (1973) had confirmed there were no differences between the susceptible and resistant biotypes of common groundsel due to herbicide uptake, distribution, or metabolism, whereas it is known that com metabolizes triazine herbicides (Shimabukuro, 1985). 

Experiments were conducted to determine how quickly a low concentration (1.4 /zM) of an aryloxyphenoxypropanoate, haloxyfop (2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoic acid) (Fig.l) could affect lipid metabolism. We used leaf discs as experimental material because it would mitigate effects of herbicide uptake and translocation. Maize (Zea mays) leaf discs were prepared as previously described (15) and incubated in C-acetate. At various times, discs were removed and cell constituents were separated into organic and aqueous soluble fractions (16). Within 30 min of application, 1.4 /zM of either the free acid or the methyl ester of haloxyfop inhibited lipid... 

The dinitroanilines are some of the most widely used herbicides. They am used almost exclusively as soil-incorporated, preemergent, selective herbicides. Uptake occurs from the soil via germinating seeds or the roots. 

Potentially, a safener could increase the tolerance of the crop by reduction of herbicide uptake and translocation, or by enhancement of metabolic herbicide inactivation in the crop tissue. Furthermore, a safener could counteract the effect of a herbicide at its biochemical target site, with a resultant reduction of crop susceptibility. Evidence for and against these potential modes of action is presented in the following sub-sections. In addition, aspects of safener specificity (crop versus weed) are covered for situations where the safener is applied in tank mix with the herbicide. 

It is usually part of the investigations on the mechanism(s) of safener action to look for possible safener interactions with the herbicide partner at the process of herbicide uptake into the crop. Looking through the relevant literature gives a complex picture. This can also be seen in a review of Davies and Caseley [2], who present an exhaustive compilation of safener effects on herbicide uptake for relevant herbicide/safener combinations developed up to that time. Only in 20% of the cases was the uptake of the herbicide reduced in combination with the... 

This observation, but also contradictory results of other studies, which showed either no effect or a stimulatory effect of N A on herbicide uptake, made it questionable that an interference with herbicide uptake plays a significant role for the mechanism of action of this safener [30, 31). It should be added that contradictory results (inhibition, stimulation or no effect on herbicide uptake) can also be found in the literature for other herbicide/safener combinations. 

Uptake studies were also carried out with the recently developed combinations of the safener mefenpyr-diethyl with the sulfonylurea herbicides mesosulfuron-methyl and iodosulfuron-methyl-sodium, which are used for selective postemergence weed control in wheat crop. In both combinations the safener had no influence on herbicide uptake [29]. 

In summary, it can be said that from present experience only in a few cases was herbicide uptake by the crop reduced in combination with a safener, and even then doubts remained as to whether the reduction of herbicide uptake was the mechanism of safener action. It is, therefore, concluded that interference with herbicide uptake by the crop has no importance as a mechanism of safener action, though it cannot be excluded that there may be cases where it plays an auxiliary role. 

A number of possible crop selectivity mechanisms have been investigated (see Brown, H. M. Pestic. Sci. 1990 (in press)). Differential uptake and/or translocation of the selective herbicide between the tolerant crop and sensitive weeds has been ruled out as the basis for crop selectivities in several specific cases. For example, Sweetser et al. ( found no ccnrelation between chlorsulfuron uptake or translocation and sensitivity to this herbicide in a study of 7 plant species, and similar conclusions were drawn in studies of thifensulfuron methyl tolerance in soybeans (28). Lichtner (29) has shown that sulfonylurea herbicide uptake and translocation in plants is not carrier-mediated, but instead depends on the physical properties of the herbicide (pKa, log P) and proceeds through an acid-trapping mechanism common to higher plants. Given this information, we conclude that differential uptake and/or translocation is unlikely to account for any of the sulfonylurea crop selectivities discovered to date. 

These results and herbicide uptake studies show that tolerance might be caused by changes in the uptake properties of the cells, possibly by changes in the membrane and wall of the tolerant cells. 

In general, it would appear that herbicide uptake and activity is adversely affected by pretreatment conditions which are unfavorable to plant growth, namely, low temperature, low light intensity, and moisture stress. 

The sites and mechanisms of surfactant action have been considered by Holloway and Stock. Surfactants may act at four possible sites in treated leaves, namely, on the surface, cuticle, and epidermis or within the internal tissues. As mentioned previously, the addition of surfactants to formulations of agrochemicals invariably improves their wetting and spreading characteristics as measured by contact angle, deposit area, spread, and surface tension. Generally, however, the effect of surfactants on coverage can seldom be related to herbicide uptake or activity. 

Studies with radiolabeled surfactants have demonstrated that a penetrating surfactant may be present in all three compartments and thus may influence any rate-limiting steps associated with herbicide uptake. Indeed, parallel studies with radiolabeled formulations containing either " C-surfactants or " C-agrochemicals indicate that nonionic surfactants which exhibit penetration characteristics may activate by copenetration or prepenetration. In the former case, activation is associated with closely related rates of uptake of surfactant and herbicide, whereas, in the latter case, activation only occurs after the surfactant has penetrated the leaf. The efficiency of these mechanisms appears to depend on the concentration of both surfactant and a.i. and on the plant species and agrochemical involved. The prepenetration requirement suggests that the need for the activating surfactant is located mainly in the inner cell wall compartment, or possibly is related to cell membrane permeability. 

Herbicide selectivity often results from a complex interaction of a number of the factors indicated above though there are many examples where a single dominant factor is responsible. There are a number of well-documented cases in which this is true of herbicide uptake and movement and of differential metabolism to less phytotoxic products. The purpose of the present chapter is to elaborate on the latter of these phenomena in... 

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