Safener action

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... 

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. 

Studies on the mode of safener action in wheat indicated that the safener enhanced the metabolic degradation of both herbicides in the crop species, while it did not significantly alter their rate of degradation in the target weed species wild oats and blackgrass Alopecurus myosuroides) [14, 21, 49]. 

Research on chemical antidotes or safeners has been summarized in several reviews and published symposia (3.-9). Most of the major developments (Table I) have resulted from impirical screening programs by Industry that may have been stimulated by observations of herbicide antagonism in plants (3, 10). However, some of the research on mode of action of antidotes has been directed at finding new ways to protect crop plants from herbicides (3). The research to be discussed in this text, namely the use of subtoxic herbicide pretreatments to improve crop tolerance to selected herbicides, arises in part from research on the mode of action of R-25788 as a selective antidote for EPIC or butylate in corn. 

In the 1970s compounds were developed which eliminated these harmful effects on the growth of crop plants. They can be applied for the treatment of the seeds or admixed with EPTC in a quantity of 5-10%. These antidotes or safener substances presumably exert their action by stimulating the functioning of the EPTC-detoxicating enzyme of crop plants, of glutathione-S-transferase. This theory is supported by the observation that sensitive and tolerant plants absorb and translocate EPTC to essentially the same extent, but in tolerant plants the active substance is rapidly metabolised oxidatively to sulfoxide, then to biologically inactive sulfon derivatives, while in sensitive plants this process is slower and the plant meanwhile dies. 

Similarly, we would expect that an adsorbed solute could be desorbed by washing the carbon with an appropriate solvent. The behavior can be envisioned as though the carbon were acting as a reservoir for the gradual and controlled release of the adsorbed substance—a concept that has been considered for various applications. As mentioned earlier, years ago carbon was employed as a carrier of medicinals to be released for ailments in the gastrointestinal tract. More recently, it has been used to safen the action of fungicides and insecticides on sensitive foliage. 

Pyrasulfotole would be the first HPPD compound for cereals and therefore a new mode of action for this crop. It is described as an innovative tool for resistance management with excellent broad-spectrum activity and excellent crop compatibility due to combination with proprietary safener technology [34]. Application rates and environmental behavior are so far not reported. 

To ensure maximum crop safety, safeners that are applied in mixture with the herbicides need to act quicker than the herbicide injury develops. The mechanism of action of safeners has received much scientific attention and will be dealt with in some detail in this chapter (Section 5.3). 

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. 

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. 

Various actions of thiocarbamates on lipid contents, lipid labeling, membrane constituents, and growth regulators have been reported (see Ref. 59). In addition, several antidotes or safeners which increase the tolerance of crop plants to thiocarbamates have been shown to reverse the effects of these herbicides on lipid metabolism. ... 

Table 3.12. Action of the Safener 5-Aminobenzotriazole in Reversing the Effects of Diallate on Fatty Acid Synthesis in Leaf Blades of Monocotyledons"...

In summary, thiocarbamates have been shown to specifically affect very long chain fatty acid (and wax/cutin/suberin) synthesis in a variety of plant tissues in vivo. This effect is most likely due to the formation of a more active metabolite (possibly a sulfoxide) and can be reversed by safening agents. It seems probable that effects on fatty acid elongation are important in the mechanism of action of thiocarbamates. 

K. K. Hatzios and R. E. Hoagland (eds.). Crop Safeners for Herbicides Development, Uses, and Mechanisms of Action, Academic Press, San Diego (1988). 

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