Herbicide translocation studies

Translocation studies with two paraquat-resistant biotypes of Hordeum lepori-num revealed that the basipetal transport of paraquat in resistant H. leporinum was much reduced compared with susceptible plants. It was concluded that the resistance to paraquat was the result of the reduced herbicide translocation out of the treated leaves [76]. One can suppose that also in this species herbicide sequestration may have been the primary cause for the altered long-distance transport. 

Biorational approaches have proven useful in the development of classes of herbicides which inhibit essential metaboHc pathways common to all plants and thus are specific to plants and have low toxicity to mammalian species. Biorational herbicide development remains a high risk endeavor since promising high activities observed in the laboratory may be nullified by factors such as limitations in plant uptake and translocation, and the instabiHty or inactivity of biochemical en2yme inhibitors under the harsher environmental conditions in the field. Despite these recogni2ed drawbacks, biorational design of herbicides has shown sufficient potential to make the study of herbicide modes of action an important and growing research area. 

It is a broad-spectrum herbicide active both pre- and post-emergent. Studies with radiolabelled compounds ( ) show it to be rapidly translocated particularly to the meristematic regions. Translocation to the underground storage organs of perennial weeds prevents regrowth of these weeds. Most herbaceous and woody plants are controlled in the field at 0.4-1.0 kg a.e./ha whereas most woody plants require 0.7-3.0 kg a.e./ha (10). 

The word "aspects is intentionally used in the title of this book because of the broadness it allows in the subject matter. It allows a discussion of principles such as in Chapter 20. Here the principles have been established, and reference has been made to their possible utilization in determining the critical surface tension of an insect. Perhaps this paper will stimulate workers to do obvious research which could yield data explaining the specificity in control of various insects. The word aspects also allows a discussion of the direct use of these physicochemical principles to solve formulation problems such as the paper by Freed and Witt on the translocation of herbicides in plants. Thirdly, this symposium contains papers dealing with phenomena that clearly are physical and colloidal but which have not as yet been the subject of rigorous physical chemical studies. For example, in Lyon s paper the effect of varying crystal size is shown to be very important in controlling... 

Definite structural requirements relating to activity (33, 34, 35, 55, 57, 58), translocation (56), and biodegradability (41) appear to exist for certain chemical groups of surfactants. Whether these also influence herbicide persistence in plants and roots requires further study. 

Through the further combined use of radiolabeled herbicides and additives in tracer and metabolic studies, it may be possible to develop new concepts regarding formulation of herbicides for differential absorption, translocation, and selective action, while at the same time minimizing chemical residues. 

The study of the efficacy and behaviour of different products and their formulations has evolved through purely empirical field trials, into various controlled environment or laboratory screening methods, to more detailed fundamental and mechanistic studies, leading to the development of models for the processes of deposition, retention, uptake and translocation [3]. This overview will consider these processes, their limitations, and their future potential. Due to the fact that most studies have been with herbicides into plants, with little or no information on uptake of pesticides by insects or padiogens, the main focus will be on the uptake of herbicides into plant foliage. 

Before Radosevich and De Villiers found in 1975 that isolated chloroplasts of resistant common groundsel were insensitive to atrazine and simazine (2), it had been erroneously assumed that all living plants would die if the herbicides could reach their target site intact. We now know that mechanisms of selectivity in crops can be due to differences in metabolism rates, uptake, translocation, site of action or avoidance mechanisms. However, the mechanisms of herbicide resistance that have evolved in weeds are usually different from the mechanisms of herbicide selectivity in most crops. This is certainly true with the most prevalent and thoroughly studied cases of herbicide resistance, including the triazines, dinitroanilines, and AHAS inhibitors. 

The influence of As species on phytotoxicity and growth was studied in rice and turnip in laboratory experiments using As ", As" MMA and DMA. In rice, DMA was readily translocated to the shoot, whereas As ", As" and MMA accumulated in the roots. MMA as monosodium or disodium salt, formerly often used as a herbicide, was the most phytotoxic compound (Marin et al. 1992). In experiments with turnip, organic arsenicals (particularly MMA) were also clearly phytotoxic (Carbon-ell-Barachina 1999), though the phytotoxicity was also dependent upon the plant species. 

Estimation of Phvtotoxicity. Several investigators have used cell cultures to study herbicide phytotoxicity (3. 5. 18-20). Callus and cell suspensions have potential in the estimation of phytotoxicity, especially in detecting the inherent phytotoxicity of a molecule that fails to penetrate into or translocate in a whole plant. Alteration of the molecule to enhance penetrability may provide a usable herbicide. Structure activity relationships have also been examined to determine the comparative potency of different plant growth regulators in cell cultures in comparison to whole plants (21). 

Cell cultures are ideal axenic physiological systems to study herbicide action without problems of cuticular transfer or complications of translocation. Still, not all metabolic systems function in all cells at all times in the cell cycle. Inhibitors of photosynthesis are often inactive in non green cells, and root-active herbicides may be degraded in green cells. Nutritional components in the medium may interfere with herbicide action. 

Use of the cucumber cotyledon bioassay is limited because of probable differences in herbicide penetration and translocation. If all DPE herbicides affect the same site(s), then discrepancies in activity could also be related to the differential ability of cucumber cotyledons to detoxify these compounds. Further structure-activity studies must utilize in vitro assays to measure intrinsic activity. 

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. 

Soil thin-layer chromatography is very useful in the investigation of the translocation and degradation of pesticides in soil. These layers combined with silica layers allow study of the movement of the pesticides and their breakdown products as well. Photochemical oxidation of aiyldialkylthiourea herbicides on the soil surface can be investigated when these plates are exposed to sunlight (143). The sorption coefficients and the mobility of C-metalaxyl was investigated on three Brazilian soil... 

Potent inhibition of the selected target may still not produce an effective herbicide. Studies of the iptake, translocation, and metabolism of the inhibitor are needed to determine if the cause of poor jn vivo performance is due to these factors or to an intrinsically poor target. Without full appreciation of each of these aspects of herbicide design, the chances for success with the target-site directed approach are reduced (Rendina, A., DuPont de Nemours and Co., Newark, DE, Personal Ocmmunication, 1992). 

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