Chester L. Foy

Interaction of Surfactants with Plant Cuticles1

Abstract: How surfactants modify the characteristics of a spray liquid is now reasonably well understood. Beneficial effects are primarily associated with reduction in surface tension. However, the mechanisms whereby surfactants enhance the diffusion of herbicides across the plant cuticle are less clear. Generally, hydrophilic surfactants with a high hydrophile/lipophile balance (HLB) are most effective at enhancing penetration of herbicides with high water solubility, whereas lipophilic surfactants with a low HLB are most effective for enhancing uptake of herbicides with low water solubility. Both high- and low-HLB surfactants are absorbed into the cuticle, but current theory suggests different mechanisms are involved in enhancing diffusion of hydrophilic and lipophilic herbicides across the cuticle. Surfactants having a high HLB are absorbed into the cuticle and enhance the water-holding capacity (hydration state) of the cuticle. With increased cuticle hydration, the permeance of hydrophilic herbicides into the cuticle is increased, which increases the herbicide diffusion rate at a constant concentration gradient. Surfactants having a low HLB are absorbed into the cuticle and increase the fluidity of waxes, as measured by a small reduction in melting point. This increased fluidity increases the permeance of lipophilic herbicides in the cuticle, which, in turn, increases their diffusion rate at a set concentration gradient. Additional index words: Herbicide diffusion, hydrophile/lipophile balance, surfactant mechanism of action, partition coefficient, permeance, postemergence herbicide absorption. Abbreviations: EO, ethylene oxide; HLB, hydrophile/lipophile balance.

Understanding the Role of Allelopathy in Weed Interference and Declining Plant Diversity1

Several weed species have been reported to have allelopathic activities. However, most of these studies indicate the probable involvement of allelochemicals but are not conducted in field settings. In addition to their adverse effects on growth and yield of many crop species, many troublesome weeds such as mugwort and lantana influence biodiversity. More studies on the ecological, physiological, and molecular aspects of weed allelopathy should be conducted in order to better understand community structure and declining biodiversity. Nomenclature: Lantana, Lantana camara L. #3 LANCA; mugwort, Artemisia vulgaris L. # ARTVU. Additional index words: Allelochemicals, plant diversity, allelopathy, phytotoxicity.

A study of ethylene and CO2 evolution from ethephon in tobacco

Abstract Buffers and leaf discs of mature tobacco ( Nicotiana tabacum L.) were utilized to study [ 14 C]-ethylene and 14 CO 2 evolution from radiolabeled ethephon, (2-chloroethyl)phosphonic acid. Metabolic fate of [ 14 C]ethephon in leaf discs was investigated by use of thin-layer chromatography, high-voltage paper electrophoresis, autoradiography, and liquid scintillation spectroscopy. The evolution of labeled ethylene generally increased with increasing buffer pH, buffer volume, and dosage of [ 14 C]ethephon. [ 14 C]Ethylene was evolved, increasingly with time, from [ 14 C]ethephon either added to the buffer or applied to leaf discs. The rate of [ 14 C]ethylene evolution was maximum during the first day and leveled off on the fourth day. More than 50% of the total [ 14 C]ethylene evolution over a 96-hr period was recovered during the first 24 hr after [ 14 C]ethephon application. No 14 CO 2 was evolved when [ 14 C]ethephon was degraded in the presence of buffer or leaf discs. Only ethephon itself, and no detectable metabolite thereof, was discovered in the methanolic extract of the leaf disc tissue. An insignificant amount of 14 C activity (approximately 2% of the extracted 14 C) was detected in the residue. By means of gas chromatography, it was confirmed that in buffers and tobacco leaf tissue ethephon breaks down to release ethylene but not CO 2 .

Metabolic fate of bioxone in cotton

Abstract The metabolic fate of the 14 C-labeled herbicide, 2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione (bioxone), in cotton ( Gossypium hirsutum L. “Acala 4-42-77”) was studied using thin-layer chromatography, autoradiography, and counting. Bioxone- 14 C was readily metabolized by cotton tissue to 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU) and 1-(3,4-dichlorophenyl)urea (DCPU). Leaf discs metabolized bioxone- 14 C rapidly; 12 hr posttreatment, 65% of the 14 C in methanol extracts was in forms other than intact herbicide. Excised leaves treated through the petiole with either heterocyclic ring-labeled or phenyl ring-labeled herbicide contained little bioxone- 14 C after 1 day; DCPMU was formed early then decreased with time. DCPU accounted for 55–70% of the 14 C in excised leaves 3 days posttreatment. In intact plants treated via the roots, the herbicide was rapidly metabolized in the roots to DCPMU and DCPU; little or no intact herbicide was translocated to the leaves. Little radioactivity accumulated in the roots with time; the radioactivity in the leaves accounted for 80–90% of the methanol-soluble 14 C 47 days posttreatment. Most of the 14 C in the leaves was recovered as DCPU (50–60%) and unidentified polar metabolite(s) which remained at the origin of the thin-layer plates (30–40%). The percentage of radioactivity which remained in cotton residue after methanol extraction increased with time. Digestion of the plant residues with the proteolytic enzyme pronase indicated that some of the nonextractable 14 C may be DCPMU and DCPU complexed with proteins. Similar metabolic patterns were noted after treatment with either heterocyclic ring-labeled or phenyl ring-labeled bioxone- 14 C. Generally, bioxone was metabolized to DCPMU which in turn was demethylated to DCPU. The herbicide and DCPMU were 20 times as toxic as DCPU to oat ( Avena sativa L.), a susceptible species.

Adjuvants: Test Design, Interpretation, and Presentation of Results1

Abstract: Adjuvant research contributes much to the knowledge and practice of weed science though the scientific process of systematically asking precise questions and subsequently making distinctions among alternative explanations. The purpose of adjuvant experimentation is to answer these questions and the purpose of associated papers and presentations is to communicate the new information. These purposes are self-evident, but are difficult to perfect. Some factors are particularly difficult for adjuvant researchers and require that researchers plan thoroughly from the formulation of the experimental question to final presentation of results. Adjuvant research requires both chemical and biological expertise that is traditionally separated in most organizations. Scientists from other disciplines or weed scientists not primarily concerned with adjuvants often direct adjuvant studies. This paper discusses mistakes that are commonly made in test design, interpretation, and presentation and suggests guidelines to improve the quality of adjuvant research.

Rapid growth responses of dwarf corn coleoptile sections to picloram

Abstract The initial and transient growth responses of coleoptile sections of Dwarf-1 corn ( Zea mays L.) to picloram (4-amino-3,5,6-trichloropicolonic acid) were studied and compared to those produced after IAA (indole-3-acetic acid) treatments. Growth was measured every 36 sec with an angular displacement transducer. Picloram-mediated and also IAA-mediated growth recorded approximately 10 min of lag period before any growth response was elicited. The rate of picloram-mediated growth then increased rapidly, reaching a maximum between 25 and 40 min after treatment. This was followed by a second maximum approximately 80 min after treatment; then the growth rate slowly decreased to a constant level. The shape of the rate curve of picloram-mediated growth was essentially the same as that obtained after IAA treatment. Analysis of growth kinetics suggests that modes of growth stimulating action of picloram may not involve IAA metabolism inplants, but suggests that the compound is itself an auxin.

Metabolism of bifenox in soil and plants

Abstract The herbicide, methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (bifenox), had a half-life of 3 to 7 days after preemergence application to a greenhouse soil mix. Metabolites identified included: 5-(2,4-dichlorophenoxy)-2-nitrobenzoic acid, 2,4-dichlorophenyl 4-nitrophenyl ether (nitrofen), and 5-(2,4-dichlorophenoxy)anthranilic acid over a 313-day sampling period. Comparison of the total 14 C in the soil to that extractable by methanol showed an increase in the proportion of bound material. The major metabolite eluted from a Frederick clay loam soil column was identified as the acid of bifenox and its mobility was associated with the short half-life of bifenox in soil. In vitro studies with shoot-tissue macerates showed that bifenox was not degraded by corn ( Zea mays L.) or soybeans ( Glycine max (L.) Merr.) and was degraded to less than 1% by velvetleaf ( Abutilon theophrasti Medic.).

Comparative uptake and metabolism of methazole in prickly sida and cotton

Abstract The comparative uptake and metabolism of 14 C-labeled 2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione (methazole), a herbicide, in prickly sida ( Sida spinosa L.) and cotton ( Gossypium hirsutum L.) were investigated as physiological bases for herbicidal selectivity, using thin layer chromatography, autoradiography, and liquid scintillation counting. Prickly sida and cotton readily absorbed and translocated 14 C from nutrient solution containing [ 14 C]methazole. Only acropetal translocation of 14 C was observed. Methazole was rapidly metabolized to 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU) and other metabolites by both species. Although metabolism appeared to be qualitatively the same, quantitative differences between species were evident. Methazole was converted to DCPMU (also phytotoxic) more readily by prickly sida than cotton; however, DCPMU was more readily detoxified to 1-(3,4-dichlorophenyl) urea (DCPU) by cotton than prickly sida. More 14 C per unit weight was present in the prickly sida shoots than in cotton shoots. Also, a larger portion of the methanol-extractable 14 C was herbicidal in the shoots of prickly sida than of cotton. Thus, the differential tolerances of prickly sida and cotton to methazole may be explained, in part, by differential uptake and metabolism of methazole and DCPMU.

Complex formation of picloram and related chemicals with metal lons

Abstract The complex formation of metal ions with pyridine carboxylic acids was estimated with polarography and spectrophotometry. Picloram (4-amino-3,5,6-trichloropicolinic acid), α-picolinic acid, fusaric acid, dipicolinic acid, and quinolinic acid formed complexes with Fe(III) or Cu(II) whose coordination involves, most probably, a lone pair of electrons of pyridine nitrogen and a carboxylic group. Picloram-metal complexes were, however, estimated to be relatively unstable compared to other pyridine-α-carboxylic acids tested. Effects of pyridine carboxylic acids on oxidation of indole-3-acetic acid (IAA) were tested in vitro in a horseradish protoheme peroxidase system. No significant effect of the pyridine carboxylic acids was observed at 2 × 10−4 M. Also, no concentration effect of picloram (10−5 to 3 × 10−3 M) was obtained. These results suggest that the phytotoxic action of picloram may not result from the depletion of free metal ions in plants nor inhibition of activity of metal-containing enzymes through strong chelation as hypothesized. Thus, auxin activity of picloram should be explained in other wasy.

Effects of temperature on triazine-resistant weed biotypes

Abstract Studies were initiated to determine the effects of different temperature regimes on triazine-resistant and -susceptible biotypes of common lambsquarters ( Chenopodium album L.) and smooth pigweed ( Amaranthus hybridus L.) from different geographical locations. Shoot height, weight, chlorophyll a and b content and fatty acid content of common lambsquarters and smooth pigweed were determined at 18/14, 26/22 or 36/26°C. Common lambsquaters biotypes from Virginia, Maryland and Switzerland were examined. These data indicate common lambsquarters biotypes had a differential growth response to temperature, but no relationship between triazine resistance or susceptibility was noted in response to temperature with the parameters used in these studies. Differences in growth were detected between triazine-resistant and -susceptible smooth pigweed biotypes which indicated that the susceptible biotypes were vigorous at lower temperatures than triazine-resistant biotypes. Chlorophyll analyses indicated that all triazine-resistant biotypes of common lambsquarters and smooth pigweed had lower chlorophyll a / b ratios than triazine-susceptible biotypes at all temperature regimes. An increase in saturated fatty acids was found in both triazine-resistant and -susceptible biotypes of common lambsquarters at higher temperatures. At higher temperatures, the triazine-susceptible biotype of smooth pigweed contained a higher unsaturated fatty acid ratio.