Oat (Avena sativa L. ‘Elgin′) seedlings were treated with combinations of diclofop-methyl {methyl ester of 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} and 2,4-D amine [dimethylamine salt of (2,4-dichlorophenoxy) acetic acid] or diclofop-methyl and chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1, 3, 5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} to determine the effect of the added herbicide on diclofop-methyl activity. Diclofop-methyl applied alone at rates of 0.50, 0.75, and 1.00 kg/ha killed the oat plants 14 days after treatment. When 2,4-D amine at 0.74 and 1.11 kg/ha was combined with diclofop-methyl, the phytotoxicity of diclofop-methyl was reduced. An antagonistic interaction between diclofop-methyl and 2,4-D was detected. Chlorsulfuron, applied alone, at 20, 40, or 60 g/ha did not affect the growth of oats. Chlorsulfuron additions did not affect the activity of diclofop-methyl. Diclofop-methyl reduced oat coleoptile elongation. Equimolar concentrations of diclofop-methyl and 2,4-D at or above 1 μM significantly reduced the 2,4-D response. A ten-fold increase of either 2,4-D or diclofop-methyl, above an equimolar concentration of 1 μM, significantly enhanced the effect of the herbicide being increased.

Greenhouse and laboratory studies were conducted to determine the basis of differential response of two corn hybrids to the chloroacetanilide herbicide metolachlor. In greenhouse experiments, metolachlor at 6.7 kg ha−1 reduced the height of the susceptible ‘Northrup-King 9283’ corn by 53% relative to untreated controls and caused extensive visible injury 14 d after treatment Under the same conditions, the height of metolachlor-treated ‘Cargill 7567’ corn seedlings was reduced by only 18% without any visible herbicide injury. The 14C-metolachlor was more rapidly absorbed by the emerging shoot of the metolachlor-susceptible hybrid, Northrup-King 9283. Thus, differential metolachlor tolerance may be due in part to processes at the level of herbicide uptake. Metabolism experiments revealed that both hybrids were able to conjugate 14C-metolachlor with glutathione at similar rates. However, glutathione S-transferase activity increased earlier during seedling development and reached higher activities in the metolachlor-tolerant hybrid, Cargill 7567.

Haloxyfop-methyl toxicity to johnsongrass was reduced when sprayed in combination with 2,4-D in greenhouse studies. The antagonism was not seen when 2,4-D application followed haloxyfop-methyl treatment by 72 h or preceded it by 24 h. The antagonism of haloxyfopmethyl activity was evident regardless of chemical form of 2,4-D used (DMA, BOE ester, IPA, or acid). Absorption of 14C-haloxyfop-methyl into johnsongrass leaves was slowed by the presence of 2,4-D in the treatment solution but total absorption after 72 h was not affected. Translocation of 14C was decreased when the 14C-haloxyfop-methyl was applied in combination with 2,4-D. More haloxyfop-methyl, less haloxyfop, and more ether-soluble haloxyfop-methyl metabolites were found in treated leaves when haloxyfop-methyl was applied with 2,4-D compared to haloxyfop-methyl applied alone. The changes in haloxyfop-methyl translocation and metabolism caused by 2,4-D can quantitatively account for the antagonism observed.

The relation of translocation and degradation to selectivity of 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) in winter wheat (Triticum aestivum L., var. Nugaines), a resistant species, and coast fiddleneck (Amsinckia intermedia Fisch. & Mey.), a susceptible species, was examined. Radioautographic and extraction analysis revealed that the label from 14C-bromoxynil was more mobile in coast fiddleneck than in wheat. Higher levels of soluble radioactivity were found in extracts of coast fiddleneck than wheat. Higher levels of insoluble label were found in tissue residues of wheat than coast fiddleneck. In both species, most of the activity remained in treated leaves. A high percentage of the soluble activity was attributed to 14C-bromoxynil in both wheat and coast fiddleneck. The evolution of 14CO2 by wheat treated with 14C-bromoxynil significantly exceeded that of coast fiddleneck. The difference in susceptibility between winter wheat and coast fiddleneck appears to be explained partly by differential translocation and degradation of bromoxynil.

Field research was conducted to determine the potential of hyperspectral remote sensing for discriminating plots of soybean intermixed with pitted morningglory and weed-free soybean with similar and different proportions of vegetation ground cover. Hyperspectral data were collected using a handheld spectroradiometer when pitted morningglory was in the cotyledon to two-leaf, two- to four-leaf, and four- to six-leaf growth stages. Synthesized reflectance measurements containing equal and unequal proportions of reflectance from vegetation were obtained, and seven 50-nm spectral bands (one ultraviolet, two visible, and four near-infrared) derived from each hyperspectral reflectance measurement were used as discrimination variables to differentiate weed-free soybean and soybean intermixed with pitted morningglory. Discrimination accuracy was 93 to 100% regardless of pitted morningglory growth stage and whether equal or unequal proportions of reflectance from vegetation existed in weed-free soybean and soybean intermixed with pitted morningglory. Discrimination accuracy was 88 to 98% when using the discriminant model developed for one experiment to discriminate soybean intermixed with pitted morningglory and weed-free soybean plots of the other experiment. Reflectance in the near-infrared spectrum was higher for weed-free soybean compared with soybean intermixed with pitted morningglory, and this difference affected the ability to discriminate weed-free soybean from soybean intermixed with pitted morningglory.

Seasonal variation in flowering of common dandelion (Taraxacum officinale Weber) was studied in 1969 and 1970 at Bowling Green, Kentucky. At this location the dandelion flowered throughout the year. Maximum flowering occurred during April and a secondary peak occurred in September and October. Number of flower stems per square meter was not significantly correlated with precipitation, temperature, or day length. Length of stem was positively correlated with temperature and day length. The plants exhibited diurnal variation in flowering habit. Number of times (or days) the heads opened and closed, length of time in which the heads remained open each day, and length of time heads remained closed before opening into mature heads (white balls) varied with time of year.

Field studies conducted over 2 yr in Louisiana determined critical periods of Rottboellia cochinchinensis interference in Zea mays. In a duration of interference study, R. cochinchinensis was allowed to compete with Z. mays for 0 (weed-free, season-long), 2, 4, 6, 8, 10, 12, or 14 (weedy, season-long) wk, after which plots were maintained weed-free for the rest of the growing season. Rottboellia cochinchinensis biomass at time of initial removal increased linearly as weeks of interference increased. For 2 wk of interference, R. cochinchinensis biomass was greater in 1993 than 1994, but for 4 wk or more of interference, biomass was greater the second year indicating environmental conditions were more conducive to R. cochinchinensis growth in 1994. Season-long R. cochinchinensis interference reduced Z. mays height by 18% compared with the weed-free check. For both years, R. cochinchinensis reduced yields 125 kg ha−1 for each week of interference. In weed-free maintenance studies, 0, 2, 4, 6, 8, 10, 12, or 14 (weed-free, season-long) wk of weed-free conditions were provided, after which R. cochinchinensis was allowed to repopulate. Zea mays yield was equivalent for the weed-free control and plots maintained free of R. cochinchinensis for 2 wk or more. In the interference studies, season-long R. cochinchinensis interference reduced Z. mays yield at least 33% compared with the season-long weed-free check.

Mechanisms of herbicide resistance were studied in a quizalofop–ethyl-resistant barnyardgrass biotype. Acetyl-coenzyme A carboxylase (ACCase) sensitivity to quizalofop-p-ethyl was measured by high-performance liquid chromatography and the trend in ACCase gene expression over time was determined using real-time polymerase chain reaction. The results showed that an insensitive ACCase was present in Geqiushan resistant plants (R), with a resistance index of 106. The basal ACCase activities in Geqiushan R and Geqiushan susceptible plants (S) were similar, at 1.20 and 1.17 ng malonyl-CoA min−1 µg−1 extract protein, respectively. Basal ACCase gene expression in Geqiushan R was similar to that in Geqiushan S. The relative expression of ACCase gene decreased after spraying quizalofop–ethyl at 60 g ai ha−1 in Geqiushan S, whereas it was almost not changed in Geqiushan R. From these results we concluded that plastid ACCase sensitivity change might be responsible for the resistance and gene overexpression does not play a role in this resistance.

Velvetleaf growth and canopy architecture were compared under a range of light conditions representative of competitive and noncompetitive environments typical of irrigated Mediterranean-type agroecosystems. Velvetleaf biomass and seed production exceeded those reported in the literature. Plants grown in full light produced 1,370 g dry weight and 44,200 seeds per plant and showed low relative variability. Velvetleaf grown with corn was reduced to 21 g dry weight and 349 seeds per plant, and had high relative variability for biomass and seed numbers. Velvetleaf grown with kidney bean, intraspecific neighbors, or under shadecloth had dry weights and seed numbers that were intermediate to plants grown in full light or with corn. Relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) were assessed utilizing Richards functions, which were fitted to the primary biomass and leaf area data by weighted regression. RGR was highest for all plants early in the season, but declined later. Dynamics of NAR and LAR appeared to be correlated with increased self-shading, shading by neighbors, leaf age, and shedding of lower canopy leaves. Dynamics of specific leaf area corresponded with light availability such that the leaves exposed to full light were thicker than those exposed to shade. The branches of plants in all treatments had random azimuths and the foliage area density was concentrated along the perimeter of the plant's canopy. Velvetleaf increased the canopy radius through extensive branching when exposed to full sunlight. Leaf area distribution and branching patterns resulted in leaf area indices of less than 1.0. Leaves maintained a perpendicular angle to the sun throughout the day, but this depended on whether leaves received a consistent directional signal from the sun and not necessarily on whether they received a high-intensity signal. When shaded, the allocation of dry matter went primarily to the stem tissue, which increased the height rather than the girth of the plants. There was a 10- to 20-d delay for allocations to seed in the case of shaded plants relative to those grown in full sunlight. In brief, velvetleaf had a wide range of growth and canopy responses to a variety of light availabilities and it should have little difficulty in becoming fully established in the irrigated agroecosystems of Mediterranean-type regions.

Weed residues can impact nitrogen (N) cycling in agro-ecosystems that primarily utilize POST weed control. Quantifying this potential N source or sink may influence weed control and fertilization practices. A laboratory experiment measured the rate and quantity of N release from common lambsquarters, common ragweed, and giant foxtail. Weeds were grown in the field at four N rates (0, 67, 134, or 202 kg N ha−1) and collected at two weed heights (10 or 20 cm) to give a range of residue chemical composition. Residue chemical composition parameters of carbon : N (C : N) ratio and total N, nitrate-N, acid detergent fiber, and neutral detergent fiber concentration were measured and correlated with N release. Nitrogen release from weed residue mixed with soil was determined over a 12-wk period. Nitrogen was released from all weed residues at 12 wk. Prior to 12 wk, N was immobilized by giant foxtail grown with no N application. Prior to 4 wk, N was immobilized by 20-cm weeds grown with no N application. Nitrogen release from weed residue was negatively correlated with C : N ratio. Weed residue with a C : N ratio of < 19 (weeds grown with N application and 10-cm weeds) released 25 to 45% total N concentration within 2 wk and may contribute N within the growing season. Weed residue with a C : N ratio > 19 (giant foxtail and 20-cm weeds grown with no N) initially immobilized N and may not contribute N within the growing season.

Two-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), and 6,7-dihydrodipyrido-(1,2-a:2’,1′-c)-pyrazidiinium salt (diquat) significantly inhibited photosynthetic C14O2-fixation to varying degrees in corn (Zea mays L., var. Dixie 18), cotton (Gossypium hirsutum L., var. Deltapine), and soybeans (Glycine max (L.) Merr., var. Lee). Atrazine treatments of 10 ppm overloaded the protective mechanisms in corn and cotton. Both 1 and 10 ppm of atrazine reduced C14O2-fixation to less than 5% of controls in soybeans, a sensitive species. One ppm of bromacil and of diquat reduced C14O2-fixation in all three crops, but diquat was less inhibitory than either bromacil or atrazine. Except for diquat on corn, herbicides had no significant effect on C14O2-fixation in the dark. Treatments in which photosynthetic C14O2-fixation was 5% or more of controls had little effect on the kinds or relative amounts of C14-labeled compounds produced. Only the total amount fixed was reduced. Treatments, in which photosynthetic C14O2-fixation was less than 5% of controls, caused a significant reduction in the relative amounts of sucrose and alanine produced; however, the relative amounts of malic, aspartic, and glutamic acids increased.

The relative tolerance of cucumber and squash to amiben was established for germinating seeds, and for young plants with both foliage and root applications. In all cases, the dosage required for 50% inhibition of growth was 6 to 8 times greater for squash than for cucumber. Root applications of carbon-14 labeled amiben on reciprocal grafts of cucumber and squash plants indicated that the shoots were spared in these experiments by differential blocking of amiben transport from the roots. Shoots of either species appeared tolerant on squash roots and susceptible on cucumber roots. Radioactivity in the shoots of both species was much greater when grafted on cucumber than on squash. Foliage applications of carbon-14 labeled amiben showed more entry into cucumber than squash, but little movement to untreated plant parts in either species.

Collections of plants of medusahead, Elymus caput-medusae L., were made from natural infestations in Nez Perce County and Washington County in northern and southern Idaho, respectively. The ash and silica content (dry weight basis) decreased as the plant matured. The total ash of the entire plant contained from 72–89 per cent silica. Similar percentages of silica in ash were obtained in the culms, heads, and seeds of the plant. X-ray diffraction patterns and polarizing microscope examinations showed the mineral form of silica to be opal. Heavy deposition of silica was found in the barbs of awns, in the epidermis of the leaves, culms, glumes and seeds, and in strands beneath the epidermis.

An understanding of the physiological action of various herbicides is based in part upon a knowledge of the exact path of movement of the substance in the plant, the rapidity of transfer from one tissue to another, and the relative proportion of the total conductive tissue that is effective in transfer under any given set of conditions. Although the intercellular distribution of radioactive isotopes in both plant and animal tissues has been studied by the use of histoautoradiography, only one brief paper describes the use of this method with plant growth regulators.

Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeri S. Watson) populations within the U.S. state of Arkansas was due solely to increased EPSPS gene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteen A. palmeri accessions were treated with 840 g ae ha−1 glyphosate. Twenty of these accessions, selected to represent a broad range of responses to glyphosate, underwent further testing. Seven of the accessions were controlled with this dose; the rest were resistant. The effective dose to cause 50% injury (ED50) for susceptible accessions ranged from 28 to 207 g ha−1. The glyphosate-resistant (GR) accessions had ED50 values ranging from 494 to 1,355 g ha−1, a 3- to 48-fold resistance level compared with the susceptible standard (SS). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene relative copy number was determined for 20 accessions, 4 plants accession−1. Resistant plants from five GR accessions (38% of resistant plants tested) did not have increased EPSPS gene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 more EPSPS gene copies than the SS. Among the accessions tested, injury declined 4% with every additional EPSPS copy. ED50 values were directly correlated with EPSPS copy number. The highly resistant accession MIS11-B had an ED50 of 1,355 g ha−1 and 150 gene copies. Partial sequences of EPSPS from GR accessions without EPSPS amplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non–target site resistance mechanisms. Therefore, elevated EPSPS gene copy number is associated with glyphosate resistance among A. palmeri from Arkansas.

Laboratory experiments were conducted to assess the influence of surfactants applied with or without nitrogen on MON 37500 foliar absorption by Bromus tectorum, Bromus japonicus, Aegilops cylindrica, Triticum aestivum, Chorispora tenella, and Lactuca serriola. MON 37500 absorption in B. tectorum and B. japonicus increased from 40% 24 h after treatment (HAT) to 48% 48 HAT, averaged across surfactants with no added nitrogen. Averaged across nitrogen source and species, nonionic surfactant, ethylated seed oil, and organosilicate provided comparable enhancement of MON 37500 absorption (56 to 68%), whereas crop oil concentrate provided only 27 to 29% absorption under the same conditions. Averaged across species and surfactant class, urea ammonium nitrate had the greatest effect on MON 37500 absorption (68%), compared to ammonium sulfate (59%) or no nitrogen (40%). Nitrogen, regardless of the type, significantly improved foliar absorption of MON 37500. MON 37500 absorption by species was 71, 63, 57, 57, 49, and 38% in C. tenella, B. japonicus, T. aestivum, A. cylindrica, B. tectorum, and L. serriola, respectively, when averaged across surfactants and nitrogen. Densely pubescent B. japonicus leaves did not retain significant amounts of MON 37500 following a primary leaf wash.

Velvetleaf was treated at the three and one-half leaf stage with one, three, or nine drops (1 μl each) of glyphosate and an amphoteric surfactant with the concentration of the droplets adjusted so that the total amount of active ingredient applied to each plant was constant. The phytotoxicity of glyphosate to velvetleaf was dependent on both the concentration of glyphosate and surfactant within the droplet A single concentrated droplet of glyphosate and surfactant was more effective in reducing the growth of velvetleaf than were more dilute droplets in greater number but with the same total quantity of herbicide. If the surfactant concentration was increased, the loss in efficacy was negated, provided the glyphosate concentration in the droplet was sufficiently high. Also, loss in efficiency was negated at increased glyphosate concentration and diluted surfactant level per droplet. The phytotoxic response of velvetleaf treated with one or three drops was greater at 45 than 90% relative humidity. This greater response at low relative humidity continued with increased surfactant concentration in each drop. However, the phytotoxic response was greater at 90 than 45% relative humidity when velvetleaf plants were treated with nine drops. When the surfactant concentration in each drop was increased, the plant's response at the two relative humidity levels did not differ.

This research examined the effect of mechanical soil disturbance (none or tilled) and legume crop residues (none or hairy vetch) on fluometuron dissipation for 2 yr from the top 0 to 8 cm of soil in a 10 yr field experiment. Soil pH in the upper 0 to 8 cm was ≤ 5.6, and soil organic matter was highest in plots not-tilled and plots which had a vetch cover crop. Calculated initial half-lives of fluometuron ranged from 19 to 38 d in the 2 yr. Neither tillage nor cover crop influenced early-season fluometuron dissipation. However, there were detectable amounts of fluometuron in all treatments 1 yr after application.

Progression of crenate broomrape population density (PD, number of emerged plants m-2) in faba bean was studied over 8 yr in Spain. Spatial dispersion and effect of climatic conditions on parasite population growth rate (PGR) also were studied. With repeated cropping of faba bean, infestations of crenate broomrape increased from an initial PD of 0.15 to an average of 26, with a maximum of about 40 to 45. The average population growth rate (PGR, ratio between the PD of any 2 consecutive years) was approximately 3. However, this figure varied widely among localities and years, from 0.8 to 7.7. A highly significant relationship (P = 0.01) was found between PGR and rainfall and soil temperatures during December to February, months of crop vegetative growth. Dispersion of crenate broomrape infestations mainly followed direction of crop rows, most likely due to the effect of tillage and harvesting operations, which were the same direction as sowing.