Terbacil tolerance was investigated in field-selected alfalfa strain ‘OK 182.’ Growth response studies indicated a 78% increase in tolerance over the unselected parental cultivar ‘Cimarron.’ Photosynthetic electron transport was inhibited similarly in both cultivars when isolated thylakoids were exposed to a wide range of terbacil concentration indicating that tolerance was not due to reduced sensitivity at the site-of-action. ‘OK 182’ took up an average of 22% less 14C-terbacil over the 6-d labeling period. A similar proportion of total radiolabel was translocated in both strains indicating that differential translocation was not a factor in the tolerance mechanism. Once in the leaves, terbacil was rapidly metabolized to a glucosidic conjugate by both strains. Terbacil and total concentration of metabolites in leaf tissues were 17% and 33% lower in ‘OK 182’ than in ‘Cimarron,’ respectively. Enhanced tolerance to terbacil in ‘OK 182’ was attributed to decreased terbacil uptake.

Greenhouse and laboratory studies were conducted to determine kochia resistance to a spectrum of acetolactate synthase (E.C.4.13.18) (ALS)-inhibiting herbicides. The chlorsulfuron-resistant biotype plants were resistant to six herbicides: triflusulfuron, thifensulfuron, halosulfuron, imazamethabenz, chlorsulfuron, and nicosulfuron. But, the resistant biotypes showed sensitivity similar to the susceptible biotypes to three herbicides: metsulfuron, imazethapyr, and imazaquin. The resistant biotypes were slightly less sensitive to primisulfuron, chlorimuron, and flumetsulam than the sensitive biotypes. The I50 values for 50% inhibition of the ALS enzyme indicated that the resistant biotype was 22, 18, and 16 times more resistant to primisulfuron, chlorsulfuron, and thifensulfuron than the susceptible biotype. In contrast, the I50 ratios (resistant/susceptible) were 3, 2, and 1 for flumetsulam, nicosulfuron, and imazethapyr, respectively. The altered ALS enzyme system of the resistant biotype showed a differential response for the ALS-inhibiting herbicides. Addition of a mixed function oxidase inhibitor, piperonyl butoxide (PBO) at 2 kg ha−1, to primisulfuron and thifensulfuron increased visual injury and reduced plant height of the chlorsulfuron-sensitive kochia biotype plants. The addition of PBO to primisulfuron enhanced visual injury of the resistant biotype at low rates of primisulfuron.

Goosegrass, wide spread throughout the tropics and subtropics, is one of the most noxious weeds known. Recently, biotypes of goosegrass have been found resistant to the dinitroaniline herbicides. An alteration in the structure/composition of a tubulin protein has been postulated as an explanation for the hyperstable microtubules and the resistant phenotype. Our study was initiated to investigate the structure of the alpha (α)-, beta (β)- and gamma (γ)-tubulin related gene sequences in resistant, intermediately resistant, and susceptible biotypes. Heterologous tubulin gene clones were used as probes of restriction endonuclease-digested genomic DNA from each biotype, to determine gene size and copy number and to screen for restriction fragment length polymorphisms. The tubulin genes are organized into gene families. There are three to five α-tubulin genes, four to seven β-tubulin genes, and four to eight γ-tubulin genes. There was no evidence of multiple copies or tandem repeats of any individual gene sequence. Although RFLPs were observed, no significant difference in the banding pattern between the resistant and the susceptible biotypes was found for either α-, β-, or γ-tubulin gene families. Therefore, it is unlikely that the difference between the herbicide-response phenotypes can be attributed to large deletions or insertions in a tubulin gene.

The inheritance of resistance to dicamba in wild mustard was determined by making reciprocal crosses between a resistant (R) population derived from a field treated repeatedly with auxin-type herbicides, and a known susceptible (S) population. The resulting F1 hybrids were selfed to produce F2 populations and backcrossed to the S parent. At the three- to four-leaf stage, parental, F1, F2, and backcross populations were screened for resistance to dicamba at three dosages (50, 200, and 400 g ai ha−1). F1 progeny survived all dosages and exhibited levels of injury similar to the R parental population. F2 populations segregated in a 3:1 ratio of R to S phenotypes. Progeny of backcrosses segregated in a 1:1 (R:S) ratio. Responses of the F1, F2, and backcross populations to treatment with dicamba indicate that resistance is determined by a single, completely dominant nuclear allele.

Copper-containing herbicides are often applied to control photoautotrophic populations to reduce biosynthesis of the earthy odorant geosmin. The objective of this investigation was to determine the effects of copper-containing herbicides on geosmin and biomass biosynthesis by heterotrophic cultures of the bacterium Streptomyces tendae and the fungus Penicillium expansum. Cultures of S. tendae treated with 12.7 mg Cu L−1 (copper sulfate) accumulated 44.6% more biomass and the mycelium contained five-fold greater concentrations of geosmin than controls. Additionally, P. expansum cultures exposed to 12.7 mg Cu L−1 (copper sulfate) accumulated 9.2% more biomass and 18-fold greater concentrations of geosmin than controls. Cultures of P. expansum and S. tendae, treated with copper chloride, copper-ethylenediamine complex, and copper chelate also accumulated more geosmin and biomass than controls. Inductively coupled plasma atomic emission spectrometry of S. tendae of untreated control cultures indicated that cellular copper concentrations were below 0.5 mg Cu kg−1. However, S. tendae cells grown on a medium containing 5 mg Cu L−1 copper sulfate contained 12.88 ± 0.54 mg Cu kg−1, suggesting that cellular accumulation of copper may be associated with increased geosmin biosynthesis. If field microbial populations exhibit similar responses to copper, copper-algicide application may contribute to geosmin-induced “off-flavors” under certain circumstances.

In the discipline of Weed Science, nuclear magnetic resonance (NMR) has been used extensively for obtaining structural information on herbicide compounds in the areas of herbicide synthesis, metabolism, and environmental degradation. However, little research has been published with regard to the utilization of NMR in determining molecular interactions in the spray solution. The molecular influence of citric add and an organosilicone adjuvant on glyphosate was analyzed with NMR spectrometry. 14C-glyphosate absorption studies showed a decrease in glyphosate absorption by sunflower when Ca2+ was added to the spray solution. This absorption antagonism was overcome with the inclusion of an organosilicone adjuvant. 1H-NMR was used to stow that the organosilicone adjuvant did not directly interact with the glyphosate molecule nor did it prevent the formation of Ca-glyphosate. Citric add was effective in overcoming the Cat2+ antagonism of glyphosate activity when the citric add concentration was 2× or 4× the Ca2+ molar concentration based on plant fresh weight and plant height, respectively. 1H-NMR was utilized to show that citric acid reacted with Ca2+ in solution to produce Ca-citrate and prevent the formation of Ca-glyphosate. NMR was an effective technique for characterizing chemical interactions among the spray solution components.

Picloram uptake, translocation, and efficacy were studied using greenhouse-grown Russian knapweed plants. Uptake of 14C-picloram, applied as discrete droplets to adaxial leaf surfaces of well-watered plants, averaged less than 10% of that applied. Most uptake occurred within 30 min of application. Uptake increased linearly with external picloram concentrations from 6.2 to 74.5 mM and was proportional to picloram concentration. Only about 10% of absorbed picloram was translocated out of the treated leaf of well-watered plants within 96 h, with approximately equal acropetal and basipetal translocation. Water stress before, at the time of, and after picloram application did not affect picloram uptake, but reduced total translocation and increased the relative amount translocated basipetally. Water stress also reduced picloram efficacy. Although Silwett L-77 increased picloram uptake into Russian knapweed leaves, it did not increase efficacy under water stress conditions.

Field experiments were conducted during the winters of 1987–88 and 1988–89 at Haryana Agricultural University, Hisar, India to evaluate the response of winter wild oat to sowing date and isoproturon application time in wheat. In another experiment, the effect of sowing time on emergence and growth of winter wild oat and wheat sown separately was studied. The treatments that provided more than 75% control of winter wild oat were: isoproturon at 0.75 kg ha−1 applied at the 2-leaf stage of winter wild oat; isoproturon 1 kg ha−1 applied at the 4-leaf stage of winter wild oat in the November 30 planting; and isoproturon 0.50 kg ha−1 applied at the 1- to 2-leaf stage of winter wild oat in the December 20 sowing. Isoproturon did not provide more than 50% control of winter wild oat in the November 10 sowing. Mortality of winter wild oat increased from 38 to 72 to 87% in November 10, November 30, and December 20 sowings, respectively. Mortality of winter wild oat was similar in November 30 and December 20 sowings but higher grain yield was recorded in November 30 sowing. Due to vigorous growth of winter wild oat in the November 10 sowing, isoproturon did not provide good control. December 20 sowing favored wheat growth due to lower density and late emergence of winter wild oat but reduction in wheat grain yield was greater. When averaged over isoproturon treatments, the grain yield of wheat was 4607, 5297, and 4457 kg ha−1 in the November 10, November 30, and December 20 sowings, respectively.

Common and giant ragweed are important weeds of soybeans in Indiana. These two weeds respond differently to imazethapyr POST treatments with common ragweed demonstrating more tolerance than giant ragweed. Both plants show initial susceptibility to imazethapyr, but common ragweed can regrow 10 to 14 days following herbicide application. Laboratory studies were conducted to determine the factors that contribute to the differential control of common and giant ragweed with imazethapyr. Differential absorption was observed at 72 h, with common ragweed absorbing 52% of the applied 14C-imazethapyr and giant ragweed absorbing 39%. The absorption of radioactivity was the same for both species by 672 h. Imazethapyr exhibited both xylem and phloem mobility by translocating both acropetally and basipetally from a treated leaf in giant and common ragweed. A higher percentage of the absorbed radioactivity accumulated in the lower foliage and roots of giant ragweed than common ragweed by 336 h. The rate of imazethapyr metabolism in common ragweed was greater than in giant ragweed. At 336 h, 81 and 68% of the identified radioactivity in the treated leaf was imazethapyr metabolites in common and giant ragweed, respectively. A higher level of the inactive glucose conjugate metabolite was found in the lower plant and root tissues of common ragweed than in giant ragweed. The differential control of common and giant ragweed with foliar applications of imazethapyr was attributed to differences in both translocation and metabolism.

Studies determined the effect of sodium bisulfate (NaHSO4) on the phytotoxicity, retention, uptake, and translocation of the suspension concentrate formulation of imazamethabenz in wild oats. NaHSO4 completely solubilized this herbicide formulation when added in an equimolar concentration and did not affect herbicidal activity at NaHSO4 concentrations below 28 mM when used in a carrier volume of 100 L ha−1. NaHSO4 improved phytotoxicity at a carrier volume of 33 L ha−1. NaHSO4 at 28 mM increased the efficacy of imazamethabenz applied as individual drops on growth chamber-grown wild oats. The herbicide retention was not changed but foliar absorption and acropetal translocation were increased, while basipetal translocation was decreased. The increase in phytotoxicity of imazamethabenz with NaHSO4 was related to absorption of imazamethabenz by the target plants.

Common ragweed is a major problem in white bean production systems in Ontario. The influence of time of emergence and density of common ragweed on white bean growth and seed yield was examined in Ontario at Elora in 1990, and at Woodstock and Staffa in 1991 and 1992. Ragweed emerged with white bean seedlings (VE) and at the second trifoliate stage of white bean (V3). Time of ragweed emergence and weed density affected white bean yield at all locations. When 1.5 ragweed seedlings m−1 of row emerged at the VE stage of crop growth 10 to 22% seed yield loss occurred. Yield losses of 4 to 9% occurred when 1.5 ragweed seedlings m−1 of row emerged at the V3 crop stage. Yield loss parameter estimates, i.e., the predicted weed-free crop yield (YWF) and the maximum yield loss (A), varied among locations and with time of ragweed emergence, whereas the parameter for yield loss at low weed density (I) was more consistent across all locations and times of weed emergence. Although I values were relatively consistent across locations and times of ragweed emergence, the standard errors associated with each estimate were large. White bean leaf area index, above-ground biomass and pod number m−2 were affected most by ragweed interference. White bean density, number of seeds per pod, and seed weight per plant were not affected by ragweed interference. Ragweed emerging at VE and V3 produced a maximum of 6000 and 1000 seeds m−2, respectively. Time of ragweed emergence may be more important than weed density when evaluating weed control options.

Studies were conducted of one of the structural factors that influences microroughness on johnsongrass leaves. Bicellular trichomes, 47 ± 5 μm long, represented 4 to 5% of all epidermal cells. They secreted a mucilagenous material that covered 8 ± 4% of the leaf surface. Bicellular trichomes occurred in longitudinal rows, intermixed with intercostal cork-silica cells, between rows of stomata. Numbers of bicellular trichomes present per unit area were inversely related to numbers of intercostal cork-silica cells. The trichomes were the panicoid type that are reported not to secrete salts. Johnsongrass trichomes, however, could be induced to discharge salt in the mucilage-type secretions when plants were grown in a soil mixture that was high in lime. Not all secretory constituents were identified, but carbohydrates and callose were found in addition to possible low concentrations of protein. The apical or cap cell of the trichomes stained positively for lipid, protein, and polysaccharide and negatively for pectin, polyphenols, steroids, and alkaloids. The presence of trichomes increases leaf surface microroughness, but the secretion covers wax crystals, decreasing leaf microroughness and likely providing another barrier to herbicide entry through the cuticle. Bicellular trichomes on grain sorghum were similar to those on johnsongrass and also discharged secretions on the leaf surface.

The competitive ability of annual round-leaved mallow was determined in spring wheat and lentil at Indian Head and Regina, Saskatchewan, in 1985 and 1986 using paired quadrats. Significant biomass and seed yield loss occurred in three of four tests in lentil and two of three tests in spring wheat. Differences in numbers of wheat tillers produced between weedy and weed-free plots were found in three of four tests. A two-variable model comprised of early season crop density loss and round-leaved mallow biomass best accounted for the majority of variation in crop yield loss for both lentil and wheat, and tiller density loss in wheat. In 1985 at Indian Head, where no yield loss occurred for either wheat or lentil, round-leaved mallow had been seeded immediately before the crop. Greater yield losses occurred at Regina, in the presence of an older, well-established infestation. In the years and locations with the greatest crop yield losses, round-leaved mallow emerged before the crop causing poor crop emergence. At Regina in 1986, crop yield losses were more than 60% in wheat and 90 to 100% in lentil because of large differences in crop density between weed-free and weedy subplots. Round-leaved mallow exhibited great variability in growth, producing more biomass per plant, more capsules per plant, and more capsules per gram of biomass in the less competitive crop, lentil, than in wheat. Density and biomass of round-leaved mallow were not correlated; with a density of 200 plants m−2, round-leaved mallow biomass in wheat ranged from 100 to 500 g m−2; while in lentil, from 200 to as high as 1000 g m−2, approximately double the range found in wheat. The type of round-leaved mallow infestation (newly seeded or well-established) and environmental conditions (mainly early season precipitation) account for differences between sites and between years.

Using starch gel electrophoresis, we examined the genetic variability of kikuyugrass collected from three golf course sites within its geographical range in California. These experiments were conducted to determine the method of spread of kikuyugrass in areas where it is considered invasive. Samples from roughs and fairways of each of these locations were compared. of the 354 plants examined, 12 different genotypes were identified by isozyme analysis, and only three of the nine putative loci varied among the genotypes. Two genotypes, representing 73% of the plants examined, were found at all three geographic locations. Our results imply that under common golf course practices, kikuyugrass is maintained by clonal reproduction. We conclude that low genetic variation does not preclude a plant species from being an aggressive invader.

Laboratory and greenhouse studies were conducted to determine the mode of action of soil- and foliar-applied UCC-C4243. Experiments demonstrated that UCC-C4243 required light for phytotoxicity, phytotoxic symptoms were similar to inhibitors of porphyrin synthesis such as acifluorfen, and UCC-C4243 potently inhibited protoporphyrinogen oxidase. Germination and emergence of field pennycress and lentil in the dark were not affected by soil-incorporated UCC-C4243 at rates more than 10 times greater than like treatments that killed all plants in the light. Soil-incorporated UCC-C4243 required light for activity and killed seedlings within 1 d after emergence; sublethal doses caused desiccation, veinal necrosis, and leaf deformation. Field pennycress and lentil were susceptible to soil-incorporated UCC-C4243 and acifluorfen in the light, but were 5 to 93 times less sensitive to the herbicides in the dark. Wheat was not affected by either herbicide in the light or dark. Injury symptoms from UCC-C4243 applied POST to redroot pigweed were similar to symptoms from diphenyl ether and bipyridinium herbicides: rapid, light-dependent chlorophyll bleaching, desiccation, and necrosis. UCC-C4243, acifluorfen-methyl, and acifluorfen acid caused light- and concentration-dependent chlorophyll bleaching and electrolyte leakage from cucumber leaf disks (I50 = 1.0, 1.8, and 4.3 μM, respectively). An inhibitor of the porphyrin synthesis pathway, 4,6-dioxoheptanoic acid, almost completely inhibited herbicide-induced electrolyte leakage. δ-Aminolevulinic acid, a tetrapyrrole precursor and stimulator of the porphyrin synthesis pathway, caused synergistic effects with each herbicide. Protoporphyrinogen oxidase from barley etioplast preparations was inhibited 50% by 40 nM UCC-C4243. Barley leaf sections treated with 100 μM UCC-C4243 accumulated protoporphyrin IX in vivo to levels > 75 times non-treated controls. These data indicate the light-requiring herbicide activity of UCC-C4243, like acifluorfen, is due to inhibition of protoporphyrinogen oxidase.

A laboratory study was conducted to evaluate the magnitude of imazethapyr resistance of field corn hybrids with altered acetolactate synthase (ALS) by comparing the responses of ALS extracted from homozygous and heterozygous resistant corn to nicosulfuron, primisulfuron, and imazethapyr. ALS isolated from Pioneer 3162 IR, Pioneer 3180 IR, Pioneer 3377 IR, and Ciba 4393 RSC hybrids was much more resistant to inhibition by these herbicides than ALS from the hybrids Pioneer 3162, Pioneer 3180, Pioneer 3377, and Ciba 4393, and the public inbred LH 74. Corn hybrids heterozygous for the Pioneer resistance gene were intermediate in response to the herbicides. ALS from ICI 8532 IT was resistant to imazethapyr but not to primisulfuron or nicosulfuron. The level of resistance of ALS from ICI 8532 IT was similar to that of ALS from Pioneer 3162 IR x Pioneer 3162 and Pioneer 3180 IR x Pioneer 3180 crosses that were also heterozygous for the resistance gene.

The impact of weed density and weed distribution on irrigated corn yield was investigated in Colorado. Weed densities examined were 0,33,50, or 100% of the indigenous weed population. A series of weed distribution treatments were achieved by varying the length of the weed-free and weedy zones within the corn row while maintaining a constant weed population of 33 or 50% of the indigenous weed level. Grain yield was affected by weed density, but not by weed distribution. Each additional weed reduced corn yield 8.5 and 2.3 kg ha−1 in 1991 and 1992, respectively. When corn yields were estimated with a computer weed/corn management model, weed densities 5 to 8 wk after planting provided a better yield reduction estimate than weed densities immediately before harvest.

A 50-year study at Fairbanks, AK, was started in 1984 to determine soil seed longevity of 17 weed species. Seed were buried in mesh bags 2 and 15 cm deep and were exhumed 0.7, 1.7, 2.7, 3.7, 4.7, 6.7, and 9.7 yr later. Viability was determined by germination and tetrazolium tests. All common hempnettle and quackgrass seed were dead after 2.7 and 3.7 yr, respectively. Less than 1% of wild oats and foxtail barley seed were viable after 3.7 yr, but > 6.7 yr were required for loss of all viability. By 9.7 yr, < 1% seed viability remained for: bluejoint reedgrass, corn spurry, pineappleweed, prostrate knotweed, and wild buckwheat From 2 to 5% of seed from common chickweed, common lambsquarters, flixweed, Pennsylvania smartweed, rough cinquefoil, marsh yellow-cress and shepherd's-purse were viable, while 62% of American dragonhead seed was still alive. Seed longevity in agricultural fields is not greater under subarctic conditions than under warmer conditions.

Trials were carried out to determine the degree of light transmittance through soil and its role in photo-induced germination of buried weed seeds. Transmittance was found to be strongly dependent on soil type and decreased with decreasing particle size and with an increase in darker components. In each soil examined, light penetration fell below 0.01% at a depth of no more than 4 mm. It was also noted that with increasing soil depth, light permeability was proportional to wavelength, leading to progressive decline in the red-far red ratio. Low light penetration played a crucial role in germination of jimsonweed seeds, which had became light-sensitive after prolonged burial. Although this effect is a function of soil type, it may represent an important ecological factor influencing seed emergence in the first few mm of upper soil layers.

Thirteen biotypes of rigid ryegrass were screened for trifluralin resistance. From these, the two most resistant biotypes, SLR 31 and SLR 10, were chosen for further studies involving exposure to other dinitroanilines, mitosis-inhibiting herbicides and 14C-trifluralin. SLR 31, and SLR 10 exhibited an approximate 10-fold reduced sensitivity to trifluralin in comparison to susceptible biotypes. Resistance to five other dinitroaniline herbicides was observed, with reduced sensitivity varying from 32-fold for ethalfluralin to 2.5-fold for isopropalin. The resistance in rigid ryegrass to other herbicides and drugs that affect mitosis were tested. Resistance comparable to that of trifluralin was recorded for the herbicides terbutol and DCPA, while low levels of cross-resistance to amiprophosmethyl was present. Trifluralin affected mitotic indices at a much lower level in the susceptible biotypes than in the resistant biotypes. No differences in the uptake and translocation of 14C-trifluralin were observed between resistant and susceptible biotypes. Most of the 14C detected in the plant material was in the root tissue. A small level of 14C was detected in the seeds, and no substantial increases were noted in coleoptile tissue. The resistance spectra in SLR 31 and SLR 10 were phenotypically similar to those occurring in an intermediate trifluralin-resistant goosegrass and trifluralin-resistant green foxtail.