The mode of action of two experimental pyridinyloxyphenoxy propionate herbicides, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} and CGA-82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid} was studied. Oat (Avena sativa L.) roots showed a rate-dependent sensitivity to both herbicides. Consequently, a series of assays was performed to study cell division, cell cycle dynamics, and nucleic acid and protein synthesis in oat root tips in order to characterize growth inhibition. Both herbicides inhibited cell division, apparently by inhibiting protein synthesis in the G2 stage of interphase. Supporting evidence for this assumption comes from the following observations: a) There was almost complete absence of leucine incorporation within 1 h of exposure to 2.7 × 10−4 M of either herbicide; b) the number of 3H-labeled dividing cells was reduced to about 20% of the control within 4 h, and essentially to zero within 8 h, after exposure to 2.7 × 10−4 M of herbicide; c) the number of unlabeled cells was reduced to less than 10% of the control within 8 h of exposure to 2.7 × 10−4 M of herbicide; d) during this 8-h period, the number of 3H-labeled cells in interphase which had been exposed to herbicide greatly exceeded the control; e) thymidine incorporation into DNA was inhibited but was not proportional to reduction in cell division; f) uridine incorporation was enhanced during the first 8 h of exposure before a reduction was evident at 12 h; and g) an oat coleoptile elongation bioassay showed that cell enlargement was not inhibited at herbicide concentrations less than 2.7 × 10−4 M.

Isopropyl m-chlorocarbanilate (chlorpropham) at 6 lb/A, the standard treatment, usually controlled dodder (Cuscuta spp.) completely for 3 to 5 weeks. Of 27 other herbicides compared with chlorpropham, seven controlled dodder completely for longer periods (6 to 18 weeks), and nine for comparable or slightly shorter periods. Eleven controlled dodder only partially or completely for a period shorter than that from chlorpropham. The period of dodder control from chlorpropham was doubled when p-chlorophenyl N-methylcarbamate at 1.5 lb/A was included with it.

The fate of foliarly applied 14C diclofop-methyl {methyl 2-[4-(2,3-dichlorophenoxy)phenoxy] propanoate} was determined in intact plants of barnyardgrass [Echinochloa crus-galli (L.) Beauv.], a susceptible (S) grass; proso millet (Panicum miliaceum L.), a moderately susceptible (MS) grass; longspine sandbur [Cenchrus longispinus (Hack.) Fern.], a tolerant (T) grass; soybean [Glycine max (L.) Merr. ‘Hark’], and cucumber (Cucumis sativus L. ‘Green Star’), both T broadleaf plants, 1,3, and 5 days after treatment (DAT). Diclofop-methyl accounted for 94% of the radioactivity washed from the leaf surfaces of all species. Plant extracts contained diclofop-methyl, diclofop, and water-soluble conjugates. Barnyardgrass (S), proso millet (MS), and soybean (T) had 75, 68, and 66% of the extracted radioactivity as diclofop and diclofop-methyl. Longspine sandbur (T) and cucumber (T) had 71 and 84% of the extracted radioactivity as water-soluble conjugates. Acid hydrolysis of the water-soluble conjugates yielded 84 and 71% diclofop in barnyardgrass (S) and proso millet (MS). Cucumber (T), soybean (T), and longspine sandbur (T) had 43, 23, and 25% ring-OH diclofop. Alkaline hydrolysis of the non-extracted plant residue yielded diclofop as the major component in all species.

Field experiments were conducted to determine the effect of DPX-PE350 on the efficacy of fluazifop-P, sethoxydim, and quizalofop. DPX-PE350 severely antagonized the activity of fluazifop-P on large crabgrass. Antagonism of sethoxydim and quizalofop was dependent on rate of graminicide and large crabgrass stage of growth at application. DPX-PE350 did not reduce control of johnsongrass with quizalofop. Sethoxydim alone controlled 36 to 71% more johnsongrass than sethoxydim mixed with DPX-PE350 at 35 to 105 g ai ha-1. Antagonism of fluazifop-P activity was dependent upon the rate of DPX-PE350 and fluazifop-P applied. Applying DPX-PE350 3 d after fluazifop-P or sethoxydim reduced antagonism. Grass species, growth stage, rate, and sequential application timing all appear to play a role in the severity of antagonism.

Greenhouse and laboratory research was conducted to determine the antagonistic effects of various tank mixtures on BAS 625 efficacy. Bensulfuron at 60 g ai ha−1 and BAS 635 at 40 g ai ha−1 did not antagonize control of Echinochloa crus-galli or Brachiaria platyphylla by BAS 625 at 30 g ai ha−1 in greenhouse experiments. Tank mixtures of BAS 625 with 1,000 g ai ha−1 bentazon reduced BAS 625 control of E. crus-galli from 100 to 40%. Antagonism of BAS 625 activity by bentazon or chlorimuron at 10 g ai ha−1 was similar with B. platyphylla, reducing control from 100 to 28 and 32%, respectively. Addition of 5% (v/v) ethanol eliminated all antagonism with any of the herbicides used with either weed species. Uptake and translocation of 14C-BAS 625 1 and 12 h after treatment was not enhanced, either alone or in tank mixtures, with the addition of ethanol. Uptake of 14C-BAS 625 1 and 12 h after treatment was lower in both species when tank-mixed with bentazon. There was no effect of any of the antagonizing herbicides or ethanol on the metabolic degradation of the BAS 625 that was taken up by the plant. The herbicide concentration for 50% inhibition of activity (I50) for BAS 625 on Triticum aestivum acetyl coenzyme A carboxylase (ACCase) was 125 µM. Bentazon, BAS 635, and NC-311 at 1 mM each did not alter the inhibition on ACCase by BAS 625. BAS 635, NC-311, and bentazon at 1 mM inhibited the activity of ACCase 12, 16, and 29%, respectively. Our results indicate that antagonism of the weed control activity of BAS 625 by bentazon may be partly caused by reduced uptake. Other mechanisms may be involved to explain the antagonism of BAS 625 by bentazon and the sulfonylurea herbicides used in this study.

Sorghum genes responsive to purple witchweed parasitism were isolated, and their expression was analyzed. Using the suppression-subtractive hybridization strategy, 30 genes that were up-regulated in response to purple witchweed parasitism were isolated from the roots of a susceptible sorghum cultivar ‘Abu 70’. The changes in the expression of each gene were investigated in the roots and leaves of the sorghum cultivars ‘Wad Ahmed’ and ‘Tabat’ were parasitized by purple witchweed. Tabat is more susceptible and Wad Ahmed is less susceptible to purple witchweed than Abu 70. Further, the changes in the gene expression and host susceptibility to purple witchweed were studied in the roots of the three sorghum cultivars after treatment with salicylic acid (SA) or methyl jasmonate (MeJA). Purple witchweed parasitism induced jasmonic acid (JA)-responsive genes and suppressed SA-responsive genes in the roots of the cultivars Abu 70 and Tabat. In contrast, purple witchweed parasitism in the less-susceptible cultivar Wad Ahmed induced SA-responsive genes and induced JA-responsive genes to a small extent. SA, but not MeJA, decreased the susceptibility of all the sorghum cultivars to purple witchweed. Systemic expression of a few genes was observed in the leaves of the purple witchweed–parasitized sorghum cultivars.

Research was conducted to establish a method to investigate the resistance level of flixweed to tribenuron-methyl and the evolved biochemical resistance mechanism. Four resistant biotypes were collected from wheat fields in Mazhuangcun, Jiacun, Dishangcun, and Bafangcun in the Hebei province of China where tribenuron-methyl had been continuously used for more than 10 yr. Two susceptible biotypes were collected from wheat fields where tribenuron-methyl was never applied. Different biotypes were assessed by petri-dish bioassay, whole-plant bioassay, and acetolactate synthase (ALS) assay. Comparisons of data indicated a similarity between methods and that experiments demonstrated that petri-dish bioassay was a feasible method to identify flixweed resistant to tribenuron-methyl. Data indicated differences among the flixweed biotypes when assessed by the petri-dish bioassay, whole-plant bioassay, or ALS enzyme assay, and a close association was obtained for the three bioassay methods. ALS resistance varied by biotypes with Mazhuangcun > Jiacun > Dishangcun > Bafangcun. Target-site enzyme assay data indicated that the resistant biotype's enhanced ALS activity was the biochemical mechanism that induced flixweed's evolved resistance to tribenuron-methyl. The concentrations of tribenuron-methyl causing 50% inhibition of ALS activity of the four resistant biotypes were 1,359, 513, 184, and 164 nM; in the susceptible biotypes these concentrations were 64 and 65 nM. Resistance indexes were 21, 8, 3, and 3 for Mazhuangcun, Jiacun, Dishangcun, and Bafangcun biotypes, respectively.

Efficacy of EPTC (S-ethyl dipropylcarbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) applied with dichlormid (N,N-diallyl-2,2-dichloroacetamide), and the former three herbicides applied with dichlormid and dietholate (O,O-diethyl-O-phenol phosphorothioate) for wild proso millet (Panicum miliaceum L. ssp. ruderale (Kitagawa) Tzevelev. # PANMI) control in corn (Zea mays L.) was evaluated in fields previously treated with EPTC + dichlormid or EPTC + dichlormid + dietholate. Cycloate + dichlormid usually controlled millet regardless of prior herbicide use. But EPTC + dichlormid was less effective than on soils not previously treated with the herbicides, suggesting enhanced soil biodegradation. The use of EPTC + dichlormid with row cultivation or applications of selected herbicides improved millet control. Following prior use of EPTC + dichlormid, wild proso millet control was improved by the addition of dietholate to EPTC + dichlormid and vernolate + dichlormid but not butylate + dichlormid. In dissipation studies using soils previously treated with EPTC + dichlormid, the relative order of herbicide persistence was EPTC + dichlormid < vernolate + dichlormid < butylate + dichlormid < EPTC + dichlormid + dietholate < vernolate + dichlormid + dietholate = butylate + dichlormid + dietholate < cycloate + dichlormid. When EPTC + dichlormid + dietholate was applied to soils previously treated with that mixture, wild proso millet control was again reduced. Dissipation studies confirmed that dietholate slowed biodegradation of EPTC in soils previously treated with EPTC + dichlormid but not with EPTC + dichlormid + dietholate.

Previous research revealed that resistance to cloransulam in at least one population of common ragweed was conferred by an altered herbicide target site, specifically, by a tryptophan-to-leucine amino acid substitution at position 574 (W574L) of acetolactate synthase (ALS). In this study, 22 common ragweed populations, several of which were suspected cloransulam resistant, were assayed to determine if the W574L ALS substitution was correlated with resistance to ALS inhibitors. From each population, 16 greenhouse-grown plants were treated with cloransulam, and another 16 were treated with imazamox. Plant dry weights were recorded 20 d after treatment and individual plants were considered resistant if their dry weight exceeded 50% of that of nonherbicide-treated controls. For each herbicide-treated plant, allele-specific primers were used in polymerase chain reactions to determine whether the ALS alleles contained leucine or tryptophan codons at position 574. Of the 352 plants treated with cloransulam, 70 were determined to be resistant, and all but two contained one or more Leu574 alleles. The frequency of imazamox resistance was much higher than that of cloransulam in the populations, with 149 of 352 plants identified as imazamox resistant. However, only about half (80) of the imazamox-resistant plants contained one or more Leu574 alleles. Correlation of imazamox resistance and Leu574 alleles was population dependent. ALS activity assays confirmed that imazamox resistance in plants from at least one population was due to an altered target site, even though plants from that population did not have a W574L substitution. These results lead to the conclusion that a Leu574 allele is the predominant basis for cloransulam resistance in common ragweed; however, other mechanisms of resistance to ALS inhibitors exist in some populations.

The combination of Aphthona spp. flea beetles and herbicides can increase leafy spurge control when compared with either method used alone, but the effect of herbicides on the biocontrol agent and conversely the effect of Aphthona on herbicides within the plant are unknown. Understanding this interaction will allow for maximum weed control with little or no negative effect on the establishment and reproduction of the biological agent. Fall application of picloram plus 2,4-D had a minimal negative effect on the A. nigriscutis population and no effect on overwintering fitness. The number of A. nigriscutis adults collected from soil cores taken from the field was similar regardless of the herbicide application date, although emergence was variable. Leafy spurge soluble and insoluble carbohydrate and soluble protein concentrations, which are indirect measurements of the herbicide's effect on nutrient availability to the insect, were similar in treated and untreated plants. Aphthona nigriscutis larval feeding did not affect the absorption or translocation of 14C–2,4-D or 14C-picloram alone or in combination in leafy spurge. The observed increase in leafy spurge control due to the combined treatment was likely an additive effect of herbicide toxicity to root tissue and A. nigriscutis larval feeding on root buds.

Field experiments were conducted in Punjab, India, in 2011 and 2012 to study the integrated effect of planting pattern [uniform rows (20-cm spacing) and paired rows (15-, 25-, and 15-cm spacing)], cultivars (PR-115 and IET-21214), and weed control treatments (nontreated control, pendimethalin 750 g ai ha−1, bispyribac-sodium 25 g ai ha−1, and pendimethalin 750 g ha−1 followed by bispyribac-sodium 25 g ha−1) on weed suppression and rice grain yield in dry-seeded rice. In the nontreated control, IET-21214 had higher grain yield than PR-115 in both planting patterns. However, such differences were not observed within the herbicide treatment. IET-21214 in paired rows, even in nontreated control, provided grain yield (4.7 t ha−1) similar to that in uniform rows coupled with the sole application of pendimethalin (4.3 t ha−1) and bispyribac-sodium (5.0 t ha−1). In uniform rows, sequential application of pendimethalin (PRE) and bispyribac-sodium (POST) provided the highest grain yield among all the weed control treatments and this treatment produced grain yield of 5.9 and 6.1 t ha−1 for PR-115 and IET-21214, respectively. Similarly, in paired rows, PR-115 in paired rows treated with sequential application of pendimethalin and bispyribac-sodium had highest grain yield (6.1 t ha−1) among all the weed control treatments. However, IET-21214 with the sole application of bispyribac-sodium produced grain yield similar to the sequential application of pendimethalin and bispyribac-sodium. At 30 days after sowing, PR-115 in paired rows coupled with pendimethalin application accrued weed biomass (10.7 g m−2) similar to the sequential application of pendimethalin and bispyribac-sodium coupled with uniform rows (8.1 g m−2). Similarly, IET-21214 with bispyribac-sodium application provided weed control similar to the sequential application of pendimethalin and bispyribac-sodium. Our study implied that grain yield of some cultivars could be improved by exploring their competitiveness through paired-row planting patterns with less use of herbicides.

Membranes define the outer boundary of the living protoplast and the internal compartmentation of plant cells. From a structural point of view, membranes consist of a lipid bilayer and proteins essential for functions such as solute transport, signal transduction, and numerous metabolic reactions. While membranes can represent a significant barrier to the free movement of many solutes, those with sufficient lipid solubility may move across membranes by dissolving into the lipid bilayer. However, selective membrane transport is generally observed for hydrophilic solutes such as mineral nutrients and cell metabolites. Such selective transport requires an input of metabolic energy, and in plants this occurs via the production of proton electrochemical gradients across the membrane by substrate- (ex. ATP, PPi) driven proton pumps. Selective solute transport is then mediated by membrane-associated secondary transport systems which utilize the proton electrochemical gradient to drive the transport process. This review of membrane structure and transport system function provides a background for a further examination of herbicide interactions with plant membranes.

Following root application of a selective rate of 3-amino-2,5-dichlorobenzoic acid-C14 (amiben-C14), the translocation of radioactivity from roots to shoots was greater in pigweed (Amaranthus retroflexus L.), a susceptible plant, than in soybeans, a resistant one. Most of the radioactivity extracted from soybean and pigweed plants was present as conjugates of amiben-C14. The conjugate extracted from soybean was chromatographically identical with an N-glycoside prepared from glucose and amiben. In soybean plants, 3-amino-2,5-dichlorobenzoic acid also was converted to N-glycosylamiben.

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.

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.

The sensitivities of chlorophyll fluorescence and fresh weight as bioassay techniques for the determination of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] concentrations in soil were compared. The ratio of the initial inflection point (I) to the variable fluorescence maximum (P) of intact oat (Avena sativa L. ‘Chilocco’) leaves was determined for plants seeded directly into herbicide-treated soil and for those transplanted into treated soil after 14 days of growth in nutrient solution. Using the chlorophyll fluorescence of transplanted oats bioassay, 0-, 0.13-, 0.25-, and 0.50-ppm concentrations could be distinguished from one another within 8 h for metribuzin, within 24 h for diuron, and within 48 h for atrazine. These distinctions between rates could not be made 17 days after seeding into treated soil when using fresh weight as the bioassay indicator. Chlorophyll fluorescence of oats seeded directly into treated soil was also a reliable technique, but required much more time to attain sufficient plant size for convenient chlorophyll fluorescence determinations.

The interaction of cultivation and photoperiod on the initiation of Canada thistle rosettes, and the effect of growth stage on control of Canada thistle with glyphosate were determined in a series of field experiments. Under the natural photoperiod occurring in the southern Canadian Prairies, rosettes of Canada thistle were initiated by cultivation to remove the shoot growth during the last week of July. Regrowth remained as rosettes without any stem elongation and formed dense clusters of leaves. Application at the “August rosette stage” improved the effectiveness of the glyphosate treatment and resulted in consistent control of Canada thistle with less than half as much herbicide as recommended for control when applied at the bud-stage. The number of shoots of Canada thistle on plots treated with glyphosate was less than on the intensively summer-fallowed check plot. Check plots received 5 cultivations during the summer-fallow season. One year after application of glyphosate at the rosette stage, the reduction in shoot density was 99% compared to the summer-fallowed check. By year three, without glyphosate applications in years two and three, the benefits of applying glyphosate at the rosette stage rather than the bud-stage were very evident. When applied at the bud-stage the shoot density on plots treated with glyphosate at 2.25 kg ha−1 was 24 m−2 compared to only 10 shoots m−2 when applied at the rosette stage, (35% vs 72% control). Glyphosate at 0.9 kg ha−1 or less than half of the rate recommended for application at the bud-stage, applied to Canada thistle in the rosette stage, resulted in consistently fewer shoots 2 and 3 yr after treatment. Physical removal of shoot top growth, simulating an in-crop herbicide treatment, improved the consistency of control. Two years after application of 0.9 kg ha−1 of glyphosate at the rosette stage, the control of Canada thistle was 98% compared to the summer-fallowed checks.

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 number of vascular bundles in the rhizomes of different ecotypes of johnsongrass (Sorghum halepense (L.) Pers.) varied from 71 to 154. The size of vascular bundles varied from 60 to 150 μ in an ecotype from California to 100 to 230 μ in an ecotype from Georgia. The size of xylem cells in these vascular bundles also varied widely, but the size of phloem cells was more constant. The number of vascular bundles in culms ranged from 43 to 123. The average number of vascular bundles in individual leaves varied from 46 to 158 for different ecotypes. The number of stomata on leaf surfaces deviated from 63 to 148/sq mm for 10 different ecotypes. An average of 95 stomata/sq mm occurred on the upper surface of johnsongrass leaves while an average of 113 stomata occurred on lower surfaces. The arrangement and size of stomata varied in different johnsongrass ecotypes.

A population of oriental mustard from Port Broughton in South Australia was reported as not being controlled by 2,4-D. Dose response experiments determined this population was resistant to both 2,4-D and MCPA, requiring greater than 20 times more herbicide for equivalent control compared to a known susceptible population (from Roseworthy, South Australia) and a population resistant only to the acetohydroxyacid synthase (AHAS)-inhibiting herbicides (from Tumby Bay, South Australia). The Port Broughton population was also found to be resistant to three chemical groups that inhibit AHAS; however, the level of resistance was lower than the known acetolactate synthase–resistant population from Tumby Bay. Herbicides from other modes of action were able to control the Port Broughton population. Assays of isolated AHAS from the Port Broughton population showed high levels of resistance to the sulfonylurea and sulfonamide herbicide groups, but not to the imidazolinone herbicides. A single nucleotide change in the AHAS gene that predicted a Pro to Ser substitution at position 197 in the protein was identified in the Port Broughton population. This population of oriental mustard has evolved multiple resistance to AHAS-inhibiting herbicides (AHAS inhibitors) and auxinic herbicides, through a mutation in AHAS and a second nontarget-site mechanism. Whether the same mechanism provides resistance to both AHAS inhibitors and auxinic herbicides remains to be determined. Multiple resistance to auxinic herbicides and AHAS inhibitors in the Port Broughton population will make control of this population more difficult.