The importance of seeds in the reproduction and maintenance of yellow nutsedge (Cyperus esculentus L. # CYPES) populations was evaluated. Isozyme analysis using starch gel electrophoresis was performed on 20 individuals of each of 10 widely separate populations in California. Genetic variation among individuals served as an indicator of the relative importance of asexual and sexual reproduction in each population. Eight enzyme systems were assayed from which 12 loci were resolvable, with four of those loci exhibiting variability. A maximum of four isozyme genotypes appeared in any population; only nine of 81 total potential genotypes were identified. Five populations were isozymically uniform, apparently composed of a single genotype. The remaining five populations were genotypically variable; these frequently deviated strongly from the genotype frequencies expected of a sexually reproducing population. These results indicate that sexually produced seeds are unimportant in the maintenance of yellow nutsedge populations in agricultural environments. Although viable seed may be produced, tubers appear to be the primary mode of reproduction in this species.

The effects of herbicide concentration, adjuvants, and temperature on the phytotoxicity of CGA-82725 {2-propynyl 2-[4-[3,5-dichloro-2-pyridinyl)oxy] phenoxy] propionate}, RO 13-8895 {acetone-O-[D-2-[p-(α,α,α-trifluoro-p-tolyl)-oxy] phenoxy] propionyl] oxime}, and BAS 9052 OH {2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} to large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] were evaluated. The addition of adjuvants increased the toxicity of BAS 9052 OH at herbicide rates below 0.14 kg ai/ha, whereas the adjuvants had little effect on the activity of CGA-82725 or RO 13-8895. There were no differences among the adjuvants in enhancing grass control with any of the herbicides. Control of large crabgrass with CGA-82725 or BAS 9052 OH was not affected by temperature at the time of spraying; RO 13-8895 was slightly more toxic at 16 C than at 32 C. Soybean [Glycine max (L.) Merr.] had good tolerance to the three herbicides at rates 3 to 10 times those recommended; no differences in tolerance were observed among seven soybean cultivars. Stage of growth and the addition of an emulsifiable oil adjuvant did not affect soybean tolerance to BAS 9052 OH.

Light after treatment was found to be required for the herbicidal activity of nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether) and oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] following postemergence applications of both herbicides to greenbean (Phaseolus vulgaris L. ‘Spartan Arrow’). The effect of photosynthate and/or light before treatment was investigated. A 24-hr dark period prior to postemergence treatment of the bean seedlings had no effect on herbicidal activity when light followed treatment. The effect of plant pigments on the activity of both herbicides was studied using mutants of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. A white mutant of corn was much more resistant to the herbicides than a greenish-yellow mutant or the normal plant. A yellow mutant of soybean was equally as susceptible as the normal type. Studies with radiolabeled foliar applications of both herbicides showed no significant difference in translocation in the light or dark.

Loss in activity of foliar-applied methyl ester of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} occurred under low soil moisture conditions. A loss in control of yellow foxtail [Setaria lutescens (Weigel) Hubb.], wild oats (Avena fatua L.), little-seed canarygrass (Phalaris minor Retz.), and barnyardgrass [Echinochloa crus-galli (L.) Beauv.], was observed under greenhouse and growth chamber conditions. When soil was maintained at 2 to 3% above wilting point as compared to near field capacity, herbicide activity was decreased by 15 to 50%. High soil moisture (at or above 67% of field capacity) for at least 2 to 4 days following treatment was needed to achieve maximum effectiveness of the herbicide. Daily furrow irrigations for a period of 10 days following treatment of barnyardgrass in the field resulted in highest activity as compared to that under single irrigation regimes within the 10-day period. The effect of low soil moisture was minimized by increased rates of herbicide application. Hoe-29152 {methyl-2-[4-(4-trifluoromethylphenoxy)phenoxy] propanoate} showed similar losses in activity associated with low soil moisture. No consistent changes in uptake or translocation of 14C-labeled diclofop could be detected in association with altered soil moisture status.

Glyphosate [N-(phosphonomethyl)glycine] toxicity to bermudagrass [Cynodon dactylon (L.) Pers.] increased as 0.84 kg/ha of the herbicide was applied in decreasing volumes of diluent from 374 to 47 L/ha. Visible injury to bermudagrass varied from 18 to 91% depending upon the spray solution formulation and diluent volume. The addition of surfactant to glyphosate (MON 0139) formulated without surfactant increased bermudagrass injury at each diluent volume tested. Glyphosate commercially formulated with surfactant (MON 2139) was the most toxic of the three formulations to bermudagrass. Regrowth of bermudagrass clipped at the soil surface 48 h after treatment ranged from 10% of the untreated control with MON 2139 at 47 L/ha to 100% of the control with MON 0139 at 374 L/ha. The MON 2139 formulation showed as much as a six-fold increase in foliage regrowth when the diluent volume was increased from 47 to 374 L/ha.

Glyphosate-resistant rigid ryegrass has been identified in California, but research has yet to elucidate the resistance mechanism. The objectives of this study were to examine the differences between sensitive and resistant rigid ryegrass in absorption and distribution of glyphosate, in vivo and in vitro absorption by chloroplasts, and shikimic acid accumulation after glyphosate treatment. Foliar absorption and distribution of 14C-glyphosate did not differ 1 to 3 d after treatment (DAT) between the susceptible (S) and resistant (R) biotypes. Absorption of 14C-glyphosate by isolated chloroplasts also did not differ between the S and R biotypes. After foliar application of 14C-glyphosate, chloroplasts were isolated from treated leaves from both biotypes. Accumulation of 14C-glyphosate in the chloroplasts did not differ between the two biotypes. Shikimic acid level increased significantly in the S biotype after treatment with glyphosate at 2.24 kg ai ha−1 to levels 10-fold greater than in the R biotype 11 DAT. Shikimic acid in the germination media at 2 to 5 mM did not affect seed germination of S and R biotypes but drastically decreased the length of coleoptiles of both at 5 DAT. Thus, biotype differences in sensitivity or metabolism of shikimic acid do not explain differences in sensitivity to glyphosate.

Transplanted tomatoes (Lycopersicon esculentum Mill. ‘Springset’) kept weed free for 36 days after transplanting, or weeded from the 24th day after transplanting, gave yields equal to those that had been kept weed free throughout the growing season. Conversely, when weeds were allowed to remain in the crop for more than 24 days after transplanting, yields were progressively reduced. Therefore, the ‘critical period’ of weed interference in tomatoes was between 24 and 36 days after transplanting. The yield of tomatoes was reduced significantly when only 5% of the weeds were allowed to remain in the plots longer than 21 days after transplanting. Weed interference appeared to have little, if any, effect on tomato juice quality as measured by color, consistency, and flavor.

In Arkansas isopropyl N–(3–chlorophenyl)carbamate (CIPC) is recommended for controlling barnyardgrass (Echinochloa crusgalli (L.) Beauv.) in rice. Method and depth of seeding, time of application, and water management after treatment are critical when using CIPC to control barnyardgrass in rice. Rice is very susceptible to CIPC when roots of the seedlings contact the herbicide. A herbicide that is more selective and less sensitive to management and environment is needed to replace CIPC.

Modern high-throughput molecular and analytical tools offer exciting opportunities to gain a mechanistic understanding of unique traits of weeds. During the past decade, tremendous progress has been made within the weed science discipline using genomic techniques to gain deeper insights into weedy traits such as invasiveness, hybridization, and herbicide resistance. Though the adoption of newer “omics” techniques such as proteomics, metabolomics, and physionomics has been slow, applications of these omics platforms to study plants, especially agriculturally important crops and weeds, have been increasing over the years. In weed science, these platforms are now used more frequently to understand mechanisms of herbicide resistance, weed resistance evolution, and crop–weed interactions. Use of these techniques could help weed scientists to further reduce the knowledge gaps in understanding weedy traits. Although these techniques can provide robust insights about the molecular functioning of plants, employing a single omics platform can rarely elucidate the gene-level regulation and the associated real-time expression of weedy traits due to the complex and overlapping nature of biological interactions. Therefore, it is desirable to integrate the different omics technologies to give a better understanding of molecular functioning of biological systems. This multidimensional integrated approach can therefore offer new avenues for better understanding of questions of interest to weed scientists. This review offers a retrospective and prospective examination of omics platforms employed to investigate weed physiology and novel approaches and new technologies that can provide holistic and knowledge-based weed management strategies for future.

Persistence of 1.12, 2.24, and 3.36 kg/ha of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied preemergence to corn (Zea mays L. ‘Funk's 4761′) was determined in three soils, each providing a range of pH from 5 to 7. Samples of surface soil from each plot were collected at intervals after atrazine application until there was no evidence of phytotoxicity to oat (Avena sativa L. ‘Florida 501′) in bioassay. Persistence (Y50) was defined as the number of days following atrazine application until bioassay indicated 50% relative growth of oats (no atrazine = 100% growth). This period ranged from 24 to 178 days, depending upon soil type, pH, and atrazine rate. Persistence of atrazine increased with increasing soil pH. The effect of soil pH was essentially the same at each atrazine rate and did not vary appreciably with years in a given soil. In McLaurin sandy loam, atrazine persisted 8 to 9 days longer with each unit increase in soil pH. In Hartsells fine sandy loam and Decatur silt loam, atrazine persistence increased 9 to 13 days and 29 days, respectively, with each unit pH.

Effects of three diphenylether herbicides [2,4-dichlorophenyl-p-nitrophenyl ether (nitrofen); 2,4,6-trichlorophenyl-4′-nitrophenyl ether (hereinafter referred to as MC-1478); and 2,4′-dinitro-4-trifluoromethyl-diphenylether (hereinafter referred to as C-6989)] were measured on phosphorylation and electron transport in spinach (Spinacia oleracea L.) chloroplasts, and mung bean (Phaseolus aureus L., var. Jumbo) and white potato tuber (Solarium tuberosum L.) mitochondria. All of the diphenylethers acted primarily as inhibitors of chloroplast noncyclic electron transport, and the coupled photophosphorylation. The compounds ranked in the following decreasing order of inhibitory effectiveness: MC-1478 ≥ C-6989 >> nitrofen. A site of action close to light reaction II was suggested. At high molar concentrations, marginal interference with cyclic electron transport or phosphorylation was obtained. In mitochondria, MC-1478 and nitrofen acted primarily as electron transport inhibitors with malate, NADH, and succinate as substrates. MC-1478 was a slightly stronger inhibitor than nitrofen. Only slight stimulation of ADP-limited oxygen uptake was obtained during the oxidation of NADH and succinate; whereas, strong inhibition of oxygen uptake was obtained with malate. C-6989 also weakly stimulated ADP-limited oxygen uptake with NADH and succinate but differed from the two chlorinated diphenylethers in that electron transport was not inhibited when ADP was present in excess. Interference with ATP generation could be one of the mechanisms through which the phytotoxicity of diphenylether herbicides is expressed.

Twelve populations of eastern black nightshade from different locations in Ontario are resistant to imazethapyr. This study aimed at determining the molecular basis of resistance in these populations and the activity of the resistant acetohydroxyacid synthase (AHAS) enzyme compared to that of the sensitive AHAS in response to different herbicides and branched-chain amino acid concentration. The results of partial AHAS sequencing indicated that all resistant populations had a cytosine331 to thymine substitution coding for an alanine205 to valine substitution. In vitro AHAS enzyme assays of one resistant population showed that the specific activity of the resistant enzyme was 56% less than that of the susceptible enzyme. AHAS from the resistant population was 72-, 70-, and 64-fold less sensitive than that of the susceptible population to imazethapyr, imazamox, and primisulfuron, respectively. Furthermore, the resistant enzyme was less sensitive to feedback inhibition from branched-chain amino acids compared to the susceptible enzyme. Results confirmed that resistance in resistant populations of eastern black nightshade was conferred by target-site modification and that the Ala205Val substitution alters the kinetics and regulation of branched-chain amino acid biosynthesis.

In pot culture experiments in the greenhouse, rice [Oryza sativa L. ‘Nova 66’], red rice [O. sativa L.], barnyardgrass [Echinochola crus-galli (L.) Beauv.], broadleaf signal-grass [Brachiaria platyphylla (Griseb.) Nash], northern jointvetch [Aeschynomene virginica (L.) B.S.P.], and hemp sesbania [Sesbania exaltata (Raf.) Cory] were seeded 1.3, 2.5, 5.1, and 10.2 cm deep in Crowley silt loam which was held at field capacity (24% moisture), saturation (34% moisture), or submerged 1.3 or 5.1 cm. At field capacity and saturation all species seeded 1.3, 2.5, and 5.1 cm deep emerged and grew well; seeding the plants 10.2 cm deep reduced emergence and growth. Submergence of the soil controlled all weeds effectively but reduced emergence and growth of dry-seeded rice.

Seed of corn (Zea mays L., var. Texas 28) and cotton (Gossypium hirsutum L., var. Deltapine smooth leaf) were planted in sand and subsequently drenched with solutions of α,α,α trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin). The levels of herbicide used inhibited growth of roots and shoots in both crops, but cotton proved more tolerant than corn. The roots failed to elongate normally, but the lateral growth that continued gave the radicle a “club-shaped” appearance, especially in corn. Stained sections of the roots were mounted as permanent slides for study. The cells at the extreme tip of the treated roots were small and dense; many were multinucleate. Immediately behind this region, the cells were abnormally large, thin-walled, and aberrant. A disorganized type of nuclear division was evident. Cell plate and cell wall formation were apparently rare. Although we found no normal sequence of mitosis in cells of treated roots, all stages of mitosis were seen in the untreated roots.

The Persistence of substituted dinitroaniline herbicides in soil varied widely. First-order kinetics were used to describe the dissipation rates. Half-lives of the herbicides in moist soil ranged from 29 to 124 days in two soil types under greenhouse conditions. Flooding the soil significantly increased the dissipation rate of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], and pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine]. Dissipation rates of dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) and butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine] were affected to a lesser extent by flooding. Volatilization of trifluralin, fluchloralin, and ethalfluralin [N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine] from Bosket sandy loam was reduced by flooding when compared to volatilization from the same soil with a moisture content equivalent to field capacity. Pendimethalin exhibited low volatility. The effect of flooding on dissipation rates is apparently not due to increased volatilization.

Velvetleaf (Abutilon theophrasti Medik. # ABUTH) is difficult to control with existing weed control strategies. Some measure of control can be obtained with a fungus, Colletotrichum coccodes (Wallr.) Hughes, used as a bioherbicide, but when the bioherbicide application was combined with the plant growth regulator thidiazuron (N-phenyl-N′-1,2,3-thidiazol-5-yl-urea), weed control was substantially improved. Thidiazuron alone interfered with normal development of velvetleaf, causing stunting and initiation of axillary bud growth. Tank mix applications of C. coccodes and thidiazuron acted synergistically to increase velvetleaf mortality. When C. coccodes was applied 10 days after thidiazuron as a split application, weed control was less than for tank mix applications. However, two applications of C. coccodes plus thidiazuron were more effective than equivalent single tank mix applications. Under laboratory conditions, high concentrations of thidiazuron (2× field application rates) inhibited growth of C. coccodes, but at field application rates thidiazuron did not reduce disease development. Combinations of thidiazuron and C. coccodes may provide effective control of velvetleaf in the field.

Laboratory and greenhouse studies were conducted on redvine and trumpetcreeper to characterize leaf surface and wax composition, determine responses of these weeds to glyphosate, characterize the nature of interactions between glyphosate and several selective postemergence herbicides (e.g., acifluorfen, bentazon, chlorimuron, imazaquin, and pyrithiobac) used in soybean and cotton, and determine the effects of various adjuvants on glyphosate activity on both species. Trumpetcreeper was consistently more susceptible to glyphosate than redvine. Glyphosate spray solution droplets had lower contact angle in trumpetcreeper than in redvine. Micro-roughness of the trumpetcreeper adaxial leaf surface was greater due to trichomes and glands compared to that of redvine, which had no trichomes or glands. Stomata or crystal wax deposition on the adaxial leaf surface were not observed in either species. The wax mass per unit area (22 to 37 µg cm−2) was similar regardless of the leaf age in both species. Epicuticular wax consisted of hydrocarbons, alcohols, acids, and triterpenes. Wax composition of young leaves of redvine was relatively hydrophilic (72% alcohols and acids, 24% hydrocarbons) compared to the hydrophobic components (23% alcohols and acids, 49% hydrocarbons) of old leaves. In contrast, wax of trumpetcreeper young leaves was relatively hydrophobic (9% alcohols and acids, 29% hydrocarbons), whereas old leaves had similar levels of hydrophilic and hydrophobic components (28% alcohols and acids, 31% hydrocarbons). Glyphosate mixed with selective postemergence herbicides were antagonistic when applied to redvine and trumpetcreeper, except acifluorfen. Various adjuvants did not increase glyphosate efficacy except ammonium sulfate, which increased glyphosate efficacy when applied alone to trumpetcreeper. These results showed that lower glyphosate efficacy was related to the more hydrophobic nature of redvine epicuticular waxes compared to that of trumpetcreeper.

Field studies were conducted in 1986 and 1987 to determine the critical period for weed control in white bean grown in Ontario. The treatments consisted of either allowing weeds to infest the crop for increasing durations after planting or maintaining plots weed free for increasing durations after planting. The beginning of the critical period was defined as the crop stage by which weed interference reduced yields by 3%. Similarly, the end of the critical period was defined as the crop stage to which the crop had to be weed free to prevent a 3% yield loss. The critical period of weed control occurred between the second-trifoliolate and first-flower stages of growth for all cultivars and years, with the exception of the cultivar ‘OAC Seaforth’ in 1986. The average number of pods per plant for both cultivars was reduced by increasing durations of weed interference after planting in both years. However, pod number of the cultivar OAC Seaforth was reduced at a greater rate in 1986 than ‘Ex Rico 23’. The beginning of the critical period corresponded with the beginning of a rapid increase in total weed biomass.

Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] applied preemergence resulted in increased phytotoxicity with increasing soil pH. In the field, fall panicum (Panicum dichotomiflorum Michx.) control, number of dead corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] plants, and crop injury ratings increased, and plant height and grain yield decreased as the soil pH increased. In the greenhouse, corn and soybean grown in soil showed decreased dry weights due to metribuzin as pH increased. By contrast, the pH response to metribuzin was not evident in nutrient-sand culture, indicating that, in soil, pH affected the availability of metribuzin for plant absorption.

The adsorption of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was measured on soil at water:soil ratio of 5:1 and 0.4:1. The adsorption isotherms for atrazine were described by the Freundlich equation. A decrease in water:soil ratio and in soil moisture content led to an increased adsorption of the herbicide. Increasing the concentration of electrolytes in the soil solution by adding solutions up to 0.1M CaCl2 increased atrazine adsorption. Greater amounts of atrazine were adsorbed at 30 than at 5 C on four soils at 0.1 bar moisture content. This indicates an endothermic reaction which was observed both before and after correction for differential atrazine solubility due to temperature. Thermodynamic quantities associated with the adsorption reaction were calculated to characterize the adsorption of atrazine on soil.