The relative phytotoxicities of 12 substituted dinitroaniline herbicides to soybeans [Glycine max (L.) Merr ‘Corsoy’], velvetleaf (Abutilon theophrasti Medic.), and either giant foxtail (Setaria faberi Herrm.) or foxtail millet [Setaria italica (L.) Beauv.] were compared under greenhouse and laboratory conditions. In these studies, dinitramine (N 4,N 4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) was most toxic to each species. Dinitramine, trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), and BAS-3921 H [N-propyl-N-(2-chloroethyl)-α,α,α-trifluoro-2,6-dinitro-p-toluidine] were most inhibitory of soybean shoot growth, while oryzalin (3,5-dinitro-N 4,N 4-dipropyl-sulfanilamide), dinitramine, and BAS-3921 H were most inhibitory of root growth. Similarly, dinitramine, oryzalin, nitralin [4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline], and BAS-3921 H inhibited velvetleaf shoot growth; and oryzalin, dinitramine, chlornidine [2,6-dinitro-N,N-di(2-dichloroethyl)-p-toluidine], nitralin, and GS-39985 (N-n-propyl-N-tetrahydrofurfuryl-4-trifluoromethyl-2,6-dinitroaniline) reduced root growth. All of the herbicides except oryzalin and nitralin inhibited foxtail millet shoot growth, while all of the herbicides reduced root growth.

The persistence of five herbicides in six soils across Nebraska can be ranked from greatest to least as follows: 5-bromo-3-isopropyl-6-methyluracil (isocil) at 5 and 25 1b/A, 2-chloro-4,6-bis-(isopropylamino)-s-triazine (propazine) at 3 and 9 1b/A, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) at 3 and 9 1b/A, trichlorobenzyl chloride (hereinafter referred to as TCBC) at 7 and 49 1b/A, and 3-(3,4-dichlorophenyl)-1-methoxyl-1-methylurea (linuron) at 3 and 9 1b/A. Soil texture differences (sandy loam, very fine sandy loam, silt loam, and silty clay loam) had a greater influence on herbicide residue carryover than did climatic differences across Nebraska during 1962 to 1968. Soil carryover of herbicide residues was greater in coarse rather than fine-textured soils and in the drier regions of western than in eastern Nebraska. Leaching of herbicides into the soil profile was an avenue of herbicide dissipation.

Herbicide treatments of tank mixtures or sequential applications were applied at different growth stages and compared with standard treatments for control of barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCM], bearded sprangletop [Leptochloa fascicularis (Lam.) Gray ♯ LEFFA], and aquatic weeds in rice (Oryza sativa L. ‘Starbonnet’). Some herbicides injured rice more than the propanil (3’,4’-dichloropropionanilide) or molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) standards, but injury for all treatments was inconsequential. Postemergence tank mixture treatments that controlled barnyardgrass (two- to four-leaf stages), bearded sprangletop (two-leaf to tillering stages), and aquatic weeds better than standard treatments were propanil plus butachlor [N-(butoxymethyl)-2-chloro-2’,6,-diethylacetanilide], oxadiazon [2-tert-butyl-4(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] thiobencarb {S-[(4-chlorophenyl)methyl]diethylcarbamothioate}, or pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine].

Buckwheat is a broadleaved annual species that is often used as a summer cover crop for its quick growth, weed suppressive ability, and ease of management. Tartary buckwheat is a species related to buckwheat, with many of the same traits valued in buckwheat as a cover crop. However, Tartary buckwheat has been reported to grow more vigorously than buckwheat, especially in cool conditions, which might fill a unique niche for vegetable farmers in Wisconsin and other northcentral states. Our research objectives were to determine the effectiveness of Tartary buckwheat relative to buckwheat for weed suppression, both during the cover-cropping phase and after cover-crop termination during cabbage production, and quantify weed suppression, soil compaction, soil nitrogen availability, and cabbage yield in no-tillage (roller-crimped or sickle-bar mowed) and conventional-tillage (rototilled) systems. Across three site-years, we found that buckwheat emerged earlier and produced 64% more shoot dry biomass than Tartary buckwheat. Pretermination weed shoot biomass (predominantly Amaranthus and Setaria spp.) in Tartary buckwheat treatments was approximately twice that of buckwheat, and did not differ from weed shoot biomass in a control fallow treatment. Cabbage yield did not differ between cover crop species nor did yield differ between conventional-tillage cover cropped and control fallow treatments. However, weed biomass was greater, and cabbage yield was reduced, in no-tillage compared to conventional-tillage treatments. We also found evidence of greater soil compaction and less nitrate–nitrogen (NO3–N) availability in no-tillage than conventional-tillage treatments. These results suggest that Tartary buckwheat is not a suitable summer cover crop alternative to buckwheat for weed suppression prior to cabbage production.

A 50-yr buried-seed study was initiated at Fairbanks, AK, in 1984. Seed of 17 weed species important in Alaskan agriculture were collected and buried 2 and 15 cm deep. Seed viability was determined initially and after recovery from the soil through a combination of germination tests and 2,3,5-triphenyltetrazolium chloride (TTC) treatments. Seed exhumed at 9 and 21 months were tested for viability and dormancy. Burial depth had a significant effect on seed longevity of corn spurry (Spergula arvensis L. # SPRAR), wild oats (Avena fatua L. # AVEFA), pineappleweed [Matricaria matricarioides (Less.) C. L. Porter # MATMT], and bluejoint reedgrass [Calamagrostis canadensis (Michx.) Nutt.]. Viability was higher for seed buried at 15 than at 2 cm. Viability of shepherdspurse [Capsella bursapastoris (L.) Medik. #CAPBP] and pineappleweed seed did not decrease significantly over the 21-month burial period. In contrast, seed viability of wild buckwheat (Polygonum convolvulus L. # POLCO), common hempnettle (Galeopsis tetrahit L. # GAETE), wild oats, quackgrass [Agropyron repens (L.) Beauv. # AGRRE], and foxtail barley (Hordeum jubatum L. # HORJU) was less than 11% at the end of 21 months. There was no significant relationship between initial seed dormancy and viability after 21 months of burial.

Integrated weed management (IWM) for agronomic and vegetable production systems utilizes all available options to effectively manage weeds. Late-season weed control measures are often needed to improve crop harvest and stop additions to the weed seed bank. Eliminating the production of viable weed seeds is one of the key IWM practices. The objective of this research was to determine how termination method and timing influence viable weed seed production of late-season weed infestations. Research was conducted in Delaware, Michigan, and New York over a 2-yr period. The weeds studied included: common lambsquarters, common ragweed, giant foxtail, jimsonweed, and velvetleaf. Three termination methods were imposed: cutting at the plant base (simulating hand hoeing), chopping (simulating mowing), and applying glyphosate. The three termination timings were flowering, immature seeds present, and mature seeds present. Following termination, plants were stored in the field in mesh bags until mid-Fall when seeds were counted and tested for viability. Termination timing influenced viable seed development; however, termination method did not. Common ragweed and giant foxtail produced viable seeds when terminated at the time of flowering. All species produced some viable seed when immature seeds were present at the time of termination. The time of viable seed formation varied based on species and site-year, ranging from plants terminated the day of flowering to 1,337 growing degree d after flowering (base 10, 0 to 57 calendar d). Viable seed production was reduced by 64 to 100% when common lambsquarters, giant foxtail, jimsonweed, and velvetleaf were terminated with immature seeds present, compared to when plants were terminated with some mature seeds present. Our results suggest that terminating common lambsquarters, common ragweed, and giant foxtail prior to flowering, and velvetleaf and jimsonweed less than 2 and 3 wk after flowering, respectively, greatly reduces weed seed bank inputs.

Recrystallized 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 3-phenyl-1,1-dimethyl-urea (fenuron), 1-n-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon), 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 5-bromo-3-isopropyl-6-methyluracil (isocil), and 5-bromo-3-sec-butyl-6-methyluracil (bromacil) were deposited in thin layers on aluminum planchets, irradiated with far, middle, and near ultraviolet light and collected in ethanol. Spectral changes in the ultraviolet region and losses were determined spectrophotometrically. The greatest changes and losses occurred with far ultraviolet and the least with near ultraviolet irradiation. Little or no spectral changes occurred in the dark, but there was some loss of each herbicide.

Acetyl-coenzyme A carboxylase (ACCase)–resistant Italian ryegrass is one of the most difficult-to-control weeds in United States wheat-production systems. Seed was collected from a suspected ACCase-resistant Italian ryegrass population in a winter wheat field with a history of ACCase-inhibitor herbicide use. This study investigated cross-resistance patterns in this Italian ryegrass population. Resistance was identified to the commercial dose of the ACCase herbicides pinoxaden, clethodim, sethoxydim, and clodinafop. Partial chloroplastic ACCase sequences revealed aspartate-to-glycine or isoleucine-to-asparagine substitutions at positions 2078 or 2041 in individuals of the resistant population. This is the first report, to our knowledge, of Asp-2078-Gly and Ile-2041-Asn substitutions in ACCase-resistant Italian ryegrass in the United States. Associating the occurrence of resistance alleles with resistance to specific active ingredients provides a better understanding of ACCase cross-resistance in Italian ryegrass and possibly options for its control.

Weeds persist and cause economic losses in agricultural systems because they exploit underused portions of that system. Reducing the effect of weeds on agroecosystems begins with minimizing the number of propagules (e.g., seeds and tubers) that are produced and returned to the soil. Purple nutsedge is a problematic weed around the globe, persisting between growing seasons as tubers in the soil. Halosulfuron is an effective herbicide for controlling purple nutsedge foliage and is used in corn and several vegetable crops. Studies were conducted to evaluate the effect of various rates of halosulfuron on purple nutsedge tuber production. Single, presprouted purple nutsedge tubers were transplanted into outdoor microplots and treated after 6 wk of growth with six rates of halosulfuron (7 to 208 g ai ha−1) POST with a nontreated control (NTC). All shoots that had emerged at the time of application were marked with plastic rings; this allowed for classification of tubers at exhumation of (1) tubers attached to shoots that had emerged by the time of application, (2) tubers attached to shoots that emerged after application, and (3) tubers without an aerial shoot during the study. Seven weeks after application, the tubers in the microplots were exhumed and tubers were classified, quantified, and their ability to sprout was evaluated. In the NTC, there were 530 total tubers, with a log-logistic regression model describing the tuber population with increasing halosulfuron rate. The rate of halosulfuron that reduced total tuber population 50% (I50) was 8 g ha−1. In the NTC, 200 tubers were attached to shoots that emerged following halosulfuron application, and this class of tubers had an I50 of 19 g ha−1. Viability of tubers with shoots that emerged following halosulfuron application was 28% at the 52 g ha−1 halosulfuron, suggesting the action of the herbicide may have rendered the tuber nonviable after new shoots were produced. The final classification of tubers was those that did not have an aerial shoot during the study. These were tubers in which apical dominance suppressed shoot development or were likely the most-recent tubers to develop. Of the three classes, the tubers without shoots were the most numerous in the NTC, with 294 tubers and an I50 of 1 g ha−1. Halosulfuron is an effective herbicide that not only controls purple nutsedge foliage but also reduces the number of new tubers produced and overall tuber viability. This could be an important component to reduce the long-term population density of the weed.

Full-season interference effects of hemp sesbania [Sesbania exaltata (Raf.) Cory. # SEBEX] on cotton (Gossypium hirsutum L. 'Stoneville 213′) were investigated on a fine sandy loam soil for 3 yr at Stoneville, MS. Experimentally induced hemp sesbania populations of ≥ 3223 plants/ha significantly reduced cotton yields. Interference of hemp sesbania at 3223, 6456, 16 114, and 32 228 plants/ha, equivalent to 1, 2, 5, and 10 plants/3.3 m, reduced seed cotton yields by 19, 25, 45, and 53%, respectively. Cotton boll weights were reduced by 9% when hemp sesbania infestations were ≥ 16 114 plants/ha. Cotton plant density, seedling vigor, and initial cotton white blooms/ha at ≤75 days after planting (DAP) did not differ with varying densities of hemp sesbania. Cotton white blooms/ha were reduced by 32 228 hemp sesbania plants/ha by 77 DAP and by all weed densities by 98 DAP. Hemp sesbania height was equal to or greater than cotton height by 55 to 65 DAP. Light penetration at 1 m above the top of the cotton canopy was 36% less than in weed-free plots when hemp sesbania densities were ≤6446 plants/ha. Likewise, light penetration was reduced at the cotton canopy top and at midcanopy but was not less at the soil surface unless hemp sesbania densities were ≤ 32 223 plants/ha.

Three-day-old soybean (Glycine max (L.) Merr., ‘Hawkeye 63′) hypocotyl segments (0.5 to 1.5 cm below the cotyledons) were utilized to evaluate the effects of the antagonistic interaction of S-ethyl dipropylthiocarbamate (EPTC) and (2,4-dichlorophenoxy)acetic acid (2,4-D) on growth and nucleic acid synthesis. EPTC inhibited growth and RNA synthesis in soybean tissue. The addition of 2,4-D with EPTC was antagonistic to EPTC inhibition of growth and caused an increase in total RNA synthesis. Analysis of soybean tissue nucleic acids by methylated-albumin-kieselguhr(MAK) column chromatography showed that EPTC inhibited ribosomal-RNA (r-RNA), DNA like RNA (D-RNA), and tenaciously-bound-RNA (TB-RNA) synthesis. The combination of 2,4-D with EPTC caused an increase in D-RNA and TB-RNA synthesis compared to the EPTC treatment alone. The 2,4-D-enhanced synthesis of D-RNA and TB-RNA in the presence of EPTC appears to be the basis of the antagonism between EPTC and 2,4-D. Preliminary analysis of r-RNA indicated that EPTC preferentially inhibited the synthesis of 18S r-RNA more than 25S r-RNA and that 2,4-D had no effect on this selective inhibition.

Amicarbazone is a photosystem II (PSII)-inhibiting herbicide in the triazolinone family, which is similar in mode of action to the triazines. Annual bluegrass is a cool-season weed and has shown resistance to some PSII-inhibiting herbicides. The objective was to evaluate triazine-resistant and -susceptible annual bluegrass populations for potential cross-resistance to amicarbazone. Two triazine-resistant (MS-01, MS-02) and triazine-susceptible (AL-01, COM-01) annual bluegrass populations were treated with amicarbazone, atrazine, and simazine at 0.26, 1.7, and 1.7 kg ai ha−1, respectively. All herbicide treatments controlled the susceptible populations greater than 94% 2 wk after treatment (WAT). No visual injury of MS-01 and MS-02 was observed at any time following herbicide treatment. Quantum yield (ΦPSII) of annual bluegrass was measured 0 to 72 h after application (HAA) to determine the photochemical effects of amicarbazone compared to other PSII inhibitors. ΦPSII of triazine-susceptible populations was reduced at all measurement times by all three herbicides. However, amicarbazone decreased ΦPSII of susceptible populations faster and greater than atrazine and simazine at most measurement times. Amicarbazone did not reduce ΦPSII of the MS-01 population. Amicarbazone significantly reduced ΦPSII of the MS-02 population during several measurement timings; however, these reductions were short-lived compared to the susceptible populations and no trend in ΦPSII reduction was observed. Sequencing of the psbA gene revealed a Ser to Gly substitution at amino acid position 264 known to confer resistance to triazine herbicides. These data indicate amicarbazone efficiently inhibited PSII of susceptible annual bluegrass populations; however, triazine-resistant annual bluegrass populations with Ser264 to Gly mutations are cross-resistant to amicarbazone.

Evidence has shown that fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl]urea} persists beyond the end of the growing season when used in continuous cotton (Gossypium hirsutum L.) production. Samples were taken from three soils following cotton production in 1980, 1981, and 1982. All three soils had been in production under the same herbicide use regime, fluometuron preemergence followed by fluometuron plus MSMA (monosodium methanearsonate), since either 1976 or 1977. The fluometuron remaining in each soil was quantified using a greenhouse bioassay and a chemical extraction technique followed by high-performance liquid chromatography determinations. The fluometuron concentrations determined by bioassay and chemical extraction methods had partial correlation coefficients of 0.62, 0.91, and 0.72 for a Sharkey silty clay, a Dundee silt loam, and a Loring silt loam, respectively. Predictive equations were determined for each soil to relate chemical extraction findings to plant response. Bioassay analysis indicated nearly 2 ppmw of fluometuron in the Sharkey silty clay in October 1980, with 1 ppmw in the Dundee silt loam, and approximately 0.27 ppmw in the Loring silt loam with annual application rates of 4.0, 2.9, and 3.5 kg/ha, respectively. Fluometuron concentrations as determined by chemical analysis were 0.83, 0.34, and 0.14 ppmw, respectively. Fluometuron concentrations declined over the winter in all three soils. Samples taken in March of 1981, 1982, and 1983 showed little difference in carryover levels in the Sharkey silty clay but more yearly variation in the other two soils. Fluometuron was found in all three soils to depths of 60 cm, but more than 55% of the fluometuron was found in the upper 15 cm of each soil. A controlled laboratory study conducted with the three soils showed that both cold and dry conditions reduced fluometuron dissipation rates. In the laboratory under conditions favorable for dissipation, fluometuron had a half-life of 26 days in the Dundee silt loam, 43 days in the Loring silt loam, and 73 days in the Sharkey silty clay. In the field, dissipation was very rapid in the Loring silt loam compared to the Dundee silt loam and the Sharkey silty clay.

Many bleaching herbicides with different core structures inhibit phytoene desaturase (PD), a membrane-bound enzyme in the carotenogenic pathway catalyzing the hydrogen abstraction step at the first C40 precursor of β-carotene. Prospects are good that new PD-active herbicides will be discovered by screening for bleaching activity. Accordingly, interest in PD enzymology and molecular genetics has increased. Although active carotenogenic cell-free systems are available, no isolation of PD has been achieved since the enzyme cannot be detected in its isolated form due to complete loss of activity. A portion of the Rhodobacter PD gene was incorporated into an appropriate plasmid which could be expressed in E. coli. This system was used to produce an antibody specific against PD from higher plants as well as Rhodobacter. All PDs assayed had an apparent molecular weight of 52 to 55 kDa. A Rhodobacter gene probe hybridized with a 3.1 kbBamH I fragment from Aphanocapsa which allowed us to sequence the PD gene from this cyanobacterium. Its DNA sequence matched with the apparent molecular weight of the PD band in the western blot, and a fusion-gene product was found to be immunoreactive with the Rhodobacter PD antibody, Anacystis mutants were produced exhibiting cross-resistance against norflurazon and fluorochloridone. Apparently, this resistance is due to an altered PD with concurrent decrease of inhibitor binding affinity. Cloning of the resistant gene into the wild type is in progress.

The formulation of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) encapsulated within a starch granule can reduce the loss in activity associated with the delayed incorporation of the emulsifiable concentrate formulation of trifluralin. Good weed control and high soybean [Glycine max (L.) Merr. ‘Wells II’] yields were obtained when a starch formulation of trifluralin was applied in the fall and incorporated at planting time the following spring. The cultural practices of shallow incorporation, delayed incorporation, and/or midseason cultivation intensified the weed control provided by a starch formulation of trifluralin. The adsorption of additional trifluralin onto the granule surface, cross-linking starch xanthate with a weak oxidant, and mixing granules with different release rates also increased initial control. Granules produced by the starch-calcium and starch-borate processes exhibited many of the same characteristics of granules produced by the older starch xanthate process, but are much less expensive to produce and do not require the use of the toxic carbon disulfide in the encapsulation process. The combination of an appropriate starch formulation of trifluralin with proper cultural practices provided good weed control from fall and spring applications of trifluralin.

We examined effects of supplementary nitrogen on young Canada thistle stands in Alberta, Canada. Urea applied at 70 or 100 kg N ha–1 increased shoot population densities, mostly through increased root growth in the top 20 cm of soil rather than through released root bud dormancy. Nitrogen fertilization, therefore, may increase the severity of a Canada thistle infestation. In 1-yr-old stands, the number of emerged and unemerged root buds/m of root was higher near the soil surface in fertilized plots than in unfertilized plots, and no underground shoots were detected at the time of sampling. In 2-yr-old stands, more unemerged root buds/m of root were found at depths below 20 cm in fertilized plots than in unfertilized plots, and twice as many underground shoots/m of root occurred between 20 and 40 cm in fertilized plots as in unfertilized plots. However, no effect of nitrogen on the number of emerged root buds was detected. Nitrogen treatment increased shoot production only for those replanted root fragments from deeper than 60 cm in 1-yr-old stands and deeper than 160 cm in 2-yr-old stands.

We developed a model and subsequently simulated robust white foxtail (Setaria viridis var. robusta-alba Schreiber) or robust purple foxtail (Setaria viridis var. robusta-purpurea Schreiber) growth. SETSIM (SETaria SIMulation) uses the GASP IV simulation language which allows for both continuously changing variables and discrete events. GASP IV provides the necessary integrations and automatic time-stepping essential in simulation. SETSIM uses the materials-flow concept to simulate foxtail growth and development on a population basis. Carbohydrate flow among six compartments (leaf, stem, and root total nonstructural carbohydrate pools; leaf, stem, and root tissue) is governed by nine physiological rates. Each rate is dependent on the physiological state of the foxtail population and the environmental conditions prevailing. Simulation of carbohydrate flow in and out of each compartment results in the net growth of that compartment. By considering all six compartments simultaneously, the vegetative growth and development of a foxtail population can be simulated. Validation of SETSIM with field data recorded over a 2-yr period has shown that this simulator can accurately predict foxtail growth parameters such as dry matter accumulation, plant height, leaf area index, and leaf to stem ratio. SETSIM could serve as a framework for other weed models because of its modular structure. Such models can benefit weed science by predicting the active stage of weed growth, predicting whether a weed could become a problem under different climatic conditions, interfacing with existing crop models to predict yield and harvest restriction, pointing out gaps in our present knowledge in weed biology, and serving as teaching aids.

Artificial weed seed banks are practical for studying seed bank depletion and weed seedling emergence because the number, depth, and species composition of seed banks can be managed. However, no studies have determined whether artificial seed banks are representative of natural seed banks. We compared the emergence of velvetleaf, giant foxtail, and common waterhemp in a natural seed bank, an artificial seed bank with stratified seeds, and an artificial seed bank with nonstratified seeds. Velvetleaf seedling emergence was higher in the nonstratified seed bank in 2001, but no differences were observed in 2002. The number of viable velvetleaf seeds at the end of the experiment was lower in the natural seed bank than in the artificial seed banks in 2002. Velvetleaf emergence occurred earlier in the natural seed bank than in the artificial seed banks. Giant foxtail emergence was higher in the artificial seed banks (58 to 82%) than in the natural seed bank (5 to 23%). Common waterhemp emergence ranged from 7 to 65% in the artificial seed banks and from 1 to 5% in the natural seed bank. In general, the distribution of emergence with time differed in the natural seed bank compared with the artificial seed banks. These differences were attributed to differences in soil temperature and soil bulk density between the natural and artificial seed banks. Artificial seed banks showed lower soil bulk density and greater temperature fluctuation than the natural seed bank. However, there was no consistent relationship between growing degree days and emergence timing in the three treatments for any of the species studied.

In the greenhouse, we examined approximately 2,700 strains of soybeans [Glycine max (L.) Merr.] for response to 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), a herbicide which inhibits photosynthesis. Duration of survival when grown in soil containing 0.84 kg/ha was the initial selection criterion. Strains thus selected as most tolerant and most susceptible were grown then for 3 to 3 ½ weeks in soil containing 0.45 kg/ha of atrazine. The dry weight of shoots expressed as a percentage of each strain's own untreated check was used to measure the strains' tolerance of atrazine. Tolerance, thus measured, generally increased as seed size increased. Regression analysis indicated that 80% of the variation in response was attributable to variation in seed size. We suggest the possibility of minimizing soybean injury from atrazine (and perhaps other herbicides) by planting large seed.

We tested for ecological differences among apomictic dandelion genotypes in Vancouver, British Columbia, Canada, in order to establish a basis for predicting potential ecological consequences of genetic variation in invading populations. A greenhouse experiment on 30 potential clonal families revealed significant among-family variation for leaf morphological traits, and molecular analyses confirmed the presence of multiple genotypes. In a field common-garden experiment on six confirmed genotypes, plant size and seed production both varied over an order of magnitude among genotypes, suggesting great potential for selection among genotypes during invasion. Genotypes also varied significantly in the timing of reproduction, which may indicate differences in the timing of resource use that could promote population performance of genotype mixtures. There was no evidence of a trade-off between adult plant fitness and seed dispersal or regeneration traits. Genetic variation in dandelion populations appears to have great potential for influencing their invasive success.