No-tillage corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production has been widely accepted in the mid-Atlantic region, favoring establishment of horseweed [Conyza canadensis (L.) Cronq.]. Within 3 yr of using only glyphosate for weed control in continuous glyphosate-resistant soybeans, glyphosate failed to control horseweed in some fields. Seedlings originating from seed of one population collected in Delaware were grown in the greenhouse and exhibited 8- to 13-fold glyphosate resistance compared with a susceptible population. There were no differences between the isopropylamine or diammonium salts of glyphosate.

This paper reviews the biology of two closely related Mediterranean annuals, yellow starthistle and common crupina, which have invaded grassland, shrub steppe, and open woodland habitats in western North America. Despite the similarity of their winter annual life cycle, the two species differ significantly in population dynamics. Common crupina has traits that favor persistence rather than rapid population growth: large, heavy achenes with an after-ripening requirement; lower fecundity but higher germination success; and reproduction regulated by vernalization and photoperiod in addition to thermal time. Persistence traits also foster invasion of undisturbed or less degraded steppe habitats. Yellow starthistle has more ruderal traits: small, light, rapidly germinating achenes; higher fecundity, with greater seedling mortality; and reproduction that is less sensitive to photoperiod and vernalization. These characteristics confer a greater adaptability for rapid spread and colonization of disturbance by yellow starthistle than by common crupina. An understanding of the relative differences in biological characters of each species and in their function in invaded environments is relevant to ecological management of these pest species.

Dormancy is a form of developmental arrest and is an adaptive trait that promotes the survival of many organisms. In flowering plants, dormancy occurs in seeds and vegetative propagules (Lang 1996). Seed dormancy increases the distribution of germination over time, thus enhancing the survival of plants in an ever-changing environment. Seed dormancy is of intrinsic interest to weed scientists because it is one of 12 adaptive characteristics associated with weeds (Baker 1974). The sporadic emergence of seedlings derived from populations of dormant and nondormant weed seeds in the soil (Benech-Arnold et al. 2000; Forcella et al. 2000) is a key factor that dictates the need to apply weed control measures repeatedly within, between, and across growing seasons. My objective in writing this paper is to provide weed scientists, advanced students, and others with limited background information, some recent findings concerning the physiological genetics of dormancy, and steps toward identifying genes that directly regulate seed dormancy and germination. Molecular aspects of dormancy and germination will not be covered here because they have been reviewed recently (Bewley 1997; Li and Foley 1997). Readers can obtain additional and more extensive information on the biology and ecology of seed dormancy and germination from several recent books and reviews (Baskin and Baskin 1998; Benech-Arnold et al. 2000; Bewley and Black 1994; Casal and Sánchez 1998; Cohn 1996, 1998; Fennell 1999; Forcella et al. 2000; Hilhorst 1995, 1998; Hilhorst and Toorop 1997; Kelley et al. 1992; Kigel and Galili 1995; Simpson 1990; Vleeshouwers et al. 1995).

The role of calcium in mediating resistance to several auxinic herbicides (i.e., 2,4-dichlorophenoxyacetic acid, [4-chloro-2-methylphenoxy] acetic acid, (±)-2-(4-chloro-2-methylphenoxy) propanoic acid [mecoprop], 3,6-dichloro-2-methoxy-benzoic acid [dicamba], or 4-amino-3, 5,6-trichloro-2-pyridinecarboxylic acid [picloram]) was investigated by modulating calcium dynamics of a susceptible (S) and resistant (R) biotype of wild mustard. The inhibitory effects of the auxinic herbicides on root length of the S seedlings were significantly reduced upon pretreatment with calcium in the presence of the calcium ionophore A23187. Conversely, the addition of verapamil, a calcium channel blocker, to the R seedlings increased their sensitivity to the auxinic herbicides. Valinomycin, a potassium channel ionophore, did not ameliorate the effect of the auxinic herbicides on both biotypes of wild mustard, thus indicating that the observed effects were specific for calcium. These results demonstrate that calcium plays a crucial role in the resistance of wild mustard to auxinic herbicides at the level of intact seedlings, thereby supporting our previous results using intact protoplasts.

Roots of dandelion were exhumed from the soil monthly from 1997 to 1999 near Scottsbluff, NE, to assess the seasonal changes in total sugars, glucose, fructose, sucrose, and fructans. In the spring, the initiation of plant growth was accompanied by increased fructose and, to a lesser degree, glucose as a percentage of total sugars. During June, July, and August, the percentage of total sugars with a middle to high degree of polymerization (DP) fructans increased. The DP in fructans changed with reduced rainfall and was associated with increased fructose and decreased high-DP fructans as a percentage of total sugars. Decreasing or freezing soil temperatures in the fall were associated with increased fructose and decreased high-DP fructans as a percentage of total sugars. When soil froze in December, the percentage of total sugars as sucrose and low-DP fructans increased and mid- to high-DP fructans decreased. The seasonal fluctuations of glucose, fructose, sucrose, and various fructan polymers may be one mechanism that allows dandelion to adapt to environmental stresses and gain a competitive advantage over other plants in the community. An understanding of these seasonal fluctuations in sugars may also allow better timing of biological, mechanical, and chemical control practices for improved plant management.

In this review, we examine current techniques and recent advances directed toward understanding cellular mechanisms involved in controlling dormancy in vegetative propagules. Vegetative propagules (including stems, rhizomes, tubers, bulbs, stolons, creeping roots, etc.) contain axillary and adventitious buds capable of producing new stems/branches under permissive environments. Axillary and adventitious buds are distinct in that axillary buds are formed in the axil of leaves and are responsible for production of lateral shoots (branches). Adventitious buds refer to buds that arise on the plant at places (stems, roots, or leaves) other than leaf axils. Both axillary and adventitious buds generally undergo periods of dormancy. Dormancy has been described as a temporary suspension of visible growth of any plant structure containing a meristem (Lang et al. 1987). Dormancy can be subdivided into three categories: (1) ecodormancy-arrest is under the control of external environmental factors; (2) paradormancy-arrest is under the control of external physiological factors within the plant; and (3) endodormancy-arrest is under the control of internal physiological factors. One common feature in all of these processes is prevention of growth under conditions where growth should otherwise continue. There is growing evidence that lack of growth is due to blockage of cell division resulting from interactions between the signaling pathways controlling dormancy and those controlling the cell cycle.

Single-leaf CO2 assimilation rate under saturating light (CA) varies as a function of leaf nitrogen content per unit leaf area (NL). Measured CA for many crop species also declines with developmental stage. Because these relationships may have strong implications for crop–weed competition, a field experiment was conducted to quantify corn and velvetleaf CA as influenced by leaf NL and stage of development. Crop and weed CA were measured on the most recent fully expanded leaves of plants grown in four nitrogen (N) application treatments. Both corn and velvetleaf CA increased with increasing NL, up to about 1.5 g N m−2. Corn and velvetleaf NL values were rarely less than 0.75 g N m−2, indicating that both species may restrict leaf growth in order to maintain constant NL under conditions of limited soil N. Corn CA declined to half its maximum by physiological maturity, whereas velvetleaf CA only declined 18%. Although velvetleaf CA was considerably lower than that of corn, the difference decreased as the growing season progressed. Because corn leaf production is complete by anthesis and velvetleaf continues vegetative production throughout its life cycle, velvetleaf will produce relatively greater quantities of biomass late in the season, which may increase competition for light.

Growth chamber experiments were conducted to evaluate the effects of relative humidity and temperature on the efficacy, absorption, and translocation of glufosinate at 205, 410, and 820 g ha−1 in Palmer amaranth, redroot pigweed, and common waterhemp. Low relative humidity decreased control of all three species by glufosinate. However, control increased as application rate increased at low relative humidity. Amaranth species grown under 21/16, 26/21, and 31/26 C day/night temperature regimes responded differently to glufosinate. At 26/21 C, glufosinate at 820 g ha−1 controlled redroot pigweed less effectively than it controlled Palmer amaranth and common waterhemp, whereas at 410 g ha−1, glufosinate controlled common waterhemp more effectively than it controlled the other two species. Neither temperature nor relative humidity altered the absorption of 14C-glufosinate in any of the three species. Most of the absorbed glufosinate remained in the treated leaves at all three temperature regimes and two relative humidity levels. However, glufosinate translocation was greater in plants grown at 90% than in those grown at 35% relative humidity, and this phenomenon coincided with greater control of the amaranth species at the high humidity level. The study showed that relative humidity had a greater effect than temperature on glufosinate toxicity to Palmer amaranth, redroot pigweed, and common waterhemp.

DNA fragment analysis, based on amplification of intersimple sequence repeats by the polymerase chain reaction (ISSR-PCR), was used to assess genetic relationships of 217 U.S. accessions of common cocklebur. Twenty-four polymorphic markers were generated from six primers. Analysis of genetic similarity by clustering procedures resulted in separation of the accessions into two main clusters. Accessions within these two clusters were designated as either northern or southern genotypes. Forty-four of 48 accessions analyzed from Washington, Michigan, Iowa, and Ohio were northern genotypes, whereas 67 of 68 accessions analyzed from Mississippi, Arkansas, South Carolina, and North Carolina were southern genotypes. Accessions from Kansas, Missouri, and Illinois included 40 and 56 northern and southern genotypes, respectively, indicating a transition zone. In Illinois, accessions collected from northern and southern counties tended to be northern and southern genotypes, respectively. We conclude that much of the genetic variation among U.S. common cocklebur accessions is distributed along a latitudinal gradient.

Several populations of smooth pigweed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine if differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption of foliar-applied cloransulam-methyl was rapid and similar in both populations of smooth pigweed. Translocation of the absorbed radioactivity out of the treated leaf was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. 14C-cloransulam-methyl metabolism was also similar in S and R2 populations. Differential tolerance to cloransulam-methyl in S and R2 populations in the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.

As a first step toward developing a genomics-based research program to study growth and development of underground adventitious shoot buds of leafy spurge, we initiated a leafy spurge expressed sequence tag (EST) database. From the approximately 2,000 clones randomly isolated from a cDNA library made from a population containing growth-induced underground adventitious shoot buds, we have obtained ESTs for 1,105 cDNAs. Approximately 29% of the leafy spurge EST database consists of expressed genes of unknown identity (hypothetical proteins), and 10% represents ribosomal proteins. The remaining 60% of the database is composed of expressed genes that show BLASTX sequence identity scores of ≥ 80 with known GenBank accessions. Clones showing sequence identity to a Histone H3, a gibberellic acid-responsive gene, Tubulin, and a light-harvesting chlorophyll a/b-binding protein were shown to be differentially expressed in underground adventitious shoot buds of leafy spurge after breaking of dormancy. RNA encoding a putative cyclin-dependent protein kinase (CDK)-activating kinase, a gene associated with cell division, and Scarecrow-like 7, a gene involved in GA signaling, were present at similar levels in dormant and growth-induced underground adventitious shoot buds. These data show how even a small EST database can be used to develop a genomics-based research program that will help us identify genes responsive to or involved in the mechanisms controlling underground adventitious shoot bud growth and development.

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.

In 1997, farmers in Ontario, Canada, reported failure of some ALS-inhibiting herbicides to provide adequate control of pigweed species. Growth room experiments were conducted to confirm resistance to ALS inhibitors in populations of Powell amaranth and redroot pigweed. Twenty-two out of 35 collected seed samples were able to grow in the presence of soil-applied imazethapyr or flumetsulam. Dose–response curves were generated for 11 and 9 populations of Powell amaranth and redroot pigweed, respectively, using foliar-applied imazethapyr and thifensulfuron. Resistance to ALS inhibitors was confirmed in nine and five populations of Powell amaranth and redroot pigweed, respectively. Within each species, comparison of the herbicide rate required to reduce plant dry weight 50% (GR50) between the resistant populations and a susceptible population was conducted to obtain resistance factors. For imazethapyr, resistance factors ranged from 4.2 to 3,438 and from 33 to 168 for Powell amaranth and redroot pigweed, respectively. High-level cross-resistance to thifensulfuron was found in two populations of each species, with resistance factors ranging from 270 to 2,416. In both species, populations could be grouped according to their cross-resistance patterns: some populations were resistant to imazethapyr only, whereas others expressed resistance to both imazethapyr and thifensulfuron. The observed patterns of cross-resistance were not correlated with known herbicide exposure history of the fields where these populations originated.

To be effective, postemergence herbicides must be absorbed and translocated to sites of action in proper form and quantity. Any factor that interferes in this process may account for differential sensitivity. Adjuvant effects on foliar absorption of BAY MKH 6561 by jointed goatgrass and downy brome were evaluated under growth chamber conditions. Absorption of BAY MKH 6561 by jointed goatgrass and downy brome without adjuvants was 41 and 30% of applied, respectively, 48 h after treatment (HAT). Herbicide absorption with methylated seed oil (MSO) was significantly higher than with nonionic surfactant (NIS) 24 and 48 HAT. The addition of urea ammonium nitrate (UAN) to MSO and NIS significantly increased absorption over MSO and NIS alone 24 HAT, but absorption was similar to that obtained with MSO 48 HAT. Averaged across adjuvant combinations, jointed goatgrass and downy brome absorbed 90 and 89% of applied BAY MKH 6561, respectively, 48 HAT. BAY MKH 6561 translocation and metabolism in jointed goatgrass, downy brome, and winter wheat were also evaluated. More 14C-BAY MKH 6561 translocated to shoot and root tissue in downy brome than in jointed goatgrass and winter wheat. Root exudation accounted for 26% of root-translocated BAY MKH 6561 in jointed goatgrass, 31% in downy brome, and 43% in winter wheat. Winter wheat, jointed goatgrass, and downy brome metabolized 82, 65, and 50% of absorbed 14C-BAY MKH 6561 12 HAT, respectively, and 97% metabolism occurred in all species 48 HAT. Exponential decay equations predicted a 7-h BAY MKH 6561 half-life in winter wheat, 10-h half-life in jointed goatgrass, and 13-h half-life in downy brome. Jointed goatgrass absorbed amounts of 14C-BAY MKH 6561 that were similar to those absorbed by downy brome, but jointed goatgrass was intermediate in translocation and metabolism compared to winter wheat and downy brome. Therefore, differential translocation and metabolism may explain differential field susceptibility observed between winter wheat, jointed goatgrass, and downy brome.

This study was conducted to ascertain movement potential of imazethapyr resistance and to measure the relative growth and productivity of imazethapyr-resistant (IR) and imazethapyr-susceptible (IS) biotypes of common sunflower under noncompetitive and competitive conditions. Susceptible biotypes of common sunflower were planted in the field in concentric circles at distances of 5.5, 8.0, 15.0, and 30.0 m around a center of densely planted IR biotypes in four locations in northeast Kansas in 1998 and 1999. Pollen movement was analyzed by sampling the IS progeny for the presence of imazethapyr resistance. The distance in which resistance is first detected from the IR pollen source, first unnatural resistant distance (FURD), ranged from 12.1 to 15.5 m. Wind direction was highly correlated with FURD; the north sections had larger FURD. Greenhouse studies were conducted to study growth of IR and IS biotypes under noncompetitive and competitive conditions. Under noncompetitive conditions, leaf area and dry weight were slightly greater for the IR than the IS biotype at early growth stages, but photosynthesis and height were similar. Under competitive conditions, photosynthesis, leaf area, height, and dry weight of IR and IS biotypes were similar. As a result, IR–IR and IS–IS intracompetition equaled IR–IS intercompetition. Gene flow from IR to IS biotypes occurred with movement up to 15.5 m. The lack of differences between growth of the IR and IS biotype at late growth stages in noncompetitive conditions and similar growth of IR and IS biotypes under competitive conditions indicated no competitive advantage from imazethapyr resistance.