Spray carrier pH affects the solubility of sulfonylurea herbicides and, therefore, could affect absorption and subsequent translocation of these compounds in weeds. Trifloxysulfuron is a sulfonylurea herbicide developed for POST weed control in cotton, sugarcane, and turfgrass with a pKa of 4.81. The objective of this study was to evaluate the absorption and translocation of foliar-applied 14C-trifloxysulfuron in Palmer amaranth and Texasweed at pH 5, 7, and 9 over a period of 4 to 72 h after treatment (HAT). For absorption, effects of time, species, and pH were significant. Absorption averaged over species and pH increased logarithmically from 4 to 72 HAT. Absorption was greater for Palmer amaranth (88%) than for Texasweed (29%) when averaged over time and pH. Absorption averaged over species and time increased in the order of pH 5 (52%) < pH 9 (60%) = pH 7 (61%). Consequently, this translated into greater translocation of 14C-trifloxysulfuron in Texasweed when sprayed with the higher pH spray solutions. These data indicate that absorption and translocation of trifloxysulfuron in some weed species may be enhanced by increasing the pH of the spray solution by 2 pH units above the pKa.

The application of the acetolactate synthase (ALS)–inhibiting herbicide metsulfuron on greenhouse- and field-grown black nightshade and greenhouse-grown ladysthumb resulted in progressive inhibition of the level of carbon dioxide (CO2) fixation, the relative quantum efficiency of electron transport through photosystem I (ΦPSI) and II (ΦPSII), and the leaf chlorophyll content. Photosynthetic-related measurements, measured 2 to 4 d after treatment (DAT) at photon flux densities of 400 to 500 μmol m−2 s−1, provided valuable information before the visual symptoms that first appeared at 7 to 10 DAT with the herbicide. Measurements of the quantum efficiency for electron transport by photosystem II and the loss in leaf chlorophyll content appeared to be two of the most practical parameters to use when designing an early detection method to assess the toxicity of metsulfuron. The use of chlorophyll fluorescence would require a comparison of steady-state ΦPSII measurements for control and treated plants, which could be realized by either measuring in time (before/after application) or space (treated/untreated patch).

Microarray analysis was used to identify changes in gene expression in corn leaves collected from plants at the V11–14 growth stage that resulted from competition with velvetleaf. The plants were grown in field plots under adequate N (addition of 220 kg N ha−1) and irrigation to minimize N and water stress. Consequently, only differences resulting from competition for micronutrients, light, and perhaps allelopathic stress were anticipated. Genes involved in carbon and nitrogen utilization, photosynthesis, growth and development, oxidative stress, signal transduction, responses to auxin and ethylene, and zinc transport were repressed in corn growing in competition with velvetleaf. Very few genes were induced because of competition with velvetleaf, and those that were provided little indication of the physiological response of corn. No differences were observed in genes responsive to water stress or sequestering/transporting micronutrients other than zinc, indicating that these stresses were not a major component of velvetleaf competition with corn at the developmental stage tested.

Fluridone has been widely used for control of the submersed aquatic weed hydrilla in Florida for over 25 years. Recently, some hydrilla biotypes were suspected of having developed resistance to fluridone. Laboratory studies were conducted to monitor changes in phytoene and β-carotene contents as a function of suspected susceptible and resistant hydrilla biotypes to fluridone treatment. Hydrilla shoot tips from each biotype were exposed to 5, 10, 15, 20, 30, and 50 μg L−1 fluridone. Higher β-carotene and lower phytoene content was observed in all resistant hydrilla biotypes compared with the susceptible. The susceptible biotype showed an increase in phytoene or a decrease in β-carotene content when treated with as little as 5 μg L−1 of fluridone, whereas much higher doses were needed for the resistant biotypes. EC50 β-carotene values of 9 and 63 μg L−1 fluridone were found in the susceptible and the most resistant biotype, respectively. Consistent levels of hydrilla injury occurred at phytoene/β carotene index values of 5.5–7 and occurred at exposure to 5–10 μg L−1 fluridone in susceptible and 50 μg L−1 in the most resistant biotype. A resistance factor (R/S) was calculated for each hydrilla biotype which ranged from as low as 2X (R1 and R2) to 7X (R5). Aggressive spread of fluridone resistant dioecious hydrilla in aquatic ecosystems can severely impact hydrilla management, and consequently cause substantial and long-lasting ecological and economic problems throughout the southern United States.

Doveweed is becoming more common in agronomic crops in North Carolina. Laboratory and greenhouse experiments were conducted to determine the effect of temperature and seed burial depth on doveweed germination and emergence. Germination of lightly scarified seed at constant temperature was well described by a Gaussian model, which estimated peak germination at 28 C. Similar maximum percentage of germination was observed for optimal treatments under both constant and alternating temperatures. Among alternating temperatures, a 35/25 C regime gave greatest germination (77%). In spite of similar average daily temperatures, germination was greater with alternating temperature regimes of 40/30 and 40/35 C (65 and 30%, respectively) than constant temperatures of 36 and 38 C (4 and 0%, respectively). No germination was observed at 38 C constant temperature or for alternating temperature regimes of 20/10 and 25/15 C. Light did not enhance germination. Greatest emergence occurred from 0 to 1 cm, with a reduction in emergence as depth increased to 4 cm. No emergence occurred from 6 cm or greater depth. This information on seedbank dynamics may aid in developing tools and strategies for management.

Rigid ryegrass is a major weed of southern Australian cropping systems. Increased knowledge about the germination biology of rigid ryegrass would facilitate development of effective weed control programs. The influence of different environmental factors on seed germination of rigid ryegrass was studied. The level of seed dormancy in five different populations, collected from different fields on the same farm, was found to be similar, suggesting consistency in seed dormancy in populations at a single location. Dormancy release was more rapid for seeds after-ripened in the field compared with those after-ripened dry in a greenhouse. Seed decay was found to be an important contributor to the loss of seeds for this species from the seed bank in the field. Seed decay was much greater for seeds after-ripened on the soil surface compared with buried seed. In contrast, germination played an important role in the loss of buried seed as compared with surface seed. Regardless of the burial depth, the dormant seed component present at the end of the growing season was quite small (4 to 16%). Seedling emergence of rigid ryegrass in the field was greater (49%) for seeds buried at 1 cm than for those on the soil surface (16%). No seedlings emerged from seeds buried at 10 cm. Seed germination of rigid ryegrass was tolerant to a wide range of pH and showed some tolerance to salt and osmotic stress. The results indicate rigid ryegrass is unlikely to become a greater weed problem in no-till compared with cultivated seeding systems.

Plant canopy radiation-use efficiency (RUE), defined as unit of dry biomass produced per unit of absorbed photosynthetically active radiation (APAR), has been widely studied both in experimental and theoretical contexts because the use of this relationship greatly simplifies estimating biomass production in plant growth models. Previous studies have indicated that RUE may be sensitive to changes in the fractions of diffuse and direct radiation; RUE has been shown experimentally to increase under conditions of increased diffuse light caused either by atmospheric conditions or by shading from other plants in intercrops. Therefore, we hypothesized that weed species RUE would be greater for weeds grown in mixed weed–crop communities than for weeds grown in more uniform and less dense monotypic communities. To address this question, field experiments were conducted during 2001 and 2002 to determine the vegetative-stage RUE of giant ragweed, velvetleaf, woolly cupgrass, and wild-proso millet grown in monotypic communities or in corn. ANOVA indicated that the effect of community type on RUE was significant (P < 0.0001) and that the interaction between species and year effects was significant (P = 0.0152). Paired comparisons showed that giant ragweed RUE differed from velvetleaf RUE in 2001 (P = 0.0381) and that giant ragweed RUE differed between years (P = 0.0455). Pooled across species types and years, RUE was approximately 50% greater for weeds grown in weed–corn communities than for weeds grown in monotypic communities. These results indicate that more complex canopy architecture in mixed-species communities (i.e., greater total leaf area index [LAI] and heterogeneity of height among individuals) was associated with greater weed RUE. Including weed RUE response to the competitive environment may be one approach to improving the predictive accuracy of process-based growth models for weed biomass accumulation in mixed-species communities.

The effects of environmental factors on seed germination and seedling emergence of Oriental mustard were studied in laboratory and field experiments. Seed germination was stimulated by light; however, some germination occurred in the dark. In light/dark, potassium nitrate at 0.02 M stimulated germination of 1-mo-old seed to a maximum level of 45% as compared with the control (18%). Oriental mustard germination was inhibited by high levels of salt and osmotic stress. Seed germination was greater than 50% over a pH range of 4 to 10. Seedling emergence of 4- to 5-mo-old seeds was highest when present on the soil surface (70%) but declined with depth, and no seedlings emerged from a soil depth of 10 mm. In the field, seedling emergence was greater under no-till than minimum tillage systems.

Experiments were conducted on the seed of turnipweed collected from southern Australia. Seed germinated over a range of temperatures (25/15, 20/12, and 15/9 C) under both light/dark and dark conditions. Regardless of the light condition, germination of naked seed (63 to 88%) was greater than the seed in intact silique (0 to 13%). Seed germination was relatively unaffected (greater than 60%) at a low level of salinity (80 mM NaCl), and some germination occurred even at 160 mM NaCl (11%). Seed germination decreased as osmotic potential decreased from 0 to −1.0 MPa; however, some germination occurred even at an osmotic potential of −1.0 MPa (22%). Seed germination was greater than 76% over a pH range of 4 to 10. Seedling emergence of turnipweed was greater (60%) for seed buried at 1 cm than on the soil surface (23%). No seedlings emerged from seed buried at 5 cm. Information gained in this study will be important in developing a better understanding of the requirements for turnipweed germination and emergence.

Understanding weed seedling emergence patterns is important for successful implementation of many weed management strategies. Identifying the sources of variation of emergence patterns could greatly improve our ability to predict emergence timing. Differences in seed dormancy levels between populations or biotypes are usually not considered when studying seedling emergence of many weed species despite evidence that dormancy levels can affect weed seedling emergence patterns. We studied the importance of seed dormancy on seedling emergence patterns of common waterhemp using three biotypes (Ames, Everly, and Ohio) that differed in dormancy regulation and level (5, 26, and 87% germination, respectively) and three tillage systems (no-tillage, chisel plow, and moldboard plow) in 2004 and 2005. Seedling emergence was at least four times greater under no-tillage than under chisel or moldboard plow conditions. Fewer seedlings emerged in moldboard plow than in other tillage systems. Furthermore, seedling emergence occurred in no-tillage over a longer period than in chisel and moldboard plow. In no-tillage the largest emergence events occurred at the end of June, whereas in other tillage systems most emergence occurred during May and the first week of June. Among biotypes, differences in number of emerged seedlings were more evident than differences in emergence timing. For Everly and Ohio biotypes, the number of emerged seedlings was the same between chisel and moldboard plow plots. For the Ames biotype, in 2004, the number of emerged seedlings was 25-fold higher in chisel plow than in moldboard plow, and the emergence pattern in no-tillage was longer and peaked later than in the other tillage systems. However, these results were not observed in 2005. Overall, we did not observe consistent differences in seedling emergence patterns among biotypes, which suggested that under field conditions, other factors can compensate for differences in seed dormancy levels.

Little mallow is becoming an increasing problem in no-till farming systems in southern Australia. There is little known about the germination ecology of this species, making management of the weed more difficult. Experiments were conducted to determine the effect of different factors on seed germination. Freshly harvested seeds had high levels of innate dormancy with only 5% of seeds germinating. Germination increased with storage time but did not reach more than 47% even 13 mo after seed harvest. Germination was not influenced by light. Germination was stimulated by scarification, which indicates inhibition of germination in this species is mainly due to the seed coat. Germination was moderately sensitive to salt and osmotic stress. Germination was completely inhibited at osmotic potentials of −0.6 to −1 MPa. Seeds of little mallow germinated over a broad range of pH between 4 and 10, but germination declined at higher pH. Potassium nitrate at 0.005 M stimulated seed germination of scarified seeds to a maximum level of 76%. Seedling emergence of little mallow was greatest at burial depths of 0.5 to 2 cm. No emergence occurred at 8 cm or deeper. Greater emergence in response to shallow burial and the requirement for scarification may both contribute to the increasing problem of small mallow in no-till farming systems in southern Australia by delaying emergence until after the crop has been sown.

Perennial pepperweed is an aggressive, exotic weed invading wetland and riparian areas in California, including the San Francisco Bay/Sacramento–San Joaquin Delta Estuary. Effective management will require detailed and accurate maps of its distribution. Remote sensing technologies offer the capability to map weed species over broad areas and with rapid return intervals. As a first step in assessing the potential to map perennial pepperweed with hyperspectral remote sensing data, this study determined its spectral uniqueness relative to co-occurring species. Spectral measurements were conducted during summer drought conditions in the Sacramento–San Joaquin Delta region. Reflectance spectra of perennial pepperweed and seven co-occurring species were collected with a portable spectrometer. Nineteen physiological indexes were calculated from the reflectance data. Physiological indexes are sensitive to narrow spectral features and encapsulate reflectance information in ecologically relevant ways. Classification trees generated from these indexes were able to discriminate both flowering and fruiting perennial pepperweed from co-occurring species with high levels of cross-validated accuracy when using the original spectrometer data and also when this data set was resampled to simulate the spectral resolution of two widely used airborne hyperspectral imagers. Perennial pepperweed's characteristic white flowers are the major component of the spectral uniqueness of this species. Phenological state influenced reflectance spectra more strongly than variation in intraseasonal maturity. Field spectrometer spectra were qualitatively and quantitatively similar to perennial pepperweed spectra extracted from airborne image data. These results suggest that hyperspectral remote sensing will be a powerful tool for the mapping and monitoring of perennial pepperweed. Future work will extend these analyses to image data encompassing the San Francisco Bay/Sacramento–San Joaquin Delta region.

Horseweed is a winter or summer annual plant, native to North America and distributed worldwide in temperate climates. This plant is considered an important agricultural weed because it can reduce agricultural yields by 90% at high densities and becomes problematic under low-tillage agriculture. Seed production is robust with an estimated 200,000 seeds produced per plant, and seed dispersal is wind-assisted. The confirmation of glyphosate-resistant horseweed in Delaware in 2001 and the rapid spread of the resistant biotype, currently covering more than 44,000 ha, has necessitated a change in the discussion about weed dispersal. Large radio-controlled airplanes were used to sample the lower atmosphere for the presence of horseweed seeds during a 3-d period in early September 2005 in southern Delaware. The collection of multiple seeds at heights ranging from 41 to 140 m above ground level strongly suggests that horseweed seeds are entering the Planetary Boundary Layer (PBL) of the atmosphere, where long-ranged transport of aerial biota frequently occurs. With wind speeds in the PBL frequently exceeding 20 m s−1, seed dispersal can easily exceed 500 km in a single dispersal event.

Roads and streams are important conduits for the spread of invasive species, and croftonweed is an invasive plant in southwest China. We established six transects along one road and stream in Huili County, Sichuan Province to determine the pattern of invasion and identify major factors potentially influencing it. Results demonstrated that the invasion (cover, abundance, and number of clusters) of croftonweed declined significantly with distance from the road and stream. However, after compensation for environmental factors, only distance from the road was significantly negatively related to number of clusters of this plant. Instead, native plant species cover and richness were significantly negatively associated with croftonweed invasion along the road; elevation and native species cover were negatively correlated with the invasion along the stream. These findings indicated that roads and streams were main conduits for the spread of croftonweed in southwest China, and reduced native species cover could facilitate invasion by croftonweed in habitats along roads and streams.

Jointed goatgrass is an important weed of wheat in the United States and other parts of the world. Under field conditions, wheat and jointed goatgrass can hybridize and produce backcross derivatives, a situation that may allow gene flow between these two species. In order to gain a better understanding of the factors governing gene flow, a study to characterize patterns of mating between these two species was undertaken. Chloroplast and nuclear microsatellite markers were used to evaluate the parentage of 413 first-generation backcross (BC1) seeds obtained from 127 wheat–jointed goatgrass F1 hybrids, produced naturally under field conditions. Of the 127 hybrids evaluated, 109 (85.8%) had jointed goatgrass as the female parent, whereas the remaining 18 F1 plants (14.2%) had wheat as the female parent. Of the 413 BC1 plants analyzed, 358 (86.7%) had wheat and 24 (5.8%) had jointed goatgrass as the male backcross parent. The male parentage of 31 BC1 (7.5%) plants could not be determined. Under natural field conditions, wheat was the prevalent pollen donor for the production of hybrids and first-generation backcross derivatives. However, hybrids and backcrosses with jointed goatgrass as the male parent also were observed. Thus, the establishment and persistence of a zone of hybridization between these species would result in the development of jointed goatgrass carrying wheat genes.

Canada thistle is a noxious weed that occurs in a wide range of habitats and is difficult to control because of its extensive root system and prolific seed production. Here, we focused on estimating the level of genetic diversity between populations in North Dakota as a first step in examining diversity across North America. Two types of genetic marker, intersimple sequence repeats (ISSRs) and microsatellites were used. Both marker types resulted in polymorphic alleles suitable for assessing diversity. Analysis of molecular variance (AMOVA), molecular diversity analyses, and cluster analysis were conducted. Highly significant variation was detected between populations (P < 0.01). The greatest variance recovered was between individuals within populations. Gene flow among populations in the Northern Great Plains was indicated by the presence of shared alleles between the North Dakota and Minnesota populations and in cluster formation. Multiple introductions and continued gene flow between populations has led to the continued success of Canada thistle as an invasive plant in North America.

The relationship among 80 different Echinochloa accessions were studied in 2000–2002 by applying amplified fragment length polymorphism (AFLP) analysis and examining the main morphological traits of these plants. The plants were collected from 40 different areas in which the main rice-growing region of northern Italy was divided on the basis of uniform environmental and agronomical conditions. Echinochloa accessions were grouped in three different species (E. crus-galli, E. erecta, E. phyllopogon) according to Pignatti's classification key and in four different species according to Carretero's taxonomy (E. crus-galli, E. hispidula, E. oryzicola, and E. oryzoides). The E. crus-galli accessions clustered as a specific group under both AFLP analysis and morphological traits analysis carried out according to Pignatti's and Carretero's keys. AFLP analysis revealed a separate group containing two accessions with a green basal stem section. These were classified as E. crus-galli by Pignatti's taxonomy and E. oryzoides according to Carretero's taxonomy. A red basal stem section, a trait not considered by either morphological classification, was the main morphological trait that characterizes all E. crus-galli accessions. All other accessions with green basal stem section clustered in a group that included E. erecta and E. phyllopogon according to Pignatti's taxonomy and E. hispidula and E. oryzicola according to Carretero's taxonomy. All species were evenly distributed in the monitored rice area, except the two E. oryzoides accessions, which were found only in the southwestern zone.

The dynamics of soil seed banks in crop rotations of corn, soybean, and winter wheat were investigated to determine whether weed seed inputs associated with the winter wheat phases of the rotation were present in the readily germinable fraction of the seed bank in subsequent phases. Two studies were conducted, each in chisel-plowed systems. In one study, we compared seed banks in plots after 8 yr of corn grown continuously and with rotation that included winter wheat and soybean. A second study followed seed bank composition and abundance for 3 yr in plots that were planted to corn and soybean in successive years after planting to winter wheat. Seed banks were measured by direct germination in a heated greenhouse. In the first study, seed banks in plots planted to continuous corn (never planted to winter wheat) and the corn phase of the rotation (winter wheat planted 3 yr earlier) did not differ in species composition or abundance despite significant differences in seed banks in plots that had been planted to winter wheat the previous season. In the second study, seed bank abundance and composition in plots planted to winter wheat in 2001 rapidly changed after planting of corn and soybean in 2002 and 2003, respectively. Data from the two experiments suggest that seed banks in annual row crops experience rapid change in composition and abundance and can be strongly influenced by the most recent crop. This could limit our ability to infer longer term trends associated with changes in management practices from studies of soil seed banks.

Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r2 = 0.21), air temperature (r2 = 0.45) or rainfall (r2 = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m−2 to a high of > 1,500 plants−2 at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations.

Weed species respond to the cumulative effect of multiple practices employed within weed management systems. However, this response is rarely studied at the system level, and the relationships between weed communities and management systems in crops are not well understood. We used multivariate analyses to assess relationships among tomato management systems and weed species identified through on-farm sampling of 59 fields. Giant foxtail, common lambsquarters, prickly sida, and carpetweed were common in all systems. Eastern black nightshade was common in the conventional processing and mixed fresh-market systems but largely absent from the organic system. Barnyardgrass and goosegrass were common in the organic fresh-market system but not in the other systems. Canonical correspondence analysis identified distinct associations between specific species and the management systems. Common purslane was strongly associated with the rain-fed, mixed fresh-market system, and barnyardgrass, goosegrass, yellow nutsedge, and green foxtail were associated with the irrigated, organic fresh-market system. Indicator species analysis identified five species that were significant indicators of the organic, fresh-market system and one species that was a significant indicator of the rain-fed, mixed fresh-market system. Weed populations persist or increase when a set of species-specific environmental conditions are met. The association of weed species in this study with particular systems supports the hypothesis that weed communities are strongly affected by management systems.