The action of trichloroacetate, a herbicide of grass-type plants such as quack grass and Johnson grass, has been studied for several years by observing the morphological responses of treated plants. Its presence in such plants has been indicated by analyses for the trichloromethyl group with a colorimetric test using pyridine and sodium hydroxide (1, 10). A review by Barrons and Hummer (1) indicates that root absorption is a much more important avenue of entry than foliar absorption. They also report differences in TCA content in the expressed sap of several species of treated plants. At 20 lbs. per acre they found “very little” TCA in corn and beans (both TCA-susceptible). At 10 lbs. per acre, they found “not a trace of TCA” in corn, “very little” in beans, but “a relatively high concentration of TCA” in peas (a relatively TCA-tolerant plant). However, Tibbitts and Holm (10) found 100 to 500 μg. of TCA per g. of fresh snap bean tissue from plants whose culture solutions contained 15 mg. of commercial grade 80% TCA as sodium salt per 150 ml. of nutrient. In order to check these observations, and to see whether TCA itself is present in the plants, and if so to determine the pattern of its distribution within the plants, as well as to look for possible metabolic products, C14-labeled TCA was synthesized and used in tracer experiments with corn and pea plants.

The observation that riboflavin overcomes the chlorosis response of higher plants to amitrole was confirmed. Amitrole was shown to be decomposed in the presence of riboflavin and light. It is suggested that this decomposition proceeds by a free radical mechanism, either in the culture medium or in the plant.

Experiments were conducted to determine the influences of temperature, light, winter storage, and water stress on seed germination of fringed sage. Seeds collected in 3 yr in central Saskatchewan were placed in sealed vials and buried in the soil after harvest, and germination was tested in spring and early summer. Seeds germinated over a wide range of temperatures with alternating 25/15 C being optimal. The range of optimal temperatures was higher for older seeds than younger seeds. The stimulating effect of light on germination varied among collections and incubating temperatures. Total germination and germination rate was limited by water stress and no seeds germinated at osmotic potentials below −0.9 MPa. Seeds hydrated in the autumn and exposed to low winter temperatures had higher germination the following spring than dry seeds exposed to the same conditions. Results suggest that sufficient soil moisture combined with moderate seedbed temperatures are optimal for fringed sage germination. Periodicity of germination may be influenced by variable germination requirements in different aging seeds.

Seven herbicides were applied to the soil surface or subsurface for control of honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell] and Macartney rose (Rosa bracteata Wendl.). Liquid formulations were applied in bands about 1 cm wide, 1.2 m or more apart, and granule or sphere formulations were applied on the soil surface either broadcast or in rows 1.2 m or more apart. On honey mesquite, karbutilate [tert-butylcarbamic acid ester with 3(m-hydroxy-phenyl)-1,1-dimethylurea] and tebuthiuron {N-[5-(1,1-dimethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} were the most effective herbicides with subsurface spray injections being generally more effective than surface applications. On honey mesquite, differences among row spacing treatments with liquid, granule, or sphere formulations of karbutilate were generally either small or insignificant. Roller chopping reduced the honey mesquite canopy but did not influence the percentage of plants killed by karbutilate at rates of 4.5 kg/ha and below. Bromacil (5-bromo-3-sec-butyl-6-methyluracil), diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], picloram (4-amino-3,5,6-trichloropicolinic acid), prometon [2,4-bis(isopropylamino)-6-methoxy-s-triazine], and 2,3,6-TBA (2,3,6-trichlorobenzoic acid) generally were ineffective as soil treatments for controlling honey mesquite. Of all the herbicides evaluated on Macartney rose, only picloram at 4.5 kg/ha and tebuthiuron at 2.2 and 4.5 kg/ha, as soil treatments, killed a significant number of plants.

Floral induction in kochia (Kochia scoparia (L.) Schrad.) was influenced more by light period and temperature than by soil fertility or soil moisture. Floral induction occurred when the light period per 24-hr cycle was shorter than a critical length. However, the critical duration of light per 24-hr cycle at which floral induction occurred varied from about 13 to 15 hr among kochia selections. The time from emergence to flowering varied from 57 to 100 days. The kochia selection from New Mexico required a shorter light period and a longer time after emergence for floral induction than did selections from North Dakota, South Dakota, and Minnesota. Kochia selections from the latter three states varied greatly in flowering date.

Research in irrigated and nonirrigated corn production systems was conducted to evaluate the effect of leaf architecture of corn hybrids on weed management. The corn hybrids used in each study were ‘Pioneer 3394’ (upright leaf) and ‘Pioneer 3260’ (horizontal leaf). In the irrigated study, residual weed control treatments included two rates of prepackaged mixtures of metolachlor plus atrazine, encapsulated acetochlor plus atrazine, nonencapsulated acetochlor plus atrazine, or a tank mixture of simazine and metolachlor plus atrazine. In the irrigated experiments, horizontal leaf architecture reduced weed density (all three sites in 1 of 2 yr), weed biomass (five of six sites), solar radiation reaching the ground (all six sites), and weed seed production (one site each year) compared with upright leaf architecture. Weed density and weed biomass did not differ between herbicide rates or acetochlor formulation at any site. Corn hybrid was significant for yield at only one site. Reduced weed biomass did not translate into yield differences. The nonirrigated study evaluated two factors at four sites over 2 yr: leaf architecture (upright or horizontal leaf) and weed management program (preemergence residual and postemergence no residual) at two application rates. Neither weed density nor weed biomass was reduced because of corn leaf architecture or herbicide rates in the nonirrigated study. No interaction was detected in either irrigated or nonirrigated studies between leaf architecture and herbicide treatments, indicating that these factors are independent of one another. On the basis of these studies, it appears that horizontal leaf architecture of corn hybrids can assist in integrated weed management in irrigated corn production but may not be beneficial when corn is grown under drought-prone conditions.

In 1982 and 1983, CGA 82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid}, fenoxaprop {(±)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxyl]propanoic acid}, fluazifop {(±)-2-[4-[[5-trifluoromethyl)-2-pyridinyl] oxy]phenoxy] propanoic acid}, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid}, and sethoxydim {2-[1-(ethoxyimino-(butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} were evaluated as postemergence treatments for the control of rhizome johnsongrass [Sorghum halepense (L.) Pers. # SORHA] in cotton (Gossypium hirsutum L. ‘Acala SJ-2′). Applications of fluazifop or haloxyfop, and the higher rates of sethoxydim or CGA 82725 or fenoxaprop, controlled 90% or more johnsongrass 20 and 52 weeks after application and prevented cotton yield losses.

Nitrogen-fixing cyanobacteria are vital photosynthetic microorganisms that contribute to soil fertility by fixing atmospheric nitrogen and are also important for maintaining ecosystem stability. These microorganisms can be very sensitive to herbicides because they possess many characteristics of higher plants. Six days after the application of monosulfuron at 0.03 to 0.3 nmol L−1 under laboratory conditions, growth of the nitrogen-fixing cyanobacteria Anabaena flos-aquae, Anabaena azollae, and Anabaena azotica was stimulated, but at higher concentrations (30 to 300 nmol L−1) protein synthesis was inhibited. The production of 16 amino acids in A. flos-aquae was reduced from 7 to 69% with increasing monosulfuron concentration. Application of monosulfuron at 3 to 300 nmol L−1 substantially inhibited in vitro acetolactate synthase (ALS) activity as indicated by 50% inhibition index values of 3.3, 65.2, and 101.3 nmol L−1 for A. flos-aquae, A. azollae, and A. azotica, respectively. In contrast, extractable ALS activity was not affected in these algal species with monosulfuron treatments ranging from 0.03 to 300 nmol L1 except in A. flos-aquae at higher concentrations (30 to 300 nmol L−1). The most sensitive species to monosulfuron was A. flos-aquae, followed by A. azollae and A. azotica. Molecular analyses showed that the genomic DNA of A. azollae and A. azotica differed in only one amino acid. Results from photogenetic analyses revealed a high degree of homology between these algae. In contrast, the genomic DNA of A. flos-aquae differed from that of A. azollae and A. azotica in 44 and 45 amino acids, respectively. Our findings support the view that monosulfuron toxicity in these three nitrogen-fixing cyanobacteria is due to interference with protein metabolism via inhibition of branch-chain amino acid biosynthesis, and particularly ALS activity.

Weed scientists are trained broadly in agronomy, botany, chemistry, soils, and other agricultural disciplines. The study of weeds, rather than the techniques used or the questions asked, unifies weed scientists around a common focus. It is often difficult for weed scientists to identify closely with any one scientific discipline, since the techniques and questions of many disciplines are needed to address problems posed by weeds. One discipline with relevance and potential for addressing weed science problems is physiological ecology. The study of the functioning or adaptation of plants in relation to environmental influences has its roots in both classical ecology and experimental physiology. Application of this discipline to weed science may take an environmental approach (e.g., studying limiting factors in the environment), a physiological approach (e.g., studying the responses of critical plant processes to environmental stress), or a more autecological approach (e.g., studying the physiological basis for the adaptation of a particular weed to a particular habitat). Many methodologies and technologies are available for both field and laboratory investigations. For example, photosynthesis, a major determinant of plant growth, can be investigated in the field at the leaf, plant, or canopy level using plant growth analysis or a portable infrared gas analyzer (IRGA) and appropriate assimilation chambers. Investigations of photosynthesis in the laboratory can focus on the plant, leaf, chloroplast, or thylakoid level using an IRGA or the techniques of polarography (measurement of evolved oxygen) or fluorometry. Application of such approaches to weed science should improve our understanding of the basis for particular weed problems and thus broaden our options for management.

Experiments were conducted in 1990 and 1991 to determine bioavailability of PRE herbicides at three soil profile depths and two soil types in established common bermudagrass sods. Prodiamine and oryzalin reduced ‘Tifgreen’ bermudagrass root growth in the 5- to 7.5-cm layer of a Bosket very fine sandy loam soil at 2 and 4 wk after treatment in 1991. These herbicides decreased bermudagrass root weight in both the Bosket and Marietta sandy clay loam both years in samples taken from the 2.5- to 5-cm depth layer. In the 0- to 2.5-cm soil layer, all herbicides reduced root weight in 1990. In a bioassay, prodiamine caused decreased Tifgreen bermudagrass root growth at concentrations as low as 4 ppb by wt in the very fine sandy loam soil, while 8 ppb was necessary in the sandy clay loam soil. Prodiamine was detected in the very fine sandy loam at 4 wk after treatment at all depths in 1991 (65, 45, and 39 ppb in the 0- to 2.5-, 2.5- to 5-, and 5- to 7.5-cm soil layers, respectively). Oryzalin was also detected at all depths in 1991 when sampled at 2 and 4 wk after treatment in the very fine sandy loam. Pendimethalin was present in concentrations of 38, 39, and 37 ppb in the sandy clay loam at 2 wk after treatment in the 0- to 2.5-, 2.5- to 5-, and the 5- to 7.5-cm soil layers, respectively. Pendimethalin was also detected in the very fine sandy loam at 2 wk after treatment at concentrations of 55, 69, and 36 ppb in the 0- to 2.5-, 2.5- to 5-, and 5- to 7.5-cm soil layers, respectively.

Using an accession of velvetleaf having a high degree of purple pigmentation as the male parent made possible the identification of seedlings resulting from controlled or natural crosses. Cross-pollination, even after flowers had opened and anthers had shed pollen, was demonstrated by transmitting pollen manually. Natural outcrossing was demonstrated in the field by growing plants of the purplepigmented accession in close proximity to locally occurring velvetleaf. Insect transmission of pollen was suspected.

Barnyardgrass seeded in field plots at monthly intervals from March through October at Shatter, CA, began to emerge in March when soil temperatures reached 17 C at a depth of 5 cm. For all plantings, most of the seeds germinated and produced seedlings within 2 weeks. Increasing day lengths favored vegetative growth, with April, May, and June plantings producing the tallest plants, the most dry matter, and the largest number of tillers. July and August plantings, the first to flower, began flowering and producing seeds within 5 to 6 weeks after planting, but the greatest number of seeds (15 000 to 25 000/plant) was produced by the larger plants of the April, May, and June plantings. September plantings produced few seeds/plant (37), when compared to March (5900), July (9800), and August (9700) plantings. The earliest that seed production would be expected from seedlings emerging in March and April would be early June, and seedlings emerging in September could produce seeds before killing frosts.

Flumetsulam sorption and mobility studies were conducted on surface (0 to 15 cm) and subsurface (30 to 46 cm) soil of several southern soils. In a batch equilibrium study using a 1:1 ratio of soil–0.01 M CaCl2, flumetsulam adsorbed ranged from 2.9 to 48.7% and from 4.2 to 63.3% on surface and subsurface soils, respectively. Herbicide soil–solution distribution (K d) and organic carbon (K oc) coefficients ranged from 0.03 to 0.95 and from 5.1 to 77.1, respectively, in surface soils and from 0.04 to 1.72 and from 7.5 to 325.5, respectively, in subsurface soils. K d and K oc were correlated with humic and organic matter in surface soils. K d was correlated with extractable Fe, whereas K oc was inversely correlated to pH in subsurface soils. A desorption study using 0.01 M CaCl2 as an extractant on the Captina silt loam surface soil demonstrated that three to four washes were required to desorb more than 94% of the flumetsulam adsorbed over several equilibration times. Mobility studies on soil thin-layer chromatography plates demonstrated that flumetsulam and imazaquin had similar values, ranging from 0.50 to 0.90 and 0.59 to 0.90, respectively, in the surface soils, and both compounds had the same range of mobility in subsurface soils, with R f values between 0.60 and 0.93. At both soil depths, K d and K oc were inversely correlated with the R f of flumetsulam.

We evaluated the use of ornamental plants as phenological indicators for predicting giant foxtail emergence and compared their performance with predictions based upon Julian day, cumulative growing degree–days (GDD), and the WeedCast program. From 1997 to 2001, we monitored giant foxtail emergence in a field experiment with and without fall and spring tillage to estimate the dates of 25, 50, and 80% emergence; we also recorded dates of first and full bloom of 23 ornamental plant species. Dates of weed emergence and ornamental blooming for 1997 to 2000 were compiled in a phenological calendar consisting of 54 phenological events for each year, and events were ordered by average (1997 to 2000) cumulative GDD (January 1 start date, 10 C base temperature). Bloom events occurring just before the giant foxtail emergence events were chosen as the phenological indicators for 2001. The Julian day method used the average (1997 to 2000) dates of foxtail emergence to predict 2001 emergence. The GDD model (October 1 start date, 0 C base temperature) was chosen by determining the combination of start date and base temperature that provided the lowest coefficient of variation for the 1997 to 2000 data. The WeedCast prediction was generated using local soil and environmental data from 2001. The rank order of the 54 phenological events in 2001 showed little deviation from the 4-yr (1997 to 2000) average rank order (R2 = 0.96). The phenological calendar indicated that, on average, 25% of giant foxtail seedlings had emerged when red chokeberry was in first bloom, and 80% of seedlings had emerged around the time multiflora rose was in full bloom. We compared the phenological calendar predictions for 25, 50, and 80% emergence with those based on Julian day, cumulative GDD, and WeedCast. The average deviation in predictions ranged from 4.4 d for the phenological calendar to 11.4 d for GDD. In addition to being generally more accurate, the phenological calendar approach also offers the advantage of providing information on the order of phenological events, thus helping to anticipate the progress of emergence and to plan and implement management strategies.

Perennial weeds constitute a serious problem in Greek cotton-growing areas, as they strongly competing against the crop and downgrade the final product. Monitoring weeds at a regional scale and relating their occurrence with abiotic factors will assist in the control of these species. Purple nutsedge, field bindweed, bermudagrass, and johnsongrass were studied in cotton crops for three consecutive growing seasons (2007 through 2009) in a large area of central Greece. Weed densities and uniformities per sampling site were assessed in relation to soil and climatic data. Abundance index (AI), which is highly dependent on abiotic factors, was also estimated, and revealed purple nutsedge to the most persistent and damaging species among the recorded weeds. Field bindweed showed the highest correlation with soil properties and especially with clay content. Furthermore, correlation analysis was used over the sampling years in order to assess the stability of weed occurrence in the sampling sites. Purple nutsedge, field bindweed, and bermudagrass proved to be stable in location and intensity. The weed density spatial distribution was evaluated by using local indicators of spatial autocorrelation (LISA) statistics, and was mapped by ordinary kriging and co-kriging interpolation methods. Only 1 to 3 spatial outliers were identified in each 1 of the 3 yr. Between the two interpolation methods co-kriging delivered better results for field bindweed and purple nutsedge, indicating that soil data could improve the estimation of weed occurrence. These co-kriging interpolated weed maps would be a very useful tool for decision makers in taking appropriate weed control measures.

The knowledge of weed distribution in a field is a key factor to manage weeds effectively. The feasibility of using weed distribution maps for site-specific weed control will largely depend on the stability of the spatial distribution of the populations. Seed banks are the most reliable way of telling the area's weediness, but the effect of regular herbicide applications on its stability is largely unknown. A field experiment was conducted during 3 yr in a winter wheat field under herbicide treatments with the aim of studying the seed bank's spatial distribution of prostrate knotweed and corn poppy and the spatiotemporal stability of their populations. Soil samples were taken each year on the same locations, and seed abundance was measured by germination in greenhouse. Both species accounted for more than 10% of the broad-leaved weed seed bank and they were selected for further analysis. Prostrate knotweed seed-bank density decreased 76% and corn poppy 88% in 3 yr. Spatial distribution was described by spherical isotropic semivariograms. Distance of spatial dependence (range) of prostrate knotweed and corn poppy decreased 33 and 11% respectively, and the spatial variability (sill) decreased 96 and 99%. Yearly spatial seed distribution was compared for each species and no temporal stability was observed over a 3-yr period. The lack of stability was attributed to the important decrease of seed density over time and the increase in the short-range variability (nugget). However, for prostrate knotweed, the location of minima and maxima were roughly the same between years, allowing farmers to extend the period of use of the weed distribution maps. Although spatial distribution of seed banks can be affected by processes that promote fast changes in the densities of weed populations, this fact does not mean that weed distribution maps could not be used in consecutive seasons.

The diversity of resistance among wild oat collected before and after commercial introduction of imazamethabenz, difenzoquat, diclofop, fenoxaprop-P, sethoxydim, and tralkoxydim was evaluated. Wild oat sampled in 1964 and 2000 from the Red River Valley of Minnesota and North Dakota were screened for resistance. Nearly 43% of the 1964 collections were susceptible (S) to all six herbicides, whereas only 9% were S by 2000. The frequency of resistance in 2000 compared with 1964 increased for all six herbicides, and 27 phenotype response groups to the six herbicides occurred in 2000 vs. 14 phenotype response groups for the 1964 collection. The proportion of resistant (R) plants increased faster for the aryloxyphenoxypropionate (APP) herbicides, diclofop and fenoxaprop-P, than for the cyclohexanedione (CHD) herbicides, sethoxydim and tralkoxydim. High diversity of resistance responses was observed in wild oat to acetyl-coenzyme A carboxylase–inhibitor herbicides, suggesting that there may be multiple APP herbicide– or CHD herbicide–specific resistance mechanisms in addition to those that confer cross-resistance. The trend of resistance response generally indicates that increased exposure to herbicides in wild oat may confer resistance to newly introduced but unrelated herbicides.

Pitted morningglory seed were collected in the fall of 2003 from Blackville, SC, and 2004 from Pendleton, SC, to assess the effect of After-Ripening and burial on light and temperature requirements for germination. Pitted morningglory germination was evaluated over a 12-mo period after maturation. Germination was neither stimulated by red light or inhibited by far-red light, nor was it reversible by red or far-red light. Light was not essential for germination of buried seed. Direct exposure to sunlight prevented germination of recently mature seed, but not once seed had sufficiently after-ripened. Pitted morningglory was capable of germination in darkness over a wide range of constant and fluctuating temperatures immediately after maturation. Germination in response to temperature varied with time of year after maturation, with the population from Pendleton having increased germination in May. Thermal fluctuations increased germination of both populations at suboptimal temperatures. Thermal amplitude regulation of germination varied over time and appeared to play a more important role in germination of after-ripened seed than recently mature ones. The ecological significance of changes in germination requirements with After-Ripening is discussed.

The advancement of biological weed control is limited by the slow development of effective, broader-spectrum biological control agents. Protoplast fusion was carried out between the Helminthosporium gramineum subsp. echinochloae (HGE) strain, HM1, and Curvularia lunata (CL) to breed new strains with improved biocontrol efficiency. The HM1 strain was derived from HGE by ultraviolet (UV) treatment. Conditions for protoplast fusion were optimized, including lytic enzyme mixtures and incubation time. The efficacy of lytic enzyme mixtures on cell wall digestion was also compared. The most effective lytic enzyme mixture for CL was 2% lywallzyme plus 2% snailase and for HM1, 2% cellulase plus 2% snailase. The optimum incubation time was 16 h for CL and 24 h for HM1. All fusant strains exhibited similar morphological and conidial properties to the HM1 parent. A total of 1,360 fusant strains were produced, 136 of which were randomly selected for characterization. Seven fusant strains showed improved spore productivity and four fusant strains had increased production of the phytotoxin ophiobolin A, compared with HM1. Random amplified polymorphic DNA (RAPD) analysis of the fusant strains showed that the seven phytoactive fusant strains had a high similarity index (95 to 99%) to the parent HM1, but low similarity (22 to 26%) to the parent CL. Ethyl acetate extract of the culture broth and mycelia effectively controlled major weeds in rice production.

Green algal cell suspensions are used in agrochemical research for screening herbicides. This paper reports on the inhibitory effects of 39 herbicides from 19 different chemical classes with nine different modes of action on green algae cultured in liquid media. The results show that herbicide screening using Chlorella as a model organism is simple, quick, and sensitive. In vitro systems can be a second screening net to ensure that potential new structures are not missed. A set of biotests using cell suspension cultures of Chlorella are suggested as a herbicide prescreen to eliminate inactive compounds before testing on higher plants in the greenhouse. The method has been shown to be suitable for the requirements of microscreen and high-throughput screening (HTS) in vitro. A large-scale microscreen has been devised, which cultures algal cells using self-made Erlenmeyer flasks containing 5 ml of liquid medium. They are particularly useful for HTS compounds with dissimilar modes of action.