To assess the influence of long-term fertilization on weed communities of early and late rice crops, the weed species composition was investigated in experimental plots initiated in 1981 at the Key Field Experimental Monitoring Station of the Reddish Paddy Soil Eco-Environment in Wangcheng, China. The treatments were (1) a control (CK), no fertilizer; (2) N–P, no K; (3) N–K, no P; (4) P–K, no N; (5) N–P–K; (6) N–P–K + Ca, N, P, and K plus lime; (7) N–P + S, N and P plus additional rice straw return; (8) N–P–K + S, N, P, and K plus additional rice straw; (9) N–K + M, N and K plus swine manure. The results indicated that weed flora composition and density were influenced by the different fertilization treatments. Multivariate analyses indicated that changes in the weed community composition were primarily due to soil-available N, followed by light intensity on the field surface, and soil-available P. More weed species and total weed density were observed in the control and P–K plots than in plots in which N, P, and K were applied together. Omission of N application had a greater effect on the weed community than the omission of P or K applications. Nutrients derived from synthetic fertilizers and organic manure or the additional application of lime had no obvious effect on the weed community of late rice crops.

We developed two leafy spurge bacterial artificial chromosome (BAC) libraries that together represent approximately 5× coverage of the leafy spurge genome. The BAC libraries have an average insert size of approximately 143 kb, and copies of the library and filters for hybridization-based screening are publicly available through the Arizona Genomics Institute. These libraries were used to clone full-length genomic copies of an AP2/ERF transcription factor of the A4 subfamily of DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEINS (DREB) known to be differentially expressed in crown buds of leafy spurge during endodormancy, a DORMANCY ASSOCIATED MADS-BOX (DAM) gene, and several FLOWERING LOCUS T (FT) genes. Sequencing of these BAC clones revealed the presence of multiple FT genes in leafy spurge. Sequencing also provided evidence that two different DAM transcripts expressed in crown buds of leafy spurge during endo- and eco-dormancy result from alternate splicing of a single DAM gene. Sequence data from the FT promoters was used to identify several conserved elements previously recognized in Arabidopsis, as well as potential novel transcription factor binding sites that may regulate FT. These leafy spurge BAC libraries represent a new genomics-based tool that complements existing genomics resources for the study of plant growth and development in this model perennial weed. Furthermore, phylogenetic footprinting using genes identified with this resource demonstrate the usefulness of studying weedy species to further our general knowledge of agriculturally important genes.

Bittercress (Brassicaceae) is one of the most prolific and costly weeds of the container nursery industry. Bittercress accessions from container nurseries throughout the major production zones in the United States were examined and compared with herbarium specimens. The identity of these weedy bittercress species were further explored using sequences of the nuclear ribosomal DNA (nrDNA) internal transcribed spacer (ITS) region and the nrDNA region for the COP1-interacting protein 7 (CIP7). Four species of bittercress were detected in the nursery industry of the United States, including New Zealand bittercress, hairy bittercress, flexuous bittercress, and little bittercress. The taxon referred to here as Cardamine flexuosa With. (flexuous bittercress) likely contains two genotypes previously reported as European C. flexuosa and Asian C. flexuosa. Phylogenetic relationships between the four species we examined, particularly in relationship to flexuous bittercress, were not fully resolved by the molecular evidence generated for this study. New Zealand bittercress is nonnative and does not appear in current keys to the species for the United States. Flexuous bittercress is also an alien species, which appears in some U.S. keys but not in all. To aid nurserymen and botanists in identification of these four closely related bittercress species, a key was developed and is accompanied by detailed descriptions and illustrations.

Practical and economical weed control measures are a necessary part of tree fruit and grape production. Weeds and grasses are serious competitors of fruit plants for moisture and nutrients; they provide cover for destructive mice, and interfere with harvest of the crop.

The floristic diversity and the vertical distribution of the weed seedbank were studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger soil seedbank (2,992 seeds m−2) than moldboard-plowed fields (1,481 seeds m−2) in the top 15 cm. This result can be explained by the larger perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 5 cm of the 15-cm soil core contained 35 and 46% of all weed seed in CT and RT systems, respectively. However, the CT system had the highest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the RT system, the same depth contained the highest concentration of perennial seeds. These results confirm that tillage systems and soil types can regulate seedbanks. Weed management programs must take this information into account.

Oats, sesbania (Sesbania macrocarpa Muhl.) and purple nutsedge (Cyperus rotundus L.) were grown as test plants in nine different soil types treated with 0, 0.5, 1, 2, 4, 8, and 16 ppm of ethyl N,N-di-n-propylthiolcarbamate (EPTC). Toxicity of EPTC was negatively correlated with the organic matter of the soil. A positive correlation was found between the sand and silt content of the soil and EPTC toxicity to oats and nutsedge. Toxicity of EPTC was not correlated with soil pH, cation exchange capacity, clay content, general reaction, or parent material. EPTC induced a dormancy of nutsedge tubers, of which the length was inversely related to the organic content of the soil.

Application of 20 to 40 lb/A dichlobenil made prior to emergence accomplished various degrees of control of pondweeds (Potamogeton diversifolius, P. foliosus, P. pectinatus, P. pusillus) and a slender naiad (Najas flexilis). Coontail (Ceratophyllum demersum) was affected only by the higher dosage. Limited control and growth inhibition was achieved on several forms of algae in early spring applications. However, applications made to rooted submersed aquatic plants and filamentous algae (Cladophora, Pithophora and Chara) at postemergence stage of development had little herbicidal effect.

Dichlobenil was not acutely toxic to fish at herbicidal concentrations. The range of median tolerance limits was 10 to 20 ppmw for pumpkinseed (Lepomis gibbosus), bluegill (L. macrochirus), redear sunfish (L. microlophus), and largemouth bass (Micropterus salmoides).

An electrical discharge system (EDS) was evaluated in field studies conducted in 1977 through 1979 in western Nebraska for its ability to control weed escapes in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′). Nine weeks after sugarbeets were planted, kochia [Kochia scoparia (L.) Schrad.] had attained a height above sugarbeets sufficient for EDS treatment. Redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters (Chenopodium album L.) generally attained sufficient height above sugarbeets 11 and 13 weeks after sugarbeet planting. Sugarbeet root yields were reduced 40, 20, and 10% from competition by kochia, common lambsquarters, and redroot pigweed, respectively. Treatment of kochia, redroot pigweed, and common lambsquarters with EDS in some cases resulted in a reduction in weed height. The EDS treatments reduced the stand of all weeds 32, 39, and 47% for 1977, 1978, and 1979, respectively. Although the EDS treatments failed to kill many weeds, it did suppress the competitive ability of the three weeds to the extent that sugarbeet yields were higher in areas receiving EDS treatments than areas receiving no EDS treatment.

In a predictable natural selection process, herbicides select for adaptive alleles that allow weed populations to survive. These resistance alleles may be available immediately from the standing genetic variation within the population or may arise from immigration via pollen or seeds from other populations. Moreover, because all populations are constantly generating new mutant genotypes by de novo mutations, resistant mutants may arise spontaneously in any herbicide-sensitive weed population. Recognizing that the relative contribution of each of these three sources of resistance alleles influences what strategies should be applied to counteract herbicide-resistance evolution, we aimed to add experimental information to the resistance evolutionary framework. Specifically, the objectives of this experiment were to determine the de novo mutation rate conferring herbicide resistance in a natural plant population and to test the hypothesis that the mutation rate increases when plants are stressed by sublethal herbicide exposure. We used grain amaranth (Amaranthus hypochondriacus L.) and resistance to acetolactate synthase (ALS)-inhibiting herbicides as a model system to discover spontaneous herbicide-resistant mutants. After screening 70.8 million plants, however, we detected no spontaneous resistant genotypes, indicating the probability of finding a spontaneous ALS-resistant mutant in a given sensitive population is lower than 1.4 × 10−8. This empirically determined upper limit is lower than expected from theoretical calculations based on previous studies. We found no evidence that herbicide stress increased the mutation rate, but were not able to robustly test this hypothesis. The results found in this study indicate that de novo mutations conferring herbicide resistance might occur at lower frequencies than previously expected.

Weed resistance to herbicides occurs when herbicides are overused and can be mitigated by reducing their use. Consensus on herbicide resistance management strategies is problematic given strong industrial profit motive links in the weed science discipline.

Annual sowthistle (Sonchus oleraceus L.) is a major weed of mungbean crops in Australia. Resistance in this weed to several herbicide groups is a challenging issue for its management. Hence, cultural weed management strategies, such as increasing the crop competitive ability through increased stand density, should be considered to reduce reliance on herbicides. It was hypothesized that a competitive crop stand may reduce the growth and seed production of S. oleraceus. Two pot studies were conducted, and each study was repeated once. The first study evaluated the effect of different mungbean [Vigna radiata (L.) R. Wilczek] densities (0, 82, 164, 246, and 328 plants m−2) on S. oleraceus growth and seed production, while the second study focused on glyphosate-resistant and glyphosate-susceptible biotypes of this weed in competition with densities of 0, 82, and 164 mungbean plants m−2. Although increasing mungbean density from 0 to 82 and 164 plants m−2 reduced S. oleraceus seed production by 55% and 78%, respectively, a large number of seeds were produced, even at the mungbean density of 328 plants m−2 (1,185 seeds plant−1). Both glyphosate-resistant and glyphosate-susceptible biotypes of S. oleraceus responded similarly to the increase in mungbean density. The results of the second study showed that height, leaves, number of inflorescence, and seed production per plant of both glyphosate-resistant and glyphosate-susceptible biotypes were reduced but not suppressed adequately. The glyphosate-resistant biotype produced fewer leaves and less biomass and, consequently, its seed production was 24% less compared with the glyphosate-susceptible biotype in the no-competition treatment. Both biotypes of S. oleraceus produced about 4,000 seeds plant−1 in competition with 164 mungbean plants m−2. The results suggest that crop competition alone cannot provide satisfactory control of S. oleraceus; therefore, for effective and adequate weed management, other practices such as PRE herbicides should be integrated with increased crop density.

Pesticide sorption by soil is among the most sensitive input parameters in many pesticide-leaching models. For many pesticides, organic matter is the most important soil constituent influencing pesticide sorption. Increased fertility, irrigation, and mowing associated with highly maintained turfgrass areas result in constant deposition of organic material, creating a soil system that can change drastically with time. Changes in soil characteristics could affect the environmental fate of pesticides applied to turfgrass systems of varying ages. Sorption characteristics of simazine and S-metolachlor were determined on five soils from bermudagrass systems of increasing ages (1, 4, 10, 21, and 99 yr) and compared to adjacent native pine and bare-ground areas. Surface soil (0 to 5 cm) and subsurface soil (5 to 15 cm) from all sites were air-dried and passed through a 4-mm sieve for separation from plant material. Using a batch-equilibrium method, sorption isotherms were determined for each soil. Data were fit to the Freundlich equation, and K d (soil sorption coefficient) and K oc (organic carbon sorption coefficient) values were determined. Sorption and soil system age were directly related to organic matter content in the soil. Sorption of both herbicides increased with age of the soil system and was greatest on the surface soil from the oldest bermudagrass soil system. Herbicide sorption decreased at greater soil depths with lower organic matter. Greater amount of 14C–simazine sorbed to subsurface soil of the oldest turfgrass system compared to 14C–S-metolachlor. Results indicate that as bermudagrass systems age and accumulate higher organic matter levels increased herbicide sorption may decrease the leaching potential and bioavailability of simazine and S-metolachlor.

The combustion apparatus described provides a rapid and inexpensive alternative for measurement of radioisotopes in plant parts. The apparatus utilized a nickel-chromium resistance wire connected to a low-voltage transformer to generate the heat required to ignite the plant tissue. The nickel-chromium wire also served as the sample holder. Combustion was performed in an O2-flushed liquid scintillation vial, and the combustion gases containing either 14CO2 or 3H2O were trapped in situ, scintillant was added, and the vials assayed for radioactivity. The percentage of the radioactivity recovered after combustion was 96.0 for 14C and 93.4 for 3H with standard deviations of 6.8 and 3.0%, respectively.

Arsenic residues in soil were examined annually over a 5-yr period following annual applications of MSMA (monosodium methanearsonate) at 4.4, 9, 13, 18, 22, 36, 72, 144, and 288 kg/ha. Elemental arsenic did not increase in any plot receiving MSMA rates less than 36 kg/ha. Plots receiving 72, 144, and 288 kg/ha of MSMA for 5 yr showed significant increases in arsenic residues. The arsenic residues for the two heaviest treatments (144 and 288 kg/ha) increased significantly during the first 4 yr, but did not increase in the fifth year.

Increasing the germination potential of dormant seeds in a population over time generally requires afterripening. Research was conducted to study the relationship between temperature and seed moisture content on afterripening of dormant leafy spurge seeds. Germination of nonafterripened seeds was 59 and 36% after 21 d for the Harwood and Fargo populations, respectively. Germination of 85 to 87% and 58 to 62% was obtained for the Harwood and Fargo populations, respectively when afterripened for 12 to 24 wk under the most effective conditions of 30 C and 2.6% seed moisture; increasing the afterripening temperature to 45 C did not increase germination. Germination decreased slightly at 30 C as the seed moisture content increased to 5.6%, but germination still exceeded that of nonafterripened seeds. Afterripening at 30 C with a seed moisture content of ≥9% greatly decreased germinability due to seed ageing. A temperature of 5 C was effective for afterripening when the moisture content was 3.3%, but germination was still slightly less than for the low moisture content seeds afterripened at 30 C. Afterripening seeds with 6 to 13% moisture at 5 C generally did not increase germination compared with the control, but did not result in seed ageing.

Cotton yield response was evaluated in 1987, 1988, and 1989 when weeds were managed with preemergence fluometuron [none (0%), band (50%), or broadcast (100%) surface coverage], cultivation (none, one, two, or three times), and postdirected fluometuron + MSMA. Weed densities (primarily prickly sida, morningglories, and hemp sesbania) varied widely among years and were directly related to early season rainfall. Postdirected herbicide application or cultivation(s) had little effect on weed density. The use of a banded fluometuron application reduced weed biomass 28 to 47%. A further decrease was observed when preemergence fluometuron was increased from banded to broadcast coverage. Seed cotton yields were low with no preemergence fluometuron. Banded fluometuron and at least one cultivation had yields similar to broadcast fluometuron only. Cotton yields were related to weed density and weed biomass in a hyperbolic relationship. Low weed densities caused more yield loss per unit weed density than higher densities.

The soil bacterium Alcaligenes eutrophus is able to degrade phenoxy herbicides such as 2,4-D [(2,4-dichlorophenoxy)acetic acid]. It has been shown that several degradative genes are located on a large plasmid harbored by this organism. We have demonstrated by HPLC and gas chromatography that 2,4-dichlorophenol is a major metabolite resulting from 2,4-D degradation by A. eutrophus. A portion of the large plasmid carrying the 2,4-D catabolic genes has been cloned and mapped by restriction endonuclease digestion. Overlapping subclones have been produced and were used in conjugation experiments in order to identify the first gene involved in 2,4-D detoxification. Once sequenced, this gene will be transferred to dicotyledonous plant cells using available vector systems.

Field experiments were conducted during 1984 and 1985 to determine the influence of 2,4-D and MCPA amine and ester formulations on control of foxtail millet with diclofop. Foxtail millet control increased as the diclofop rate increased from 0.6 to 1.7 kg ai/ha whether diclofop was applied alone or with 2,4-D or MCPA amine and ester formulations; however, foxtail millet control was lower when diclofop was applied with 2,4-D or MCPA amine and ester formulations compared to diclofop applied alone. MCPA ester, MCPA amine, 2,4-D ester, or 2,4-D amine at 0.14 kg ai/ha added to a diclofop spray mixture reduced foxtail millet control 8, 15, 20, and 30% compared to diclofop applied alone, averaged over diclofop rates of 0.6, 0.8, 1.1, 1.4, and 1.7 kg/ha. Adding a petroleum oil to diclofop increased foxtail millet control with diclofop at 0.6 kg/ha but not at 0.8 kg/ha. The petroleum oil additive did not overcome the antagonism of foxtail millet control when diclofop at 0.6 kg/ha was applied with 2,4-D or MCPA amine and ester formulations. The amount of 14C-diclofop absorbed by oats was greater when diclofop was applied with the ester formulations of 2,4-D or MCPA compared to the amine formulations. Either once-refined sunflower oil or sunflower oil methyl ester applied with 14C-diclofop increased the amount of 14C absorbed and translocated in oat plants compared to 14C-diclofop applied alone.

Differential tolerances to dalapon (2,2-dichloropropionic acid), amitrole (3-amino-s-triazole) plus ammonium thiocyanate, and glyphosate [N-(phosphonomethyl)glycine] of 3- to 10-week-old seedlings of creeping red fescue (Festuca rubra L. ‘Boreal’) and redtop (Agrostis alba L.), which are desirable ditchbank grasses, and reed canarygrass (Phalaris arundianacea L.), a troublesome ditchbank weed, were evaluated in the field. Within each of four treatment dates (growth stages), dalapon affected all species similarly; effects among treatment dates ranged from none to severe. Amitrole at 4.5 kg/ha plus ammonium thiocyanate at 4.1 kg/ha reduced the stand of 3-week-old seedlings of redtop, creeping red fescue, and reed canarygrass by 0, 42, and 94%, respectively, but had little effect on older seedlings. Glyphosate at 1.1 kg/ha did not significantly reduce the stand of 5- to 10-week-old creeping red fescue seedlings, but killed nearly all reed canarygrass seedlings. Redtop was intermediate in its tolerance to glyphosate. Applications of amitrole plus ammonium thiocyanate or glyphosate should be effective in converting ditchbank vegetation from an undesirable stand of reed canarygrass to a desirable stand of redtop and creeping red fescue, respectively.

All actively metabolizing cells have an electrical potential difference, negative on the interior, across their membranes. This electrochemical potential gradient is generated primarily by proton-pumping ATPases and provides the driving force for the transport of various ionic and neutral solutes. It is a key element in the energy metabolism of cells. Such factors as alteration of transport processes, energy metabolism, cytoplasmic pH, and membrane permeability have a direct effect on the magnitude of the membrane potential. In a brief survey, diclofop-methyl, diclofop, hydroxydiclofop, CGA 82725, haloxyfop-methyl, haloxyfop, bentazon, dinoseb, 4-hydroxy CIPC, and 2-hydroxy CIPC caused rapid depolarizations of the membrane potential of oat coleoptiles. Chlorsulfuron, dimethipin, propham, CIPC, dicamba, alachlor, metolachlor, napthalic anhydride, and paraquat had no measurable effects. The depolarizing effects of diclofop-reported earlier are used to illustrate the methods and interpretation of plant cell membrane potential measurements. Diclofop and diclofop-methyl affect the membrane properties of sensitive plant cells. Diclofop irreversibly depolarized the membrane potential and increased the proton permeability of sensitive cells but not resistant cells. It also increased the ATPase activity of isolated membrane vesicles. The mechanism through which diclofop exerted its effect is not fully understood. The equipment and techniques required for the intercellular recording of membrane potentials and resistance are described as well as the limitations of the techniques. A method not used in herbicide studies but with great potential for studies of herbicide interactions with membranes is patch clamp. A brief introduction to the methods will be given.