The benzoylcyclohexane-1,3-diones, the triketones, are potent bleaching herbicides whose structure-activity relationships and physical properties are substantially different from classical bleaching herbicides, which affect phytoene desaturase. The first clue to their unique mechanism of action was the discovery that rats treated with a triketone were found to be tyrosinemic. Additionally, examination of the rat urine revealed the accumulation of p-hydroxyphenylpyruvate (HPP) and p-hydroxyphenyllactate. These results suggested that this chemically induced tyrosinemia was the result of the inhibition of p-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27), and this suggestion was confirmed when a triketone was shown to be a potent inhibitor of rat liver HPPD. In plants, HPPD is a component of the biosynthetic pathway to plastoquinone (PQ), which in turn is a key cofactor of phytoene desaturase. The expectation that triketone-treated plants should accumulate tyrosine while having reduced PQ levels was dramatically demonstrated in the meristematic tissue of ivyleaf morningglory. Plant HPPD, like the mammalian enzyme, was inhibited in vitro by triketones. These biochemical effects provide evidence that the triketone herbicidal mechanism of action is HPPD inhibition leading to a deficiency of PQ, a key cofactor for carotenoid biosynthesis. Other chemical classes of bleaching herbicides were also examined for their ability to elevate tyrosine and deplete PQ as a definitive means of establishing their mode of action and for delineating the structural and physical chemical requirements for an HPPD herbicide. Evidence is provided to support the claim that a 2-benzoylethen-1-ol substructure is the minimum substructure required for a potent HPPD inhibitor.

An enzyme-directed approach has led to finding specific inhibitors of imidazoleglycerolphosphate dehydratase (IGPD), which catalyzes a dehydration reaction of imidazoleglycerol phosphate (IGP) to form imidazoleacetol phosphate in the biosynthesis of histidine. These newly synthesized IGPD inhibitors showed wide-spectrum, postemergence herbicidal activity at application rates ranging from 0.05 to 2 kg ai ha−1. IGPD from wheat germ was purified to apparent homogeneity and characterized, and IGPD cDNAs were isolated from Arabidopsis and wheat cDNA libraries. Both the purified plant enzyme and the recombinant enzymes, which have been prepared by expressing the isolated IGPD cDNAs in the baculovirus/insect cell system, were used for establishment of an in vitro enzyme assay for inhibitor evaluation. Enzyme inhibitors were designed and synthesized by considering the structure of the substrate, IGP, and the inhibition activities were enhanced by a systematic structure—activity relationship study in vitro. Three triazole phosphonates, IRL 1695, IRL 1803, and IRL 1856, inhibited the reaction of the enzyme with Ki values of 40 ± 6.5 nM, 10 ± 1.6 nM, and 8.5 ± 1.4 nM, respectively, and were highly cytotoxic to cultured plant cells. Plant cell growth inhibition was completely reversed only by l-histidine, proving that the cytotoxicity was primarily caused by the inhibition of histidine biosynthesis. These new IGPD inhibitors have demonstrated that the specific inhibition of plant IGPD is lethal to plants. Further, treatment of Arabidopsis with IRL 1803 affected gene expression of eight enzymes involved in the biosynthesis of aromatic amino acids, histidine, lysine, and purines. These effects were reversed by the addition of l-histidine, indicating that the changes in gene expression were caused by the inhibition of l-histidine biosynthesis. These results indicate that higher plants, like microorganisms, are capable of cross-pathway metabolic regulation.

SCB-1, a semicarbazone, is a phytotoxin with both preemergence and postemergence activity against broadleaves and grasses. It effectively displaced [3H]naptalam bound to a solubilized plasma membrane fraction, with the concentration required for 50% dissociation being 3.5 nM. The intrinsic dissociation constant (Kd) for binding of [3H]naptalam to zucchini plasma membranes was estimated to be 9.7 nM. Binding of [3H]naptalam to the solubilized protein fraction from the plasma membrane was rapid, time dependent, saturable, and reversible. The Kd for binding was estimated at 4 nM. Based on the maximum binding of [3H]naptalam, concentration of the binding protein was calculated to be 23.6 and 10.8 pmol mg−1 protein in the plasma membrane and solubilized fraction, respectively. SCB-1 also blocked the efflux of [14C]IAA, resulting in more than 800% accumulation of radiolabel in zucchini squash hypocotyl segments at 5 μM. This accumulation was quenched by excess cold IAA and 2,4-D. Collectively, these results suggest that SCB-1 is a potent inhibitor of auxin efflux and that it competes with naptalam for binding to the plasma membrane protein.

Attempts to discover new active ingredients and target sites within the aromatic pathway have resulted in the synthesis of potent enzyme inhibitors, but no herbicides. As an aid in identifying a new target for inhibitor design and screening, we have determined the mode of action of a compound (6-methyl anthranilate) that exhibits noncommercial levels of herbicidal activity. Our evidence suggests that 6-methyl anthranilate is converted in vivo, by traversing the tryptophan biosynthetic sequence, to 4-methyl tryptophan, which inhibits anthranilate synthase. Inhibitors synthesized by design and those found by target-based screening converged on analogs of tryptophan and anthranilate. None, however, was more herbicidal than 6-methyl anthranilate.

Greenhouse-grown soybean cultivars varied in their tolerance to preemergence application of sulfentrazone. The cultivars Ransom, Hutcheson, Kato, Gasoy 17, and Cobb exhibited relatively low tolerance to 0.5 kg ai ha−1 sulfentrazone with 38, 41, 46, 50, and 58% height reduction compared to respective controls. The growth of tolerant cultivars Centennial, Edison, and Hartz 5164 was not affected by this treatment. However, the growth of all cultivars was reduced at the excessive rate of 2.0 kg ha−1 preemergence application of sulfentrazone. No differences in root uptake or translocation of [14C] sulfentrazone were observed between the relatively tolerant and less tolerant cultivars tested. Centennial and Hutcheson cultivars rapidly metabolized sulfentrazone via oxidative degradation of the 3-methyl group on the triazolinone ring of the herbicide. Only 4.7 and 4.9% of the active ingredient remained in the foliage of Hutcheson and Centennial 24 h after treatment, respectively. While there were no differences in Protox inhibition or Proto IX accumulation between the two cultivars, Hutcheson was more sensitive than Centennial to peroxidative stresses induced by either Proto IX accumulation or rose bengal. Therefore, tolerance to sulfentrazone is due to rapid metabolism of the herbicide; however, the intraspecific difference in response to sulfentrazone appears to be due to intrinsic differential tolerance to the herbicide-induced peroxidative stress.

Differential response of three weed species to prosulfuron was examined through studies of acetolactate synthase (ALS) sensitivity, uptake, translocation, and metabolism of the herbicide. Sensitivity of common cocklebur, common lambsquarters, and sicklepod ALS to prosulfuron was similar with I50 values of 3.3, 4.5, and 8.8 nM, respectively. Absorption of the herbicide increased with time for all three weed species. Percent absorption 2 d after application for common cocklebur, common lambsquarters, and sicklepod was 8, 33, and 26%, respectively. An average of 53% of the absorbed herbicide remained in the treated leaves of common cocklebur, 80% in common lambsquarters, and 93% in sicklepod. Approximately 33, 15, and 5% of the absorbed herbicide was translocated into the upper shoots of common cocklebur, common lambsquarters, and sicklepod, respectively. Metabolism of 14C-prosulfuron 1, 2, and 4 d after application was greater in sicklepod than in common lambsquarters or common cocklebur. The results suggest a combination of differential rates of translocation and metabolism may account for the differing sensitivity of these weeds at the whole plant level.

Fresh weight reductions of greenhouse-grown soybeans and corn treated with post-emergence thifensulfuron at 4.4 g ai ha−1 were synergistically enhanced when the herbicide was mixed with formulated chlorpyrifos or malathion insecticides, but the enhancement was not observed with formulated methomyl insecticide. Thifensulfuron plus formulants of either chlorpyrifos or malathion reduced fresh weights no more than did the herbicide applied alone. Growth rate of hydroponically grown soybeans was reduced by root-applied thifensulfuron in combination with a foliar-applied formulation of chlorpyrifos or malathion, but not methomyl. Postemergence-applied thifensulfuron reduced the growth rate of hydroponically grown soybeans and corn with chlorpyrifos, malathion, or methomyl applied postemergence 1 d before thifensulfuron and with procedures identical to those used for absorption, translocation, and metabolism experiments. None of the insecticides applied 1 d before thifensulfuron altered foliar absorption of 14C-thifensulfuron or its translocation in unifoliolate soybeans or three-leaf corn. Thin-layer chromatography of soybean extracts revealed one primary thifensulfuron metabolite, presumably the deesterified free acid. 14C-thifensulfuron metabolism in corn produced about five unidentified metabolites in appreciable amounts. Levels of unmetabolized 14C-thifensulfuron 24 h after herbicide application were highest in insecticide-treated soybeans and corn. Over all experiments, enhancement of injury and inhibition of thifensulfuron metabolism generally were greatest in soybeans with chlorpyrifos but were greatest in corn with chlorpyrifos or malathion. Synergistic enhancement of thifensulfuron injury to soybeans and corn by chlorpyrifos and malathion appears to result from insecticide inhibition of thifensulfuron detoxication.

A new phytotoxin, pyridazocidin, was produced in shake-flask cultures of a Streptomyces sp. soil isolate. A combination of solvent partitioning, size-exclusion chromatography, and high-pressure liquid chromatography (HPLC) was used to isolate the material. The structure was assigned based on mass spectroscopy and nuclear magnetic resonance (NMR) experiments. Greenhouse evaluations indicated rapid necrosis and chlorosis of treated weeds, particularly of giant foxtail. Isolated chloroplasts consumed oxygen in the presence of pyridazocidin over a concentration range also inhibiting plant growth. Pyridazocidin represents the first reported natural product that appears to act via reversible oxidation/reduction linked to photosynthetic electron transport.

Field bindweed biotypes varying in susceptibility to glyphosate were studied to determine the role of glyphosate absorption and translocation in susceptibility differences. Absorption and translocation of radiolabeled glyphosate were measured after application to leaves of glyphosate-tolerant and -sensitive field bindweed biotypes grown under ambient, low, or high relative humidity (RH). Glyphosate injury and cuticle mass were also determined at both low and high RH. To avoid potential differences in susceptibility due to differential foliar absorption, glyphosate was fed to roots of hydroponically grown plants, and uptake, translocation, and injury patterns were also determined. Under ambient RH, no differences in foliar-applied glyphosate absorption or translocation were detected between field bindweed biotypes. The RH under which plants were grown influenced cuticle amounts, glyphosate absorption, and injury. Plants of both biotypes had less cuticular material when grown at high RH (80%) than at low RH (30%); however, more radioactivity was absorbed by the susceptible biotype at low RH. Regardless of RH regime, glyphosate injury was greater in the susceptible biotype than in the tolerant biotype. Glyphosate fed to roots of hydroponically grown plants was absorbed and translocated equally in the biotypes, yet relative differential glyphosate sensitivities were retained. These results suggest that differences in glyphosate susceptibility between field bindweed biotypes cannot be fully accounted for by differences in glyphosate absorption or translocation.

The objective of this study was to evaluate growth and seed production of giant foxtail under different N sources (NO3 and NH4) and N fertilizer application rates. Nitrate and NH4 fertilizers plus nitrification inhibitor were applied at 56, 112, and 225 kg N ha−1 under field conditions, and in the greenhouse four N rates (1, 5, 10, and 25 mM N) were applied in three NO3: NH4 ratios (100 : 0, 50 : 50, 0 : 100). Growth of giant foxtail was affected by N rates under both greenhouse and field conditions. In 1993, abundant rainfall in May and June allowed a rapid and earlier uptake of N by giant foxtail, resulting in larger plants with greater N accumulation and higher numbers of heads and seeds than in 1994. Total dry weight increased with increasing N rates; however, seed production reached a maximum at approximately 150 kg N ha−1. Nitrogen translocation efficiency decreased with increasing N rates. Giant foxtail did not show any preference to N form; however, seed production was reduced when the high N rate was applied as NH4 compared to NO3. These results suggest that NH4 fertilizer applications with a long-term nitrification inhibitor could reduce the seed production of giant foxtail and its contribution to the soil seedbank for subsequent growing seasons.

Greenhouse studies were carried out to determine the effects of different shading levels on biomass accumulation of purple and yellow nutsedge. Artificial shading levels provided were 20, 40, 60, and 80% of incident sunlight in addition to the full sunlight control (0% shading). Yellow nutsedge shoot and tuber number, plant height, and shoot and tuber dry weight were less affected by 20% shading than were these variables for purple nutsedge. Shoot and tuber dry biomass for both species responded linearly to shading level. In yellow nutsedge, 80% shading reduced dry matter partitioning to the tubers and increased partitioning into the shoots. In contrast, biomass partitioning to purple nutsedge tubers was decreased under high shade conditions without increases in partitioning to the shoots. The light compensation point was lower for yellow nutsedge than for purple nutsedge. This could possibly explain the wide distribution of yellow nutsedge in relatively low light intensity environments, in contrast with purple nutsedge, which generally occurs in tropical areas where higher light intensities are the norm.

Broom and threadleaf snakeweed reduce rangeland productivity throughout the western United States. These closely related species are only distinguishable at flowering and are highly variable in morphology and phenology. The objective of this study was to evaluate the genetic variation among snakeweed populations from Arizona, New Mexico, Texas, and Wyoming using isozyme analysis. Plants from each population were transplanted into pots and maintained in a greenhouse for 6 mo prior to analysis. Protein was extracted from young leaf tissue and separated using starch gel electrophoresis. Genetic variation was assessed based on expressed isozyme patterns of 10 enzymes. Of 10 enzymes tested, 16 loci were identified. Total genetic diversity (HT) was large for the eight polymorphic loci in broom snakeweed and three polymorphic loci in threadleaf snakeweed. Genetic variation among populations (DST) of both species contributed the major portion of diversity, but diversity among individuals within each population (HS) was also present. Phylogenetic analysis using Nei's genetic identity and distance determined broom snakeweed populations from New Mexico and Texas were more alike than those from Wyoming and Texas, or those from Wyoming and New Mexico. Compared to genetic differences among New Mexico broom snakeweed populations defined in a previous study, genetic diversity was greater among populations from different states. In addition, genetic variation was smaller in threadleaf snakeweed populations relative to broom snakeweed populations. Results concurred with previous research designating broom snakeweed as the progenitor of threadleaf snakeweed. Local populations of each species have probably developed due to long-term environmental selection.

Greenhouse experiments were conducted to determine the extent of purple nutsedge tuber depletion and shoot regrowth when shoots were removed periodically from plants sprouting from tubers of different original weights. Presprouted purple nutsedge tubers were planted in 0.5-L plastic containers filled with potting medium. Tuber weight categories were 0.25, 0.50, 0.75, and 1 g per tuber. Shoots were removed by cutting at soil level beginning 6, 12, 18, 24, or 30 d after transplanting (DAT). Shoot removal occurred every 6 d after the first removal with a minimum of seven removals. Tuber weight had an effect on both shoot number and dry weight when the first removal was imposed either 6 or 12 DAT. When initial shoot removal was begun 6 DAT, total tuber depletion was achieved after the fifth removal for tubers weighing 0.25 g and after the seventh removal for tubers weighing 0.50 g. When the initial removal was delayed until 12 DAT, only tubers weighing 0.25 g were depleted after the sixth removal. No tuber depletion occurred for tubers weighing 0.75 g or more at any initial removal time. Greater initial sprouting vigor was observed among large tubers compared to smaller ones. Early removal (6 DAT) promoted the formation of more shoots from tubers weighing 0.75 or 1 g.

On-farm implementation of IWM includes the biological and economic rationalization of weed management decisions. This, in turn, requires the ability to predict how weed populations and pressures will change with farm management options such as tillage method. We have developed a set of algorithms that simulate seedling emergence of mixed populations of green and redroot pigweed on the basis of the ecophysiological responses of seed germination and shoot elongation to temperature and, for germination, to moisture. The algorithms were calibrated using field data from 1993 and evaluated with data from two locations in 1994 and 1995. Over both sites and years, for four tillage systems, cumulative emergence was predicted with an overall root mean square error of 1.7%. Overprediction in one year was attributed to moisture shortage. Errors were greater for moldboard plowed plots than for those no-tilled. This decreased ability to predict emergence with increased tillage (soil disturbance) suggests that the algorithms should be modified to account for increased heterogeneity in weed seed distribution and soil moisture within disturbed soil. The algorithms, which were explicitly designed for incorporation within a crop growth model (e.g., CROPSIM), could become a useful part of a decision support system to rationalize weed management. Importantly, they could help predict changes in weed populations as farm management is adjusted, thereby reducing economic and environmental risk in agroecosystems.

Two greenhouse experiments were conducted to compare the growth of individual sulfonylurea-resistant and -susceptible kochia under noncompetitive conditions. Aboveground leaf and stem dry weight, and leaf area per plant were measured weekly 14 times, starting 14 d after planting. Data were fitted using the Richards function for shoot dry weight per plant, a polynomial exponential for leaf area per plant and splined function to calculate leaf area ratio, leaf weight ratio, and leaf/stem ratio. Absolute and relative growth rates, and net assimilation rate were derived from these functions. Growth and development of individual sulfonylurea-resistant and -susceptible kochia plants under noncompetitive conditions was the same. Final shoot dry weight and leaf area were unaffected by aceto lactate synthase enzyme differences in the kochia biotypes. However, more resources were partitioned to leaves than stems in resistant than in susceptible kochia. If competitive abilities of sulfonylurea-resistant and -susceptible kochia are different, it is not the consequence of differential growth and ontogeny of the two types of plants.

Laboratory experiments were conducted to determine the effect of light, oxygen, and submergence on bur beggarticks seed germination. Fresh and dry stored seeds exhibited dormancy, requiring thiourea and light for germination. Red light was more effective than far-red or blue light in breaking dormancy. Both an oxygen-saturated and anoxic atmosphere completely inhibited germination. Hypoxic conditions (5 and 10% oxygen) led to an increase in germination compared to normoxia (21% oxygen). Primary dormancy was eliminated by 1 yr of soil burial. Seeds exhumed after soil burial had greater germination percentage than fresh or lab-stored seeds after a brief far-red irradiation, but did not germinate in the dark. The induction of secondary dormancy increased with increasing durations of submergence. Finally, sowing at different soil depths (0 to 8 cm) showed that germination and emergence occurred mainly with shallow burial (0.5 to 1.0 cm).

Field experiments were conducted to determine the growth, development, and seed yield response of scentless chamomile when planted at various dates throughout the growing season. Scentless chamomile emerged within 6 to 15 d of planting and required 75 to 131 growing degree days (GDD). Flowering occurred within 56 to 82 d of planting and required 584 to 845 GDD. Plants that emerged in mid-July and onward did not flower in that season but survived as winter annuals. Spring-planted scentless chamomile matured within 97 to 127 d and required 906 to 1,176 GDD. Plants that emerged in May and June attained more leaf area and biomass than plants that emerged in mid-summer. Plants that emerged early in the growing season produced three-to fourfold more seed than plants that emerged at later dates. Scentless chamomile seed production ranged from 71,000 to 256,000 plants−1 depending on planting date and existing environmental conditions. Results are discussed in terms of its potential to become an increasingly troublesome weed in crops of the Canadian prairies and possible management strategies for its control.

Many growers perceive that reduced herbicide inputs and greater reliance on mechanical methods will result in increased weed management problems over time. Previous research has shown short-term benefits of integrated weed management strategies, but information concerning the long-term implications of these strategies is lacking. Therefore, research was conducted from 1992 through 1995 to determine the influence of full-rate preemergence broadcast, half-rate preemergence broadcast plus cultivation, and full-rate preemergence band plus cultivation treatments on weed population and seedbank dynamics in no-tillage (NT), chisel plow (CP), and moldboard plow (MP) systems in both continuous corn (CC) and corn—soybean (CS) rotation. Seventeen weed species were identified in the plant population and seedbank across all treatments, but common lambsquarters, giant foxtail, and redroot pigweed dominanted. Vertical distribution of weed seeds in soil was influenced by tillage, with 74, 59, and 43% of the total viable seed of these species less than 10 cm deep in NT, CP, and MP, respectively. In contrast, seed dormancy in the spring was not influenced greatly by tillage and averaged 79, 10, and 42% for common lambsquarters, giant foxtail, and redroot pigweed, respectively, across tillage methods. Crop rotation was not a major factor influencing weed population and seedbank dynamics. From 1992 to 1995, reduction of shoot biomass and seed production of the predominant weed species were similar among weed management treatments in most tillage and crop rotation systems. The influence of weed management treatments on the seedbank was also similar, with up to 50, 95, and 92% less seed of common lambsquarters, giant foxtail, and redroot pigweed, respectively, in the seedbank in 1995 than in 1992. Corn and soybean grain yields were similar among weed management treatments and were greater than the nontreated check in each tillage and crop rotation treatment. These results indicate that reduced herbicide inputs plus interrow cultivation were as effective as full-rate herbicide inputs for the management of several annual weed species in both conventional- and conservation-tillage systems over 4 yr.