Croftonweed is a major invasive weed in China and, although of subtropical origin, has invaded into regions with colder climates. Freezing tolerances of nine croftonweed populations from different geographies were studied using a freezing injury index. Physiological responses to freezing temperature were determined to elucidate mechanisms of freezing tolerance. Plants from Baise, Guangxi (BSG), and Qujing, Yunnan (QJY), China, showed the most freezing injury symptoms, whereas plants from Huangguoshu, Guizhou (HGG), China, displayed the least. Under freezing stress, physiological changes, including increases in malondialdehyde (MDA) and total soluble protein contents; reductions in total soluble sugar, chlorophyll contents, and ratios of variable chlorophyll fluorescence to maximum chlorophyll fluorescence (Fv : Fm); and fluctuation of superoxide dismutase (SOD) activity, were observed among all nine populations. However, different degrees of physiological responses were found among populations with diverse low-temperature sensitivities. After 4 d of treatment at −5 C, MDA content increased 25-fold in leaves of the sensitive BSG population compared with untreated leaves, whereas a range of 0.8-fold to ∼5.3-fold increase was found in other populations. Total soluble protein content in leaves of the tolerant HGG population increased to the highest value among the nine populations. SOD activity of the freezing-sensitive BSG population decreased 36% of the control, whereas the tolerant HGG population reduced to 70%. Moreover, soluble sugar of the tolerant HGG population decreased 29%, less than the sensitive BSG population (87%). There were fewer declines in the percentages of chlorophyll content and Fv : Fm value in HGG than in BSG (less 44% and 32%). Freezing injury index had significant negative correlations with Fv : Fm values and chlorophyll contents (−0.619 and −0.622, respectively). These results suggest that croftonweed has evolved into different ecotypes with regard to freezing tolerance through physiological adaptation during their invasion of southwest regions of China. The freezing-tolerant croftonweed population would have more chances to invade distant northeastern areas in the future.

Controlled – environment studies were conducted to determine the influence of temperature, light, seed weight, depth of planting, substrate pH, and salinity on the germination and emergence of silverleaf nightshade (Solanum elaeagnifolium Cav.). The effects of planting date, top removal, and root-cutting length on establishment were studied in the field. Fluctuating temperatures of 20 to 30C produced 57% germination. Maximum germination, 59%, occurred between pH 6 and 7. NaCl concentrations greater than 2500 ppmw caused germination to decrease 85% or more compared to the control. Light was not important for germination. Mean seedling emergence was 46% from depths of 1.0 to 2.5 cm. Silverleaf nightshade seedling emergence from a 2-cm depth was 31% greater in a sandy soil than a loam. Delayed seeding caused a decline in dry-weight accumulation of above-ground parts. Fifteen percent of seedlings clipped 15 days after emergence were capable of regrowth, and after 30 days or more, 90% of the clipped plants regrew.

Myrothecium verrucaria, isolated from sicklepod, was investigated for bioherbicide potential against a wide range of economically important weed species from agronomic, pasture, and horticultural systems. A number of different weed species from a range of plant families were highly susceptible to sprays of crude preparations of the fungus. A small number of species, primarily monocots, were tolerant, showing no damage symptoms or insignificant effects on biomass. Symptoms developed very rapidly in susceptible hosts, suggesting the activity of toxins, several of which are known to be produced by Myrothecium spp. The activity of crude harvests of M. verrucaria was not diminished when they were filtered to remove fungal mycelium and spores. In contrast, washed conidia had relatively little impact on weed species. We conclude that the activity of M. verrucaria is primarily caused by the activity of metabolites produced by the fungus in culture and not due to infection by the fungus per se. Myrothecium verrucaria cultural preparations have extremely potent herbicidal properties, but given its ability to produce mammalian toxins, we caution against its use.

Seedlings of kudzu were grown at 350, 675, or 1000 μl/L CO2 in controlled-environment chambers. At elevated CO2 concentrations, maximum leaf expansion rates were approximately 40% greater, leaves were fully expanded several days sooner, fully expanded leaves were larger at each leaf position, and leaf production rates were increased 12%. Peak starch accumulation was much greater in plants grown at elevated CO2 concentrations. Total xylem water potentials were higher (less negative) at full hydration, and osmotic potentials were decreased (more negative) by CO2 enrichment. At 1000 μl/L CO2, leaf turgor pressure was twice that at 350 μl/L CO2. Results suggest that leaf expansion rates and leaf expansivity may have been increased due to higher turgor pressure at the higher CO2 concentrations. The potential for successful seedling establishment may be enhanced as the atmospheric CO2 concentration continues to rise, increasing kudzu invasiveness.

Field experiments were conducted over a 4-yr period, 1989 to 1992, in southern Manitoba to investigate potato (5. tuberosum) response to simulated drift of the dimethylamine salt of dicamba, 2.8 to 22.2 g ai ha–1; alkanolamine salt of clopyralid, 4 to 32 g ai ha–1; and tribenuron, 0.15 to 1.2 g ai ha–1. Rate ranges investigated corresponded to 2 to 16% of the recommended field rate for each herbicide. Dicamba at 22.2 g ai ha–1, clopyralid at 32 g ai ha–1, and tribenuron at 1.2 g ai ha–1 reduced total tuber yields 40, 29, and 41%, respectively, and marketable yields 74, 75, and 53%, respectively. Average weight of marketable-size tubers was unaffected by any treatment in any year. No tuber malformations were observed. Tubers produced on treated plants were planted the year following treatment to investigate the effects of simulated drift on subsequent tuber regrowth. Injury symptoms were evident for each herbicide in 2 out of 3 yr, but neither total nor marketable yields were affected.

Evidence in the literature has generally supported either of two paraquat resistance mechanisms: an increase in activity of oxygen radical-scavenging enzymes in resistant plants which affords protection from active oxygen species formed by paraquat; and sequestration of paraquat away from its site of action in the chloroplast. Evidence for the first model relies primarily on measurement of increased enzyme activity and cross-resistance to other oxygen radical-generating stresses in resistant plants. The sequestration model is supported by data showing decreased translocation of paraquat and absence of paraquat injury in plant systems that do not have increased levels of protective enzymes. An alteration in paraquat transport at one of several plant cell membranes could confer resistance by modifying movement of paraquat into the compartment bounded by that membrane. Properties of the plasmalemma, chloroplast envelope, and tonoplast that may be important to paraquat transport are discussed and data supporting or discounting specific membrane alterations in resistant plants are presented. Finally, the possibility that both mechanisms may work in concert is addressed.

For many years, virtually all pharmaceutical companies had an agrochemical division. This was partly to maximize the benefits of expensive chemical synthesis efforts by searching for many types of useful biological activities. Leads for pharmaceuticals and pesticides often overlap, in some cases leading to similar compounds used for human health and weed management purposes. This review will focus on herbicides and herbicide classes that have potential pharmaceutical properties, both as therapeutic agents that act through human molecular target sites and those that act on infectious agents. An example of the first case is compounds that target plant acetyl coenzyme A carboxylases, inhibiting fatty acid synthesis, and similar compounds used in humans as anti-inflammatory agents. Another such example is the triketone class of compounds that can act both as herbicides and as treatments for the genetic disease tyrosinemia, targeting the same enzyme in both cases. Examples of the second case are the relatively large number of herbicides that have activity against the malaria protozoan (Plasmodium spp.). It turns out that Plasmodium spp. and related disease organisms have an organelle that is apparently analogous to the plant plastid, the apicoplast. Herbicides such as dinitroanilines are active against several protozoan parasites by the same mechanism by which they kill plants, interaction with tubulin to halt cell division and other tubulin-dependent processes. These and other multiple activities of various herbicides and herbicide classes provide perspective on the broad biological activity of herbicides and related compounds.

Root sections of seven Canada thistle (Cirsium arvense (L.) Scop.) ecotypes were grown under 8, 12, 14, and 16-hr photoperiods at 16, 21, and 27 C. Flowering occurred in all ecotypes under a 16-hr photoperiod. At the 14-hr photoperiod five ecotypes flowered; flowering in three of them was temperature-dependent. Shoot and root development and plant height varied with ecotype. Both the root-to-shoot ratios and the number of shoot buds formed on the roots were inversely related to temperature and length of photoperiod. Herbicides tested for their effects on Canada thistle were 4-amino-3,5,6-trichloropicolinic acid (picloram), 3,6-dichloro-o-anisic acid (dicamba), and (2,4-dichlorophenoxy)acetic acid (2,4-D). Control of top growth increased with increasing temperature. Similarly, root control was maximum at 27 C, at which temperature there were few fleshy roots. Picloram, unlike 2,4-D and dicamba, caused little leaf damage but completely destroyed the root system.

Two separate studies were conducted to determine the relative importance of root and shoot competition in dry direct-seeded rice growing with junglerice and ludwigia. By growing rice in pots placed within larger pots such that the roots of the plants were either separated from or free to mingle with those of neighboring weeds, or by growing rice in the same pots but in the absence of weeds, the relative importance of shoot and root competition can be described. When rice was grown together with either weed species, shoot competition reduced the growth and yield of rice more than root competition. Results suggest that shoot competition for light may be the primary mechanism determining competitive outcomes between dry direct-seeded rice and junglerice or ludwigia. Junglerice was more competitive than ludwigia, which may reflect the C4 metabolism efficiency of junglerice compared to the C3 metabolism of ludwigia. Rice grain yield was highly correlated with above- and belowground biomass. The results also suggest the importance of measuring the whole plant when seeking to understand differences in the competitive ability of dry direct-seeded rice.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, pH, water stress, and planting depth on crowfootgrass germination. When treated with constant temperature, crowfootgrass germinated over a range of 15 to 40 C, with the optimum germination occurring at 30 C (42%). Onset, rate, and total germination (94%) were greatest in an alternating 20 and 35 C temperature regime. Germination decreased as pH increased, with greatest germination occurring at pH 4 and 5. Germination was reduced when seed was subjected to water stress, and no germination occurred below −0.8 mPa. Emergence was similar when seed were placed on the soil surface or buried at depths of 0.5 or 1 cm. Germination decreased with burial depth, and no seed emerged from 10 cm. These data suggest that crowfootgrass may emerge later in the season with warmer temperatures and after a precipitation event, and may emerge rapidly. These attributes could contribute to poor control later in the season by soil-applied herbicides or allow crowfootgrass to emerge after final postemergence treatments are made.

Simazin, 2–chloro–4,6–bis(ethylamino)–s–triazine, is a relatively new herbicide that has shown much promise for pre-emergence weed control in corn (3,4,7). It is a white crystalline compound with a solubility of 5 ppm in water and 900 ppm in chloroform. This compound is non-flammable, non-corrosive, and has an acute oral toxicity greater than 5 g/kg for mice. Simazin is said to be highly toxic to cucumber, of intermediate toxicity to cotton, and non-toxic to corn. Schneider (6) reported it to be effective only when absorbed through the root system. This study was begun to determine whether the differences in susceptibility of these plants and the limitation of effectiveness of simazin to the root system could be correlated with its absorption and translocation.

The influence of water stress on toxic alkaloid concentrations in threadleaf groundsel (Senecio longilobus Benth.) was investigated in a controlled environment by monitoring leaf xylem water potential during a 32-day period, and then analyzing for alkaloid concentration by plant part. Total plant alkaloid concentration increased with increasing severity of water stress. The most severe water stress (−2.0 MPa) for the 32-day period increased alkaloid concentration by 4.6 times, compared to the least stress imposition (−1.0 MPa). Averaged across watering regimes, alkaloid concentrations (percent dry weight) were highest in threadleaf groundsel roots (0.088%) and occurred in similar concentrations in stems (0.046%) and leaves (0.050%). These results indicate the importance of considering the influence of plant water stress when investigating secondary plant compounds.

Research involved the behavior of BAS-9052 OH {2[1-(ethoxyimino)butyl]-5-[2(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} in soybean [Glycine max (L.) Merr.] and johnsongrass [Sorghum halepense (L.) Pers.] plants, and the fate of 14C-BAS-9052 OH in intact plants and cell cultures of both species. Microscopic examination of seedling johnsongrass plants (two- to three-leaf stage) treated with foliar applications of 0.48 μg/plant revealed disorganized apical regions and necrotic cells within the apex and leaf primordia of the shoot. Necrotic zones were also evident at the base of expanding leaves and in root apices 1 day after treatment. Following application of 14C-BAS-9052 OH, the same radioactive products were isolated from cell cultures and intact plants. Products included BAS-9052 OH and three unidentified metabolites. Greater proportions of unchanged BAS-9052 OH were extracted from the apical leaves, roots, and cell cultures of johnsongrass than of soybean. BAS-9052 OH was thermal and photo-labile, and a large portion of 14C may not have entered the plant as BAS-9052 OH. However, tolerant soybean plants and cell cultures appeared to have metabolized the herbicide more rapidly than susceptible johnsongrass plants and cell cultures. An average of 64% of the 14C remained in the treated leaf of johnsongrass compared with 48% in soybean. About one-half as much 14C was translocated to the apical leaves of susceptible johnsongrass than tolerant soybean. Therefore, differential uptake and translocation cannot account for the selectivity of BAS-9052 OH.

Methyl isothiocyanate (MIT) consistently killed large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] seed at concentations of 4.0 mM or greater. Concentrations of 0.6 to 1.0 mM MIT delayed germination of large crabgrass seed but ultimately allowed the majority of seed to germinate. Dormant large crabgrass seed were killed at concentrations of MIT similar to those required to kill nondormant seed. MIT stimulated germination of dormant large crabgrass seed at sublethal concentrations (0.1 to 1.0 mM). Experiments with metham (sodium methyldithiocarbamate) in the greenhouse and field (metham rapidly degrades to MIT in soils) confirmed results of laboratory experiments with MIT.

Research was conducted from 1979 through 1982 in east-central Alabama to monitor changes in sicklepod (Cassia obtusifolia L. ♯ CASOB) seed numbers in the soil under various cropping and weed management systems in soybeans [Glycine max (L.) Merr.]. Significant declines in sicklepod seed numbers were attained only when mechanical summer fallow each year prevented replenishment of seed in soil. Repeated disking favored germination and emergence of sicklepod seed and thus caused a reduction in seed numbers. Chemical summer fallow, which relied on postemergence control of emerged sicklepod, did not result in a disturbed seedbed and was less effective in reducing seed numbers. Permitting subcompetitive densities of sicklepod, 0.45 and 0.90 plants/m2, to reach maturity each year resulted in increases in seed numbers in the soil. Increases were more dramatic in conventionally planted soybeans than in no-till culture. Sicklepod produced more pods per plant in tilled culture than in no-till. Sicklepod seed distribution in the upper 30 cm of the soil was not affected by the type of primary tillage.

Biological control of parthenium, a major weed in grazing areas in Australia, was initiated in the mid 1970s. Since then, nine species of insects and two rust fungi have been introduced. Evaluation using pesticide exclusion at two sites (Mt. Panorama and Plain Creek) in Queensland, Australia, revealed that classical biological control had a significant negative effect on the target weed, but the impact varied between years. In this study, I quantified the effects of biological control of parthenium on grass production. Grass production declined with the increase in parthenium biomass. Significant increase in grass production due to biological control was observed, but only in 1 of 4 yr at Mt. Panorama and 2 of 4 yr at Plain Creek. At Mt. Panorama, there was a 40% increase in grass biomass in 1997 because of defoliation by Zygogramma bicolorata and galling by Epiblema strenuana. At Plain Creek, grass biomass increased by 52% in 1998 because of E. strenuana and by 45% in 2000 because of combined effects of E. strenuana and the summer rust Puccinia melampodii. This study provides evidence on the beneficial effects of biological control of parthenium in areas under limited grazing.

Experiments were conducted in the field and greenhouse to evaluate the effects of cultivar and sulfentrazone rate on soybean injury and yield from soil-applied sulfentrazone and to determine soybean inheritance of sulfentrazone tolerance. Excessive rainfall and cool growing conditions in 1996 enhanced sulfentrazone injury and caused yield reduction in the susceptible cultivars ‘KS4895’ and ‘Hutcheson’ at 0.56 kg ai ha−1 and KS4895 at 0.42 kg ha−1. Yields of tolerant cultivars ‘Deltapine 3478’ and ‘Manokin’ were not reduced. Sulfentrazone at 0.42 kg ha−1 reduced soybean stand 17 and 35% for tolerant and susceptible cultivars, respectively. Field and greenhouse inheritance studies were conducted on plant families developed from two crosses: Manokin (tolerant) by ‘Asgrow A4715’ (susceptible) and ‘Northrup King S59–60’ (tolerant) by KS4895 (susceptible). The sulfentrazone tolerance of Manokin and Northrup King S59–60 appeared to be controlled by a single gene, with tolerance being dominant to susceptibility. The allelic relationship of tolerance from these two sources needs to be determined.

This invitational paper on the α-chloroacetamides is the first of a series of historical reports on each of the major classes of chemistry that have been developed for herbicides.

Absorption and translocation of three commercial formulations of glyphosate, the isopropylamine salt formulated as Roundup Ultra™ (IPA1) and Roundup UltraMax™ (IPA2) and the diammonium salt formulated as Touchdown™ IQ (DA), were compared in three- to five-leaf velvetleaf, common waterhemp, and pitted morningglory. Absorption of 14C-glyphosate in velvetleaf was not significantly different among the three formulations up to 50 h after treatment (HAT). More absorption of 14C-glyphosate occurred in the IPA1 (26.0%) vs. the IPA2 (17.7%) formulation over 74 h. Of the total 14C-glyphosate absorbed, 20 to 35% was translocated from the treated leaf to the rest of the plant. Initial absorption of 14C-glyphosate was rapid in common waterhemp with the IPA1 (42.7%) and IPA2 (30.7%) formulations; both were higher compared with absorption of the DA formulation (11.5%) by 2 HAT. These differences continued up to 26 HAT, but no differences were evident by 74 HAT. Up to 65% of the 14C-glyphosate absorbed was translocated out of the treated leaf by 74 HAT, with roots the primary sink. Initial absorption of 14C-glyphosate was slow in pitted morningglory compared with the other species. More foliar absorption occurred in plants treated with the DA (13.6%) vs. the IPA2 formulation (4.9%) by 6 HAT. Absorption beyond 26 HAT was not different among the three glyphosate formulations. Translocation of 14C-glyphosate to roots was 27% greater as the DA salt than IPA1 and IPA2 by 74 HAT. The distribution pattern of glyphosate was similar in all species; phosphorimages demonstrated movement both acropetal and basipetal, with accumulation in roots greater than in any other plant parts. An efficacy study parallel to the 14C study showed no difference among the three glyphosate formulations on the species investigated at both 74 HAT and 2 wk after treatment.

Broadleaf weeds apparently have patchy distributions within a field while POST control decisions are made assuming a regular spatial distribution. As a result, yield loss from weed competition may be overestimated, possibly leading to mistakes in choosing the optimal control treatment. Data on distribution of broadleaf weeds in 14 soybean fields were used in simulation experiments to investigate the potential for improving decision making with information about weed patchiness. The feasibility of modeling weed distribution in individual fields was also examined. Overall, the cost of assuming a regular distribution when making POST decisions was found to be low. Errors that occurred most often involved recommending more intensive control than was actually required, although in a few cases less intensive control was recommended. Error in the yield loss estimated for the uncontrolled population did not indicate the potential for a mistake in decision making for a field. Accurately modeling distribution of weeds within fields may be difficult as a result of correlations between distributions of individual species within a field and variation in distributions between fields.