A biotype of rigid ryegrass (Lolium rigidum G. ♯ LOLRI) has become resistant to amitrole and atrazine after 10 yr of exposure to a mixture of these herbicides. Resistance has also been demonstrated to the chloro-s-triazines: simazine, cyanazine, propazine; the methylthios-triazines: ametryn, prometryn; the substituted ureas: chlortoluron, isoproturon, metoxuron, diuron, fluometuron, methazole; and the triazinone herbicide metribuzin. The biotype remains susceptible to chlorsulfuron, metsulfuron, sulfometuron, sethoxydim, diclofop, fluazifop, glyphosate, carbetamide, and oxyfluorfen. Inhibition of oxygen evolution by atrazine, diuron, and metribuzin was similar in thylakoids isolated from both resistant and susceptible biotypes. Therefore, resistance to the photosystem II inhibitors is not caused by an alteration of the target site of these herbicides. Resistant plants treated with a 3-h pulse of 0.12 mM chlortoluron recover photosynthetic activity more rapidly than susceptible plants. This suggests that the basis for resistance is enhanced metabolism or sequestration of the herbicide within the leaf.

CGA-154281 and temperature effects on metolachlor absorption and metabolism were studied in corn seedlings grown in untreated soil or soil treated with metolachlor, CGA-154281, or both. Seedlings were grown under a cool (21/13 C, 16-h day) or warm (30/21 C, 16-h day) temperature regime and exposed to 14C-metolachlor for 10 min at either 21 or 30 C. Corn grown under the cool temperature regime absorbed slightly more 14C-metolachlor than corn grown under the warm temperature regime. Corn held at 21 C during a 10-min 14C-metolachlor exposure period metabolized metolachlor more slowly than corn held at 30 C. Decreased corn tolerance to metolachlor observed at lower temperatures may be due in part to slower metolachlor metabolism. Corn grown in the presence of metolachlor plus CGA-154281 metabolized 14C-metolachlor to the glutathione conjugate twice as fast as corn grown in the presence of metolachlor alone. The increase in metabolism rate was due to a fivefold increase in glutathione-S-transferase (GST, EC 2.5.1.18) activity and not to an increase in glutathione (GSH) content. Results are consistent with the hypothesis that CGA-154281 protects corn from metolachlor injury by enhancing GST activity, which accelerates metolachlor detoxification via GSH conjugation.

Based on chlorophyll content, hydroponically cultured soybean seedlings were 254, 66, and 13 times more tolerant to clomazone than velvetleaf, corn, and smooth pigweed, respectively. Clomazone, at concentrations that inhibited chlorophyll, did not affect fresh weight accumulations of any species except velvetleaf. However, in velvetleaf, fresh weight accumulation was only half as sensitive to clomazone as the leaf chlorophyll content. Uptake of 14C-clomazone from nutrient solution by 72 h after treatment (HAT) (pigweed > velvetleaf > soybean > corn) indicates that differential absorption cannot account for selectivity. Shoot:root ratios of 14C recovered from soybean, corn, velvetleaf, and pigweed by 72 HAT were 0.39, 0.84, 1.67, and 2.37, respectively. The limited acropetal clomazone translocation in soybean seedlings may account to a small degree for soybean tolerance to clomazone. Conversion of clomazone to more polar metabolites was rapid in all four species. There were no significant differences among species in the percentage of 14C activity recovered as clomazone from root tissue by 72 HAT. Of the 14C activity recovered from shoots of soybean, corn, pigweed, and velvetleaf seedlings by 72 HAT, 46, 59, 35, and 54%, respectively, was clomazone. Differences in clomazone uptake, distribution, and metabolism among the four species were either insignificant or poorly correlated to selectivity, and therefore cannot account for the tremendous differences in clomazone sensitivity among these species. These observations indicate, indirectly, that differences at the site of action may account for selectivity.

Effects of ammonium and methylammonium on foliar uptake of 14C-thifensulfuron, in the presence of 0.2% nonionic surfactant by vol, into velvetleaf were investigated. Total 14C uptake into velvetleaf 24 h after treatment was only 15% without addition of nitrogen, but addition of 0.32 M NH4Cl, urea ammonium nitrate, or CH3NH2·HCl increased total 14C uptake to 31, 38, or 59%, respectively. Total 14C uptake 24 h after treatment was increased 3- and 4-fold by addition of 0.08 and 0.16 M CH3NH2·HCl, respectively, but was not increased further by greater CH3NH2·HCl concentrations. Although the increase in 14C uptake was followed by an increase in total 14C translocated, the relative amount of 14C translocated (expressed as a percentage of total 14C uptake) was not affected by nitrogen additives. Enhancement of uptake of 14C-thifensulfuron by CH3NH2·HCl was partially reversed by addition of 0.16 M CaCl2 to the application solution.

The herbicide clomazone caused ultrastructural damage to etioplasts of cotton cotyledons. Etioplast envelopes were irregular, prothylakoids were absent or irregular, stroma density was low, and there were abnormal stromal vesicles. Further damage occurred upon exposure to light. Clomazone greatly slowed the conversion of chlorophyllide to chlorophyll in cotton, suggesting that phytol synthesis was affected. Neither synthesis of protochlorophyllide nor phototransformation of protochlorophyllide to chlorophyllide was affected by clomazone. Clomazone completely inhibited carotenoid synthesis without an accumulation of phytoene. However, the sesquiterpenoids hemigossypol and the dimeric sesquiterpenoid gossypol accumulated in greater amounts in clomazone-treated than in control tissues. These results indicate that this herbicide inhibits synthesis of terpenoids after the formation of farnesyl pyrophosphate.

Effects of several inorganic ions on short-term glyphosate absorption by cultured plant cells were determined by measuring the glyphosate content of cells exposed to inorganic salts in a 0.01-mM glyphosate solution buffered at pH 5.7. Within 10 min, glyphosate absorption by potato and velvetleaf cells was stimulated about equally by 1.5 mM CaCl2 or 1.5 mM MgCl2 but was not affected by 1.5 mM KCl. Glyphosate absorption by both species increased linearly with log10 CaCl2 concentration; for velvetleaf cells, the response was greatest for CaCl2 concentrations greater than 0.2 mM. Over time of absorption, glyphosate concentration within velvetleaf cells approached an asymptote. Addition of 0.1 mM KH2PO4 inhibited glyphosate absorption by potato cells in the presence or absence of Ca2+ and Mg2+ salts; all other K+ salts tested had no effect. Glyphosate absorption by potato, velvetleaf, soybean, corn, and rice cells was stimulated by 5.0 mM CaCl2 but was inhibited by 0.1 mM KH2PO4 in the presence of 0.05 or 5.0 mM CaCl2.

Rhizome johnsongrass grown in the greenhouse and treated with glyphosate at 1680 g ai ha−1 at an early (3- to 4-leaf) or late (6- to 8-leaf) growth stage displayed injury within a week. Plants treated with CGA-136872 or DPX-V9360 at 40 g ai ha−1 at both growth stages displayed injury 1 to 2 weeks later. CGA-136872 did not prevent regrowth at either growth stage. No regrowth occurred from DPX-V9360 or glyphosate-treated plants. Foliar absorption by greenhouse-grown plants within 24 h of application was greater with 14C-glyphosate than with 14C-DPX-V9360 or 14C-CGA-136872. More 14C-DPX-V9360 was absorbed than 14C-CGA-136872. Growth stage influenced glyphosate absorption (more by younger plants) but not CGA-136872 or DPX-V9360 absorption. Translocation of the 14C-CGA-136872 and 14C-DPX-V9360 out of the treated leaf was less than 20% of the absorbed label and was less than glyphosate translocation. Growth stage of rhizome johnsongrass at the time of treatment had no effect on the distribution of radiolabeled herbicides within 24 h.

Field experiments were established near Portageville, MO, to determine the effects of giant ragweed interference in soybeans. Threshold densities of giant ragweed that reduced soybean yield were less than two plants 9 m−1 of soybean row. This density reduced soybean seed yield 46 and 50% in 1988 and 1989, respectively, after full-season interference. The critical duration of giant ragweed interference in soybeans was between 4 and 6 weeks after emergence (WAE) in 1988 and between 2 and 4 WAE in 1989. Full-season giant ragweed interference at densities of 220 000 and 360 000 plants ha−1 in 1988 and 1989, respectively, resulted in almost complete soybean yield loss. Eight to 10 weeks after emergence of giant ragweed-free conditions were required to prevent soybean yield reductions.

Red rice at 20 plants m−2 was grown with two rice cultivars for 0, 40, 60, 80, 100, or 120 days after emergence. Red rice interference for 120 days after rice emergence reduced straw dry weights of Lemont and Newbonnet 58 and 34%, respectively. Grain yield of Lemont and Newbonnet was reduced 86 and 52%, respectively, by red rice interference for 120 days after emergence. Regression analyses indicated that red rice interference reduced straw dry weights of Newbonnet and Lemont 25 and 50 kg ha−1 day−1, respectively, for interference durations of 40 to 120 days after emergence. Grain yield of Newbonnet and Lemont was reduced 60 and 93 kg ha−1 day−1, respectively, for interference durations of 60 to 120 days. Negative linear relationships occurred between interference durations of red rice and plant height, panicles m−2, spikelets panicle−1, filled grains panicle−1, or panicle length of both cultivars. However, all parameters were reduced more for Lemont than for Newbonnet as interference duration increased. Head rice (whole kernels) and total milled rice yields of both cultivars were reduced by season-long red rice interference. Red rice straw dry weight and number of culms m−2 were greater when red rice was grown with Lemont than when grown with Newbonnet.

Several weed-free and barnyardgrass competition periods were established and maintained in cotton in field plots to evaluate their influence on cotton yield and seed production of barnyardgrass. When barnyardgrass was sprayed with fluazifop 3, 6, 9, or 12 weeks after cotton emergence, only plots sprayed at 3 weeks yielded as much cotton as plots kept weed free. Barnyardgrass permitted to compete for 6, 9, 12, and 25 weeks reduced cotton yields an average of 21, 59, 90, and 97%, respectively. When barnyardgrass was seeded into weed-free plots at 3, 6, 9, and 12 weeks after cotton emergence, a weed-free period of 9 weeks was required to prevent significant cotton yield reduction. Cotton in plots weed free for 3 and 6 weeks yielded an average of 13 and 87% as much seed cotton, respectively, as weed-free controls. Barnyardgrass produced 15 to 85% as many seeds as weedy check plots in plots weed free for less than 9 weeks and in plots where control was delayed for 9 and 12 weeks. Cotton grades were reduced because of grass in plots weed free for 3 and 6 weeks, and in plots where barnyardgrass competed for 9 and 12 weeks.

Growth of perennial sowthistle was examined under different levels of soil pH, soil moisture content, light intensity, and temperature. Soil pH ranging from 5.2 to 7.2 had little effect on the number of leaves, rosette diameter, plant height, and number of capitula. However, total dry weight was 30% less in plants grown in soil of pH 5.2 compared to those grown at higher soil pH levels. Perennial sowthistle demonstrated a consistent positive growth response to increasing soil water including saturation. Severe reduction in vegetative and reproductive growth occurred in plants grown in soil below field capacity. Plants grown under full light (1015 μE m−2 s−1 photosynthetic photon flux density) developed a fourfold increase in the number of capitula per plant and a 50% increase in total dry weight compared to plants grown at 285 μE m−2 s−1. Initiation of reproduction was delayed 4 weeks for plants grown at 580 μE m−2 s−1 and 285 μE m−2 s−1. Plants grown under less than full light developed fewer but larger leaves. Plants grown under a day/night temperature of 20/15 C grew more rapidly than those under 30/25 C or 10/5 C. Plants at 30/25 C began to senesce 7 to 8 weeks after planting. Net carbon assimilation, leaf conductance, transpiration, and water use efficiency decreased as soil moisture and light intensity decreased. Plants at 30/25 C had the highest rate of transpiration and the lowest water use efficiency. The observed optimum for perennial sowthistle growth occurred at a soil pH of 6.2 or 7.2, water-saturated soil, high light intensity, and a temperature of 20/15 C, day/night.

Dormant weed seeds are difficult to kill and, with the exception of soil sterilization methods, current weed control techniques are not effective against dormant weed seeds. This study was conducted to investigate the effects of temperature, light, soil moisture, burial depth, ethephon, and potassium nitrate (KNO3), and their interactions on the germination of false chamomile seeds under controlled conditions. False chamomile was photoblastic and thermophilic and germinated rapidly within the first 4 days of incubation under all conditions. Regression analysis indicated that ethephon and KNO3 had quadratic effects on false chamomile germination. Ethephon enhanced germination more than KNO3. There was a linear reduction in germination with increased storage time, and it is projected that zero germination should occur after 9 yr of storage at room temperature. Soil moisture had a positive linear effect and a positive interaction with ethephon on false chamomile germination. Very little germination occurred when seeds were buried 5 cm deep even with high soil moisture.

Studies were undertaken to determine effects of scarification, temperature, stratification, pH, and osmotic potential on seed germination; to determine the effect of planting depth on emergence; to describe the emergence sequence; and to quantify seedling development of bigroot morningglory. Mechanical and chemical scarification caused increased germination but cool, moist stratification did not break dormancy. Optimum germination occurred at 20 and 25 C and with alternating temperatures of 20/10 and 30/20 C. The optimum pH range for germination was between 6 and 8.5. Increasing the solution osmotic potential decreased germination linearly from 90% at −0.08 MPa to 0% at −0.8 MPa. Plant establishment was best at 2 to 4 cm and decreased at greater seeding depths. Seedling emergence was observed and recorded and found not to fit the classical models of hypogeal or epigeal emergence. Only 3% of 5-day-old seedlings sprouted after being clipped to the root crown. When seedlings were 11 days old at clipping, over 85% of the plants sprouted. These results indicate that bigroot morningglory has the potential to successfully reproduce by seed and that the rapid development of the plant should be considered when designing control programs.

The effects of quackgrass shoot populations shortly after emergence, and dry weight at harvest on yield of canola were determined in four field experiments conducted near Vegreville, Alberta in 1986 and 1988. A hyperbolic model described the data well in most cases and indicated little intraspecific quackgrass interference among quackgrass shoots until populations exceeded 200 shoots m−2. Magnitudes of predicted canola yield losses among experiments were close, even though quackgrass-free yields varied. A pooled hyperbolic equation, based on shoot density, predicted that an intermediate quackgrass infestation of 50 to 100 shoots m−2 would reduce canola yield by 18 to 32%. An economic threshold model based on the hyperbolic model provides a means of estimating when control of quackgrass with herbicides is economical.

Field trials were conducted in 1986 and 1987 to determine the effects of moisture on cinmethylin activity. The herbicide was applied preemergence at rates of 0.0, 0.3, 0.6, and 0.9 kg ai ha−1 to both dry and moist sandy loam soil, followed by varying irrigation regimes. Plant species used in the study included soybean, snap bean, cotton, peanut, and cucumber. Peanut was the most tolerant to cinmethylin, followed by cotton, soybean, snap bean, and cucumber. When cinmethylin was applied to a moist soil, less crop injury resulted than when it was applied to a dry soil. If 7.6 cm of water was applied shortly after cinmethylin application to a dry soil, severe crop injury occurred. When 2.5 cm of irrigation was applied within 8 h or at 5 days after cinmethylin application to a dry soil, crop injury was reduced when compared to applying 7.6 cm irrigation.

Field experiments were conducted in 1986 and 1987 to evaluate the potential of growth regulators mepiquat chloride and chlormequat chloride as seed treatments to protect cotton from fluometuron injury. Fluometuron at two and three times the recommended use rate reduced cotton stand and height on Taloka and Convent silt loam soils both years. Cotton grown on a Sharkey silty clay soil was not injured by fluometuron. Mepiquat chloride and chlormequat chloride increased cotton stands on a Taloka silt loam soil when averaged over rates and years. In general, fluometuron injury to cotton was not reduced by treating seed with 1000 ppmw concentrations of chlormequat chloride or mepiquat chloride. Chlormequat chloride reduced chlorosis and necrosis of cotton treated with fluometuron, but neither growth regulator eliminated cotton injury or yield reduction caused by fluometuron at two or three times the recommended rates.

The effects of 1.24 kg ha−1 ammonium sulfate, 5 g ae ha−1 of the isopropylamine salt of imazapyr, and air temperatures of 18, 27, or 35 C at 40 or 100% relative humidity (RH) on the absorption and translocation of the ammonium salt of 14C-imazethapyr applied postemergence to pitted morningglory were evaluated. Absorption of 14C-imazethapyr was greater at 100 than at 40% RH (88 vs. 47%). At 40% RH, absorption was increased to 79% by the addition of ammonium sulfate. At 100% RH absorption was similar with all treatments. Translocation of 14C-imazethapyr applied alone was greater at 100 than at 40% RH (34 vs. 17%). Addition of ammonium sulfate increased translocation at 40% RH but not at 100%. Addition of imazapyr did not affect 14C translocation. Distribution of 14C throughout the plant was more acropetal than basipetal with the greatest distribution at 35 C.

The effect of soil composition on adsorption of chloridazon by 18 samples of natural soils, with different chemical and textural characteristics, obtained from west-central Spain, was investigated. The effect of removal of organic matter on adsorption and adsorption of the herbicide by the active components of the soils (montmorillonite and humic acid) was also studied. A highly significant correlation was found between the distribution coefficient (Kd) and organic matter content when all the soils or the soils with organic matter content >2% were considered. According to the determination coefficients (r2), organic matter content accounted for 72% of the variance in adsorption in the former case and 92% in the latter. In soils with an organic matter content <2%, there was no correlation between Kd and organic matter content. A significant correlation was observed between Kd and smectite content for soils containing smectite in their clay fraction. Both simple and multiple correlations showed that clay content, excluding the smectite fraction, had a relatively small effect on adsorption of the herbicide. Adsorption of chloridazon by oxidized soils, montmorillonite, and humic acid confirms the effect of organic matter and smectite on adsorption in natural soils.

An enzymatic (amylase) pretreatment was developed that allows quantitative recovery of the parent compound from soil samples receiving the pesticide in starch-encapsulated formulation. The pretreatment involves adding amylase, dissolved in a phosphate buffer solution, to soil samples and incubating the resulting slurry at 50 C for 1 h. Efficiency of this pretreatment was tested on two soils, a silty clay and a loamy sand, receiving atrazine and alachlor in the starch-encapsulated formulation. The inexpensive method adds approximately 1 h to the extraction procedure and a twofold increase in percentage recovery of starch-encapsulated atrazine. The enzyme pretreatment is not necessary for quantitative recovery of pesticides that have rapid rates of release such as alachlor. The method will be useful for studying starch encapsulation pesticide behavior in the soil environment.