Characteristics of glyphosate [N-(phosphonomethyl)glycine] uptake for as long as 3 days by plant cells were studied using suspension-cultured potato (Solanum tuberosum L. cv. ‘Red Pontiac’ and S. brevidens Phil.) cells. An experimental system was developed wherein uptake was assayed in a reduced salt medium that minimized cell growth yet maintained cell viability. Glyphosate uptake was nonlinear over time; an immediate rapid uptake phase was followed by uptake at a decreased rate that continued through 72 h. Glyphosate uptake was the same in both an experimental uptake medium (reduced salt and sucrose, no hormones) and in the complete growth medium used for routine cell growth. The pattern of glyphosate uptake was similar with both Red Pontiac and S. brevidens cells, though uptake was greater in Red Pontiac cells. Glyphosate uptake was not inhibited by 1 mM CaSO4, MgSO4, MnSO4, or ZnSO4. Rather, uptake was inhibited by the absence of salts at 8 h, and ZnSO4 and MnSO4 increased uptake. Similarly, increasing concentrations (0 to 5 mM) of CaSO4 and CaCl2 did not inhibit uptake, whereas the absence of either salt of Ca2+ decreased uptake. Increasing concentrations of (NH4)2 SO4 (0 to 150 mM) did not affect glyphosate uptake, but (NH4)2 SO4 at 50 mM and above was toxic to the cells.

Twelve analogs of the strigol D-ring (butenolides) were evaluated at 0.1 mM as germination regulators of 26 weed and crop seeds. In general, the butenolides either inhibited or did not affect monocot germination, although the hydroxybutenolide stimulated germination of dormant cheat and nondormant perennial ryegrass and sorghum seeds. The response rate for dicots was greater but still primarily inhibitory. Dormant lettuce seeds (both light-sensitive and light-insensitive), Palmer amaranth, and redroot pigweed showed several significant responses, both inhibitory and stimulatory. Some of the butenolides that had not been previously assayed against witchweed were tested as 10-4 to 10-11 M dilution series. The allyloxybutenolide demonstrated activity comparable to strigol (30 to 60% germination at 10-6 to 10-11 M); the sec-butyloxybutenolide was somewhat less active (20 to 40% germination) but still effective at 10-9 M. In general, the butenolide analogs of strigol were less effective in stimulating seed germination than the multiring analogs.

Atrazine metabolism was studied in four warm-season forage grasses to determine if metabolism was the basis for differential atrazine tolerance among the grasses. Big bluestem and switchgrass are atrazine tolerant while indiangrass and sideoats grama are atrazine susceptible in the seedling stage. Metabolism of atrazine in big bluestem and switchgrass occurred primarily by glutathione conjugation. The major metabolic product isolated from indiangrass and sideoats grama was the N-deethylated metabolite of atrazine. Glutathione conjugation by big bluestem and switchgrass occurred at a faster rate than N-dealkylation of atrazine in indiangrass and sideoats grama. Differential tolerance to atrazine among the grasses studied was probably due to the metabolic route by which they detoxify atrazine and the rate of metabolism for that specific route. Intraspecific differences in atrazine tolerance in indiangrass were due to the amount of metabolite produced in relationship to the amount of parent atrazine remaining in the shoot tissue. The more tolerant indiangrass lines had a higher metabolite to parent atrazine ratio than susceptible lines. This study confirmed differences in seedling atrazine tolerance of four indiangrass lines observed in previous greenhouse studies.

A field investigation was conducted to determine the influence of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] and trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] herbicides on soybean [Glycine max (L.) Merr. ‘Centennial’] — Bradyrhizobium japonicum symbiosis. Pendimethalin and trifluralin applied at rates of 1.1, 1.7, and 2.2 kg ai/ha delayed emergence and injured soybean seedlings grown on an Olivier silt loam. Nodule number, dry weight, and nitrogen (N2) fixation measured by acetylene (C2H2) reduction were decreased by all herbicide rates during the vegetative growth stages in 1984, with occasional decreases in nodulation noted during the late reproductive growth stage. Seedling injury was less severe in 1985 than in 1984, with most injury occurring at 1.7 and 2.2 kg/ha for both herbicides. Nodule number and dry weight in 1985 were decreased from vegetative through the early reproductive growth stages with little influence on N2(C2H2) fixation. Inhibitory effects of the herbicides on nodulation and N2(C2H2) fixation did not influence seed formation since seed yield was not affected either year.

Leafy spurge (Euphorbia spp.), an aggressive noxious perennial weed of North America, is a complex group that has been designated as several different species, including E. esula L. (# EPHES) and E. virgata Wald. & Kit. [E. waldsteinii (Sojak) Radcliffe-Smith]. Current classification keys are unsatisfactory in assigning plants in the field to specific taxa (or to a single taxon). These keys rely heavily upon morphological characteristics of leaves, but great variation in the leaves has been noted by us and previously reported by others. In this study we demonstrate qualitatively and quantitatively that the within-plant, within-clone, among-clone, and among-site variation in leaf morphology and triterpenoid content of the latex of leafy spurge is inherently high. Leaf characters were of little value in separating any of the accessions considered in our study. Latex triterpenoid profiles were useful in distinguishing E. lucida W. & K. × salicifolia Host. and E. salicifolia from European E. esula, E. waldsteinii, and E. sequieriana Neck. ssp. seguieriana, and all Montana accessions previously described from morphological studies. We concluded that Montana leafy spurge and the European E. esula, E. waldsteinii, and E. sequieriana belong to a single taxon: Euphorbia esula L.

The antagonism of the methyl ester of haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy propanoic acid} by bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide] was evaluated through studies of absorption and translocation in yellow foxtail [Setaria lutescens (Weig. ex Stuntz.) Hubb. # SETLU]. Leaf penetration of haloxyfop was reduced 37% in the presence of bentazon. Bentazon had no effect on translocation from the treated leaf. Direct chemical interactions and contact activity by bentazon do not account for the reduced uptake of haloxyfop. Experimental evidence shows the benzothiadiazione structural class of chemistry to be general antagonists of absorption for foliarly applied herbicides.

Hydrophobic exudate from roots of sorghum [Sorghum bicolor (L.) Moench. #3 SORVU ‘IS 8768’] contain four p-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica (L.) Kuntz. # STRLU]. The three minor p-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the major p-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.

Experiments were conducted on uptake and translocation of butyl ester of 14C-fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} in green foxtail [Setaria viridis (L.) Beauv. # SETVI]. Absorption of 14C-fluazifop reached 67.6% 12 h after treatment with the petroleum oil additive compared to 44.0% when applied alone. Maximum 14C-fluazifop absorption detected was 74.2% after 48 h with the oil additive compared to 58.6% after 72 h when applied alone. Translocation of 14C-fluazifop was greater when applied with the oil additive than alone, but translocation did not increase after 12 h. Absorption and translocation of 14C-fluazifop by green foxtail was greater when applied with the oil additive than alone in all environments. The oil additive overcame the reduced absorption of 14C-fluazifop applied alone with low temperature (20 C), relative humidity (40%), and soil fertility (0.25× strength Hoagland's solution), but not the reduced absorption and translocation with an increase in stress to −10 bars. 14C-fluazifop lost as vapors from the treated green foxtail leaf was 4.1% when applied alone compared to 0.6% when applied with an oil additive.

The addition of surfactant I (trimethylnonylpolyethoxyethanol) or surfactant II (4-isopropenyl-1-methyl cyclohexane) at 0.5% (v/v) to the spray solution or 1:1 mixtures of benazolin [4-chloro-2-oxo-3(2H)-benzothiazole-aceticacid], picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), or triclopyr {[(3,5,6-trichioro-2-pyridinyl) oxy] acetic acid} with clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) each at 0.28 kg ae/ha enhanced the phytotoxicity of clopyralid on honey mesquite (Prosopis glandulosa Torr. # PRCJG) compared to clopyralid applied alone. Addition of surfactant I, surfactant II, picloram, or triclopyr to clopyralid enhanced its absorption into leaves by 4 h after treatment and movement into upper-stem phloem by all additives by 1 day after treatment. When combined at equal rates, more clopyralid than picloram was transported into stems.

Ragweed parthenium (Parthenium hysterophorus L. #3 PTNHY) growing at six localities between 28°46′N and 13°O'N latitudes in India produced polymorphic ray achenes that varied in size and weight. Frequency of small achenes (1.9 to 2.2 mm) was higher at lower latitude (13°O'N) and that of large achenes (2.5 to 2.8 mm) at higher latitude (28°46′N). Achene weight was positively correlated with its germination percentage. Large-heavy achenes showed better germination than small-light ones.

The yield potential and the effect on yield loss of canola of different densities of volunteer barley were investigated at three locations in western Canada. Field studies were conducted from 1982 to 1986. Rectangular hyperbolic models based on data pooled over years, locations, and canola cultivars, and incorporating different densities of volunteer barley and canola accurately portrayed field responses in most instances. Results indicated that volunteer barley severely reduced canola yield. However, financial losses due to reduced canola yield were partly offset by the volunteer barley crop.

Volunteer corn (Zea mays L. # ZEAMX) is a troublesome weed in soybeans [Glycine max (L.) Merr.] grown in the Corn Belt. Field studies were conducted in 1983 and 1984 at Champaign, IL, to determine the soybean growth and yield parameters affected by volunteer corn interference. Soybean yield was inversely and linearly related to volunteer corn clump density, with yield losses reaching 25% at 5380 clumps/ha (10 plants/clump). Volunteer corn clumps of 1, 4, 7, and 10 plants reduced soybean yields by 6, 16, 21, and 22%, respectively, at a density of 5380 clumps/ha. Treatments of either the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} or glyphosate [N-(phosphonomethyl)glycine] were applied at various dates between 2 and 12 weeks after soybean emergence (WAE) to a volunteer corn infestation of 5380 clumps/ha (10 plants/clump). Volunteer corn caused soybean yield losses of 2, 6, 12, 19, and 27% when herbicides were applied at 2, 4, 6, 8, and 10 WAE, respectively. Volunteer corn interference reduced soybean seed yield, plant dry weight, pod number, seed number, leaf fresh weight, and leaf area index, within 40 cm of a clump of 10 volunteer corn plants. A clump of 10 volunteer corn plants reduced soybean pod number/plant within a radius of 86 cm.

The freezing resistance of overwintering buds of four perennial weed species was evaluated in artificial freezing experiments conducted in the winters of 1986 and 1987. Survival, dry weight of surviving shoots, and number of shoots produced/surviving section were determined at −4, −8, −12, −16, and −20 C for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes. The temperature required to reduce survival (LT50) and total dry weight (GR50) by 50% was determined. The response to freezing temperatures varied among species and in some cases between years. The LT50 was −7, −13, colder than −20, −17, and colder than −20, and the GR50 was −5, −11, −14, −15, and −13 for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes, respectively. The relative susceptibility to freezing among species appears to be directly related to the depth at which the overwintering buds generally are located.

The presence of a common cocklebur (Xanthium strumarium L. # XANST) population resistant to organic arsenical herbicides was confirmed in three adjoining counties of South Carolina's ten-county cotton-growing region, but little evidence of resistance was found in the remaining seven counties. This population was resistant to both disodium salt of methylarsonic acid (DSMA) and monosodium salt of methylarsonic acid (MSMA) but not to herbicides from nine other chemical groups. A much greater level of resistance was apparent in the field than under greenhouse conditions and larger plants were more resistant than small plants.

Greenhouse studies were conducted to determine the extent of translocation from the foliage to fleshy roots, the inherent toxicity, and the fate of radiolabeled and nonlabeled dicamba, picloram, and triclopyr in horsenettle. Roots of horsenettle acted as the major sink for photosynthate accumulation at the 0.2- to 0.5-bloom growth stages as determined by autoradiography. Dicamba, picloram, and triclopyr were translocated into the roots of horsenettle and accumulation continued for at least 16 days. 14C associated with each herbicide found in the roots ranged from 1.3% at 4 days to 3.8% at 16 days. After 16 days, slightly more 14C from plants treated with dicamba and triclopyr (3.8 and 3.6%) than picloram (3.0%) was translocated to roots. These compounds were metabolized slowly in the foliage and roots as determined by thin-layer chromatography (TLC) and autoradiography. In translocation studies with horsenettle shoots, picloram at 1.12 kg/ha killed the treated and untreated shoots and roots. Dicamba and triclopyr at the highest rates killed the treated shoots and partially destroyed the root system. Symptoms were noted on the untreated shoots, but full recovery occurred at 8 weeks. Since each of the herbicides was metabolized slowly and only slight differences in their translocation were observed, the relatively higher herbicidal effectiveness of picloram must be attributed to its greater inherent potency.

Greenhouse studies were conducted to determine the influence of glyphosate [N-(phosphonomethyl)glycine] and surfactant concentration on Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] control. Five, 10, and 30% (v/v) solutions of the commercial formulation of glyphosate (356 g ae/L glyphosate and 178 g ai/L MON 0818) applied in 2-μl droplets to Canada thistle leaves at an equal dose per plant did not reduce plant growth, whereas a 2.5% solution reduced growth by 76%. Varying the glyphosate and the polyethoxylated tallow amine surfactant (MON 0818) concentrations independently showed that low glyphosate and MON 0818 concentrations controlled Canada thistle better than high concentrations. High glyphosate concentration (108 μg/μl), high MON 0818 concentration (54 μg/μl), and large droplet size (2 μl) reduced 14C-glyphosate absorption and translocation compared with low glyphosate concentration (9 μg/μl), low MON 0818 concentration (4.5 μg/μl), and small droplet size (0.2 μl). High glyphosate and high MON 0818 concentrations may cause rapid tissue toxicity resulting in reduced translocation and poor perennial weed control.

Dryland and irrigated field experiments were established to measure differences in soil water relations throughout the growing season between plots in which cotton was grown with and without silverleaf nightshade interference. Soil moisture readings were taken weekly at 15-cm increments to a maximum depth of 120 and 150 cm during 1984 and 1985, respectively. When cotton was grown with silverleaf nightshade, soil water loss was greater at the lower portion of the soil profile earlier in the growing season than when cotton was grown alone. In the irrigated environment in 1985 when precipitation was higher than normal and the soil moisture content remained above normal, soil water loss did not differ. Cotton lint yield, plant height, and boll size reflected the amount of soil moisture available for growth and development of the crop. A statistical technique for quantifying soil water relations between the crop growing alone and growing with interference from the weed is demonstrated.

Seedling growth of soybean [Glycine max (L.) Merr. ‘Forrest’], common cocklebur (Xanthium strumarium L. # XANST), and tall morningglory [Ipomoea purpurea (L.) Roth. # PHBPU] was compared in field studies. Root growth was determined by measuring roots restricted to the inside of a permeable membrane that contained no soil. Leaf area duration and net assimilation rate between 4 and 9 weeks were greatest for common cocklebur compared to the other species. At 9 weeks, shoot and root dry weight and leaf area were greatest for common cocklebur compared to soybean and tall morningglory. Relative growth rate of common cocklebur roots was greater than the other species between 4 and 9 weeks. Tall morningglory had greater shoot length or height at 4 and 9 weeks and greater shoot or height elongation rate between 2 and 4 weeks, and 4 and 9 weeks than the other species.

The relative activities of (R) and (S) enantiomers of the methyl ester of haloxyfop were determined on annual grasses. Samples enriched in the (S) enantiomer were markedly less active than the (R) in petri dish evaluations and foliar applications. The pure (S) enantiomer was estimated by regression to be 1000-fold or less active than the (R). The activity of the (S) enantiomer was found to be equivalent to that of the (R) following preemergence applications. Isolation and characterization of haloxyfop from soil treated with the methyl ester of haloxyfop indicated inversion of the (S) enantiomer to the (R) enantiomer within 7 days. Field trials confirmed the differential activity of enantiomers applied postemergence and their equivalence when applied preemergence. These findings indicate that inversion of the (S) enantiomer to the (R) occurs in soil following preemergence applications.

The methyl ester of thiameturon {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] amino] sulfonyl]-2-thiophencarboxylic acid} toxicity to kochia [Kochia scoparia (L.) Schrad. # KCHSC] increased with surfactant concentration in the spray to 0.25% (v/v). High soil nitrogen fertility (165 ppm) enhanced kochia susceptibility to thiameturon compared to low nitrogen fertility (65 ppm). Nitrogen plus surfactant in the spray only partially overcame the effect of low soil nitrogen but did enhance thiameturon toxicity to kochia at all fertilities. Petroleum oil plus surfactant enhanced thiameturon so that kochia fresh weight reduction was similar regardless of soil fertility. Thiameturon toxicity to kochia generally increased with an increase in temperature regardless of relative humidity (RH) or adjuvants. An RH of 90 to 95% enhanced thiameturon toxicity to kochia regardless of temperature or adjuvants, except for petroleum oil plus surfactant with the low (10:8 C day:night) temperature regime. A simulated rain of 2 mm within 24 h after treatment reduced thiameturon toxicity to kochia. Drought stress (≤25% of field capacity) after thiameturon application reduced toxicity regardless of spray additives. The results of the various experiments indicated that thiameturon phytotoxicity would be greatest to kochia growing with high temperature and high RH, adequate soil moisture and nitrogen fertility, and when applied with petroleum oil plus surfactant.