Lipid metabolism and other biochemical responses to alachlor [2-chloro-2’,6’-dimethyl-N-(methoxymethyl) acetanilide] and the protectant flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid, (phenylmethyl ester)] in germinated grain sorghum [Sorghum bicolor (L.) Moench. ‘MFA GS 301’] were studied. Treatments included flurazole, applied to grain sorghum seed at 2.5 g ai/kg, alachlor at 8.2 × 10−6 M, and alachlor plus flurazole at these rates. Treatments did not alter total lipid synthesis or fatty acid composition. Seeds that germinated for 48 h in media containing alachlor or alachlor plus flurazole had a greater triglyceride:phospholipid ratio than those treated with flurazole or the control seeds, indicating that alachlor plus flurazole retarded catabolism of storage lipid. Oxygen consumption of seeds was reduced after the herbicide plus protectant treatment. Oxidation of 14C-pyruvate to CO2 was inhibited by alachlor and alachlor plus flurazole treatments, but not by flurazole alone.

Redroot pigweed is a troublesome weed in the sorghum-growing regions of North America. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of redroot pigweed density and environmental conditions on physiological determinants of redroot pigweed growth: duration of plant growth, light interception, radiation-use efficiency, and dry matter partitioning. In addition, specific leaf area was determined. Redroot pigweed was seeded at monoculture densities of 2, 4, and 12 plants m−1 of row each year at each location. Duration of redroot pigweed growth was not influenced by plant density. Light interception was defined as a simple exponential function of leaf area index. Specific leaf area did not change over the season and averaged 135 cm2 g−1. Partitioning of redroot pigweed dry matter was not influenced by plant density or environmental conditions but did not change within vegetative and reproductive stages. Radiation-use efficiency was not influenced by redroot pigweed density; the most reliable estimate was 1.74 g dry matter MJ−1 of intercepted photosynthetically active radiation. Physiological determinants described were not affected by redroot pigweed density or environmental conditions and therefore provide a starting point for the development of a redroot pigweed growth module. The module could be coupled with available crop growth models (e.g., the sorghum growth model SORKAM) to simulate redroot pigweed–sorghum competition.

Studies in outdoor, circular pools have shown that one shoot tip of hydrilla (Hydrilla verticillata Royle) produced as much plant dry weight during a 16-week period as 16 tips planted under the same conditions. Dry weight of plants 16 weeks after planting was as high as 1,561 times that of the shoot tips planted. Tubers were present within 8 weeks after planting the tips; higher numbers of these vegetative propagules were associated with the higher number of tips planted. Plants under a photoperiod of 11.5 h or less produced more tubers than those exposed to 12 h or more of light.

The phytotoxicity of soil-applied terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] to wheat (Triticum aestivum Vill.) was significantly affected by soil moisture and soil temperature. Distribution coefficients (Kd) provided a better indication of the phytotoxicity of terbutryn to wheat than any single measured parameter contributing to herbicide adsorption by the soil. Soil temperatures and soil moisture levels suitable for good plant growth tended to enhance the phytotoxicity of terbutryn. No phytotoxic levels of terbutryn to wheat were detected in Teller sandy loam after 20 weeks of incubation at above 10C and 14% soil moisture by weight. However, phytotoxicity to wheat was observed in air-dry terbutryntreated soil after an incubation period of 20 weeks, regardless of incubation temperature. Significant quantities of terbutryn may remain in the field under dry soil conditions.

The growth and seed return of auxin herbicide resistant (R) wild mustard was compared to that of a susceptible (S) biotype in wheat in the field. In the absence of herbicide, the S biotype accumulated shoot dry matter more quickly than the R biotype throughout most of the growing season. However, in only one of the two years did the S biotype set substantially more seed than the R biotype (3120 versus 2520 seeds plant−1). The recommended dosage of 2,4-D for wild mustard control (420 g ai ha−1) killed all S plants in both years of the study, and severely inhibited growth and seed return of R plants. Shoot dry matter accumulation and seed return of treated R plants were reduced 75 to 90% compared to the untreated control. However, at a density of 20 plants m−2 R seed return was still very high; 9000 and 5700 seeds m−2 in 1992 and 1993, respectively. The recommended dosage of dicamba (300 g ha−1) did not inhibit the growth and seed return of either S or R wild mustard to the same extent as 2,4-D. Dicamba at 300 g ha−1 reduced S shoot dry matter and seed return 80 to 90%, while R shoot dry matter and seed return was reduced 60 to 65%. The results of this study indicate a very high selection pressure for R wild mustard at recommended dosages of 2,4-D. Despite a high selection pressure, and considering the long history of phenoxy herbicide usage on the Prairies, the relatively rare occurrence of phenoxy herbicide resistant weeds implies that the frequency of resistant individuals is very low. From a mathematical model it was determined that the frequency of R wild mustard in an unselected population may be in the order of 10−30.

Treatment of tobacco (Nicotiana tabacum L., var. NC-402) with 2,3,5-trichloro-4-pyridinol (pyriclor) leads to progressive disruption of chloroplast ultrastructure. Initial changes include the inception of a spherical form, swelling of the fret system, and commencement of loss of starch. More advanced changes include the complete disappearance of starch, further swelling and disorganization of the fret membrane system, followed by swelling and disruption of the membranes of the granal discs and rupture of the chloroplast envelope. The changes induced by pyriclor on chloroplast ultrastructure in tobacco are analagous to those caused by 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) on kidney bean (Phaseolus vulgaris L.) and barnyardgrass (Echinochloa crusgalli L. Beauv.). The shape and size of mitochondria were not altered by treatment with the chemical; however, there appeared to be a significant increase in the number of mitochondria present when alterations in chloroplast structure could be seen. The appearance of visual symptoms of phytotoxicity (chlorosis) correlated with the onset of disruptive changes in chloroplast ultrastructure.

Imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} absorption, translocation, and toxicity were investigated in yellow nutsedge (Cyperus esculentus L. # CYPES) and purple nutsedge (C. rotundus L. # CYPRO) after foliar treatment. Imazaquin rates from 0.125 to 0.375 kg ai/ha were sprayed on nutsedge plants at the four-leaf stage. Compared to untreated plants, these rates reduced the main shoot growth of yellow nutsedge by 70 to 86% and that of purple nutsedge by 80 to 92% at 28 days after application (DAA). However, the regrowth as measured by tiller production was significantly greater at 0.125 and 0.188 kg/ha rates than at higher rates. The plants treated with 0.313 and 0.375 rates had no rhizomes or tubers at the 28 DAA sampling. In yellow nutsedge, absorption of 14C-imazaquin increased from 36% at 1 DAA to 57% at 8 DAA. of the total absorbed 14C, 12% translocated from the treated area by 8 DAA. Roots and rhizomes accumulated equal amounts of radioactivity. In purple nutsedge, the absorption of imazaquin increased from 17% at 1 DAA to 53% at 8 DAA. Translocation of absorbed 14C in purple nutsedge at 8 DAA was 21%. Rhizomes accumulated significantly greater amounts of 14C than the roots.

Bioassay and gas chromatographic procedures were evaluated for their effectiveness in detecting fluridone [1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone] in soil. Bioassay procedures were limited to a detection range of 0.05 to 0.45 ppmw if grain sorghum (Sorghum bicolor (L.) Monech) was used. Visual estimations of chlorophyll content in affected plants, used as an indicator of fluridone concentration, were as reliable as spectrophotometric determinations. Gas-liquid chromatographic (GLC) procedures were less variable than the bioassay and had no upper limit of detection. Recovery of fluridone from treated soil ranged from 60 to 100 percent, depending on soil type. Soil water content affected percent recovery. Fluridone did not leach more than 1 cm in clay or sandy loam soils when up to 10 cm of water was passed through a soil column containing the herbicide, as determined by GLC. Extensive downward movement, 12 to 16 cm, occurred in a coarse sand when 5 or 10 cm of water was passed through the column.

Greenhouse studies were conducted to study the competition between soybeans [Glycine max (L.) Merr. ‘Vansoy’] and barnyardgrass [Echinochloa crus-galli (L.) Beauv.]. The competition was initiated or terminated when soybeans were 2, 3, 4, 5, 6, 7, 8, 9, and 10 weeks old. The reproductive efficiency of soybeans was not lowered by 4 to 7 weeks of competition after planting of soybeans. Competition between the two species for the entire growing season resulted in a significant decrease in plant height of soybeans. The total dry matter per plant was negatively correlated with an increase in the length of competition. Competition during the first 5 to 9 weeks of the life cycle of soybeans was more deleterious to soybean productivity than the last 5 to 9 weeks of its life cycle.

Because soil characteristics and weed densities vary within agricultural landscapes, determining which subfield areas are most favorable to weed species may aid in their management. Field and greenhouse studies were conducted to determine whether subfield environments characterized by higher soil organic carbon (SOC), or ridge vs. furrow microsites, affect common sunflower seed germination after winter burial, seedling emergence, or the control afforded by a preemergence herbicide in a ridge-tillage corn production system. Among seeds buried in situ during winter months and germinated in the laboratory, no differences in common sunflower seed germination or mortality were detected between high-SOC (1.8% mean) and low-SOC (1.1% mean) locations. However, seeds buried at 5-cm depth had about 40% laboratory germination compared with about 10% for seeds stratified on the soil surface or under crop residues. In field emergence and survival experiments, the SOC main-plot effect indicated 25% greater seedling survival in high- than in low-SOC locations. In the absence of herbicide, both emergence and survival were ≥ 35% greater in the ridge than in the furrow microsite, and seedling survival was 48% greater in high- vs. low-SOC furrow environments. However, common sunflower seedling survival was similar between herbicide-treated high- and low-SOC ridges. Greenhouse studies indicated a 13 to 24% increase in common sunflower seedling biomass per 1% increase in SOC under three atrazine doses. Altered or additional weed management tactics should be considered for common sunflower in high-SOC environments to offset the greater potential for seedling survival and growth.

Under Texas conditions, the potassium salt of 4-amino-3,5,6-trichloropicolinic acid (picloram) usually was more effective than the isooctyl ester formulation for control of live oak (Quercus virginiana Mill.), yaupon (Ilex vomitoria Ait.), winged elm (Ulmus alata Michx.), huisache (Acacia farnesiana (L.) Willd.), and honey mesquite (Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell). Possible reasons for the difference in effectiveness of picloram formulations were studied in the laboratory. Extensive degradation of the ester of picloram (96%) occurred in open Petri dishes after 72 hr exposure to ultraviolet (uv) light, compared to a loss of 26% for the salt of picloram. Forty-five percent of the isooctyl ester of picloram was lost at high temperatures (60 C), whereas only 2% of the potassium salt of picloram was lost after 1 week at 60 C from open Petri dishes in a dark oven. Application of the ester to soils reduced thermal and ultraviolet light degradation losses compared to losses from open Petri dishes. Loss of the ester was greater when applied in diesel oil to Petri dishes than in either water or paraffin oil. The salt of picloram leached most after 12.5 cm simulated rainfall in soil columns to the 17.5 to 30-cm level (907 μg), followed by the acid (360 μg), and last the isooctyl ester (0 μg). However, considerable acid (161 μg) was recovered at the 32.5 to 45-cm depth from the isooctyl ester treatment exposed to wet soils for 3 days, indicating hydrolysis of the ester to acid.

Ivyleaf morningglory [Ipomoea hederacea (L.) Jacq. # IPOHE], planted in 1984 and 1985 the first of each month from April through August, began emerging in cotton (Gossypium hirsutum L. ‘Acala SJ-2’) 1 week after planting. Morningglory, at a density of 1 plant/2 m of row, emerging in April and May became so competitive to cotton by harvest in September that the total crop was lost. June plantings, although less competitive than earlier plantings, still reduced yields by 11%. Later plantings did not reduce yields of cotton. Morningglory plantings that produced seed (April through July) began flowering within 7 weeks, and viable seed was collected for the first time 9 weeks after planting. Ivyleaf morningglory plants that emerged in April and May produced about 11 000 seed/plant by cotton harvest. This compared to 3000 and 93 seed/plant for morningglory that emerged in June and July, respectively. The earliest that morningglory might be expected to produce seed in cotton planted in early April would be late May to early June, and morningglory emerging as late as the first of July could produce some seed by cotton harvest.

Genetics can be a powerful adjunct to just about any kind of physiological study, including weed physiology or weed/herbicide interactions. Making, mapping, and reverting mutations is simple and straightforward. Making mutants can be as simple as isolating variant individuals from the “wild”, as uncomplicated as doing seed mutagenesis in your laboratory, or as sneaky as recovering mutants as sectors in whole plants. The overall principles for successful development of a protocol for seed mutagenesis of weeds are described and potential problem areas noted. These generalities are illustrated with a specific case history, that of chlorsulfuron. Although chlorsulfuron is accurately described as an inhibitor of the synthesis of branched chain amino acids, careful physiological examination suggests that it kills plant cells, not by starvation for amino acids, but by active toxicity of a metabolite, α-amino butyric acid, produced from a precursor available for diversion in cells with inhibited acetolactate synthase (EC 4.1.3.18, ALS). The story of dominant resistance due to an altered ALS enzyme is well known; analysis using additional mutants fleshes out the story of how chlorsulfuron works. Such analysis has the potential to help unravel other problems in weed physiology.

Effects of bearded sprangletop interference durations on Lemont and Newbonnet rice cultivars were studied. Interference durations of 63, 70, and 130 d after rice emergence reduced Lemont grain yields 11, 21, and 50%, respectively, and lowered Newbonnet grain yields 11, 13, and 37%, respectively. Interference durations of 21 to 56 d after emergence did not reduce grain yields of either cultivar. Bearded sprangletop grown in Lemont rice produced more biomass than that in Newbonnet. Season-long interference reduced plant height and straw dry weight of Lemont more than that of Newbonnet.

The addition of surfactant (trimethylnonylpolyethoxyethanol) at 0.5% (v/v) of the spray solution caused a twofold increase in the deposit of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) applied at 0.14 or 0.28 kg ae/ha on greenhouse-grown honey mesquite (Prosopis glandulosa Torr. # PRCJG) and increased transport into the plants compared to clopyralid applied alone. Application of equal rates of benazolin [4-chloro-2-oxo-3(2H)-benzothiazoleacetic acid] with clopyralid at 0.28 + 0.28 kg/ha enhanced transport of clopyralid into honey mesquite. Spray deposit, absorption, and translocation of clopyralid was unaffected by addition of 0.5% (v/v) of d'limonene (4-isopropenyl-1-methyl-cyclohexane). Greater spray deposit of clopyralid on plant surfaces after addition of surfactant was associated with reduced liquid surface tension and greater percentage of spray volume in small droplets (<204-μm diam).

The effects of eight s-triazines on respiration of (Phaseolus vulgaris L. ‘Black Valentine’) bean and rat liver mitochondria and on cyclic photophosphorylation of pea (Pisum sativum L. ‘Thomas Laxton’) chloroplasts were determined. All triazines inhibited state 3 respiration and cyclic photophosphorylation. The degree of inhibition was similar in both rat liver and bean mitochondria. Prometryne [2,4-bis-(isopropylamino)-6-(methylthio)-s-triazine] was the most potent inhibitor of both respiration and cyclic photophosphorylation. Triazines with the methylthio group showed the most activity, followed by those with the methoxy and chloro substituents in that order.

Postemergence treatments with 3-[p-(p-chlorophenoxy)phenyl]-1,1-dimethylurea [chloroxuron], for broadleaf weed control in soybeans [Glycine max (L.) Merr.] often partially defoliate, and temporarily stunt the crop. Chloroxuron was applied at 3 lb/A and at 1, 1½, and 3 lb/A with surfactant over the tops of weed-free soybeans in the cotyledon, unifoliate, and first trifoliolate stages of growth in 1967, 1968, and 1969. Relatively small but statistically significant yield reductions, particularly from treatments in the unifoliate stage, resulted in 1967. No significant yield reductions resulted in 1968 or 1969. These studies demonstrated the ability of soybeans to withstand initial injury from early postemergence treatments with chloroxuron without serious reductions in yield.

Dichlobenil (2,6-dichlorobenzonitrile), 3-tert-butyl-5-chloro-6-methyluracil (terbacil), and 3-tert-butyl-5-bromo-6-methyluracil (hereinafter referred to as DP-733) were applied annually for 3 years as soil surface or incorporated treatments for weed control in young peach (Prunus persica (L.) Batsch., var. Redhaven) trees. Average monthly ratings showed significant increases in bermudagrass (Cynodon dactylon (L.) Pers.) control with incorporation of all three herbicides. Treetrunk diameters in incorporated dichlobenil plots were greater than those in surface-applied dichlobenil plots. Incorporation in the soil reduced loss of dichlobenil, terbacil, and DP-733. The herbicides did not accumulate in the 0 to 15-cm soil layer. Low concentrations were detected in the 30 to 60-cm soil depth 1 year after the third annual application of 6.72 kg/ha of dichlobenil and 4.48 kg/ha of DP-733. Terbacil was not present in detectable amounts at 30 to 60 cm but was present in the 15 to 30-cm layer of 4.48 kg/ha plots.

The pre-emergence use of 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron) offers a means of controlling weeds in cotton during the extremely critical period of stand establishment (1, 2, 6, 8, 9, 10, 11). The amount of diuron required to give effective weed control without injury to cotton has been observed to vary between locations. Factors such as temperature, moisture, light conditions, and physical and chemical properties of the soil may be involved in causing variation in the activity of this herbicide.

Enhancement of 3(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) toxicity to oat plants was not dependent on type—anionic, cationic, or nonionic—of surfactant. However, specific surfactant and concentration were important. Concentrations of surfactant above 0.1% were required for consistent phytotoxic effects. Concentration of diuron in solution at time of application was not an important factor in phytotoxicity. Lowering the temperature from 72 to 60 F slowed the expression of toxicity and increased the chances of plant recovery.