Shattercane interference in irrigated soybean was evaluated during 1987, 1988, and 1989 at Clay Center, NE, using ‘Rox’ forage sorghum to simulate shattercane. Soybean yield reduction did not occur if shattercane was removed by 2 wk after emergence in 1987 and 6 wk after emergence in 1988 and 1989. Shattercane interference with soybean began when shattercane height exceeded soybean height. Soybean yield was reduced up to 25% before the height differential reached 30 cm, the minimum difference required for selectively applying glyphosate with a wiper applicator. Soybean nodes per stem, pods per stem, and beans per pod decreased as duration of interference increased. A direct relationship between soybean yield loss and shattercane density fit a rectangular hyperbolic function. Yield loss per shattercane plant was highest at low shattercane densities. Soybean plant height, biomass, nodes per stem, pods per stem, pods per node, and beans per pod decreased as shattercane density increased. An interference model for estimation of soybean yield and economic loss based on shattercane density was developed.

The phenolic content of purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (Cyperus esculentus L.) was determined. The major phenolic components were p-coumaric, ferulic, p-hydroxybenzoic, syringic, vanillic, salicylic, proto-catechuic, and caffeic acids, and eugenol. High concentrations of eugenol and salicylic acid inhibited the sprouting of nutsedge tubers. The lack of activity at lower concentrations argues against a major role for these compounds in maintaining nutsedge tuber dormancy. Exogenous abscisic acid inhibited nutsedge tuber sprouting and may be a natural dormancy factor in nutsedge.

The influence of mineral nutrition on the parasite-host relationship of witchweed (Striga lutea Lour.) and sorghum (Sorghum vulgare Pers. ‘Texas Bighead’) was investigated in greenhouse and growth chamber studies. Sorghum was grown in Eustis loamy sand, either infested or not infested with witchweed, and treated with one of three levels of mineral nutrients. Witchweed reduced host-shoot yields by about 70% at the low nutrient levels and by about 45% at the highest nutrient level. Witchweed produced seed at all nutrient levels, whereas the host produced seed at the highest level only. Increased nutrition also increased witchweed yields. Regardless of nutrient level, an early attack by witchweed was more destructive to the host than a late attack. Growth of witchweed in soil also was studied after the parasite was detached from the host or the host shoot was removed. When only the host shoot was removed, (a) witchweed growth was inhibited; (b) feeding of sucrose to witchweed did not restore parasite growth; (c) mineral nutrients were toxic to the parasite; and (d) high relative humidity around witchweed protected it from the nutrient toxicity. High relative humidity enhanced survival of the detached parasite but markedly inhibited growth of witchweed in the intact parasite-host relationship.

Field studies were conducted in 1979 and 1980 to determine yield losses caused by perennial sowthistle (Sonchus arvensis L. # SONAR) in rapeseed (Brassica napus L., B. campestris L.) fields in Saskatchewan and Manitoba. The relationship between percentage yield loss and density of perennial sowthistle was expressed by the linear regression equation y = −3.81 + 13.76 rdx. Weed survey data indicated that perennial sowthistle occurred in 39% of the rapeseed fields but infested only 7% of the hectares surveyed. Using both the survey data and the yield loss equation, the average annual loss in rapeseed yield was estimated to be 9.4 million kg in Saskatchewan and 6.1 million kg in Manitoba, with a value of 2.6 million and 1.7 million dollars (Cdn.), respectively. Perennial sowthistle occurs with a similar frequency and density in other crops so that the total loss from this weed would be much greater.

Three experiments were conducted in the greenhouse to study the effect of 1,1′-dimethyl-4,4′-bipyridinium salt (paraquat) when sprayed directly on seeds at the soil surface. Germination of alfalfa (Medicago sativa L., var. DuPuit) and red clover (Trifolium pratense L., var. Kenland) was not affected at 1 1b/A paraquat, but germination of Kentucky bluegrass (Poa pratensis L., var. Newport) and perennial ryegrass (Lolium perenne L., var. Linn) was severely reduced at that rate. A protecting layer of soil approximately 0.25 inch thick was completely effective in protecting the seeds from the effect of paraquat. Germination of seven other grass species was reduced by 0.5 1b/A paraquat.

A 2-year field study was conducted to determine the effects of fertilization on elodea (Elodea canadensis Michx.), eurasian watermilfoil (Myriophyllum spicatum L.), and heartleaf pondweed (Potamogeton pulcher Tuckerm.). Plants, water, and sediment were sampled and inorganic mineral contents determined. Water concentrations of NO3, NH4, P, and K increased sharply following fertilization and generally reached maximum values within 7 days after treatment. Concentrations of these nutrients decreased rapidly after reaching maximum values. In the first year, plant growth was not increased due to fertilization while in the second year only heartleaf pondweed produced significantly greater yields. Elodea and eurasian watermilfoil grew better in the control environment, presumably due to less competition from algae, whose growth was increased by the addition of fertilizer. At the end of the study the top 1.3 cm of bottom sediment had increased in cation exchange capacity (C. E. C.), organic matter, total N, and available P for all treatments while the underlying sediment remained relatively unchanged. Available P was greater in the surface of the fertilized sediment than in the surface of the control sediment.

Clodionafop, an acetyl-coenzyme A carboxylase (ACCase) inhibitor, changed the shape of the chlorophyll fluorescence induction curve (Kautsky curve) in barley and oat in greenhouse experiments. Biomass ED50, based on log-logistic dose–response curves, for barley was considerably higher than that for oat in all experiments. Biomass ED50 and relative potency (ED50 [barley]/ED50 [oat]) were consistent among experiments when sprayed at the same phenological stage of plant development. Especially at high doses, clodinafop changed the shape of the Kautsky curve more for oat than for barley. From the numerous parameters that can be derived from the OJIP steps of the Kautsky curve, we found that (1) F vj, the relative changes at the J step [F vj = (F m − F j)/F m], (2) area between Kautsky curve and maximum fluorescence (F m), and (3) F v/F m, maximum quantum efficiency of Photosystem II [F v/F m = (F m − F 0)/F m], were closely linked to the biomass dose–response relationships for both species. The linkage between biomass and the fluorescence parameters may be used to shorten the screening period for ACCase inhibitors.

Six herbicides including DNP (2,4-dinitrophenol) were tested for their effects upon nitrate metabolism in green leaves. It was found that these herbicides which are not classified as “photosynthetic inhibitors” permitted nitrite to accumulate within green leaves in darkness but not in light. It is postulated that these herbicides have an uncoupling effect, similar to that of DNP, which indirectly affects the process of nitrate reduction. A new technique which demonstrates this effect is presented and discussed.

Inhibition of growth and photosynthesis in sugarbeet (Beta vulgaris L. var. ‘USH10’) treated with desmedipham [ethyl m-hydroxycarbanilate carbanilate (ester)] was most severe between 25 and 30 C and decreased with higher or lower temperatures. Transfer of sugarbeet plants grown at temperatures from 10 to 35 C to higher temperatures after treatment increased injury and photosynthetic inhibition. Higher temperatures prior to treatment reduced injury at all posttreatment temperatures. When the temperature was changed from 25 to 40 C, inhibition was most severe immediately after treatment. Two days after treatment this 15 C temperature change did not cause additional injury. High posttreatment light intensities caused greater inhibition of photosynthesis than low light intensities.

The phenology of reproduction was highly variable among 23 selections of medusahead (Taeniatherum asperum (Sim.) Nevski). Selections with markedly early and late maturity were observed. The phenology generally was consistent during 4 years of testing at Reno, Nevada, and during 2 years at Davis, California. Individual selections differed greatly in phenology between two locations. Some of the selections exhibited phenotypic plasticity in phenology when grown in competition with other weeds.

Copper sulfate (CuSO4 · 5H20) was used as a tracer to determine soil sample variation and the effects of incorporation and seedbed preparation practices on the horizontal distribution of broadcast herbicide sprays. A Dubbs very fine sandy loam soil was sprayed with 14.1 kg/ha copper (as CuSO4 solution) and 19 mm diameter soil core samples were taken to a depth of 15 cm after spraying, after single or double disk-harrow incorporation, and after incorporation followed by bedding and row leveling. Cores were systematically located along and across the center two rows of the plots and numbered according to position. Soil cores were weighed, extracted with 0.05 M Na2 EDTA and the extract was analyzed by atomic absorption spectrophotometry. For all samples, cores had a range of copper between 50 and 1000 μg, with a coefficient of variation of 47%. Disk-harrow incorporation decreased the copper found in the center of the harrow path by nearly 50%. Bedding followed by row leveling did not change the tracer content of cores but decreased soil core densities. Disk-harrow incorporation may redistribute a herbicide that is initially applied evenly, producing areas of poor weed control and/or crop damage. This observation was confirmed in a field bioassay using trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and wheat (Triticum aestivum L. ‘Coker 6815′).

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.

Picloram uptake, translocation, and efficacy were studied using greenhouse-grown Russian knapweed plants. Uptake of 14C-picloram, applied as discrete droplets to adaxial leaf surfaces of well-watered plants, averaged less than 10% of that applied. Most uptake occurred within 30 min of application. Uptake increased linearly with external picloram concentrations from 6.2 to 74.5 mM and was proportional to picloram concentration. Only about 10% of absorbed picloram was translocated out of the treated leaf of well-watered plants within 96 h, with approximately equal acropetal and basipetal translocation. Water stress before, at the time of, and after picloram application did not affect picloram uptake, but reduced total translocation and increased the relative amount translocated basipetally. Water stress also reduced picloram efficacy. Although Silwett L-77 increased picloram uptake into Russian knapweed leaves, it did not increase efficacy under water stress conditions.

The relative competitiveness of triazine-resistant (TR) and -susceptible (TS) common waterhemp, as a component of fitness, was assessed using TR and TS biotypes collected from the same field in Fillmore County, Nebraska. Competition between biotypes was evaluated under greenhouse conditions using a set of replacement series experiments with five ratios of TR:TS (1:0, 3:1, 1:1, 1:3, and 0:4) and four plant densities (50, 100, 150, and 300 plants m−2). No difference in final individual plant weight between TR and TS plants was observed in the absence of interbiotypic competition at all plant densities examined. Interbiotypic competition was indicated by ANOVA of plant weight and modified relative crowding coefficients (RCC) for plant weight, height, and volume. Replacement series diagrams showed TS plants were more competitive than TR plants, as measured by plant weight at densities of 50, 100, and 150 plants m−2. TS and TR plants were equally competitive at 300 plants m−2. Individual TS plant weight for 50, 100, and 150 plants m−2 was greatest at the 3:1 ratio (TR:TS) and decreased as the proportion of susceptible plants increased. Individual TR plant weight for 50, 100, and 150 plants m−2 was greatest at the 1:0 ratio and decreased as the TR proportion decreased. Lack of interbiotypic competition at 300 plants m−2 and decreasing TS plant weight with increasing ratios suggest intrabiotypic competition. RCC values for volume and height were calculated weekly during the experiment and indicated the competitive advantage of TS plants was constant from transplanting through harvest.

Reduced effectiveness of carbamothioate (thiocarbamate) herbicides in certain soils has been attributed to rapid herbicide degradation by soil microorganisms. Studies were conducted to determine if greater populations of EPTC (S-ethyl dipropyl carbamothioate)-degrading microorganisms were responsible for increased rates of degradation observed following repeated applications of EPTC to a Grenada silt loam soil. EPTC-degrading microorganism populations, measured with a 14C-MPN (most-probable-number) technique, were not larger in soils with accelerated rates of EPTC degradation, and degrader populations did not increase after application of 6 mg EPTC/kg of soil. Degrader populations increased after application of 60 mg EPTC/kg of soil only in soil previously treated for 6 yr with EPTC. Increased rates of metabolism of EPTC were apparently responsible for the increased rates of degradation, rather than increased populations of degraders.

Freshly harvested Florida pusley (Richardia scabra L.) seeds do not germinate in continuous dark, but do germinate if exposed to more than 2 hr of light each day. Increasing periods of illumination up to 16 hr per day increases the germination percentages. Scarification increases the rate of germination in the presence of light. Scarification also causes germination of seeds stored for 8 months or longer in total darkness. The seeds fail to germinate at constant temperatures of 15 C or less and at 40 C. Almost complete germination occurs at a constant temperature of 30 C, or at alternating temperatures of 20 and 30 C. The seeds germinate equally well in the pH range of 3 to 8. Increasing depths of planting reduces percent emergence, and none of the seedlings emerge from a depth of 1.5 cm or more. Air-dried seeds can be stored either at 5 or at 25 C without losing viability, at least up to a period of 1 year after harvest. Moist storage of seeds at 5 C reduces germinability.

Enzymatically isolated leaf cells from cotton (Gossypium hirsutum L. ‘Stoneville’) and soybean (Glycine max (L) Merr. ‘Kino’) were used to study the effect of SAN 6706 [4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] on photosynthesis, protein, RNA, and lipid synthesis during the first 19 hr of herbicidal treatment. Cotton plants are tolerant to SAN 6706, whereas soybean plants are susceptible. SAN 6706 inhibited the incorporation of 14C-bi-carbonate, uridine, and acetate into cotton leaf cells but stimulated incorporation of leucine. Incorporation of all of these precursors into soybean cells was inhibited. Cotton and soybean cells each took up equivalent amounts of labeled SAN 6706. The incorporation activity of the cotton and soybean cells appeared to regulate the entrance of the precursors into the cells by depleting the intracellular precursor pools. It is suggested that the inhibition of RNA and lipid synthesis in cotton and soybean cells is an indirect result of inhibited photosynthesis. Isolated cotton cells were not tolerant of the herbicide.

Field experiments established that tetraploid soybean [Glycine max (L.) Merr.] plants were relatively tolerant while diploid plants were highly susceptible to metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] applied both preemergence and postemergence. Sensitivity of diploids and tolerance of tetraploids was also found in growth chamber experiments. Autoradiographs prepared 4 days after 14C-metribuzin application showed a high level of 14C-translocation to all parts of diploid plants including meristems. In contrast, only low levels of 14C were translocated in tetraploid plants, with no 14C-movement into meristems. Tetraploid plants rapidly transformed absorbed metribuzin to nontoxic products. Diploids were incapable of inactivating absorbed metribuzin at a rate sufficient to prevent injury. Differences in absorption, translocation, and metabolism of metribuzin appeared to be the main factors in the diploid and tetraploid differential response in field and growth chamber experiments. Differences in the rate of metribuzin metabolism appeared to be the factor responsible for the differential response in diploid and tetraploid cell suspension cultures. The primary polar metabolites were conjugates of metribuzin and deaminated metribuzin [6-tert-butyl-3-(methylthio)-1,2,4-triazin-5(4H)-one] with a ratio of 8:1, respectively.

Quackgrass (Agropyron repens (L.) Beauv.) was defoliated for 25 to 30 days with applications of 1.0 lb/A of l,l′-dimethyl-4–4′-bipyridinium salt (paraquat). Applications were more effective made prior to darkness than at midday. Phytotoxicity was increased by adding surfactants in the field, and experiments utilizing 14C-paraquat under controlled environmental conditions indicated that the surfactant X-77 increased foliar absorption by 20%. Quackgrass plants grown in nutrient solutions exhibited chronic paraquat toxicity which increased as the nitrogen levels increased. Paraquat also inhibited the growth of lateral buds on quackgrass rhizome segments cultured in the dark. Greater movement of 14C-paraquat in quackgrass leaves occurred under 6 hr of light as compared to 6 hr of darkness following application. However, a 6-hr dark period after application prior to the light period further enhanced the movement from the treated leaf.

Lithium salt of (2,4-dichlorophenoxy)acetic acid (2,4-D) (1000 ppm) was sprayed on plants of curly dock (Rumex crispus L.) 12 days before anthesis, at an thesis, 7 days after anthesis, and 34 days after anthesis. Treatment 12 days before anthesis completely prevented the formation of viable seeds. Two percent of the seeds had minute embryos after treatment at anthesis, but even these seeds did not germinate. Embryos were present in 91% of the seeds from plants treated 7 days after anthesis. These embryos ranged in size from 0.5 to 2.5 mm. The total weight of seeds obtained was significantly less than the control and only 5 to 15% of these seeds were capable of germination. Spraying the inflorescences 34 days after anthesis did not affect the number of seeds produced or seed viability. Almost all plants remained alive after the harvest.