The 14C-AC 263,222 was foliar applied to common cocklebur, johnsongrass, peanut, sicklepod, and soybean in order to study translocation and metabolism characteristics in each species. Differential absorption of 14C-AC 263,222 between species was evident 4 h after application. At 48 h after application, sicklepod and peanut absorbed more 14C-AC 263,222 than johnsongrass, common cocklebur, or soybean. Translocation of 14C-AC 263,222 and its metabolites out of the treated leaves increased with time for all species. The 14C-AC 263,222 and its metabolites appear to be both xylem- and phloem-mobile based on patterns of movement. Absorption and translocation differences occurred between species; however, they did not appear to explain differential species response. Metabolism of 14C-AC 263,222 differed greatly among species. Common cocklebur metabolized less 14C-AC 263,222 than any other species 96 h after application. Johnsongrass and sicklepod metabolized 24 and 28% of the 14C-AC 263,222, respectively, 96 h after application. Peanut and soybean metabolized 76 and 62%, respectively, of the 14C-AC 263,222 96 h after treatment. The half-life of 14C-AC 263,222 was 1.1, 2.5, 7.7, 11.6, and 34.5 d in peanut, soybean, sicklepod, johnsongrass, and common cocklebur, respectively. Differential tolerance of species appears to be directly related to the half-life of AC 263,222 in the plant.

Paraquat is known to affect all green plants and other eukaryotic organisms including mammalian cells. The aim of this study was to improve the understanding of paraquat toxicity in nonphotosynthetic plant cells using dark-grown kidney bean cells in tissue culture. It is shown that uptake of paraquat is an active process, and that paraquat inhibits cell growth, reduces DNA synthesis, and inhibits the activity of hydroxypyruvate reductase while enhancing the activity of glutathione reductase which is involved in cellular defense against oxidant stress. Additionally, it is demonstrated that iron ions are involved in paraquat toxicity. We conclude that uptake of paraquat into cells is via polyamine channels and that the deleterious effects of paraquat on these nonphotosynthetic cells are mediated by iron.

Greenhouse and growth chamber experiments with potted plants were conducted to determine the effects of interspecific root and canopy interference, light intensity, and soil moisture on water uptake and biomass of soybean, common cocklebur, and sicklepod. Canopy interference and canopy plus root interference of soybean with common cocklebur increased soybean water uptake per plant and per unit leaf area. Root interference with soybean decreased common cocklebur water uptake per plant. Canopy interference of soybean with sicklepod increased soybean water uptake per unit leaf area, while root interference decreased uptake per plant. Combined root and canopy interference with soybean decreased water uptake per plant for sicklepod. Soybean leaf area and shoot weight were reduced by root interference with both weeds. Common cocklebur and sicklepod leaf area and shoot weight were reduced by root and canopy interference with soybeans. Only common cocklebur root weight decreased when canopies interfered and roots did not. The relationship between light intensity and water uptake per unit leaf area was linear in both years with water uptake proportional to light intensity. In 1991 water uptake response to tight was greater for common cocklebur than for sicklepod. The relationship between soil moisture level and water uptake was logarithmic. Common cocklebur water uptake was two times that of soybean or sicklepod at −2 kPa of pressure potential. In 1991 common cocklebur water uptake decreased at a greater rate than soybean or sicklepod in response to pressure potential changes from −2 to −100 kPa.

Lower leaves of greenhouse-grown common cocklebur and velvetleaf were shaded to 5% of full light over a 12-d period while upper leaves remained exposed to full light to determine weed foliar and branching responses to partial shading similar to that encountered in soybean crops. Shading increased lower leaf senescence and specific leaf area, and decreased branch length and number of second-order leaves in both species compared to unshaded controls. Common cocklebur branched more extensively along the lower portion of its stem than velvetleaf under both shaded and unshaded conditions. Upper leaves of partially shaded velvetleaf were held in a more perpendicular position to the light source beginning 3 days after treatment (DAT) compared to upper leaves of unshaded plants. Shading of lower leaves caused an increase in upper (unshaded) leaf area beginning 3 and 6 DAT in velvetleaf and common cocklebur, respectively. Petiole length of upper leaves also increased in response to shading in both species. Total plant dry weight at 12 DAT was unaffected by shading in velvetleaf but was reduced 10% by shading in common cocklebur. While common cocklebur maintained greater lower shoot growth in the presence of shade than velvetleaf, there was a greater change in upper leaf angle by velvetleaf in response to shading than by common cocklebur. These results support previous field observations of apparent greater shade tolerance of common cocklebur compared to velvetleaf and indicate that both species have the ability to compensate for shading of lower leaves by altering upper shoot growth.

Research was conducted to determine the influence of soil fertilization and mowing on persistence of Indian mockstrawberry and common blue violet in an old, weakened tall fescue lawn. Fertilization treatments included: F0 = none applied; F1 = spring and fall application of 0.49 kg N, 0.21 kg P, and 0.41 kg K 100 m-2; and F2 = spring and fall application of twice the F1 rate. Mowing treatments were: M0 = not mowed; and M1 and M2 = mowing heights of approximately 4 and 6 cm, respectively, at biweekly intervals. Persistence of both weeds was influenced more by mowing than by fertilization. Survival of Indian mockstrawberry was greatest when mowed at 6 cm and not fertilized and lowest when highly fertilized and not mowed. Blue violets were virtually eliminated by either mowing treatment and did not respond to fertilization. Mowing and fertilization effectively reduced survival of Indian mockstrawberry and common blue violet in the tall fescue lawn.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m-2. Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter rye biomass and seed yield. Downy brome reduced yields most when it emerged within 3 wk of rye, but densities of more than 100 downy brome m-2 were required to reduce yields by 20 to 30%. The greatest reductions in rye biomass (28%) and seed (33%) yields over the 3 yr occurred when 400 downy brome m-2 emerged with the crop. Downy brome, at densities up to 400 plants m-2, emerging 6 wk after rye or in early spring, reduced rye biomass and seed yield less than 10% in all years. Winter rye effectively shaded downy brome (40 to 90%) for much of the growing season.

Progression of crenate broomrape population density (PD, number of emerged plants m-2) in faba bean was studied over 8 yr in Spain. Spatial dispersion and effect of climatic conditions on parasite population growth rate (PGR) also were studied. With repeated cropping of faba bean, infestations of crenate broomrape increased from an initial PD of 0.15 to an average of 26, with a maximum of about 40 to 45. The average population growth rate (PGR, ratio between the PD of any 2 consecutive years) was approximately 3. However, this figure varied widely among localities and years, from 0.8 to 7.7. A highly significant relationship (P = 0.01) was found between PGR and rainfall and soil temperatures during December to February, months of crop vegetative growth. Dispersion of crenate broomrape infestations mainly followed direction of crop rows, most likely due to the effect of tillage and harvesting operations, which were the same direction as sowing.

A fundamental component of modeling crop interference is the effect of understory photosynthetic photon flux density (PPFD) on weed architecture and growth. The effect of decreased PPFD on spaced redroot pigweed dry matter accumulation, distribution, and plant architecture was quantified by increasing corn density and delaying weed planting date. As canopy-transmitted PPFD declined, total dry matter accumulation decreased and relative dry matter distribution was greater to main-stem components than to branch components. Increased rectangularity in understory weed architecture was associated with a concomitant decrease in branch number. The proportion of leaf area and dry matter in the upper segment of the redroot pigweed increased as PPFD declined with increased corn density. Results suggest that changes in plant architecture, as influenced by canopy-transmitted PPFD, may be as important as those of total dry matter and leaf area when describing and predicting the effects of crop-weed interference.

Sicklepod was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. Maximum dry weight, leaf area, plant height, node number, and leaf number after 46 d occurred at 29/26 and 34/26 C. Temperatures of 29/21 C or lower reduced dry weight by more than 50%. Leaf number, leaf weight, and leaf area were more sensitive to changes in day temperature, whereas plant height and root, stem, and total dry weight were more sensitive to night temperature. Dry matter production was more closely correlated with leaf area duration than with its other component, net assimilation rate. Leaf appearance rate and dry matter production were linearly related to average daily temperature. The low-temperature threshold for leaf production was 13 ± 1 C. Observations of plant development in photoperiods ranging from 10 to 16 h confirmed that sicklepod is a short-day plant with a critical day length of 13 to 14 h. No reproductive development occurred in photoperiods of 15 or 16 h. Seedlings that emerged in 10-h photoperiods required more than 1-wk exposure to short days to initiate and continue reproductive development. Plants from a North Carolina population flowered earlier than plants from a Florida population in photoperiods of 12, 13, or 14 h, but in an 11-h photoperiod the two populations flowered at the same time.

Adaxial and abaxial epidermal surfaces of johnsongrass leaves were studied to determine which cells contribute to leaf microroughness. Cork-silica cell (CSC) pairs, three types of prickles, macrohairs, bicellular trichomes, stomata, and ordinary short and long epidermal cells were found and described. CSC pairs made up about 22% of all cells and probably contribute more to microroughness than any other single type because each cork cell produces 11 ± 3 wax filaments that are up to 100 μm long. Bicellular trichomes represented 4 to 5% of the total cells but decreased leaf roughness by secreting a type of mucilage that covers microscopic wax crystals. Stomatal complexes comprised 15 to 18% of all cells and contributed to leaf roughness because they are slightly recessed below the leaf surface. Long prickles occur primarily over veins and represent less than 1% of all cells. Small prickles were present primarily on adaxial surfaces and represent only 3% of all cells. Macrohairs were the largest appendages, 237 ± 104 μm, but they represent far less than 1% of all cells and occur primarily over veins. Ordinary short cells comprised 6 to 13% of all cells. Long cells were most common (41%) of all cells. Short and long cells contribute to leaf roughness because the surface is often convex. A typical johnsongrass leaf may contain more than 25 million appendages on each surface that increase the roughness already caused by epicuticular wax crystals.

Leaf development and expansion are important factors in determining the outcome of crop-weed interference. The comparative effects of temperature and corn canopy-induced shading on the rate of leaf appearance (RLA) of redroot pigweed were quantified in this study. Growth cabinet results indicated a linear increase in RLA with increased temperature. Weed RLA was predicted utilizing both this function and field temperature data. The ratio of observed to predicted RLA of redroot pigweed grown in field experiments decreased in 1990 and 1991 as shading increased with increased corn density and delayed weed planting date. Results indicated that RLA is substantially affected by canopy-induced shading in addition to temperature.

Interference between redstem filaree and round-leaved mallow was studied under controlled environmental conditions. Each species was grown in monoculture at densities of 2, 4, 8, and 12 plants per 20-cm-diam pot and in mixtures at all possible combinations of these densities. Leaf area per plant was similar for both species but round-leaved mallow grew taller and produced more shoot biomass than redstem filaree when each was grown in monoculture. Mixed culture responses varied with the proportion density of each species. A reciprocal yield model was tested and modified to account for this significant density interaction. When grown in mixture, round-leaved mallow usually gained in leaf area and shoot biomass at the expense of redstem filaree indicating that it was the superior competitor. Calculated competition ratios indicate that round-leaved mallow was about twice as competitive as redstem filaree under the growing conditions of this study.

An experiment was established at three sites to determine the longevity and distribution of rhizome buds of two quackgrass biotypes in the soil profile under two tillage systems (no-till or fall-plow). The experiments were established in May 1987 at sites initially free of quackgrass. Rhizome segments with three nodes were planted 2 to 5 cm deep to give an initial density of 21 buds m-2. Production of new buds was prevented in subsequent years. Rhizome bud populations were sampled every 6 mo between October 1987 and October 1989. Buds were produced until snow cover, indicating the need to control quackgrass until late fall to prevent a bud population increase. Under no-till conditions, 94% of quackgrass buds occurred in the top 10 cm of soil while 68 and 19% were found below 10 and 20 cm, respectively, in the fall-plow treatments. A control program against quackgrass should aim at keeping the majority of rhizomes in the top 10 cm in order to facilitate control by having most shoots emerging at the same time. There was no statistically significant difference in bud longevity between biotypes, sites, and tillage systems. Viability of the rhizome buds declined rapidly during the first year. In most cases it took 2 yr for the bud populations to reach extinction although 0.6% of buds of one biotype survived as long as 30 mo in one of the three sites. Therefore, a minimum of 2 yr of total control would be required to eradicate quackgrass from most locations.

Effects of proso millet interference with irrigated dry beans were evaluated in Nebraska over a 2-yr period. Dry bean yield reduction ranged from 12 to 31% from a wild proso millet density of 10 plants m-2. As density increased, dry bean yield reduction could be predicted with a rectangular hyperbola regression model. Ten wild proso millet plants m-2 growing with dry beans produced 14 780 to 21 420 seed m-2. Dry bean yields were reduced 41 and 11% in 1990 and 1991, respectively, when wild proso millet removal was delayed 6 wk after dry bean planting. Four weeks of weed-free maintenance were sufficient to provide dry bean yields comparable to plots kept weed free all season.

The photosynthetic responses of rice (C3) and three Echinochloa species (C4), barnyardgrass, early watergrass, and late watergrass, to changes in CO2 intercellular partial pressure, light intensity, and leaf temperature were investigated under laboratory conditions. The three Echinochloa species exhibited photosynthetic responses characteristic of C4 plants. The three weedy species showed higher efficiency for CO2 utilization at low CO2 intercellular partial pressure (CO2i) than rice. Compensation and saturation of CO2i for photosynthesis were lower in the weedy species than in rice. The maximum photosynthetic rates at high light intensity were 33.5, 32.7, 30.5, and 21.5 μmol CO2 m-2s-1 for barnyardgrass, early watergrass, late watergrass, and rice, respectively. Photosynthesis temperature optimum was 35 to 37 C for the three Echinochloa species and 33 C for rice. Overall, under simulated summer conditions, the four taxa showed a photosynthetic ability hierarchy with regard to gas exchange performance as follows: barnyardgrass ≥ early watergrass > late watergrass > rice.

Field studies in 1990 and 1991 evaluated treatment timings of nicosulfuron and bentazon alone and in mixture for weed control, peanut injury, and yield. The experimental areas were treated with a broadcast PPI application of benefin at 1.68 kg ai ha-1. Nicosulfuron was applied at 40 g ai ha-1 and bentazon at 560 g ai ha-1. Nicosulfuron mixed with bentazon applied 1 wk after crop emergence (WAE) controlled yellow nutsedge 81%. Bentazon or bentazon plus nicosulfuron applied 1 or 3 WAE controlled smallflower morningglory at least 94%. Nicosulfuron controlled sicklepod at least 76% in 1990. However, control of sicklepod was 71% from 1 WAE nicosulfuron application in 1991 and less than 26% for 3, 5, or 7 WAE application. Bentazon controlled Ipomoea spp. at least 70% but did not control Florida beggarweed and sicklepod. Nicosulfuron alone controlled Florida beggarweed and Ipomoea spp. Nicosulfuron reduced early-season peanut growth, but peanut recovered by mid-August. Bentazon reduced nicosulfuron control of Florida beggarweed when applied 1, 3, or 5 WAE, but not 7 WAE. Bentazon mixed with nicosulfuron applied 3 WAE in 1990 reduced weed-free peanut yields compared to nicosulfuron alone. Nicosulfuron and bentazon did not significantly reduce peanut yields under weed-free conditions in 1991.

Preemergence control of itchgrass in sugarcane with nonincorporated pendimethalin and prodiamine increased as rate for each herbicide increased from 2.2 to 3.4 kg ai ha-1. Itchgrass control at 3.4 kg ha-1 was similar for both herbicides, ranging from 78 to 94% (86% mean) for pendimethalin in six field experiments and 83 to 91% (86% mean) for prodiamine in three experiments. Incorporated trifluralin at 2.2 kg ha-1, the standard, gave 99% mean control of itchgrass. None of the nonincorporated treatments with pendimethalin or prodiamine consistently prevented a decrease in the sugarcane stalk population at harvest. Asulam applied postemergence at 3.7 kg ae ha-1 controlled itchgrass that survived the preemergence herbicides, and sugarcane stalk populations were maintained. Preemergence control of johnsongrass with nonincorporated treatments of pendimethalin or prodiamine at 2.8 kg ha-1, in a mixture with atrazine at 2.2 kg ai ha-1, ranged from 86 to 95% (89% mean) for pendimethalin and 80 to 95% (88% mean) for prodiamine. Metribuzin at 2.2 kg ha-1, the standard, gave 97% mean control of johnsongrass. Mixtures of pendimethalin with metribuzin or terbacil generally gave more effective control of johnsongrass than mixtures with atrazine. When used in conjunction with other herbicides, nonincorporated pendimethalin and prodiamine were effective alternatives to soil-incorporated treatments for weed control in sugarcane.

The purpose of this study was to quantify hysteresis during adsorption and desorption of atrazine as a function of incubation time for a Sharkey clay soil. Adsorption was carried out using one day batch equilibration and was followed by incubation periods ranging from 1 to 24 d. Incubation was subsequently followed by six consecutive desorption steps where each step represented 1 d. The Freundlich equation (S = K CN where S is the amount of atrazine retained, μg g-1; C is concentration, μg ml-1; K is the distribution coefficient, cm3g-1; and N is a dimensionless parameter) was used to describe batch results. Both adsorption and desorption isotherms were well described by the Freundlich model. Fitted K parameter values for desorption isotherms were consistently higher than those associated with adsorption. The opposite trend was observed for the exponential parameter N. The results revealed that desorption deviated significantly from adsorption data. The deviation, which is commonly referred to as hysteresis, was more pronounced as incubation time increased. Batch equilibration results also indicated that the extent of hysteresis was not influenced by soil sterilization. Attempts to quantify the extent of hysteresis using a simplified approach are presented. We found that, for a given batch data set, hysteresis can be quantified provided that Freundlich N from adsorption and desorption isotherms is known.

The persistence and movement of sethoxydim residues were determined in the top 45 cm of an Estherville sandy loam (sl), Port Byron silt loam (sil), and Webster clay loam (cl) in the field. Analysis of sethoxydim residues simultaneously quantified parent and eight metabolites by conversion to a common volatile end product, 3-[2(ethylsulfonyl)propyl]-pentanedioic acid dimethyl ester, followed by analysis using gas chromatography (GC) and a flame photometric detector. Recovery of residues from soil spiked with 100 μg kg-1 was 86 ± 21%. Residues remaining in soil 21 days after treatment (DAT) in 1989 were 37, 51, and 29% of the applied sethoxydim in the sl, sil, and cl, respectively, while in 1990, 86, 52, and 24% remained in the sl, sil, and cl, respectively. Alachlor remaining 21 DAT in 1989 was 66, 52, and 65% of that applied in the sl, sil, and cl, respectively, while in 1990, it was 16, 13, and 100% in the sl, sil, and cl, respectively. Atrazine had the greatest % of applied chemical remaining 21 DAT in 1989: 100, 71, and 73% in the sl, sil, and cl, respectively. of the atrazine applied in 1990, atrazine remaining 21 DAT was 87% in the sl, 42% in the sil, and 100% in the cl. Over all soils and years, the amount of total sethoxydim residues remaining 137 DAT was the least of the three herbicides. In terms of leaching, sethoxydim residues showed least movement, with minimal detections below 0 to 15 cm. Although sethoxydim appears to be advantageous over alachlor and atrazine in terms of potential impact on ground water quality, additional information regarding composition of the detected residues is needed to better assess its environmental impact.