Three chlorsulfuron-resistant kochia accessions were tested for levels of resistance to sulfonylurea and imidazolinone herbicides, based on whole plant response and sensitivity of the target enzyme. The resistant Minot and Chester accessions were not affected by treatment with 175 g ha−1 chlorsulfuron, and I50 values for the Chester accession ranged from 22-fold (metsulfuron-methyl) to 196-fold (chlorsulfuron) higher than the susceptible Bozeman accession. However, the Chester accession was 1.5- to 2-fold more resistant than Minot to five sulfonylurea herbicides, as determined by acetolactate synthase (ALS) I50 values. The third resistant accession (Power) displayed an intermediate response and was only 2- to 5-fold more resistant than the susceptible Bozeman accession to all sulfonylurea herbicides tested. The Minot and Chester accessions were slightly cross-resistant to four imidazolinone herbicides, ranging from 2-fold (imazamethabenz, imazethapyr, and imazaquin) to 6-fold (imazapyr) more resistant than the Bozeman accession, but cross-resistance levels did not differ appreciably between the Minot and Chester accessions. The Power accession was not cross-resistant to the four imidazolinone herbicides tested. The results demonstrate that degrees of ALS resistance and cross-resistance are highly variable among kochia populations: these differences may be due to the type of mutation in the gene encoding ALS.

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.

Greenhouse and laboratory studies were conducted to determine the degree of dominance of the monogenic sulfonylurea herbicide resistance trait in diploid sugarbeet by comparing the response of homozygous and heterozygous resistant sugarbeet to primisulfuron, thifensulfuron, and chlorimuron on the whole plant and acetolactate synthase (ALS) enzyme level. Progeny tests suggested that the monogenic sulfonylurea herbicide resistance was semidominant. Subsequently, heterozygous resistant (R-1) and homozygous resistant (R-2) sugarbeet lines were sprayed with increasing rates of primisulfuron, thifensulfuron, and chlorimuron, and herbicide rates required for 50% growth reduction (GR50) were determined. GR50 values were also determined for homozygous susceptible sugarbeet lines (S-1 and S-2). The GR50 values indicated that the R-2 sugarbeet was 377, 269, and 144 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than susceptible S-2 sugarbeet. In contrast, R-1 sugarbeet was only 107, 76, and 57 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than S-1 sugarbeet, indicating at least a twofold difference in the magnitude of resistance between homozygous resistant and heterozygous resistant sugarbeet lines. ALS enzyme activity analysis were consistent with whole plant results. Thus, based on these two, maximum crop resistance can be obtained by developing homozygous resistant cultivars.

Hemp dogbane is sensitive to fluroxypyr and tolerant to clopyralid. Absorption, translocation, and metabolism of clopyralid and fluroxypyr were studied in hemp dogbane to determine if differences in these processes could be responsible for differential sensitivity. In addition, the effect of growth stage on herbicide absorption and translocation was evaluated. The 14C-herbicides were applied to the adaxial side of a single leaf located near the midpoint of hydroponically cultured plants. Uptake of fluroxypyr was more rapid than clopyralid. At 72 h after treatment (HAT), fluroxypyr and clopyralid absorption was 62 and 38%, respectively. Clopyralid was much more mobile than fluroxypyr, with 75% of the absorbed 14C from 14C-clopyralid recovered outside the treated leaf compared to only 45% for fluroxypyr 72 HAT. Relative to fluroxypyr, a higher percentage of 14C-clopyralid recovered outside the treated leaf translocated acropetally, especially when plants were treated during the vegetative stage. Treatment during the early reproductive stage increased basipetal and reduced acropetal translocation relative to the vegetative stage. Neither herbicide was metabolized rapidly. Approximately 60 and 90% of the recovered 14C was attributable to unaltered fluroxypyr and clopyralid, respectively, 72 HAT. Some differences in absorption, translocation, and metabolism between clopyralid and fluroxypyr exist, but they cannot fully account for differential sensitivity of hemp dogbane to these two herbicides. Differences in activity at the target site may be responsible for differential activity of these herbicides on hemp dogbane.

Bromoxynil efficacy, uptake, translocation, and spray retention were investigated when bromoxynil was applied in conventional water volumes of 234 L ha−1 and in simulated sprinkler irrigation at 127 000 L ha−1 to common lambsquarters. Bromoxynil controlled common lambsquarters similarly regardless of water volume, whereas injury to spearmint, a tolerant crop, was greatest using low water volume. Spray retention was 18 and 38 times greater on common lambsquarters and spearmint, respectively, when bromoxynil was applied in 234 L ha−1 than 127 000 L ha−1. Two weeks after applying bromoxynil in 127 000 L ha−1 water volume, common lambsquarters dry weight was 60% of the nontreated check where only soil was treated but was 5% of the nontreated check where only leaves were treated. Roots of lambsquarters absorbed 22% of 14C-bromoxynil applied to hydroponic solution by 7 d, but only 2% was translocated to the shoots. Percent absorption and translocation of foliar-applied 14C-bromoxynil were 15 and 6% greater, respectively, from 0.0096 g L−1 than from a 1.2 g L−1 bromoxynil solution by 24 h after application. Uptake of bromoxynil was 13% greater through lower than upper leaf surfaces. These results suggest efficacy of bromoxynil applied in large spray volumes is, in part, due to root uptake, efficient foliar uptake and translocation, and uptake from lower leaf surfaces.

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.

The fact that quackgrass may occasionally escape control by the herbicide glyphosate is thought to result from the wide range in growth rate and sink activity among rhizome buds, especially in older portions of the rhizome. To study growth of rhizome structures, we supplied whole plants with 14CO2 throughout a 10-h light period and determined the amount of labeled carbon accumulated by the end of the subsequent 14-h night. Growth of rhizome structures during this 24-h period was estimated by determining their growth rate coefficients: the amount of labeled carbon accumulated per unit of carbon present in the structure. Growth rate coefficients generally were high for the rhizome tip that is enclosed in a sheath and the adjacent bud and rhizome segment, with values decreasing rapidly in a basipetal direction. However, extensive differences in the level and pattern of assimilate accumulation among rhizome structures were observed as rhizome development continued. Glyphosate accumulation generally paralleled the level of assimilate accumulation even though the range among rhizome structures for both increased with rhizome age. As a result of the increased variability among buds, some of the older buds will accumulate only a small, perhaps sublethal, amount of glyphosate and this may explain the tendency of the buds in older regions to escape control by glyphosate.

Field trials indicated that addition of ammonium sulfate to imazethapyr plus nonionic surfactant increased quackgrass control, especially at low imazethapyr rates. In greenhouse experiments, approximately twice as much imazethapyr was absorbed by quackgrass leaves when the herbicide was applied in combination with nonionic surfactant plus ammonium sulfate than when the herbicide was applied with nonionic surfactant alone. Black Mexican Sweet maize (BMS) suspension-cultured cells were used to evaluate the effects of ammonium sulfate and nonionic surfactant on cellular absorption of imazethapyr in the absence of a cuticular barrier. Imazethapyr absorption by BMS cells was diffusion-mediated, energy-dependent, and exhibited a pH optimum of approximately 3. Over the concentration range of 0.1 to 10.0 μM, the equilibrium concentration of imazethapyr in BMS cells was a linear function of the external concentration. Addition of ammonium sulfate to the external medium of BMS cells enhanced both the rate of imazethapyr uptake and medium acidification. There was a linear correlation between the ability of ammonium sulfate (0.5 to 10 mM) to promote medium acidification and imazethapyr uptake by BMS cells. The ammonium sulfate-induced stimulation of imazethapyr absorption in BMS cells was sensitive to plasma membrane adenosine triphosphatase inhibitors (sodium vanadate, diethylstilbestrol), the uncoupler carbonyl cyanide m-chlorophenylhydrazone, and energy metabolism inhibitors (sodium azide, nitrogen gas), demonstrating that this effect was dependent on ATP production and the functioning of the plasma membrane ATPase. It is hypothesized that cytoplasmic acidification in BMS cells due to ammonium assimilation stimulates the plasma membrane ATPase to pump protons across the plasma membrane which in turn acidifies the cell wall promoting cellular accumulation of imazethapyr by ion-trapping. Cell wall acidification due to ammonium assimilation may contribute to the ability of ammonium sulfate to enhance the efficacy of imazethapyr and other foliar-applied herbicides.

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.

Broadleaf weeds, including spurred anoda, emerge after direct-seeded chile peppers are thinned. Field experiments were conducted in 1989, 1990, and 1991 to determine the effect of spurred anoda density on green and red pepper yield, quality, and ease of hand harvest. Spurred anoda was established immediately after peppers were thinned at initial densities of 0, 3, 6, 12, 24, or 48 plants 9 m−1 row. The 1991 experiment also evaluated the influence of delayed pepper thinning and concurrent spurred anoda establishment on the competitive effect of spurred anoda. Spurred anoda were beginning to flower at green harvest and senescing at red harvest regardless of planting date. Spurred anoda were taller and accumulated more biomass when planted at a pepper thinning stage of 10 cm compared to 20 cm. Spurred anoda that emerged after thinning peppers reduced yield and ease of harvest of green and red peppers but not the quality of green peppers. Yield reduction at the highest spurred anoda density was 31 to 49% and 12 to 27% when peppers were thinned at 10 or 20 cm, respectively. Yield reduction was smaller when peppers were thinned at 20 cm tall than 10 cm tall and appeared to be associated with reduced spurred anoda biomass. Time required to hand harvest 1 kg of green or red peppers increased as spurred anoda density increased when peppers were thinned at 10 cm.

Investigations of smooth crabgrass growth and fenoxaprop-ethyl retention, foliar penetration, translocation, and metabolism were conducted at various soil moisture levels using a polyethylene glycol (PEG) semipermeable membrane system. The activity of fenoxapropethyl was significantly reduced at higher levels of moisture stress and this antagonistic effect was greater with increased duration of water deficit following herbicide application. Fenoxaprop-ethyl spray retention decreased linearly (23% total reduction) as soil matric potential (Ψm) decreased from −0.01 to −0.1 MPa. Foliar penetration and translocation of 14C-fenoxaprop-ethyl applied on the third true leaf were not affected by level or duration of moisture stress. Only 2% of the absorbed radioactivity was translocated out of the treated leaf for each moisture stress level and duration. As the soil Ψm decreased (−0.01 to −1.0 MPa) the relative levels of fenoxaprop-ethyl increased by 76 and 65% after a 48- and 96-h postapplication moisture stress period, respectively. In contrast, fenoxaprop acid decreased by 59 and 44% after 48 and 96 h of moisture stress, respectively. The relative level of fenoxaprop acid was linearly correlated to the antagonistic effect on shoot dry weight. These results suggest that decreased spray retention and, particularly, alterations in fenoxaprop-ethyl metabolism contribute to reduced fenoxaprop-ethyl activity observed in moisture-stressed smooth crabgrass.

Effectiveness of cyclical in vitro selection for imazethapyr tolerance in tomato was studied. Cotyledons and leaf explants from shoots regenerated on media containing imazethapyr were used to initiate three successive cycles of selection. Increases in tissue viability, callus initiation, and callus proliferation were observed following three cycles of selection on 10−7 M imazethapyr. Dry matter accumulation of unselected tomato seedlings was reduced by 50% when imazethapyr was applied postemergence at a rate of 28 g ae ha−1. Progeny of eight tomato lines selected after one cycle of selection had greater tolerance to imazethapyr than control plants (60 to 70% dry matter accumulation versus 50%, respectively). Variability was also generated for imazethapyr tolerance among progeny of the same tomato line.

Field observations indicate that sulfonylurea-resistant kochia may germinate at lower soil temperatures and/or germinate more rapidly than susceptible kochia in the absence of herbicide. To investigate this possibility, seeds from three resistant and two susceptible kochia accessions were germinated at temperatures ranging from 4.6 to 13.2 C on thermal gradient plates. At 4.6 and 13.2 C, germination rates of all resistant accessions were higher than susceptible accessions, while germination rates of one resistant accession were higher than susceptible accessions at 7.2 and 10.5 C. Percent germination of all resistant accessions was significantly higher than susceptible accessions after 48 h at 4.6 C. At higher temperatures, percent germination of some resistant accessions was higher after 12 or 24 h, but germination of all accessions was similar at later times. HPLC analysis revealed that seeds from resistant accessions contained about 2-fold higher free levels of branched chain amino acids than seeds from susceptible accessions. The results indicate that mutations conferring resistance to sulfonylurea herbicides in these kochia accessions may concomitantly reduce or abolish acetolactate synthase sensitivity to normal feedback inhibition patterns, resulting in elevated levels of branched chain amino acids available for cell division and growth during early germination.

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.

The translocation pattern and distribution in rhizomes of 14C-glyphosate, 14C-quizalofop, and 14C-sucrose was examined in five quackgrass biotypes. Translocation of radioactivity in the different plant parts varied among biotypes. Translocation in the whole plant after treatment with the three 14C-chemicals varied among biotypes but was not correlated with their tolerance to herbicides. Detailed analysis of distribution of radioactivity in the primary rhizome showed that more 14C was found in the apical sections after treatment with 14C-sucrose. A similar pattern was observed after 14C-glyphosate application with all biotypes except one. Very low radioactivity was found in rhizomes after 14C-quizalofop application, but a preferential accumulation in apical sections of the primary rhizome was detected in two biotypes. The tolerance of one biotype to glyphosate was explained by the absence of radioactivity accumulation in the apical sections of the rhizome.

Field studies were conducted in 1986 and 1987 to determine the critical period for weed control in white bean grown in Ontario. The treatments consisted of either allowing weeds to infest the crop for increasing durations after planting or maintaining plots weed free for increasing durations after planting. The beginning of the critical period was defined as the crop stage by which weed interference reduced yields by 3%. Similarly, the end of the critical period was defined as the crop stage to which the crop had to be weed free to prevent a 3% yield loss. The critical period of weed control occurred between the second-trifoliolate and first-flower stages of growth for all cultivars and years, with the exception of the cultivar ‘OAC Seaforth’ in 1986. The average number of pods per plant for both cultivars was reduced by increasing durations of weed interference after planting in both years. However, pod number of the cultivar OAC Seaforth was reduced at a greater rate in 1986 than ‘Ex Rico 23’. The beginning of the critical period corresponded with the beginning of a rapid increase in total weed biomass.

Field studies were conducted in 1987 and 1988 to evaluate the vegetative response of soybeans to early postemergence applications of lactofen and to measure crop recovery. Soybean injury 4 d after treatment (DAT) averaged 29 and 34% with 0.22 and 0.44 kg ai ha−1 of lactofen, respectively. Severity of leaf injury depended on leaf size at the time of lactofen application. Leaf area of soybean trifoliates one through three were reduced 30 to 45% at 12 DAT but recovered by 26 DAT. Later emerging trifoliates were not affected. Fresh weights of lactofen-treated soybeans were less than those of untreated plants 4, 8, and 16 DAT, and dry weights were less at 16 and 26 DAT. Lactofen reduced stomatal conductance of expanded leaves but had no effect on unexpanded leaves. Soybean yields did not differ with lactofen treatment.

Mixtures of chlorimuron and 2,4-DB were additive with respect to crop injury and were either additive or slightly antagonistic with respect to weed control in greenhouse experiments. Absorption and translocation of 14C following application of 14C-chlorimuron and 14C-2,4-DB were not affected by the presence of the other unlabeled herbicide, except in Florida beggarweed and peanut where 2,4-DB affected distribution of 14C-chlorimuron in the treated leaf. In field studies, maximum efficacy was obtained with mixtures of chlorimuron plus 2,4-DB applied 7 or 9 wk after planting. Florida beggarweed control was greatest with chlorimuron or chlorimuron mixtures while the addition of 2,4-DB to chlorimuron improved morningglory and sicklepod control. At 9 and 11 wk after planting, addition of 2,4-DB to chlorimuron controlled Florida beggarweed better than chlorimuron alone. Peanut yields were increased by the addition of 2,4-DB at later applications.

Annual application of AC-94377 at 3.4 kg ha−1 in the field reduced survival of shallowly buried (1.25 cm deep), undisturbed wild mustard seed compared to untreated check seed in two 4-yr-long trials. By the second fall, greater than twofold more untreated check seed survived as did AC-94377-treated seed. Moreover, no AC-94377-treated seed survived beyond year three following treatment in fall alone or fall plus spring in each of 3 yr. In contrast, 25% of untreated check seed survived into the fall of year four. AC-94377 applied in spring alone, fall alone, or both spring and fall for each of 4 yr progressively reduced seed survival. Seed survival expressed as a percent of the initial number of seed buried was best modeled as a negative exponential function of time in years. In the greenhouse, more wild mustard seed on the soil surface established after AC-94377 treatment at 3.4 kg ha−1 when enclosed in large seed packets (5 by 12 cm), like those used in the field, than when in small seed packets (5 by 6.25 cm), whether or not the packets contained soil. When soil was added to either sized seed packet, fewer seed survived compared to seed not in seed packets or seed in packets without soil. Thus, it is likely that the field seed survival study underestimated effectiveness of AC-94377 to reduce wild mustard seed survival.

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.