A biotype of rigid ryegrass (Lolium rigidum G. ♯ LOLRI) has become resistant to amitrole and atrazine after 10 yr of exposure to a mixture of these herbicides. Resistance has also been demonstrated to the chloro-s-triazines: simazine, cyanazine, propazine; the methylthios-triazines: ametryn, prometryn; the substituted ureas: chlortoluron, isoproturon, metoxuron, diuron, fluometuron, methazole; and the triazinone herbicide metribuzin. The biotype remains susceptible to chlorsulfuron, metsulfuron, sulfometuron, sethoxydim, diclofop, fluazifop, glyphosate, carbetamide, and oxyfluorfen. Inhibition of oxygen evolution by atrazine, diuron, and metribuzin was similar in thylakoids isolated from both resistant and susceptible biotypes. Therefore, resistance to the photosystem II inhibitors is not caused by an alteration of the target site of these herbicides. Resistant plants treated with a 3-h pulse of 0.12 mM chlortoluron recover photosynthetic activity more rapidly than susceptible plants. This suggests that the basis for resistance is enhanced metabolism or sequestration of the herbicide within the leaf.

Studies were conducted to test if dicamba reduces haloxyfop herbicidal activity and to determine if this interaction by auxin-type herbicides or bentazon was caused by preventing the haloxyfop-induced inhibition of acetyl-CoA carboxylase (ACCase). Addition of dicamba at concentrations of 2.17 or 4.34 mM reduced haloxyfop activity on tall fescue shoots. Haloxyfop at 14 to 20 μM inhibited the activity of ACCase in cell-free extracts from tall fescue by nearly 50%. Dicamba did not prevent the inhibitory effect of haloxyfop on ACCase activity in these cell-free enzyme preparations. Dicamba alone at concentrations as high as 3.2 mM had no significant effect on the in vitro activity of the enzyme. Bentazon concentrations up to 10 mM did not prevent haloxyfop inhibition of ACCase activity. Bentazon alone or mixed with haloxyfop significantly inhibited ACCase activity at bentazon concentrations starting at 1 mM with near 40% inhibition at 10 mM. of the numerous possible explanations for the interference of dicamba or bentazon with haloxyfop action, interaction directly at the enzyme level appears to have been eliminated.

Concentrations of clomazone in nutrient solution causing 50% reduction in shoot fresh weights were 325 and 2 μM for soybean and velvetleaf, respectively. Root uptake of 14C–clomazone from nutrient solution and translocation to shoots were similar for the two species. Soybean and velvetleaf contained similar amounts of 14C per plant; however, because velvetleaf had lower tissue weights, concentration of 14C was higher in velvetleaf. Clomazone did not accumulate in roots but was rapidly translocated to shoots of both species. No more than 30% of absorbed clomazone remained in roots at any time in the experiment. Soybean and velvetleaf leaves contained 4.2 and 1.7 nmol clomazone g−1 fresh weight, respectively, and 3.7 and 6.1 nmol clomazone metabolites g−1 fresh weight, respectively. Thus, velvetleaf metabolized clomazone more rapidly than soybean did. Treatment of clomazone metabolites with β-glucosidase demonstrated that metabolism occurred by oxidative cleavage and conjugation of the benzyl moiety in both plant species. One aglycone cochromatographed with 2–chlorobenzylalcohol. The results suggest that reduced absorption or translocation or increased detoxication of clomazone by soybean seedlings is probably not responsible for their tolerance to this herbicide.

The effects of chlorimuron as related to application timing were evaluated on sicklepod and soybean in the glasshouse. Plants were sprayed with chlorimuron at 8.8 g ai ha–1 at 14, 21, 28, and 35 days after planting (DAP). Early (14 DAP) chlorimuron application reduced sicklepod dry weight, leaf area, and water use to levels significantly less than those of plants treated at 28 and 35 DAP. Soybean dry weight and leaf area were not affected by chlorimuron. However, soybean water use was initially reduced at application timings of 28 DAP or earlier but recovered by 4 weeks after treatment (WAT). Sicklepod net assimilation rate (NAR) and relative growth rate (RGR) were reduced by chlorimuron regardless of application timing while soybean was not affected.

Twenty accessions within 11 perennial Glycine species plus soybean were tested for tolerance to 2,4-D. Soybean was severely injured by 2,4-D, but 13 of the Glycine accessions had 15% or less injury 4 weeks after 2,4-D application. Greatest 2,4-D tolerance occurred with accessions of G. latifolia and G. microphylla. There was no difference among tolerant accessions of G. latifolia and G. microphylla and susceptible accessions of G. canescens in recovery or absorption of 14C-2,4-D 1, 3, 7, or 14 days after treatment (DAT). Distribution of 14C from 14C-2,4-D in various plant parts was similar among accessions. Metabolism of 2,4-D in the tolerant accessions (81 to 89% 1 DAT) was higher and more rapid than in susceptible accessions (approximately 50%, 1 DAT). The same five metabolites plus parent 2,4-D were extracted from the treated leaf of all accessions at all sampling dates. However, relative distribution between metabolites differed between tolerant and susceptible accessions. More rapid metabolism of 2,4-D in treated leaves of tolerant Glycine accessions can explain differential 2,4-D responses.

CGA-154281 and temperature effects on metolachlor absorption and metabolism were studied in corn seedlings grown in untreated soil or soil treated with metolachlor, CGA-154281, or both. Seedlings were grown under a cool (21/13 C, 16-h day) or warm (30/21 C, 16-h day) temperature regime and exposed to 14C-metolachlor for 10 min at either 21 or 30 C. Corn grown under the cool temperature regime absorbed slightly more 14C-metolachlor than corn grown under the warm temperature regime. Corn held at 21 C during a 10-min 14C-metolachlor exposure period metabolized metolachlor more slowly than corn held at 30 C. Decreased corn tolerance to metolachlor observed at lower temperatures may be due in part to slower metolachlor metabolism. Corn grown in the presence of metolachlor plus CGA-154281 metabolized 14C-metolachlor to the glutathione conjugate twice as fast as corn grown in the presence of metolachlor alone. The increase in metabolism rate was due to a fivefold increase in glutathione-S-transferase (GST, EC 2.5.1.18) activity and not to an increase in glutathione (GSH) content. Results are consistent with the hypothesis that CGA-154281 protects corn from metolachlor injury by enhancing GST activity, which accelerates metolachlor detoxification via GSH conjugation.

Primary root cells from five dinitroaniline-resistant (R) and three susceptible (S) goosegrass biotypes from North Carolina and South Carolina were observed by transmission electron microscopy to determine whether resistance was associated with changes in cell wall formation. Cell wall malformations were found in some cells from two of the R-biotypes and in one of the S-biotypes. Malformations consisted of partially deposited cell walls and the inclusion of cell wall material in the cytoplasm. Some of the affected cells also had abnormal, lobed nuclei and malformed mitochondria. There seems to be little or no correlation between dinitroaniline resistance and cell wall malformations.

Diclofop–methyl and haloxyfop–methyl (0.001 to 10 μM) caused 9 to 61% inhibition of 14C–acetate incorporation into lipids in corn leaf segments within 1 h of herbicide treatment, while neither herbicide affected this process in bean leaf segments. The herbicides did not affect 14C-malonate incorporation into lipids in corn leaf segments. Diclofop-methyl and haloxyfop-methyl reduced 14C-acetate incorporation into polar lipids and triglycerides in corn while incorporation into sterols was increased. In vitro activity of acetyl-coenzyme A carboxylase (EC 6.4.1.2) was inhibited from 26 to 94% within 5 min of exposure to the herbicides (1 to 10 μM). Diclofop acid inhibited this enzyme activity more than did haloxyfop acid. Differences in field activity between diclofop-methyl and haloxyfop-methyl are not related to differential sensitivity of acetyl–coenzyme A carboxylase to the two herbicides.

Based on chlorophyll content, hydroponically cultured soybean seedlings were 254, 66, and 13 times more tolerant to clomazone than velvetleaf, corn, and smooth pigweed, respectively. Clomazone, at concentrations that inhibited chlorophyll, did not affect fresh weight accumulations of any species except velvetleaf. However, in velvetleaf, fresh weight accumulation was only half as sensitive to clomazone as the leaf chlorophyll content. Uptake of 14C-clomazone from nutrient solution by 72 h after treatment (HAT) (pigweed > velvetleaf > soybean > corn) indicates that differential absorption cannot account for selectivity. Shoot:root ratios of 14C recovered from soybean, corn, velvetleaf, and pigweed by 72 HAT were 0.39, 0.84, 1.67, and 2.37, respectively. The limited acropetal clomazone translocation in soybean seedlings may account to a small degree for soybean tolerance to clomazone. Conversion of clomazone to more polar metabolites was rapid in all four species. There were no significant differences among species in the percentage of 14C activity recovered as clomazone from root tissue by 72 HAT. Of the 14C activity recovered from shoots of soybean, corn, pigweed, and velvetleaf seedlings by 72 HAT, 46, 59, 35, and 54%, respectively, was clomazone. Differences in clomazone uptake, distribution, and metabolism among the four species were either insignificant or poorly correlated to selectivity, and therefore cannot account for the tremendous differences in clomazone sensitivity among these species. These observations indicate, indirectly, that differences at the site of action may account for selectivity.

Intraspecific and interspecific interference effects on growth, gas exchange, and water potential of wild oat and spring barley were measured under greenhouse conditions using a 1:1.06 barley to wild oat replacement series. Intraspecific barley interference affected barley growth more than interspecific wild oat interference. Interspecific wild oat interference with barley reduced wild oat growth more than intraspecific interference. Wild oat plant height surpassed barley plant height near barley anthesis. Growth and gas exchange of barley and wild oat responded the same to short-term water stress.

Inhibition of net photosynthesis of jointed goatgrass and downy brome protoplasts by metribuzin and its ethylthio analog (ethyl-metribuzin) was greater at 25 than at 10 C. As temperature increased from 10 to 25 C, the concentration of ethyl-metribuzin required to inhibit net photosynthesis 50% (I50) decreased by a factor of 3.5 and 4.3, respectively, in jointed goatgrass and downy brome. I50 values for metribuzin decreased by a factor of 1.5 and 2.5 in jointed goatgrass and downy brome, respectively, for the same 15 C increase in temperature. Based on I50 values at 10 C, metribuzin was nine times more inhibitory than ethyl-metribuzin in jointed goatgrass and eight times more inhibitory in downy brome. At 25 C, metribuzin was only 4.7 and 3.9 times more inhibitory than ethyl-metribuzin in jointed goatgrass and downy brome, respectively. Thus, cold temperatures reduced the activity of ethyl-metribuzin more than metribuzin. The activity of both herbicides was reduced less in protoplasts of jointed goatgrass than in protoplasts of downy brome over the 15 C range.

Individual herbicides and all double combinations of imazaquin, imazethapyr, and imazapyr were applied alone and in combination to the foliage of pitted morningglory plants in order to evaluate potential influences on translocation. Unlabeled imazethapyr at 5.62 μg plant−1 stimulated basipetal translocation of both 14C-imazaquin and 14C–imazapyr. Unlabeled imazaquin at 7.49 μg plant−1 also increased basipetal translocation of 14C–imazethapyr but did not influence 14C–imazapyr translocation. No combination affected the amount of 14C herbicide remaining in treated leaves or moving into foliage above treated leaves. Unlabeled imazapyr at 0.11 μg plant−1 did not affect movement of either imazaquin or imazethapyr.

Field experiments were conducted during the winters of 1986–87 and 1987–88 at Haryana Agricultural University, Hisar, India to classify the ability of winter wheat cultivars to compete with wild oat. Wild oat reduced winter wheat grain yield by 17 to 62% depending upon cultivar. WH-147 and HD-2285 were the most competitive cultivars. Winter wheat dry matter accumulation and grain yield were negatively correlated with wild oat dry matter. A high number of tillers, particularly in HD-2009, WH-291, and S-308, did not always translate into grain yield advantage in wild oat-infested plots. Wheat height and dry matter accumulation per unit area during early crop growth were better characters than number of tillers for predicting the competitive ability of wheat cultivars to wild oat.

Photosynthetic productivity parameters were determined for mayweed chamomile, a troublesome annual weed of the cropping systems in the Pacific Northwest. At a photosynthetic photon flux density of 1800 μE m−2 s–1, maximum net photosynthetic rate of greenhouse-grown plants was 35 mg CO2 dm−2 h–1 and maximum transpiration rate was 6.7 μg H2O cm−2 s–1. Dark respiration rate was 1.4 mg CO2 dm−2 h–1 and the light compensation point was 17.5 μE m−2 s–1. Carbon dioxide compensation point increased from 25 ppm at 15 C to 43 ppm at 30 C. At saturating photosynthetic photon flux densities, optimum leaf temperature for net photosynthesis was about 25 C. Maximum net photosynthesis of leaves of field-grown plants averaged 15.8 mg CO2 dm−2 h–1. After a 24-h exposure to 0.075 kg ha–1 metribuzin, maximum net photosynthesis and transpiration were reduced 85 and 40%, respectively. Soil water deficits reduced maximum net photosynthesis about 50%.

Effects of ammonium and methylammonium on foliar uptake of 14C-thifensulfuron, in the presence of 0.2% nonionic surfactant by vol, into velvetleaf were investigated. Total 14C uptake into velvetleaf 24 h after treatment was only 15% without addition of nitrogen, but addition of 0.32 M NH4Cl, urea ammonium nitrate, or CH3NH2·HCl increased total 14C uptake to 31, 38, or 59%, respectively. Total 14C uptake 24 h after treatment was increased 3- and 4-fold by addition of 0.08 and 0.16 M CH3NH2·HCl, respectively, but was not increased further by greater CH3NH2·HCl concentrations. Although the increase in 14C uptake was followed by an increase in total 14C translocated, the relative amount of 14C translocated (expressed as a percentage of total 14C uptake) was not affected by nitrogen additives. Enhancement of uptake of 14C-thifensulfuron by CH3NH2·HCl was partially reversed by addition of 0.16 M CaCl2 to the application solution.

Research was conducted to determine foliar absorption of 14C-clethodim and its polar degradation products with five adjuvants and to compare the relative herbicidal activity of clethodim and its polar degradation products. Differential 14C-clethodim and 14C-polar degradation products absorption occurred among the five adjuvants. Adjuvants that mediated rapid foliar absorption of clethodim did not always mediate rapid foliar uptake of polar degradation products. Herbicidal activity of clethodim exceeded activity of polar degradation products regardless of time of day of application or whether adjuvant was used. Inclusion of adjuvant was required for herbicidal activity of polar degradation products. Clethodim activity declined when applied at 0800 h EDT compared to applications at 2000 h EDT. To maximize herbicidal activity of clethodim, adjuvants should be selected that mediate rapid foliar absorption of both clethodim and the polar degradation products of clethodim and that minimize photodegradation of clethodim.

Laboratory studies demonstrated that naptalam safens cucumber against the phytotoxic effects of chloramben. In petri dish studies, cucumber seedlings grown from seeds exposed to chloramben plus naptalam had greater shoot growth, root growth, and dry weight than seedlings grown from seeds exposed to chloramben alone. Naptalam also partially reversed the reduction in dry weight of various plant parts caused by exposure of roots of hydroponically grown seedlings to chloramben. More radioactivity from root-applied 14C-chloramben remained in cucumber roots and less was translocated to shoots with a 14C-chloramben plus naptalam treatment than with a 14C-chloramben alone treatment. Naptalam appeared to influence chloramben metabolism. In various plant parts, concentrations of chloramben and its metabolites differed between the two treatments.

A biotype of the annual grass weed hare barley infesting an alfalfa field with a 24-yr history of the use of the bipyridylium herbicides paraquat and diquat, was investigated for resistance to these herbicides. Rates of up to 800 g ai ha–1 of each herbicide caused no mortality in the hare barley plants from this field. The same species, collected from an adjacent pasture field with no history of bipyridylium herbicide application, exhibited LD50's of 57 and 160 g ai ha–1 for paraquat and diquat, respectively. Tiller numbers and dry matter production in the biotype from the alfalfa field were not affected by the normal rate recommended for both herbicides. These results clearly show that hare barley from the alfalfa field is resistant to paraquat and diquat. Both biotypes were equally sensitive to fluazifop, glyphosate, and sethoxydim.

The herbicide clomazone caused ultrastructural damage to etioplasts of cotton cotyledons. Etioplast envelopes were irregular, prothylakoids were absent or irregular, stroma density was low, and there were abnormal stromal vesicles. Further damage occurred upon exposure to light. Clomazone greatly slowed the conversion of chlorophyllide to chlorophyll in cotton, suggesting that phytol synthesis was affected. Neither synthesis of protochlorophyllide nor phototransformation of protochlorophyllide to chlorophyllide was affected by clomazone. Clomazone completely inhibited carotenoid synthesis without an accumulation of phytoene. However, the sesquiterpenoids hemigossypol and the dimeric sesquiterpenoid gossypol accumulated in greater amounts in clomazone-treated than in control tissues. These results indicate that this herbicide inhibits synthesis of terpenoids after the formation of farnesyl pyrophosphate.

Measurements of net photosynthesis, chlorophyll a fluorescence and solute leakage were evaluated as techniques for rapid detection of grass tolerance to fenoxaprop-ethyl and sethoxydim. Net photosynthesis measured as net CO2 uptake of the youngest fully expanded leaf of Calamagrostis arundinacea ‘Karl Foerster’ (Feather Reed Grass) detected tolerance to fenoxaprop-ethyl and susceptibility to sethoxydim 4 days after treatment and prior to the observation of visual injury. Sethoxydim reduced photosynthesis compared to untreated controls but fenoxaprop-ethyl did not. Measurements of older leaf (second most recently fully expanded leaf) photosynthesis were less consistent than those of the younger leaf. Chlorophyll a fluorescence of Calamagrostis leaf segments following treatment with fenoxaprop-ethyl and sethoxydim was ineffective for detecting grass tolerance. Solute leakage from leaf discs treated with 50 to 500 μg ml−1 fenoxaprop–ethyl and 10 to 100 μg ml−1 sethoxydim indicated differential tolerance of Calamagrostis and johnsongrass to fenoxaprop–ethyl and centipedegrass and johnsongrass to sethoxydim. Fenoxaprop–ethyl increased solute leakage from susceptible johnsongrass at 100 μg ml−1 while the 500 μg ml−1 concentration was required to cause substantial solute leakage from tolerant Calamagrostis. Sethoxydim caused greater leakage from johnsongrass than from centipedegrass at 50 and 100 μg ml−1. The best potential rapid screening technique was solute leakage measurements.