Pitted morningglory and entireleaf morningglory treated with 14C-imazaquin translocated 14C to areas above and below the treated leaf. Pitted morningglory absorbed and translocated more 14C from 14C-imazaquin than entireleaf morningglory. Translocation of 14C from root-supplied 14C-imazaquin was similar in both species 1 d after treatment, with 14C moving rapidly to the shoots. Entireleaf morningglory metabolized slightly more imazaquin than pitted morningglory in treated leaves. Greater tolerance of entireleaf morningglory than pitted morningglory to postemergence applications of imazaquin is attributed to reduced absorption and translocation and increased metabolism of the herbicide in the entireleaf morningglory.

Effects of the herbicides SC-0224 and glyphosate on the pool levels of free amino acids, soluble protein, and protein synthesis in inflated duckweed were compared. SC-0224 caused larger increases than glyphosate in the pool levels of amino acids; the increases caused by SC-0224 were similar, however, to those caused by trimethylsulfonium iodide (TMS-I). Expressed on a per gram fresh weight basis, none of the treatments changed soluble protein or the incorporation of 14C-leucine into soluble protein. On a per flask basis (allowing for decreased growth in treated flasks), both herbicides and TMS-I decreased soluble protein and 14C-leucine incorporation. Decreases in soluble protein and 14C-leucine incorporation were equal for SC-0224 and TMS-I but larger than for glyphosate. These data indicate that differences in the phytotoxicity of SC-0224 and glyphosate may be due to the action of the trimethylsulfonium portion of the SC-0224 structure.

Low rates of diphenylether herbicides were previously shown to increase efficacy of glyphosate. Experiments were conducted to determine effect of lactofen on uptake of glyphosate and movement into tissues distal from the treatment area as measured by shikimate accumulation. As expected, glyphosate caused shikimate to accumulate, and sublethal rates of lactofen increased this effect. Cuticle abrasion had no effect on activity of glyphosate, alone or with lactofen, suggesting that the influence of lactofen was not in aiding penetration into the leaf.

Chlorimuron antagonized the activity of quizalofop on broadleaf signalgrass when applied in greenhouse studies as a postemergence tank mix. In vitro leaf disc assays utilizing 14C-acetate or 14C-pyruvate as substrates were conducted to ascertain the effect of clorimuron and quizalofop on fatty acid biosynthesis and to determine if antagonism between the two herbicides occurs at the biochemical sites of action. Incorporation of 14C-acetate in control treatments was linear with time to 120 min. Acetate incorporation in the presence of quizalofop (1.1 μM) was also linear but was inhibited 30 min after initialization of the reaction. The concentration of quizalofop that inhibited 14C-acetate incorporation 50% (I50) was 0.54 μM. Chlorimuron, up to 155 μM, had no effect on 14C-acetate incorporation. A mixture of quizalofop (1.1 μM) and chlorimuron (4.8 μM) inhibited 14C-acetate incorporation similar to that of quizalofop alone at 1.1 μM. Quizalofop I50 for incorporation of 14C-pyruvate was 1.1 μM, and clorimuron at 4.8 μM decreased incorporation 15%. Excess unlabeled pyruvate (5 μM) had no effect on either 14C-acetate or 14C-pyruvate incorporation in the presence of both herbicides. It is believed that antagonism of quizalofop by clorimuron is not due to an excess pool of pyruvate resulting from inhibition of acetolactate synthase by chlorimuron.

The effect of ammonium sulfate (AMS) on efficacy and on absorption and translocation of sethoxydim in wild oats and barley was investigated under controlled environmental conditions. Increases in efficacy in the presence of AMS were relatively small. The rate of sethoxydim required to reduce dry weight 50% was reduced from 0.08 kg a.i. ha−1, for both species, to 0.07 and 0.06 kg ha−1 for wild oats and barley, respectively, in the presence of added AMS. In short-term experiments, uptake of 14C from 14C-labeled sethoxydim 0.5 and 1 h after application was increased approximately twofold with the addition of AMS. AMS had little effect on sethoxydim absorption beyond 1 h after application. Translocation at 1 h after application was unaffected by AMS. However, translocation 12 and 24 h after application was increased with AMS. It is postulated that the translocation increase results indirectly from the increase in absorption. The observed increase in absorption with the addition of AMS is evident only when crop oil concentrate (COC) is also present In model systems, the addition of AMS enhanced sorption of sethoxydim into Isolated cuticles but had no effect on movement into excised roots.

Intact wheat plants and wheat calli responded similarly to varying concentrations of imazapyr. Fifty percent growth inhibition of wheat callus occurred with 0.05 μM imazapyr after 70 d. As imazapyr concentration increased from 0 to 10 μM, the free isoleucine, leucine, and valine decreased from 160 to 35, 260 to 49, and 310 to 59 pmol mg−1, respectively. Resistant calli, which had relative growth rates exceeding a calculated upper prediction interval, were obtained by in vitro selection at 2 and 5 μM imazapyr. Resistant calli growing on 2 μM imazapyr had free isoleucine, leucine, and valine concentrations intermediate to the control and susceptible callus.

A triazine-resistant (TR) biotype of Powell amaranth was discovered in 1989 in a potato field treated with metribuzin. A survey of all agricultural counties in Idaho showed that the TR Powell amaranth infestation was localized in the southeastern corner of Gooding county in southern Idaho. To determine the mechanism of triazine resistance, I50 values for inhibition of photosystem II were determined for atrazine, metribuzin, and diuron using thylakoids isolated from TR and triazine-susceptible (TS) biotypes. TR/TS ratios based on I50 values were 134 for atrazine, 62 for metribuzin, and 1.9 for diuron. Results of binding studies with atrazine and metribuzin were consistent with the I50 studies, indicating that resistance was due to reduced binding of triazines to the thylakoid membrane D1 protein. Sequencing the chloroplast psbA gene from TR and TS biotypes revealed a serine 264 to glycine change in the TR biotype. The mutation presumably resulted in reduced hydrogen bonding between triazine herbicides and the Dl protein.

A laboratory bioassay was conducted to determine the differential tolerance of cole crops to clomazone as measured by extractable total chlorophyll and carotenoids. Clomazone concentrations causing 50% inhibition (I50) in the biosynthesis of total chlorophyll in broccoli, cauliflower, and green and red cabbage cotyledons were 16, 11, 3, and 11 μM, respectively, while I50 values for carotenoid levels were 20, 10, 4, and 8 μM clomazone, respectively. Therefore, broccoli was the most tolerant to clomazone based upon extractable chlorophyll and carotenoid concentrations. Further laboratory studies were performed to investigate the basis for differential clomazone tolerance in 3-wk-old cole crop seedlings. No differences in total root uptake of 14C-clomazone were observed between these crops after 24 h. There were no differences in rate of metabolism of 14C-clomazone to methanol-soluble metabolites in roots of these crops. Percentage of polar metabolites in roots remained fairly constant over time. There were also no differences between crops in percentage of methanol-soluble 14C-clomazone metabolites formed in shoots between 24 and 96 h. In all crops, levels of 14C-clomazone decreased in a similar manner over time in methanolic extracts of roots and shoots while nonextractable 14C levels increased, indicating a conversion of clomazone to insoluble, nonextractable forms. Differential uptake, translocation, and metabolism do not appear to account for clomazone selectivity differences between cole crop seedlings.

Ammonium fertilizers, petroleum oil concentrate, and nonionic surfactant were evaluated as postemergence spray additives to improve giant foxtail and volunteer corn control by 28 g ai ha−1 of the ethyl ester of quizalofop or 56 g ha−1 sethoxydim. Additions of 0.25% by vol nonionic surfactant or 2.5% petroleum oil concentrate improved grass control, but additions of 10% urea ammonium nitrate (28-0-0), 10% ammonium polyphosphate (10-34-0), or 0.1M ammonium sulfate (21-0-0-24S) did not consistently affect grass control. In laboratory studies with corn, greatest 14C absorption from leaf-applied 14C-quizalofop (8 h after treatment) was found with additions of petroleum oil concentrate (80% absorbed) or nonionic surfactant (18% absorbed), while less absorption was observed with treatments containing either no additive, urea ammonium nitrate, ammonium polyphosphate, or ammonium sulfate (8 to 13% absorbed). Surface tension and droplet size of spray solutions were affected primarily by additions of nonionic surfactant, petroleum oil concentrate, and the formulated herbicides. Solution density, solute potential, pH, and buffering capacity were primarily affected by fertilizer additions.

A triazine-resistant Powell amaranth biotype collected in Idaho was approximately six times more resistant to terbacil and sixteen times more resistant to bromacil than a normal susceptible biotype when planted into terbacil- or bromacil-treated soil. The concentration of terbacil required to reduce photosystem II activity by 50% (I50) in isolated thylakoids was 0.24 and 13.33 μM for the susceptible and resistant biotypes, respectively. Likewise, the I50 values for bromacil were 0.33 and 18.4 μM for the susceptible and resistant biotypes, respectively. More 14C-terbacil was bound to isolated thylakoids of the susceptible than the resistant biotype with binding constants (Kb) of 0.26 and 12.9 μM, respectively, indicating that resistance was at the chloroplast level.

Uptake, translocation, and metabolism of 14C-terbacil was investigated in 12-leaf (tolerant) and 3-leaf (susceptible) field violet plants. Field violets with 12 leaves absorbed less 14C-terbacil g−1 of fresh weight from solution culture than did plants with three leaves. Plants with three leaves translocated twice as much radioactivity to foliage than did plants with 12 leaves. Most 14C in roots (77%) and foliage (57%) of field violet plants with 12 leaves was in polar metabolites. Metabolism studies indicated that most 14C (79%) in foliage extracts from field violet plants with three leaves was 14C-terbacil. Polar metabolites were not detected in roots of field violet plants with three leaves.

Greenhouse and laboratory studies were conducted to determine the basis of differential response of two corn hybrids to the chloroacetanilide herbicide metolachlor. In greenhouse experiments, metolachlor at 6.7 kg ha−1 reduced the height of the susceptible ‘Northrup-King 9283’ corn by 53% relative to untreated controls and caused extensive visible injury 14 d after treatment Under the same conditions, the height of metolachlor-treated ‘Cargill 7567’ corn seedlings was reduced by only 18% without any visible herbicide injury. The 14C-metolachlor was more rapidly absorbed by the emerging shoot of the metolachlor-susceptible hybrid, Northrup-King 9283. Thus, differential metolachlor tolerance may be due in part to processes at the level of herbicide uptake. Metabolism experiments revealed that both hybrids were able to conjugate 14C-metolachlor with glutathione at similar rates. However, glutathione S-transferase activity increased earlier during seedling development and reached higher activities in the metolachlor-tolerant hybrid, Cargill 7567.

Root absorption of subtoxic levels of metribuzin and its ethylthio analog (ethyl-metribuzin) by downy brome, jointed goatgrass, and winter wheat increased by a factor of three to five times as temperature increased from 10 to 20 C. Absorption of ethyl-metribuzin per gram dry weight was similar for all three species. Absorption and distribution of ethyl-metribuzin, but not metribuzin, were similar per gram dry weight in downy brome and jointed goatgrass. Absorption of metribuzin per gram dry weight was lower for winter wheat than for the other two species at 20 C. In general, the ratio of absorbed ethyl-metribuzin detected in shoots to that in roots was less in winter wheat and jointed goatgrass than in downy brome. The absorption by roots of 14C-herbicides relative to water was similar for winter wheat and jointed goatgrass. Absorption of both 14C-herbicides by winter wheat and jointed goatgrass was nonpreferential with respect to water absorption at 10 and 15 C. However, at 20 C 14C-herbicide absorption was reduced 5 to 30% with respect to water absorption. Downy brome absorption of 14C-herbicides with respect to water was 30 to 50% less than that of the other two species.

A diclofop-methyl-resistant biotype of Italian ryegrass was characterized to determine the expression and inheritance of herbicide resistance and whether this trait was due to the presence of a diclofop-insensitive form of acetyl-coenzyme A carboxylase (ACCase). At the whole plant level, the resistant biotype was > 93-fold more resistant to diclofop-methyl than the susceptible biotype. Crosses of diclofop-resistant and –susceptible plants were performed to produce F1 plants. No maternal effects were evident in responses of reciprocal F1 plants to diclofop. GR50 diclofop rates determined for resistant, F1, and susceptible plants were 15, 6.3, and 0.16 kg ha−1, respectively. F2 populations treated with a 7.5 kg ha−1 rate of diclofop exhibited three injury response phenotypes 3 wk after treatment: a susceptible (S) phenotype which was killed, an intermediate resistance (I) phenotype with severe injury, and a resistant (R) phenotype with little or no injury. Testcross progeny exhibited only I and S phenotypes. Observed segregation of phenotypes in F2 and testcross populations conformed to segregation ratios predicted for a trait with inheritance controlled by a single partially dominant nuclear gene. ACCase activity determined in crude cell-free extracts of resistant, F1, and susceptible biotypes exhibited I50 values of 50, 20, and 0.7 μM diclofop, respectively. A positive relationship between the injury response phenotype and site of action (ACCase) response to diclofop was evident in both F1 and F2 populations. In extracts from R, I, and S phenotype F2 plants, 20 μM diclofop acid inhibited ACCase-mediated incorporation of 14C by 27.1, 45.1, and 78.9%, respectively. The ACCase data are consistent with the hypothesis that diclofop resistance in Italian ryegrass is conferred by a diclofop-insensitive form of ACCase.

The basis for increased phytotoxicity of foliar-applied atrazine at high temperature in common bean and redroot pigweed was investigated. Plants were grown under low (15/10 C), medium (25/20 C), or high (35/30 C) temperature regimes. Atrazine absorption by plants grown under different temperatures increased with increasing temperatures in both species. Greater than 90% of absorbed atrazine remained in treated leaves and translocation was not altered by temperature in both species. Metabolism of atrazine by both hydroxylation and glutathione-conjugation was greater in plants grown at 35/30 than 15/10 C in both species. Foliar-applied atrazine reduced extractable photosystem II (PS II) activity as temperature increased in both species. Studies were also conducted on thylakoid membranes from plants not treated with atrazine. The I50 for atrazine inhibition of PS II decreased and affinity of atrazine binding to thylakoid membranes increased as temperature increased in both species. We concluded that the increased phytotoxicity of atrazine at high temperatures is caused by enhanced foliar absorption and greater affinity of atrazine for the binding site.

Although drought-stressing green foxtail before and up to 1 d after herbicide application did not reduce control with fluazifop-P in growth chamber trials, extending the drought stress for 2 to 4 d after herbicide application reduced control of green foxtail with fluazifop-P by 40 and 57%, respectively. Absorption and translocation of 14C-fluazifop-P 1, 4, and 10 d after herbicide application were similar between nonstressed and drought-stressed plants. Drought-stressed green foxtail converted slightly more of the parent acid of fluazifop-P to polar conjugates 4 d after herbicide application than did nonstressed plants, but these differences in metabolism were probably not adequate to explain large differences in efficacy.

Photoacoustic spectroscopy was used to study effects of atrazine and diuron on excised leaves of triazine-susceptible (S) and -resistant (R) biotypes of black nightshade. Changes of oxygen and photothermal components were compared to photochemical fluorescence quenching obtained by fluorimetry. After 1 h incubation in an aqueous solution of atrazine (0 to 200 μM), oxygen component of the photoacoustic signal was strongly decreased in the S biotype while the R biotype was not affected. Also, reoxidation of the primary quinone acceptor (QA−) of photosystem (PS) II of the S biotype was lower than that of the R biotype. With diuron treatments, changes in the characteristics of these biophysical signals were the same in both R and S biotypes. Both oxygen component and photochemical fluorescence quenching were decreased in treated leaves of the R and S biotypes. By using modulated oxygen and heat emissions, and the ratio of the initial inflection point (I) to the fluorescence maximum (P) as herbicide bioassay indicators, we showed that the photoacoustic spectroscopy was also a reliable technique for whole plant studies. Inhibition of photosynthesis was maximal 2 d after onset of treatment with atrazine (200 μM). Inhibitors of PSII did not induce a significant increase of heat emission in leaves which otherwise showed phytotoxic symptoms after treatment By using the photoacoustic technique, it was possible to obtain useful information on photosynthetic activity under herbicide stress, suggesting that pulsed oxygen emitted by leaves could be used to quantify susceptibility or to detect resistance to many types of photosynthetic inhibitors in weeds and crop plants.

Tests with 18 growth regulators and chemicals revealed that the cytokinins [zeatin, kinetin, and 6-benzylaminopurine (BAP)], ethylene, and sodium hypochlorite stimulate germination of cowpea witchweed seeds at various concentrations. Witchweed seeds preconditioned in different concentrations of sodium hypochlorite for 3 wk had germination rates of 25 to 89% after incubation in distilled water, 10−5M solution of kinetin, 50 mg L−1 of 2-chloroethylphosphonic acid, and exudates from cowpea roots. Witchweed seeds preconditioned in distilled water and subsequently incubated in distilled water did not germinate.

Experiments were conducted to confirm the bentazon susceptibility of corn inbred GA209 and determine the physiological basis of this susceptibility. Bentazon (1.1 to 4.4 kg ha-1) plus crop oil concentrate (1% by vol) did not cause visible injury or dry weight loss of corn inbred B73 but caused 66 to 89% visual injury and 62 to 70% dry weight reduction of GA209 1 wk after treatment. Bentazon (2.2 kg ha-1) inhibited variable chlorophyll fluorescence decay in GA209 and B73 8 h after treatment. Variable fluorescence decay recovered in B73, but not in GA209, 96 h after treatment. Absorption and translocation of 14C from 14C-bentazon was greater in GA209 than B73 during a 48-h time course. Both inbreds converted bentazon to a polar metabolite which formed 6-hydroxybentazon upon hydrolysis with β-glucosidase. However, 63% of absorbed 14C from 14C-bentazon remained in the parent form in GA209 compared to 25% in B73 over a 72-h time course. A decreased ability of GA209 to metabolize bentazon may explain bentazon sensitivity of this inbred compared to B73.

Studies were conducted to compare changes in germinability and/or viability of yellow starthistle achenes buried (in packets) in soil or stored dry in the laboratory, and to determine the rate of achene depletion from a soil seed bank. In one study, after 72 mo, 0 to 96% germinable achenes remained in packets buried 5 cm deep, and a mean above 99% after dry storage. In another study, total live (germinable plus viable) achenes declined from 100% at harvest to 77.6% after 24 mo of burial. Germinable pappus-bearing achenes remaining in packets increased with depth of burial. After 12 mo of burial, maximum survival of achenes occurred at depths of 5 cm or more. The density of yellow starthistle achenes and seedlings in a natural soil seed bank declined, when achene rain was prevented, to 3.9 and 1.1% of initial density, respectively, after 36 mo. Decline was attributed primarily to seedling emergence and achene death. Achene density in the upper 2.5 cm of soil appeared to be a good predictor of seedling emergence after autumn rains.