A biotype of johnsongrass cross resistant to clethodim, sethoxydim, quizalofop-P, and fluazifop-P was identified in several fields in Washington County, MS. Absorption, translocation, and metabolism studies using 14C-clethodim and acetyl-coenzyme A carboxylase (ACCase) activity assays were conducted to determine the resistance mechanism. Absorption of 14C-clethodim was higher in the resistant than the susceptible biotype 4 hours after treatment (HAT), but at 24, 48, and 72 HAT, similar levels of radioactivity were detected in both johnsongrass biotypes. Consequently, resistant plants had more radioactivity present in the treated leaves at 4 and 24 HAT. However, there was no difference between resistant and susceptible biotypes in the translocation of 14C out of the treated leaf at 4, 8, 24, 48, and 72 HAT as a percentage of total absorbed. Metabolism of clethodim was similar in the resistant and susceptible biotypes. There was no difference in the specific activity of ACCase from the susceptible and resistant johnsongrass biotypes (means of 0.221 and 0.223 nmol mg−1 protein min−1, respectively). ACCase from the susceptible biotype was sensitive to clethodim, with an I 50 value of 0.29 μM clethodim. The ACCase enzyme from the resistant biotype was less sensitive, with an I 50 value of 1.32 μM clethodim. The resultant R/S ratio for clethodim was 4.5. These results indicate that resistance to clethodim in this johnsongrass biotype resulted from an altered ACCase enzyme that confers resistance to clethodim.

The rust pathogen, Puccinia thlaspeos, is used for biocontrol of dyer's woad. Rust infection results in chlorotic leaves and abnormal flower and seed production of this noxious weed during its second year of growth. Auxin-like and cytokinin-like activity of healthy and infected plants (roots, shoots, and leaves) were determined during different growth stages. These samples were analyzed by use of the mung-bean rooting bioassay (auxin-like activity) and the soybean-callus bioassay (cytokinin-like activity). Rooting activity in the mung-bean bioassay was significantly higher in the infected-root extracts from Harvest 1 (rosette stage) compared with the healthy-root extracts and significantly higher in the healthy-shoot extracts from Harvest 2 (bolting plants) compared with infected-shoot extracts. Infected plants collected during Harvest 1 had greatly reduced cytokinin-like activity in the roots, shoots, and leaves compared with the healthy plants. Cytokinin-like activity increased slightly in the infected plants collected in subsequent harvests, with the highest activity recorded in root extracts. The results suggest potential ways that the rust-causing fungal pathogen is able to influence some physiologic processes in dyer's woad and so affect its growth.

Controlled-environment experiments were conducted to determine giant foxtail epicuticular wax (ECW) deposition and fluazifop-P absorption under different environmental conditions and with two adjuvants. Drought stress and low temperature increased leaf ECW content, whereas low light intensity decreased ECW content compared with medium light intensity. Drought stress conditions decreased the fatty acid and primary alcohol content of ECW and increased the hydrocarbon content compared with field capacity. Compositional changes would make the ECW more hydrophobic and reduce leaf wetting by herbicide spray. Increasing air temperature decreased the aldehyde content of ECW, whereas decreasing light intensity increased ECW fatty acid and aldehyde content while decreasing primary alcohols and esters. Compositional changes under low light intensity would make the ECW more hydrophilic and increase leaf wetting by a herbicide spray. Drought stress reduced fluazifop-P absorption regardless of the temperature but could not further reduce fluazifop-P absorption under low light intensity. Fluazifop-P absorption by plants under low light and drought stress conditions was similar to plants under low or medium light intensity and field capacity conditions. Similarly, the rate of fluazifop-P absorption was less under drought stress and low light conditions. Fluazifop-P absorption was greater when crop oil concentrate was added compared with 28% urea ammonium nitrate or no additive. Crop oil concentrate, added to the herbicide solution, overcame reduced fluazifop-P absorption under the low light conditions and in one of the two drought stress regimes but could not overcome reduced fluazifop-P absorption with the high temperature regime.

Spray carrier pH affects the solubility of sulfonylurea herbicides and, therefore, could affect absorption and subsequent translocation of these compounds in weeds. Trifloxysulfuron is a sulfonylurea herbicide developed for POST weed control in cotton, sugarcane, and turfgrass with a pKa of 4.81. The objective of this study was to evaluate the absorption and translocation of foliar-applied 14C-trifloxysulfuron in Palmer amaranth and Texasweed at pH 5, 7, and 9 over a period of 4 to 72 h after treatment (HAT). For absorption, effects of time, species, and pH were significant. Absorption averaged over species and pH increased logarithmically from 4 to 72 HAT. Absorption was greater for Palmer amaranth (88%) than for Texasweed (29%) when averaged over time and pH. Absorption averaged over species and time increased in the order of pH 5 (52%) < pH 9 (60%) = pH 7 (61%). Consequently, this translated into greater translocation of 14C-trifloxysulfuron in Texasweed when sprayed with the higher pH spray solutions. These data indicate that absorption and translocation of trifloxysulfuron in some weed species may be enhanced by increasing the pH of the spray solution by 2 pH units above the pKa.

Tolerance to glufosinate has been bioengineered into cotton through the expression of a gene encoding the enzyme phosphinothricin acetyl transferase (PAT). Studies were conducted to determine thermal limitations on herbicide efficacy in bioengineered cotton. The 50% inhibition (I50) of glufosinate of the target-site enzyme glutamine synthetase was thermally dependent with the lowest values between 25 and 35 C. Larger values of I50 were measured above and below the 25 to 35 C range. The apparent Michaelis constant KM of the enzyme PAT was relatively stable from 15 to 30 C and increased more rapidly from 30 to 45 C. The two components in combination suggest the aggregate tolerance to glufosinate would not be thermally limited between 15 and 45 C. The thermal dependence of the aggregate tolerance in cotton suggests that glufosinate would not damage the crop over a range of temperatures. This prediction is in agreement with the results of field studies carried out over a number of years, which showed the glufosinate-tolerant cotton to be undamaged by glufosinate over a wide range of temperatures.

Inhibitors of acetolactate synthase (ALS) are important herbicides for control of wild mustard, a common weed of the north central United States and Canada. Wild mustard that survived treatments with the ALS inhibitors cloransulam, imazethapyr, and thifensulfuron was sampled from a North Dakota soybean field in 1999. The mechanism of resistance and response of this wild mustard biotype to ALS-inhibiting herbicides was investigated. In vitro enzyme-inhibition experiments confirmed a resistance mechanism associated with the ALS enzyme; imazethapyr or imazamox at 1 × 10−4 M caused only 10 to 11% and 12 to 16% reductions in ALS activity, respectively. ALS from a susceptible wild mustard biotype was inhibited 50% (I50) with imazethapyr at 8 × 10−7 M or imazamox at 1.1 × 10−6 M. Whole-plant greenhouse treatments confirmed cross-resistance across ALS-inhibitor classes. Treatment with twice-normal field rates of thifensulfuron, ethametsulfuron, triflusulfuron, imazamox, imazethapyr, flumetsulam, cloransulam, flucarbazone, and imazamethabenz reduced biomass of the susceptible biotype at least 96% 28 d after treatment. Biomass of the resistant biotype was reduced 49% by triflusulfuron and 35% by thifensulfuron, but was not reduced by other herbicides. DNA sequence analysis of ALS genes from resistant and susceptible biotypes revealed a point mutation inferring a Trp-to-Leu amino acid substitution in ALS of the resistant biotype. This mutation, corresponding to position 574 of the Arabidopsis ALS amino acid sequence, is known to confer cross-resistance to ALS-inhibiting herbicides and is the probable cause of resistance in the wild mustard biotype. Phylogenetic analysis of wild mustard and canola ALS sequences confirmed that the Trp574 mutation arose within wild mustard and was not derived via introgression from imidazolinone-resistant canola. The results of this research indicate a naturally occurring target-site point mutation responsible for conferring cross-resistance to ALS-inhibiting herbicides in this wild mustard biotype.

African rue is an exotic, herbaceous perennial established in several western states that tolerates harsh, water-stressed conditions. The influence of water-deficit stress on herbicide response and subsequent herbicide fate within the plant were compared. African rue seedlings were deprived of water for 0 to 7 d to establish a gradient of water-deficit levels before treatment with hexazinone, imazapyr, or metsulfuron. At herbicide application, water-deficit treatments reduced plant water potential values from −1.0 MPa to −4.7 MPa, causing concomitant reductions in photosynthesis. Thirty-five days after treatment, dry weight of imazapyr- and metsulfuron-treated plants was reduced in plants exposed to more than 4 d water-deficit stress before herbicide application. In contrast, hexazinone-treated plants had less dry weight than water-stressed, nonsprayed control plants regardless of water-deficit stress. Seventy-two hours after herbicide application, African rue leaves absorbed from 5 to 42% of herbicide applied; however, herbicide absorption did not correlate to efficacy. Less than 12% of absorbed herbicide translocated out of the treated leaf, regardless of herbicide. Radiolabel translocated from the treated leaf to acropetal or root tissue did not differ among herbicide treatments, regardless of water deficit before herbicide application. However, compared to other herbicides, translocation to basipetal shoot tissue was greatest in imazapyr-treated seedlings with the largest water deficit at herbicide application. Increased translocation occurred at higher levels of water stress than were necessary to increase herbicide efficacy, suggesting differential translocation was not involved in enhanced efficacy. In summary, African rue seedlings absorbed and mobilized three different herbicides at all levels of water-deficit stress. In addition, the efficacy of metsulfuron and imazapyr increased as water-deficit stress increased, but efficacy of hexazinone was not influenced by plant water status. This unusual relationship between water-deficit stress and herbicide performance may enable improved African rue management under stressful environments.

Microarray analysis was used to follow changes in gene expression coinciding with seasonal changes in the dormancy status of crown buds of field-grown leafy spurge. Known cold-regulated genes were induced, and numerous gibberellic acid–responsive genes were down-regulated during the transition from paradormancy to endodormancy. Genes involved in photomorphogenesis were induced during endodormancy. Also, ethylene signaling responses were observed during ecodormancy rather than endodormancy. These results provide additional insights into the signals regulating expression of several genes previously associated with transition from paradormancy to growth in root buds.

Differential sensitivity to paraquat was observed between cucumber cultivars and leaf age. Physiological responses to paraquat, including antioxidative enzyme activity, were investigated in leaf age classes of cucumber to identify mechanisms of paraquat tolerance. Leaf injury for ‘Naeseosamcheok’, ‘Daehandadagi’, ‘Baekgwangdadagi’, ‘Sangrokheukjinju’, and ‘Eunseongbaekdadagi’ cultivars was less than that of six other cultivars tested, averaged over leaf age and herbicide rate. The level of foliar injury caused by paraquat was Leaf 1 > 2 > 3 > 4 in seven of 11 cultivars used, where 4 was the youngest leaf. There was a positive correlation between leaf age and its relative susceptibility to paraquat, regardless of growth stage. Lipid peroxidation was less in the youngest leaf (Leaf 4) than in the older leaves at all herbicide concentrations. The youngest leaf had higher values for apparent photosynthesis than the oldest leaf. Differential leaf response to paraquat was partially correlated with the change in superoxide dismutase, ascorbate peroxidant, and glutathione reductase activities in treated leaves. Enzyme activity of ascorbate peroxidase (APX) was higher in Leaf 4 than in Leaves 1, 2, or 3 in untreated plants and after exposure to paraquat. APX isozymes were more abundant in treated than in untreated leaves and produced in higher amounts in younger than in older leaves. Application of ascorbate and glutathione before paraquat treatment protected cucumber leaves from paraquat injury.

Landoltia was collected and cultured from a canal in Lake County, Florida, where diquat was used repeatedly during the past 20–30 yr for duckweed control. Recent applications of diquat failed to provide adequate control of duckweed, and a new commercial formulation of diquat was suspected. The new formulation was not the cause of reduced efficacy. Static exposures (48 h) to various concentrations of diquat were used to compare the susceptibility of the Lake County landoltia accession to one never exposed to diquat. These static tests indicated that landoltia, from a population with no prior history of herbicide treatment, was extremely susceptible to diquat. The accession from Lake County, FL had developed resistance to diquat, and was also cross resistant to paraquat. The resistance factor was 50 for diquat and 29 for paraquat. The Lake County accession also exhibited reduced ion leakage after diquat exposure under light and dark conditions. This suggests the resistance mechanism to the bipyridylium herbicides in landoltia is independent of photosynthetic electron transport. This research documents the first aquatic plant species that has developed resistance to the bipyridylium herbicides.

This article discusses the concept of relative potency of herbicides in bioassays where individual dose–response curves can be similar or nonsimilar, often denoted parallel and nonparallel curves, and have different upper and lower limits. The relative potency is constant for similar dose–response curves and measures the relative horizontal displacement of curves of a similar shape along the dose axis on a logarithmic scale. The concept of similar dose–response curves has been used extensively to assess results from herbicide experiments, for example, with the purpose of adjusting herbicide doses to environmental conditions, formulations, and adjuvants. However, deeming dose–response curves similar when they are not may greatly affect the calculation of the relative potency at response levels such as effective dosage (ED)90, which is relevant for effective weed control, or ED 10, which is used in crop tolerance studies. We present a method for calculating relative potencies between nonsimilar dose–response curves at any response level. It also is demonstrated that if the upper, lower, or both limits among response curves are substantially different, then the ED 50 or any other ED level cannot be used indiscriminately to compute the relative potency. Rather, the relative potency should be viewed as a function of the response level.

The application of the acetolactate synthase (ALS)–inhibiting herbicide metsulfuron on greenhouse- and field-grown black nightshade and greenhouse-grown ladysthumb resulted in progressive inhibition of the level of carbon dioxide (CO2) fixation, the relative quantum efficiency of electron transport through photosystem I (ΦPSI) and II (ΦPSII), and the leaf chlorophyll content. Photosynthetic-related measurements, measured 2 to 4 d after treatment (DAT) at photon flux densities of 400 to 500 μmol m−2 s−1, provided valuable information before the visual symptoms that first appeared at 7 to 10 DAT with the herbicide. Measurements of the quantum efficiency for electron transport by photosystem II and the loss in leaf chlorophyll content appeared to be two of the most practical parameters to use when designing an early detection method to assess the toxicity of metsulfuron. The use of chlorophyll fluorescence would require a comparison of steady-state ΦPSII measurements for control and treated plants, which could be realized by either measuring in time (before/after application) or space (treated/untreated patch).

Experiments were conducted to identify acetolactate synthase (ALS, EC 2.2.1.6 [formerly EC 4.1.3.18]) mutation sites in eight biotypes of smooth pigweed and correlate these mutations with patterns of herbicide cross-resistance. Four herbicide-resistant smooth pigweed biotypes (R5, R6, R7, R8) collected from fields in Virginia, Delaware, and Maryland, showed a similar response to postemergence applications of the ALS-inhibitors imazethapyr, pyrithiobac, chlorimuron, thifensulfuron, and cloransulam. These R biotypes ranged from 261- to 537-fold resistant to imazethapyr and 29- to 88-fold resistant to pyrithiobac. The biotypes also had reduced sensitivity to chlorimuron and thifensulfuron of 2- to 14-fold and 10- to 25-fold, respectively, relative to a susceptible smooth pigweed biotype (S). Biotypes R6, R7, and R8 had reduced sensitivity of 3- to 10-fold to cloransulam relative to the S biotype, whereas R5 had increased sensitivity. All of these biotypes were found to have a serine to asparagine substitution at amino acid position 653, as numbered relative to the protein sequence of Arabidopsis thaliana. This stands in contrast to four other imidazolinone (IMI)-resistant smooth pigweed biotypes (R1, R2, R3, R4) that were collected from fields in Somerset County, Maryland. These biotypes were found to have an alanine to threonine substitution at position 122 of the ALS enzyme and were previously characterized at the whole-plant level with high-level resistance to IMI herbicides, increased sensitivity to pyrimidinylthiobenzoate and triazolopyrimidine sulfonanilide herbicides, and low to no cross-resistance to sulfonylurea herbicides.

Microarray analysis was used to identify changes in gene expression in corn leaves collected from plants at the V11–14 growth stage that resulted from competition with velvetleaf. The plants were grown in field plots under adequate N (addition of 220 kg N ha−1) and irrigation to minimize N and water stress. Consequently, only differences resulting from competition for micronutrients, light, and perhaps allelopathic stress were anticipated. Genes involved in carbon and nitrogen utilization, photosynthesis, growth and development, oxidative stress, signal transduction, responses to auxin and ethylene, and zinc transport were repressed in corn growing in competition with velvetleaf. Very few genes were induced because of competition with velvetleaf, and those that were provided little indication of the physiological response of corn. No differences were observed in genes responsive to water stress or sequestering/transporting micronutrients other than zinc, indicating that these stresses were not a major component of velvetleaf competition with corn at the developmental stage tested.

A series of dose–response experiments were performed at low humidity to determine if glufosinate efficacy could be increased by lengthening the drying time through the addition of humectants. Of several humectants evaluated, only 5% glycerol or 5% triethylene glycol when applied with glufosinate produced dry weight reductions and mortality similar to exposure to high humidity. 14C-glufosinate movement through isolated wild oat cuticles was greater at high humidity, poorest at low humidity, but intermediate at low humidity in the presence of 5% glycerol in the spray solution. The increases in uptake observed at high humidity and with 5% glycerol at low humidity were characterized by greater initial uptake that continued much longer than that observed at low humidity without humectant.

Laboratory studies were conducted to examine the leaf surface, epicuticular wax content, and spray droplet behavior on common lambsquarters, common purslane, and velvetleaf. Adaxial and abaxial leaf surfaces were examined using scanning electron microscopy, and leaf wax was extracted and quantified for all three weed species. The spread of 1-μl droplets of distilled water, primisulfuron solution (without surfactant), primisulfuron solution with a nonionic low foam wetter/spreader adjuvant (0.25% v/v), and with an organosilicone wetting agent (0.1% v/v) was determined on the adaxial leaf surfaces of each of the weed species. Glands and trichomes were present on both the adaxial and abaxial leaf surfaces of velvetleaf. Common purslane had neither glands nor trichomes on either side of the leaf. Common lambsquarters did not have any glands or trichomes, but it had globular bladder hairs on both adaxial and abaxial leaf surfaces. Stomata were present on both adaxial and abaxial leaf surfaces in all three weed species. Common purslane had a much lower number of stomata per unit area of leaf as compared with velvetleaf or common lambsquarters. Common lambsquarters had the highest epicuticular wax content on the leaf surface (274.5 μg cm−2), followed by common purslane (153.4 μg cm−2) and velvetleaf (7.4 μg cm−2). There were no significant variations in the spread of the 1-μl droplet of distilled water and primisulfuron (without adjuvant) among the species. Spread of primisulfuron droplets with surfactant was highest on the leaf surface of velvetleaf that had the lowest wax content. Droplet spread was greatest with organosilicone surfactant followed by the nonionic surfactant.

Biological and biochemical methods, based on glufosinate inhibitory effects on plant growth and nitrogen metabolism, were examined for their applications to detect this herbicide. Dose–response analysis of radicle growth inhibition showed that, among six vegetables tested, Chinese mustard and edible amaranth were the most sensitive to glufosinate. Field mustard and cruciferous Ching-Geeng were found to be more sensitive to this herbicide than the other four vegetables when three-leaf seedlings were tested in another bioassay. In three-leaf seedlings of Ching-Geeng, accumulation of ammonium, a biochemical marker for glufosinate toxicity because of its inhibition of glutamine synthetase, showed a linear regression to the log-transformed concentrations of glufosinate ranging from 7.5 × 10−5 to 1.5 × 10−3 M. For the detection of glufosinate lower than 7.5 × 10−6 M, a linear regression was observed between ammonium accumulation and the applied concentration, instead of the log-transformed value, of glufosinate. A similar relationship was observed between the accumulation in Ching-Geeng seedlings of glyoxylate, another biochemical marker, and glufosinate but with a narrower range than that for ammonium accumulation. The applicability of ammonium accumulation in three-leaf seedlings of Ching-Geeng to detect glufosinate residue in water and soil was confirmed by high-performance liquid chromatography (HPLC).

A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied to 5- to 13-cm-tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha−1 controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse, I 50 values (rate necessary for 50% inhibition) for visual control and shoot fresh weight, expressed as percentage of the nontreated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype.

Herbicidal activities of clove oil and its primary constituent eugenol on broccoli, common lambsquarters, and redroot pigweed and the role of crystalline leaf epicuticular wax (LEW) in susceptibility and retention of these essential oils were studied. Clove oil (2.5%) and eugenol (1.5%) were applied to leaves of greenhouse-grown broccoli, common lambsquarters, and redroot pigweed seedlings and effects on seedling growth and leaf cell membrane integrity were studied. Compared with eugenol, clove oil caused greater inhibition of seedling growth in all species. Both eugenol and clove oil caused greater loss of membrane integrity and inhibition of seedling growth in redroot pigweed, which has no crystalline LEW, compared with common lambsquarters, which has a thick layer of crystalline LEW. In broccoli seedlings with LEW, clove oil caused greater inhibition of growth than eugenol. Both clove oil and eugenol caused greater electrolyte leakage from broccoli leaves without LEW than in the leaves with LEW. Removal of LEW increased electrolyte leakage, an indicator of cell membrane damage, by 97% in eugenol-treated and 26% in clove oil–treated broccoli leaves. Susceptibility of broccoli seedlings and possibly some weed species may, therefore, be affected by factors (e.g., genetic, environmental) that influence the amount of LEW. Although the presence of LEW greatly reduced the retention of the essential oil solutions, there was no significant difference between the retention of clove oil and eugenol solutions, indicating that differences in their phytotoxicity to broccoli leaves was not due to differential foliar retention.

Fluridone has been widely used for control of the submersed aquatic weed hydrilla in Florida for over 25 years. Recently, some hydrilla biotypes were suspected of having developed resistance to fluridone. Laboratory studies were conducted to monitor changes in phytoene and β-carotene contents as a function of suspected susceptible and resistant hydrilla biotypes to fluridone treatment. Hydrilla shoot tips from each biotype were exposed to 5, 10, 15, 20, 30, and 50 μg L−1 fluridone. Higher β-carotene and lower phytoene content was observed in all resistant hydrilla biotypes compared with the susceptible. The susceptible biotype showed an increase in phytoene or a decrease in β-carotene content when treated with as little as 5 μg L−1 of fluridone, whereas much higher doses were needed for the resistant biotypes. EC50 β-carotene values of 9 and 63 μg L−1 fluridone were found in the susceptible and the most resistant biotype, respectively. Consistent levels of hydrilla injury occurred at phytoene/β carotene index values of 5.5–7 and occurred at exposure to 5–10 μg L−1 fluridone in susceptible and 50 μg L−1 in the most resistant biotype. A resistance factor (R/S) was calculated for each hydrilla biotype which ranged from as low as 2X (R1 and R2) to 7X (R5). Aggressive spread of fluridone resistant dioecious hydrilla in aquatic ecosystems can severely impact hydrilla management, and consequently cause substantial and long-lasting ecological and economic problems throughout the southern United States.