Two red rice accessions from Arkansas have been found to be resistant to the labeled rate of imazethapyr, which is used to control red rice in ClearfieldTM rice. Full-length amplification of the acetolactate synthase (ALS) gene in imazethapyr-resistant red rice revealed a coding sequence of 1,935 base pairs, which is the same as that of the cultivated rice. Coding sequences were generated from four red rice accessions collected from different geographical regions in Arkansas, consisting of accessions that were either resistant or susceptible to imazethapyr. Nucleotide sequence alignments identified six base polymorphisms, three of which resulted in amino acid substitutions in the ALS gene. One amino acid substitution, Gly654Glu, involves a residue required for imazethapyr binding to the ALS. The other substitution, Val669Met, implies conformational changes in the ALS structure that enhances binding of thiamine diphosphate, an ALS cofactor. These novel amino acid substitutions first reported for ALS-resistant red rice accessions support the hypothesis that ALS-resistant red rice can evolve with sustained herbicide selection pressure. Thus, it behooves growers to integrate the Cleafield rice technology with other tools to achieve a successful, long-term weed management program.

The occurrence of glyphosate-resistant weeds has been reported after more than 20 yr of extensive use. Rigid ryegrass that evolved resistance to glyphosate was found in Australia and in California. Glyphosate-resistant rigid ryegrass plants were collected from northern California and selected through generations 8 and 5 to segregate the most resistant (R) and sensitive (S) biotypes. The eighth generation of R and the fifth generation of S biotypes survived 6.72 and died from 0.11 kg ae ha−1 glyphosate, respectively. The objectives of this study were to evaluate the role of metabolism in the observed resistance, to study the effect of glyphosate on the activity of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS; EC 2.5.1.19), and to characterize the EPSPS gene in R and S rigid ryegrass. Neither quantitative nor qualitative difference was observed in the metabolism of 14C-glyphosate between the biotypes. Activity of constitutive EPSPS decreased more significantly in the S than R biotype in the presence of 5, 50, 500, and 5,000 µM glyphosate. Inhibition of 50% (I50) of the EPSPS activity by glyphosate was more than 90-fold in S compared to R biotype. Decreased EPSPS sensitivity in the R biotype appeared to be a major contributor to glyphosate resistance in rigid ryegrass from California. Fragments of the EPSPS gene containing 1,320 nucleotides were isolated from mRNA of S and R biotypes. A single nucleotide mutation from cytosine (C) to thymine (T) was identified at nucleotide 301 of the truncated EPSPS gene of the R biotype. This mutation changed the amino acid code from proline (Pro) to serine (Ser), which was similar to that reported for the glyphosate-resistant goosegrass from Malaysia and correlated with glyphosate insensitivity of EPSPS.

Red rice is a major weed problem in rice production of the southern United States and other rice-producing countries. One hundred thirty red rice accessions from 26 rice-growing counties in Arkansas were tested for tolerance to imazethapyr in seed- and whole-plant response bioassays. The red rice accessions were compared with imazethapyr-resistant (ClearfieldTM) rice cultivars (‘CL121’, ‘CL161’, and ‘CL-XL8’) and conventional rice cultivars (‘Bengal’, ‘Dongjin’, ‘Drew’, and ‘Wells’). Red rice accessions 79, 84, and 118 showed 17-, 48-, and 37-fold more tolerance to imazethapyr, respectively, than the standard susceptible red rice accession (82) in whole-plant bioassays. The imazethapyr-resistant rice cultivars, CL121, CL161, and CL-XL8 were 41-, >177-, and 48-fold more resistant to imazethapyr, respectively than the susceptible standard. The imazethapyr-tolerant red rice and ClearfieldTM cultivars were generally cross tolerant to other acetolactate synthase (ALS; EC 4.1.3.18) inhibiting herbicides such as imazapyr, imazaquin, imazamox, and pyrithiobac. The tolerance level of red rice or rice to imidazolinone herbicides was highest with imazaquin and lowest with imazapyr. The imazethapyr-tolerant red rice accessions and ClearfieldTM rice were susceptible to glufosinate and glyphosate. The ALS enzyme of tolerant red rice accessions was less sensitive to imazethapyr than the susceptible standard, but tolerance at the enzyme level was less than at the whole-plant level. Therefore, tolerance of red rice to imazethapyr may involve other mechanisms besides an insensitive target site. We learned that a few imazethapyr-tolerant red rice populations existed probably before ClearfieldTM rice was introduced, supporting the hypothesis that evolution of herbicide-resistant red rice populations can happen with intensive herbicide selection pressure.

Few studies on herbicide resistance report data to establish unambiguously the correlation between genotype and phenotype. Here we report on the importance of the EPSPS prolyl106 point mutation to serine (P106S) in conferring resistance to glyphosate in a goosegrass population from Davao, Mindanao Island, the Philippines (Davao). Initial rate-response studies showed clear survivors within the Davao population at glyphosate rates that completely controlled the standard sensitive goosegrass population (STD1). Assessment of potential resistance mechanisms identified the presence of P106S mutant individuals in the Davao population. Polymerase chain reaction (PCR) amplification of specific alleles (PASA) analysis established that the mixed-resistant Davao population was comprised of 39.1% homozygous proline wild-type (PP106), 3.3% heterozygous serine mutant (PS106), and 57.6% homozygous serine mutant (SS106) genotypes. Further rate-response studies on plants with a predetermined genotype estimated the Davao SS106 individuals to be approximately 2-fold more resistant to glyphosate compared to Davao PP106 individuals. Extensive analysis at different goosegrass growth stages and glyphosate rates established strong correlation (P < 0.001) between presence of P106S in EPSPS and the resistant phenotype. Importantly, no differences in the pattern of absorbed or translocated 14C–glyphosate were observed between PP106 and SS106 Davao genotypes or Davao and STD1 individuals, suggesting that glyphosate resistance in the Davao population was attributable to an altered target site mechanism. This study demonstrates that whilst P106S in EPSPS confers a moderate resistance level to glyphosate, the mechanism is sufficient to endow glyphosate failure at the recommended field rates.

Reports arose from major Chinese wheat production regions that flixweed was not controlled by tribenuron after the herbicide was continuously used for several years. Flixweed seeds were collected from wheat fields that had been treated with tribenuron repeatedly over 3 to 15 yr or from road sides and remote hills that had never received tribenuron in Jiangsu, Hebei, Shanxi, Sichuan, Shandong, Shaanxi, and Henan provinces, and Tianjin and Beijing metropolises in China. The response of various biotypes to tribenuron was determined by whole plant experiments in the greenhouse. The experiments demonstrated that 11 of 32 flixweed biotypes were susceptible to tribenuron. The remaining 21 biotypes expressed moderate to high levels of tribenuron resistance with resistance indices ranging from 4 to > 1,500. DNA sequence analysis of acetolactate synthase (ALS) genes of selected biotypes 6, 7, 27, and 29 revealed a point mutation at position 197 of the ALS gene, as numbered relative to the protein sequence of Arabidopsis, where proline was substituted by leucine in biotype 7 and by threonine in biotype 29. These mutations are known to confer resistance to ALS-inhibiting herbicides and are responsible for the high resistance of these biotypes to tribenuron. The results also indicate that tribenuron resistance in flixweed is widespread in China and management programs to control these resistant populations are warranted.

Hydrilla is one of the most serious aquatic weed problems in the United States, and fluridone is the United States Environment Protection Agency (USEPA)-approved herbicide that provides relatively long-term systemic control. Mature (6-wk-old) and young (freshly planted from 10-cm apical shoot apices) hydrilla were grown in 540 L fiberglass vaults under short- (natural 8 to 10 hr light/14 to 16 hr dark photoperiod) or long- (artificially extended 16 h light/8 h dark photoperiod) day greenhouse conditions. Fluridone treatments of 0, 1, 5, and 10 µg L−1 were applied after 2 wk and maintained within each population and photoperiodic regime throughout the study. Short days promoted subterranean turion formation, but this effect was reduced by long days and 5 and 10 µg L−1 fluridone. Fluridone caused a reduction in chlorophyll and carotenoid levels but the effect on anthocyanin content was variable. Short days caused elevated anthocyanin, and this effect was diminished by fluridone. Fluridone reduced the abscisic acid content of mature apical stems and was higher under short days in younger plants. These studies provide further evidence that fluridone can be used as a fall herbicide application to reduce turion production.

SHOOTMERISTEMLESS (STM) encodes a member of the class I KNOX homeodomain protein family that is required for meristem development and maintenance. We have isolated both genomic and cDNA clones of STM from the perennial weed leafy spurge. A comparison to other class I KNOX genes indicates that EeSTM represents an orthologue of AtSTM and not one of the other class I KNOX gene family members. 5′ rapid amplification of cDNA ends (RACE) indicated that the transcription initiation site is close to the start of translation and is conserved between arabidopsis and leafy spurge. Putative cis-acting elements were identified in the EeSTM promoter, including a tuber-specific sucrose-responsive element, which could play a major role in the expression of EeSTM in root tissue.

Ten years of Oregon Seed Certification Service (OSCS) preharvest field inspections converted from a nonspatial database to a geographic information system (GIS) were analyzed for patterns in spatial distribution of occurrence and severity of the 36 most common weeds of grass seed crops. This was done under the assumptions that those patterns would be primarily consequences of interactions among farming practices, soil properties, and biological traits of the weeds, and that improved understanding of the interactions would benefit the grass seed industry. Kriging, Ripley's K-function, and both Moran's I spatial autocorrelation and Getis-Ord General G high/low clustering using the multiple fixed distance band option all produced roughly similar classifications of weeds possessing strongest and weakest spatial clustering patterns. When Moran's I and General G analyses of maximum weed severity observed within individual fields over the life of stands were conducted using the inverse distance weighting option, however, results were highly sensitive to the presence of a small number of overlapping fields in the 10-yr record. Addition of any offset in the range from 6 to 6,437 m to measured distances between field centroids in inverse distance weighting matrices removed this sensitivity, and produced results closely matching those for the multiple fixed distance band method. Clustering was significant for maximum severity within fields over the 10-yr period for all 43 weeds and in 78% of single-year analyses. The remaining 22% of single-year cases showed random rather than dispersed distribution patterns. In decreasing order, weeds with strongest inverse-distance spatial autocorrelation were German velvetgrass, field bindweed, roughstalk bluegrass, annual bluegrass, orchardgrass, common velvetgrass, Italian ryegrass, Agrostis spp., and perennial ryegrass. Of these nine weeds, distance for peak spatial autocorrelation ranged from 2 km for Agrostis spp. to 34 km for common velvetgrass. Weeds with stronger spatial autocorrelation had greater range between distance of peak spatial autocorrelation and maximum range of significance. Z-scores for General G high/low clustering were substantially lower than corresponding values for Moran's I spatial autocorrelation, although the same two weeds (German velvetgrass and field bindweed) showed strongest clustering using both measures. Simultaneous patterns in Moran's I and General G implied that management practices relatively ineffective in controlling weeds usually played a greater role in causing weeds to cluster than highly effective practices, although both types of practices impacted Italian ryegrass distribution. Distance of peak high/low clustering among perennial weeds was smallest (1 to 3 km) for Canada thistle, field bindweed, Agrostis spp., and western wildcucumber, likely indicating that these weeds occurred in patchy infestations extending across neighboring fields. Although both wild carrot and field bindweed doubled in average severity over the period from 1994 to 2003, wild carrot was the only weed clearly undergoing an increase in spatial autocorrelation. Soil chemical and physical properties and dummy variables for soil type and crop explained small but significant portions of total variance in redundancy and canonical correspondence analysis of weed occurrence and severity. Fitch-Morgoliash tree diagrams and Redundancy Analysis (RDA) and Canonical Correspondence Analysis (CCA) ordinations revealed substantial differences among soil types in weed occurrence and severity. Gi∗ local hot-spot clustering combined with feature class to raster conversion protected grower expectations of confidentiality while describing dominant spatial features of weed distribution patterns in maps released to the public.

Sicklepod is a competitive and prolific weed that emerges throughout the crop season. Glyphosate applications to flowering sicklepod greatly reduce seed production, but there is limited information on glyphosate translocation in flowering weeds. Therefore, a laboratory study was conducted to document the absorption and translocation of 14C-glyhosate in flowering sicklepod. Flowering sicklepod plants were treated with a 14C-glyphosate solution, and 14C-glyphosate absorption and translocation was quantified by scintillation spectrometry. Absorption and translocation of 14C-glyphosate in flowering sicklepod were similar for both 0.21 and 0.42 kg ae ha−1 glyphosate. Although the treated leaflets retained approximately 50% of the recovered 14C-glyphosate, movement of the herbicide was both acropetal and basipetal, with the highest 14C-glyphosate concentrations in the shoot below the treated leaf and the roots. By 96 h after treatment, sicklepod buds and flowers had abscised, but analysis of the structures revealed accumulation of 2% of the recovered 14C-glyphosate. Based on results of the study, effects of glyphosate accumulation in buds and flowers combined with plant stress associated with the primary and secondary effects of glyphosate result in bud and flower abscission, drastically reducing sicklepod seed production.

Laboratory studies were conducted in 2006 and 2007 to evaluate the effects of temperature and propanil on alligatorweed control with penoxsulam. Biomass reduction of alligatorweed at 42 d after treatment (DAT) compared to nontreated was greatest at 21/11 C (day/night) compared to 26/18 C or 30/25 C for all treatments. Propanil plus penoxsulam reduced biomass less than penoxsulam applied alone, independent of temperature. At 21 and 27 C, delaying propanil application 3 d after penoxsulam provided similar biomass reduction to penoxsulam applied alone. At 27 C and 30 C, delaying propanil application 10 d after treatment was required to achieve biomass reduction greater or equal to penoxsulam applied alone. Absorption and translocation of 14C penoxsulam indicated that propanil reduced absorption of penoxsulam into the treated leaf of alligatorweed 48 h after treatment. This research demonstrates the potential for propanil to antagonize penoxsulam when applied to alligatorweed. Under the most severe antagonistic conditions (30 C) propanil applications following penoxsulam needed to be delayed 10 d to avoid antagonism.

Growth room studies were conducted to determine the physiological basis of reduced glyphosate efficacy under low soil nitrogen using velvetleaf, common ragweed, and common lambsquarters as model species. Glyphosate dose–response experiments of weeds grown under low (1.5 mM) and high (15 mM) soil N were conducted. Velvetleaf and common lambsquarters grown under low N required 169 g ae ha−1 glyphosate for a significant reduction in biomass, but only 84 g ae ha−1 were required when grown under high N. However, when common ragweed was grown under low or high soil N there was no significant difference in response to glyphosate at all doses tested. The reduced glyphosate efficacy on velvetleaf and common lambsquarters under low N was primarily due to decreased herbicide translocation to the meristem. It appears that low N may decrease the net assimilation of carbon in plants, resulting in a decrease in the net export of sugars and hence glyphosate from mature leaves. Understanding the relationship between soil N and herbicide efficacy may help explain observed weed control failures with glyphosate and may contribute to our knowledge of the occurrence of weed patchiness in fields. This is the first report illustrating a physiological basis for decreased glyphosate efficacy under low soil N in selected weed species.

Greenhouse and laboratory experiments were conducted to evaluate the tolerance of four commercial sugarbeet varieties to s-metolachlor and dimethenamid-P, determine the principle site of absorption of these herbicides, and determine the physiological basis for differential tolerance among varieties to these herbicides. ‘Beta 5833R’ was the most tolerant sugarbeet variety, and ‘Hilleshog 7172RZ’ was the most susceptible sugarbeet variety to injury from s-metolachlor and dimethenamid-P. The primary site of s-metolachlor and dimethenamid-P absorption was through the sugarbeet roots; however, some absorption occurred through the sugarbeet hypocotyl. Sugarbeet injury was greater from dimethenamid-P than s-metolachlor when sugarbeet was grown in soil. However, when sugarbeet was grown hydroponically, injury from the herbicides was similar, indicating that the relative availability of these herbicides in the soil greatly influenced sugarbeet injury. Reduced translocation and slower metabolism of 14C-dimethenamid-P in both the roots and shoots of the sugarbeet plants most likely contributed to the greater susceptibility of sugarbeet to dimethenamid-P compared with s-metolachlor. Metabolism of 14C-herbicides in sugarbeet shoots was 0.7 to 2.1 h slower in the more susceptible sugarbeet varieties compared with the more tolerant variety, Beta 5833R. This was the most significant factor contributing to differences in sugarbeet variety tolerance to both s-metolachlor and dimethenamid-P.

Several field studies have observed increased foramsulfuron efficacy for the control of dallisgrass when foramsulfuron is applied after MSMA. Therefore, laboratory studies were conducted with mature dallisgrass to study the absorption and translocation of 14C-foramsulfuron, and then examine the impact of preliminary applications (preapplications) of MSMA or foramsulfuron on herbicide absorption and movement. Herbicide absorption increased rapidly through 4 h, and by 8 h, differences in absorption between pretreated and control plants were evident. After 48 h, foramsulfuron absorption in non-pretreated plants was 55%, whereas plants that received either pretreatment absorbed 70% of the herbicide. Translocation above (younger tissue) and below (older tissue) the treated leaf was 0.65 and 0.62% for non-pretreated plants, respectively. Pretreatment with foramsulfuron resulted in the translocation of 2.12 and 1.55% of applied radioactivity above and below the treated leaf, respectively. Pretreatment with MSMA resulted in the translocation of 2.33 and 2.34% of applied radioactivity above and below the treated leaf, respectively. These data indicated that pretreatment of mature dallisgrass with either foramsulfuron or MSMA results in an increase in both uptake and translocation of foramsulfuron applied 2 wk after pretreatment. The increase in absorption and translocation of foramsulfuron in the pre–MSMA-treated plants may explain the increase in control observed in the field when comparing it to the pre–foramsulfuron-treated dallisgrass plants.

Selection by herbicides has resulted in widespread evolution of herbicide resistance in agricultural weeds. In California, resistance to glyphosate was first confirmed in rigid ryegrass in 1998. Objectives of this study were to determine the current distribution and level of glyphosate resistance in Italian ryegrass, and to assess whether resistance could be due to an altered target site. Seeds were sampled from 118 populations and seedlings were treated with glyphosate at 866 g ae ha−1. Percentage of survivors ranged from 5 to 95% in 54 populations. All plants from 64 populations died. One susceptible (S) population, four putatively resistant (R) populations, and one S accession from Oregon were used for pot dose–response experiments, shikimic acid analyses, and DNA sequencing. Seedlings were treated with glyphosate at eight rates, ranging from 108 to 13,856 g ae ha−1. Shoot biomass was evaluated 3 wk after treatment and fit to a log-logistic regression equation. On the basis of GR50 (herbicide rate required to reduce growth by 50%) values, seedlings from putatively R populations were roughly two to 15 times more resistant to glyphosate than S plants. Shikimic acid accumulation was similar in all plants before glyphosate treatment, but at 4 and 7 DAT, S plants from California and Oregon accumulated approximately two and three times more shikimic acid, respectively, than R plants. Sequencing of a cDNA fragment of the EPSPS coding region revealed two different codons, both of which encode proline at amino acid position 106 in S individuals. In contrast, all R plants sequenced exhibited missense mutations at site 106. Plants from one population revealed a mutation resulting in a proline to serine substitution. Plants from three R populations exhibited a mutation corresponding to replacement of proline with alanine. Our results indicate that glyphosate resistance is widespread in Italian ryegrass populations of California, and that resistance is likely due to an altered target enzyme.

Croftonweed is a major invasive weed in China and, although of subtropical origin, has invaded into regions with colder climates. Freezing tolerances of nine croftonweed populations from different geographies were studied using a freezing injury index. Physiological responses to freezing temperature were determined to elucidate mechanisms of freezing tolerance. Plants from Baise, Guangxi (BSG), and Qujing, Yunnan (QJY), China, showed the most freezing injury symptoms, whereas plants from Huangguoshu, Guizhou (HGG), China, displayed the least. Under freezing stress, physiological changes, including increases in malondialdehyde (MDA) and total soluble protein contents; reductions in total soluble sugar, chlorophyll contents, and ratios of variable chlorophyll fluorescence to maximum chlorophyll fluorescence (Fv : Fm); and fluctuation of superoxide dismutase (SOD) activity, were observed among all nine populations. However, different degrees of physiological responses were found among populations with diverse low-temperature sensitivities. After 4 d of treatment at −5 C, MDA content increased 25-fold in leaves of the sensitive BSG population compared with untreated leaves, whereas a range of 0.8-fold to ∼5.3-fold increase was found in other populations. Total soluble protein content in leaves of the tolerant HGG population increased to the highest value among the nine populations. SOD activity of the freezing-sensitive BSG population decreased 36% of the control, whereas the tolerant HGG population reduced to 70%. Moreover, soluble sugar of the tolerant HGG population decreased 29%, less than the sensitive BSG population (87%). There were fewer declines in the percentages of chlorophyll content and Fv : Fm value in HGG than in BSG (less 44% and 32%). Freezing injury index had significant negative correlations with Fv : Fm values and chlorophyll contents (−0.619 and −0.622, respectively). These results suggest that croftonweed has evolved into different ecotypes with regard to freezing tolerance through physiological adaptation during their invasion of southwest regions of China. The freezing-tolerant croftonweed population would have more chances to invade distant northeastern areas in the future.

A threefold glyphosate tolerance was identified in two Italian ryegrass populations, T1 and T2, from Mississippi. Laboratory experiments were conducted to characterize the mechanism of glyphosate tolerance in these populations. The T1 population absorbed less 14C-glyphosate (43% of applied) compared to the susceptible (S) population (59% of applied) at 48 h after treatment (HAT). The T2 population absorbed 14C-glyphosate at levels (56% of applied at 48 HAT) that were similar to both T1 and S populations, but tended to be more comparable to the S population. The amount of 14C-glyphosate that remained in the treated leaf was significantly higher in both T1 (67% of absorbed) and T2 (65% of absorbed) populations compared to the S population (45% of absorbed) at 48 HAT. The amount of 14C-glyphosate that moved out of treated leaf to shoot and root was lower in both T1 (25% of absorbed in shoot and 9% of absorbed in root) and T2 (25% of absorbed in shoot and 11% of absorbed in root) populations compared to the S population (40% of absorbed in shoot and 16% of absorbed in root) at 48 HAT. There were no differences in epicuticular wax mass among the three populations. Treating a single leaf with glyphosate solution at the field use rate (0.84 kg ae ha−1) as 10 1-µl droplets killed the S plant but not the T1 and T2 plants (33 and 55% shoot fresh-weight reduction, respectively). Shikimic acid accumulated rapidly at higher levels in glyphosate-treated leaf segments of the S population compared to the T1 population up to 100 µM glyphosate. However, above 500 µM glyphosate, the levels of shikimate were similar in both the S and T1 populations. Furthermore, shikimic acid content was three- to sixfold more in whole plants of the S population treated with 0.22 kg ae ha−1 glyphosate compared to the T1 and T2 populations. No degradation of glyphosate to aminomethylphosphonic acid was detected among the tolerant and susceptible populations. These results indicate that tolerance to glyphosate in the T1 population is partly due to reduced absorption and translocation of glyphosate and in the T2 population it is partly due to reduced translocation of glyphosate.

Purple-leaf button weed and Indian heliotrope are widespread and common weed species of rain-fed rice in many tropical countries. The influence of various environmental factors on seed germination of these species was studied. Seeds of both species germinated at a range of alternating temperatures (25/15, 30/20, and 35/25 C day/night). Germination of purple-leaf button weed was similar among light conditions after an after-ripening period of 3 mo, whereas germination of Indian heliotrope was always greater in light. Seed germination of both species was not affected by a high level of salt and a range of pH between 5 and 9, but was sensitive to high degrees of water stress. Seed burial strongly inhibited germination and emergence of these species. Seedling emergence of purple-leaf button weed (82 to 86%) and Indian heliotrope (70%) was optimal when seeds were sown in the top 0.2-cm soil layer. A burial depth of 2 cm completely inhibited emergence of Indian heliotrope, whereas, for purple-leaf button weed, this depth was 5 cm. Most of these seeds germinated, however, when brought to the soil surface. The information gained from this study would help in predicting the potential of these species for spreading into new areas, and could contribute to their control.

Control of common lambsquarters with POST applications of glyphosate in glyphosate-resistant crops generally has been effective. In 2002, common lambsquarters plants from Westmoreland County, VA, were identified after not being controlled with a POST glyphosate application in glyphosate-resistant soybean. Plants from this site that survived glyphosate were collected in both 2002 and 2004. The objective of this research was to evaluate the susceptibility of F1 and F2 progeny from these common lambsquarters plants, relative to the susceptibility of common lambsquarters collected in Montgomery County, VA. F1 progeny of the Westmoreland plants from both the 2002 and 2004 collections showed reduced response to glyphosate relative to the Montgomery collection. Vigor reduction of F1 progeny from three 2004 Westmoreland source plants with 0.84 kg ae ha−1 of glyphosate ranged from 66 to 85% at 28 d after treatment (DAT), compared with 89% for the Montgomery collection. Evaluation of four Westmoreland F2 common lambsquarters lines derived from 2002 collections indicated significant differences in glyphosate sensitivity. Fifteen F2 lines were generated from 2004 collections from each of three Westmoreland source plants and from the Montgomery source. For the least sensitive Westmoreland source, vigor reduction ranged from only 24 to 36% across F2 lines in response to 1.68 kg ha−1 of glyphosate at 28 DAT relative to 55 to 100% for the Montgomery source. I50 estimates for fresh weight reduction were 0.91 and 0.32 kg ha−1, for these sources, respectively. Sequential treatments of 0.42, 1.26, and 1.68 kg ha−1 applied at 3-wk intervals to the least susceptible 2004 Westmoreland F2 line resulted in only 37% vigor reduction and no mortality among 360 treated plants.

In order to explore the physiological mechanism of paraquat resistance in tall fleabane, a widespread weed in Taiwan where resistance to this herbicide has been observed since 1980, the role of the antioxidative system was assessed. The susceptible (S) and resistant (R) biotypes of tall fleabane were distinguished clearly by the relative distribution frequency of injury index caused by 78 µM paraquat. Although malondialdehyde, an indicator for peroxidation damage to the plant, in the the R-biotype was not changed, in the S-biotype malondialdehyde increased within 2 h after treatment of 50 µM paraquat. Analysis of several antioxidants and pertinent enzymes revealed that ascorbate peroxidase activity was decreased by paraquat treatment in the S-biotype; and a lower basal level of ascorbate was present in the S-biotype as well. The maintenance of a high ratio of reduced glutathione to total glutathione, coupled with a pronounced and rapid increase of glutathione reductase (GR) activity in the the R-biotype, suggests that an active reduced glutathione/oxidized glutathione (GSH/GSSG) cycle is critical to paraquat resistance of tall fleabane. The decisive contribution of a functional GSH/GSSG cycle to paraquat resistance through an enhancement of GR activity in this weed was further confirmed by an experiment of exogenous application of ascorbate.