Reduced translocation is involved in resistance to glyphosate and paraquat in Conyza bonariensis and Conyza canadensis from California

Resistance to glyphosate and paraquat has evolved in some populations of Conyza spp. from California, USA. This study evaluated whether herbicide absorption and translocation were involved in the mechanism of resistance to both herbicides. Three lines of each species were used: glyphosate‐paraquat‐susceptible (GPS), glyphosate‐resistant (GR) and glyphosate‐paraquat‐resistant (GPR). Radiolabelled herbicide was applied to a fully expanded leaf, and absorption and movement out of the treated leaf were monitored for up to 24 h for paraquat and 72 h for glyphosate. Plants treated with paraquat were incubated in darkness for the first 16 h and then subjected to light conditions. More glyphosate was absorbed in C. bonariensis (52.9–58.3%) compared with C. canadensis (28.5–37.6%), but no differences in absorption were observed among lines within a species. However, in both species, the GR and GPR lines translocated less glyphosate out of the treated leaf when compared with their respective GPS lines. Paraquat absorption was similar among lines and across species (71.3–77.6%). Only a fraction of paraquat was translocated in the GPR lines (3% or less) when compared with their respective GPS or GR lines (20% or more) in both species. Taken together, these results indicate that reduced translocation is involved in the mechanism of resistance to glyphosate and paraquat in C. bonariensis and C. canadensis.     

Foliar absorption and translocation of nicosulfuron and rimsulfuron in five annual weed species

Ambrosia artemisüfolia L. (common ragweed) and Digitaria ischaemum Schreb. (smooth crabgrass) are not controlled by nicosulfuron and rimsulfuron at the highest recommended application rates, whereas Panicum miliaceum L. (wild proso millet), Amaranthus retroflexus L. (redroot pigweed) and Avena fatua L. (wild oat) are susceptible. The foliar absorption and translocation of ¹⁴C-nicosulfuron and ¹⁴C-rimsulf uron were studied in these weed species up to 48 h after treatment (HAT). Differences in herbicide uptake and translocation were not correlated with the species susceptibility. By 48 HAT, more than 50% of both herbicides remained on the treated leaf surface. Foliar absorption of rimsulfuron was greater than that of nicosulfuron in A. retroflexus, P. miliaceum and A. artemisüfolia. Most of the absorbed herbicide remained in the treated leaf of each weed species. Export of ¹⁴C–nicosulfuron ranged from 28 to 54% of that absorbed, in contrast to 15 to 39% for ¹⁴C–rimsulfuron. The absorption and translocation rates of both herbicides were highest within the initial 6 HAT, and decreased thereafter. Both herbicides showed approximately the same distribution pattern within each weed species.     

Uptake, translocation and metabolism of aminocyclopyrachlor in prickly lettuce, rush skeletonweed and yellow starthistle

BACKGROUND: Aminocyclopyrachlor is a new herbicide proposed to control broadleaf weeds and shrubs in non‐crop and rangeland systems. To gain a better understanding of observed field efficacy, the uptake and translocation of foliar‐applied aminocyclopyrachlor (DPX‐MAT28) and aminocyclopyrachlor methyl ester (DPX‐KJM44) were evaluated in two annuals, prickly lettuce (Lactuca serriola L.) and yellow starthistle (Centaurea solstitialis L.), and one perennial, rush skeletonweed (Chondrilla juncea L.). RESULTS: Absorption and translocation varied between species. While absorption of DPX‐KJM44 was greater than absorption of DPX‐MAT28, rush skeletonweed absorbed the most, followed by yellow starthistle and prickly lettuce. Overall, the total translocation of either herbicide was highest in yellow starthistle, followed by rush skeletonweed and prickly lettuce. Proportional herbicide movement between species was similar, with the majority translocating to developing shoots. However, in rush skeletonweed, early translocation was directed to root tissue. In rush skeletonweed, no DPX‐MAT28 metabolism occurred, while DPX‐KJM44 was rapidly de‐esterified and translocated as DPX‐MAT28. CONCLUSION: Aminocyclopyrachlor absorption and translocation are dependent on active ingredient structure and species sensitivity. Highly sensitive species such as prickly lettuce absorb and translocate less material than relatively less sensitive species such as rush skeletonweed. De‐esterification of DPX‐KJM44 appears to delay translocation of the resulting acid in yellow starthistle and rush skeletonweed. Copyright © 2011 Society of Chemical Industry     

Protoporphyrinogen oxidase-inhibiting activity of the new,wheat-selective isoindoldione herbicide,cinidon-ethyl

Cinidon-ethyl (BAS 615H) is a new herbicide of isoindoldione structure which selectively controls a wide spectrum of broadleaf weeds in cereals. The uptake, translocation, metabolism and mode of action of cinidon-ethyl were investigated in Galium aparine L, Solanum nigrum L and the tolerant crop species wheat (Triticum aestivum L). When plants at the second-leaf stage were foliarly treated with cinidon-ethyl equivalent to a field rate of 50 g ha⁻¹ for 48 h, the light requirement for phytotoxicity and the symptoms of plant damage in the weed species, including rapid chlorophyll bleaching, desiccation and necrosis of the green tissues, were identical to those of inhibitors of porphyrin synthesis, such as acifluorfen-methyl. The selectivity of cinidon-ethyl between wheat and the weed species has been quantified as approximately 500-fold. Cinidon-ethyl strongly inhibited protoporphyrinogen oxidase (Protox) activity in vitro, with I₅₀ values of approximately 1 nM for the enzyme isolated from the weed species and from wheat. However, subsequent effects of herbicide action, with accumulation of protoporphyrin IX, light-dependent formation of 1-aminocyclopropane-1-carboxylic acid-derived ethylene, ethane evolution and desiccation of the green tissue, were induced by cinidon-ethyl only in the weed species. After foliar application of [¹⁴C] cinidon-ethyl, the herbicide, due to its lipophilic nature, was rapidly adsorbed by the epicuticular wax layer of the leaf surface before it penetrated into the leaf tissue more slowly. No significant differences between foliar and root absorption and translocation of the herbicide by S nigrum, G aparine and wheat were found. After foliar or root application of [¹⁴C]- cinidon-ethyl, translocation of ¹⁴C into untreated plant parts was minimal, as demonstrated by combustion analysis and autoradiography. Metabolism of [¹⁴C]cinidon-ethyl via its E-isomer and acid to further metabolites was more rapid in wheat than in S nigrum and G aparine. After 32 h of foliar treatment with 50 g ha⁻¹ of the [¹⁴C]-herbicide, approximately 47%, 36%, and 12% of the absorbed radioactivity, respectively, were found as unchanged parent or its biologically low active E-isomer and acid in the leaf tissue of G aparine, S nigrum and wheat. In conclusion, cinidon-ethyl is a Protox-inhibiting, peroxidizing herbicide which is effective through contact action in the green tissue of sensitive weed species. It is suggested that a more rapid metabolism, coupled with moderate leaf absorption, contribute to the tolerance of wheat to cinidon-ethyl. © 1999 Society of Chemical Industry     

Interaction between saflufenacil and imazethapyr in red rice (Oryza ssp.) and hemp sesbania (Sesbania exaltata) as affected by light intensity

BACKGROUND: Saflufenacil is a broadleaf herbicide for preplant burndown and pre‐emergence applications in various crops. This study was established to evaluate the absorption and translocation of saflufenacil in hemp sesbania and imazethapyr in red rice as a function of their post‐emergence interaction and light intensity. RESULTS: Imazethapyr plus saflufenacil provided a greater uptake (30%) and translocation (35%) of ¹⁴C‐imazethapyr than imazethapyr alone. In the section above treated leaf (ATL), a higher percentage of the absorbed imazethapyr (23%) was quantified in the imazethapyr plus saflufenacil treatment after 168 h. Faster basipetal movement of imazethapyr was identified under higher light availability. Absorption of ¹⁴C‐saflufenacil ranged from approximately 40 to 60% among herbicide and light intensity treatments. At 12 and 24 h after treatment (HAT) a greater percentage (15–20%) of the absorbed saflufenacil was quantified above the treated leaf at the two lower light intensities. Similar trends were observed for basipetal movement of saflufenacil. CONCLUSION: Saflufenacil enhanced absorption, overall translocation and acropetal movement of imazethapyr in the TX4 red rice. Basipetal movement of imazethapyr was faster under higher light intensities. Overall, imazethapyr improved absorption of saflufenacil in hemp sesbania plants. Reduction in light intensity resulted in greater translocation of saflufenacil, promoting acropetal and basipetal distribution at the two lower light intensity treatments. Copyright © 2011 Society of Chemical Industry     

Differences in flowering phenology and seed production of four morning glory (Ipomoea) species in Japan

The spread of morning glory (Ipomoea spp.) in soybean fields in Japan has severely decreased soybean yield. Yet, current control measures do not control the proliferation of Ipomoea spp. As little is known about the flowering period and seed production among the different invading Ipomoea spp., it is challenging to create targeted control measures based on ecological characteristics. This study aimed to reveal the characteristics of the flowering phenology and seed production of four morning glory species, namely, Ipomoea coccinea L. (red morning glory), Ipomoea lacunosa L. (pitted morning glory), Ipomoea hederacea L. Jacq. var. integriuscula A. Gray (entireleaf morning glory), and Ipomoea triloba L. (three-lobe morning glory). Between 2017 and 2019, the four selected study species were grown under similar conditions of soil quality, irrigation, and environmental influences and their flowering phenology and seed data were recorded. The flowering period ranged from 36 to 40 days, and the initial flowering of I. triloba was approximately 2 weeks later than the others. I. coccinea had the highest flowering number and seed production, followed by I. lacunosa, I. triloba, and I. hederacea var. integriuscula. The fruit setting rate of I. triloba decreased later in the reproductive stage but tended to increase as the daily mean temperature increased on each flowering day. Thus, we report that the flowering phenology and seed production differed greatly among the Ipomoea spp. These findings can provide crucial insights into designing targeted species-specific control measures against the spread of Ipomoea spp. in Japan.     

Glyphosate resistance in common ragweed ( A mbrosia artemisiifolia L.) from Mississippi,USA

Glyphosate is one of the most commonly used broad‐spectrum herbicides over the last 40 years. Due to the widespread adoption of glyphosate‐resistant (GR) crop technology, especially corn, cotton and soybean, several weed species have evolved resistance to this herbicide. Research was conducted to confirm and characterize the magnitude and mechanism of glyphosate resistance in two GR common ragweed ( A mbrosia artemisiifolia L.) biotypes from Mississippi, USA. A glyphosate‐susceptible (GS) biotype was included for comparison. The effective glyphosate dose to reduce the growth of the treated plants by 50% for the GR1, GR2 and GS biotypes was 0.58, 0.46 and 0.11 kg ae ha^?1, respectively, indicating that the level of resistance was five and fourfold that of the GS biotype for GR1 and GR2, respectively. Studies using ¹⁴ C‐glyphosate have not indicated any difference in its absorption between the biotypes, but the GR1 and GR2 biotypes translocated more ¹⁴ C‐glyphosate, compared to the GS biotype. This difference in translocation within resistant biotypes is unique. There was no amino acid substitution at codon 106 that was detected by the 5‐enolpyruvylshikimate‐3‐phosphate synthase gene sequence analysis of the resistant and susceptible biotypes. Therefore, the mechanism of resistance to glyphosate in common ragweed biotypes from Mississippi is not related to a target site mutation or reduced absorption and/or translocation of glyphosate.     

Aminocyclopyrachlor sorption–desorption and leaching in soil amended with organic materials from sugar cane cultivation

The correct application of a new herbicide depends on knowledge concerning its behaviour within the cultivation system. The objective of this study was to evaluate the sorption–desorption process of aminocyclopyrachlor in soils with the addition of three aged organic materials from sugar cane and their transport via leaching. Sugar cane straw (12 t/ha), filter cake (90 t/ha) and vinasse (200 m³/ha) were added to a clayey soil 15, 30 and 60 days before herbicide application. Sorption and desorption were evaluated by the batch equilibrium method. For leaching assessments, the materials were applied to the soil surface. Sorption was relatively low in all treatments (K_d = 0.17–0.41 L/kg), although significantly higher in soil without organic material addition. A negative correlation between herbicide sorption and increased soil base saturation was observed, indicating competition for sorption sites. With the addition of vinasse, 71% of the herbicide reached the leachate, while <50% reached the leachate in the other treatments. Aminocyclopyrachlor availability was not reduced with organic material addition to the soil, which may be favourable for weed control. However, the presence of vinasse leads to the risk of leaching to deeper soil layers than the seed bank.     

Mechanism of resistance to bensulfuron-methyl in Alisma plantago-aquatica biotypes from Portuguese rice paddy fields

Two Alisma plantago‐aquatica biotypes resistant to bensulfuron‐methyl were detected in rice paddy fields in Portugal’s Mondego (biotype T) and Tagus and Sorraia (biotype Q) River valleys. The fields had been treated with bensulfuron‐methyl‐based herbicide mixtures for 4–6 years. In order to characterize the resistant (R) biotypes, dose–response experiments, absorption and translocation assays, metabolism studies and acetolactate synthase (ALS) activity assays were performed. There were marked differences between R and susceptible (S) biotypes, with a resistance index (ED₅₀R/S) of 500 and 6.25 for biotypes Q and T respectively. Cross‐resistance to azimsulfuron, cinosulfuron and ethoxysulfuron, but not to metsulfuron‐methyl, imazethapyr, bentazone, propanil and MCPA was demonstrated. No differences in the absorption and translocation of ¹⁴C‐bensulfuron‐methyl were found between the biotypes studied. Maximum absorption attained 1.12, 2.02 and 2.56 nmol g⁻¹ dry weight after 96 h incubation with herbicide, for S, Q and T biotypes respectively. Most of the radioactivity taken up by the roots was translocated to shoots. Bensulfuron‐methyl metabolism in shoots was similar in all biotypes. The R biotypes displayed a higher level of ALS activity than the S biotype, both in the presence and absence of herbicide and the resistance indices (IC₅₀R/S) were 20 197 and 10 for biotypes Q and T respectively. These data confirm for the first time that resistance to bensulfuron‐methyl in A. plantago‐aquatica is target‐site‐based. In practice, to control target site R biotypes, it would be preferable to use mixtures of ALS inhibitors with herbicides with other modes of action.     

Absorption and translocation of glyphosate in Spermacoce verticillata and alternative herbicide control

Glyphosate has been associated with control failures for Spermacoce verticillata in some Brazilian States. The objective of this work was to evaluate and determine the possible causes of this and propose alternative herbicides to use. Glyphosate was applied at three plant stages of development (2–4 leaves, 4–6 leaves and full bloom) where foliar absorption and translocation of glyphosate to various plants parts and time were analysed using the ¹⁴C technique. Data were submitted to nonlinear regressions and analysis of variance, where means were compared by a Tukey test at 5% probability. Plant control by the application of different herbicides (19) in the same three stages of development of weed was evaluated. Twenty‐one days after herbicide application, control was visually evaluated and data analysed and means were compared. Due to absorption and/or translocation problems, S. verticillata plants were not controlled by glyphosate. Plants with 4–6 leaves showed lower absorption and translocation of the herbicide to the leaf/root regions compared with plants at the beginning of their development. Plants at full bloom showed lower translocation of the herbicide to the root. In addition to the application of glyphosate at early stages of development, the application of paraquat, flumioxazin and mixtures of glyphosate with flumioxazin or cloransulam is recommended. Late applications could result in poor control, principally if glyphosate is used. Therefore, early applications are strongly recommended for control of this species.     

Leaf chlorophyll fluorescence discriminates herbicide resistance in Echinochloa species

Timely detection of herbicide resistance at an early stage of crop cultivation is essential to help farmers find alternative solutions to manage herbicide resistance in their fields. In this study, maximum quantum yield of PS II [F_v/F_m = (F_m – F_o)/F_m] was measured at the 4–5 leaf stage to discriminate between herbicide‐resistant and susceptible biotypes of Echinochloa species. The differences in F_v/F_m between herbicide‐resistant and susceptible Echinochloa spp. were consistent with the whole‐plant assay based on I₅₀ (herbicide doses causing a 50% inhibition of F_v/F_m) and GR₅₀ (herbicide doses causing a 50% reduction in plant fresh weight) values and R/S ratios (herbicide resistance index), regardless of the mode of action of the tested herbicides. A PS II inhibitor caused the fastest inhibition of F_v/F_m, compared with ACCase and ALS inhibitors, after herbicide treatment. The required time for discrimination between herbicide‐resistant and susceptible Echinochloa spp. was 64 h after PS II inhibitor treatment, much shorter than those of ACCase and ALS inhibitor‐treated plants, which required 168 and 192 h respectively. The leaf chlorophyll fluorescence assay provided reliable diagnostics of herbicide resistance in Echinochloa spp. with significant time savings and convenient measurement in field conditions compared with the conventional whole‐plant assay.     

The uptake,translocation and metabolism of epronaz in selected species

The absorption, translocation and metabolism of the selective pre- or early post- emergence herbicide epronaz (N-ethyl-N-propyl-3-propylsulphonyl-1,2,4-triazole-1-carboxamide) were investigated using selected crop and weed species. The pattern of tolerance to epronaz of both germinating seeds and 10-day-old plants grown in nutrient solution, was found to be soybean (Glycine max L.) > maize (Zea mays L.) > cotton (Gossypium hirsutum L.) > rice (Oryza sativa L.) > barnyard grass [Echinochloa crus-galli (L.) Beauv.]. In all species, absorption and translocation of ¹⁴C from a nutrient solution containing [¹⁴C]epronaz (0.02 μCi ml^?1) increased with time. Autoradiographic and liquid scintillation analysis indicated the presence of radioactivity in the apical regions of all species after 4 h. Interspecies variation in uptake and distribution did not appear to be a major factor explaining selectivity, although the resistance of cotton may be partly due to compartmentalisation of ¹⁴C in the lysigenous glands in stem and leaves. Analysis of extracts from plants treated with [¹⁴C]epronaz indicated the presence of epronaz, its major degradation product [3-propylsulphonyl-l,2,4-triazole (BTS 28 768)] and certain unknown radio-labelled compounds. The major metabolite (Unknown I) was believed to be a conjugate of certain plant components with either epronaz or BTS 28 768. The rate of formation of Unknown I corresponded to the relative resistance and susceptibility to epronaz of soybean, rice and barnyardgrass. The level of the herbicide remained much higher in cotton than in the other species, possibly reflecting compartmentalisation and inactivation of epronaz in the lysigenous glands. For maize, high levels of uptake, exudation and degradation in the nutrient solution were recorded.     

Uptake, translocation and fate of 2,4-D and chlorsulfuron in Silene vulgaris (Moench) Garcke

Experiments were conducted to examine the up take, translocation and metabolism by S. vulgaris of two distinctly different herbicides: 2,4-D, a phenoxyalkanoic acid with growth regulator activity to which this species exhibits complete tolerance, and chlorsulfuron, a sul-fonylurea to which S. vulgaris is highly sensitive. Despite their structural dissimilarities 2,4-D and chlorsulfuron was readily absorbed by S. vulgaris with 65 and 69%, respectively, of the applied dosage being absorbed within 72 hours after treatment. Approximately 35% of the 2,4-D and 10% of the chlorsulfuron label was translocated out of the treated leaf after 72 hours. Neither herbicide accumulated in the terminal bud. Seventy-two hours after treatment 63% of the recovered ¹⁴C remained as unaltered 2,4-D in S. vulgaris, while in tomato, a 2,4-D sensitive species, 65% of the recovered ¹⁴C remained as intact herbicide. In S. vulgaris approximately 86% of the radioactivity remained as intact chlorsulfuron 72 hours after treatment compared to 12% in the tolerant wheat. The tolerance of S. vulgaris to 2,4-D could not be accounted for by limited absorption, translocation nor metabolic degradation of the herbicide. The sensitivity of S. vulgaris to chlorsulfuron would appear to be related to the inability of this species to metabolize the herbicide molecule.     

Metabolism of the herbicide glufosinate‐ammonium in plant cell cultures of transgenic (rhizomania‐resistant) and non‐transgenic sugarbeet (Beta vulgaris), carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium)

The metabolism of the herbicide glufosinate‐ammonium was investigated in heterotrophic cell suspension and callus cultures of transgenic (bar‐gene) and non‐transgenic sugarbeet (Beta vulgaris). Similar studies were performed with suspensions of carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium). ¹⁴C‐labelled chemicals were the (racemic) glufosinate, L ‐glufosinate, and D ‐glufosinate, as well as the metabolites N‐acetyl L ‐glufosinate and 3‐(hydroxymethylphosphinyl)propionic acid (MPP). Cellular absorption was generally low, but depended noticeably on plant species, substance and enantiomer. Portions of non‐extractable residues ranged from 0.1% to 1.2% of applied ¹⁴C. Amounts of soluble metabolites resulting from glufosinate or L ‐glufosinate were between 0.0% and 26.7% of absorbed ¹⁴C in non‐transgenic cultures and 28.2% and 59.9% in transgenic sugarbeet. D ‐Glufosinate, MPP and N‐acetyl L ‐glufosinate proved to be stable. The main metabolite in transgenic sugarbeet was N‐acetyl L ‐glufosinate, besides traces of MPP and 4‐(hydroxymethylphosphinyl)butanoic acid (MPB). In non‐transgenic sugarbeet, glufosinate was transformed to a limited extent to MPP and trace amounts of MPB. In carrot, D stramonium and D purpurea, MPP was also the main product; MPB was identified as a further trace metabolite in D stramonium and D purpurea. © 2001 Society of Chemical Industry     

Influence of multiple herbicide resistance on growth in Amaranthus tuberculatus

Plant defence traits, such as herbicide resistance mutations, may incur a fitness cost to plants that become evident when the trait is not needed. However, individuals with multiple herbicide resistance traits may decrease fitness beyond that of plants with a single herbicide resistance mutation. Multiple herbicide‐resistant (MHR) Amaranthus tuberculatus populations are becoming more prevalent in Midwest United States agroecosystems. The objective was to determine whether selected MHR A. tuberculatus populations express differential development when grown in a herbicide‐free environment. The hypothesis was that MHR A. tuberculatus populations become increasingly less fit when additional herbicide resistances evolve. Multiple herbicide‐resistant and herbicide‐susceptible A. tuberculatus populations were grown in a herbicide‐free field for 20 weeks for two seasons. Differences (P < 0.001) in apical growth were detected 5 and 7 weeks after transplanting for all populations in 2016 and 2017 respectively. Gender and population influenced (P < 0.001) flowering date, with males flowering up to 1.5 weeks earlier than females, but did not cause pollination asynchrony. Shoot biomass was not different (P = 0.84) across A. tuberculatus populations, but there were differences (P < 0.001) for gender and year. Seed production was different amongst A. tuberculatus populations (P = 0.001), but was not influenced by the number of MHR traits. Conversely, a negative quadratic relationship between seed mass and the number of MHR traits was observed (r² = 0.32; P < 0.001). The experiment results demonstrate that MHR in A. tuberculatus populations is not incurring a fitness penalty that will remove the populations in the immediate future.     

Extended leaf phenology presents an opportunity for herbicidal control of invasive forest shrubs

For many of the shrub species invading temperate deciduous forests, extended leaf phenology contributes substantially to annual carbon gains, helping to make possible rapid growth and spread. We carried out a pair of proof‐of‐concept studies to evaluate the susceptibility of such shrubs to foliar herbicide treatment during the period of delayed senescence, i.e. well after it is typically attempted. We first evaluated leaf‐level physiology in four species that lose leaves late in autumn; photosynthetic rates were comparable (80–111%) to those reported for the same species in summer. While preliminary, this finding provides a strong indication that diverse shrub taxa remain susceptible to control into late autumn. In addition, we conducted a field trial involving one of these species (Lonicera maackii) to directly evaluate the effectiveness of foliar treatment in November; applications included glyphosate and two concentrations of aminocyclopyrachlor plus metsulfuron methyl. Treatments killed (72%) or severely injured (14%) target plants, inducing greater cambial damage in plants that retained greater fractions of their canopy or had smaller canopy widths; there were no statistically significant differences among application types. This study demonstrated that late autumn can be a viable period in which to treat weedy species that delay senescence strongly, such as the many invasive Lonicera species in North America. Given that climate change and urbanisation are further delaying senescence in invasive plant populations, our study serves as a call for further investigation into what promises to be an increasingly viable opportunity for weed control in deciduous forests.     

Resistance Mechanism of Propanil-Resistant Barnyardgrass: II. In-vivo Metabolism of the Propanil Molecule

Propanil-resistant barnyardgrass populations, previously verified in Arkansas rice fields and in greenhouse tests, were examined in the laboratory to ascertain if the resistance mechanism in this weed biotype was herbicide metabolism. Propanil-resistant barnyardgrass was controlled >95% in the greenhouse when carbaryl (an aryl acylamidase inhibitor) was applied two days prior to propanil. Laboratory studies with ¹⁴C-radiolabelled propanil indicated that the herbicide was hydrolysed in propanil-resistant barnyardgrass and rice to form 3,4-dichloroaniline, but no detectable hydrolysis occurred in susceptible barnyardgrass. Two additional polar metabolites were detected in propanil-resistant barnyardgrass and rice and tentatively identified by thin layer chromatography. Overall, metabolites in the resistant barnyardgrass had R_f values similar to those in rice, indicating similar metabolism for both species. These data, coupled with data from a previous report on the resistant biotype showing no differential absorption/translocation or molecular modification of the herbicide binding site in the resistant biotype, indicate that the resistance mechanism is metabolic degradation of propanil. © of SCI.     

Waterhemp (Amaranthus tuberculatus) control under drought stress with 2,4‐dichlorophenoxyacetic acid and glyphosate

Waterhemp (Amaranthus tuberculatus) is a common and troublesome weed in cropping systems throughout the United States. With the potential for future periods of low rainfall or drought, the need for improved weed control under drought stress is necessary. Drought stress typically reduces herbicide efficacy by reducing the foliar uptake of herbicides and their translocation. The objectives of this research were to determine the efficacy of 2,4‐dichlorophenoxyacetic acid (2,4‐D) and glyphosate, applied alone or when tank‐mixed, on waterhemp under varying levels of drought stress, the effect of the timing of drought stress in relation to herbicide application and the absorption and translocation of each herbicide in drought‐stressed waterhemp. At reduced herbicide rates, 2,4‐D had a greater level of control of waterhemp under drought stress, compared to glyphosate. The level of herbicide efficacy was lower when the amount of water that was applied to the plants was reduced. The level of waterhemp control was greatest when drought stress occurred before the herbicide application and when the plants were watered to saturation after the application, compared to when drought stress occurred after the herbicide application or restricted watering levels occurred throughout the entire study. Glyphosate absorption and translocation were reduced in the drought‐stressed plants, but 2,4‐D absorption and translocation were not altered. The absence of a reduction in 2,4‐D translocation in the drought‐stressed weeds has not been previously reported. Applying herbicides prior to a rainfall event could increase the weed control level, even if the weed is stressed. Determining how and why 2,4‐D absorption and translocation levels, compared to those of glyphosate, are unaffected by drought stress in waterhemp can aid in improving the control of drought‐stressed weeds with other postemergence herbicides.     

Absorption and fate of imazapyr in leafy spurge (Euphorbia esula)

Imazapyr absorption, translocation, root release and metabolism were examined in leafy spurge (Euphorbia esula L.). Leafy spurge plants were propagated from root cuttings and [¹⁴C]imazapyr was applied to growth-chambergrown plants in a water + 28% urea ammonium nitrate + nonionic surfactant solution (98.75 + 1 + 0.25 by volume). Plants were harvested two and eight days after herbicide treatment (DAT) and divided into: treated leaf, stem and leaves above treated leaf, stem and leaves below the treated leaf, crown, root, dormant and elongated adventitious shoot buds. Imazapyr absorption increased from 62.5% 2 DAT to 80.0% 8 DAT. Herbicide translocation out of the treated leaf and accumulation in roots and adventitious shoot buds was apparent 2 DAT. By the end of the eight-day translocation period only 14% of applied ¹⁴C remained in the treated leaf, while 17% had translocated into the root system. Elongated and dormant adventitious shoot buds accumulated 3.2- and 1.8-fold more ¹⁴C, respectively, 8 DAT than did root tissue based on Bq g^?1 dry weight. Root release of ¹⁴C was evident 2 DAT, and by 8 DAT 19.4% of the ¹⁴C reaching the root system was released into the rooting medium. There was no metabolism of imazapyr in crown, root or adventitious shoot buds 2 DAT; however, imazapyr metabolism was evident in the treated leaf 2 and 8 DAT. Imazapyr phytotoxicity to leafy spurge appears to result from high imazapyr absorption, translocation to underground meristematic areas (roots and adventitious shoot buds), and a slow rate of metabolism.     

Patterns of defoliation and their effect on the plant growth and photosynthetic characteristics of Ipomoea cairica

In order to determine the susceptibility of Ipomoea cairica to herbivory, the compensatory growth and photosynthetic characteristics of I. cairica plants were measured after simulated herbivory by leaf trimming in three patterns: leaf‐apex removal, leaf‐edge removal, and perforation. The leaf‐edge removal resulted in a significantly reduced total biomass and root biomass of the plants, but the leaf‐apex removal and perforation had no significant influence on the plant growth. The defoliation patterns had significant effects on the photosynthesis of I. cairica. The net photosynthetic rate and stomatal conductance of the plants whose leaf edges had been removed were the highest among the three defoliation patterns and the fraction of absorbed light that is used in Photosystem II photochemistry increased greatly, while the fraction of light energy that is dissipated thermally decreased. The increased photosynthetic rate as a result of the leaf‐edge removal treatment could be attributed to a decrease in stomatal limitation and an increase in the Rubisco content, as well as higher photosynthetic efficiency and less light energy being dissipated as heat. Increased photosynthesis in the plants whose leaf edges had been removed changed the carbon allocation and resulted in less root development. As the expansion of I. cairica primarily depends on clonal growth, smaller roots could limit its uptake of nutrients from the soil. These direct and indirect effects indicate that leaf‐edge‐feeding herbivores could have potential in the biological control of I. cairica.