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A wild radish population (R) has been recently confirmed to be cross-resistant to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides without previous exposure to these herbicides. This cross-resistance is endowed by enhanced metabolism. Our study identified one 2-oxoglutarate/Fe(II)-dependent dioxygenase gene (Rr2ODD1) and two P450 genes (RrCYP704C1and RrCYP709B1), which were significantly more highly expressed in R versus susceptible (S) plants. Gene functional characterization using Arabidopsistransformation showed that overexpression of RrCYP709B1conferred a modest level of resistance to mesotrione. Ultra-performance liquid chromatography–tandem mass spectrometry analysis showed that tissue mesotrione levels in RrCYP709B1 transgenic Arabidopsisplants were significantly lower than that in the wild type. In addition, overexpression of Rr2ODD1or RrCYP704C1in Arabidopsisendowed resistance to tembotrione and isoxaflutole. Structural modeling indicated that mesotrione can bind to CYP709B1 and be easily hydroxylated to form 4-OH-mesotrione. Although each gene confers a modest level of resistance, overexpression of the multiple herbicide-metabolizing genes could contribute to HPPD-inhibiting herbicide resistance in this wild radish population.

The introduction of harvest weed seed control (HWSC) techniques and associated machinery has enabled the routine use of an alternative weed control technology at a novel weed control timing in global grain cropping fields. Driven by the significant threat of widespread populations of annual ryegrass (Lolium rigidum ) with multiple-herbicide resistance, in the 1990s Australian growers and researchers developed techniques to target, at grain harvest, the seed production of annual ryegrass and other important weed species. The HWSC approach to weed management is now routinely used by a majority of Australian grain producers as an integral component of effective weed control programs. Here we detail the development and introduction of current HWSC systems and describe their efficacy in Australian grain production systems. The use of HWSC has likely contributed to lower annual ryegrass population densities and thus mitigated the impacts of herbicide resistance as well as slowing further evolution of resistance. In addition, low weed densities enable the introduction of site-specific weed control technologies and the opportunity to target specific in-crop weeds with non-selective alternative weed control techniques. With an awareness of the evolutionary potential of weed species to adapt to all forms of weed control, there is an understanding that HWSC treatments need to be judiciously used in grain cropping systems to ensure their ongoing efficacy. The successful use of Australian developed HWSC systems has attracted global interest and there is now a considerable international research effort aimed at introducing this alternative weed control approach and timing into the world’s major cropping systems.

Weeds are a major contributing factor to crop yield loss. Weed control is regularly practiced during the growing season, with many growers making a conscious effort to minimise weed-seed return to the soil seedbank during the cropping program. However, growers may be unintentionally introducing weed seeds through sowing of contaminated crop seed. Using samples of crop seed obtained from 29 growers across two Western Australian grain-growing regions, 81 samples were hand-cleaned to determine weed-seed contamination levels. Of those samples, 41% were weed-free, and in the remaining 59%, the main contaminant was Lolium rigidum (annual ryegrass), occurring in 49% of contaminated samples. Crop type and cleaning method had significant effects on the level of weed-seed contamination, with barley having higher levels of contamination than other crops, and professional contractors providing lower contamination than other methods of cleaning. However, any seed-cleaning method provided significantly cleaner grain samples than no seed cleaning. This study established that crop-seed contamination was evident on Western Australian farms and that growers may be unintentionally sowing weed seeds with their crops. Seed cleaning combined with judicious paddock selection and weed-seed removal during the growing season can lead to weed-free crop seed.

There have been many studies on target-site resistance (TSR) to PSII-inhibiting herbicides, but only a few on the non-target-site resistance (NTSR). Here, we reported both TSR and NTSR to metribuzin in a wild radish population. Dose–response studies revealed a higher level of resistance to metribuzin in the resistant (R) compared to the susceptible (S) population. Sequencing of the target psbAgene revealed the known Ser-264-Gly mutation in R plants. In addition, a higher level of [14C]-metribuzin metabolism and, consequently, a lower level of [14C] translocation were also detected in the R plants. These results demonstrated that both psbAgene mutation and enhanced metabolism contribute to metribuzin resistance in this wild radish population. Furthermore, this resistant population showed resistance to ALS-inhibiting herbicides due to multiple ALSgene mutations. This is the first report in wild radish of metabolic herbicide resistance, in addition to the target-site psbAgene mutation.

Synthetic auxin herbicides, such as 2,4-dichlorophenoxyacetic acid (2,4-D), are widely used for selective control of broadleaf weeds in cereals and transgenic crops. Although the troublesome weed wild radish (Raphanus raphanistrum) has developed resistance to 2,4-D, no populations have yet displayed an enhanced capacity for metabolic detoxification of the herbicide, with both susceptible and resistant wild radish plants readily metabolizing 2,4-D. Using mass spectrometry and nuclear magnetic resonance, the major 2,4-D metabolite was identified as the glucose ester, and its structure was confirmed by synthesis. As expected, both the endogenous and synthetic compounds retained auxin activity in a bioassay. The lack of detectable 2,4-D hydroxylation in wild radish and the lability of the glucose ester suggest that metabolic 2,4-D resistance is unlikely to develop in this species.

This study confirmed the first case of glyphosate resistance in Tridax procumbensand investigated the glyphosate-resistance mechanisms. Sequencing and cloning of the full 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) coding sequences revealed a point mutation (ACC to TCC) at amino acid position 102, resulting in a novel Thr-102-Ser substitution. Other possible resistance mechanisms (i.e., target-site EPSPS-gene overexpression, nontarget-site differential glyphosate uptake and translocation) were also examined and were unlikely to be involved in resistance in this population. Structural modeling of the wild-type and mutant EPSPS in complex with glyphosate and phosphoenolpyruvate (PEP) revealed that the Thr-102-Ser substitution weakly decreased EPSPS affinity to glyphosate, but sharply increased EPSPS affinity to the natural substrate, PEP. Therefore, this novel mutation is very likely responsible for the observed glyphosate resistance in this tetraploid weed species via dual mechanisms of reducing glyphosate binding and favoring PEP binding to EPSPS.

Resistance to the pre-emergence herbicide trifluralin is increasing in Australian annual ryegrass (Lolium rigidum) populations. Three L. rigidumpopulations (R1, R2, and R3) collected from Australian grain fields were identified with trifluralin resistance. Both target-site and nontarget-site resistance mechanisms were investigated. No target-site α-tubulin mutations were detected in populations R1 and R3, while an Arg-243-Lys mutation was found in R2. Compared with the three trifluralin-susceptible populations, enhanced [14C]-trifluralin metabolism, quantified by measuring the amount of [14C] label partitioning into the polar phase of a hexane:methanol system, was identified in all the three resistant populations. This is the first report of metabolic resistance to trifluralin. Coevolution of target-site and nontarget-site resistance to trifluralin is occurring, and metabolic resistance is not rare in L. rigidumpopulations in Australia. A method was established for trifluralin metabolic resistance detection, overcoming the difficulties of quantifying this highly volatile herbicide by chromatographic methods.

AbstractIn Australia, widespread evolution of multi-resistant weed populations has driven the development and adoption of harvest weed seed control (HWSC). However, due to incompatibility of commonly used HWSC systems with highly productive conservation cropping systems, better HWSC systems are in demand. This study aimed to evaluate the efficacy of the integrated Harrington Seed Destructor (iHSD) mill on the seeds of Australia’s major crop weeds during wheat chaff processing. Also examined were the impacts of chaff type and moisture content on weed seed destruction efficacy. Initially, the iHSD mill speed of 3,000 rpm was identified as the most effective at destroying rigid ryegrass seeds present in wheat chaff. Subsequent testing determined that the iHSD mill was highly effective (>95% seed kill) on all Australian crop weeds examined. Rigid ryegrass seed kill was found to be highest for lupin chaff and lowest in barley, with wheat and canola chaff intermediate. Similarly, wheat chaff moisture reduced rigid ryegrass seed kill when moisture level exceeded 12%. The broad potential of the iHSD mill was evident, in that the reductions in efficacy due to wide-ranging differences in chaff type and moisture content were relatively small (≤10%). The results from these studies confirm the high efficacy and widespread suitability of the iHSD for use in Australian crop production systems. Additionally, as this system allows the conservation of all harvest residues, it is the best HWSC technique for conservation cropping systems.

Palmer amaranth is the most troublesome weed problem in mid-southern US crop production. Herbicides continue to be the most commonly employed method for managing Palmer amaranth, despite the weed’s widespread resistance to them. Therefore, farmers need research and extension efforts that promote the adoption of integrated weed management (IWM) techniques. Producers, crop consultants, educators, and researchers would be more likely to deploy diversified chemical and nonchemical weed management options if they are more informed about long-term biological and economic implications via user-friendly decision-support software. Described within is a recently developed software that demonstrates the effects of Palmer amaranth management practices on soil seedbank, risk of resistance evolution, and economics over a 10-year planning horizon. Aiding this objective is a point-and-click interface that provides feedback on resistance risk, yield potential, profitability, soil seedbank dynamics, and error checking of management options.

Avena is a problematic weed of cropping regions of southern Australia and many areas of the world. In 2010, a random survey was conducted across 14million hectares of the Western Australian grain belt to monitor the change in herbicide resistance levels by comparing resistance frequency results with a survey conducted in 2005. Screening Avena populations with herbicides commonly used to control this weed revealed that 48% of Avena populations displayed resistance to the commonly used acetyl-Co A carboxylase-inhibiting herbicides, which was lower than that found in 2005 (71%). The broad-spectrum herbicides glyphosate and paraquat provided good control of all Avena populations. Resistance to acetolactate synthase-inhibiting herbicides and to flamprop were detected for the first time in Western Australia in this survey. Therefore, a wide range of weed management options that target all phases of the cropping program are needed to sustain these cropping systems in the future.

Raphanus raphanistrum is a problematic weed, which has become increasingly difficult to control in Australian cropping regions. In 2010, a random survey was conducted across 14million ha of the Western Australian grain belt to establish the frequency of herbicide resistance in R. raphanistrum and to monitor the change in resistance levels by comparing results with a previous survey in 2003. Screening R. raphanistrum populations with herbicides commonly used to control this weed revealed that most populations (84%) contained individual plants resistant to the acetolactate synthase-inhibiting herbicide chlorsulfuron, whereas 49% of populations also had plants resistant to the imidazolinone herbicides. Resistance to other mode of action herbicides (2,4-D (76%) and diflufenican (49%)) was also common. Glyphosate, atrazine and pyrasulfotole+bromoxynil remained effective on most R. raphanistrum populations. These results demonstrate that resistance to some herbicides has increased significantly since 2003 when the values were 54% for chlorsulfuron and 60% for 2,4-D; therefore, a wide range of weed management options that target all phases of the cropping program are needed to sustain these cropping systems in the future.

Transgenic glyphosate-resistant canola was first commercially grown in Western Australia (WA) in 2010, providing an opportunity to obtain important baseline data regarding the level of glyphosate resistance in weeds following the exclusive use of glyphosate for in-crop weed control. In this study, two surveys (2010 and 2011) were conducted across the 14Mha of the grainbelt of WA. The 2010 survey was carried out at the late-flowering stage of glyphosate-resistant canola, whereas the 2011 survey was conducted at an earlier growth stage (6–8 leaves), ~2–3 weeks after the second in-crop glyphosate application. During the surveys, 239 fields were visited, representing an estimated combined area of 24000ha. The 2011 survey alone represented a subsample of 23% of the total glyphosate-resistant canola planting in the WA grainbelt for that season. Glyphosate resistance was identified in one population of wild radish (Raphanus raphanistrum L.) and in eight annual ryegrass (Lolium rigidum L.) populations. None of the tested capeweed (Arctotheca calendula (L.) Levyns) populations were glyphosate-resistant. In this survey, no populations of barley grass (Hordeum spp.), brome grass (Bromus spp.), wild oat (Avena spp.) or small-flowered mallow (Malva parviflora L.) survived glyphosate application. Despite a long history of pre-seeding and fallow glyphosate use in WA, this survey found that glyphosate still provides excellent in-crop control of most species; however, some resistance is evident, requiring diverse weed control techniques to limit their spread.

Random surveys conducted in the Western Australian (WA) grain belt have shown that herbicide-resistant Lolium rigidum and Raphanus raphanistrum are a widespread problem across the cropping region. In 2010, a random survey was conducted to establish the levels of herbicide resistance for common weed species in crop fields, including the minor but emerging weeds Bromus and Hordeum spp. This is the first random survey in WA to establish the frequency of herbicide resistance in these species. For the annual grass weed Bromus, 91 populations were collected, indicating that this species was present in >20% of fields. Nearly all populations were susceptible to the commonly used herbicides tested in this study; however, a small number of populations (13%) displayed resistance to the acetolactate synthase-inhibiting sulfonylurea herbicides. Only one population displayed resistance to the acetyl-coenzyme A carboxylase-inhibiting herbicides. Forty-seven Hordeum populations were collected from 10% of fields, with most populations being susceptible to all herbicides tested. Of the Hordeum populations, 8% were resistant to the sulfonylurea herbicide sulfosulfuron, some with cross-resistance to the imidazolinone herbicides. No resistance was found to glyphosate or paraquat, although resistance to these herbicides has been documented elsewhere in Australia for Hordeum spp. (Victoria) and Bromus spp. (Victoria, South Australia and WA).

Global grain production is under threat from the escalating evolution of herbicide‐resistant weed populations. Worldwide, herbicide‐reliant grain crop production systems have driven the proliferation of herbicide resistant populations of major weed species. Alternatives or adjuncts to herbicides are needed for sustainable control of crop weeds in grain crops worldwide. Here we introduce and prove the ability of a new weed control tool, the Harrington Seed Destructor (HSD), to intercept and destroy weed seeds during grain crop harvest. The interception and destruction of weed seeds exiting the grain harvester in the chaff fraction during grain crop harvest is a hitherto unrealized opportunity. We have developed a cage mill‐based chaff processing unit that consistently destroys weed seed infesting grain crop chaff fractions. The subsequent construction of the HSD incorporating this unit had >95% weed seed destruction efficacy when used during commercial harvest of three major grain crops, wheat (Triticum aestivumL.), barley (Hordeum vulgareL.), and lupin (Lupinus angustifoliusL.). The HSD system has the potential for a dramatic impact on global grain production by providing the unique combination of effective weed control with complete retention of grain crop harvest residues. Only herbicides offer this same combination of highly effective weed control and complete residue retention in large scale grain crop production systems.

Narrow-leafed lupin (Lupinus angustifolius L.) is an important grain legume crop in Australia. Metribuzin is an important herbicide used to control weeds in lupin crops. This study investigated metribuzin tolerance mechanism in narrow-leafed lupin by comparing two induced mutants (Tanjil-AZ-33 and Tanjil-AZ-55) of higher metribuzin tolerance with the susceptible wild type. Sequencing of the highly conserved region of the chloroplast psbA gene (target site) revealed that the sequences of the wild type and the mutants were identical and therefore metribuzin tolerance is not target site based. Photosynthetic activity was measured and the leaf photosynthesis of the two tolerant mutants was initially inhibited after metribuzin treatment, but recovered within 2.5 days whereas that of the susceptible plants remained inhibited. The photosynthetic measurements confirmed the target site chloroplast was susceptible and the tolerance mechanism is non-target site based. Investigation with known cytochrome P450 monooxygenase inhibitors (omethoate, malathion and phorate) showed that tolerance could be reversed in both mutants, indicating the tolerance mechanism in two tolerant mutants may involve cytochrome P450 enzymes. Interestingly, the inhibitor tridiphane reversed metribuzin tolerance of only one of the two tolerant mutants, indicating diversity in metribuzin tolerance mechanisms in narrow-leafed lupin. These results signify that further investigation of metribuzin metabolism in these plants is warranted. In conclusion, metribuzin tolerance mechanism in lupin mutants is non-target site based, likely involving P450-mediated metribuzin metabolism.

In 2005, a random survey was conducted across 14million hectares of the Western Australian grain belt to establish the frequency and distribution of herbicide-resistant wild oat (Avenaspp.) in cropping fields. In total, 677 cropping fields were visited, with wild oat populations collected from 150 fields. These wild oat populations were screened with several herbicides commonly used to control this weed. Most of the wild oat populations (71%) were found to contain individuals resistant to the ACCase-inhibiting herbicide diclofop-methyl. Resistance to other ACCase-inhibiting herbicides was markedly lower. Herbicides of alternative modes of action were effective on all wild oat populations. Overall, wild oat resistance to diclofop-methyl was found to be widespread across the Western Australian grain belt, but resistance to other herbicides was relatively low. Therefore, through diversity in herbicide use and with cultural management, it is possible to maintain wild oat populations at a low level and/or minimise herbicide resistance evolution.

Crop-topping, the practice of applying non-selective herbicides at crop maturity, has proved to be an effective management technique in preventing the input of seed into the seedbank for some annual weed species of southern Australian crop production systems. However, the efficacy of this practice on the dominant broad-leaf weed of these systems, wild radish, is not well understood. These studies investigated the effect of crop-topping and swathing on the viable seed production of wild radish. Crop-topping with either glyphosate or sprayseed (paraquat 135g/L+diquat 115g/L) can provide large reductions of 80–90% in viable seed production of wild radish plants present in crops at the end of the growing season. However, the efficacy of this practice was found to be highly variable and therefore, cannot be relied upon to consistently produce these large reductions in seed numbers. Similarly, swathing also produced large reductions in viable seed production but results from this practice were even less consistent than crop-topping treatments. For all treatments, early application timings of growth stage 6.5 or earlier, were optimum for targeting wild radish seed production. However, these treatment timings also resulted in large crop yield losses of ~30%. To preserve at least 90% of crop yield, crop-topping and swathing treatments need to be delayed until wild radish growth stage 8.5, with expected reductions in seed numbers of up to 70%. However, in high-density infestations the need to preserve grain yield will be less important than preventing substantial inputs of wild radish seed into the seedbank.

In 2003, a random survey was conducted across the Western Australian wheatbelt to establish the frequency and distribution of herbicide resistance in ryegrass populations infesting crop fields. Five hundred cropping fields were visited at crop maturity, and ryegrass seed was collected in 452 of these fields. Subsequently, each crop field population was screened with herbicides of various modes of action that are commonly used for ryegrass control in Australian cropping systems. Most of these ryegrass populations were found to be resistant to the ACCase-inhibitor herbicide diclofop-methyl (68%) and the ALS-inhibitor herbicide sulfometuron (88%). A comparison of resistance levels in the same agronomic zones surveyed 5 years earlier determined that there had been an increase of 20 percentage points in the frequency of resistance over this 5-year period. This survey also determined that the majority (64%) of populations were found to be multiple resistant to both diclofop-methyl and sulfometuron. The distribution patterns of the collected populations indicated that there were higher frequencies of resistant and developing resistance populations occurring in the intensively cropped regions of the wheatbelt, which had greater herbicide selection pressure. Of concern is that 24% and 8% of populations were found to be developing resistance to trifluralin and clethodim, respectively. Currently these herbicides are heavily relied upon for control of ACCase and ALS herbicide resistant ryegrass. Nearly all populations remain susceptible to glyphosate. Ryegrass across the WA wheatbelt now exhibits multiple resistance across many but not all herbicides, posing severe management and sustainability challenges.

Experiments were conducted to investigate the effect of Lolium rigidum (annual ryegrass) seed developmental stage and application rate of glyphosate and SpraySeed (paraquat 135g/L + diquat 115g/L) on the number, germinability, and fitness of seeds produced. Glyphosate (450g/L) was most effective when applied at a rate of 0.5?1L/ha during heading and anthesis, reducing the number of filled seeds produced compared with unsprayed plants. Application post-anthesis, when seeds were at the milk to soft dough stage, was less effective. SpraySeed was most effective when applied post-anthesis, during the milk and early dough stages of seed development at a rate of 0.5?1L/ha, resulting in the production of few viable seeds. Although some filled seeds were produced, most of the seeds were dead. Application during anthesis or once the seeds reached soft dough stage was less effective. For both herbicides, those seeds that were capable of germinating were smaller and had slower radicle and coleoptile growth, resulting in slower early seedling growth and reduced biomass production within the first month of growth. Additionally, glyphosate application reduced the proportion of seeds exhibiting dormancy. The anticipated reduction in seed competitive ability and altered emergence timing resulting from late-season herbicide application, even when application timing is not optimal, could be exploited to reduce the likelihood of successful L. rigidum establishment in the following season.

Failure of the herbicide carbetamide to adequately control annual grasses upon repeated annual application was linked to rapid degradation of this herbicide in soil. Rapid carbetamide degradation in soil was clearly evident following application at normal field rates and when evaluated at time intervals reflecting normal agronomic use. The DT50of carbetamide in soil was reduced from 54 to 9days when evaluated one year after a single carbetamide application to previously untreated soil. Rapid carbetamide degradation was transferred to previously untreated soil by addition of 1% rapid carbetamide-adapted soil and was inhibited following pre-treatment with the broad-spectrum antibiotic chloramphenicol. In carbetamide-adapted soil mineralisation of [14C-ring] labelled carbetamide to 14CO2occurred at a faster rate than with previously untreated soil. It was concluded that rapid carbetamide degradation upon repeated application is probably linked to the adaptation of soil bacteria which can utilise carbetamide as a source of carbon and energy.

Three Australian Sisymbrium orientale and one Brassica tournefortii biotypes are resistant to acetolactate synthase (ALS)-inhibiting herbicides due to their possession of an ALS enzyme with decreased sensitivity to these herbicides. Enzyme kinetic studies revealed no interbiotypic differences within species in Km (pyruvate) (the substrate concentration at which the reaction rate is half maximal) but a greater Vmax (the rate when the enzyme is fully saturated with substrate) for two of the resistant S orientale biotypes over susceptible levels. F1 hybrids from reciprocal crosses between resistant and susceptible biotypes of S orientale showed an intermediate response to chlorsulfuron compared to the parental plants. ALS herbicide resistance in S orientale segregated in a 3:1 (resistant:susceptible) ratio in F2 plants with a single rate of chlorsulfuron, indicating that resistance is inherited as a single, incompletely dominant nuclear gene. Two regions of the ALS structural gene known to vary in ALS-resistant biotypes were amplified and sequenced. Resistant S orientale biotypes NS01 and SS03 contained a single nucleotide substitution in Domain B, predicting a Trp (in susceptible) to Leu (in resistant) amino acid change. Two adjacent nucleotide substitutions (CCT to ATT) predicting a Pro (in susceptible) to Ile (in resistant) change in the primary amino acid sequence were identified in Domain A of resistant S orientale biotype SS01. Likewise, a single nucleotide substitution at the same site in the resistant B tournefortii biotype predicts a Pro (in susceptible) to Ala (in resistant) substitution. No other interbiotypic nucleotide differences predicted amino acid changes in the sequenced regions, suggesting that the amino acid substitutions reported above are responsible for resistance to ALS-inhibiting herbicides in the respective biotypes. © 1999 Society of Chemical Industry

Biotype SLR 31 ofLolium rigidumhas developed resistance and cross-resistance to a wide range of herbicides. Herbicide resistance in SLR 31 has been demonstrated to be the result of multiple mechanisms of resistance. Despite much speculation, the mechanisms endowing resistance to the acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicide diclofop-methyl have not been fully elucidated. Two subsets of this population were isolated based on root length of individuals germinated on agar containing 3 μM of the ACCase-inhibiting herbicide fluazifop-P-butyl. Those individuals with normal root length were highly resistant to all ACCase-inhibiting herbicides and contained a herbicide-resistant target enzyme, ACCase. Those individuals with highly stunted roots were resistant to diclofop-methyl and fluazifop-P-butyl, but not to sethoxydim, and contained a herbicide-sensitive ACCase. Biotype SLR 31 also has enhanced metabolism of diclofop acid. Amitrole, a non-ACCase-inhibiting herbicide, was demonstrated to be an inhibitor of diclofop acid metabolism in both SLR 31 and the susceptible biotype and synergised the effect of diclofop-methyl on both biotypes. This demonstrates that enhanced diclofop acid metabolism can confer substantial resistance to diclofop-methyl inL. rigidum.In contrast, amitrole was unable to synergise the effect of chlorsulfuron, an acetolactate synthase-inhibiting herbicide, to which SLR 31 is resistant also due to enhanced herbicide metabolism. Amitrole also did not inhibit chlorsulfuron metabolism in SLR 31. Resistance to diclofop-methyl in biotype SLR 31 is due to at least two mechanisms: enhanced metabolism, which provides 26-fold resistance, and a less sensitive target site, which provides 9-fold resistance. When the two mechanisms are combined, individuals are 239-fold resistant compared to the susceptible population.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.

Earlier studies from our laboratory strongly suggested that a single or similar cytochrome P450 isoform(s) isolated from microsomes of wheat catalyze the oxidation of the medium-chain fatty acid, lauric acid, and the wheat selective herbicide diclofop. Lauric acid is mainly hydroxylated at the subterminal position C11 (ω-1) and for that reason P450-dependent reactions were initially designated lauric acid (ω-1)-hydroxylase ((ω-1)-LAH). This report presents data on thein vitroandin vivoirreversible inhibition of both lauric acid and diclofop oxidation by mechanism-based inhibitors targeting lauric acid hydroxylation. Incubation of microsomes from etiolated wheat seedlings with 10-dodecynoic acid (10-DDYA) produces a dramatic inhibition of lauric acid hydroxylation. The inhibition is both time- and concentration-dependent in a process typical for mechanism-based inhibitors. A half-life of 3 min and an apparent inhibition constant of 14 μMwere determined from pseudo-first order kinetic studies of (ω-1)-LAH inhibition. Similar results were obtained by incubating microsomes with a terminal acetylene, 11-dodecynoic acid (11-DDYA). Based on results ofin vitroexperiments we have developed a series of new mechanism-based inhibitors to inhibit diclofop metabolism in developing wheat seedlings. To protect the inhibitors from degradation by α- and β-oxidation systems, water-soluble sodium salts of undec-10-yne-1-sulfonic acid (10-UDYS), undec-9-yne-1-sulfonic acid (9-UDYS), and undecan-1-sulfonic acid (SULAUR) were synthesized. Following treatment of wheat coleoptiles for 6 hr with these modified inhibitors, diclofop and chlortoluron metabolism was measured. Both compounds selectively inhibited diclofop metabolism without affecting chlortoluron metabolism. The importance of triple bonds in the inhibition process is clearly demonstrated by the fact that SULAUR had very small inhibitory effects. A 100 μMconcentration of both inhibitors resulted in about 50% inhibition of diclofop metabolism. Increasing the concentration of inhibitors to 2.5 mMincreased inhibition of diclofop oxidation to about 90%. Our results clearly show that one or similar forms of cytochrome P450 that are naturally involved in lauric acid oxidation are responsible for diclofop hydroxylation.

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO2 uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (Fv,692) while the instantaneous fluorescence (Fo,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as FM,692 and Fv,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO2 pressure or decreased O2 pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.

A biotype ofLolium rigidumGaudin (VLR 69) shows multiple resistance to at least nine dissimilar herbicide chemistries. This biotype has enhanced metabolism to herbicides that inhibit Photosystem II, acetolactate synthase, and acetyl-coenzyme A carboxylase. The potential for malathion and piperonyl butoxide (PBO) to act as synergists for herbicides from five different chemical classes was investigated in potted plants. Of the herbicide/synergist combinations examined, PBO only synergized chlorotoluron, and malathion only synergized chlorsulfuron in this resistant biotype. The ability of PBO, malathion, 1-aminobenzotriazole (ABT), and tetcyclacis to inhibit metabolism of herbicidesin vivowas also examined. ABT, PBO, and tetcyclacis inhibited metabolism of simazine and chlorotoluron, but malathion did not. Malathion alone inhibited metabolism of chlorsulfuron, ABT inhibited metabolism of diclofop, and none of these compounds affected tralkoxydim metabolism. These results suggest that at least four different herbicide-metabolizing enzymes have increased activity in this resistant biotype. In addition to enhanced metabolism, this biotype also contains a resistant form of acetyl-coenzyme A carboxylase which shows 31-, 4-, and 20-fold resistance to diclofop acid, fluazifop acid, and haloxyfop acid, respectively, but which shows no resistance to sethoxydim or tralkoxydim. Multiple resistance in this biotype ofL. rigidumis clearly the result of the accumulation of several resistance mechanisms.

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO2-saturated rate of CO2 uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (FM) and the variable (FV) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (FO) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.

An Avena sterilis biotype was found to be highly resistant to aryloxyphenoxypropionate (APP) herbicides, especially diclofop-methyl. At the enzyme level, this biotype contained a modified acetyl-coenzyme A carboxylase (ACCase) with six-fold resistance to diclofop acid. Absorption and translocation of [¹⁴C]diclofop-methyl applied to the leaf axil of the two-leaf stage plants were similar in both susceptible and resistant biotypes. However, the rate of metabolism of [¹⁴C]diclofop was increased 1·5-fold in this resistant biotype compared to the susceptible. Experiments with tetcyclacis, a cytochrome P450 monooxygenase inhibitor, indicated that inhibition of this enhanced diclofop metabolism increased diclofop-methyl phytotoxicity in this biotype. Studies with ten individual families of the resistant biotype indicated that both mechanisms of resistance, an altered target site and enhanced metabolism, are present in each individual of the population. Hence, it is likely that these two mechanisms of resistance both contribute to resistance in this biotype. © 1997 SCI.

The article provides details on the Global Herbicide Resistance Challenge Conference to be held in Fremantle, Western Australia on February 18-22, 2013, a gathering of experts from around the world aiming to address the threat posed by the evolution of herbicide-resistance weeds to global crop production. Among the speakers presenting during the event is Dr. Jason Norsworthy, who will discuss how diversity is crucial to the success of weed management programs in the U.S. and Australia.

It is likely that transgenic canola expressing genes encoding resistance to glyphosate and glufosinate ammonium will be introduced into the Australian cropping system in the next few years. One risk associated with the introduction of such cultivars is the release of herbicide resistance genes into weedy relatives of canola. This review examines the currently available experimental information regarding the possibility of gene flow from canola to weedy relatives. Three species are identified as having the potential to outcross with canola, Brassica juncea, B. rapa, and Raphanus raphanistrum. Two of these species are not yet widespread weeds of the southern Australian cropping zone. In contrast, R. raphanistrum is already a major weed in Australia with existing resistance to ALS-inhibiting herbicides. Information is urgently needed to determine whether successful hybrids between B. napus and R. raphanistrumcan be produced under Australian conditions. Major deficiencies in the existing information are identified in relation to some other important weed species within the southern Australian cropping zone. Further studies are required to determine the out-crossing potential of canola to B. tournefortii, Diplotaxis tenuifolia, Sisymbrium officinale, and S. orientale if transgenic canola is to be safely and responsibly introduced into Australia.

A biotype of Avena sterilis ssp. ludoviciana is highly resistant to a range of herbicides which inhibit a key enzyme in fatty acid synthesis, acetyl-CoA carboxylase (ACCase). Possible mechanisms of herbicide resistance were investigated in this biotype. Acetyl-CoA carboxylase from the resistant biotype is less sensitive to inhibition by herbicides to which resistance is expressed. I50 values for herbicide inhibition of ACCase were 52 to 6 times greater in the resistant biotype than in the susceptible biotype. This was the only major difference found between the resistant and susceptible biotypes. The amount of ACCase in the meristems of the resistant and susceptible is similar during ontogeny and no difference was found in distribution of ACCase between the two biotypes. Uptake, translocation and metabolism of [14C]diclofop-methyl were not different between the two biotypes. In vivo, ACCase activity in the meristems of the susceptible biotype was greatly inhibited by herbicide application whereas only 25% inhibition occurred in the resistant biotype. Depolarisation of plasma membrane potential by 50 μM diclofop acid was observed in both biotypes and neither biotype showed recovery of the membrane potential following removal of the herbicide. Hence, a modified form of ACCase appears to be the major determinant of resistance in this resistant wild oat biotype.