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Echinochloa crus-galli is a major weed in rice fields in China, and quinclorac has been long used for its control. Over-reliance of quinclorac has resulted in quinclorac resistance in E. crus-galli. Two resistant (R) E. crus-galli populations from Hunan, China were confirmed to be at least 78-fold more resistant to quinclorac than the susceptible (S) population. No difference in foliar uptake of 14C-labelled quinclorac was detected between the R and S plants. However, a higher level of 14C translocation and a lower level of quinclorac metabolism were found in the R plants. Basal and induced expression levels of β-cyanoalanine synthase (β-CAS) gene and β-CAS activity were not significantly different between the R and S plants. However, the induction expression of 1-aminocyclopropane-1-carboxylic acid oxidase (ACO1) gene by quinclorac treatment was evident in the S plants but not in the R plants. Quinclorac resistance in the two resistant E. crus-galli populations was not likely to be related to foliar uptake, translocation or metabolism of quinclorac, nor to cyanide detoxification via β-CAS. Thus, target-site based quinclorac signal reception and transduction and regulation of the ethylene synthesis pathway should be the focus for further research. [ABSTRACT FROM AUTHOR]

An Echinochloa colona population from Western Australia has evolved resistance to glyphosate. This current study investigates the physiological, molecular and biochemical basis of glyphosate resistance in this population. To minimise genetic differences the susceptible (S) and resistance (R) phenotypes were isolated from within this resistant population. The S phenotype was found to accumulate significantly more shikimate in the leaf tissue than the R phenotype following glyphosate treatment. Target-site EPSPS gene sequencing revealed no resistance mutations and EPSPS gene expression was similar between the S and R phenotypes. Thus, glyphosate resistance in this population is unlikely target-site based. Similarly, there were no significant differences between the S and R phenotypes in glyphosate leaf uptake and translocation at the whole plant level. It is also unlikely that the resistance is associated with glyphosate metabolism as no major glyphosate metabolites were detected in leaf tissue of the S and R phenotypes. Despite much effort the exact glyphosate resistance mechanisms in this R population remain unclear, and novel resistance mechanisms are to be determined. [ABSTRACT FROM AUTHOR]

Weed population simulations can be useful to predict the effects of alternative management practices on herbicide resistance (HR) evolution. Almost all previous simulations have ignored the possibility of within-field spatial structure in a weed population, instead making the implicit assumption of perfect dispersal and spatial homogeneity in population density and genetics. The effects of this simplifying assumption have not been examined, despite the fact that dispersal limitations and spatial structure within the population are likely to occur and to affect the evolution of resistance. Therefore, we developed a new spatially-explicit model called SOMER, and examined how changing the following factors affected the predicted evolution of resistance: the degree of spatial resolution used in the model; whether resistance was semi-dominant or fully-dominant; distances of pollen and natural seed dispersal; and inadvertent collection and grain harvester weed seed dispersal (GHWSD). Simulations showed that spatial resolution is important when modelling HR evolution, with the size of sub-population divisions, the pollen dispersal parameter, the level of dominance, and GHWSD all being important factors in predicting the rate and type of HR evolution. Our results show that accounting for spatial structure and dispersal does affect predictions of HR evolution, with the non-spatial model generally predicting faster resistance evolution compared to the more realistic equivalent spatial model. Most importantly, GHWSD increased the speed of HR evolution. Our spatial model also allowed us to investigate the dynamics of density and genetic structure within patches of herbicide resistant weeds, and we found that resistance genes were spread several times wider than the visible patch, and that homozygous mutations were commonly found in more centrally located weeds. We conclude that an ‘integrated spatial modelling’ approach that accounts for spatial structure should be considered when modelling HR evolution, and the evolution of resistance in general. [ABSTRACT FROM AUTHOR]

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F 1 and F 2 families. [ABSTRACT FROM AUTHOR]

Abstract: The potential for low rates of diclofop-methyl to result in rapid evolution of herbicide resistance in a herbicide-susceptible Lolium rigidum (annual ryegrass) population was demonstrated in a recent crop-field study. In this present study, the data from the crop-field study was used together with simulation modelling to identify possible genetics of the herbicide resistance that was selected for. This analysis clearly indicated that the herbicide resistance was polygenic. Subsequently, the estimated genetic possibilities were used to parameterise a model of herbicide resistance evolution in a simulated crop-field situation, and the potential of different rates of diclofop-methyl (ACCase herbicide) to cause herbicide-resistance evolution in L. rigidum was explored and compared using the calibrated model. The calibrated model outputs indicated that the evolution of diclofop-methyl resistance would generally be faster at low herbicide rates than at higher rates due to the rapid selection of minor gene herbicide resistance traits at low rates and their subsequent recombination by cross-pollination. The results of the study therefore indicate potential risks in herbicide rate cutting and highlight the need for careful scientific evaluation of any herbicide use rate for its potential to select for minor gene herbicide resistance from a weed population. [Copyright &y& Elsevier]

Abstract: Dormancy release in freshly matured, imbibed annual ryegrass (Lolium rigidum) seeds is inhibited by light and involves a decrease in seed sensitivity to abscisic acid. Other processes involved in dormancy release in the dark were investigated by measuring seed storage compound mobilisation and the activity of cell wall-degrading enzymes. Activities of endo-β-mannanase and total peroxidase were higher in dark-stratified compared to light-stratified seeds, indicating that weakening of the structures constraining the embryo was accelerated in the dark. A dramatic degradation of storage proteins in light-stratified seeds, accompanied by induction of a high molecular mass protease, suggests that maintenance of storage(-like) proteins is also important in dark-mediated dormancy release. α-Amylase activity was induced in dark-stratified seeds at least 48h prior to radicle emergence upon transfer to conditions permitting germination, or in light-stratified seeds supplied with exogenous gibberellin A4. This suggests that (a) α-amylase is involved in stimulation of germination of non-dormant L. rigidum seeds, and (b) dark-stratified seeds have an increased sensitivity to gibberellins which permits the rapid induction of α-amylase activity upon exposure to germination conditions. Overall, it appears that a number of processes, although possibly minor in themselves, occur in concert during dark-stratification to contribute to dormancy release. [Copyright &y& Elsevier]

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In Australian grain-growing regions, the evolution of weeds resistant to multiple herbicides threatens the sustainability of existing cropping systems. As a result, herbicide susceptibility is considered to be a potentially exhaustible resource. Based on a survey of 132 Western Australian grain growers, perceptions of various factors relating to the herbicide resource were measured. These include the rate of resistance evolution, cost of resistance, the future availability of replacement herbicides, reductions in the level of resistance in a population over time, and mobility of resistance. Growers were found to have a generally high level of awareness of herbicide resistance and consider it to have a high potential cost. However, many perceive that the stock of effective herbicides is likely to be renewed in the short term, either through new herbicide development or resistance reversion. Potential opportunities for targeted extension to improve farmer decision-making are identified. [Copyright &y& Elsevier]

Abstract: The physiological and biochemical bases of seed dormancy in Lolium rigidum (annual ryegrass) are largely unknown, and study of this process is complicated by the outcrossing nature of the species and the strong influence of environment on seed dormancy. In order to identify heritable biochemical factors contributing to seed dormancy in L. rigidum, seeds from a field-collected population were used to select sub-populations with consistently low or high seed dormancy over four generations. Low-dormancy seeds showed constitutive α-amylase activity prior to imbibition, higher concentrations of polyphenols and cis-zeatin, and lower abscisic acid and cis-zeatin riboside concentrations than high-dormancy seeds. Selection for high dormancy was associated with a reduction in response to dark-stratification for 21d at 20°C (an effective means of releasing dormancy in the original, unselected population) over successive generations, but fluridone remained effective in breaking dormancy. Crossing of low- and high-dormancy populations indicated that dormancy level was not dependent upon the maternal genotype of the seed, and that the constitutive α-amylase activity and high seed anthocyanin concentrations characteristic of the low-dormancy populations were not correlated to high basal germination ability. [ABSTRACT FROM AUTHOR]

Abstract: Multi-species RIM, a bio-economic model that simulates the population dynamics of two annual weed species over a 20-year period, was used to investigate the value of transgenic glyphosate-resistant canola in the management of herbicide resistant annual ryegrass and wild radish in a Western Australian dryland farming system. The perceived advantage of growing this crop is the potential to control post-emergent weeds with the broad-spectrum herbicide glyphosate, and without the yield penalty evident in triazine-resistant canola. We found that the economic value of glyphosate-resistant canola is consistently higher than that of the commonly grown triazine-resistant canola. However, the benefits of glyphosate-resistant canola would need to be weighed up against potential risks to marketability (due to consumer resistance) and risks of increased weed resistance to glyphosate (due to increased selection pressure). [Copyright &y& Elsevier]

• Trifluralin resistance inheritance in a Lolium rigidum population is determined. • Plants in this population contain the α-tubulin mutation Val-202-Phe. • Resistance is largely inherited as a monogenic, recessive nuclear trait. • Polygenic resistance may also be involved, especially at low herbicide rates. • Recessive resistance has implications in resistance evolution and management. The increasing number of weedy species resistant to dinitroaniline herbicides warrants studies on the evolutionary factors contributing to resistance evolution, including genetic inheritance of resistance traits. In this study, we investigated the genetic control of trifluralin resistance in a well-characterised Lolium rigidum Gaud. population from Western Australia. This population was purified to contain plants homozygous for the Val-202-Phe α-tubulin mutation, and used as the resistant (R) parents and crossed with susceptible (S) parents to produce eight reciprocal F1 families. Trifluralin dose response curves of the eight F1 families indicate that trifluralin resistance in this population is inherited as an incomplete recessive nuclear trait. The F1 plants were crossed within each families to establish eight pseudo-F2 (ψ -F2) families. Segregation ratio of resistance and susceptibility in ψ -F2 families were determined using the discriminating trifluralin rates of 120 and 480 g a.i. ha−1. At 480 g a.i. ha−1 trifluralin, the segregation ratio in almost all ψ -F2 families (except one) was fit to 1:3 (resistance: susceptibility) one recessive gene control model. However, at 120 g a.i. ha−1 trifluralin, the segregation ratios in half of the families did not fit this model, indicating involvement of one or more genes in resistance at the lower rate. These results showed complexity of genetic inheritance of trifluralin resistance in this L. rigidum population possessing the Val-202-Phe α-tubulin mutation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Semi-dominant alleles segregate at one major locus for pyroxasulfone resistance. [•] Cross-resistance is also conferred by major locus(i). [•] A combination of herbicide options can reduce pyroxasulfone selection pressure. [•] This study will enable more accurate predictions by computational modelling. [•] Inheritance studies of novel herbicides can help sustain their longevity. [ABSTRACT FROM AUTHOR]

The RIM (resistance and integrated management) model is presented. RIM represents a wide diversity of herbicide and non-herbicide options for management of Australia''s most important crop weed, Lolium rigidum, in the context of the non-irrigated extensive farming system of southern Australia. Enterprise choices in the model include cereals, lupins, canola and three types of pastures for grazing by sheep. Users of RIM may specify the enterprise sequence and any feasible combination of the 35 weed treatment options each year over 10 or 20 years. Weed treatment options include selective herbicides (11), non-selective herbicides (5), non-chemical treatments (16) and user-defined treatments (3). The model represents weed and seed bank population dynamics, weed-crop competition, weed treatment impacts (including phytotoxicity), agronomic details, and financial details. Economic and biological model results are presented for scenarios with differing levels of availability of selective herbicides and different rotational sequences. [Copyright &y& Elsevier]