Characterisation of low-level pyrasulfotole resistance and the role of herbicide translocation in wild radish (Raphanus raphanistrum).

The synthetic auxin 2,4-D and the 4-hydroxyphenylpyruvate dioxygenase inhibitor pyrasulfotole are phloem-mobile post-emergence herbicides, the latter applied in co-formulation with either bromoxynil (a contact herbicide causing leaf desiccation) or MCPA (another synthetic auxin). Previous studies have shown a wide range of 2,4-D translocation phenotypes in resistant populations of the agricultural weed Raphanus raphanistrum, but it was hypothesised that enhanced movement out of the apical meristem could contribute to resistance. Little is known about pyrasulfotole translocation or the effect of bromoxynil on pyrasulfotole movement. Therefore, the behaviour of pyrasulfotole and 2,4-D applied to the growing point of susceptible and resistant R. raphanistrum seedlings was assessed, along with the effect of bromoxynil on pyrasulfotole translocation. The small amount of herbicide directly contacting the growing point after spraying was sufficient to induce herbicide symptoms, and there was no enhancement of translocation away from the growing point in either pyrasulfotole- or 2,4-D-resistant populations. Bromoxynil had a slightly inhibitory effect on pyrasulfotole translocation in some populations, somewhat negating the minor differences observed among populations when pyrasulfotole was applied alone. Resistance to pyrasulfotole could not explained by enhanced metabolism or vacuolar sequestration of the herbicide. Overall, differential translocation in either the treated leaves or apical meristems does not appear to be a major determinant of resistance to pyrasulfotole or 2,4-D.

Plasma membrane receptor-like kinases and transporters are associated with 2,4-D resistance in wild radish.

BACKGROUND AND AIMS: Resistance to the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) in wild radish (Raphanus raphanistrum) appears to be due to a complex, multifaceted mechanism possibly involving enhanced constitutive plant defence and alterations in auxin signalling. Based on a previous gene expression analysis highlighting the plasma membrane as being important for 2,4-D resistance, this study aimed to identify the components of the leaf plasma membrane proteome that contribute to resistance. METHODS: Isobaric tagging of peptides was used to compare the plasma membrane proteomes of a 2,4-D-susceptible and a 2,4-D-resistant wild radish population under control and 2,4-D-treated conditions. Eight differentially abundant proteins were then targeted for quantification in the plasma membranes of 13 wild radish populations (two susceptible, 11 resistant) using multiple reaction monitoring. KEY RESULTS: Two receptor-like kinases of unknown function (L-type lectin domain-containing receptor kinase IV.1-like and At1g51820-like) and the ATP-binding cassette transporter ABCB19, an auxin efflux transporter, were identified as being associated with auxinic herbicide resistance. The variability between wild radish populations suggests that the relative contributions of these candidates are different in the different populations. CONCLUSIONS: To date, no receptor-like kinases have been reported to play a role in 2,4-D resistance. The lectin-domain-containing kinase may be involved in perception of 2,4-D at the plasma membrane, but its ability to bind 2,4-D and the identity of its signalling partner(s) need to be confirmed experimentally. ABCB19 is known to export auxinic compounds, but its role in 2,4-D resistance in wild radish appears to be relatively minor.

2,4-D and dicamba resistance mechanisms in wild radish: subtle, complex and population specific?

Background and Aims: Resistance to synthetic auxin herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) is increasing in weed populations worldwide, which is of concern given the recent introduction of synthetic auxin-resistant transgenic crops. Due to the complex mode of action of the auxinic herbicides, the mechanisms of evolved resistance remain largely uncharacterized. The aims of this study were to assess the level of diversity in resistance mechanisms in 11 populations of the problem weed Raphanus raphanistrum, and to use a high-throughput, whole-genome transcriptomic analysis on one resistant and one susceptible population to identify important changes in gene expression in response to 2,4-D. Methods: Levels of 2,4-D and dicamba (3,6-dichloro-2-methoxybenzoic acid) resistance were quantified in a dose-response study and the populations were further screened for auxin selectivity, 2,4-D translocation and metabolism, expression of key 2,4-D-responsive genes and activation of the mitogen-activated proein kinase (MAPK) pathway. Potential links between resistance levels and mechanisms were assessed using correlation analysis. Key Results: The transcriptomic study revealed early deployment of the plant defence response in the 2,4-D-treated resistant population, and there was a corresponding positive relationship between auxinic herbicide resistance and constitutive MAPK phosphorylation across all populations. Populations with shoot-wide translocation of 2,4-D had similar resistance levels to those with restricted translocation, suggesting that reduced translocation may not be as strong a resistance mechanism as originally thought. Differences in auxin selectivity between populations point to the likelihood of different resistance-conferring alterations in auxin signalling and/or perception in the different populations. Conclusions: 2,4-D resistance in wild radish appears to result from subtly different auxin signalling alterations in different populations, supplemented by an enhanced defence response and, in some cases, reduced 2,4-D translocation. This study highlights the dangers of applying knowledge generated from a few populations of a weed species to the species as a whole.

2,4-D resistance in wild radish: reduced herbicide translocation via inhibition of cellular transport.

Resistance to auxinic herbicides is increasing in a range of dicotyledonous weed species, but in most cases the biochemical mechanism of resistance is unknown. Using (14)C-labelled herbicide, the mechanism of resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) in two wild radish (Raphanus raphanistrum L.) populations was identified as an inability to translocate 2,4-D out of the treated leaf. Although 2,4-D was metabolized in wild radish, and in a different manner to the well-characterized crop species wheat and bean, there was no difference in metabolism between the susceptible and resistant populations. Reduced translocation of 2,4-D in the latter was also not due to sequestration of the herbicide, or to reduced uptake by the leaf epidermis or mesophyll cells. Application of auxin efflux or ABCB transporter inhibitors to 2,4-D-susceptible plants caused a mimicking of the reduced-translocation resistance phenotype, suggesting that 2,4-D resistance in the populations under investigation could be due to an alteration in the activity of a plasma membrane ABCB-type auxin transporter responsible for facilitating long-distance transport of 2,4-D.

A potential role for endogenous microflora in dormancy release, cytokinin metabolism and the response to fluridone in Lolium rigidum seeds.

BACKGROUND AND AIMS: Dormancy in Lolium rigidum (annual ryegrass) seeds can be alleviated by warm stratification in the dark or by application of fluridone, an inhibitor of plant abscisic acid (ABA) biosynthesis via phytoene desaturase. However, germination and absolute ABA concentration are not particularly strongly correlated. The aim of this study was to determine if cytokinins of both plant and bacterial origin are involved in mediating dormancy status and in the response to fluridone. METHODS: Seeds with normal or greatly decreased (by dry heat pre-treatment) bacterial populations were stratified in the light or dark and in the presence or absence of fluridone in order to modify their dormancy status. Germination was assessed and seed cytokinin concentration and composition were measured in embryo-containing or embryo-free seed portions. KEY RESULTS: Seeds lacking bacteria were no longer able to lose dormancy in the dark unless supplied with exogenous gibberellin or fluridone. Although these seeds showed a dramatic switch from active cytokinin free bases to O-glucosylated storage forms, the concentrations of individual cytokinin species were only weakly correlated to dormancy status. However, cytokinins of apparently bacterial origin were affected by fluridone and light treatment of the seeds. CONCLUSIONS: It is probable that resident microflora contribute to dormancy status in L. rigidum seeds via a complex interaction between hormones of both plant and bacterial origin. This interaction needs to be taken into account in studies on endogenous seed hormones or the response of seeds to plant growth regulators.

Pyroxasulfone Metabolism in Resistant Lolium rigidum: Is It All Down to GST Activity?

Resistance to the herbicide pyroxasulfone has slowly but steadily increased in agricultural weeds. The evolved resistance of one Lolium rigidum population has been attributed to the conjugation of pyroxasulfone to reduced glutathione, mediated by glutathione transferase (GST) activity. To determine if GST-based metabolism is a widespread mechanism of pyroxasulfone resistance in L. rigidum, a number of putative-resistant populations were screened for GST activity toward pyroxasulfone, the presence of GSTF13-like isoforms (previously implicated in pyroxasulfone conjugation in this species), tissue glutathione concentrations, and response to inhibitors of GSTs and oxygenases. Although there were no direct correlations between pyroxasulfone resistance levels and these individual parameters, a random forest analysis indicated that GST activity was of primary importance for L. rigidum resistance to this herbicide.

Selection for low dormancy in annual ryegrass (Lolium rigidum) seeds results in high constitutive expression of a glucose-responsive α-amylase isoform.

BACKGROUND AND AIMS: α-Amylase in grass caryopses (seeds) is usually expressed upon commencement of germination and is rarely seen in dry, mature seeds. A heat-stable α-amylase activity was unexpectedly selected for expression in dry annual ryegrass (Lolium rigidum) seeds during targeted selection for low primary dormancy. The aim of this study was to characterize this constitutive activity biochemically and determine if its presence conferred insensitivity to the germination inhibitors abscisic acid and benzoxazolinone. METHODS: α-Amylase activity in developing, mature and germinating seeds from the selected (low-dormancy) and a field-collected (dormant) population was characterized by native activity PAGE. The response of seed germination and α-amylase activity to abscisic acid and benzoxazolinone was assessed. Using an alginate affinity matrix, α-amylase was purified from dry and germinating seeds for analysis of its enzymatic properties. KEY RESULTS: The constitutive α-amylase activity appeared late during seed development and was mainly localized in the aleurone; in germinating seeds, this activity was responsive to both glucose and gibberellin. It migrated differently on native PAGE compared with the major activities in germinating seeds of the dormant population, but the enzymatic properties of α-amylase purified from the low-dormancy and dormant seeds were largely indistinguishable. Seed imbibition on benzoxazolinone had little effect on the low-dormancy seeds but greatly inhibited germination and α-amylase activity in the dormant population. CONCLUSIONS: The constitutive α-amylase activity in annual ryegrass seeds selected for low dormancy is electrophoretically different from that in germinating seeds and its presence confers insensitivity to benzoxazolinone. The concurrent selection of low dormancy and constitutive α-amylase activity may help to enhance seedling establishment under competitive conditions.

Enhanced production of water-soluble cinmethylin metabolites by Lolium rigidum populations with reduced cinmethylin sensitivity.

BACKGROUND: Cinmethylin, a pre-emergence herbicide inhibiting fatty acid thioesterase activity, has recently been introduced to Australian cereal cropping for the control of Lolium rigidum Gaud. (annual ryegrass). To date, there have been no confirmed cases of cinmethylin resistance identified in this species, but some populations exhibit reduced sensitivity to this herbicide. To explore the mechanism which contributes to reduced sensitivity of annual ryegrass to cinmethylin, the extent and nature of cinmethylin metabolism, using carbon-14 (14 C)-labelled herbicide, were analysed in three reduced-sensitivity annual ryegrass populations, alongside a susceptible population and cinmethylin-tolerant wheat as controls. RESULTS: All samples showed the same metabolite profile, with the extent of production of a specific water-soluble metabolite being correlated to the level of herbicide sensitivity. Application of the cytochrome P450 inhibitor phorate caused a decrease in water-soluble metabolite production as well as seedling growth in the presence of cinmethylin, indicating that reduced cinmethylin sensitivity in annual ryegrass could be wholly or partially due to oxidative modification of cinmethylin. CONCLUSION: Because annual ryegrass has the potential to metabolize cinmethylin in the same way as wheat, careful stewardship is required to ensure the longevity of this herbicide. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Identification and characterisation of seed storage protein transcripts from Lupinus angustifolius.

BACKGROUND: In legumes, seed storage proteins are important for the developing seedling and are an important source of protein for humans and animals. Lupinus angustifolius (L.), also known as narrow-leaf lupin (NLL) is a grain legume crop that is gaining recognition as a potential human health food as the grain is high in protein and dietary fibre, gluten-free and low in fat and starch. RESULTS: Genes encoding the seed storage proteins of NLL were characterised by sequencing cDNA clones derived from developing seeds. Four families of seed storage proteins were identified and comprised three unique α, seven ß, two γ and four δ conglutins. This study added eleven new expressed storage protein genes for the species. A comparison of the deduced amino acid sequences of NLL conglutins with those available for the storage proteins of Lupinus albus (L.), Pisum sativum (L.), Medicago truncatula (L.), Arachis hypogaea (L.) and Glycine max (L.) permitted the analysis of a phylogenetic relationships between proteins and demonstrated, in general, that the strongest conservation occurred within species. In the case of 7S globulin (ß conglutins) and 2S sulphur-rich albumin (δ conglutins), the analysis suggests that gene duplication occurred after legume speciation. This contrasted with 11S globulin (α conglutin) and basic 7S (γ conglutin) sequences where some of these sequences appear to have diverged prior to speciation. The most abundant NLL conglutin family was ß (56%), followed by α (24%), δ (15%) and γ (6%) and the transcript levels of these genes increased 103 to 106 fold during seed development. We used the 16 NLL conglutin sequences identified here to determine that for individuals specifically allergic to lupin, all seven members of the ß conglutin family were potential allergens. CONCLUSION: This study has characterised 16 seed storage protein genes in NLL including 11 newly-identified members. It has helped lay the foundation for efforts to use molecular breeding approaches to improve lupins, for example by reducing allergens or increasing the expression of specific seed storage protein(s) with desirable nutritional properties.

Selection for low or high primary dormancy in Lolium rigidum Gaud seeds results in constitutive differences in stress protein expression and peroxidase activity.

Seed dormancy in wild Lolium rigidum Gaud (annual ryegrass) populations is highly variable and not well characterized at the biochemical level. To identify some of the determinants of dormancy level in these seeds, the proteomes of subpopulations selected for low and high levels of primary dormancy were compared by two-dimensional polyacrylamide gel electrophoresis of extracts from mature, dry seeds. High-dormancy seeds showed higher expression of small heat shock proteins, enolase, and glyoxalase I than the low-dormancy seeds. The functional relevance of these differences in protein expression was confirmed by the fact that high-dormancy seeds were more tolerant to high temperatures imposed at imbibition and had consistently higher glyoxalase I activity over 0-42 d dark stratification. Higher expression of a putative glutathione peroxidase in low-dormancy seeds was not accompanied by higher activity, but these seeds had a slightly more oxidized glutathione pool and higher total peroxidase activity. Overall, these biochemical and physiological differences suggest that L. rigidum seeds selected for low dormancy are more prepared for rapid germination via peroxidase-mediated cell wall weakening, whilst seeds selected for high dormancy are constitutively prepared to survive environmental stresses, even in the absence of stress during seed development.

Rapid On-Farm Testing of Resistance in Lolium rigidum to Key Pre- and Post-Emergence Herbicides.

Overreliance on herbicides for weed control is conducive to the evolution of herbicide resistance. Lolium rigidum (annual ryegrass) is a species that is prone to evolve resistance to a wide range of herbicide modes of action. Rapid detection of herbicide-resistant weed populations in the field can aid farmers to optimize the use of effective herbicides for their control. The feasibility and utility of a rapid 7-d agar-based assay to reliably detect L. rigidum resistant to key pre- and post-emergence herbicides including clethodim, glyphosate, pyroxasulfone and trifluralin were investigated in three phases: correlation with traditional pot-based dose-response assays, effect of seed dormancy, and stability of herbicides in agar. Easy-to-interpret results were obtained using non-dormant seeds from susceptible and resistant populations, and resistance was detected similarly as pot-based assays. However, the test is not suitable for trifluralin because of instability in agar as measured over a 10-d period, as well as freshly-harvested seeds due to primary dormancy. This study demonstrates the utility of a portable and rapid assay that allows for on-farm testing of clethodim, glyphosate, and pyroxasulfone resistance in L. rigidum, thereby aiding the identification and implementation of effective herbicide control options.

Pyroxasulfone-Resistant Annual Ryegrass (Lolium rigidum) Has Enhanced Capacity for Glutathione Transferase-Mediated Pyroxasulfone Conjugation.

The herbicide pyroxasulfone was widely introduced in 2012, and cases of evolved resistance in weeds such as annual ryegrass (Lolium rigidum Gaud.) and tall waterhemp [Amaranthus tuberculatus (Moq.) Sauer] have started to emerge. Pyroxasulfone is detoxified by tolerant crops, and by annual ryegrass that has been recurrently selected with pyroxasulfone, in a pathway that is hypothesized to involve glutathione conjugation. In the current study, it was confirmed that pyroxasulfone is conjugated to glutathione in vitro by glutathione transferases (GSTs) purified from susceptible and resistant annual ryegrass populations and from a tolerant crop species, wheat. The extent of conjugation corresponded to the pyroxasulfone resistance level. Pyroxasulfone-conjugating activity was higher in radicles, roots, and seeds compared to coleoptiles or expanded leaves. Among the GSTs purified from annual ryegrass radicles and seeds, an orthologue of Brachypodium distachyon GSTF13 was >20-fold more abundant in the pyroxasulfone-resistant population, suggesting that this protein could be responsible for pyroxasulfone conjugation.

A Val-202-Phe α-tubulin mutation and enhanced metabolism confer dinitroaniline resistance in a single Lolium rigidum population.

BACKGROUND: A Lolium rigidum population collected from Western Australia was previously reported as highly resistant to dinitroaniline herbicides mainly due to a Val-202-Phe substitution in the target site α-tubulin protein. To further determine the contribution of the 202 mutation to resistance, two sub-populations, respectively comprising the 202 mutant and wild-type (WT) individuals, were isolated from within the same resistant population and subject to dinitroaniline herbicide doses. A rice transgenic study was conducted to demonstrate whether the amino acid substitution at the 202 residue confers resistance. In addition, as indicated in the phenotyping and genotyping study, non-target enhanced trifluralin metabolism was further examined in the same population. RESULTS: The 202 mutants were more resistant than the wild-type plants. Rice calli transformed with the L. rigidum mutant α-tubulin gene (Val-202-Phe) were more resistant to dinitroaniline herbicides relative to calli transformed with the wild-type gene. Also, enhanced trifluralin metabolism was detected in the 202 mutants in comparison to the susceptible seedlings. CONLCUSION: Both target-site Val-202-Phe α-tubulin mutation and non-target-site enhanced trifluralin metabolism co-exist in this dinitroaniline-resistant L. rigidum population. © 2019 Society of Chemical Industry.

Loss of trifluralin metabolic resistance in Lolium rigidum plants exposed to prosulfocarb recurrent selection.

BACKGROUND: Resistance to the dinitroaniline herbicide trifluralin in Lolium rigidum (annual ryegrass) often is mediated by the enhanced capacity to metabolize the herbicide to less toxic polar conjugates and/or by functionally recessive target-site mutations in α-tubulin. RESULTS: In two L. rigidum populations possessing enhanced trifluralin metabolism, resistance was largely reversed by recurrent selection with the thiocarbamate herbicide prosulfocarb (i.e. plant survival was two- to >20-fold lower). Their ability to metabolize trifluralin was significantly decreased (by ≈2.3-fold) following recurrent prosulfocarb selection, to levels comparable to those observed in susceptible plants or when trifluralin metabolism was inhibited by treatment with the insecticide phorate. CONCLUSIONS: This study provides evidence that trait(s) enabling efficient trifluralin metabolism in L. rigidum are purged from the population under prosulfocarb recurrent selection. The level of trifluralin metabolism in vitro and its inhibition caused by phorate action on trifluralin-metabolizing enzyme(s) is equivalent to the effect produced by prosulfocarb selection. The hypothetical link between the two phenomena is that the putative monooxygenase(s) conferring trifluralin metabolic resistance also mediate the activation of prosulfocarb to its toxic sulfoxide. Thus, we speculate that survival to prosulfocarb via a lack of metabolic herbicide activation, and survival to trifluralin conferred by enhanced herbicide metabolism, are mutually exclusive. These findings not only open up a new research direction in terms of the interaction between different herbicide resistance mechanisms in L. rigidum, but also offer strategies for immediate management of the population dynamics of metabolism-based resistance in the field. © 2020 Society of Chemical Industry.

ABA inhibits germination but not dormancy release in mature imbibed seeds of Lolium rigidum Gaud.

Dormancy release in imbibed annual ryegrass (Lolium rigidum Gaud.) seeds is promoted in the dark but inhibited in the light. The role of abscisic acid (ABA) in inhibition of dormancy release was found to be negligible, compared with its subsequent effect on germination of dormant and non-dormant seeds. Inhibitors of ABA metabolism had the expected effects on seed germination but did not influence ABA concentration, suggesting that they act upon other (unknown) factors regulating dormancy. Although gibberellin (GA) synthesis was required for germination, the influence of exogenous GA on both germination and dormancy release was minor or non-existent. Embryo ABA concentration was the same following treatments to promote (dark stratification) and inhibit (light stratification) dormancy release; exogenous ABA had no effect on this process. However, the sensitivity of dark-stratified seeds to ABA supplied during germination was lower than that of light-stratified seeds. Therefore, although ABA definitely plays a role in the germination of annual ryegrass seeds, it is not the major factor mediating inhibition of dormancy release in imbibed seeds.

Green and blue light photoreceptors are involved in maintenance of dormancy in imbibed annual ryegrass (Lolium rigidum) seeds.

Light plays an important role in two separate processes within the seeds of Lolium rigidum (annual ryegrass). Dormant seeds of L. rigidum remain dormant when imbibed in the light, but once seeds have lost dormancy through dark-stratification, light stimulates their germination. This study characterizes the light qualities and quantities which are effective in maintenance of dormancy. Dormant seeds were stratified under narrow- and broad-waveband light to identify the potential photoreceptors involved in dormancy maintenance, and to determine whether dark-induced dormancy loss is reversible by light. Blue and green light both mediated dormancy maintenance in a far-red-independent manner. Red light resulted in dormancy maintenance only when far-red wavelengths were excluded, suggesting a redundant function of phytochrome. At low fluence rates, white light was more effective than monochromatic light, suggesting the action of multiple photoreceptors in dormancy maintenance. By contrast, nondormant seeds did not germinate unless provided with red light. These results indicate that seed dormancy maintenance is potentially mediated through the actions of blue and green light photoreceptors. Seed dormancy could thus be added to the growing list of plant responses that may be mediated by green light in a cryptochrome-independent manner.

Identity and Activity of 2,4-Dichlorophenoxyacetic Acid Metabolites in Wild Radish ( Raphanus raphanistrum).

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.

Weed resistance to synthetic auxin herbicides.

Herbicides classified as synthetic auxins have been most commonly used to control broadleaf weeds in a variety of crops and in non-cropland areas since the first synthetic auxin herbicide (SAH), 2,4-D, was introduced to the market in the mid-1940s. The incidence of weed species resistant to SAHs is relatively low considering their long-term global application with 30 broadleaf, 5 grass, and 1 grass-like weed species confirmed resistant to date. An understanding of the context and mechanisms of SAH resistance evolution can inform management practices to sustain the longevity and utility of this important class of herbicides. A symposium was convened during the 2nd Global Herbicide Resistance Challenge (May 2017; Denver, CO, USA) to provide an overview of the current state of knowledge of SAH resistance mechanisms including case studies of weed species resistant to SAHs and perspectives on mitigating resistance development in SAH-tolerant crops. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Wheat genotypes show contrasting abilities to recover from anoxia in spite of similar anoxic carbohydrate metabolism.

Physiological and metabolic responses to anoxia and reaeration were compared for 4-7-day-old seedlings of 11 genotypes of wheat (Triticum aestivum) with reputed differences in waterlogging tolerance. Genotypes differed in seminal root elongation, and recovery of root tissue K(+) concentration, during reaeration following 72 h anoxia. Post-anoxic recovery ranged from complete (100% retention of seminal root elongation potential) to almost nil (death of all seminal root apices and inability to recover K(+) concentration). The anoxia tolerance ranking of the genotypes based on these parameters corresponded with that of their reputed waterlogging tolerance, but with some exceptions. However, the differences in anoxia tolerance of the seedlings could not be explained by differences in capacity for ethanol production. A decreased ability to utilise seed starch reserves under anoxia, due to inadequate levels of alpha-amylase activity at the time anoxia was imposed, was apparent in all genotypes.

Intensive cropping systems select for greater seed dormancy and increased herbicide resistance levels in Lolium rigidum (annual ryegrass).

BACKGROUND: Lolium rigidum (annual ryegrass) is a widespread annual crop weed that has evolved high levels of resistance to selective herbicides. Anecdotal evidence suggests that intensive cropping also leads to higher seed dormancy in L. rigidum. This was quantified by measuring dormancy levels in L. rigidum populations collected from paired sites (one with nil to low cropping intensity, the other intensively cropped) located throughout the Western Australian grain belt. RESULTS: Populations from non-cropped fields or those with low cropping intensity showed higher and faster germination than populations from fields with a medium- or high-intensity cropping regime. Resistance to selective herbicides was also higher in the medium- and high-intensity cropping fields than in the low-intensity cropping fields. CONCLUSION: High-intensity cropping systems are likely to impose greater selection pressures for seed dormancy and selective herbicide resistance, because late-emerging seedlings avoid preplanting weed control practices (tillage and non-selective herbicide application) but are exposed to selective in-crop herbicides.