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The balance of pollination competition and facilitation among co-flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant-pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio-temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi-scale plant-pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio-temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect-pollinated plant species (L’équilibre des relations de compétition et de facilitation entre plantes pour la pollinisation et la disponibilité des ressources abiotiques affectent le succès reproducteur des espèces et des individus de plantes. La succession temporelle et l’hétérogénéité spatiale des ressources florales modifient les interactions plantes‐pollinisateurs à différentes échelles écologiques (individu végétal, assemblage plantes‐pollinisateurs local, réseau d’interactions des infrastructures écologiques à travers la ferme). Les variations intraspécifiques de phénologie de floraison peuvent moduler le succès reproducteur individuel en déterminant le niveau d’hétérogénéité spatio‐temporelle de densité de donneurs de pollen, des interactions plantes‐pollinisateurs et des ressources florales auxquelles un individu de plante est exposé. Nous avons mené une expérimentation pour tester comment la production de graines des plantes sauvages est affectée par les interactions plantes‐pollinisateurs à différentes échelles écologiques, ces interactions étant modulées par la phénologie florale et l’hétérogénéité spatio‐temporelle des ressources florales (découlant des infrastructures agroécologiques). Nous avons transplanté 144 individus de deux espèces végétales entomophiles (

Book of abstract p.372; International audience; The carry-over effect of cover crops on weeds and crop productivity in the subsequent crops has been related to cover crop composition and cover crop termination methods but their interaction with soil resource availability remains poorly documented, as well as the relative importance of each of these factors. This study investigated the effect of cover crop management (i.e. cover crop mixture, fertilisation, irrigation, termination method and their combinations) on weed biomass and crop productivity in two subsequent crops (spring barley followed by winter linseed). We hypothesised that cover crop management could affect productivity of the subsequent crops through both weed suppression and nitrogen supply. Two experiments spanning a duration of two years were set-up, on two different fields in two different years, to investigate the effect of cover crop mixture (2 or 8 species including or not legume species, plus a bare soil control), water and nitrogen availability at cover crop sowing and cover crop termination methods (rolling, herbicide-use and winter-kill control) on weed biomass and crop productivity of the two subsequent unweeded, unfertilised and directly seeded crops. Weed biomass and crop productivity in both subsequent crops were affected by multiple interactions between cover crop mixture, soil resource availability, cover crop termination method and experiment. In experiment 1, combinations of cover crop management alternative to the reference (i.e. bare soil, without fertilisation and irrigation, winter-killed) mainly showed beneficial carry-over effects (i.e. lower weed biomass and higher crop productivity) in the subsequent spring barley while having no effect in winter linseed. In experiment 2, alternative combinations of cover crop management mainly showed no effects or detrimental carry-over effects (i.e. higher weed biomass and lower crop productivity) in spring barley while having some positive effects in winter linseed (i.e. only when cover crops were terminated with herbicide-use). Crop productivity was mainly affected by weed biomass which was significantly reduced almost only when cover crops were terminated with herbicide-use. Crop productivity was also affected but to a lesser extent by cover crop soil-mediated effects (e.g. nitrogen supply). These results highlight complex interactions between cover crop management and environmental conditions on the carry-over effects of cover crops in the subsequent crops. Cover crops may not play an essential role for weed management in no-till and herbicide-free systems, particularly at low levels of cover crop biomass production.

Cover crop (CC) mixtures appear as a promising agroecological tool for weed management.Although CC supress weed growth by competing for resources, their suppressive effect undercontrasting levels of soil resource availability remains poorly documented. A two field: yearexperiment was conducted to investigate the weed suppressive effect of four CC mixtures.They were composed of 2 or 8 species including or not legume species and compared to abare soil control. The experiment included two levels of irrigation and nitrogen fertilisation atCC sowing. The objectives were to assess (i) weed and CC aboveground biomass response toCC mixtures and resource availability, (ii) the weed suppressive effect of CC mixtures across agradient of CC biomass and (iii) weed community response to CC mixtures and resourceavailability. CC and weed biomass were mainly influenced by interactions between CCmixtures and fertilisation and between CC mixtures and irrigation, with contrasted effectsbetween field: years. Nitrogen fertilisation increased biomass of non-legume based CCmixtures but this only resulted into a further reduction of weed biomass of little biologicalrelevance. Legume-based CC mixtures suppressed weeds to a greater extent without nitrogenfertilisation in year 2 but not in year 1, possibly due to contrasted initial soil nitrogenavailability (lower in year 2). Weed biomass generally benefited more from irrigation than CCmixtures. Among the 33 weed species recorded, weed communities in each plot weredominated by wheat volunteers, Geranium dissectum, Veronica persica and Echinochloa crusgalli, whose biomass varied depending on the interaction between year, CC mixture andresource availability. Our results revealed that competitive outcomes between CC mixturesand weed species were driven by a complex interaction between resource availability andspecies traits. Further experiments focusing on plant traits should improve our understandingof weed: CC competitive outcomes under various levels of resource availability.

Raphanus raphanistrum (wild radish) is a highly competitive weed in winter crops. Integrated weed management practices and decision-making tools benefit from the ability to predict seed longevity and dormancy status in the soil seed bank, as well as time and density of emergence in the field. We wondered if unique values taken from databases could serve for modelling purposes, whatever the origins of the weed populations. We investigated dormancy and longevity of fruits buried in the soil over a four-year seed burial experiment of two highly contrasted populations that differ by their biogeographical origin (oceanic versus continental climate), their habitat (arable field versus undisturbed) and their fruit size (thick versus thin fruit wall). High viability persisted over two years, followed by a rapid reduction, especially for the ‘continental-undisturbed-thin’ population. Dormancy cycling, with dormancy at its lowest in October, was observed for the ‘oceanic-arable-thick’ population, whilst a slow decrease without clear seasonal pattern was found for the other population. These results indicate different ways of regulating seed persistence in the soil, which might be taken into account when building demographic models. These differences could be due to interactions between fruit wall thickness and the other factors; it is possible that a thicker fruit wall increases longevity by promoting dormancy by physical restriction, but depending on temperature. Thinner fruit walls would make plants display other adaptive strategies to maximize survival time in the soil seed bank. Studies involving more populations and isogenic material are needed to address this specific question.

Better managing crop : weed competition in cropping systems while reducing both nitrogen and herbicide inputs is a real challenge that requires a better understanding of crop and weed root architecture in relation to soil-nitrogen availability. An original approach was used which considered the parameters of a simulation model of root architecture as traits to analyse (a) the interspecific diversity of root system architecture, and (b) its response to soil-nitrogen availability. Two greenhouse experiments were conducted using three crop and nine weed species grown at two contrasted concentrations of soil-nitrogen availability. Plant traits were measured to characterise both overall plant growth and root architecture, with a focus on primary root emergence, root elongation and branching. The studied root traits varied among species (from a twofold to a fourfold factor, depending on the trait), validating their use as indicators to analyse the interspecific variability of root architecture. The largest interspecies differences were for two traits: ‘maximal apical root diameter’ and ‘interbranch distance’ (distance between two successive laterals on the same root). Conversely, most of the studied root traits varied little with soil-nitrogen availability (from no variation to a 1.1-fold factor, depending on the trait) even though soil-nitrogen availability varied with a 17-fold factor and impacted the overall shoot and root biomass. So, the root traits used in this article are stable whatever soil-nitrogen availability. As they reflect processes underlying root system architecture, this low effect of nitrogen suggests that the rules governing root architecture are little affected by plant nitrogen status and soil-nitrogen availability. We propose that the determinants of differences in root system architecture between soils with contrasted nitrogen availability mainly originate from differences in the amount of carbon allocated to and within the root system. Characterising each plant species by a combination of root traits gave insights regarding the potential species competitive ability for soil resources in agroecosystems.

National audience; Crop mixtures may decrease reliance on pesticides but reducing herbicide use might increase weeds. Individual-based crop models can provide management guidelines. In heterogeneous canopies, light availability of individual plants depends on their dominant or dominated position. Estimating nitrogen demand at the plant scale, independently of light environment, is a challenge for modelling. A relationship linking shoot nitrogen amount to leaf biomass (or leaf area) at optimal nitrogen nutrition was investigated to establish if it could allow estimating nitrogen demand at the plant scale independently of its light environment. Crop and weed species were grown in greenhouse under various nitrogen treatments and, for two species, under two light levels. At the plant scale, shoot nitrogen amount was proportional to leaf biomass (or leaf area) at optimal nitrogen nutrition at vegetative stage. At reproductive stage, the relationship was allometric. The effect of light on shoot nitrogen amount per leaf biomass was minor and greater when using leaf area. Using data from previous experiments showed that the relationship using leaf biomass was stable across diverse growing conditions. The relationship linking shoot nitrogen amount to leaf biomass at optimal nitrogen nutrition can be used to model nitrogen demand of individual plants in heterogeneous canopies.

SPE EA BIOmE GESTAD INRA SUPDAT; International audience; How plant species associations affect the rhizosphere microbiota, in comparison to each plant species, has been overlooked. Here we investigated how bi-species plant associations affect total bacterial and N-cycling microbial communities (nitrate reducers and ammonia-oxidizers), as an example of functional communities, at low and high soil N levels. At low N level, the total bacterial abundance in the rhizosphere of the bi-species associations was significantly higher than that of either plant on its own for half of the bi-species associations. For the other associations, the bacterial abundance was not different from the corresponding mono-species cultures. The abundance of several N-cycling guilds was also enhanced by some of the bi-species associations with increases of up to 125% and no negative effects were recorded, which highlights the importance of plant diversity in agro-ecosystems.

18 pages; International audience; The ecological consequences of seed size variation have been studied extensively in plants. Curiously, little attention has been paid to the qualitative and quantitative variation of the seed-stored molecules and on their ecological significance. Here, we analysed the oil content and oil composition of ca. 200 weed seed species from agricultural fields in France based on single seed accessions, concentrating on interspecies differences and ignoring within-species variation. The relationships between seed weight, oil %, fatty acids (FAs) and the energetic value of the seed and its antioxidant properties were also investigated. The antioxidant activity could contribute to protect the oily seed reserves from alteration over time. Among the species analysed, we found a considerable quantitative (oil%) and qualitative variation of FAs stored in the seeds. Such variation was largely related to the plant family of the different species, but intrafamily variation was also found. Heavier seeds contained less oil on a per gram basis than lighter seeds, suggesting a trade-off between seed weight and oil ratio in the seed and that oil storage strategy depends on seed size. Moreover, oily seeds contained more polyunsaturated FAs. However, contrary to our hypothesis, we did not found a higher antioxidant capability in oily seed extracts than in non-oily seeds, nor to the quantitative or to the qualitative variation of FAs in the seeds. Considering the role of these important trait variations on weed ecological strategies, such as germination period, seed predation rate and competition–colonisation trade-off, could improve the sustainable management of weed communities.

International audience; Cornflower (Cyanus segetum), an iconic weed species, was used to investigate the implications of climate change on the behavior of long day dependent species while earlier cereal harvest may prematurely end the period of flowering and seed production in weeds. Variability in flowering, with or without vernalization, of European C. segetum populations situated along a 45-55 degrees North latitudinal gradient was studied. Long day requirement was confirmed. Vernalization requirements are not stringent in C. segetum the proportion of plants that were not sensitive or that responded strictly or weakly to vernalization varied across populations. The mean sum of growing degree days (GOD) for first flowering under a 14h day length at 15 degrees C was 1344 and 2072, with and without vernalization, respectively, whatever the population. A growth temperature at 25 degrees C had no effect on GDD except for a population from mountains. Advancing the date of harvest by ten days in a field experiment resulted in one-third fewer seeds produced, which could accelerate its decline, but polymorphism for vernalization and temperature responses could allow adaption.

International audience; We assessed the contributions of target site- and non-target site-based resistance to herbicides inhibiting acetyl-coenzyme A carboxylase (ACC) in Alopecurus myosuroides (black grass). A total of 243 A. myosuroides populations collected across France were analysed using herbicide sensitivity bioassay (24 300 seedlings analysed) and ACC genotyping (13 188 seedlings analysed). Seedlings resistant to at least one ACC-inhibiting herbicide were detected in 99.2% of the populations. Mutant, resistant ACC allele(s) were detected in 56.8% of the populations. Among the five resistant ACC alleles known in A. myosuroides, alleles containing an isoleucine-to-leucine substitution at codon 1781 were predominant (59.5% of the plants containing resistant ACC alleles). Comparison of the results from herbicide sensitivity bioassays with genotyping indicated that more than 75% of the plants resistant to ACC-inhibiting herbicides in France would be resistant via increased herbicide metabolism. Analysis of herbicide application records suggested that in 15.9% of the populations studied, metabolism-based resistance to ACC-inhibiting herbicides was mostly selected for by herbicides with other modes of action. Our study revealed the importance of non-target site-based resistance in A. myosuroides. Using herbicides with alternative modes of action to control populations resistant to ACC-inhibiting herbicides, the recommended management approach, may thus be jeopardised by the widespread occurrence of metabolism-based resistance mechanisms conferring broad-spectrum cross-resistance

International audience; Plant species are important drivers of soil microbial communities. However, how plant functional traits are shaping these communities has received less attention though linking plant and microbial traits is crucial for better understanding plant-microbe interactions. Our objective was to determine how plant-microbe interactions were affected by plant traits. Specifically we analyzed how interactions between plant species and microbes involved in nitrogen cycling were affected by plant traits related to nitrogen nutrition in interaction with soil nitrogen availability. Eleven plant species, selected along an oligotrophic-nitrophilic gradient, were grown individually in a nitrogen-poor soil with two levels of nitrate availability. Plant traits for both carbon and nitrogen nutrition were measured and the genetic structure and abundance of rhizosphere microbial communities, in particular the ammonia oxidizer and nitrate reducer guilds, were analyzed. The structure of the bacterial community in the rhizosphere differed significantly between plant species and these differences depended on nitrogen availability. The results suggest that the rate of nitrogen uptake per unit of root biomass and per day is a key plant trait, explaining why the effect of nitrogen availability on the structure of the bacterial community depends on the plant species. We also showed that the abundance of nitrate reducing bacteria always decreased with increasing nitrogen uptake per unit of root biomass per day, indicating that there was competition for nitrate between plants and nitrate reducing bacteria. This study demonstrates that nitrate-reducing microorganisms may be adversely affected by plants with a high nitrogen uptake rate. Our work puts forward the role of traits related to nitrogen in plant-microbe interactions, whereas carbon is commonly considered as the main driver. It also suggests that plant traits related to ecophysiological processes, such as nitrogen uptake rates, are more relevant for understanding plant-microbe interactions than composite traits, such as nitrophily, which are related to a number of ecophysiological processes.

Summary The baseline toxicity of 22 acetolactate synthase (ALS)inhibiting herbicides and the cross-resistance patterns of chlorsulfuron- and imazapyr-resistant (R) lines on these 22 ALS-inhibiting herbicides were investigated using the model species Arabidopsis thaliana. The 22 herbicides consisted of 18 sulfonylureas (SU), three imidazolinones (IMI) and one triazolopyrimidine (TP). The ED50 values (doses of herbicides required to reduce dry matter by 50%) of the post-emergence-treated Col and Ler susceptible (S) lines ranged from 22 to 4822 mg ha )1 and from 17 to 3143 mg ha )1 respectively. The csr1-1 chlorsulfuron-resistant line (substitution of Pro197 to Ser) conferred a high resistance to the only TP tested as well as to nine SU herbicides (R:S ratio ‡30), a low resistance to two SU herbicides (R:S ‡5 and

In grasses, residues homologous to residues Ile-1,781 and Ile-2,041 in the carboxyl-transferase (CT) domain of the chloroplastic acetyl-coenzyme A (CoA) carboxylase (ACCase) from the grass weed black-grass (Alopecurus myosuroides [Huds.]) are critical determinants for sensitivity to two classes of ACCase inhibitors, aryloxyphenoxypropionates (APPs) and cyclohexanediones. Using natural mutants of black-grass, we demonstrated through a molecular, biological, and biochemical approach that residues Trp-2,027, Asp-2,078, and Gly-2,096 are also involved in sensitivity to ACCase inhibitors. In addition, residues Trp-2,027 and Asp-2,078 are very likely involved in CT activity. Using three-dimensional modeling, we found that the side chains of the five residues are adjacent, located at the surface of the inside of the cavity of the CT active site, in the vicinity of the binding site for APPs. Residues 1,781 and 2,078 are involved in sensitivity to both APPs and cyclohexanediones, whereas residues 2,027, 2,041, and 2,096 are involved in sensitivity to APPs only. This suggests that the binding sites for these two classes of compounds are overlapping, although distinct. Comparison of three-dimensional models for black-grass wild-type and mutant CTs and for CTs from organisms with contrasted sensitivity to ACCase inhibitors suggested that inhibitors fitting into the cavity of the CT active site of the chloroplastic ACCase from grasses to reach their active sites may be tight. The three-dimensional shape of this cavity is thus likely of high importance for the efficacy of ACCase inhibitors.

Dominance of a resistance trait can be defined as a measure of the relative position of the phenotype of the heterozygote RS compared with the phenotype of the two corresponding homozygotes, SS and RR. This parameter has been shown to have primary importance in the dynamics of pesticide resistance evolution. Literature on insecticide resistance suggests that dominance levels in the presence of insecticide vary greatly from completely recessive to completely dominant. With insecticides, both the chemical applied and the dosages used have been demonstrated to affect the dominance. By contrast, almost all herbicide resistances have been found to be inherited as partially to totally dominant traits. This discrepancy between weeds and insects may partly result from the methodologies applied to measure the dominance, ie a single dose for herbicide versus several doses for insecticide. Using two well-known resistances (csr1-1 and csr1-2) to acetolactate synthase (ALS) inhibitors in Arabidopsis thaliana (L) Heynh (mouse-ear cress), we used several herbicide doses to determine the dominance level to six ALS-inhibiting herbicides. The dominance level in the presence of herbicide varied from completely dominant to completely recessive, depending on the resistance allele and the herbicide tested. The dominance of the csr1-1 and csr1-2 resistance alleles ranged from 0 (completely recessive) to 1.1 (dominant) and from 0 to 0.3 (partially dominant), respectively. The recessivity of some resistance alleles in the presence of herbicide could lead to the development of improved resistance management in order to delay or avoid herbicide resistance evolution, especially in the control of outcrossing weed species.

A simple method based upon allele-specific PCR was developed to detect an isoleucine-leucine substitution in the gene encoding chloroplastic acetyl-coenzyme A carboxylase (ACCase) in two gramineous weeds: Lolium rigidum Gaud and Alopecurus myosuroides Huds. Analysis of 1800 A myosuroides and 750 L rigidum seedlings showed that the presence of ACCase leucine allele(s) conferred cross-resistance to the cyclohexanedione herbicide cycloxydim and to the aryloxyphenoxypropionate herbicides fenoxaprop-P-ethyl and diclofop-methyl. Seedlings containing ACCase leucine allele(s) could be either sensitive or resistant to the aryloxyphenoxypropionate herbicides haloxyfop-P-methyl and clodinafop-propargyl. Successful detection of resistant plants in a field population of A myosuroides was achieved using this PCR assay. Using it with basic molecular biology laboratory equipment, the presence of resistant leucine ACCase allele(s) can be detected within one working day.

International audience; Plant nitrophily plays a key role in weed community assembly. To date, the determinants of the success of nitrophilic weeds and of the decline of oligotrophic weeds in intensive cropping systems are not fully understood. This study investigated which ecophysiological traits related to carbon and N nutrition explain plant nitrophily in field annual species. Twelve species (consisting of monocotyledonous and dicotyledonous species, as well as crops and weeds) covering the oligotrophic-nitrophilic continuum were grown in a glasshouse experiment at two contrasted conditions of soil-nitrogen supply. From measured plant growth variables, ecophysiological traits were calculated to account for plant nutritional strategies. Globally, whatever the soil-nitrogen supply, increased nitrophily was associated with a decreased allocation of carbon to root vs. shoot. At high soil-nitrogen only, increased nitrophily was also generally associated with an increased nitrogen uptake activity per unit of root biomass, pointing to a trade-off between oligotrophic species with a preferential carbon investment in root structure and nitrophilic species with a preferential carbon investment in root activity. Beyond these global trends explaining nitrophily, results also show that different strategies are possible for plant species to be considered as nitrophilic. This study provides evidence on why adjusting N fertilisation in quantity, in space (with nitrogen fertiliser placement on the row) or in time can be useful strategies to manage the most nitrophilic weeds, which are also the most problematic in intensive cropping systems.

BACKGROUND: Target-site-based resistance to acetyl-CoA carboxylase (ACCase) inhibitors in Alopecurus myosuroides Huds. is essentially due to five substitutions (Isoleucine-1781-Leucine, Tryptophan-2027-Cysteine, Isoleucine-2041-Asparagine, Aspartate-2078-Glycine, Glycine-2096-Alanine). Recent studies suggested that cross-resistance patterns associated with each mutation using a seed-based bioassay may not accurately reflect field resistance. The authors aimed to connect the presence of mutant ACCase isoform(s) in A. myosuroides with resistance to five ACCase inhibitors (fenoxaprop, clodinafop, haloxyfop, cycloxydim, clethodim) sprayed at the recommended field rate. RESULTS: Results from spraying experiments and from seed-based bioassays were consistent for all mutant isoforms except the most widespread, Leucine-1781. In spraying experiments, Leucine-1781 ACCase conferred resistance to clodinafop and haloxyfop. Some plants containing Leucine-1781 or Alanine-2096 ACCase, but not all, were also resistant to clethodim. CONCLUSION: Leucine-1781, Cysteine-2027, Asparagine-2041 and Alanine-2096 ACCases confer resistance to fenoxaprop, clodinafop and haloxyfop at field rates. Leucine-1781 ACCase also confers resistance to cycloxydim at field rate. Glycine-2078 ACCase confers resistance to all five herbicides at field rates. Only Glycine-2078 ACCase confers clethodim resistance under optimal application conditions. It may be that Leucine-1781 and Alanine-2096 ACCases may also confer resistance to clethodim in the field if the conditions are not optimal for herbicide efficacy, or at reduced clethodim field rates. Copyright © 2008 Society of Chemical Industry

We have investigated the process of evolution of target-site-based resistance to herbicides inhibiting acetyl-CoA carboxylase (ACCase) in nine French populations of black-grass (Alopecurus myosuroides Huds). To date, two different ACCase resistant alleles are known. One contains an isoleucine-to-leucine substitution at position 1781, the second contains an isoleucine-to-asparagine substitution at position 2041. Using phylogenetic analysis of ACCase sequences, we showed that 1781Leu ACCase alleles evolved from four independent origins in the nine black-grass populations studied, while 2041Asn ACCase alleles evolved from six independent origins. No geographical structure of black-grass populations was revealed. This implies that these populations, although geographically distant, are, or have until recently been, connected by gene flows. Comparison of biological data obtained from herbicide sensitivity bioassay and molecular data showed that distinct resistance mechanisms often exist in a single black-grass population. Accumulation of different resistance mechanisms in a single plant was also demonstrated. We conclude that large-scale evolution of resistance to herbicides in black-grass is a complex phenomenon, resulting from the independent selection of various resistance mechanisms in local black-grass populations undergoing contrasted herbicide and agronomical selection pressures, and connected by gene flows whose parameters remain to be determined. Copyright © 2003 Society of Chemical Industry

Chloroplastic acetyl CoA-carboxylase (ACCase) is the target of widely used, specific graminicide herbicides: cyclohexanediones (CHDs) and aryloxyphenoxypropionates (APPs). Resistance to these compounds is a worldwide, increasing problem. Population genetic studies aimed at understanding the dynamics of this situation and the diffusion of resistance genes within and between weed populations are challenging because biological assays are not adequate for this purpose, and because different mechanisms of resistance confer a similar resistance phenotype. Molecular markers for specifically detecting resistance genes are therefore urgently needed to conduct such studies. For this purpose, we cloned and sequenced the whole gene encoding chloroplastic ACCase in Alopecurus myosuroides Huds. (Black-grass). We identified two point mutations at nucleotide 5,341 that both cause an isoleucine-leucine substitution at position 1,781. Three bi-directional allele-specific PCR assays were developed, each detecting two distinct ACCase alleles with a single PCR reaction. The sensitivity of 1,190 seedlings of A. myosuroides to one CHD and one APP was determined. Genotyping revealed that, although resistant plants were only selected by APPs, the (1,781)Leu ACCase allele is a widespread, dominant gene of resistance to both APPs and CHDs. No other ACCase allele associated with resistance could be identified in this work. Useful applications of allele-specific PCR markers are population genetic studies as well as routine molecular diagnosis of herbicide resistance.

summary The morphology, flowering behaviour and esterase genotype was examined for six F2 progenies of hybrids between the two main variants of Poa annua L. (namely, annual-erect and perennial-prostrate). The association of flowering time with leaf and tiller morphology and Est 1 allozyme genotype, previously described for the two variants in populations from Burgundy, was fully broken in the F2 generation, indicating that no linkage blocks were involved. Whether selection maintains the association of isozyme genotype and growth form is discussed.