Evolution and spread of glyphosate resistance in Conyza bonariensis in California and a comparison with closely related Conyza canadensis

Glyphosate‐resistant weeds are an increasing problem in perennial cropping systems in the Central Valley of California, USA. To elucidate the evolutionary origins and spatial spread of resistance, we investigated the geographical distribution of glyphosate resistance and the population genetic diversity and structure of Conyza bonariensis and compared the results with previously studied C. canadensis. Thirty‐five populations from orchards and vineyards across the Central Valley were sampled. Population genetic structure was assessed using microsatellite markers. Population‐level resistance was assessed in glasshouse screening of plants grown from field‐collected seed. Bayesian clustering and analyses of multilocus genotypes indicated multiple origins of resistance, as observed in C. canadensis. Pairwise F_ST analysis detected spatial spread of resistance in the south of the Central Valley, also similar to C. canadensis. The results strongly indicate that the southern valley was an environment markedly more suitable than the northern valley for resistance spread and that spread in Conyza species was driven by increased uniformity of strong selection in the southern valley, due to recent regulation on herbicides other than glyphosate. Accordingly, resistant C. canadensis individuals occurred at high frequencies only in the southern valley, but interestingly resistant C. bonariensis occurred at high frequencies throughout the valley. Expression of resistance showed varying degrees of plasticity in C. bonariensis. The lower selfing rate and substantially greater genotypic diversity in C. bonariensis, relative to C. canadensis, indicate greater evolutionary potential over shorter time periods. Interspecific hybridisation was detected, but its role in resistance evolution remains unclear.     

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

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

Factors affecting seed germination and emergence of Gomphrena perennis

Controlled growth chamber experiments were conducted to determine factors affecting seed germination and emergence of the troublesome weed Gomphrena perennis. The objective of this research was to examine the effects of temperature, light, moist chilling, osmotic potential, dry storage and depth of seed burial on G. perennis germination and emergence. The optimum temperature for germination was around 15–20°C. Seeds showed germination rates above 90% under 20/10 and 25/15°C temperature regimes. The minimum exposure to light needed to stimulate germination was 1 min. However, the light requirement was reduced after a long storage period. Furthermore, germination was high (>90%) in all moist‐chilling treatments tested. Germination was highly sensitive to increasing osmotic stress. The highest germination percentage (94%) was achieved at 0 MPa, and decreasing osmotic potential from 0 to ?0.3 MPa reduced germination to 11%. The highest seedling emergence occurred for seeds placed from 0 to 1 cm deep, and no seedlings emerged from a 5‐cm burial depth. Gomphrena perennis has a suitable environment in a no‐till soybean field, where seeds remaining on the surface have the required temperature, light and depth needed for germination.     

A hydrothermal seedling emergence model for Conyza bonariensis

Conyza bonariensis is a South American native annual Asteraceae that has been introduced to the Mediterranean, where it behaves as a ruderal plant and a weed that is difficult to control in several crops. The development of predictive models can contribute to control measures at early growth stages, but currently there are no studies to predict seedling emergence of Conyza species. Our objectives were to develop and evaluate a model for predicting emergence response of C. bonariensis to the soil hydrothermal environment. A hydrothermal seed germination model was fitted to time course germination data from germination tests carried out at different constant temperatures and water potentials with the aim of establishing the hydrothermal parameters characterising C. bonariensis seed germination. The relationship between cumulative seedling emergence and cumulative hydrothermal time under field conditions was analysed using the Gompertz function. Model development was based on 2 years' data from a field experiment. Base temperature and base water potential for seed germination were estimated at 10.6°C and ?0.70 ± 0.151 MPa, respectively. The emergence model showed a very good fit to the experimental data. According to this model, seedling emergence starts at 15 accumulated hydrothermal time (HTT) after sowing, and 50 and 95% emergence is completed at 53 HTT and 105 HTT, respectively. For model evaluation, independent field experiments were carried out in two localities. Cumulative seedling emergence was accurately predicted by the model. Results indicate that this model can be useful as a predictive tool contributing to effective control of C. bonariensis populations.     

Seed germination response to temperature for a range of international populations of Conyza canadensis

Conyza canadensis is a surface‐germinating ruderal facultative winter annual with recruitment that is highly susceptible to changes in microsite conditions. A key adaptive characteristic for a facultative winter annual species, like C. canadensis, is germination response to temperature. The objective of this study was to determine the germination response to temperature for C. canadensis seed sourced from regions around the world with differing climates and, by doing so, gain insight into the role that seed germination biology plays in the adaptiveness and weediness of facultative winter annual weeds. Seed was sourced from populations in Málaga, Spain, Hertfordshire, UK, Shiraz, Iran and southern Ontario, Canada, and grown out in a common garden under controlled conditions to produce seed for this study. These seeds were then subjected to temperatures from 6.5 to 20°C at 1.5°C increments using a thermogradient plate. Cumulative daily germination counts for 30 days were recorded. Results indicated that temperature and source location had a significant effect on germination response. Estimated base germination temperature ranges were significantly different among the populations [Ontario (8–9.5°C), Iran (9.5–11°C), Spain (12.5–14°C), UK (11–12.5°C)], as were accumulated growing degree days (GDDs; d°C) required to reach 50% germination. For three of the four populations, estimated base germination temperature range values were below those previously reported in the literature. These differences are most likely rapid evolutionary adaptations to local climate and highlight the potential C. canadensis has to be problematic as a native and invasive species.     

Influence of environmental factors on seed germination and emergence of Iresine diffusa

Iresine diffusa has become more abundant under no‐till soyabean in Argentina. The influence of temperature, light, cold‐wet storage, osmotic potential, dry storage and depth of seed burial on germination and emergence of I. diffusa was examined in a growth chamber experiment. Iresine diffusa seeds germinated at the highest proportion (>0.80) in all fluctuating day/night temperatures tested. Conversely, under a constant temperature regime, maximum germination rates occurred at 15 (0.78) and 20°C (0.82), and minimum germination rates occurred at 10 (0.19) and 30°C (0.36). Seed germination was not influenced by light exposure. However, germination decreased after 12 (0.76) and 16 (0.65) weeks in cold‐wet storage. To reduce germination significantly, ?0.4 MPa of osmotic potential (induced by PEG‐6000) or 120 mmol L^?1 of salt (NaCl) concentration was required. Seeds of I. diffusa showed high viability (0.85) after 720 days of dry storage. Low emergence was recorded for seeds buried at 2 cm, and seedling emergence was completely inhibited when seeds were buried at 5 and 10 cm. Iresine diffusa seeds had high viability and were capable of emerging in a broad range of environmental conditions. The thermal germination conditions, shallow soil depths and high moisture conditions in germination phase for I. diffusa are congruent with the conditions in Argentina no‐tillage soyabean. Thus, no‐tillage could provide better conditions for germination than conventional tillage systems. However, due to the fact that I. diffusa can reproduce by rhizomes, further research should be conducted to understand the relative importance of the vegetative reproductive strategy in relation to the presence and persistence of this weed in fields.     

Growth and development of fall- and spring-planted populations of Conyza canadensis and C. bonariensis

The growth and phenological development of spring- and fall-planted Conyza canadensis and C. bonariensis were studied in Fresno, CA, USA. A glyphosate-resistant (GR) and a glyphosate-susceptible (GS) population of each species were used. Time taken by each plant to reach the rosette, bolting, appearance of first bud, appearance of first open flower, and initial seed set were converted to growing degree days (GDDs). The fall- and spring-planted C. canadensis required similar GDDs to complete their life cycles but the GR type required fewer GDDs than the GS type. In C. bonariensis, the GDDs required to complete its life cycle differed between the fall- and spring-plantings but not between the GR and GS types. The total aboveground biomass of both fall- and spring-planted Conyzas at initial seed set was similar but the spring-planted GS C. canadensis produced more biomass than the GR type, whereas the fall-planted GS C. bonariensis produced more biomass than the GR type. Plant development based on GDD models can have global implications and help in planning for timely applications of post-emergence herbicides.     

Inter‐ and intrapopulation variation in dormancy of Oryza sativa (weedy red rice) and allelic variation in dormancy‐linked loci

Oryza sativa (weedy red rice), the same species as cultivated rice, is a serious problem in rice production worldwide. Seed dormancy contributes to its persistence. We determined the effect of germination temperature and after‐ripening period on germination capacity (GC) of red rice seeds from Arkansas rice fields in three production zones. We also determined the gene diversity (GD) of dormancy‐linked loci among selected populations. The germination behaviour was evaluated at three temperatures (1°C, 15°C and 35°C) and four after‐ripening periods (0, 30, 60 and 90 days) in two independent experiments. Germination response to temperature and after‐ripening time differed among and within populations in each production zone. Overall, populations from the Delta and Grand Prairie were more dormant than those from White River. Regardless of ecotype or production zone, incubation at 35°C (mean GC = 84–100%) favoured the germination of seeds after‐ripened for 60 days. Germination of these seeds was most variable at suboptimal temperature (15°C), with mean GC ranging from 44 to 97%; at 1°C, none of the seeds germinated. Primary dormancy was released in the majority of populations after 90 days of after‐ripening. Blackhull populations generally had lower mean GC than strawhull populations, regardless of temperature, and required longer after‐ripening time to release dormancy. They also showed a higher inter‐ and intrapopulation variation in germination and after‐ripening than strawhulls and had the highest gene diversity (GD = 0.55–0.58) among test populations. Non‐dormant strawhulls were most distant (D = 0.63) from dormant blackhulls. Ecotype influenced genotypic clustering more than the dormancy trait.     

Untersuchungen zur Keimungsbiologie von sechs tropischen Segetalarten

Investigation of the germination of six tropical arable weeds With the six tropical arable weeds studied, Ageratum conyzoides, Blechum brownei, Crassocephalum crepidioides, Mikania micrantha, Paspalum conjugatum and P. paniculatum, the lowest temperatures at which germination occurred were in the range 10–15(20)°C, the highest up to 40°C, with the optimum between 20°C and 35°C. Ageratum conyzoides and C. crepidioides had lower limits of germination temperature than the other species, which explains their comparatively greater incidence at high altitudes. The germination temperatures of both representative temperate species, Alopecurus myosuroides and Chenopodium album were 5 (minimum), 15–25 (optimum) and 40°C (maximum). The germination of A. conyzoides, C. crepidioides and M. micrantha was reduced by an osmotic potential of - 1 bar. At -4·7 bar only P. paniculatum of the tropical species germinated to a small extent whilst Al. myosuroides and Ch. album were not affected. After storage for a year in dry conditions all six tropical species only germinated when exposed to light. After 20 months, C. crepidioides and P. paniculatum would also do so in the dark.     

Effects of after-harvest period and environmental factors on seed dormancy of Amaranthus species

The effects of seven constant temperatures (10–40°C at 5°C intervals) and seven after‐harvest periods (30–540 days after harvest) were evaluated on seed germination of nine Amaranthus species (A. albus, A. blitoides, A. cruentus, A. deflexus, A. graecizans, A. hybridus, A. lividus, A. retroflexus and A. viridis). Seeds of A. blitoides and A. viridis were also tested at alternating temperatures of 10/30°C (12/12 h thermoperiod) in continuous darkness and in an alternating 12/12 h dark/light photoperiod. With the exception of A. blitoides and A. viridis, germination increased as temperature increased from 20 to 35°C; the latter representing the optimum temperature (70–100% germination). At 10 and 15°C constant temperatures, no significant seed germination occurred in A. albus, A. deflexus, A. graecizans and A. lividus, while in A. cruentus, A. hybridus and A. retroflexus there was no germination at 10°C, but at 15°C more than 60% germination occurred. Germination was influenced strongly by after‐ripening period in A. cruentus, A. hybridus and A. retroflexus, partially in A. deflexus, and barely in A. graecizans and A. lividus. Seeds of A. blitoides and A. viridis required alternating temperatures and light to achieve high germination percentage (>90%). Primary dormancy in Amaranthus plays a fundamental role in extending germination over a longer period, so that the probability of seedling survival is maximised. The present study adds to the understanding of the environmental control and germination ecology of Amaranthus species and provides data that can contribute to predicting weed emergence dynamics.     

Factors affecting the seed germination and seedling emergence of muskweed (Myagrum perfoliatum)

Myagrum perfoliatum is a noxious broad‐leaved weed in western Iranian farming systems. A better understanding of the timing of seedling emergence would facilitate the development of better control strategies for this weed. Therefore, the objective of this study was to examine the effects of different factors on muskweed seed germination. Only 2.8% of the seeds of this species, which are encapsulated in siliques, germinated by, while the seeds that had been removed from the siliques had a 50% germination rate. The immersion of muskweed fruits in concentrated sulfuric acid for 110 min was the best treatment for promoting germination. Gibberellic acid stimulated the germination of the naked seeds by 29.1%, potassium nitrate (40 mmol L^‐1) increased the germination rate to 71%, while higher concentrations of potassium nitrate inhibited germination. The optimum germination temperature for the naked seeds was 20/10°C (day/night) and light was not required for germination. No seedling emerged when the seeds were buried 6 cm deep. The seeds were sensitive to both osmotic and salinity stress, but they germinated to 46–49% over a pH range of 4–10. The results of this study revealed that the seeds of M . perfoliatum have physiological dormancy and that it is slowly broken via after‐ripening. However, the fruit wall can prevent germination after physiological dormancy is broken. Thus, this species has the potential to form a persistent seed bank because of the presence of the fruit wall.     

Germination ecology of seeds of the annual weeds Capsella bursa-pastoris and Descurainia sophia originating from high northern latitudes

Low temperatures may inhibit dormancy break in seeds of winter annuals, therefore it was hypothesized that seeds of Capsella bursa‐pastoris and Descurainia sophia that mature at high latitudes in late summer–early autumn would not germinate until they had been exposed to high summer temperatures. Consequently, germination would be delayed until the second autumn. Most freshly matured seeds of both species collected in August and September in southern Sweden were dormant. After 3 weeks of burial at simulated August (20/10°C) and September (15/6°C) temperatures, 28 and 27%, respectively, of the C. bursa‐pastoris and 56 and 59%, respectively, of the D. sophia seeds germinated in light at 15/6°C. In contrast, in germination phenology studies conducted in Sweden, only a few seeds of either species germinated during the first autumn following dispersal. However, there was a peak of germination of both species the following spring, demonstrating that dormancy was lost during exposure to the low habitat temperatures between late summer and early autumn and spring. Nearly 100% of the seeds of both species subjected to simulated annual seasonal temperature changes were viable after 30.5 months of burial. In the burial study, exhumed seeds of C. bursa‐pastoris were capable of germinating to 98–100% in light at the simulated spring–autumn temperature regime (15/6°C) in both spring and autumn, while those of D. sophia did so only in autumn. In early spring, however, seeds of D. sophia germinated to 17–50% at 15/6°C. Thus, most seeds of these two annual weeds that mature in late summer do not germinate in the first autumn, but they may do so the following spring or in some subsequent autumn or spring.     

Asian spiderflower (Cleome viscosa) germination ecology in southern Iran

Cleome viscosa is one of the most important weeds of warm‐season crops in southern Iran. Laboratory experiments were conducted to assess the impact of environmental factors on seed germination of C. viscosa . Freshly harvested seeds exhibited dormancy that was relieved (>90%) after immersion for 20 min in concentrated sulfuric acid. Regardless of the temperature regime, the final percentage of germination in light/dark (69.3%) was significantly higher than in complete darkness (58.3%). The optimum temperature for germination was 35/25°C in both light and dark. No germination was observed at constant temperatures of either 15 or 45°C. The thermal thresholds for seed germination, the base (T _b) and the mean ceiling germination temperatures (T _c(50)) were estimated to be 18.8 and 39.9°C, respectively. A base water potential ( Ψ _b(50) ) of ?0.96 MPa was identified for C. viscosa seeds. The response threshold of C. viscosa to reduce 50% of maximum germination for salinity was estimated to be 255 mM. Seeds that were placed on the soil surface had the highest percentage of seedling emergence (77.3%), and no seedlings emerged from seeds placed at a depth of 6 cm. The findings of this study could help to improve the integrated weed management strategies for this species.     

Effect of soil moisture during stratification on dormancy release in seeds of five common weed species

Although the effects of cold stratification on the release of physiological dormancy in seeds have been studied extensively, knowledge of the role of soil moisture content on seed dormancy release during cold stratification is limited. Our study determined seed dormancy characteristics and the effect of soil moisture content on seed dormancy breakage during cold stratification in the five common weed species Amaranthus retroflexus, Chenopodium album, Chenopodium hybridum, Plantago lanceolata and Setaria glauca. Seeds of all five species were dormant at the time of harvest and their germination response to light and temperature varied. Soil moisture content had a significant effect on seed dormancy release of all species except P. lanceolata. Germination percentage of A. retroflexus, C. album, C. hybridum increased and then decreased as soil moisture content increased, regardless of germination test temperature. The optimal soil moisture content and seed moisture content for dormancy breakage of A. retroflexus, C. album, C. hybridum were 8%, 12%, 8% and 22.0%, 37.7%, 25.7% respectively. Dry storage (after‐ripening) significantly increased germination of S. glauca. Moreover, increasing soil moisture content first slowed and then increased dormancy breakage in S. glauca. These results suggest that data on soil moisture content should be incorporated into models that predict weed seed dormancy breakage and timing of seedling emergence as well as those for weed management.     

Improved chemical control of Conyza bonariensis in wheat limits problems in the following fallow

Conyza bonariensis is a major weed infesting zero‐tilled cropping systems in subtropical Australia, particularly in wheat and winter fallows. Uncontrolled C. bonariensis survives to become a problem weed in the following crops or fallows. As no herbicide has been registered for C. bonariensis in wheat, the effectiveness of 11 herbicides, currently registered for other broad‐leaved weeds in wheat, was evaluated in two pot and two field experiments. As previous research showed that the age of C. bonariensis, and to a lesser extent, the soil moisture at spraying affected herbicide efficacy, these factors also were investigated. The efficacy of the majority of herbicide treatments was reduced when large rosettes (5–15 cm diameter) were treated, compared with small rosettes (<5 cm diameter). However, for the majority of herbicide treatments, the soil moisture did not affect the herbicide efficacy in the pot experiments. In the field, a delay in herbicide treatment of 2 weeks reduced the herbicide efficacy consistently across herbicide treatments, which was related to weed age but not to soil moisture differences. Across all the experiments, four herbicides controlled C. bonariensis in wheat consistently (83–100%): 2,4‐D; aminopyralid + fluroxypyr; picloram + MCPA + metsulfuron; and picloram + high rates of 2,4‐D. Thus, this problem weed can be effectively and consistently controlled in wheat, particularly when small rosettes are treated, and therefore C. bonariensis will have a less adverse impact on the following fallow or crop.     

Seed germination, seedling establishment and growth patterns of wrinklegrass (Ischaemum rugosum Salisb.)

Several laboratory and glasshouse experiments were conducted to assess seed germination, seedling establishment and growth patterns of wrinklegrass (Ischaemum rugosum Salisb.) influenced by temperature and light regimes, and chemical media. Wrinklegrass was a positively photoblastic species, and seed germination was temperature‐dependent and light‐mediated. Seeds soaked in distilled water for 24 h, or oven‐dried at the respective temperature regimes of 15, 20, 25, 30, 35, or 40°C prior to treatment in distilled water and incubated in darkness, failed to germinate. Likewise, no germination prevailed when the seeds were exposed to similar temperature regimes and treated with 0.2 m KNO₃, 5% H₂O₂ or 0.01 m HNO₃, and incubated under continuous darkness. Seeds treated with 5% H₂O₂ at 30°C, or oven‐dried and treated with 0.01% M HNO₃ at 35°C registered 10 and 20% germination. Approximately 75 and 90% of the light‐exposed seeds for all treatments germinated in the first three and six days at 25°C. No germination occurred at 15°C in the first three days after treatment. Seeds subjected to 40°C for six days after treatment recorded 36% germination. The optimum temperatures for seed germination were 25–30°C. Seed drying and soaking treatments widened the windows of the optimal temperatures for wrinklegrass germination. The acidic media of KNO₃, H₂O₂ or HNO₃ favored seed germination. Less than 5% of seed germination occurred with burial or water inundation at depths exceeding 2 cm. Seed burial or inundation at ≥2 cm depths inhibited seed germination. Seeds sown onto moist paddy soils registered ca. 50% germination. Free‐floating seeds on the water surface registered ca. 98% germination within the first six days after seeding. The mean number of seedlings that survived was inversely proportional to water depths, with close to 100% mortality at the 14 cm depths of inundation. Both plant height and seedling survival were linearly proportional to the amount of root mass of seedlings which penetrated the soil. The weed was a prolific seed producer (ca. 6000 seeds/genet or 18 000 seeds/genet per year). The vegetative and reproductive efforts of each wrinklegrass plant registered values of 0.68 and 0.32, respectively.     

Germination ecology of five arable Ranunculaceae species

Germination and emergence are critical life stages for annual plants and so their full understanding is essential for managing arable plant populations. This study investigated the most important species‐specific environmental cues that regulate seed germination and emergence (temperature and light) of the arable Ranunculaceae species Consolida orientalis, Consolida pubescens, Delphinium gracile, Delphinium halteratum ssp. verdunense and Nigella gallica, to propose management strategies for their preservation in agro‐ecosystems. Growth chamber and outdoor pot experiments were conducted for two consecutive seasons to analyse light (complete darkness or 12 h light) and temperature (5/10, 5/15 and 10/20°C) requirements and emergence patterns. The relative light germination requirement (ΔG_light), which extends from ?100 (complete darkness) to 100 (light), was estimated. Weibull functions were fitted to observed emergence (%) in pots. For all species, germination was higher in complete darkness than with a light regime (?60 < ΔG_light < ?95). This dark requirement indicates better germination for buried seeds. A tillage operation just after seed shed is therefore recommended. Consolida spp. germinate and emerge almost exclusively in autumn–winter, while Delphinium spp. and N. gallica can also germinate in spring. These arable plants would be able to adapt to delayed sowings, an important strategy for avoiding early‐emerging competitive weeds. Facultative winter‐germinating species could face early herbicide treatments if sufficient emergence occurs in winter–spring. These results bring new information to help develop conservation strategies for these species in agro‐ecosystems.     

Germination behaviour in seeds of Diplotaxis erucoides and D. virgata

The seed germination behaviour of four populations each of Diplotaxis erucoides (L.) DC. and D. virgata (Cav.) DC, two weedy Brassicaceae species widely distributed in the Mediterranean region, was studied under controlled light, temperature and moisture conditions. Germination rates in D. virgata were generally higher at alternating (25/15^C) and low (15^C) temperatures, whereas in D. erucoides optimal germination rates were achieved at higher temperatures (25^C). No correlation between seed weight and germination was found in either species. In D. virgata, the geographic origin of the seed had an important effect on germination percentage. Great germination variability was found among the populations and even among seeds belonging to the same population. In this species, the application of gibberellic acid stimulated germination, especially in the populations with higher dormancy levels. Dormancy was found in mature D. virgata seeds but not in those of D. erucoides. In both species, germination percentages decreased after a certain period of storage at low temperatures. This period varied depending on the species and the population considered.     

Effects of environmental factors and burial depth on seed germination and emergence of two populations of Caucalis platycarpos

Caucalis platycarpos is a weed species in irrigated and dry land farming systems in East Azerbaijan and Kermanshah provinces of Iran. Experiments were undertaken to compare C. platycarpos seed germination and emergence of a population from each province over a range of environmental factors, burial depth and crop residue treatments. The Azerbaijan population required lower temperatures (20/10°C day/night temperature) for its highest (90%) germination, compared with the Kermanshah population (88% germination at 25/15°C day/night temperature). In both populations, germination was 84–90% over a wide range of light/dark periods (10–24 h light), but considerable reduction (up to 42%) occurred under continuous darkness. The osmotic potential required for 50% inhibition of germination was ?0.54 and ?0.40 MPa for Azerbaijan and Kermanshah populations respectively. The NaCl concentration of 8.83 and 5.71 dS m^?1 caused 50% inhibition of germination in Azerbaijan and Kermanshah populations respectively. The X₅₀ parameter (the burial depth at which emergence is reduced by 50%) for Azerbaijan and Kermanshah population was 2.18 and 2.86 cm respectively. Crop residues had more inhibitory effects on the Azerbaijan than Kermanshah population. Adaptive differentiation of C. platycarpos populations has also resulted in smaller seeds of the Azerbaijan compared with the Kermanshah population and resulted in higher emergence for Kermanshah population seedlings from greater soil depths. These results suggest that differences in germination requirements, drought and salinity tolerance of C. platycarpos populations are correlated with environmental conditions of the habitats of the studied populations.     

Dormancy,germination and emergence of Urochloa panicoides regulated by temperature

Urochloa panicoides is an annual weed of summer crops. In Argentina, in subhumid areas with monsoon rainfall, it germinates and establishes in a single flush. To (i) identify the environmental factors that modify its seed dormancy level and germination and (ii) quantify the parameters describing the thermal behaviour of the germination and emergence dynamics of this weed under non‐limiting water conditions, we established a set of germination experiments performed (i) under controlled conditions using seeds after ripened for 3 or 6 months in different thermal and hydric conditions and (ii) under field conditions, where the soil temperature was modified by applying different shading levels. Seed dormancy level remained high with 3 months after ripening in all treatments. After 6 months, seeds stored at 4°C in dry conditions did not germinate at any temperature, while seeds stored at 25°C in dry conditions and in situ germinated c. 20% and 60% respectively. Germination percentage was higher in seeds harvested before their natural dispersal. The base, optimum and maximum temperatures for seed germination were 6, 35 and 45°C respectively. Shading reduced the number of emerged seedlings, possibly by reducing the soil thermal amplitude. The results explained the dormancy‐breaking mechanism of U. panicoides that allows a high germination rate in the field when rainfall occurs.