Factors affecting seed germination and emergence of Aegilops tauschii

Information on seed germination and emergence ecology of Aegilops tauschii is scant, despite it being a widespread invasive weed in China. We conducted this study to determine the effects of various factors on seed germination and seedling emergence in three A. tauschii populations. Seeds germinated across a wide range of temperatures (5–35°C), with germination of over 90% at 15–20°C. Germination was completely inhibited when dry seeds were exposed to a temperature of 160°C for 5 min; a similar response was observed for pre-soaked seeds at 100°C. Light was neither required for nor inhibited germination. Germination was not significantly affected by pH. Aegilops tauschii was relatively tolerant to low osmotic potential and high salt stress: over 80% of seeds germinated at −0.3 MPa, and all three populations germinated in the presence of 400 mM salt (NaCl) although salt tolerance varied among the populations. Seeds buried at depths of 1–3 cm emerged well, but emergence was completely inhibited at depths greater than 8 cm. The addition of maize straw caused a linear reduction in seedling emergence, although the rate of reduction varied among the populations. The results of this study have contributed to understanding the requirements of A. tauschii germination and emergence and optimising an integrated management system for this weed in Huang–Huai–Hai Plain of China. In addition, our study provides data for development of models to predict the geographical distribution of this weed.     

The influence of temperature on germination of gorse (Ulex europaeus L.)

The germination of U. europaeus seed was investigated at temperatures between 4 and 40°C on a thermal gradient bar. At constant temperature the germination rate increased linearly with temperature from a minimum near 0°C to an optimum at 18°C and then decreased to a maximum at 26°C. At still higher temperatures seeds became imbibed but only germinated when transferred to cooler conditions. Above 35°C viability was lost. The rate of imbibition prior to germination increased with temperature over the whole range. Germination was not increased either by a light treatment or by potassium nitrate. Germination was not stimulated by a wide range of alternating temperature regimes and at mean temperatures below the optimum the germination rate could be adequately predicted from a formula based on performance at constant temperature. At higher temperatures the observed rates became progressively slower than predicted. Seeds from three different sources were tested, all showing similar relationships between germination rate and temperature but differing in the proportion of hard seed. The proportion was least with seed extracted from the soil, intermediate with a collection from the surface and greatest from pods. The findings are discussed in relation to germination in the field following clearing of gorse thicket.     

Dormancy studies in three populations of Poa annua L. seeds

Seeds of Poa annua from original collections in Louisiana, Maryland and Wisconsin were grown together in Louisiana over a 3-year period. The freshly harvested seeds and samples stored in moist soil at 30°C were tested for germination at a range of temperatures to compare dormancy and germination characteristics. Seeds of the Louisiana population were dormant over the germination temperature range of 5–25°C, and imbibed storage for 2 weeks did not break dormancy. Freshly harvested seeds of the Maryland population germinated well (78%) at 10°C. With 1 week of imbibed storage at 30°C, germination was good over the range from 5 to 15°C and near 50% at 20°C. Storage for 2 weeks had little further effect. Freshly harvested seeds of two Wisconsin populations germinated above 50% throughout the range of temperatures, and imbibed storage for 2 weeks at 30°C had no effect on germination. The variations in the dormancy of freshly harvested seeds and the varying responses of dormancy breaking from storing imbibed seeds at 30°C suggests that these populations have adapted to avoid high summer temperatures in Louisiana and Maryland but to grow as a summer annual in Wisconsin.     

Karrikins promote germination of physiologically dormant seeds of Chrysanthemoides monilifera ssp. monilifera (boneseed)

Physiological dormancy in weed species has significant implications for weed management, as viable seeds may persist in soil seedbanks for many years. The major stimulatory compound in smoke, karrikinolide (KAR₁), promotes germination in a range of physiologically dormant weed species allowing targeted eradication methods to be employed. Control of Chrysanthemoides monilifera ssp. monilifera (boneseed), a Weed of National Significance in Australia, may benefit from adopting such an approach. In this study, we hypothesised that seeds of C. monilifera ssp. monilifera exhibit physiological dormancy, germinate more rapidly as dormancy is alleviated, show fluctuations in sensitivity to KAR₁ and form a persistent soil seedbank. Seeds responded to 1 μM KAR₁ (40–60% germination) even during months (i.e. March, April, July, August) when seeds were observed to be more deeply dormant (control germination: 7–20%). Seeds germinated readily over a range of cooler temperatures (i.e. 10, 15, 20, 20/10 and 25/15°C) and were responsive to KAR₂ (~50% germination) as well. Eradication efforts for C. monilifera ssp. monilifera may benefit from use of karrikins to achieve synchronised germination from soil seedbanks, even at times of the year when C. monilifera ssp. monilifera seeds would be less likely to germinate, allowing more rapid depletion of the soil seedbank and targeted control of young plants.     

Germination response of Phalaris paradoxa L. seed to different light qualities

The influence of different light regimes on the germination of Australian and English populations of Phalaris paradoxa L. (awned canary‐grass) seed was investigated to determine the impact of changing tillage practices on weed infestation. Seeds of all biotypes were highly viable, but differed in levels of innate dormancy (26–99%). In one experiment seed from a single Australian biotype, either enclosed in the spikelet glumes or having the spikelet glumes removed, were exposed to nine light treatments. Germination was stimulated by red and white light, but was inhibited by far‐red light. Time to 50% germination was less for seed enclosed in the spikelet glumes than for naked caryopses, although the final percentage of seed germinating when still enclosed in the spikelet glumes was significantly lower than for naked caryopses. In another experiment, six Australian and English biotypes with varying dormancy characteristics were exposed to eight light treatments. Red light did not stimulate germination in the deeply dormant biotype, however stimulated all other biotypes. Germination in darkness was below 20% in all biotypes except for one where germination was 51%. To overcome dormancy seeds were imbibed and placed in darkness at 16°C for either 7 or 14 days prior to exposure to red or white light for a single 15‐min period. Dormancy in all biotypes was overcome indicating that a period of burial may decrease the dormancy level and increase seed sensitivity to light. This increased light sensitivity suggests that exposure to light during tillage may stimulate germination in P. paradoxa seed.     

Temperature requirements for germination of buried seeds of Aethusa cynapium L.

Freshly-collected mature mericarps of Aethusu cynapium were dormant, but some germinated at alternating (16 h low/8 high) temperatures when the seed coverings were removed. Burial during winter increased percentage germination and the temperature range over which it took place. In late spring the range narrowed, first at low and then at higher temperatures, widening again in autumn. Moist storage at both low (4°C) and high (30°C) temperatures overcame dormancy, but exposure to 30°C inhibited subsequent germination at low temperatures. Germination of intact mericarps was consistently lower than that of de-coated seeds. The cyclic change in dormancy status of the seeds appears to interact with the restricting effects of the seed coverings and perhaps other factors in determining the consistent pattern of spring emergence in A. cynapium.     

Factors affecting the seed germination and seedling emergence of redflower ragleaf (Crassocephalum crepidioides)

Redflower ragleaf (Crassocephalum crepidioides) is a weed, as well as a minor vegetable, in tropical and subtropical regions of the world. The influence of environmental factors and seed conditions on the germination and emergence of redflower ragleaf have been evaluated in order to help understand its distribution and to develop effective management strategies. The seeds germinated at a constant temperature in the range of 10–30°C and reached a maximum at 15–20°C. The highest germination rate was recorded at an alternating temperature of 20/15°C (day/night). The seeds germinated over a wide pH range (2–12), with the highest germination rate at between 4 and 10. Germination under saturated and flooded conditions was also high. The germination of seeds from opened (mature) capitula was significantly higher than from partially opened or unopened capitula. The germination of seeds without a pappus was significantly higher than for seeds with a pappus. The germination rate of 1 year old seeds decreased drastically when compared to that of freshly harvested seeds. The seedling emergence rate was ～63% for those seeds placed on the soil surface, but no seedling emerged from a depth of ≥1 cm. These results indicate that redflower ragleaf seeds can germinate in various environmental conditions, but that the percentage that germinates will be different in different environments. Regeneration could be effectively prevented by at least a 1 cm soil covering or by destroying the plant before the capitula open. In contrast, freshly harvested seeds from opened capitula should be sown on the soil surface when redflower ragleaf is to be cultivated as a vegetable.     

Germination characteristics of Amaranthus quitensis as affected by seed production date and duration of burial

Thermal requirements for the germination of Amaranthus quitensis, a common annual weed in Argentina, were studied. In addition, temporal changes in dormancy from seeds produced at different times during the growing season were examined. For this second objective, thermal and light requirements for germination were tested in seeds buried at different depths, with or without crop residues. Base and optimum temperatures for germination rates were 12.8°C and 37°C respectively. At dispersal time, maximum percentage germination was 60–70% and this was generally recorded at 35°C/25°C in a 14-h photoperiod. Seed germination tended to increase in later seed collection dates. Seeds of A. quitensis showed seasonal changes in germinability in the soil. In winter, germination of retrieved seeds increased to over 90% until summer, after which there was a decrease until the following winter when germination was close to 40%. There were no differences in germinability between burial depths and crop residue levels. Germination requirements for alternating temperatures and light tended to disappear after burial. Initial viability was 99% and declined slightly during burial. Soil temperature seems to play a crucial role not only by regulating seasonal changes in dormancy, but also by defining the percentage and the germination rate in non-dormant seeds.     

Seed germination ecology of creeping mannagrass (Glyceria acutiflora) and response to POST herbicides

Creeping mannagrass is a perennial grass weed widely distributed in China and is becoming increasingly problematic in nurseries and landscapes in some regions. Understanding the germination ecology and response to commonly available POST herbicides of this weed is critical to determining its adaptive capabilities and potential for infestation, and assist in the development of effective control strategies. In the light/dark regime, creeping mannagrass germinated over a wide range of temperatures (15/5 to 30/20°C), with maximum germination at 20/10°C (95%). No seed germinated at 35/25 or 10/0°C. The time required for 50% of maximum germination increased as temperature decreased. Compared with the light/dark conditions, germination was slightly stimulated when seeds were placed in the dark. Creeping mannagrass is moderately tolerant to osmotic and salt stress, which had 53 and 50% germination rates at ?0.6 mPa osmotic potential and 200 mM NaCl concentration, respectively. Seedling emergence of the seeds buried at a depth of 0.5 cm (86%) was higher than those sowing on the soil surface (17%), but declined with burial depth increasing. There were no differences in the emergence rates from a burial depth 0.5–2 cm. Few seeds (4%) could emerge when seeds were sowed at a depth of 8 cm. POST application of haloxyfop‐R‐methyl, quizalofop‐p‐ethyl, sethoxydim, and pinoxaden provides 100% control of creeping mannagrass at the three‐leaf to five‐leaf stages. To achieve 80% control with clodinafop‐propargyl, mesosulfuron‐methyl, and fenoxaprop‐p‐ethyl, herbicides had to be applied at the three‐leaf stage.     

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.     

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.     

The role of light and alternating temperatures on germination of Polygonum aviculare seeds exhumed on various dates

The annual dormancy cycle was investigated in buried seeds of Polygonum aviculare L. exposed to natural temperature changes in Lexington, Kentucky, U.S.A. Seeds were exhumed monthly from December 1984 to February 1987 and tested in light (14-h daily photoperiod) and continuous darkness at 12/12-h daily alternating temperature regimes of 15/6, 20/10, 25/15, 30/15 and 35/20°C. During autumn and winter, seeds became non-dormant, and in March 1985 they germinated to 95-100% at all thermoperiods in light and to 7-61% in darkness. Seeds remained non-dormant during spring but became more specific in their germination requirements in early summer. During July and August 1985, seeds germinated to 17-53% in light at 30/15 and 35/20°C but to 0-10% at all other test conditions. By September, about 65% of the seeds were dormant, but the others were able to germinate under the higher alternating temperatures in light. A similar seasonal cycle was recorded in the following year through to the spring of 1987. The results confirm the seasonal pattern of dormancy in this species (Courtney, 1968) but indicate that alternating temperatures combined with light are important in determining germination potential in P. aviculare.     

A high temperature requirement for after ripening of imbibed dormant Poa annua L. seeds

Freshly harvested seeds of Poa annua L. collected in south Louisiana were stored in moist soil at seven temperatures between 5°C and 35°C. At monthly intervals, seed lots were removed and germinated at each of the seven temperatures. Seed were dormant for at least 1 month at all test temperatures. Seeds stored for 2 months at 30 and 35°C showed conditional dormancy; there was 100% germination at 10 or 15°C, and poorer germination at 5 or 20°C. Seeds started to lose viability after 2 months at 35°C and were dead after 7 months. In seeds stored at 10–30°C, there were increased percentages and a wider range of germination temperatures as storage time or storage temperatures increased. Seeds stored at 10°C remained dormant for 9 months, but by 12 months of storage the seeds germinated only at 5 or 10°C. Nearly all seeds stored at the same temperatures in air dry soil remained dormant for 6 months, regardless of storage temperature. These results differ from other reports of low temperatures breaking seed dormancy in Poa annua L. and suggest an adaptation to subtropical climates.     

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.     

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.     

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.     

Longevity,dormancy and germination of Cyanus segetum

Cyanus segetum is an iconic, colourful weed in arable fields that provides ecological and societal services. To understand better both the infestation dynamics of C. segetum as an abundant, harmful weed and maintain sustainable populations where it provides beneficial services, we compared information on seed dormancy, seed longevity and germination conditions in two populations. Persistence of seeds buried in the soil was low, with <10% viable after 3 years. Periodic dormancy cycling was observed over the 4 years in the soil, with a maximum of dormant seeds in the spring and a minimum in the autumn; however, 20% of the seeds were non‐dormant all the time. Seeds of C. segetum were positive photosensitive, but light requirement varied among populations. Base water potential for germination was ?1 MPa. Base temperature ranged from 1 to 2°C. Optimum temperature for germination was about 10 to 15°C, but the mean thermal time varied greatly between populations, from 80 to 134 day °C. Photoperiod and temperature combinations had no effect on germination percentage, but both reduced the germination rate. Burial deeper than 2 cm greatly reduced germination and seedling emergence strongly decreased at depths >0.5 cm. No seeds buried deeper than 8 cm emerged. Low seed longevity and a wide range of germination conditions could partly explain the rapid disappearance of C. segetum populations after herbicide application began in western Europe. However, yearly sowing in restoration areas does not seem to be essential.     

Modelling Chloris virgata germination and emergence under different temperature and light quality conditions

Chloris virgata is a problematic weed around the world. Prediction of weed germination rates could be a useful strategy to optimise timing of weed control actions. We studied the germination and emergence of C. virgata collected seeds under different after-ripening treatments and different exhumation dates after seed dispersal, to estimate seed dormancy level and predict weed emergence dynamics under field conditions. Three experiments were conducted under controlled conditions to determine base, optimum and maximum germination temperatures (T_b, T_o and T_m respectively) and comprised: (a) exposure of seeds to gradually increasing and decreasing temperatures between 5 and 35°C; (b) exposure of seeds to different constant temperatures; and (c) exposure of seeds to different light quality conditions (red – far red ratio) and temperature regimes (constant and alternating temperatures). To explore genuine environmental conditions, a field experiment was performed to determine weed emergence under different shading levels. Finally, with the data obtained, a thermal time model for dormancy release was used to predict C. virgata seedling emergence in the Argentine Pampas region. Seeds after-ripened in cold and wet conditions and constant 25°C showed the highest germination percentages. The values of T_b (7°C), T_o (28°C) and T_m (40°C) remained constant at all exhumation dates. Neither light quality nor thermal regime modified the final germination percentages. However, shading delayed seedling emergence under field conditions, even when it was adjusted by thermal time. These results may allow predicting C. virgata emergence in temperate regions and help to improve weed control in integrated weed management strategies.     

Seed production and subsequent seed germination of Senecio vulgaris (groundsel) grown alone or in autumn-sown crops

Senecio vulgaris is a common weed of agriculture in the UK, but is also of food value to invertebrates and birds. Thus, it may be beneficial to retain it within agricultural ecosystems to enhance overall biodiversity. A less intensive approach to weed management requires a sound understanding of weed population dynamics so as to avoid unacceptable population growth. Experiments were carried out in 2003 and 2004 to assess seed production, and subsequent germination, by S. vulgaris growing alone, in winter wheat, or in winter field beans. Plant and seed samples were collected during May and June. There was a strong allometric relationship between capsule number and plant weight, irrespective of the year or the presence of crop competition. Numbers of seeds/capsule varied slightly from 51 to 66 seeds per capsule. Plants growing alone were estimated to produce 8471 to 12 887 seeds per plant, whilst those in wheat only 923 to 2156. Germination tests in Petri dishes in incubators showed that virtually all seeds were viable and germinated under daily alternating light:dark conditions within 10 days. Seeds in continuous dark germinated less readily, reaching only 30% after 21 days. On the basis of this and other published work, it would appear that the retention of S. vulgaris in arable fields will not pose a major threat to the long-term viability of crop production.     

Germination of proximal and distal seeds of Poa trivialis L. from contrasting habitats

Seeds of Poa trivialis L. were collected from one grassland and two arable habitats. Seeds from the grassland population were less dormant than the arable populations. Distal (upper) seeds were consistently more dormant than proximal (basal) seeds. Dry storage at 4°C and 15°C for 4 weeks after shedding resulted in a slightly greater loss of dormancy than storage at 23°C. Germination was enhanced by subjecting seeds stored at 15°C to repeated hydration and dehydration cycles. Germination of P. trivialis seeds was density-dependent. Germination of distal seeds was particularly impaired at high densities. The ecological implications of these results are discussed in relation to seed survival strategies.