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1.
Pyrenophora semeniperda, an important pathogen in Bromus tectorum seed banks in semi‐arid western North America, exhibits >4‐fold variation in mycelial growth rate. Host seeds exhibit seasonal changes in dormancy that affect the risk of pathogen‐caused mortality. The hypothesis tested is that contrasting seed dormancy phenotypes select for contrasting strategies for increasing pathogen fitness, and that increased fitness on nondormant seeds involves a resource trade‐off between toxin production and growth. The strategy for successfully attacking rapidly germinating nondormant seeds at high inoculum loads in autumn involves increased post‐infection aggressiveness to prevent seed escape through germination. An earlier study demonstrated that slow‐growing strains caused higher mortality than faster‐growing strains on nondormant host seeds at high inoculum loads. In this study, production of the toxin cytochalasin B was significantly higher in slower‐growing strains, and was induced only in seeds or in seed‐constituent‐containing media. Its production was reduced in vivo by Bromus tectorum seeds, suggesting direct involvement in pathogenesis on seeds. Fast‐growing strains caused significantly higher mortality than slow‐growing strains at low inoculum loads on dormant seeds, which apparently have resistance that is overcome at high loads or through rapid mycelial proliferation. In a co‐inoculation study, the fast‐growing isolate produced 3 × more stromata than the slow‐growing isolate on dormant seeds, whereas the slow‐growing isolate was twice as successful on nondormant seeds. These results provide evidence that mycelial growth rate variation and associated variation in cytochalasin B production represent a trade‐off maintained through temporally varying selection resulting from seasonal variation in host seed dormancy status.  相似文献   

2.
Galinsoga quadriradiata (hairy galinsoga) and Galinsoga parviflora (smallflower galinsoga, gallant soldier) are very troublesome weeds in many vegetable row crops in Europe. To optimise management strategies for Galinsoga spp. control, an in‐depth study of germination biology was performed. Germination experiments were conducted to evaluate the impact of light and alternating temperatures on germination of a large set of Galinsoga populations. Seedling emergence was investigated by burying seeds at different depths in a sandy and sandy loam soil. Dormancy of fresh seeds harvested in autumn was evaluated by studying germination response in light at 25/20°C with and without nitrate addition. Seed longevity was investigated in an accelerated ageing experiment by exposing seeds to 45°C and 100% relative humidity. Galinsoga spp. seeds required light for germination; light dependency varied among populations. Seedling emergence decreased drastically with increasing burial depth. Maximum depth of emergence varied between 4 and 10 mm depending on soil type and population. In a sandy soil, emergence percentages were higher and seedlings were able to emerge from greater depths than in a sandy loam soil. Freshly produced G. parviflora seeds, harvested in autumn, showed a varying but high degree of primary dormancy and were less persistent than G. quadriradiata seeds that lack primary dormancy. Lack of primary dormancy of freshly harvested G. quadriradiata seeds and light dependency for germination may be used to optimise and develop Galinsoga management strategies.  相似文献   

3.
Seeds of Viola arvensis collected in different years and in different months within those years were buried in soil under natural seasonal temperature cycles, and changes in their germination requirements monitored. Seeds were dormant at maturity in May or June, but nondormant by autumn. During winter, some seeds entered dormancy, while others entered conditional dormancy, i.e. retained the ability to germinate at 15/6 and 20/10oC but not at other thermoperiods. Dormant and conditionally dormant seeds became nondormant the following summer. Seeds collected in 1981 exhibited an annual dormancy:nondormancy cycle, while those collected in 1982 exhibited an annual conditional dormancy:nondormancy cycle. The type of dormancy cycle found in these seed lots during their first year of burial persisted in subsequent years. Thirty–five and 36% of seeds collected in May 1983 and 1986, respectively, were conditionally dormant the following May, while only 5 and 9% of those collected in the same field in June 1983 and 1986, respectively, were conditionally dormant. Dormant seeds collected in 1981,1982 and 1984 and buried at 5oC during summer germinated to 0, 33 and 0% respectively, at 15/6oC in autumn. After the 1982 seeds became nondormant during summer, only 25% entered conditional dormancy when buried at 5oC, but after the 1981 and 1984 seeds became nondormant, 100% entered conditional dormancy at 5oC. Thus, the persistent seed bank of V. arvensis at a population site may consist of seeds with an annual dormancy:mondormancy cycle and others with an annual conditional dormancy:nondormancy cycle. This is the first report of the two types of annual seed dormancy cycles in the same species.  相似文献   

4.
The seasonal changes in percentage of dormant seeds of Echinochloa crus-galli in the field were recorded for 4 years. The lots of seeds were wrapped in nylon fabric, buried 20 cm under the grass sward and exhumed at monthly intervals. The proportion of seeds germinating under light conditions at a constant temperature of 25 °C fluctuated between 0% and 96%, with maxima in May–July and minima in September–November. Small between-year differences in the course of summer dormancy induction and its winter termination were probably caused by variation of weather conditions.
Attributes of dormancy innate to seeds after maturation (primary dormancy) and dormancy induced in buried seeds during the summer (secondary dormancy) were compared by investigating the rate of dormancy termination during storage of (a) dry seeds at 25 °C, (b) imbibed seeds at 5°C and (c) in seeds buried under field conditions during October–June. Percentage of germination increased faster in secondary than primary dormant seeds at both constant 25 °C and 5 °C. The seeds with primary and secondary dormancy also differed in the response to `germination pre-treatment', a 10-day exposure of imbibed seeds at 25 °C that causes germination of the non-dormant fraction of seed materials. After this treatment the time to resuming germination in primary dormant seeds was substantially increased, whereas the secondary dormant seeds were much less affected. Annual variation in the proportion of germinable seeds explains the low efficiency of autumn soil cultivation for decreasing reserves of E. crus-galli seeds in the soil.  相似文献   

5.
Eupatorium rugosum (Ageratina altissima), commonly known as white snakeroot, is a weedy plant that invades woodland areas in North America, Korea, and Japan. In order to examine the inheritance of seed dormancy in this species, seeds from a single population were screened for their differential germination response to stratification. After two cycles of recurrent selection, the seed from the shallow–dormant lines had 4.4 times greater germination prior to stratification than did the seed from the deep–dormant lines. The seed from the deep–dormant lines showed 3.4 times greater germination after stratification, compared to the seed from the shallow–dormant selections. This suggests that primary dormancy in the seed of white snakeroot is under some degree of genetic control. This perennial species produces overwintering rhizomes that give rise to adventitious, vegetative buds each spring. The plants selected for the production of seeds with lower levels of dormancy in the fall were observed to generate rhizomatous buds that were released from dormancy earlier in spring, compared to the plants that produced seeds with higher levels of dormancy. A statistically significant positive correlation also was observed between seed and bud dormancy in a naturally occurring population of white snakeroot. Common regulatory elements might be influencing dormancy in both the seeds and vegetative buds of this species.  相似文献   

6.
Trials were carried out to study the germination and dormancy of Cuscuta campestris Y. (dodder) seeds and factors influencing the success of early parasitisation of sugarbeet. Primary dormancy can be removed by seed scarification. Germination was negligible at 10°C and optimal at 30°C, while it was not influenced by light. Seed burial induced a cycle of induction and breaking of secondary dormancy. Seedling emergence was inversely proportional to the depth of seed burial and only seed buried within 5 cm of the soil surface emerged. Storage of C. campestris seeds in a laboratory for 12 years resulted in the loss of primary dormancy, enabling the germination of all viable seeds. Host infection (i.e. protrusion of parasite haustoria from host tissue) was heavily influenced by host growth stage. Tropism towards a host was due to the perception of light transmitted by green parts of sugarbeet plants. Insertion of a transparent glass sheet between host leaves and parasite seedlings did not modify this response. This phototropism permitted Cuscuta to identify host plants with high chlorophyll content as a function of the lower red/far red ratio of transmitted light.  相似文献   

7.
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.  相似文献   

8.
Competition from native species is a key mechanism for biotic resistance to invasion. Accelerated germination to pre‐empt resources or delayed germination and induced dormancy until the next growing season are two alternative strategies for annual invasive plants to avoid the drawbacks of competition at the seed stage. In Ambrosia artemisiifolia, both of these tactics could theoretically increase its long‐term fitness. However, their relative importance has never been tested. We studied the germination pattern of A. artemisiifolia seeds in various competitive environments by experimentally modifying the life stage (seed, seedling, adult), density (low, high) and also the identity (intraspecific and 3 interspecific competitors) of neighbours in controlled conditions. When facing competition of seeds and seedlings at high densities or of particular identity, A. artemisiifolia accelerated its germination. In contrast, A. artemisiifolia followed a competition avoidance strategy in the presence of established adult heterospecific neighbours by delaying germination and reducing the germination fraction through induction of secondary dormancy. By testing the seedlings' performance in the same competition situations as those of seeds, we showed that the germination responses were beneficial in the case of heterospecific, but not of conspecific neighbours.  相似文献   

9.
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.  相似文献   

10.
The transition from seed dormancy to germination is a multi‐step process. However, distinguishing between physiological processes involved in seed dormancy alleviation and those involved in germination has been difficult. We studied the seed dormancy alleviation process in Amaranthus tuberculatus, an important weed species in midwestern USA. Using three A. tuberculatus biotypes that differ in dormancy level, it was determined that stratification reduced seed dormancy from a high to a low level. Temperature alternation alleviated low seed dormancy and triggered germination. Exogenously applied abscisic acid (ABA) and gibberellic acid (GA) had no effect on seeds with high dormancy. However, ABA and paclobutrazol (a GA biosynthesis inhibitor) significantly reduced germination of seeds with low dormancy. Hormones could not replace the effects of stratification or temperature alternation on dormancy alleviation. Based on our results, we propose a seed dormancy–germination transition model in which the dormancy of A. tuberculatus seeds is progressively reduced from a high to a low level; but environmental conditions (i.e. stratification) can accelerate the dormancy alleviation process. Under low dormancy levels, the seed is more sensitive to environmental cues that are responsible for removing dormancy and triggering germination (i.e. temperature alternation). Finally, ABA and GA regulation occurs primarily during the final transition from low dormancy to germination rather than the alleviation of high dormancy.  相似文献   

11.
Sesamum mulayanum is a wild relative of cultivated sesame, Sesamum indicum, and sometimes grows in sesame crop fields as an associated weed. This species shows deep seed dormancy and is characterized by conspicuous purple pigmentation on the lower lip of the corolla. The present study examined the inheritance mode of seed dormancy by using reciprocal progeny from crosses between the two species. The seeds of S. indicum and F1 (S. indicum×S. mulayanum) showed good germination, but those of S. mulayanum and F1 (S. mulayanum×S. indicum) showed deep dormancy. The F2 seeds from both reciprocal crosses showed deep dormancy. These results, combined with the maternal inheritance of seed‐coat characteristics, indicated that the seed dormancy of S. mulayanum can be attributed to its seed‐coat structure (coat‐enhanced dormancy). The F3 (S. indicum×S. mulayanum) seeds varied in their depth of seed dormancy and those seeds with deep dormancy (<50% germination) and those with no or shallow dormancy (≥50% germination) occurred in the expected ratio of 3:1, indicating that this trait is polygenic but is controlled by a single dominant major gene. The purple pigmentation of the corolla was expressed in both reciprocal F1 plants and the presence and absence of pigmentation was segregated among the F2 plants at the expected ratio of 3:1, indicating that this trait is also controlled by a single dominant gene. The segregation of the major gene controlling seed dormancy and that controlling purple pigmentation was not independent (9:3:3:1), indicating that these genes are linked, providing insights on sesame domestication.  相似文献   

12.
Alien grape (Vitis, Vitaceae) species and recently described hybrids are recognised as major environmental weeds in southern Europe, based on their tendency to spread into natural and semi‐natural habitats, often behaving as invasive species. Despite a high number of agronomic, genetic and phytopathological studies on Vitis cultivars, regeneration from seed has been poorly investigated, although it plays a key role in plant distribution and evolution. To this end, in this study we exposed seeds of eight wild grapes in Europe (three species and five nothospecies, including the native Vitis vinifera), to different temperature treatments in the laboratory. Fresh seeds from all studied taxa were dormant and germinated only after cold and/or warm plus cold stratification, but showing significant differences between the species. Seeds of V. vinifera showed an intermediate complex morphophysiological dormancy, while all other tested Vitis taxa exhibited a deep complex morphophysiological dormancy. Dark conditions reduced the germination percentage in Vitis labrusca, Vitis riparia and V. vinifera and their hybrids, and alternating temperatures elicited the highest germination percentages in all populations. Our results demonstrated for the first time that Vitis hybrids are capable of regeneration by seed, even in the absence of dispersal by animals. The germination processes studied here help understand the current expansion of alien Vitis taxa in Europe outside the viticultural areas. Consequently, germination requirements contribute to a better understanding of the mechanisms underlying alien Vitis taxa establishment and invasion.  相似文献   

13.
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.  相似文献   

14.
Secondary seed dormancy has been linked to seedbank persistence of volunteer oilseed rape (Brassica napus) in western Canada. It has been suggested that there is a genetic component to secondary seed dormancy expression in oilseed rape, but little is known of its importance in relation to non‐genetic factors. In a series of experiments we investigated the relative importance of genotype, seed size, time of windrowing and pre‐ and post‐harvest environment on the expression of secondary seed dormancy. We found that genotype contributed between 44 and 82% to the total variation in secondary seed dormancy. A broad range in secondary seed dormancy expression was observed among 16 genotypes examined. Nevertheless, three‐quarters of the genotypes investigated exhibited relatively high potential for the expression of secondary seed dormancy (back‐transformed mean 71% dormant seeds). Seed size contributed 21% to the total variation, while the influence of seed maturity (harvest regime) on secondary seed dormancy expression was negligible. Despite diverging environmental conditions during the four growing seasons spanning these experiments, the influence of pre‐harvest environment on seed dormancy expression was relatively small and ranged from 0.1% to 4.5%. Secondary seed dormancy potential decreased over time during seed storage. This decrease was greatest when seeds were stored at ambient temperatures and least when seeds were stored at ?70°C.  相似文献   

15.
Plant species invasiveness is frequently associated with rapid proliferation and production of seeds that can persist in the soil for long periods of time. Leucaena leucocephala (Fabaceae) is an alien and invasive species, for example in Brazilian forest and savannah ecosystems. This study quantified the invasive potential of this species by analysing its seed rain (using seed collectors), seed longevity in the soil (stored in buried bags) and the germination capacity of the soil seedbank (by collecting soil samples in the study area). Our results showed that seed rain occurred throughout the year, although more intensely from July to September, with about 5500 seeds m?2 year?1 being released. The numbers of seeds in the buried bags diminished over time and intact seeds showed low germinability (approximately 15%), although their viability remained >80% of the recovered seeds after two years of in situ storage. The germinability of seeds collected directly from the soil was approximately 40%, indicating that more than half of the seeds of soil seedbank were dormant (physical dormancy). Leucaena leucocephala produces large numbers of seeds and is able to form a persistent short‐lived seedbank (viability 1–5 years). These factors may contribute significantly to its invasive potential, which makes it difficult to control this species once it becomes established. As control costs become higher over time, immediate public efforts are needed to counter this threat.  相似文献   

16.
This study examined germination and dormancy in Galeopsis speciosa (Lamiaceae), a common summer annual weed in cold‐temperate areas. Seeds collected in southern Sweden were subjected to several experiments. The seeds were dormant at maturity. Seeds sown outdoors after collection produced a small number of seedlings that emerged early in the spring. After long cold stratification or stratification outdoors over two winters, the maximum germination was 40–50%; germination occurring over a wide range of temperatures. Warm stratification preceding cold stratification had no effect on germination, but repeated warm and cold periods seemed to promote germination. Gibberellic acid (GA) stimulated germination, but full germination was only achieved after more than 2 months of incubation at the most suitable temperature regime tested. Excised embryos grew and developed into normal seedlings. With these results, the species does not fit into the currently used system for seed dormancy classifications. The response to GA and the growth of excised embryos indicate non‐deep or intermediate physiological dormancy, but dormancy alleviation by stratification was not in line with the guiding principles for these classifications. Galeopsis speciosa has a strong dormancy that is sufficiently alleviated during the winter to allow germination of only part of a seed batch each year; hence a stepwise germination pattern occurs over a period of several years.  相似文献   

17.
Seed dormancy and germination in sand burr (Cenchrus longispinus (Hack,) Fern,) were investigated in laboratory and field studies. The burrs contain two types of seeds which differed in their innate dormancy. Primary seeds formed in the upper spikelet usually germinated within a year. Secondary seeds from lower spikelets germinated slowly and remained dormant for longer periods. Dormancy was enforced at low and high temperatures, and secondary seeds apparently developed an induced dormancy when continuously exposed to high temperatures. More than 94 % of the seedlings established during spring. Light suppressed germination, and secondary seeds also developed an induced dormancy when stored in the light. Burrs sown on the soil surface had an extended period of germination lasting for more than 3 years. However, over 96 % of the seeds sown below the surface of bare soil germinated within 2 years. Deep burial did not enforce dormancy, but germination was suppressed by the presence of live vegetation. It is concluded that treatments which disturb the soil and bury the burrs will stimulate the germination of dormant seeds.  相似文献   

18.
Seed dormancy and persistence in the soil seedbank play a key role in timing of germination and seedling emergence of weeds; thus, knowledge of these traits is required for effective weed management. We investigated seed dormancy and seed persistence on/in soil of Chenopodium hybridum, an annual invasive weed in north‐western China. Fresh seeds are physiologically dormant. Sulphuric acid scarification, mechanical scarification and cold stratification significantly increased germination percentages, whereas dry storage and treatments with plant growth regulators or nitrate had no effect. Dormancy was alleviated by piercing the seed coat but not the pericarp. Pre‐treatment of seeds collected in 2012 and 2013 with sulphuric acid for 30 min increased germination from 0% to 66% and 62% respectively. Effect of cold stratification on seed germination varied with soil moisture content (MC) and duration of treatment; seeds stratified in soil with 12% MC for 2 months germinated to 39%. Burial duration, burial depth and their interaction had significant effects on seed dormancy and seed viability. Dormancy in fresh seeds was released from October to February, and seeds re‐entered dormancy in April. Seed viability decreased with time for seeds on the soil surface and for those buried at a depth of 5 cm, and 39% and 10%, respectively, were viable after 22 months. Thus, C. hybridum can form at least a short‐lived persistent soil seedbank.  相似文献   

19.
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.  相似文献   

20.
Changes in dormancy of Spergula arvensis seeds were studied during pre-incubation at constant temperatures and under a temperature regime that condensed the annual temperature cycle into 73 days. Each day in the regime represented the mean day and night temperatures and day lengths of 5 successive days of an average year in The Netherlands. Incubation occurred in water or loamy sand, in darkness. Germination of the seeds was tested in water or KNO3 over a range of temperature. Seeds were irradiated with saturating doses of red light. In half of the treatments, pre-incubated seeds were dehydrated at the transfer to the conditions of the germination test. Breaking of dormancy occurred under conditions of 'spring'. It did not depend on exposure to low‘winter’temperatures, but was induced by rising 'spring’temperatures. Seeds developed secondary dormancy in late‘autumn'. The expression of the changes in dormancy that were induced during pre-incubation depended on the conditions of the germination test. Light, nitrate and dehydration stimulated germination. The experiments predicted that field emergence from nitrate-poor soils that have not been dehydrated will be restricted to a short period in autumn, whereas disturbance of nitrate-rich soils followed by a dry spell will stimulate germination of S. arvensis seeds from early spring to late autumn. The data presented good explanations for the cosmopolitan character and the serious weediness of this species. Its classification as a summer or winter annual is discussed.  相似文献   

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