Loss Of Dormancy Research Articles

Seed dormancy, after-ripening and light requirements of four annual Asteraceae in south-western Australia.

The role of dormancy, temperature and light in the regulation of seed germination of four annual Asteraceae from south-western Australia was investigated. The experiments aimed to identify after-ripening patterns, and to relate these to climatic conditions of the habitat in which the species occur. Seeds of all species were strongly dormant at maturity and maintained high levels of dormancy for time periods corresponding to the duration of summer in south-western Australia. Dry after-ripening was promoted best by temperatures lower than those prevailing in the dry season, although differences among storage temperatures were mostly insignificant. Germination percentages were highest at average winter temperatures (15 degrees C). A logistic model revealed significant differences in germinability among species, but not between incubation temperatures or light and dark treatments across species. Three species with seeds >0.5 mg germinated better in darkness than in light, whereas germination in darkness was almost inhibited in the species with the smallest seeds (0.14 mg). The course of dormancy loss, tested over a range of fluctuating incubation temperatures (7-30 degrees C), showed that seeds of three species came out of dormancy first at temperatures that prevail in south-western Australia during the winter (10-15 degrees C). Seeds from one species, introduced from South Africa, first lost dormancy at the lowest temperature (7 degrees C). All species showed after-ripening patterns of Type 1, typical of species growing in Mediterranean climates. The germination characteristics of the investigated species can be interpreted as ensuring that initial growth and establishment occur during the winter growing season, thereby avoiding the hot and dry summer conditions that follow seed dispersal.

Persistence of microscopic human cancers in mice: alterations in the angiogenic balance accompanies loss of tumor dormancy.

Some human tumor lines do not form visible tumors when inoculated into immunosuppressed mice. The fate of these human tumor lines was followed by transfecting them with green fluorescence protein before inoculating them into mice. Although the tumor lines failed to grow progressively, they formed small dormant microscopic foci maintained at constant mass by balanced proliferation and apoptosis. Transfecting the cells with either VEGF165 or activated c-Ha-ras induced loss of dormancy, which correlated with a shift in the angiogenic balance toward increased vascularity with reduced tumor cell apoptosis. These results support a model in which loss of dormancy is controlled in part by a switch to an angiogenic phenotype. These tumor lines may serve as models for investigating the cellular mechanisms controlling dormancy and identifying those factors that promote the loss of balanced proliferation and apoptosis. Finally, these models may prove useful in the design and testing of therapies directed toward eradicating dormant tumors and preventing tumor recurrence.

Abscisic acid, phaseic acid and gibberellin contents associated with dormancy and germination in barley.

Analyses of abscisic acid (ABA), ent-kaurenoids and gibberellins (GAs) showed that there were major changes in the contents of these compounds associated with germination of after-ripened barley (Hordeum vulgare cv. Schooner and cv. Proctor) grain but not in hydrated dormant grain. Embryos from dormant and after-ripened dry grain contained similar amounts of ABA, of ent-kaurenoids and of GAs, determined by gas chromatography-mass spectrometry-selected ion monitoring. In embryos of after-ripened grain, ABA content decreased rapidly after hydration and ABA appeared to be metabolized (inactivated) to phaseic acid (PA) rather than diffusing into the endosperm or the surrounding medium as previously thought. Similar changes in ABA occurred in hydrated dormant grain during germination in darkness. Accumulation of ent-kaurenoids and GAs, including GA1, the first biologically active GA in the early 13-hydroxylation biosynthetic pathway, occurred to a much greater extent in after-ripened than in dormant grain and these changes occurred mainly after 18 h of hydration when ABA had already decreased and germination was occurring. The block in ent-kaurenoid and GA synthesis in dormant grain appeared to occur prior to ent-kaurene in the biosynthetic pathway. These results are consistent with the view that ABA is the primary effector of dormancy and that after-ripening involves the development of the ability to reduce the amount of ABA quickly following hydration. Accumulation of GAs does not appear to be causally related to loss of dormancy but it does appear to be related to germination.

Non‐deep simple morphophysiological dormancy in seeds of the weedy facultative winter annual Papaver rhoeas

Embryos in freshly matured seeds of the facultative winter annual Papaver rhoeas are underdeveloped and physiologically dormant; thus, seeds have morphophysiological dormancy (MPD). Seeds lost physiological dormancy during 12 weeks of burial in moist soil at 12 h/12 h daily alternating temperature regimes of 15/5°C, 20/10 °C and 25/15 °C but not at 1 °C. Physiological dormancy was not broken in seeds stored dry at room temperature for 12 weeks. After physiological dormancy was broken, seeds required light for embryo growth (i.e. for loss of morphological dormancy) and consequently for germination. After a 12‐week period of burial in soil at 25/15 °C, seeds that matured in 1997 germinated to 100% in light at 25/15 °C, demonstrating that cold stragification temperatures (≈ 0.5–10 °C) are not required for embryo growth. Thus, seeds have non‐deep simple MPD. During exposure to low winter temperatures (5/1 °C, 1 °C), 52% of the seeds with physiologically non‐dormant embryos entered conditional dormancy and thus lost the ability to germinate at 25/15 °C but not at 15/5 °C or 20/10 °C. The peak of germination for seeds sown in southern Sweden was in autumn, but some also germinated in spring. A higher percentage of seeds that matured in a relatively warm, dry year (1997) came out of MPD and germinated than did those that matured in a relatively cool, wet year (1998) at the same site.

Ecology and ecological genetics of seed dormancy in downy brome

Downy brome, an obligately selfing winter annual, has invaded a variety of habitats in western North America. Seeds are at least conditionally dormant at dispersal in early summer and lose dormancy through dry after-ripening. In the field, patterns of germination response at dispersal vary among populations and across years within populations. Degree of dormancy at summer temperatures in recently harvested seeds, as well as rate of dormancy loss during dry storage, can be related to the risk of premature summer germination in different habitats. Patterns of dormancy loss are predictable and can be modeled using hydrothermal time concepts. To assess the relative contribution of genotype and maturation environment, multiple parental lines from contrasting populations were grown for three generations under manipulated greenhouse conditions. Significant germination response differences among populations were observed, as well as major differences among full-sib families. Among-population variation accounted for over 90% of the variance in germination traits, whereas within-family variance accounted for 1% or less. Populations from predictable extreme environments (subalpine meadow and warm desert margin) showed significantly less variation among families than did populations from less predictable environments (cold desert, foothill, and plains). Environmental conditions that shortened the seed ripening period (water stress and high temperature) resulted in reduced seed dormancy level at maturation, but there were strong inbred line–environment interactions. For fully after-ripened seeds, inbred line and environmental effects were no longer evident, indicating that differences in genotype and maturation environment function mainly to regulate dormancy and dormancy loss rather than to mediate response patterns of nondormant seeds.

Effects of desiccation and a change in temperature on germination of immature grains of wheat (Triticum aestivum L.)

A study was made of the effects of desiccation and a change in temperature on the germination of wheat grains harvested 20 days after anthesis. When the germination test was performed immediately after harvesting, germination percentages ranged from 5.2% to 10.7%. Germination percentages increased to 48.2% to 90.3% after grain had been desiccated at 20 °C and then hydrated at 10 °C. This increase occurred even if grains had been desiccated in an atmosphere of high relative humidity. The germination percentage of non-desiccated grains depended on the germination temperature. When the pericarp and testa were removed from embryos, the germination percentage of grains incubated at 20 °C and then at 10 °C was higher than that of grains incubated at 10 °C. In general, a low germination temperature is believed to be effective in breaking dormancy of wheat grains. However, a change in temperature stimulated germination to a greater extent than a constant low temperature. The germination percentage of 5-day cycle alternating temperature was greater than that with 1-day, 2-day and 10-day cycles. Although the germination of immature wheat grains was stimulated by both high and low germination temperatures, it is likely that cycles shorter and, also, longer than a critical period induce limited germination. Loss of dormancy commonly occurs when development of wheat grains proceeds at a high temperature, with imbibition at a low temperature. However, germination ability of non-desiccated immature wheat grain was enhanced by a change in temperature during germination.

Predicting Preharvest Sprouting Susceptibility in Barley

Preharvest sprouting (PHS) susceptibility in cereals is a consequence of low grain dormancy before harvest. Dormancy loss rate depends on genotype and may also be affected by environmental conditions during seed formation. To establish a quantitative relationship between temperature and PHS susceptibility, a malting barley (Hordeum vulgare L.) cultivar, ‘Quilmes Palomar’, was sown on different dates over a 3‐yr period to obtain a range of thermal conditions during grain filling (soil type: Aeric Argiudoll). The period from pollination to physiological maturity (PM) was adjusted to a thermal time (TT) scale, which was then divided into 50°C‐day intervals. Mean temperature within each interval was calculated for the different sowing dates. Grain dormancy was monitored using a germination index (GI). We sought a linear relationship between temperature during grain filling and GI at some moment after PM. The strongest correlation (P < 0.0001) was obtained between mean temperature values within the TT interval ranging from 300 to 350°C‐day (Tm300–350) and GI values 12 d after PM (GI12DAPM). This indicates that temperature during this sensitivity window explains variability in dormancy level among years and locations for this cultivar, and may therefore explain differences in PHS susceptibility. A regression model (GI12DAPM = 7.14 × (Tm300–350) − 99; r2 = 0.95, n = 9) was generated for predicting GI values 12 d after PM, and tested on commercial plots. The linear relationship between temperature and GI after PM was confirmed, though the effect of one or more undescribed, environmental factors differing among tested locations was revealed.

The effects of different maturation conditions on seed dormancy and germination ofCenchrus ciliaris

AbstractSeeds ofCenchrus ciliarisL. were produced under different hydro–photo–thermal environments with and without fertilizer. Dormancy loss of spikelets and extracted caryopses was tested during dry after-ripening at 40°C and 43% equilibrium relative humidity. Caryopses had higher initial germination and lost their dormancy faster than spikelets. Dormancy of both caryopses and spikelets generally decreased with an increase of maturation temperature and fertility, whereas dormancy increased if water stress was imposed during maturation. The latter effect was smaller when the mother plants were exposed to water stress after caryopses were fully formed than when water stress cycles were applied throughout maturation. Daylength extension (to 14 or 18 h d-1) by artificial light increased dormancy of both caryopses and spikelets. The effect of long days declined when plants were exposed to natural daylight for more than 10 h d-1. The after-ripening curves were consistent with the hypothesis that dormancy periods of individual seeds are normally distributed within each seed lot. Rates of loss of dormancy were quantified by the slopes of these curves. In a given experiment, these rates were identical for caryopses but not always for spikelets that matured in diverse environments. Even for caryopses, however, the slopes varied between experiments. Therefore, the results do not support the hypothesis that a dormancy model can be applied universally to all seed lots ofCenchrus ciliaris. Methods of predicting the period of after-ripening required to achieve desired levels of dormancy for reseeding degraded rangelands are discussed.

Using hydrothermal time concepts to model seed germination response to temperature, dormancy loss, and priming effects in Elymus elymoides

AbstractHydrothermal time (HTT) describes progress toward seed germination under various combinations of incubation water potential ( ) and temperature (T). To examine changes in HTT parameters during dormancy loss, seeds from two populations of the bunchgrass Elymus elymoides were incubated under seven temperature regimes following dry storage at 10, 20 and 30°C for intervals from 0 to 16 weeks. Fully after-ripened seeds were primed for 1 week at a range of s. Data on germination rate during priming were used to obtain a HTT equation for each seed population, while data obtained following transfer to water were used to calculate HTT accumulation during priming. HTT equations accurately predicted germination time course curves if mean base water potential, b(50), was allowed to vary with temperature. b(50) values increased linearly with temperature, explaining why germination rate does not increase with temperature in this species. b(50) showed a linear decrease as a function of thermal time in storage. Slopes for the T × b(50) relationship did not change during after-ripening. This thermal after-ripening time model was characterized by a single base temperature and a constant slope across temperatures for each collection. Because the difference between initial and final b(50)s was uniform across tempera-tures, the thermal after-ripening requirement was also a constant. When seeds were primed for 1 week at −4 to −20 MPa, accumulation of HTT was a uniform 20% of the total HTT requirement. When primed at 0 to −4 MPa, HTT accumulation decreased linearly with decreasing priming potential, and a hydrothermal priming time model using a constant minimum priming potential adequately described priming effects. Use of these simple HTT relationships will facilitate modelling of germination phenology in the field.

Seed germination ecology of the annual grass Leptochloa panicea ssp. mucronata and a comparison with L. panicoides and L. fusca

Abstract Leptochloa panicea ssp. mucronata is an annual grass that grows in relatively dry habitats. Requirements for dormancy loss and germination were determined for seeds of this species and compared to those of two species from wet habitats. Seeds of L. panicea were dormant at maturity in autumn, but when exposed to actual or simulated autumn temperatures (e.g. 20/10, 15/6 °C), they entered conditional dormancy and thus germinated to high percentages in light at 35/20 °C. Seeds buried in non-flooded soil exposed to natural seasonal temperature changes in Kentucky (USA) were non-dormant by the following summer and germinated to 80–100 % in light at 25/15, 30/15 and 35/20 °C. Seeds buried in non-flooded soil exhibited an annual conditional dormancy/non-dormancy cycle, with seeds mostly germinating to 80–100 % in light at 30/15 and 35/20 °C throughout the year but to 80–100 % in light at 25/15 °C only in summer. Results for L. panicea were compared to published data for L. panicoides and L. fusca. Whereas seeds of L. panicea buried in flooded soil failed to come out of dormancy, those of L. panicoides, an annual of moist habitats such as mudflats, exhibited an annual conditional dormancy/non-dormancy cycle, and those of L. fusca, a semi-aquatic, required flooding for both dormancy loss and germination. Differences in dormancy breaking and germination responses of seeds of Leptochloa species may help to explain why this genus occupies a wide range of habitats with regard to soil moisture conditions.

Kinetics of dormancy release and the high temperature germination response in Aesculus hippocastanum seeds

The kinetics of primary dormancy loss were investigated in seeds of horse chestnut (Aesculus hippo-castanum L.) harvested in four different years. Freshly collected seeds from 1991 held for up to 1 year at temperatures between 2 degrees C and 42 degrees C exhibited two peaks in germination (radicle growth), representing a low temperature (2-8 degrees C) and a high temperature response (31-36 degrees C), Germination at 36 degrees C generally occurred within 1 month of sowing, but was never fully expressed in the seedlots investigated. At low temperatures (2-8 degrees C), germination started after around 4 months. Generally, very low levels of germination were observed at intermediate temperatures (11-26 degrees C). Stratification at 6 degrees C prior to germination at warmer temperatures increased the proportion of seeds that germinated, and the rate of germination for all seedlots. Within a harvest, germination percentage (on a probit scale) increased linearly with stratification time and this relationship was independent of germination temperature (16-36 degrees C), However, inter-seasonal differences in the increases in germination capacity following chilling were observed, varying from 0.044 to 0.07 probits d(-1) of chilling at 6 degrees C, Increased sensitivity to chilling was associated with warmer temperatures during the period of seed filling. The estimated base temperature for germination, T-b for newly harvested seeds varied slightly between collection years, but was close to 25 degrees C. For all seedlots, T-b decreased by 1 degrees C every 6 d of chilling at 6 degrees C. This systematic reduction in T-b with chilling ultimately facilitated germination at 6 degrees C after dormancy release.

Ecological genetics of seed germination regulation in Bromus tectorum L. : II. Reaction norms in response to a water stress gradient imposed during seed maturation.

The probability that a seed will germinate depends on factors associated with genotype, maturation environment, post-maturation history, and germination environment. In this study, we examined the interaction among these sets of factors for 18 inbred lines from six populations of Bromus tectorum L., a winter annual grass that is an important weed in the semi-arid western United States. Seeds of this species are at least conditionally dormant at dispersal and become germinable through dry-afterripening under summer conditions. Populations and inbred lines of B. tectorum possess contrasting dormancy patterns. Seeds of each inbred line were produced in a greenhouse under one of three levels of maturation water stress, then subjected to immediate incubation under five incubation regimes or to dry storage at 20°C for 4 weeks, 12 weeks, or 1 year. Dry-stored seeds were subsequently placed in incubation at 20/30°C. Narrow-sense heritability estimates based on parent-offspring regressions for germination percentage of recently harvested seeds at each incubation temperature were high (0.518-0.993). Germination percentage increased with increasing water stress overall, but there were strong interactions with inbred line and incubation temperature. Inbred lines whose seeds were non-dormant over the full range of incubation temperatures when produced at low maturation water stress showed reaction norms characterized by little or no change as a function of increasing stress. For inbred lines whose dormancy status varied with incubation temperature, incubation treatments where seeds exhibited either very low or very high levels of dormancy showed the least change in response to maturation water stress. Inbred lines also varied in their pattern of dormancy loss during storage at 20°C, but maturation water stress had only a minor effect on this pattern. For fully afterripened seeds (1 year in storage at 20°C), inbred line and maturation water stress effects were no longer evident, indicating that differences in genotype and maturation environment function mainly to regulate dormancy and dormancy loss in B. tectorum, rather than to mediate response patterns of non-dormant seeds.

A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

(Roberts, 1988; Murdoch and Ellis, 1992). In seeds of parasitic plants such as Striga and Orobanche, a moist Seeds of three species of Orobanche were conditioned pretreatment known as ‘conditioning’ is a prerequisite for (i.e. stored fully imbibed in darkness) for periods up to germination (Joel et al., 1991). The seeds become respons210 d in order to model relief of primary dormancy and ive to a germination stimulant which, in the natural induction of secondary dormancy. The data were con- environment, originates from the host plant. Extending sistent with the hypothesis that periods for loss and the pretreatment also results in reduced germination due induction of dormancy and loss of viability are normally to secondary dormancy which was called ‘wet dormancy’ distributed in populations of imbibed seeds and that by Vallance (1950). For example, Brown et al. (1951) these three processes are independent. There was a reported that O. minor Sm. seeds conditioned at 25°C positive, linear relationship between the rate of loss showed a maximum loss of dormancy after 14 d and that of primary dormancy and temperature from 10‐30°C the germination capacity of some seed lots decreased in O. aegyptiaca and O. cernua and 10‐25° Ci n rapidly thereafter. Van Hezewijk (1994) also reported O. crenata. In all three species, the rate of induction that prolonged conditioning resulted in a decrease in of secondary dormancy was highest at 10°C and germination of O. crenata Forsk. This decrease in gerdecreased with increase of temperature up to about mination was greater at 15°C and 10°C than at 20° Ci n 20°C, above which there was little further change in one of the two seed lots tested. Vallance (1950) observed the rate with temperature. The resulting model that germination of Striga hermonthica (Del.) Benth. explained over 90% of the variation in germination seeds was lower when the conditioning period was longer after conditioning in both O. aegyptiaca and O. cernua. than that necessary to obtain maximum germination, In O. crenata, however, this model was only satisfact- which at 22°C was 6 d. ory at 10 and 15°C. At higher temperatures, dormancy During conditioning the two processes of loss of primwas relatively stable for periods of conditioning from ary dormancy and induction of secondary dormancy are 70 to about 154 d. Possible explanations for this are occurring perhaps simultaneously, perhaps independently discussed. Applications of these models for estimating and certainly at diVerent rates. As such, there is a close the time required to reduce Orobanche infestations in analogy with the prechilling response of Rumex and Picea the field are also briefly discussed.

A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

Dormancy/Nondormancy Cycles in Spores of the Liverwort Sphaerocarpos texanus

The presence of a dormancy/nondormancy cycle was investigated in spores of the winter, annual, liverwort, Sphaerocarpos texanus. To test for loss of dormancy, mature laboratoryderived spores were held at three daily alternating incubation temperature regimes (thermoperiods) of 35/20, 30/15, and 25/15?C. For 22 intervals (varying from one week to 91 weeks), spores were transferred from each of these three thermoperiods onto a wet substrate in a germination chamber held at 16/10?C. Loss of dormancy, indicated by the spores ability to germinate, increased as the length of time spores kept in the incubation thermoperiods increased. Loss of dormancy in spores held at 35/20?C increased faster than spores held at 30/15 and 25/15?C. Spores held at each of the three thermoperiods germinated best when transferred to 16/100C and failed to germinate when transferred to 35/20 and 30/15?C. Spores subjected to simulated seasonal temperature changes were induced back into secondary dormancy when subjected to low temperatures. The loss of dormancy and subsequent induction of dormancy suggest that S. texanus has a dormancy/nondormancy spore cycle similar to that found in seeds of obligate winter annuals. The occurrence of dormancy/nondormancy cycles in seeds of winter annual seed plants has been extensively documented (Baskin & Baskin 1985, 1989, 1998; Egley 1995 and ref. therein). These annual plants can tolerate the low temperatures of winter, but not the high temperatures and drought conditions of summer. They produce seeds that are innately dormant (seeds not germinating under any set of natural conditions), which prevents germination at a time when the chance of seedling survival is low (summer). Exposure to high temperatures in summer are required for these seeds to lose their dormancy. By autumn, seeds are nondormant and the mild autumn temperatures allow for seed germination when the habitat favors completion of the life cycle. Low winter temperatures induce seeds that did not germinate into dormancy again (secondary dormancy). Secondary dormancy prevents germination in spring when temperatures are mild, but insufficient time is available for life cycle completion. For innate and induced secondary dormant seeds to germinate, they must first become nondormant (through physiological and/or biochemical changes). Dormancy appears to be rare in bryophytes. No evidence of spore dormancy was found by Mogensen (1981) in over 60 tested species. A similar observation of a lack of spore dormancy was observed for 20 bryophytes species by Longton and Schuster (1983). Nevertheless, dormancy in spores has been reported in a few bryophyte species and was discussed by During (1979). Spores of Riccia nigrosquamata germinated a few at a time reaching 2% after 12 weeks (Berrie 1975). Optimum germination was obtained if spores were placed first in dry storage (Thompson 1941) or, placed in 250C in water in the dark (Steiner 1969) in Riella affinis and Sphaerocarpos donnellii, respectively. Several environmental factors have been found to influence spore germination including light (Hoffman 1964; Krupa 1964) and temperature (Krupa 1964; Longton & Greene 1979). Further experiments are needed to determine whether bryophyte spores that fail to germinate are innately dormant or whether they are nondormant, but fail to germinate due to unfavorable environmental conditions. Coupling such studies within the context of the timing of a species life cycle and the prevailing environmental conditions will provide insight into the roles that dormancy and germination patterns play in maintaining a species population. The liverwort, Sphaerocarpos texanus, is a winter annual that germinates in autumn, and the gametophore senesces in early spring. Freshly-matured spores collected from senescing plants in the field or from laboratory-grown plants failed to germinate when sown in simulated autumn temperatures. Since the life cycle of S. texanus is similar to that of winter annual seed plants, this study was initiated and designed to determine if spores of S. texanus have a similar dormancy/nondormancy cycle as the seeds of winter annual seed plants. Specifically, I tested for 1) temperature dependence and rate of loss of dormancy, 2) temperature conditions for germination, and 3) presence of secondary dor-

Patterns of Seed Softening in Subterranean Clover in a Cool, Temperate Environment

AsbtractsEnvironmental conditions affect seed softening patterns of annual legumes. These patterns have not been well established for cool, temperate environments. This work examines the effect of genotype and of growing and softening environments on the short‐term loss of dormancy in subterranean clover (Trifolium subterraneum L.) during the summer and early autumn. Burrs of six cultivars and one accession of subterranean clover that varied widely in maturity and hardseed level were placed to soften on the soil surface, over the summerautumn period, under two contrasting environments in western Victoria, Australia: Hamilton and Telopea Downs. There were significant differences (P < 0.05) in the rate of hardseed breakdown both among cultivars and at the two locations. At the cooler site, Hamilton, the levels of residual hardseed after the summer averaged 75% (over all cultivars and growing sites), compared with 66% at Telopea Downs. There was no interaction between cultivar and location of softening. At the end of the period of field exposure the ranking from lowest to highest residual hardseed was ‘Enfield’ < ‘Leura’ = ‘Gosse’ < ‘Dalkeith’ < ‘York’ = SE008 = ‘Nungarin’. High levels of embryo dormancy in Leura indicate that this cultivar has the capacity for delayed germination via embryo dormancy. The results suggest that in cool temperate environments high seed banks of subterranean clover can be achieved due to a lower percentage of seed softening, which in turn means a potentially greater long‐term persistence of subterranean clover and the possibility of conducting 1:1 pasture‐crop rotations in a rainfall zone where consistently high grain yields may be expected.

A simulation model to predict seed dormancy loss in the field for Bromus tectorum L

A simulation model to predict seed dormancy loss in the field for Bromus tectorum L

A simulation model to predict seed dormancy loss in the field for Bromus tectorum L.

Bromus tectorum L. (cheatgrass) is an invasive winter annual whose seeds lose dormancy through dry after-ripening. In this paper a thermal after-ripening time model for simulating seed dormancy loss of B. tectorum in the field is presented. The model employs the hydrothermal time parameter mean base water potential (ψ b (50)) as an index of dormancy status. Other parameters of the hydrothermal time equation (the hydrothermal time constant θ HT , the standard deviation of base water potentials σ ψb , and the base temperature T b ) are held constant, while ψ b (50) is allowed to vary and accounts for changes in germination time-course curves due to stage of after-ripening or incubation temperature. To obtain hydrothemal time parameters for each of four collections, seeds were stored dry at 20°C for different intervals, then incubated in water (0 MPa) or polyethylene glycol (PEG) solutions (-0.5, -1.0, -1.5 MPa) at 15 and 25°C. Germination data for the thermal after-ripening time model were obtained from seeds stored dry in the laboratory at 10, 15, 20, 30, 40, and 50°C for 0 to 42 weeks, then incubated at two alternating temperatures in water. Change in ψ b (50) was characterized for each collection and incubation temperature as a linear function of thermal time in storage. Measurements of seed zone temperature at a field site were combined with equations describing changes in ψ b (50) during after-ripening to make predictions of seed dormancy loss in the field. Model predictions were compared with values derived from incubation of seeds retrieved weekly from the field site. Predictions of changes in ψ b (50) were generally close to observed values, suggesting the model is useful for simulating seed dormancy loss during after-ripening in the field.

Ecological aspects of seed dormancy loss

AbstractAdvances in seed biology include progress in understanding the ecological significance of seed dormancy mechanisms. This knowledge is being used to make more accurate predictions of germination timing in the field. For several wild species whose seedlings establish in spring, seed populations show relevant variation that can be correlated with habitat conditions. Populations from severe winter sites, where the major risk to seedlings is frost, tend to have long chilling requirements or to germinate very slowly at low temperatures. Populations from warmer sites, where the major risk is drought, are non-dormant and germinate very rapidly under these same conditions. Seed populations from intermediate sites exhibit variation in dormancy levels, both among and within plants, which spreads germination across a considerable time period. For grasses that undergo dry after-ripening, seed dormancy loss can be successfully modelled using hydrothermal time. Dormancy loss for a seed population is associated with a progressive downward shift in the mean base water potential, i.e., the water potential below which half of the seeds will not germinate. Other parameters (hydrothermal time requirement, base temperature and standard deviation of base water potentials) tend to be constant through time. Simulation models for predicting dormancy loss in the field can be created by combining measurements of seed zone temperatures with equations that describe changes in mean base water potential as a function of temperature. Successful validation of these and other models demonstrates that equations based on laboratory data can be used to predict dormancy loss under widely fluctuating field conditions. Future progress may allow prediction of germination timing based on knowledge of intrinsic dormancy characteristics of a seed population and long-term weather patterns in the field.

Role of Temperature in Dormancy Break and/or Germination of Autumn‐maturing Achenes of Eight Perennial Asteraceae from Texas, U.S.A.

Abstract Achenes of Aster ericoides, Baccharis neglecta, Brickellia dentata, Eupatorium havavense, Gymnosperma glutinosum, Helianthus maximilianii, Verbesina virginica and Viguiera dentata were collected at maturity in autumn in southcentral Texas and tested for germination over a range of thermoperiods (15/6, 20/10, 25/15, 30/15 and 35/20°C)simulating those in the habitat from September to May. Achenes of Aster, Baccharis, Brickellia, Eupatorium and Gymnosperma were nondormant, thus cold stratification did not change the range of thermoperiods over which they germinated. However, cold stratification increased the percentage of Brickellia achenes germinating at 15/6 and 20/10°C. Achenes of Helianthus, Verbesina and Viguiera were conditionally dormant at maturity, and cold stratification decreased the minimum thermoperiod for germination of Helianthus, Verbesina and Viguiera achenes to 15/6°C and increased the maximum thermoperiod for germination of Verbesina achenes to 35/20°C.Thus, achenes of Helianthus and Viguiera exhibited a type 2 germination response pattern during dormancy break and those of Verbesina a type 3. Germination percentages of Helianthus, Verbesina and Viguiera achenes increased during 6–8 wk of incubation at 20/10 and 15/60°C, which simulate habitat temperatures from November to February. Therefore, winter field temperatures are favorable for dormancy loss as well as for germination. If the soil is too dry for germination of conditionally‐dormant achenes in autumn, a reduction of the minimum temperature for germination during winter would allow them to germinate in midto late winter, several months before the onset of the high temperature and drought conditions of early summer.