Conditional Dormancy Research Articles

Germination of fresh and frost-treated seeds from dry Central Asian steppes

We tested the germination of fresh and frost-treated seeds of 26 species of southern Mongolian mountain and desert steppes, covering the major growth forms of woody and herbaceous perennials and short-lived species in the region. In the field, germination depends on rains that are largely restricted to the summer months between June and August. Thus, germination tests were performed at alternating conditions of 10°C in darkness and 20°C in light (12 h/12 h), which correspond to temperatures at the study site in early and late summer. Seeds of both woody and herbaceous perennials germinated well under the chosen conditions and apparently did not require stratification or scarification. In contrast, germination of annual species was mostly below 30%, while seed viability was equally high in all three growth forms. Winter conditions, simulated by freezing dried seeds at −18°C, hardly changed seed germination in the perennial species, but several short-lived species responded with increased germination. Short-lived species are not abundant in the real vegetation, which is governed by perennials. Thus, we conclude that the important species in Mongolian mountain steppes germinate readily without a dormancy-breaking treatment. A review of the available literature revealed that a complete lack of dormancy, or presence of only conditional dormancy, is also widely described for other species of Central Asian deserts and steppes, which is in contrast to data from North American prairies.

Germination and dormancy states of seeds in the environmental weed Rumex vesicarius (Polygonaceae). I. Seed polymorphism and germination of extracted seeds

In Australia, Rumex vesicarius (Ruby Dock; subgenus Acetosa, section Vesicarii, family Polygonaceae) is an environmental weed, with the potential to have a significant impact on the natural flora and fauna in areas where it grows. Ruby Dock seeds are polymorphic (light and dark of various shades) and of high potential viability. Seeds are enclosed within showy, papery fruiting valves at maturation. Extracted seeds are characterised by non-deep physiological dormancy. Light and dark seeds usually require an after-ripening period of several months, but thereafter germinate at any time of the year in a light–dark rhythm. Light seeds also show excellent germination in constant darkness; they are non-dormant. Dark seeds show conditional dormancy, where germination is inhibited in darkness, but not in a light–dark rhythm. Scarification experiments indicate that the conditional dormancy of dark seeds is related to the pericarp. The pericarp may restrict oxygen consumption by the embryo, contain chemical inhibitors and/or impede radicle protrusion. A range of environmental variables is likely to affect the specific germination requirements of particular seed types. Environmental conditions may induce secondary dormancy, in both light and dark seeds.

Morphophysiological dormancy in seeds of six endemic lobelioid shrubs (Campanulaceae) from the montane zone in Hawaii

The purpose of this study was to investigate seed dormancy breaking and germination requirements of six woody Hawaiian endemic lobelioids (Campanulaceae). Seeds of all species had underdeveloped, physiologically dormant embryos and thus morphophysiological dormancy. Fresh seeds of Clermontia pyrularia Hillebr. and Trematolobelia macrostachys (Hook. & Arnott) A. Zahlbr. did not germinate during 4 weeks of incubation in light at 15/6, 20/10, or 25/15 °C, whereas those of Clermontia fauriei H. Lev., Clermontia hawaiiensis (Hillebr.) Rock, Clermontia kakeana Meyen, and Cyanea angustifolia (Cham.) Hillebr. germinated to 61%–85% but only at 25/15 °C. Since seeds of the latter four species eventually germinated to 84%–100% when incubated for 12–36 weeks at the three temperature regimes, fresh seeds had conditional dormancy; the physiological component of morphophysiological dormancy was nondeep. Seeds of Trematolobelia macrostachys also came out of dormancy (and germinated to 90%) during 18 weeks of incubation at each of the three temperatures regimes, whereas those of Clermontia pyrularia did so only at 15/6 °C. Simulated seasonal temperature variations did not promote dormancy break and germination in any species except Clermontia pyrularia , in which a 12-week incubation period at 25/15 °C resulted in 90% germination after seeds were moved to 20/10 °C. Seeds of all species had an absolute light requirement for germination except those of Clermontia pyrularia, which germinated to 48% in darkness. Since seeds of the six species only require high temperatures for embryo growth and the breaking of physiological dormancy, they have nondeep simple morphophysiological dormancy.

Seed Dormancy in Three Wild Rye (Elymus) Species

Wild rye (Elymus) contains several species of cool season grasses that are important components of forest and woodland ecosystems. Little specific information is known about seed dormancy in wild rye species, but cool season grasses generally display endogenous, non-deep physiological dormancy that would normally be satisfied by moist chilling during winter to permit early spring germination. However, few studies have documented the effect of extended chilling stratification on dormancy release in cool season grasses. Therefore, the objective of this study was to document the dormancy condition of representative wild rye species and to observe the impact of chilling stratification on dormancy release. Three species of wild rye (E. virginicus, E. macgregorii, and E. villosus) were selected based on their taxonomic and ecological relationships. All species showed conditional dormancy with respect to germination temperature. At 15 °C, E. virginicus, E. macgregorii, and E. villosus germinated at 75%, 81%, and 40%, respectively, compared to 5%, 3%, and 12% for each species at 20 and 25 °C. Chilling stratification at 10 °C improved germination compared to non-stratified seeds to 95% and 94% for E. m acgregorii and E. villosus, but had no effect or reduced germination in E. virginicus. Stratification at 5 °C was not as effective as 10 °C for dormancy release and appeared to cause chilling injury in E. virginicus and E. macgregorii. The data suggest that these wild rye species express a form of conditional endogenous, non-deep physiological dormancy that is most pronounced when seeds are germinated at non-optimal temperatures.

Dormancy and viability of Phalaris minor seed in a rice–wheat cropping system

SummarySeed placement, soil temperature and soil moisture content influenced the process of after‐ripening in Phalaris minor seeds. Seeds of P. minor collected from the soil just after wheat harvesting exhibited higher germination than seeds from P. minor threshed directly. There was a pronounced impact of periodic inhabitation of seed into the soil on germination after its dispersal. Germination was strongly inhibited when the seed was kept in soil at more than field capacity (FC) or in water. Maximum germination of seed incubated in soil at FC occurred at 30°C while a temperature of 40°C favoured after‐ripening of seed when mixed with dry soil or kept dry without any medium. Release from conditional dormancy was quicker in the seed retrieved from the soil kept at 20°C than at 10°C. Seed release from conditional dormancy and germination increased with a rise in temperature from 30 to 40°C when the seed was retrieved from incubation in soil at FC for 70 days. The seed kept immersed in water was least responsive to a rise in temperature. Seed recovered from dry soil, or kept without any medium, responded quickly at both temperatures. Light enhanced the germination of Phalaris minor seed. The seedbank subjected to rice (Oryza sativa) field management conditions lost vigour in comparison with the seed stored in laboratory. There was significant variability in seed viability when exposed to differential water management conditions in rice.

Seed dormancy and germination in threatened Iberian Coincya (Brassicaceae) taxa

:This study analyzes the influence of temperature, light conditions, seed age, and seed position in the fruit on germination of four Coincya taxa endemic to the south-central Iberian Peninsula: C. rupestris subsp. rupestris (two populations), C. rupestris subsp. leptocarpa, C. longirostra, and C. monensis subsp. orophila (two populations). The first three taxa are endangered. Germination was considerably lower in darkness than with a photoperiod in all taxa analyzed. Freshly matured seeds of C. rupestris subsp. leptocarpa and C. rupestris subsp. rupestris from main population showed primary dormancy, failing to germinate at any temperature or light condition tested. In the other taxa and populations analyzed, fresh seeds showed conditional physiological dormancy, germinating at low and middle but not at high temperatures (32/18 °C). In most taxa and populations, germination capability increased with seed age, and dormancy was finally broken, which suggests that Coincya taxa have non-deep physiological dormancy. The hypothesis that seeds can be induced into conditional dormancy with seed age was confirmed in a population of C. monensis subsp. orophila, where seeds lost the ability to germinate at 5 °C from the fourth month of dry-storage. In most populations tested, short-aged seeds (≤ 2 months) showed a more pronounced dormancy level when they were collected from the valvar dehiscent portion than from the indehiscent beak. However, the hypothesis that thick-beaked taxa have beak seeds with higher germination than seeds from the valvar portion cannot be accepted as a general trend.

DORMANCY AND GERMINATION IN A GUILD OF SONORAN DESERT ANNUALS

To investigate bet hedging and species coexistence in a guild of Sonoran Desert winter annuals, we subjected seeds of eight species to factorial combinations of summer treatments (varying temperature and precipitation), germination conditions (representative of early, middle, and late germination season temperatures and day lengths), and experiment trial dates (spanning the germination season). In keeping with bet hedging theory, we found that many viable seeds would not germinate in response to any combination of treatments (germination usually <50%). In keeping with the storage effect model, we found that these coexisting species differed in their germination response to our experimental manipulations and also differed in how the experimental variables interact. Field germination data from a long-term project on population and community dynamics of this guild show that germination fractions are similar between field and growth chamber and that species that tend to germinate under early season conditions in the growth chamber also tend to do so under unmanipulated field conditions. Some species are nondormant during the summer and only acquire dormancy at the onset of the autumn germination season, while others appear to have either innate or conditional dormancy until the onset of the germination season.

Recovery directions for monoecious and endangered Bertya ingramii using autecology and comparisons with common B. rosmarinifolia (Euphorbiaceae)

There is much interest in the utility of incorporating a common congener in studies of rare plants, because differences between species may be informative for conservation management. We propose here that it is the similarities between such species and even the parameters in which the rare species is comparatively more successful that can provide much useful information and opportunities. This is a benefit not often recognised within these species' comparisons. Bertya ingramii (Euphorbiaceae) is a threatened shrub known only from two small populations (<70 plants in each) in northern New South Wales, Australia. Both populations are in decline with grazed and senescing individuals and very few seedlings. In contrast, Bertya rosmarinifolia is widespread and was found to produce five-fold more seedlings than B. ingramii. A comparative study of these species involving pre- and post-seed release factors (bud to fruit ratios, flowering synchrony, breeding system, seed-set, seed viability and seed removal rates) was undertaken to determine whether there are key factors that reduce the pre- and post-dispersal reproductive potential in B. ingramii. Both species are monoecious, self-compatible and flower from August to November. In both species female flowers appear first and last, with ca. 8 weeks of intermittent synchrony between female and male flowers. In 1996, B. rosmarinifolia produced more than 100% more fruit than B. ingramii, which in part is due to bud production which is two-fold higher in B. rosmarinifolia. Seeds in both species exhibited ca. 97% viability. Germination treatments (80, 100 °C , scarified and control) revealed that for both species manual scarification produced a higher germination rate than heat or controls. An additional treatment of GA3 improved germination rates of B. ingramii seeds over most non-GA3 treatments, indicating that seeds of B. ingramii contain a conditional dormancy mechanism. Almost 100% of B. ingramii seeds were removed from experimental trays by ants and more B. ingramii seeds were removed than B. rosmarinifolia. To break the extinction vortex in which B. ingramii is involved, we propose human-assisted seed dispersal to safe sites, the augmenting of populations with ex situ germinated seedlings and the exclusion of herbivores from B. ingramii populations. The incorporation of the common congener B. rosmarinifolia was useful because it revealed that it too experienced bud loss to moths and seed removal by ants. Threat abatement for B. ingramii, particularly of moth damage, could be trialled on common B. rosmarinifolia.

Use of the hydrothermal time model to analyse interacting effects of water and temperature on germination of three grass species

The temperate grass species, red fescue (Festuca rubrassp.litoralis), perennial ryegrass (Lolium perenne) and Kentucky bluegrass (Poa pratensis), are often sown in mixtures for turfgrass. Differences in germination response to environmental conditions often result in different establishment success. The three species were compared in a study of imbibition at reduced water potential (Ψ) and in a study of the effects of Ψ and temperature on germination. In all three species, imbibition rate and extent were reduced with decreasing Ψ, causing a prolonged lag-phase before germination, particularly in Kentucky bluegrass. In perennial ryegrass and Kentucky bluegrass, water content at radicle emergence decreased non-linearly with decreasing Ψ, with a lower asymptote expressing the critical water content for germination. Seeds were germinated in factorial combinations of two temperatures (10 and 25°C) and nine Ψ levels (–2 to 0 MPa). Kentucky bluegrass required a longer thermal time to germinate than the other species and exhibited conditional dormancy at 25°C, which affected the use of the thermal time model. The effects of Ψ were described by a hydrotime model. Kentucky bluegrass had higher base water potential (Ψb) and required a longer hydrotime for germination than the other species. However, Ψbincreased with temperature in Kentucky bluegrass and red fescue, but decreased in perennial ryegrass, and the base temperatures also changed with Ψ. The interacting effects of Ψ and temperature affected the ability of the hydrothermal time model to predict germination performance across all temperature and Ψ conditions.

Ecological significance of seed germination characteristics in flood-meadow species

Summary In climate chamber and outdoor experiments we analysed germination traits of 42 vascular plant species typical of floodmeadows of the alliance Cnidion. In Central Europe such flood-meadows and their highly endangered character species are confined to large lowland river corridors with a dry and warm climate. Due to the prevailing ecological conditions, particularl y the extremely high variability in soil moisture potential, it was hypothesised that flood-meadow species exhibit specific strat egies in terms of germination phenology, temperature requirements and primary dormancy to avoid constraints on seedling recruitment imposed by flooding, drought and competition from established vegetation. Furthermore, we expected that germination characteristics could be a key for the causal understanding of range size, confinement to large river corridors, recent rarification processes, seed bank persistence and establishment success in restoration projects. PCA ordination of germination traits reflected a continuous gradient of increasing primary dormancy levels ranging from species with a high capacity for fast and almost complete germination over a wide range of temperatures to those with high and narrow temperature requirements and/or a delayed and asynchronous emergence. Many of the studied species germinated only at relatively high temperature in the year of shedding, but the primary conditional dormancy disappeared in most cases after a period of cold wet stratification. Requirements for chilling and/or high germination temperature were revealed to be the most common strategies for avoidance of harmful autumn and winter germination. The majority of the studied species tends to exploit particularly favourable regeneration niches in early spring. Surprisingly, many species with large long-term persistent soil se ed banks exhibited relatively low dormancy levels in light ; this was correlated with small seed size and a consequent higher probability of burial. We found no relationship between germination characteristics and river corridor confinement or Central European range size. However, there was a significant trend in endangered species towards higher temperature requirements and delayed, asynchronous germination. This is presumably disadvantageous under the environmental conditions of subcontinental flood-meadows.

Parameters affecting germinability and seed bank dynamics in dimorphic achenes of Centaurea solstitialis in California

Yellow starthistle (Centaurea solstitialis L.) is a major rangeland weed pest in California and other western states. Two complementary aspects of seed biology were examined: germination and seed bank dynamics. Achenes were tested for changes in germinability over time using various light, temperature, and moisture treatments. Soil cores were collected over time to monitor changes in achene density. Both plumed and nonplumed achenes exhibited photo reversible responses to red and far-red light. White and red light enhanced germination above that occurring in darkness. Achenes showed seasonal changes in germinability, which were characteristic of conditional dormancy. Exposure of achenes to field conditions during summer alleviated primary dormancy, allowing germination to occur at higher temperatures and lower substrate moisture. Nonplumed achenes collected in winter showed reduced germinability at or above moderate temperatures under both single and multiple light exposures. Imbibition and moderately low temperatures with subsequent drying induced secondary conditional dormancy in some achenes. Plumed achenes dominated the soil seed bank before the rainy season, then declined rapidly following rain. Nonplumed achenes peaked after rains began, then declined. Although nearly all achenes were depleted after 4 years, both types remained, showing annual cycles of high and low germinability in autumn and late spring, respectively.Key words: light, phytochrome, conditional dormancy, longevity.

Contrasting seed germination patterns for intracontinental disjuncts of Heuchera (Saxifragaceae) in North America: the eastern temperate H. parviflora and western montane H. cylindrica

Heuchera parviflora var. parviflora grows primarily in southeastern North America and Heuchera cylindrica var. cylindrica in northwestern North America. Seeds of H. parviflora were collected in Kentucky in November 1995 and 1996 during the dispersal stage of the life cycle, and those of H. cylindrica were collected in Washington in August 1996. The purpose of the study was to compare the germination patterns of these two intracontinental disjunct species of Heuchera. During 2–12 weeks of incubation over a range of thermoperiods (15:6, 20:10, 25:15, 30:15, 35:20°C (12 h : 12 h)) that simulated habitat temperatures, fresh H. parviflora seeds germinated to 62–87% at 20:10 – 30:15°C in light. Cold stratification at 5°C effectively broke conditional dormancy in H. parviflora seeds: 76–96% germination took place during 2 weeks of incubation at 15:6 – 35:20°C in light following 12 weeks of cold stratification in light. In contrast, fresh H. cylindrica seeds germinated to 69–94% at 15:6 – 25:15°C in light during 2–12 weeks of incubation in light, but cold stratification at 5°C did not improve germination. Peak germination of H. parviflora seeds sown in November 1996 in an ambient-temperature greenhouse occurred in early March 1997, whereas that of H. cylindrica seeds sown in September 1996 occurred in late October 1996. Adaptive divergence of these species apparently has occurred with respect to their seed germination ecology. Their dormancy and germination characteristics are similar to the many other species growing in the same vegetation types.Key words: adaptive divergence, Heuchera cylindrica, Heuchera parviflora, physiological dormancy, Saxifragaceae, seed 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.

Modelling the germination of Stellaria media using the concept of hydrothermal time.

The germination characteristics of Stellaria media (common chickweed) were investigated over a range of constant temperatures and degrees of moisture stress in order to assess the suitability of hydrothermal time as a basis for modelling germination under field conditions. Maximum percentage germination occurred over a much narrower temperature range around the optimum temperature than previously seen for cultivated crop seed. The entire final percentage germination response to temperature in water was well described by two probit curves, and this model was extended to describe the data at all water potentials at a temperature close to the optimum. The implications of the reduction in germination at nonoptimal temperatures are discussed with respect to the interpretation of germination progress curves and conditional dormancy. After adjusting for maximum percentage germination, a hydrothermal time model was found to fit the data set well within the conditions normally encountered in horticultural seedbeds. This separation of the final percentage germination presents a flexible modelling approach that allows for the different levels of dormancy typically expressed within weed populations. By contrast with many previously reported species, S. media had a synchronous germination rate within the population at any given temperature/water potential combination. This synchronous germination of at least a proportion of the population over a wide range of temperature and water potentials might have ecological significance for the opportunistic germination behaviour of this weed species.

SHORT COMMUNICATION Increased sensitivity to green light during transition from conditional dormancy to nondormancy in seeds of three species ofSolidago(Asteraceae)

AbstractGermination responses of nonstratified (conditionally dormant) and of cold-stratified (nondormant) seeds of threeSolidagospecies were tested in darkness at 30/15°C following various lengths of exposure [0 (control), 5 and 30 min, 1, 6 and 14 h] to green (mostly 500–600 nm) or white (400–700 nm) light. Prior to exposures, nonstratified seeds were imbibed in darkness at laboratory conditions for 24 h, and cold-stratified seeds were kept in darkness at 5°C on a moist substrate for 8 wk. Nonstratified seeds ofS. altissima,S. nemoralisandS. shortiigerminated to 0–46%, 0–25% and 36–99%, respectively, in darkness following 5 min to 14 h of green or white light, whereas cold-stratified seeds germinated to 53–98%, 31–85% and 82–100%, respectively. Thus, sensitivity of seeds to both green and white light increased as they passed from conditional dormancy to nondormancy. Moreover, sensitivity to green and white light was similar to the hierarchy of seed size. Green safe lights must be used with caution in interpreting the results of studies on light/dark responses of seeds.

Experimental evidence for a persistent spore bank in Sphagnum.

Spore capsules of four Sphagnum species were buried at different depths in peat on a bog. Spore viability was determined after 0, 1, 2 and 3 yr. Viability generally declined with time, but viable spores were still found at all depths after 3 yr. The light-coloured spores of S. balticum and S. tenellum retained their viability better than the darker spores of S. fuscum and S. lindbergii. Survival was highest under wet but aerobic conditions, but was also high under humid or periodically desiccated conditions. By contrast, most spores stored under wet, anaerobic conditions died within 2-3 yr. These results, and predictions from them, are not consistent with earlier results for spores of long-lived and dominant bryophytes, or for seeds of phanerogams of undisturbed wetlands and forests. There was no correlation between spore size and longevity across species, but the small spores from small capsules of S. balticum and S. tenellum generally showed higher viability than those from the medium-sized and large capsules of the same species. This suggests a positive intraspecific relationship between longevity and dispersal distance. There was an indication of conditional dormancy, controlled by weather, in Sphagnum spores. The experiments indicate that Sphagnum spores can form a long-term persistent spore bank under suitable conditions, with a half-life of between 1 and 20 yr (mean across species of 2.6 and 5.0 yr at two depths studied), and with potential values in individual spore capsules of several decades, or even of centuries. Sphagnum spores kept refrigerated showed 15-35% viable spores after 13 yr. The capacity to form a persistent spore bank that can be activated whenever favourable conditions occur might help explain the wide geographical distribution of many Sphagnum species in the boreal and temperate zones, where they have managed to colonize almost every suitable patch of acidic, nutrient-poor wetland.

Germination Traits and Adaptive Regeneration Strategies of the Three Carpinus Species

Germination tests in the three sympatricCarpinus species,C. laxiflora, C. tshonoskii, andC. cordata, were carried out to clarify their germination traits in relation to the regeneration strategies. Seeds did not germinate just after ripening under any conditions in the three species. After one or two rounds of cold stratification, however,C. laxiflora andC. tshonoskii showed high germination percentages (>80%) at alternating temperature in light whileC. cordata did not. After a long-term (10 months) cold stratification, about 100% ofC. cordata seeds germinated, although about 70% of these seeds were in “conditional dormancy” and cannot germinate in the dark. Seed germination in the threeCarpinus species was enhanced by alternating temperature, but not by light exposure. These results suggest that the seeds of the three species are dormant just after ripening and the dormancy is broken by cold stratification. with intra-generic variations in the breakage. Germination traits in each species may be closely related to the regeneration strategy;C. cordata maintains a persistent seed bank with seed dormancy and regenerates after disturbances as a seed bank strategist, whereas the other two species germinate in the next spring after seedfall and regenerate depending either on existing disturbed-sites or sites of recent disturbances after seedling establishment as a seed rain strategist (C. tschonoskii) or seedling bank strategist (C. laxiflora).

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.

Seed germination responses to some environmental factors in the seagrass Zostera capricorni from eastern Australia

The germination requirements of Zostera capricorni Aschers. were studied in relation to temperature, salinity and oxygen potential. In treatments maintained at 16°C (corresponding to winter mean water temperature) and a salinity of 15‰ (half that of normal seawater), the cumulative percentage germination in both aerobic and anaerobic treatments was over 80%. However, seed germination was much faster under anaerobic conditions than aerobic conditions (cf. mean time-to-germinate (MTG) of <41 and >136 days, respectively). Under anaerobic conditions, at a constant 16°C, salinity level did not affect germination which was high (>80%) and rapid (MTG<41 days) at both 15‰ and 30‰. However, under aerobic conditions, percentage germination did not exceed 60% at the higher salinity, 30‰, at the end of the experiment. Germination was also slower at the higher salinity, 30‰, compared to 15‰ (MTG=159.6 and 136.3 days, respectively) but in both cases this was considerably slower than that recorded under anaerobic conditions. In all treatments held at 26°C (corresponding to summer mean water temperature) the MTG of seeds was high (>82 days) and the cumulative percentage germination did not exceed 40%. The effects of temperature on germination were more marked under anaerobic conditions than aerobic conditions. When Z. capricorni seeds were transferred to 16°C, after 24 days at 26°C (where germination rate was very slow), rapid germination followed in the anaerobic treatments. In contrast, when seeds were similarly transferred from 16°C to 26°C, the germination rate decreased to a negligible level following a 1-week lag period immediately after transfer when they continued to germinate rapidly. After 5 months cold stratification in artificial seawater at 6°C, both final germination and germination rate under anaerobic conditions were broadly similar to that recorded in unstratified seeds. In contrast, under aerobic conditions despite the fact that final germination at both salinity levels did not exceed 60%, stratified seeds germinated much quicker than unstratified seeds (average MTG value across both salinity levels, 51.2 and 148 days, respectively) indicating a reduction in the conditional dormancy originally maintained by aerobic conditions.

Reimposition of conditional dormancy during air-dry storage of prechilled Sitka spruce seeds

AbstractPrechilling seeds of Sitka spruce (Picea sitchensis[Bong.] Carr.) at 4°C with 30% moisture content for 12–14 weeks (84–98 d) removed conditional dormancy (i.e. they were then able to germinate at 10°C). The non-dormant status was preserved after redrying to 6% moisture content. However, conditional dormancy was gradually reimposed during subsequent air-dry storage at 4°C and 6% seed moisture content in all five seed lots tested. Further investigations with one seed lot showed that reimposition was reversed by a second prechill treatment, but was reimposed again during subsequent air-dry storage. The trend of dormancy reimposition within seed lots over time was quantified by negative exponential relations between ability to germinate at 10°C and duration of air-dry storage. The progress of dormancy reimposition was influenced by seed storage moisture content and was most rapid at 4–10%. At higher moisture contents (15 and 20%) the rate of the reimposition of conditional dormancy was much reduced, while at moisture contents of 25 and 30% further loss in dormancy occurred. Thus it is clear that dormancy reimposition occurred during storage at low water potential rather than solely during desiccation from high to low water potential.