Epicotyl morphophysiological dormancy in seeds of Lilium polyphyllum (Liliaceae)

Dormancy-breaking and seed germination studies in genus Lilium reveal that the majority of Lilium spp. studied have an underdeveloped embryo at maturity, which grows inside the seed before the radicle emerges. Additionally, the embryo, radicle or cotyledon has a physiological component of dormancy; thus, Lilium seeds have morphophysiological dormancy (MPD). A previous study suggested that seeds of Lilium polyphyllum have MPD but the study did not investigate the development of the embryo, which is one of the main criteria to determine MPD in seeds. To test this hypothesis, we investigated embryo growth and emergence of radicles and epicotyls in seeds over a range of temperatures. At maturity, seeds had underdeveloped embryos which developed fully at warm temperature within 6 weeks. Immediately after embryo growth, radicles also emerged at warm temperatures. However, epicotyls failed to emerge soon after radicle emergence. Epicotyls emerged from >90% seeds with an emerged radicle only after they were subjected to 2 weeks of cold moist stratification. The overall temperature requirements for dormancy-breaking and seed germination indicate a non-deep simple epicotyl MPD in L. polyphyllum.     

Post-dispersal embryo development, germination phenology, and seed dormancy in Cardiocrinum cordatum var. glehnii (Liliaceae s. str.), a perennial herb of the broadleaved deciduous forest in Japan

In an investigation of seed germination in Cardiocrinum cordatum var. glehnii, embryos in fresh seeds in October were underdeveloped and did not grow until September of the following year. Then, they grew rapidly and had fully elongated by early November. In the second spring after dispersal, radicles emerged under snow in late March and after snowmelt in April. Cotyledons emerged soon after radicles. In several laboratory experiments, embryos grew at 15°/5°C (light 12 h/ dark 12 h) following 25°/15°C. Radicles emerged from seeds with fully elongated embryos at 5°-15°C after cold stratification at 0°-5°C. Cotyledons emerged in 2 wk from seeds with a radicle at 15°/5°C to 30°/20°C. Although seeds require c. 18-19 mo after dispersal to germinate in nature, under controlled conditions, they required only 9 mo with a sequence of 25°/15°C → 15°/5°C → 0°-5°C → 15°/5°C. This is practical knowledge for propagation of plants from seeds. GA(3) treatment partially substituted for the high temperature requirement. Based on dormancy-breaking requirements, the seeds have deep simple morphophysiological dormancy (MPD). A literature review of seed dormancy in taxa of Liliaceae s. str. showed that phylogenetic position in this case is not a good predictor of level of MPD.     

Interchangeable effects of gibberellic acid and temperature on embryo growth, seed germination and epicotyl emergence in Ribes multiflorum ssp. sandalioticum (Grossulariaceae)

Morphophysiological dormancy was investigated in seeds of Ribes multiflorum Kit ex Roem et Schult. ssp. sandalioticum Arrigoni, a rare mountain species endemic to Sardinia (Italy). There were no differences in imbibition rates between intact and scarified seeds, suggesting a lack of physical dormancy, while methylene blue solution (0.5%) highlighted a preferential pathway for solution entrance through the raphe. Embryos were small at seed dispersal, with an initial embryo:seed ratio (E:S) of ca. 0.2 (embryo length, ca. 0.5 mm), whereas the critical E:S ratio for germination was three times longer (ca. 0.6). Gibberellic acid (GA(3), 250 mg · l(-1)) and warm stratification (25 °C for 3 months) followed by low temperature (<15 °C) enhanced embryo growth rate (maximum of ca. 0.04 mm · day(-1) at 10 °C) and subsequent seed germination (radicle emergence; ca. 80% at 10 °C). Low germination occurred at warmer temperatures, and cold stratification (5 °C for 3 months) induced secondary dormancy. After radicle emergence, epicotyl emergence was delayed for ca. 2 months for seeds from three different populations. Mean time of epicotyl emergence was affected by GA(3) . Seeds of this species showed non-deep simple (root) - non-deep simple (epicotyl) morphophysiological dormancy, highlighting a high synchronisation with Mediterranean seasonality in all the investigated populations.     

Recalcitrancy and a new kind of epicotyl dormancy in seeds of the understory tropical rainforest tree Humboldtia laurifolia (Fabaceae, Ceasalpinioideae)

We report a new kind of seed dormancy and identify the storage behavior category for an important understory rainforest tree that also is used as an ornamental. While studying seed dormancy of Fabaceae species in Sri Lanka, we observed a considerable delay in emergence of the plumule following radicle emergence in Humboldtia laurifolia. Because epicotyl dormancy has not been reported in Fabaceae, we undertook a detailed morphological study of seed germination in this species. Our aims were to document desiccation tolerance/intolerance and epicotyl dormancy in seeds of H. laurifolia. Drying and low temperature storage were used to evaluate storage behavior of the seeds and imbibition, germination, and seed coat anatomy to categorize seed dormancy in two seed collections. Plumule development before its emergence and effects of light and temperature on plumule emergence were monitored. All seeds that were dried to 15% moisture content or stored at -1°C lost viability. Plumules began to grow 20 ± 5 d from radicle emergence and emerged after 40 ± 3 d. Dark and high illuminance further delayed plumule emergence. Seeds are recalcitrant and have a hitherto unreported kind of epicotyl dormancy, for which we propose the formula .     

Temperature requirements differ for the two stages of seed dormancy break in Aegopodium podagraria (Apiaceae), a species with deep complex morphophysiological dormancy

Only a few studies have considered the possibility that low temperature requirements may vary among stages of dormancy break in seeds with morphophysiological dormancy (MPD). We show that this lack of consideration in previous studies on seed dormancy and germination of Aegopodium podagraria might explain the low germination percentages and/or the relatively long periods of incubation needed for germination. Under natural temperatures, embryos began to grow in September and were fully elongated by late December; most growth occurred when the average daily mean temperature was about 10°C. Radicles emerged under snow in late winter, and cotyledons emerged after snowmelt in early spring. In laboratory experiments, 100% of the embryos grew to full length at both 0 and 5°C, whereas 0°C was much more effective than 5°C in overcoming the physiological dormancy in seeds after embryos were fully elongated. Following radicle emergence, cotyledons emerged readily in a wide range of temperatures ≥5°C. GA(3) did not substitute for the low temperature requirement for dormancy break. Seed dormancy in A. podagraria fits Nikolaeva's formula for deep complex MPD, i.e., C(3)B-C(3). Better germination of seeds pretreated at 0° than at 5°C has practical implications for cultivating this species.     

Sequential temperature control of multi-phasic dormancy release and germination of Paeonia corsica seeds

Aims The physiological responses during dormancy removal and multi-phasic germination were investigated in seeds of Paeonia corsica (Paeoniaceae).Methods Seeds of P. corsica were incubated in the light at a range of temperatures (10–25 and 25/10°C), without any pre-treatment, after W (3 months at 25°C), C (3 months at 5°C) and W + C (3 months at 25°C followed by 3 months at 5°C) stratification, and a GA 3 treatment (250 mg·l^-1 in the germination substrate). Embryo growth, time from testa to endosperm rupture and radicle emergence were assessed as separate phases. Epicotyl–plumule emergence was evaluated incubating the germinated seeds at 15°C for 2 weeks, at 5 and 25°C for 2 months on agar water before transplanting to the soil substrate at 10, 15 and 20°C and at 15°C for 2 months on the surface agar water with GA 3 .Important findings Embryo growth, testa rupture, endosperm rupture (radicle emergence) and growth of the epicotyl were identified as four sequential steps in seeds of P. corsica. Gibberellic acid alone and warm stratification followed by 15°C promoted embryo growth and subsequent seed germination. Cold stratification induced secondary dormancy, even when applied after warm stratification. After radicle emergence, epicotyl–plumule emergence was delayed for ca. 3 months. Mean time of epicotyl–plumule emergence was positively affected by cold stratification (2 months at 5°C) and GA 3. P. corsica seeds exhibited differential temperature sensitivity for the four sequential steps in the removal of dormancy and germination processes that resulted in the precise and optimal timing of seedling emergence.     

Discovering the type of seed dormancy and temperature requirements for seed germination of Gentiana lutea L. subsp. lutea (Gentianaceae)

Aims There are a number of mechanisms that regulate germination; among these, seed dormancy, one of the most important, is an adaptative mechanism in plants to promote survival by dispersing germination in space and time until environmental conditions are favourable for germination. The main goals of this study were to determine the temperature requirements for seed dormancy release and germination of Gentiana lutea subsp. lutea, to identify the class and level of seed dormancy and to suggest an optimal germination protocol.Methods Seeds belonging to two different localities were subjected to various pre-treatments, including cold stratification (0 and 5°C), warm stratification (25/10°C) and different combinations of these, and then incubated at a range of constant temperatures (5–25°C) and 25/10°C. Embryo growth during pre-treatments and incubation conditions were assessed at different times by measuring the embryo to seed length ratio (E:S ratio). The final germination percentage (FGP) and the germination rate (t 50) were calculated.Important findings Fleshy mature seeds of G. lutea subsp. lutea have linear underdeveloped embryos. Cold stratification at 0°C was effective in overcoming the physiological dormancy (PD) and promoted embryo growth and subsequent germination. After cold stratification at 0°C, both the root and the shoot emerged readily under a wide range of temperatures. G. lutea subsp. lutea seeds showed an intermediate complex morphophysiological dormancy (MPD). As regards the optimal germination protocol for this taxon, we suggest a period of cold stratification at ca. 0°C followed by seed incubation at 10–20°C. The optimal germination temperatures found for seeds of this taxon, as well as its pre-chilling requirement at 0°C, suggest that it is well adapted to a temperate climate; this behavior highlights an increasing threat from global warming for G. lutea, which could reduce the level of natural emergence in the field, prejudicing also the long-term persistence of the natural populations in Sardinia.     

Physiology, morphology and phenology of seed dormancy break and germination in the endemic Iberian species Narcissus hispanicus (Amaryllidaceae)

Background and Aims

Only very few studies have been carried out on seed dormancy/germination in the large monocot genus Narcissus. A primary aim of this study was to determine the kind of seed dormancy in Narcissus hispanicus and relate the dormancy breaking and germination requirements to the field situation.

Methods

Embryo growth, radicle emergence and shoot growth were studied by subjecting seeds with and without an emerged radicle to different periods of warm, cold or warm plus cold in natural temperatures outdoors and under controlled laboratory conditions.

Key Results

Mean embryo length in fresh seeds was approx. 1·31 mm, and embryos had to grow to 2·21 mm before radicle emergence. Embryos grew to full size and seeds germinated (radicles emerged) when they were warm stratified for 90 d and then incubated at cool temperatures for 30 d. However, the embryos grew only a little and no seeds germinated when they were incubated at 9/5, 10 or 15/4 °C for 30 d following a moist cold pre-treatment at 5, 9/5 or 10 °C. In the natural habitat of N. hispanicus, seeds are dispersed in late May, the embryo elongates in autumn and radicles emerge (seeds germinate) in early November; however, if the seeds are exposed to low temperatures before embryo growth is completed, they re-enter dormancy (secondary dormancy). The shoot does not emerge until March, after germinated seeds are cold stratified in winter.

Conclusion

Seeds of N. hispanicus have deep simple epicotyl morphophysiological dormancy (MPD), with the dormancy formula C_1bB(root) – C₃(epicotyl). This is the first study on seeds with simple MPD to show that embryos in advanced stages of growth can re-enter dormancy (secondary dormancy).     

Dormancy and the fire-centric focus: germination of three Leucopogon species (Ericaceae) from South-eastern Australia

BACKGROUND AND AIMS: Germination studies of species from fire-prone habitats are often focused on the role that fire plays in breaking dormancy. However, for some plant groups in these habitats, such as the genus Leucopogon (Ericaceae), dormancy of fresh seeds is not broken by fire cues. In the field, these same species display a flush of seedling emergence post-fire. Dormancy and germination mechanisms therefore appear complex and mostly unknown. This study aimed to identify these mechanisms by establishing dormancy class and testing the effects of a set of typical germination cues, including those directly related to fire and entirely independent of fire. METHODS: To classify dormancy, we assessed seed permeability and embryo morphology, and conducted germination experiments at seasonal temperatures in incubators. To test the effects of fire cues on germination, factorial combinations of smoke, heat and dark treatments were applied. Ageing treatments, using burial and seasonal incubation, were also tested. Germination phenology was established. KEY RESULTS: Seeds were dormant at release and had underdeveloped embryos. Primary dormancy of the study species was classified as morphophysiological. Seasonal temperature changes overcame primary dormancy and controlled timing of germination. Fire cues did not break primary dormancy, but there was a trend for smoke to enhance germination once this dormancy was overcome. CONCLUSIONS: Despite the fact that fire is a predominant disturbance and that many species display a flush of emergence post-fire, seasonal temperatures broke the primary physiological dormancy of the study species. It is important to distinguish between fire being responsible for breaking dormancy and solely having a role in enhancing levels of post-fire germination for seeds in which dormancy has been overcome by other factors. Biogeographical evidence suggests that morphological and physiological factors, and therefore seasonal temperatures, are likely to be important in controlling the dormancy and patterns of post-fire germination of many species in fire-prone regions.     

Interpopulation variability on embryo growth,seed dormancy break,and germination in the endangered Iberian daffodil Narcissus eugeniae (Amaryllidaceae)

The main goal of the study was to assess germination requirements in a threatened daffodil to elaborate a detailed protocol for plant production from seeds, a key tool for conservation. Experiments were carried out both in the laboratory and outdoor conditions. In Pseudonarcissi section, endemic Iberian species of Narcissus studied heretofore have different levels of morphophysiological dormancy (MPD). Embryo length, radicle emergence, and shoot emergence were analyzed to determine the level of MPD. Both interpopulational variability and seed storage duration were also studied. Mean embryo length in fresh seeds was 1.32 mm and the embryo had to grow until it reached at least 2.00 mm to germinate. Embryo growth occurs during warm stratification, after which the radicle emerges when temperatures go down. Seed dormancy was broken in the laboratory at 28/14°C in darkness followed by 15/4°C, but the germination percentage varies depending on the population. In outdoor conditions, seed dispersal occurs in June, the embryo grows during the summer and then the radicle emerges in autumn. The radicle system continues to grow during the winter months, but the shoot does not emerge until the beginning of the spring because it is physiologically dormant and requires a cold period to break dormancy. Early cold temperatures interrupt embryo growth and induce dormancy in seeds with an advanced embryo development. Seeds of N. eugeniae have deep simple epicotyl MPD. In addition, we found that embryo growth and germination were improved by seed storage duration.     

Diversity of epicotyl dormancy among tropical montane forest species in Sri Lanka

• Fruiting season of many Sri Lankan tropical montane species is not synchronised and may not occur when conditions are favourable for seedling establishment. We hypothesised that species with different fruiting seasons have different seed dormancy mechanisms to synchronise timing of germination with a favourable season for establishment. Using six species with different fruiting seasons, we tested this hypothesis.
• Germination and imbibition of intact and manually scarified seeds were studied. Effect of GA₃ on germination was examined. Embryo length:seed length (E:S) ratio of freshly matured seeds and of those with a split seed coat was determined. Time taken for radicle and plumule emergence and morphological changes of the embryos were recorded.
• The radicle emerged from Ardisia missionis, Bheza nitidissima and Gaetnera walkeri seeds within 30 days, whereas it took >30 days in other species. Embryos grew in seeds of B. nitidissima and G. walkeri prior to radicle emergence but not in Microtropis wallichiana, Nothapodytes nimmoniana and Symplocos cochinchinensis. A considerable delay was observed between radicle and plumule emergence in all six species. Warm stratification and/or GA₃ promoted germination of all species.
• All the tested species have epicotyl dormancy. Seeds of B. nitidissima and G. walkeri have non‐deep simple morphophysiological epicotyl dormancy, and the other four species have non‐deep physiological epicotyl dormancy. Differences in radicle and epicotyl dormancy promote synchronisation of germination to a favourable time for seedling development. Therefore, information on dormancy‐breaking and germination requirements of both radicle and epicotyl are needed to determine the kind of dormancy of a particular species.

     

Complex combination of seed dormancy and seedling development determine emergence of Viburnum tinus (Caprifoliaceae)

BACKGROUND AND AIMS: The shrub Viburnum tinus is widely distributed in mattoral vegetation of the Mediterranean basin. The purpose of the present study was to classify the seed dormancy type and examine the requirements for embryo growth, root protrusion and shoot emergence. METHODS: Overwintered fruits were collected in western Spain in April 2001 and prepared in three ways: entire pericarp was removed, exocarp and mesocarp were removed or fruits were left intact. Fruits treated in these three ways were subjected to artificial annual temperature cycles or to constant temperature regimes for 1.5 years. KEY RESULTS: Removal of exocarp and mesocarp was necessary for embryo growth and germination. High temperature favoured dormancy alleviation and embryo growth, intermediate to low temperatures favoured root protrusion, and intermediate temperature shoot emergence. There was substantial germination at constant temperature regimes, indicating an overlap between temperature intervals suitable for the different stages of embryo and seedling development. Functionally, V. tinus has the same root and shoot emergence pattern that is described for other Viburnum species considered to have epicotyl dormancy. However, the requirement for high and low temperatures for radicle protrusion and epicotyl emergence, respectively, was missing in V. tinus; these characters are the foundation for the epicotyl dormancy classification. CONCLUSIONS: It is concluded that V. tinus does not have epicotyl dormancy. Instead, there is a combination of a weak morphophysiological dormancy and a slow germination process, where different temperatures during an annual cycle favour different development stages. The present study suggests that the first complete seedlings would emerge in the field 1.5 years after fruit maturation in October, i.e. seed dispersal during winter, embryo growth during the first summer, root protrusion and establishment during the second autumn and winter, and cotyledon emergence during the second spring.     

Role of warm stratification in promoting germination of seeds of Empetrum hermaphroditum (Empetraceae), a circumboreal species with a stony endocarp

The broad objective of this research was to define the role of warm (≥15°C) stratification in breaking dormancy in seeds with stony endocarps that require warm-plus-cold (～0°-10°C) stratification for germination. This question was addressed using seeds (true seed + endocarp, hereafter called seeds) of Empetrum hermaphroditum. Only 2-5% of freshly matured seeds collected in September and October at five sites in Sweden germinated in light at daily alternating temperature regimes of 15°/6°, 20°/10°, and 25°/15°C. Dormancy was not due to impermeability of the stony endocarp surrounding each seed, and embryos did not grow prior to radicle emergence. Thus, seeds did not have physical, morphological, or morphophysiological dormancy. Long periods of either cold stratification (20 or 32 wk) or warm stratification (16 wk) resulted in a maximum of 22-38 and 10% germination, respectively, in light at 25°/15°C. After 12 wk warm stratification plus 20 wk cold stratification, 83-93% of the seeds germinated in light at the three temperature regimes. For a cold stratification period of 20 wk, germination increased with increase in length of the preceding warm stratification treatment. Gibberellic acid (GA(3)) promoted germination of 77-87% of the seeds. Based on dormancy-breaking requirements and response to GA(3), 62-78% of the seeds had intermediate physiological dormancy; the others had nondeep physiological dormancy. Contrary to suggestions of several other investigators that warm stratification is required to make the endocarp permeable to water via its breakdown by microorganisms, our results with E. hermaphroditum show that this is not the case. In this species, warm stratification is part of the dormancy-breaking requirement of embryos in seeds with intermediate physiological dormancy.     

Temperature controls seed germination and dormancy in the European woodland herbaceous perennial Erythronium dens-canis (Liliaceae)

We examined the germination ecology and the temperature requirements for germination of Erythronium dens-canis, under both outdoor and laboratory conditions. E. dens-canis is a spring flowering woodland geophyte widely distributed across Europe. Germination phenology, including embryo development and radicle and cotyledon emergence, were investigated in a natural population growing in Northern Italy. Immediately after harvest, seeds of E. dens-canis were either sown on agar in the laboratory under simulated seasonal temperatures or placed in nylon mesh sachets and buried in the wild. Embryos, undifferentiated at the time of seed dispersal, grew during summer and autumn conditions in the laboratory and in the wild, culminating in radicle emergence in winter when temperatures fell to ≈ 5 °C. Emergence of cotyledons did not occur immediately after radicle emergence, but was delayed until the end of winter. Laboratory experiments showed that temperature is the main factor controlling dormancy and germination, with seeds becoming non-dormant only when given warmth, followed by cold stratification. Unlike seeds of E. dens-canis that germinate in winter, in other Erythronium species radicle emergence occurs in autumn, while in some it is delayed until seeds are transferred from winter to spring conditions. Our results suggest that there is genetic and environmental control of the expression of seed dormancy amongst Erythronium species, which is related to local climate.     

Dormancy Release,Germination, and Electrolyte Leakage from Apple Embryos during Stratification in the Presence of Sucrose

The dynamics of dormancy release during the stratification of apple (Malus domestica Borkh.) seeds was quantitatively described by three characteristics of seeds germination: the percentage of seeds that germinated by the tenth day, mean germination time, and the sum of seeds germinated in each of ten days (Timson's parameter), which allowed the assessment of the viability, the rate of dormancy release, and seed heterogeneity. We showed that apple seeds were characterized by a combined (physical and physiological) type of dormancy, with the seed coat and the embryo envelope being involved in the maintenance of physical dormancy. The addition of sucrose to the stratification medium accelerated the release of seed dormancy and improved all characteristics that determine seed germinability. Electrolyte leakage from embryos hardly changed during stratification, which agrees with the fact that all seeds remained viable throughout the entire period of dormancy. We assume that the release of seed dormancy is not a single-stage process.     

Dissecting seed dormancy and germination in Aquilegia barbaricina,through thermal kinetics of embryo growth

• Threshold‐based thermal time models provide insight into the physiological switch from the dormant to the non‐dormant germinating seed.
• This approach was used to quantify the different growth responses of the embryo of seeds purported to have morphophysiological dormancy (MPD) through the complex phases of dormancy release and germination. Aquilegia barbaricina seeds were incubated at constant temperatures (10–25 °C) and 25/10 °C, without pre‐treatment, after warm+cold stratification (W+C) and GA₃ treatment. Embryo growth was assessed and the time of testa and endosperm rupture scored. Base temperatures (T_b) and thermal times for 50% (θ₅₀) of embryo growth and seed germination were calculated.
• W+C enabled slow embryo growth. W+C and GA₃ promoted rapid embryo growth and subsequent radicle emergence. The embryo internal growth base temperature (T_be) was ca. 5 °C for W+C and GA₃‐treated seeds. GA₃ treatment also resulted in similar T_b estimates for radicle emergence. The thermal times for embryo growth (θ_e50) and germination (θ_g50) were four‐ to six‐fold longer in the presence of GA₃ compared to W+C.
• A. barbaricina is characterised by a multi‐step seed germination. The slow embryo growth during W+C reflects continuation of the maternal programme of development, whilst the thermal kinetics of both embryo and radicle growth after the removal of physiological dormancy are distinctly different. The effects of W+C on the multiphasic germination response in MPD seeds are only partially mimicked by 250 mg·l^?1 GA₃. The thermal time approach could be a valid tool to model thermal kinetics of embryo growth and radicle protrusion.

     

Molecular mechanisms and hormonal regulation underpinning morphological dormancy: a case study using Apium graveolens (Apiaceae)

Underdeveloped (small) embryos embedded in abundant endosperm tissue, and thus having morphological dormancy (MD) or morphophysiological dormancy (MPD), are considered to be the ancestral state in seed dormancy evolution. This trait is retained in the Apiaceae family, which provides excellent model systems for investigating the underpinning mechanisms. We investigated Apium graveolens (celery) MD by combined innovative imaging and embryo growth assays with the quantification of hormone metabolism, as well as the analysis of hormone and cell-wall related gene expression. The integrated experimental results demonstrated that embryo growth occurred inside imbibed celery fruits in association with endosperm degradation, and that a critical embryo size was required for radicle emergence. The regulation of these processes depends on gene expression leading to gibberellin and indole-3-acetic acid (IAA) production by the embryo and on crosstalk between the fruit compartments. ABA degradation associated with distinct spatiotemporal patterns in ABA sensitivity control embryo growth, endosperm breakdown and radicle emergence. This complex interaction between gibberellins, IAA and ABA metabolism, and changes in the tissue-specific sensitivities to these hormones is distinct from non-MD seeds. We conclude that the embryo growth to reach the critical size and the associated endosperm breakdown inside MD fruits constitute a unique germination programme.     

Deep and intermediate complex morphophysiological dormancy in seeds of Ferula gummosa (Apiaceae)

We tested the hypothesis that seeds of the monocarpic perennial Ferula gummosa from the Mediterranean area and central Asia have deep complex morphophysiological dormancy. We determined the water permeability of seeds, embryo morphology, temperature requirements for embryo growth and seed germination and responses of seeds to warm and cold stratification and to different concentrations of GA₃. The embryo has differentiated organs, but it is small (underdeveloped) and must grow inside the seed, reaching a critical embryo length, seed length ratio of 0.65–0.7, before the seed can germinate. Seeds required 9 weeks of cold stratification at <10°C for embryo growth, dormancy break and germination to occur. Thus, seeds have morphophysiological dormancy (MPD). Furthermore, GA₃ improved the germination percentage and rate at 5°C and promoted 20 and 5% germination of seeds incubated at 15 and 20°C, respectively. Thus, about 20% of the seeds had intermediate complex MPD. For the other seeds in the seed lot, cold stratification (5°C) was the only requirement for dormancy break and germination and GA₃ could not substitute for cold stratification. Thus, about 80% of the seeds had deep complex MPD.     

Morphological dormancy in seeds of the autumn-germinating shrub Lonicera caerulea var. emphyllocalyx (Caprifoliaceae)

To better understand the germination ecophysiology of the genus Lonicera , the dormancy class, temperature requirements for embryo growth and radicle emergence and phenology of seedling emergence were determined for Lonicera caerulea var. emphyllocalyx . At maturity, seeds have an underdeveloped embryo (approximately 28% of the length of full-grown embryos). Embryos in fresh seeds grew to full length at 15, 20, 20/10 and 25/15°C within 3 weeks, but failed to grow at ≤ 10°C and at 30°C. Radicles emerged from 86–100% of freshly matured seeds in light at 15, 20, 20/10 and 25/15°C within 28 days, but failed to emerge at 10°C. Radicles emerged equally well in a 12 h photoperiod and in continuous darkness at 25/15°C. Rapid embryo growth and germination over a range of conditions indicate that seeds of this taxon have morphological dormancy (MD); this is the first report of MD in a species of Lonicera . Seeds are dispersed in summer, at which time high temperatures promote embryo growth. Embryos grow to the critical length for germination in approximately 1 month; the peak of seedling emergence occurs in early autumn. Radicles emerged within 2 months from 98% of seeds buried at soil depths of 2 cm and 10 cm in the field in August in Sapporo, Japan; thus, seeds have no potential to form a persistent soil seed bank. However, seeds sown too late in autumn for embryos to grow remained viable and germinated the following summer when temperatures were high enough to promote embryo growth.     

Dormancy-breaking and germination requirements for seeds of Symphoricarpos orbiculatus (Caprifoliaceae)

Fruits (drupes) of Symphoricarpos orbiculatus ripen in autumn and are dispersed from autumn to spring. Seeds (true seed plus fibrous endocarp) are dormant at maturity, and they have a small, linear embryo that is underdeveloped. In contrast to previous reports, the endocarp and seed coat of S. orbiculatus are permeable to water; thus, seeds do not have physical dormancy. No fresh seeds germinated during 2 wk of incubation over a 15°/6°-35°/20°C range of thermoperiods in light (14-h photoperiod); gibberellic acid and warm or cold stratification alone did not overcome dormancy. One hundred percent of the seeds incubated in a simulated summer → autumn → winter → spring sequence of temperature regimes germinated, whereas none of those subjected to a winter → spring sequence did so. That is, cold stratification is effective in breaking dormancy only after seeds first are exposed to a period of warm temperatures. Likewise, embryos grew at cold temperatures only after seeds were exposed to warm temperatures. Thus, the seeds of S. orbiculatus have nondeep complex morphophysiological dormancy. As a result of dispersal phenology and dormancy-breaking requirements, in nature most seeds that germinate do so the second spring following maturity; a low to moderate percentage of the seeds may germinate the third spring. Seeds can germinate to high percentages under Quercus leaf litter and while buried in soil; they have little or no potential to form a long-lived soil seed bank.