Seed dormancy release and germination characteristics of <i>Corispermum lehmannianum</i> Bunge,an endemic species in the Gurbantunggut desert of China

Seed dormancy release and germination of Corispermum lehmannianum Bunge were tested using various treatments: temperature, cold stratification, gibberelins (GA3), dry storage and sand burial. Results showed that temperature and light did not affect the germination of fresh seeds, cold stratification and GA3 could improve seed germination, whereas dry storage and sand burial did not. The germination percentage was highest at 35/20 °C after the cold stratification and GA3 treatments. Corispermum lehmannianum seeds were classified as non-deep, Type-2, physiological dormancy (PD), whose seed dormancy could be released by cold stratification and GA3.     

Effects of warming on seed germination of woody species in temperate secondary forests

• Seed germination, a critical stage of the plant life cycle providing a link between seeds and seedlings, is commonly temperature-dependent. The global average surface temperature is expected to rise, but little is known about the responses of seed germination of woody plants in temperate forests to warming.
• In the present study, dried seeds of 23 common woody species in temperate secondary forests were incubated at three temperature sequences without cold stratification and after experiencing cold stratification. We calculated five seed germination indices and the comprehensive membership function value that summarized the above indicators.
• Compared to the control, +2 and +4 °C treatments without cold stratification shortened germination time by 14% and 16% and increased the germination index by 17% and 26%, respectively. For stratified seeds, +4 °C treatment increased germination percentage by 49%, and +4 and +2 °C treatments increased duration of germination and the germination index, and shortened mean germination time by 69%, 458%, 29% and 68%, 110%, 12%, respectively. The germination of Fraxinus rhynchophylla and Larix kaempferi were most sensitive to warming without and with cold stratification, respectively. Seed germination of shrubs was the least sensitive to warming among functional types.
• These findings indicate warming (especially extreme warming) will enhance the seedling recruitment of temperate woody species, primarily via shortening the germination time, particularly for seeds that have undergone cold stratification. In addition, shrubs might narrow their distribution range

     

Comparative germination ecology of two altitudinal vicariant Saxifraga species endemic to the north of Spain

Seeds of high‐mountain species are thought to germinate rapidly, synchronously and at high percentages after a cold period, with limited dependence on the external environment; yet, empirical evidence only partially supports this behaviour. We performed a comparative study of the germination response of two closely related taxa along an altitude gradient in northern Spain. Seeds from several maternal families of six populations of Saxifraga trifurcata (lowland species) and S. canaliculata (highland species) were subjected to temperature and stratification treatments. Germination percentages and germination rates were analysed using generalised linear mixed modelling and accelerated failure‐time modelling. We found that germination percentages and germination rates were high and dependent on incubation temperature in both species. Within species, seeds from higher altitudes had higher germination percentages under all conditions. Cold–wet stratification negatively affected germination success, particularly in the lowland species. Overall, the highland species was less responsive to the experimental treatments and showed more synchronous germination patterns. We conclude that seeds from these two Saxifraga species germinate as efficiently as species from other habitats, but have a narrower germination response, probably due to the stronger selective pressures in their harsh environments. Finally, a cold, wet stratification period is not a prerequisite for the germination of high‐mountain S. canaliculata, and its strong negative effect on the germination of its lowland relative S. trifurcata may contribute to the altitudinal segregation of these two species.     

TWO TYPES OF MORPHOPHYSIOLOGICAL DORMANCY IN SEEDS OF TWO GENERA (OSMORHIZA AND ERYTHRONIUM) WITH AN ARCTO-TERTIARY DISTRIBUTION PATTERN

Temperature requirements for embryo growth and germination were determined for seeds of Osmorhiza occidentalis, O. chilensis, and Erythronium grandiflorum collected in western North America (Utah). Initially, embryos were 1.2, 0.6, and 0.8 mm in length, respectively, and they grew to 9.4, 9.2, and 4.1 mm, respectively, before germination occurred. Embryo growth and germination occurred during cold stratification (1, 5, 5/1 C), without a warm stratification pretreatment. However, warm stratification pretreatments at 30/15 C increased rates of embryo growth in O. occidentalis and E. grandiflorum seeds moved to low temperatures and germination rates in all three species. Optimum germination temperatures were 1, 5, or 5/1 C; gibberellic acid did not substitute for cold stratification. Thus, seeds of the three species have deep complex morphophysiological dormancy (MPD). In comparison, two species each of Osmorhiza and Erythronium from eastern North America have nondeep complex MPD and require warm followed by cold stratification for germination. Thus, disjunct species in genera with an Arcto-Tertiary distribution pattern can have different types of MPD. It is suggested that deep complex may have been derived from nondeep complex MPD.     

Tools for <Emphasis Type="Italic">Carex</Emphasis> revegetation in freshwater wetlands: understanding dormancy loss and germination temperature requirements

Carex is a globally distributed genus with more than 2000 species worldwide and Carex species are the characteristic vegetation of sedge meadow wetlands. In the mid-continental United States, Carex species are dominant in natural freshwater wetlands yet are slow to recolonize hydrologically restored wetlands. To aid in Carex revegetation efforts, we determined the dormancy breaking and temperature germination requirements of 12 Carex species. Seeds were cold stratified at 5/1°C for 0–6 months and then incubated in light at 5/1°C, 14/1°C, 22/8°C, 27/15°C, or 35/30°C. We found that all Carex species produced conditionally dormant seeds. The optimal temperature for germination for all but three species was 27/15°C. As is the case in other species with physiological dormancy, cold stratification increased germination percentages, broadened the temperature range suitable for germination, and increased germination rates for most species, but the magnitude of the effects varied among species. Many species germinated to 80% at 27/15°C without cold stratification and at 22/8°C with ≤1 month of stratification but required much longer stratification (up to 6 months depending on the species) to germinate to 80% at 14/1°C and 35/30°C. Our findings illustrate how a stratification pretreatment can greatly benefit Carex seed sowing efforts by triggering rapid germination to higher percentages. We recommend that cold stratification be targeted towards species with strong dormancy or used across a wider range of species when seed supplies for restoration are limiting. For Carex revegetation, establishing Carex canopies rapidly may help to prevent the invasion of undesirable species such as Phalaris arundinacea.     

Linking seed germination and plant height: a case study of a wetland community on the eastern Tibet Plateau

• Seed germination is the earliest trait expressed in a plant's life history, and it can directly affect the expression of post‐germination traits. Plant height is central to plant ecological strategies, because it is a major determinant of the ability of a species to compete for light. Thus, linking seed germination and plant height at the community level is very important to understanding plant fitness and community structure.
• Here, we tested storage condition and temperature requirements for germination of 31 species from a wetland plant community on the eastern Tibet Plateau and analysed correlation of germination traits with plant height in relation to storage condition.
• Germination percentage was positively related to plant height, and this relationship disappeared when seeds were incubated at a low temperature (i.e. 5 °C) or after they were stored under wet‐cold conditions. The response of seeds to dry+wet–cold storage was negatively related to plant height. Based on the scores of each species on the first two principal components derived from PCA, species were classified into two categories by hierarchical clustering, and there was a significant difference between germination and plant height of species in these two categories.
• These results suggest that the requirements for seed germination together with seasonal change in environmental conditions determine the window for germination and, in turn, plant growth season and resource utilisation and ultimately plant height.

     

Seed germination requirements of Buck’s beard [Aruncus dioicus (Walter) Fernald (Rosaceae)]

Aruncus dioicus (Walter) Fernald (Rosaceae) is a perennial herbaceous plant whose young shoots are traditionally collected in the wild and consumed as a food in NE Italy. The aim of this study was to determine the germination requirements of its seeds in order to start its cultivation, and to assess the germination of six accessions of the species. Viability of seeds ranged from 86 to 97% in the various accessions. Germination rate was almost null in seeds of two accessions, and ranged from 10.5 to 37.3 in the other ones. The seed coat was permeable to water. Treatments with GA₃, KNO₃ and mechanical scarification did not enhance the germination, while the cold stratification treatment at 2 °C for different periods improved the germination rate and the mean germination time as compared with the untreated seeds. With 45 days of cold stratification, the germination rate and mean germination time (respectively, 90.1% and 7.7 dd) of seeds were different from those of the untreated seeds. Cold stratified seeds germinated under artificial light and did not germinate in the dark. Seeds of A. dioicus displayed an intermediate physiological dormancy, removable by a cold stratification treatment, requiring both light and cold conditions.     

Can seed characteristics or species distribution be used to predict the stratification requirements of herbs in the Australian Alps?

The germination requirements of 19 herbs in the Australian Alps were investigated to determine which species may be sensitive to predicted climate changes. Seeds were subjected to factorial treatments of cold stratification for 0, 4, 8 and 12 weeks, followed by incubation at constant temperatures of 10, 15, 20 and 25 °C and alternating temperatures of 20/5 and 20/10 °C. Germination responses were used to identify stratification‐dependent species, to classify dormancy and to determine optimum conditions for laboratory germination. Ordinal logistic regression was used to determine whether the duration of stratification required for ≥ 50% germination could be predicted by seed weight, seed length, embryo : seed ratio or species distribution (latitudinal range, altitudinal range and maximum altitude). The Kruskal–Wallis test was used to determine any significant differences in stratification requirement between endospermic and non‐endospermic seeds. Species varied considerably in their response to the treatment combinations, and therefore their dormancy class. No significant predictors of stratification requirement were identified by ordinal logistic regression (P > 0.9); however, there was a significant difference in stratification requirement between endospermic and non‐endospermic seeds (P = 0.003). Species with non‐endospermic seeds did not require any stratification to germinate well over a range of temperatures, and appear most likely to remain stable or expand in range in response to climate warming. Conversely, the need for ≥ 8 weeks of cold stratification was associated with the presence of endosperm and either a restricted distribution or upland ecotypes of widely distributed species. Alpine species with endospermic seed and a restricted distribution are most likely to contract in range under climate change and would be appropriate to prioritize for ex situ conservation. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172 , 187–204.     

Germination of Prosopis juliflora (Sw) DC seeds after scarification treatments

Invasive plant species are the second most important threat to global biodiversity loss after land‐use change. Invasive species can modify native community composition, deplete species diversity and affect ecosystem processes. The Caatinga is one of the most human‐affected Brazilian ecosystems owing to non‐sustainable use of its natural resources. Prosopis juliflora is an important invasive plant species in the Caatinga ecosystem. Seed germination is a critical stage in plant life cycles and is a major factor in the establishment and success of invasive plant species. Among the factors that affect seed germination and dormancy, coat‐imposed seems to be the most important for P. juliflora. In Prosopis species, the ingestion of fruits by wild and domestic animals may promote and accelerate germination, enhancing the dispersal of seeds and fruits of these species. We investigated the germination capacity of P. juliflora seeds after artificial mechanical and chemical scarification and analyzed the changes in seedling vigor caused by the scarification treatments. Germination rate, germination time (TMG) and germination synchrony (E) differed significantly with the length of the scarification treatments in H₂SO₄ for both seeds with endocarps and seeds without endocarps (non‐endocarp seeds). Sulfuric acid affected plant survival more strongly than germination rate, particularly in non‐endocarp seeds.     

Comparative <Emphasis Type="Italic">in vitro</Emphasis> germination ecology of <Emphasis Type="Italic">Calopogon tuberosus</Emphasis> var. <Emphasis Type="Italic">tuberosus</Emphasis> (Orchidaceae) across its geographic range

Seed responses to temperature are often essential to the study of germination ecology, but the ecological role of temperature in orchid seed germination remains uncertain. The response of orchid seeds to cold stratification have been studied, but the exact physiological role remains unclear. No studies exist that compare the effects of either cold stratification or temperature on germination among distant populations of the same species. In two separate experiments, the role of temperature (25, 22/11, 27/15, 29/19, 33/24°C) and chilling at 10°C on in vitro seed germination were investigated using distant populations of Calopogon tuberosus var. tuberosus. Cooler temperatures promoted germination of Michigan seeds; warmer temperatures promoted germination of South Carolina and north central Florida seeds. South Florida seed germination was highest under both warm and cool temperatures. More advanced seedling development generally occurred at higher temperatures with the exception of south Florida seedlings, in which the warmest temperature suppressed development. Fluctuating diurnal temperatures were more beneficial for germination compared to constant temperatures. Cold stratification had a positive effect on germination among all populations, but South Carolina seeds required the longest chilling treatments to obtain maximum germination. Results from the cold stratification experiment indicate that a physiological dormancy is present, but the degree of dormancy varies across the species range. The variable responses among populations may indicate ecotypic differentiation.     

Seed germination ecology of the threatened endemic Iberian <Emphasis Type="Italic">Delphinium fissum</Emphasis> subsp. <Emphasis Type="Italic">sordidum</Emphasis> (Ranunculaceae)

Seeds of Delphinium fissum subsp. sordidum are physiologically dormant at maturity, with underdeveloped embryos; thus they have morphophysiological dormancy (MPD). The aims of this study were to determine the requirements for embryo growth, dormancy break and germination, to characterise the type of seed dormancy and to evaluate the effects of light, seed age, pollination mechanism, and inter-annual and inter-population variability on germinative ability. After 3 months of incubation at 5°C (cold stratification) in darkness conditions, the mean embryo length increased from 5.6 to 2.07 mm, with 76% of seeds germinating. Conversely, embryos of seeds incubated during 3 months at 20/7 or 28/14°C hardly grew and no germination was recorded. Since cold stratification was the only requirement for the loss of MPD, and both dry storage in laboratory conditions and warm stratification prior to cold stratification shortened the cold stratification period required for germination, it could be concluded that D. fissum subsp. sordidum seeds have intermediate complex MPD. Cold stratification and incubation in darkness conditions promoted higher germination percentages than those in light. In addition, germinative ability increased with seed age up to 8 months (reaching 96% at 5°C in darkness), showed a pronounced inter-annual and inter-population variability, as well as a significant decrease in seeds coming from pollination by geitonogamy. High temperatures (25/10 or 28/14°C) induced seeds to secondary dormancy, so seedling emergence in the greenhouse was restricted to February–March. The requirements for dormancy break and germination reflect an adaptation to trigger germination in late winter. This study is the first one to document a gradual increase in germination percentage with seed age for plant species with intermediate complex MPD.     

NONDEEP COMPLEX MORPHOPHYSIOLOGICAL DORMANCY IN SEEDS OF OSMORHIZA CLAYTONII (APIACEAE)

Freshly matured seeds of Osmorhiza claytonii exhibit a type of morphophysiological dormancy (MPD). Under natural conditions, embryo growth begins in late September and early October and continues until mid***- to late February, with the peak in October and November. Most seeds germinate between mid-February and late March. Embryos did not grow in seeds incubated for 24 weeks at 30/15 (warm stratification) or 5 C (cold stratification). However, in seeds given 12 weeks at 30/15 and then 12 weeks at 5 C, embryo length increased 1,246% while seeds were at 5 C. Zero to 7 days of warm followed by 24 weeks of cold stratification resulted in 2%–27% germination of fresh seeds, whereas 2 to 12 weeks of warm followed by 24 weeks of cold stratification resulted in 80%–98% germination. Warm plus cold stratification was required for embryo growth and germination of seeds that remained undispersed for a year in the field. GA₃ was partially effective in substituting for warm stratification. The name “nondeep complex MPD” is proposed for the type of MPD found in O. claytonii and a few other species, making a total of eight types of MPD presently known.     

Effects of the duration of cold stratification on early life stages of the Mediterranean alpine plant Silene ciliata

Cold stratification provided by snow cover is essential to break seed dormancy in many alpine plant species. The forecast reduction in snow precipitation and snow cover duration in most temperate mountains as a result of global warming could threaten alpine plant populations, especially those at the edge of their species distribution, by altering the dynamics of early life stages. We simulated some effects of a reduction in the snow cover period by manipulating the duration of cold stratification in seeds of Silene ciliata, a Mediterranean alpine specialist. Seeds from three populations distributed along an altitudinal gradient were exposed to different periods of cold stratification (2, 4 and 6 months) in the laboratory and then moved to common garden conditions in a greenhouse. The duration of the cold stratification treatment and population origin significantly affected seed emergence percentage, emergence rate and seedling size, but not the number of seedling leaves. The 6‐month and 4‐month cold stratification treatments produced higher emergence percentages and faster emergence rates than seeds without cold stratification treatment. No significant cold stratification duration x seed population origin interactions were found, thus differential sensitivity to cold stratification along elevation is not supported.     

Sympatric species of Hibbertia (Dilleniaceae) vary in dormancy break and germination requirements: implications for classifying morphophysiological dormancy in Mediterranean biomes

Background and Aims

Several ecologically important plant families in Mediterranean biomes have seeds with morphophysiological dormancy (MPD) but have been poorly studied. The aim of this study was to understand the seed ecology of these species by focusing on the prominent, yet intractably dormant Australian genus Hibbertia. It was hypothesized that the slow germination in species of this genus is caused by a requirement for embryo growth inside the seed before germination, and that initiation of embryo growth is reliant upon a complex sequence of environmental cues including seasonal fluctuations in temperature and moisture, and an interplay with light and smoke. Using the results, the classification of the MPD level in species of Hibbertia is considered.

Methods

Four species of Hibbertia in winter rainfall south-western Australia were selected. These species, whilst differing in geographic distributions, are variously sympatric, and all are important understorey components of plant communities. The following aspects related to dormancy break, embryo growth and germination were investigated: temperature and moisture requirements; effects of karrikinolide, gibberellic acid and aerosol smoke; and phenology.

Key Results

Following exposure to wet/dry cycles at low or high temperatures, embryo growth and germination occurred, albeit slowly in all species at low temperatures when moisture was unlimited, corresponding to winter in south-west Australia. Photo regime influenced germination only in H. racemosa. Aerosol smoke triggered substantial germination during the 1st germination season in H. huegelii and H. hypericoides.

Conclusions

Although the study species are con-generic, sympatric and produce seeds of identical morphology, they possessed different dormancy-break and germination requirements. The physiological component of MPD was non-deep in H. racemosa but varied in the other three species where more deeply dormant seeds required >1 summer to overcome dormancy and, thus, germination was spread over time. Embryos grew during winter, but future studies need to resolve the role of cold versus warm stratification by using constant temperature regimes. To include Mediterranean species with MPD, some modifications to the current seed-dormancy classification system may need consideration: (a) wet/dry conditions for warm stratification and (b) a relatively long period for warm stratification. These outcomes have important implications for improving experimental approaches to resolve the effective use of broadcast seed for ecological restoration.     

Seed Dynamics of Early and Late Successional Tree Species in Tropical Abandoned Pastures: Seed Burial as a Way of Evading Predation

We explored different treatments to enhance the probability of sowed seeds of two early successional (ES, Cecropia obtusifolia and Ochroma pyramidale) and two late successional (LS, Brosimum costaricanum and Dialium guianense) species to escape predation and germinate in abandoned cattle‐raising pasture fields in Southeastern Mexico. ES species were sown in groups of 50 seeds under three treatments: invertebrate exclusion, burial, and exposition to seedeaters. LS species were sown in groups of 10 seeds under three treatments: vertebrate exclusion, burial, and exposition to seedeaters. We registered seed predation and germination 2, 4, 8, 16, 32, and 64 days after the initial sowing. Overall, ES showed higher predation rates (mean ± SE = 0.45 ± 0.07 seed seed^?1 day^?1; n = 3) than LS species (0.09 ± 0.02 seed seed^?1 day^?1). Cecropia obtusifolia was completely predated in all treatments after 8 days. Burial and exclusion treatments reduced final predation in circa 6% for O. pyramidale, relative to that of exposed seeds (85% after 8 days); most germination occurred in buried seeds (3.7%). In B. costaricanum, burial enabled germination by 10%; exposed and excluded seeds were removed 100%. Dialium guianense showed 12% germination in buried seeds and circa 20% of the seeds were not removed after 64 days. Direct sowing would be a recommended rainforest restoration practice for species with relatively large seeds if deposited in groups and buried. Studies which address variation across numerous sites are necessary in order to generate more consistent seed predation patterns and rainforest restoration principles in tropical pastures.     

Germination niche breadth varies inconsistently among three Asclepias congeners along a latitudinal gradient

• Species responses to climate change will be primarily driven by their environmental tolerance range, or niche breadth, with the expectation that broad niches will increase resilience. Niche breadth is expected to be larger in more heterogeneous environments and moderated by life history. Niche breadth also varies across life stages. Therefore, the life stage with the narrowest niche may serve as the best predictor of climatic vulnerability. To investigate the relationship between niche breadth, climate and life stage we identify germination niche breadth for dormant and non‐dormant seeds in multiple populations of three milkweed (Asclepias) species.
• Complementary trials evaluated germination under conditions simulating historic and predicted future climate by varying cold–moist stratification temperature, length and incubation temperature. Germination niche breadth was derived from germination evenness across treatments (Levins B_n), with stratified seeds considered less dormant than non‐stratified seeds.
• Germination response varies significantly among species, populations and treatments. Cold–moist stratification ≥4 weeks (1–3 °C) followed by incubation at 25/15 °C+ achieves peak germination for most populations. Germination niche breadth significantly expands following stratification and interacts significantly with latitude of origin. Interestingly, two species display a positive relationship between niche breadth and latitude, while the third presents a concave quadratic relationship.
• Germination niche breadth significantly varies by species, latitude and population, suggesting an interaction between source climate, life history and site‐specific factors. Results contribute to our understanding of inter‐ and intraspecific variation in germination, underscore the role of dormancy in germination niche breadth, and have implications for prioritising and conserving species under climate change.

     

Post‐burning germination responses of woody invaders in a fire‐prone ecosystem

It has been frequently recognised that there is a positive feedback between plant invasion and fire underlying invasion success in fire‐prone ecosystems. Accordingly, the response of woody alien species germination to fire may have direct implications on their invasiveness in those ecosystems, particularly when fruit ripening occurs in the fire season. Here, we experimentally evaluated the germination response of some of the main woody invaders of the Chaco Serrano dry woodlands (Gleditsia triacanthos, Cotoneaster glaucophyllus, Ligustrum lucidum, Pyracantha angustifolia and Melia azedarach), which fruit in the seasons of highest fire frequency. Seeds were subjected to heat‐shock treatments that simulated a range of heat intensities, and the species were classified according to their germination response as heat sensitive, tolerant or stimulated. Since Gleditsia triacanthos has indehiscent fruits that fall from the plant and might be exposed to flames, its germination response was also assessed of seeds exposed to fruit burning. Germination responses to heat varied among the invasive species. G. triacanthos seeds experienced increased germination under very low and low heat indexes; it was therefore classified as heat stimulated. The other four species showed no change in germination under very low heat indexes and were therefore considered heat tolerant. However, all species were sensitive to high heat as indicated by their significant decline in germination. G. triacanthos would have limited capacity to recruit from seeds following flaming combustion of its fruits. The prevalence of heat‐tolerant rather than heat‐stimulated germination responses suggests that the occurrence of frequent and seasonal fires in this subtropical savanna system might delay rather than boost the expansion of these invasive species in the system. Yet, the presence of heat‐stimulated germination in one of the studied species warns against generalisation, even within the same ecosystem, and further supports the idiosyncratic nature of invasion success. Abstract in Spanish is available with online material.     

Achene germination of the spring ephemeroid species Carex physodes in the Gurbantunggut Desert

Much of the seed germination research on Carex has focused on wetland species, and little is known about the species of arid habitats. Here, we investigated seed dormancy and germination of Carex physodes, which is an important component of the plant community of the Gurbantunggut Desert of the Junggar Basin in Xinjiang, China. Our studies included the effects of mechanical and chemical scarification, dry storage, treatment with GA₃, wet‐cold stratification, and burial in the field. No freshly matured achenes germinated over a range of temperature regimes after treatment with GA₃, 6 months of dry storage or removal of part of the endosperm. The mechanical scarification resulted in < 5% achene germination, however, higher percentage of achene germination occurred after removal of the pericarp (60%), H₂SO₄ scarification (30%) or scarification in 10% NaOH (85%). Six and nine months of wet‐cold stratification promoted < 40% achene germination. The optimal germination temperatures ranged from 25/10°C to 35/20°C. Maximum germination after 9 months of burial at a depth of 3 cm in the field was 36%. These results indicate that the seeds have non‐deep physiological dormancy (PD) and that the pericarp contains germination inhibitors and has strong mechanical resistance to germination.     

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.

     

Factors controlling seed germination of the Iberian critically endangered Pseudomisopates (Antirrhinaceae)

Many seeds are dormant when shed from the mother plant. This unique characteristic of plants poses challenges in conservation and many different treatments have traditionally been used to break dormancy. When germination only occurs under certain circumstances, recruitment may be insufficient and the viability of plant populations may be threatened. A marked dormancy was previously identified in the Iberian critically endangered species Pseudomisopates rivas-martinezii. The present study aimed to determine the magnitude of dormancy by estimating seed viability under different germination treatments: heat, ash, stratification, gibberellin addition, and darkness. Our results indicate that there were significant differences in viability across plant populations and treatments. Maximum seed germination was obtained under ash addition, although cold stratification alone gave a considerable enhancement. However, gibberellic acid did not improve germination and darkness had a diminishing effect. In conclusion, a cold period is required for the species to germinate, coupled with the effects of fire: (1) ash, (2) vegetation clearings, and (3) resprouting, which are major factors triggering seed germination. Although these conditions are widely found in the field, we hypothesize that a more dramatic situation may have tackled this species before the increase of human-mediated fires in the past millennia.