Evidence for physiological seed dormancy cycling in the woody shrub Asterolasia buxifolia and its ecological significance in fire‐prone systems

• Dormancy cycling is a key mechanism that contributes to the maintenance of long‐term persistent soil seed banks, but has not been recorded in long‐lived woody shrub species from fire‐prone environments. Such species rely on seed banks and dormancy break as important processes for post‐fire recruitment and recovery.
• We used germination experiments with smoke treatments on fresh seeds and those buried for 1 year (retrieved in spring) and 1.5 years (retrieved the following late autumn) to investigate whether Asterolasia buxifolia, a shrub from fire‐prone south‐eastern Australia with physiologically dormant seeds, exhibited dormancy cycling.
• All seeds had an obligation for winter seasonal temperatures and smoke to promote germination, even after ageing in the soil. A high proportion of germination was recorded from fresh seeds. but germination after the first retrieval was significantly lower, despite high seed viability. After the second retrieval, germination returned to the initial level. This indicates a pattern of annual dormancy cycling; one of the few observations, to our knowledge, for a perennial species. Additionally, A. buxifolia’s winter temperature and smoke requirements did not change over time, highlighting the potential for seeds to remain conditionally dormant (i.e. restricted to a narrow range of germination conditions) for long periods.
• For physiologically dormant species, such as A. buxifolia, we conclude that dormancy cycling is an important driver of successful regeneration, allowing seed bank persistence, sometimes for decades, during fire‐free periods unsuitable for successful recruitment, while ensuring that a large proportion of seeds are available for recruitment when a fire occurs.

     

Diversity of germination strategies and seed dormancy in herbaceous species of campo rupestre grasslands

The effects of fire on the vegetation vary across continents. However, in Neotropical fire‐prone grasslands, the relationship between fire and seed germination is still poorly understood, while their regeneration, especially after strong anthropogenic disturbance, is challenging for their conservation. In the present study, we assessed diversity of germination strategies in 15 dominant herbaceous species from Neotropical altitudinal grasslands (locally known as campos rupestres). We exposed seeds to several fire‐related treatments. We also compared germination between regularly and post‐fire fruiting species. Finally, we investigated the diversity of dormancy classes aiming at better understanding the biogeography and phylogeny of seed dormancy. Germination strategies varied among families. Velloziaceae and Xyridaceae produced non‐dormant, fast‐germinating seeds. Cyperaceae and Poaceae showed an extremely low or null germination due to a high proportion of unviable or embryo‐less seeds. The seeds of campo rupestre grasslands are fire resistant, but there is no evidence that fire triggers germination in this fire‐prone ecosystem. Although heat and charred wood did not promote germination, smoke enhanced germination in one grass species and decreased the mean germination time and improved synchrony in Xyridaceae and Velloziaceae. Fire had a positive effect on post‐fire regeneration by stimulating fruit set in some Cyperaceae and Poaceae species. These species produced faster germinating seeds with higher germination percentage and synchrony compared to regularly fruiting Cyperaceae and Poaceae species. This strategy of dispersion and regeneration seems to be an alternative to the production of seeds with germination triggered by fire. Physiological dormancy is reported for the first time in several clades of Neotropical plants. Our data help advance the knowledge on the role of fire in the regeneration of Neotropical grasslands.     

Evidence for direct effects of fire‐cues on germination of some perennial forbs common in grassy ecosystems

In grassy ecosystems of south‐eastern Australia, fire maintains richness of native forbs. It is commonly thought that fire promotes regeneration indirectly by reducing competition for light and providing gaps for recruitment, rather than directly stimulating germination. However, physiological dormancy and morphophysiological dormancy are common, and few studies have explored responses to fire‐cues among dormant or hard‐to‐germinate forbs. Recent studies from other fire‐prone ecosystems suggest that in some cases, fire‐cues may not alleviate physiological or morphophysiological dormancy, but instead promote germination in combination with treatments which alleviate dormancy. We experimentally tested the prevailing hypothesis that perennial forbs common in south‐eastern Australian grassy ecosystems do not germinate in direct response to fire. Responses to fire‐cues both inherently and in combination with treatments which alleviate dormancy were investigated for seven species. Two fire‐cues (smoke and heat) plus a treatment of both heat + smoke were applied to fresh seed at three temperatures (35/25°C, 30/20°C and 25/15°C). Following this, the effect of fire‐cues on seed that had undergone warm stratification, cold stratification and dry‐after‐ripening was investigated. Three species — Arthropodium strictum, Cheiranthera cyanea and Dianella revoluta — responded to fire‐cues inherently, although germination in C. cyanea was low. High germination of D. revoluta was found when fire‐cues were combined with warm stratification. Fire‐cues had no effect on germination of Brunonia australis, Burchardia umbellata and Eryngium ovinum. Germination of Stypandra glauca was zero following all treatment combinations. Our finding that fire‐cues promote germination of three of the seven study species did not provide sufficient evidence to reject the current hypothesis that germination of perennial forbs is not typically promoted by fire‐cues. However, this study highlights the important direct role fire‐cues can play in promoting germination of some perennial forbs both inherently and in combination with treatments used to alleviate physiological dormancy.     

Echinochloa crus‐galli seed physiological dormancy and germination responses to hypoxic floodwaters

• Hypoxic floodwaters can seriously damage seedlings. Seed dormancy could be an effective trait to avoid lethal underwater germination. This research aimed to discover novel adaptive dormancy responses to hypoxic floodwaters in seeds of Echinochloa crus‐galli, a noxious weed from rice fields and lowland croplands.
• Echinochloa crus‐galli dormant seeds were subjected to a series of sequential treatments. Seeds were: (i) submerged under hypoxic floodwater (simulated with hypoxic flasks) at different temperatures for 15 or 30 days, and germination tested under drained conditions while exposing seeds to dormancy‐breaking signals (alternating temperatures, nitrate (KNO₃), light); or (ii) exposed to dormancy‐breaking signals during hypoxic submergence, and germination monitored during incubation and after transfer to drained conditions.
• Echinochloa crus‐galli seed primary dormancy was attenuated under hypoxic submergence but to a lesser extent than under drained conditions. Hypoxic floodwater did not reinforced dormancy but hindered secondary dormancy induction in warm temperatures. Seeds did not germinate under hypoxic submergence even when subjected to dormancy‐breaking signals; however, these signals broke dormancy in seeds submerged under normoxic water. Seeds submerged in hypoxic water could sense light through phytochrome signals and germinated when normoxic conditions were regained.
• Hypoxic floodwaters interfere with E. crus‐galli seed seasonal dormancy changes. Dormancy‐breaking signals are overridden during hypoxic floods, drastically decreasing underwater germination. In addition, results indicate that a fraction of E. crus‐galli seeds perceive dormancy‐breaking signals under hypoxic water and germinate immediately after aerobic conditions are regained, a hazardous yet less competitive environment for establishment.

     

Fire and flood: Soil‐stored seed bank and germination ecology in the endangered Carrington Falls Grevillea (Grevillea rivularis,Proteaceae)

Abstract Seed germination is dependent on the interaction between the dormancy state of a seed and the presence of favourable environmental conditions. Thus, the spectacular pulse of seedling recruitment in many Australian vegetation communities following disturbances such as fire can be attributed to changes in microsite conditions and/or the dormancy‐breaking effect of the disturbance on accumulated seed banks. Grevillea rivularis is a threatened species endemic to the area immediately above Carrington Falls in the NSW Southern Highlands. Most of the population is confined to the riparian vegetation zone in woodland and heath, and is therefore subject to periodic disturbance from fire and flood. For this species, a pulse of seedling recruitment has been recorded after fire, flood and mechanical soil disturbance. The aims of this study were to examine the density and vertical distribution of the soil‐stored seed bank and to investigate the role of heat and scarification as cues for germination of fresh and soil‐stored seed. There was a large seed bank under the canopies of established individuals (194 ± 73 seeds m^?2) and most seeds were found in the 0–2 cm and leaf‐litter layers of the soil profile. The germination response of soil‐stored and fresh seed was examined using a hierarchical series of laboratory experiments. Seeds of G. rivularis showed marked dormancy polymorphism. Thirty‐six percent of soil‐stored seed germinated without treatment, whereas no untreated fresh seeds germinated. Scarification or heating caused significant germination of dormant soil‐stored seed, but only scarification resulted in germination of dormant fresh seeds. These results highlight important differences in the dormancy state of soil‐stored and fresh seed. Thus, being a riparian species in a fire‐prone environment, the dormancy mechanisms in seeds of G. rivularis suit this species to disturbance by both fire and flood.     

Mapping of QTL for seed dormancy in a winter oilseed rape doubled haploid population

Following winter oilseed rape cultivation, considerable numbers of volunteer oilseed rape plants may occur in subsequent years in following crops. The appearance of volunteer oilseed rape plants is based on the capability of the seeds to become secondary dormant and to survive in this stage for many years in the soil. Genetic reduction of secondary seed dormancy in oilseed rape could provide a means to reduce the frequency of volunteer plants and especially the dispersal of transgenic oilseed rape. The objective of the present study was to analyse the inheritance of primary and secondary seed dormancy in a winter oilseed rape doubled haploid population derived from the cross Express 617 × R53 and to study correlations to other seed traits. Field experiments were performed in Germany for 2 years at two locations with two replicates. Seeds harvested from open pollinated plants were used for all analyses, including a laboratory test for seed dormancy. A previously developed molecular marker map of the doubled haploid population was used to map QTL of the relevant traits. For primary, secondary and total seed dormancy, the results showed significant effects of the genotypes and their interactions, with years and locations. Two, four and five QTL were detected for primary, secondary and total seed dormancy which explained 19, 35 and 42 % of the phenotypic variance, respectively. Results show that secondary seed dormancy is a heritable trait and that selection for low secondary seed dormancy is possible.     

How much seed remains in the soil after a fire?

Soil seed banks that persist after a fire are important in fire-prone habitats as they minimise the risk of decline or local extinction in plants, should the fire-free interval be less than the primary juvenile periods of the species. In two common woody plant genera (Acacia and Grevillea) in southeastern Australia, we examined the size and location of the residual seed bank after fire across areas of varying seedling densities at three locations in comparison to the distribution of seeds in the soil at an unburnt site. We found viable dormant seeds remaining in the soil after fire (evidence of residual soil seed bank). A significantly lower proportion of seeds remained in the top 5 cm of soil than at 5–10 cm or 10–15 cm soil depths, independent of seedling density or plant genus. This was due to greater germination, and possibly some seed mortality, near the soil surface. Reduced germination below 5 cm was probably due to the reduced efficacy of the fire cues that break seed dormancy, a declining ability of seeds to emerge successfully from such depths, and the lower abundance of seeds in the soil at such depths. The magnitude of the residual seed bank was similar across 0–5, 5–10 and 10–15 cm soil depths in Acacia suaveolens. For two Grevillea species, most residual seeds were at 0–5 and 5–10 cm. The residual soil seed bank in the top 10 cm of soil after fire varied across sites with estimates of 0, 19 and 27% in G. speciosa and 23, 35, and 55% in A. suaveolens. At two sites, both species had similar residual seed bank sizes, while at a third, there were large differences between the species (0–55%). The observed patterns imply that the fire-related cues that break seed dormancy generally declined with soil depth. For Acacia, seed dormancy is broken by heat shock, a fire-cue that declines with soil depth. Some 250 species (approx 15% of the fire-prone flora) in the region are thought to have dormancy broken by heat shock. For Grevillea, where seed dormancy is broken by the interaction of smoke and heat shock, at two sites, we suggest three possibilities: (i) the smoke cue declined with soil depth; (ii) both heat and smoke are obligatory for breaking seed dormancy; or (iii) the cues may be independent and additive and below the zone of soil heating, only a proportion of available seeds had dormancy broken by smoke alone. At a third site (no residual seed bank detected) the smoke cue was predicted not to have declined with soil depth. Up to 900 species (just under half the fire-prone flora) in the study region are thought to have seed dormancy broken by the interaction of heat and smoke during the passage of a fire.     

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.     

Osmoconditioning prevents the onset of microtubular cytoskeleton and activation of cell cycle and is detrimental for germination of Jatropha curcas L. seeds

• Jatropha curcas is an oilseed crop renowned for its tolerance to a diverse range of environmental stresses. In Brazil, this species is grown in semiarid regions where crop establishment requires a better understanding of the mechanisms underlying appropriate seed, seedling and plant behaviour under water restriction conditions. In this context, the objective of this study was to investigate the physiological and cytological profiles of J. curcas seeds in response to imbibition in water (control) and in polyethylene glycol solution (osmoticum).
• Seed germinability and reactivation of cell cycle events were assessed by means of different germination parameters and immunohistochemical detection of tubulin and microtubules, i.e. tubulin accumulation and microtubular cytoskeleton configurations in water imbibed seeds (control) and in seeds imbibed in the osmoticum.
• Immunohistochemical analysis revealed increasing accumulation of tubulin and appearance of microtubular cytoskeleton in seed embryo radicles imbibed in water from 48 h onwards. Mitotic microtubules were only visible in seeds imbibed in water, after radicle protrusion, as an indication of cell cycle reactivation and cell proliferation, with subsequent root development. Imbibition in osmoticum prevented accumulation of microtubules, i.e. activation of cell cycle, therefore germination could not be resumed.
• Osmoconditioned seeds were able to survive re‐drying and could resume germination after re‐imbibition in water, however, with lower germination performance, possibly due to acquisition of secondary dormancy. This study provides important insights into understanding of the physiological aspects of J. curcas seed germination in response to water restriction conditions.

     

Seed moisture content affects afterripening and smoke responsiveness in three sympatric Australian native species from fire‐prone environments

Germination of freshly collected seeds of three sympatric herbaceous species native to fire‐prone environments in south‐western Australia was significantly improved through the application of novel combinations of dry heat, gibberellic acid, smoke water and dry afterripening. For fresh seeds, combinations of dry heat, gibberellic acid and/or smoke water resulted in >80% germination in Austrostipa elegantissima (Poaceae) and Stylidium affine (Stylidaceae) seeds and >60% germination in Conostylis candicans (Haemodoraceae) seeds, compared with <10% germination of control seeds. For fresh seeds, two broad germination patterns were observed in response to smoke water: nil – low germination for both control and smoke water‐treated seeds (A. elegantissima and S. affine); and a significant smoke response (35%) compared with control seeds (1%) (C. candicans). During afterripening, high germination for A. elegantissima seeds was achieved following 3 months storage of seeds at equilibrium relative humidities of 23–75%, but seeds stored at 5–13% equilibrium relative humidities took 6–36 months to achieve similar levels of germination. Germination of C. candicans seeds also increased after 3 months storage, to >60% at each equilibrium relative humidity and further increases over time were slight. For S. affine seeds >60% germination was achieved only after 36 months storage at 50% equilibrium relative humidity. Seeds from all three species were smoke‐responsive at some point, but the interaction/effects of afterripening on the smoke response varied significantly between species. This study highlights an apparent effect of seed dormancy status on response to smoke and a surprisingly high level of ecological variation in pre‐germination requirements (cues) for these co‐occurring species that may relate to variation(s) in microsite selection forces operating on the soil seed bank of the different species.     

Temperature thresholds of physically dormant seeds and plant functional response to fire: variation among species and relative impact of climate change

Variation in dormancy thresholds among species is rarely studied but may provide a basis to better understand the mechanisms controlling population persistence. Incorporating dormancy‐breaking temperature thresholds into existing trait frameworks could improve predictions regarding seed bank persistence, and subsequently species resilience in response to fire, climate change and anthropogenic management. A key ecological strategy for many species from fire‐prone ecosystems is the possession of a long‐lived seed bank, ensuring recovery after fire. Physical dormancy is dominant in these ecosystems and maintaining this dormancy is directly linked to seed bank persistence. We identified a suite of seed‐related factors relevant to maintaining populations in fire‐prone regions for 14 co‐occurring physically dormant species. We measured variation in initial levels of dormancy and then applied experimental heating treatments, based on current seasonal temperatures and those occurring during fires, to seeds of all study species. Additionally, higher seasonal temperature treatments were applied to assess response of seeds to temperatures projected under future climate scenarios. Levels of germination response and mortality were determined to assess how tightly germination response was bound to either fire or seasonal cues. Six species were found to have dormancy cues bound to temperatures that only occur during fires (80°C and above) and were grouped as having obligate pyrogenic dormancy release. The remaining species, classified as having facultative pyrogenic dormancy, had lower temperature dormancy thresholds and committed at least 30% of seeds to germinate after summer‐temperature treatments. Evidence from this study supports including dormancy‐breaking temperature thresholds as an attribute for identifying functional types. High temperature thresholds for breaking dormancy, found in our obligate pyrogenic group, appear to be a fire‐adapted trait, while we predict that species in the facultative group are most at risk to increased seed bank decay resulting from elevated soil temperatures under projected climate change.     

Fire‐related cues break seed dormancy of six legumes of tropical eucalypt savannas in north‐eastern Australia

Abstract This paper describes an assessment of the effect of exposure to fire‐related cues (heat shock, smoke and nitrate) and the interactions between the cues on seed dormancy release of tropical savanna legumes in north‐eastern Australia. Ten legume species were tested, comprising both native and exotic species. The ten species responded variously to the treatments. Brief exposure to temperatures between 80 and 100°C was found to break the seed dormancy of the native ephemeral herbs Chamaecrista mimosoides, Crotalaria calycina, Crotalaria montana, Indigofera hirsuta and Tephrosia juncea, as well as the exotic ephemeral herb Crotalaria lanceolata. Exposure to 80°C combined with treatment with a nitrate solution produced an additive effect on the germination of Chamaecrista mimosoides and Crotalaria lanceolata. However, the four species with the heaviest seeds, two exotic ephemeral herbs (Chamaecrista absus and Crotalaria pallida) and two native perennials (Galactia tenuiflora and Glycine tomentella) displayed no significant increase in germination with exposure to fire‐related cues. Exposure to 120°C for 5 min produced seed mortality in all species tested. Two of the largest seeded species, Crotalaria pallida and Galactia tenuiflora, displayed the lowest tolerance to heat shock, with seed mortality after exposure to 100°C for 5 min. These data indicate that fire can promote the germination of some tropical savanna legumes. As a proportion of seeds of each species displayed no innate dormancy, some germination may occur in the absence of fire, especially of exotic species.     

Effects of smoke,heat and charred wood on the germination of dormant soil‐stored seeds from a Eucalyptus baxteri heathy‐woodland in Victoria,SE Australia

The effects of dry heat, wet heat, charred wood and smoke on the germination of dormant soil‐stored seeds from a Eucalyptus woodland in western Victoria were tested by using a glasshouse seed‐bank germination experiment. Seedling density, species richness and species composition were compared between replicated treated and control samples. A total of 5922 seedlings, comprising 59 plant species, was recorded from the soil samples over a period of 150 days. While a few species dominated (including Centrolepis strigosa, Wahlenbergia gracilenta and Ixodia achillaeoides), 26 species were represented by fewer than five seedlings and 18 species were restricted to single treatment types. With the exception of charred wood, all treatments led to a significant increase in seed germination relative to the control. The highest number of germinants was obtained for the smoke treatment, with a mean (± SE) of 12 547 ± 449 seedlings m^–2. Heat treatments yielded intermediate densities, with means (± SE) varying between 7445 ± 234 and 9133 ± 445 seedlings m^–2. In comparison with the estimates of seed‐bank sizes from other fire‐prone ecosystems, these densities are high. Species richness differed significantly among treatments. Highest mean richness was recorded in the smoke treatment and lowest for the control and charred wood treatments. There were significant differences in seed‐bank species composition between treatment types based on analysis of similarity (Anosim) using Bray–Curtis similarity. While heat was a specific requirement for triggering germination in hard‐seeded species (e.g. Fabaceae), smoke was the most effective trigger for species from a broad range of other families. The potentially confounding effect of physical and chemical mechanisms of germination stimulation in heated bulk soil samples is raised as an issue requiring further investigation in relation to the role of smoke as a germination trigger.     

Seed germination and dormancy traits of forbs and shrubs important for restoration of North American dryland ecosystems

• In degraded dryland systems, native plant community re‐establishment following disturbance is almost exclusively carried out using seeds, but these efforts commonly fail. Much of this failure can be attributed to the limited understanding of seed dormancy and germination traits.
• We undertook a systematic classification of seed dormancy of 26 species of annual and perennial forbs and shrubs that represent key, dominant genera used in restoration of the Great Basin ecosystem in the western United States. We examined germination across a wide thermal profile to depict species‐specific characteristics and assessed the potential of gibberellic acid (GA₃) and karrikinolide (KAR₁) to expand the thermal germination envelope of fresh seeds.
• Of the tested species, 81% produce seeds that are dormant at maturity. The largest proportion (62%) exhibited physiological (PD), followed by physical (PY, 8%), combinational (PY + PD, 8%) and morphophysiological (MPD, 4%) dormancy classes. The effects of chemical stimulants were temperature‐ and species‐mediated. In general, mean germination across the thermal profile was improved by GA₃ and KAR₁ for 11 and five species, respectively. We detected a strong germination response to temperature in freshly collected seeds of 20 species. Temperatures below 10 °C limited the germination of all except Agoseris heterophylla, suggesting that in their dormant state, the majority of these species are thermally restricted.
• Our findings demonstrate the utility of dormancy classification as a foundation for understanding the critical regenerative traits in these ecologically important species and highlight its importance in restoration planning.

     

Polyploidy affects the seed,dormancy and seedling characteristics of a perennial grass,conferring an advantage in stressful climates

• Polyploidy (the state of having more than two genome copies) is widely distributed in flowering plants and can vary within species, with polyploid races often associated with broad ecological tolerances. Polyploidy may influence within‐species variation in seed development, germination and establishment. We hypothesized that interactions between polyploidy and the seed developmental environment would affect subsequent dormancy, germination and early growth traits, particularly in stressful environments.
• Using seeds developed in a common garden under ambient and warmed conditions, we conducted germination trials under drought and temperature stress, and monitored the subsequent growth of seedlings. The study species, Themeda triandra, is a widespread, keystone, Australian native grass and a known polyploid complex.
• Tetraploid plants produced heavier, more viable seeds than diploids. Tetraploids were significantly more dormant than diploids, regardless of seed developmental environment. Non‐dormant tetraploids were more sensitive to germination stress compared to non‐dormant diploids. Finally, tetraploid seedlings were larger and grew faster than diploids, usually when maternal plants were exposed to developmental temperatures atypical to the source environment.
• Seed and seedling traits suggest tetraploids are generally better adapted to stressful environments than diploids. Because tetraploid seeds of T. triandra are more dormant they are less likely to germinate under stress, and when they do germinate, seedling growth is rapid and independent of seed developmental environment. These novel results demonstrate that polyploidy, sometimes in interaction with developmental environment and possibly also asexuality, can have within‐species variation in seed and seedling traits that increase fitness in stressful environments.

     

Seed dormancy of Cardiospermum halicacabum (Sapindaceae) from three precipitation zones in Sri Lanka

• This study investigated seed germination of Cardiospermum halicacabum, a medicinally important invasive species.
• We compared mass, moisture content (MC), dormancy and dormancy‐breaking treatments and imbibition and germination of scarified and non‐scarified seeds of C. halicacabum from a low‐elevation dry zone (DZ), low‐elevation wet zone (WZ1) and mid‐elevation wet zone (WZ2) in Sri Lanka to test the hypothesis that the percentage of seeds with water‐impermeable seed coats (physical dormancy, PY) decreases with increased precipitation.
• Seed mass was higher in WZ2 than in DZ and WZ1, while seed MC did not vary among the zones. All scarified DZ, WZ1 and WZ2 and non‐scarified DZ and WZ1 seeds imbibed water, but only a few non‐scarified WZ2 seeds did so. When DZ and WZ1 seeds were desiccated, MC and percentage imbibition decreased, showing that these seeds have the ability to develop PY. GA₃ promoted germination of embryos excised from fresh DZ and WZ1 seeds and of scarified WZ2 seeds.
• At maturity, seeds from DZ and WZ1 had only physiological dormancy (PD), while those from WZ2 had combinational dormancy (PY+PD). Thus, our hypothesis was not supported. Since a high percentage of excised embryos developed into normal seedlings; this is a low‐cost method to produce C. halicacabum plants for medicinal and ornamental purposes.

     

Quantification of plant water uptake by water stable isotopes in rice paddy systems

Aims

Maintaining variation in germination response provides a selective advantage, by spreading risk during recruitment. In fire-prone regions, physically dormant (PY) species vary their response to dormancy-breaking fire-related heat cues at the intra-population level. However little is known about physiologically dormant (PD) species, which respond to smoke cues. These contrasting dormancy types reflect different evolutionary developmental pathways and we considered whether intra-population variation in germination of Boronia floribunda (PD) occurs in response to smoke.

Methods

Seeds were collected from individual plants. We assessed germination magnitude and rate of seeds from each individual in response to a single aerosol smoke treatment, and three concentrations of smoke water, using replicate seed lots in temperature-controlled incubators.

Results

The magnitude and onset of germination differed significantly among individuals in response to the same smoke treatment. Seeds from different individuals varied in their sensitivity to smoke water concentration, with some responding to very low doses, and others obligated to high doses.

Conclusions

Variation in germination response to smoke highlights a mechanism by which PD species spread risk, by allowing some seeds to emerge quickly, while others remain dormant in the soil seed bank. The similarity to heat-cued variation displayed by PY species suggests that this could represent a convergent functional response.

     

Seed-coat dormancy in Grevillea linearifolia: little change in permeability to an apoplastic tracer after treatment with smoke and heat

BACKGROUND AND AIMS: Seeds of Grevillea linearifolia germinate following fire, and have seed-coat dormancy broken by smoke and heat shock. Smoke breaks seed coat dormancy in Emmenanthe penduliflora by altering the permeability of the seed coat to an internal germination inhibitor, which subsequently escapes. This model was tested for in G. linearifolia by investigating the permeability of the seed coat to diffusion of high-molecular-weight compounds, and whether this changed after exposure to fire cues. METHODS: Germination response of the seeds to heat shock, smoke or heat + smoke was tested. Penetration of Lucifer Yellow dye into intact seeds was examined after 24 and 48 h of exposure, and penetration of the dye from the inside of the seed coat outwards was examined after 24 h. Histochemical staining with Nile Red and Acridine Orange was used to locate cuticles, suberin and lignin. KEY RESULTS: Twenty-three per cent of untreated seeds germinated; heat shock and smoke increased germination additively up to approx. 80 % for both cues combined. Lucifer Yellow did not penetrate fully through the seed coat of untreated seeds, whether diffusing inwards or outwards. Three barriers to diffusion were identified. Treatment with heat or smoke slightly increased penetration of the dye, but did not completely remove the barriers. Suberin was identified in secondary walls of exotestal and mesotestal cells, and was absent from primary cell walls. Movement of Lucifer Yellow occurred through the middle lamella and primary cell wall of suberized cells; movement of the dye was impeded where suberin was absent. CONCLUSIONS: Fire cues did not significantly decrease barriers to diffusion of high-molecular-weight compounds in the seed coat of Grevillea, and must be breaking dormancy by another mechanism.     

The effect of stratification temperatures on the level of dormancy in primary and secondary dormant seeds of two <Emphasis Type="Italic">Carex</Emphasis> species

The effect of temperature on the level of dormancy of primary and secondary dormant Carex pendula and Carex remota seeds was investigated. Primary dormant and secondary dormant seeds were stratified for 4 weeks at 5, 11, 13, and 15 °C, respectively, and tested for germination at 15/5 °C in light. To obtain secondary dormant seeds, primary dormant seeds were stratified at 5 °C and afterwards at 25 °C for 4 weeks. Germination tests were carried out in water and in 25 μmol KNO₃-solution to examine differences in sensitivity to nitrate between seeds relieved from primary and secondary dormancy. In both species, seeds with primary and with induced secondary dormancy showed no significant differences in germination. The two sedges showed significant differences in the effect of stratification temperatures between primary and secondary dormant seeds. Primary dormant seeds of C. pendula showed high germination (>80%) in nitrate-solution after stratification at all temperatures, while only temperatures of 5, 11, and 13 °C led to higher germination in nitrate-solution in secondary dormant seeds. Germination percentages of primary and of secondary dormant C. pendula seeds in water increased to a higher extent only after stratification at 5 and 11 °C; stratification of 11 °C was more effective in secondary than in primary dormant seeds. The only temperature that relieved primary dormancy in C. remota seeds was 5 °C where germination in water and nitrate-solution was >90%. Germination of secondary dormant seeds was increased by stratification at 11 °C independent of the test solution but higher germination after stratification at 13 °C occurred only in nitrate-solution. The results support the existence of physiological differences in the regulation of primary and secondary dormancy by temperature, and in the reaction of nitrate, at least in C. remota.