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.     

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.     

Structural Diversity and its Adaptive Significancein Seeds of Vigna minima (Roxb.) Ohwi Ohashi and its Allies (Leguminosae-Papilionoideae)

The surface features of seed coat, hilum and strophiole, histologicalaspects of hilum and strophiole, and mechanism of seed dormancyinVigna minima, V. umbellata, V. dalzelliana and V. calcarataauct. pi. have been investigated. Scanning electron microscopyhas revealed a substantial variability in seed coat microtopography,hilum and strophiole, which is of taxonomic and evolutionarysignificance. The histology of the hilum is uniform in all fourtaxa, but the structure of the strophiole shows differencesbetween wild and cultivated species. All four taxa have hardseeds; those ofV. umbellata have no dormancy but those of wildspecies have water-impermeable dormancy. Seed germination experimentsusing araldite-coated, and hilum-micropyle- and strophiole-pluggedhard seeds of V. umbellata and water-impermeable seeds of V.minima, which were pin-pricked through the strophiolar groove,demonstrate that the zone of weakness to the permeability ofwater in hard seeds of these species is the strophiolar region,and not the seed coat, hilum or micropyle. The parenchymatoustissue of the strophiolar plug in the seeds ofV. umbellataisassociated with the lack of dormancy, whereas the sclerenchymatoustissue of the strophiolar plug in the seeds ofV. minima is responsiblefor the water-impermeable seed dormancy. Under natural conditions,the water-impermeable dormancy of wild species breaks down asa result of the degradation of the hard seed coat, by microbialaction, at the strophiolar groove.     

Combinational dormancy in seeds of Sicyos angulatus (Cucurbitaceae,tribe Sicyeae)

Seeds with a water‐impermeable seed coat and a physiologically dormant embryo are classified as having combinational dormancy. Seeds of Sicyos angulatus (burcucumber) have been clearly shown to have a water‐impermeable seed coat (physical dormancy [PY]). The primary aim of the present study was to confirm (or not) that physiological dormancy (PD) is also present in seeds of S. angulatus. The highest germination of scarified fresh (38%) and 3‐month dry‐stored (36%) seeds occurred at 35/20°C. The rate (speed) of germination was faster in scarified dry‐stored seeds than in scarified fresh seeds. Removal of the seed coat, but leaving the membrane surrounding the embryo intact, increased germination of both fresh and dry‐stored seeds to > 85% at 35/20°C. Germination (80–100%) of excised embryos (both seed coat and membrane removed) occurred at 15/6, 25/15 and 35/20°C and reached 95–100% after 4 days of incubation at 25/15 and 35/20°C. Dry storage (after‐ripening) caused an increase in the germination percentage of scarified and of decoated seeds at 25/15°C and in both germination percentage and rate of excised embryos at 15/6°C. Eight weeks of cold stratification resulted in a significant increase in the germination of scarified seeds at 25/15 and 35/20°C and of decoated seeds at 15/6 and 25/15°C. Based on the results of our study and on information reported in the literature, we conclude that seeds of S. angulatus not only have PY, but also non‐deep PD, that is, combinational dormancy (PY + PD).     

Investigations into seed dormancy in Grevillea linearifolia, G. buxifolia and G. sericea: anatomy and histochemistry of the seed coat

BACKGROUND AND AIMS: Seeds of east Australian Grevillea species generally recruit post-fire; previous work showed that the seed coat was the controller of dormancy in Grevillea linearifolia. Former studies on seed development in Grevillea have concentrated on embryology, with little information that would allow testing of hypotheses about the breaking of dormancy by fire-related cues. Our aim was to investigate structural and chemical characteristics of the seed coat that may be related to dormancy for three Grevillea species. METHODS: Seeds of Grevillea linearifolia, Grevillea buxifolia and Grevillea sericea were investigated using gross dissection, thin sectioning and histochemical staining. Water movement across the seed coat was tested for by determining the water content of embryos from imbibed and dry seeds of G. sericea. Penetration of intact seeds by Lucifer Yellow was used to test for internal barriers to diffusion of high-molecular-weight compounds. KEY RESULTS: Two integuments were present in the seed coat: an outer testa, with exo-, meso- and endotestal (palisade) layers, and an inner tegmen of unlignified sclerenchyma. A hypostase at the chalazal end was a region of structural difference in the seed coat, and differed slightly among the three species. An internal cuticle was found on each side of the sclerenchyma layer. The embryos of imbibed seeds had a water content six times that of dry seeds. Barriers to diffusion of Lucifer Yellow existed at the exotestal and the endotestal/hypostase layers. CONCLUSIONS: Several potential mechanisms of seed coat dormancy were identified. The embryo appeared to be completely surrounded by outer and inner barriers to diffusion of high-molecular-weight compounds. Phenolic compounds present in the exotesta could interfere with gas exchange. The sclerenchyma layer, together with strengthening in the endotestal and exotestal cells, could act as a mechanical constraint.     

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.     

Seed Germination Traits of Loquat (<i>Eriobotrya japonica</i> Lindl.) as Affected by Various Pre-Sowing Treatments (Cutting of Cotyledons,Removal of Perisperm,Moist Chilling and/or Exogenous Application of Gibberellin)

The purpose of this study was to investigate the effects of various presowing treatments on the germinability (final germination percentage) and germination rate of loquat seeds in order to increase seedling production in nurseries (applied research) as well as provide answers for important physiological issues related to loquat seeds and their seed coat (basic research). Three experiments were carried out with various pre-sowing treatments. These treatments included full or partial removal of seed coat (perisperm), partial cutting of cotyledons as well as moist chilling at 5°C for 13 days and/or soaking the seeds in water or 250 ppm gibberellic acid (GA3) solution for 24 h. According to the results, cotyledons excision resulted in delayed germination, regardless of the presence or absence of the seed coat in comparison with the decoated seeds that demonstrated the highest germination rate amongst them. In addition, even the partial excision of seed coats affected positively both the germinability and the germination rate, compared to the control-intact seeds. Furthermore, control-intact seeds had a higher germination percentage when exposed to moist chilling independently of the application or not of gibberellin; while the combination of gibberellin application and moist chilling improved both the percentage and the rate of germination of decoated seeds. In conclusion, the role of perisperm (seed coat) in the germination procedure of loquat seeds seems to be important, indicating the existence of seed coat-imposed dormancy on loquat seeds. Finally, the existence of a mild endogenous embryo-dormancy on loquat is also discussed.     

Impacts of fire cues on germination of Brassica napus L. seeds with high and low secondary dormancy

• Agricultural burning is used in farm management operations; however, information about the impact of fire cues on the release and/or induction of secondary dormancy in crop seeds is scarce.
• Seeds from two oilseed rape cultivars were induced for high (HD) or low (LD) secondary dormancy using polyethyleneglycol (PEG) pre‐treatment, and their germination after exposure to various fire cues was compared to control PEG pre‐treated and non‐dormant seeds.
• Non‐dormant seed germination was unaffected by various fire cues. Low doses of aerosol smoke released secondary dormancy in HD seeds, while higher doses increased dormancy of LD seeds. Dilute smoke water also released HD seed secondary dormancy, but concentrated smke water enhanced dormancy in both LD and HD seeds. The concentrated aqueous extracts from charred oilseed rape straw only promoted germination of HD seeds, while dilution inhibited LD seed germination. Heat shock (80 °C, 5 min) released secondary dormancy in HD seeds; however, higher temperatures and/or increased exposure time was associated with seed death. GC‐MS analyses of smoke water revealed two butenolides and an array of monoaromatic hydroxybenzene compounds with potential germination inhibitor or promoter activity.
• The extent of secondary dormancy induction in seeds affects their subsequent responses to fire cues. Both aerosol smoke and smoke water have both germination promoter and inhibitor activity. Lacking any butenolides, aqueous extracts of charred straw contain a potential germination stimulating steroid, i.e. ergosterol. The significance of fire‐derived cues on behaviour of oilseed rape seeds in the soil seed bank is discussed.

     

Relationships between seed germination requirements and ecophysiological characteristics aid the recovery of threatened native plant species in Western Australia

Summary One of the foremost technical issues addressed in reintroduction and restoration projects is the feasibility of establishing living plants. To advance the recovery process, the germination requirements of 201 threatened Western Australian seed‐bearing taxa representing a range of life forms, families and ecophysiological characteristics were studied. Procedures used to stimulate germination in otherwise dormant seed involved pretreatment using thermal shock, scarification, seed coat removal, soaking in an aqueous smoke solution and/or additions of the growth hormone gibberellic acid (GA₃). Sixty‐one taxa germinated under the basic trial conditions of light (12‐ h photoperiod), temperature (constant 15°C) and moisture, without additional pretreatments. These taxa were generally those with canopy‐stored seeds in the families Proteaceae and Casuarinaceae, and small‐seeded taxa in Myrtaceae. Taxa with soil‐stored seeds required single or multiple cues to stimulate germination. Seeds in the families Fabaceae and Mimosaceae were dependent on cracking of the seed coat, mechanically through nicking of the testa or through thermal shock, as were several non‐leguminous species of the Sterculiaceae and Liliaceae. Complete or partial removal of seed coats, in conjunction with GA₃ enhanced germination percentage in some taxa of the Myoporaceae, Lamiaceae and Myrtaceae. Application of GA₃ also enhanced germination percentage in members of the Epacridaceae. Several taxa previously stimulated by aqueous smoke solutions were equally responsive to additions of GA₃ after complete seed coat removal. In general, species with seed weights greater than 10 mg germinated better under a range of conditions than those with lighter seeds. There was no difference in germinability between resprouter and seeder species, and no obvious relationship between seed weight and germination rate. In the light of previous studies these results indicate that the relationship between germination requirements and ecophysiological characteristics is similar for both threatened and common species. These data will enable better prediction of likely dormancy breaking cues for other related species and will greatly assist the process of recovery and restoration work for mining operations and community bushland regeneration as well as single species reintroductions.     

The Interaction of Heat and Smoke in the Release of Seed Dormancy in Seven Species from Southwestern Western Australia

Exposure to high temperatures (40–60 °C) over severaldays to months (simulating summer soil conditions) or to extremelyhigh temperatures (80–120 °C) for periods rangingfrom minutes to several days (simulating the heat of an extremefire) promoted germination in five (including one legume species)of the seven species tested from southwestern Australia. Heat,a collective term for high temperature (HT) and extremely hightemperature (EHT), has previously been used to promote germinationin species with impermeable seed coats (many of which are legumes).If heat is found to promote germination in species with permeableseed coats, it could be of use in the release of dormancy forcommercial development of native plants. Here we report thatelevated storage temperatures (50 °C for 45 and 90 d or60 °C for 45 d) and EHT (100 °C for 3 h and 120 °Cfor 30 min) alone, and in addition to smoke, directly promotegermination of the refractory species Actinotus leucocephalus,Anigozanthos manglesii, Gompholobium knightianum, Loxocaryastriatus and Stylidium affine. Germination of Sowerbaea laxifloraseed was not improved with heat alone but was improved by thecombination of heat and smoke. Seed of Schoenus unispiculatusdid not germinate in response to any of the treatments. Interactionsbetween the effect of temperature, the duration of heat exposureor smoke were detected in all species. Whilst most species demonstratedan interaction between heat and time, not all species were affectedby smoke alone. This was illustrated by an interaction betweensmoke and HT found only in Actinotus leucocephalus, Anigozanthosmanglesii and Stylidium affine or smoke and EHT in A. manglesiiandS. affine . Smoke and exposure time to heat influenced thegermination of Actinotus leucocephalus,Anigozanthos manglesiiand L. striatus in response to HT and only S. affine appearedto demonstrate an interaction between smoke and EHT. The variationsfound in the interactions between treatments suggest that germinationevents occur over a range of conditions both spatially and temporallywhich may facilitate seedling survival and limit competitionbetween emergents. Copyright 2001 Annals of Botany Company Smoke, heat, fire, seedbank, seed dormancy, seed germination, Western Australia     

Spatial and Developmental Variation in Seed Dormancy Characteristics in the Fire-responsive Species Anigozanthos manglesii(Haemodoraceae) from Western Australia

Anigozanthos manglesii(Haemodoraceae) is a colourful, herbaceousperennial exhibiting intra- and inter-populational variationin germination in response to smoke and heat. This study investigatedthe extent and nature of this variation in A. manglesii populationscollected along a 550 km latitudinal cline from kwongan scrub(30°S) to more mesic jarrah forest habitats (34°S) insouthwestern Australia. Variation in seed germination alonga maturing inflorescence was also investigated. Germinationof this species is known to be prolific following fire, andtwo germination treatments, aerosol smoke and heat, were usedas germination cues. There was a trend of increasing responsivenessof A. manglesii to smoke with increasing latitude along a clinalrange, but there were no differences in germination of seedsalong the cline in response to control or heat treatments. Therewas no significant difference in intra-population seed germinationin response to any treatment. Lastly, a significant and increasingresponse to smoke for seeds from the apex to base of the inflorescencewas detected. This latter trend may be attributed to higherresource allocation and an accelerated after-ripening of basipetalcompared to acropetal seeds. Possible reasons for the clinalvariation are discussed.Copyright 2001 Annals of Botany Company Anigozanthos manglesii, populations, geographical cline, inflorescence level, germination, smoke, heat, fire, seed development     

Dormancy in seeds of Phellodendron wilsonii is Mediated in Part by Abscisic Acid

A regime of temperatures alternating between 35°C (8 h)and 10°C (16 h) proved to be a specific method for breakingdormancy of seeds of Phellodendron wilsonii. The relationshipbetween the germination capacity after about 13 days of incubationunder this regime and the logarithm of the amount of ABA inthe seed before incubation yielded a high correlation coefficient.The final germination capacity under this regime was poorlycorrelated with the amount of ABA in seeds before incubation.These observations suggest that at least part of the germinationbehavior is controlled by the amount of abscisic acid in theseed. Stratification decreased the amount of ABA but resultedin only 8–25% germination relative to other dry storedseeds failed to germinate under suboptimal conditions, namely,incubation at 22°C with 12 h light. This phenomenon indicatesthat some unknown mechanism is the major factor that controlsthe dormant state, and the effect of this mechanism are overcomeonly by the specific alternating-temperature regime. Fluctuationsin temperature failed, however, to reduce the ABA content ofseeds during the incubation period. We conclude that some unknownfactor contributes to the dormancy of seeds of P. wilsonii althoughthe level of ABA definitely plays a minor role in the maintenanceof seed dormancy. (Received January 25, 1993; Accepted November 25, 1993)     

Ecological mechanisms involved in dormancy breakage in Ulex parviflorus seeds

Dormancy in the hard seed coats of Mediterranean species is considered a strategy that enables persistent seed banks to be formed in the soil. An important factor related to seed coat fracture and dormancy breakage in Mediterranean ecosystems is heat. Nevertheless, the effect of factors other than heat on dormancy breakage in these species has hardly been studied. To investigate the different ecological factors involved in germination, in the laboratory we applied several scarification treatments to seeds with chromatic polymorphism. We evaluated the effect of soil seed depth during experimental burns by sowing seeds at −1, −3 and −5 cm in the soil profile, and we also studied the effect of seed origin on the posterior germination of seeds from 4 and 10 year-old shrubs as well as from the soil seed bank. U. parviflorus shows clear chromatic polymorphism: its brown seeds present higher dormancy levels than its yellow seeds. The different techniques of dormancy breakage result in different degrees of germination; the highest degree of germination is generated by the mechanical treatment, followed by the acid and the heat treatments, in that order. The depth of the seeds in the soil determines the temperature thresholds and the residence times of these temperatures and whether they stimulate a massive germination at the −1 cm soil profile or only a slight germination at the −5 cm depth. Seeds recently produced by the plant show higher dormancy levels than seeds extracted from soil seed banks. Dormancy levels also depend on the shrubland age used for extracting the soil samples (3>9 years old). In effect, from the point of view of dormancy, the germination behaviour of U. parviflorus seeds seems to follow a multiresponse strategy based on different seed populations and involving both biological and abiotic processes to break dormancy.     

Interactive effects of heat shock and smoke on germination of nine species forming soil seed banks within the Sydney region

Abstract The germination response of seeds from fire‐prone vegetation to fire‐related cues such as heat shock and smoke has usually been studied by applying the cues singly. The few studies that have applied the cues in combination have shown that interactions between the cues are possible. Here, the response of seeds from a number of species to combined heat shock and smoke is reported. Heat shock (25, 50, 75 and 100°C) and aerosol smoke (0, 5, 10 and 20 min) were applied factorially to nine species that form soil seed banks in the Sydney region of south‐eastern Australia. These species were from Epacridaceae (four species), Myrtaceae (four species) and Cyperaceae (one species) and ranged from fire‐sensitive obligate seeders to fire‐tolerant facultative resprouters. Germination of Dracophyllum secundum R. Br and Sprengelia monticola (A. Cunn. ex DC.) Druce was low and did not respond to the germination cues. The positive response of Gahnia sieberiana Kunth and Kunzea ambigua (Sm.) Druce to heat shock and smoke was independent and additive. The positive response of Kunzea capitata Rchb. to the interaction between heat shock and smoke was synergistic, and the response of Baeckea diosmifolia Rudge and Baeckea imbricata (Gaertn.) Druce was unitive, with germination increase only occurring following combined heat and smoke application. Epacris coriacea A. Cunn. ex DC. and Epacris obtusifolia Sm. had low levels of dormancy and hence it was not possible to find a fire response. Gahnia sieberiana and K. capitata responded differently to the combination of heat shock and smoke than has previously been reported. Germination of species from habitats that are infrequently burnt was not affected by heat shock or smoke. Low‐intensity fire or patches within fire may be important for seedling recruitment as the 50°C heat shock stimulated germination in four of the five species that responded to the heat cue, and germination of Baeckea imbricata declined within the 100°C heat shock treatment. Germination of one species, Baeckea imbricata, was only stimulated by a specific combination of cues, indicating that regeneration niches may be narrow for some species and that the application of a range of heat and smoke doses is required to find such responses. Of the species positively responding to heat shock and smoke, a requirement for both cues was prevalent, therefore the response to these cues in isolation cannot be relied upon to give a true indication of the fire response of a species.     

Influence of multiple fire‐related germination cues on three Sydney Grevillea (Proteaceae) species

Abstract Many species found in fire‐prone habitats that possess a soil‐stored seedbank only recruit seedlings in large numbers following a fire. Fire‐related germination cues are presumably used by these seeds as signals that a fire has occurred, and would include the heating that occurs in the soil and the combustion products of burning vegetation, smoke and charcoal. Three Sydney species, Grevillea buxifolia (Sm.) R. Br., Grevillea sericea (Sm.) R. Br. and Grevillea speciosa (Knight) McGillivray, were studied for the interactive effects of these cues on their germination. The germination of all species was found to increase with both smoke and heat treatments. While smoke always had a greater influence than heat, the relationship between the two treatments varied with species. The presence of two fire‐related germination cues should allow these species to take better advantage of the recruitment opportunities of the post‐fire environment.     

The size,distribution and germination requirements of the soil-stored seed-bank of Grevillea barklyana (Proteaceae)

Abstract Fire-triggered release from seed dormancy is a characteristic of many Australian plant species. We investigated aspects of the seed-bank dynamics and dormancy characteristics in seeds of Grevillea barklyana, an understorey shrub of coastal sclerophyll vegetation in the Jervis Bay Region on the south coast of New South Wales. We used two soil core sizes to compare the number and distribution of stored seeds in soil cores taken from underneath and outside the limits of plant canopies at three study sites. Core size did not affect the estimate of seed density. No seeds were found outside the existing canopies. Even in under-canopy samples, seed numbers were small. Mean seed densities were estimated as 10. 9, 14. 1 and 4. 3 seeds per for the three sites. A hierarchical series of laboratory experiments was used to test the germination response of both fresh and soil-stored seeds. Likewise, we attempted to simulate ‘natural’ disturbance conditions in a glasshouse experiment. Both laboratory and glasshouse experiments indicated polymorphism in germination behaviour. A constant proportion of seeds exhibited enforced dormancy, when moist at room temperature, while a smaller number of seeds showed either an induced dormancy or a non-seed-coat linked innate dormancy. The majority (75%) of seeds were innately dormant due to a hard seed-coat. This dormancy was broken when the seed-coat was damaged, for instance, by heat. The level of polymorphic germination behaviour will be dependent on the length of the inter-fire periods. We conclude that the expression of polymorphism within and between species across a range of environments and fire regimes is an important consideration for any further study attempting to assess the role of the seed-bank.     

Seed Dormancy and Germination Responses of Nine Australian Fire Ephemerals

Fire ephemerals are short-lived plants with seeds that persist in the soil and germinate after a fire or physical soil disturbance. Ex situ germination of many Australian fire ephemerals has previously been difficult. Dormancy was present in most of the nine fire ephemerals examined. Alyogyne hakeifolia (Giord.) Alef. and Alyogyne huegelii (Endl.) Fryxell (Malvaceae) seeds had physical and possibly also physiological dormancy, Actinotus leucocephalus Benth. (Apiaceae) seeds had morphophysiological dormancy, Austrostipa compressa (R.Br.) S.W.L. Jacobs & J. Everett and Austrostipa macalpinei (Reader) S.W.L. Jacobs & J. Everett (Poaceae) seeds were either non-dormant or possessed physiological dormancy, and seeds of all remaining species possessed physiological dormancy. A proportion of the Alyogyne hakeifolia, Alyogyne huegelii, Austrostipa compressa and Austrostipa macalpinei seed populations were non-dormant because some seeds could germinate at the various incubation temperatures without further treatment. At 20 °C, artificial methods of inducing germination such as manual or acid scarification were among the optimal treatments for Austrostipa compressa, Austrostipa macalpinei, Alyogyne huegelii, Actinotus leucocephalus and Grevillea scapigera A.S. George (Proteaceae), and gibberellic acid induced maximum germination of Tersonia cyathiflora (Fenzl) J.W. Green (Gyrostemonaceae) seeds. Heat (70 °C for 1 h) and smoke water was one of the most effective treatments for germinating Actinotus leucocephalus and Codonocarpus cotinifolius (Desf.) F. Muell. (Gyrostemonaceae) seeds. Germination of Grevillea scapigera, Codonocarpus cotinifolius, Gyrostemon racemiger H. Walter (Gyrostemonaceae) and Tersonia cyathiflora did not exceed 40% and may require other treatments to overcome dormancy. Although the nine fire ephemerals examined require fire to germinate under natural conditions, a range of germination responses and dormancy types was observed.     

Germinable soil seed banks in a tropical savanna: seasonal dynamics and effects of fire.

Abstract The germinable soil seed bank of a tropical eucalypt savanna of north‐eastern Australia was found to be dominated by grasses and forbs, with seed bank density ranging from 58 to 792 seeds per square metre, from a total of 53 species. Late dry season fires and the fire‐related cues, heat shock and smoke, broke the seed dormancy of a range of tropical savanna species. Heat shock promoted the germination of the species groups natives, exotics, subshrubs, ephemeral and twining perennial forbs, and the common species Indigofera hirsuta, Pycnospora lutescens and Triumfetta rhomboidea. Exposure to smoke at ambient temperature promoted germination from the soil seed bank of the species groups combined natives, upright perennial forbs and grasses, as well as the common grasses Digitaria breviglumis and Heteropogon triticeus. The germinable soil seed bank varied seasonally, increasing from the mid wet season (February) and early dry season (May) to a maximum in the late dry season (October). The effect of recent fire history on soil seed bank dynamics was limited to the immediate release of some seed from dormancy; a reduction in seed densities of subshrubs and monocots, other than grasses, in recently burnt savanna; and enhanced seed density of the ephemeral I. hirsuta in the year following fire. The seed banks of most savanna species were replenished in the year following burning.     

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.     

Breaking of Seed Dormancy by Nitrate as a Gap Detection Mechanism

Germination of Planlago lanceolata seeds buried in a chalk grasslandwas higher in bare soil than in vegetated soil, and measurementof soil nitrate concentrations showed that they were high enoughto account for this stimulation. When seeds of P. lanceolatawere sown in pots of soil with or without plants, and wateredwith nutrient solution containing either no nitrate, or 14 mMnitrate (sufficiently high that not all nitrate was absorbedby the plants), the presence of plants inhibited germinationonly when the nutrient solution contained no nitrate. It wasconcluded that breaking of seed dormancy by nitrate can functionas a gap detection mechanism if nitrate concentrations in baresoil are high enough to break seed dormancy, but are too lowto break dormancy when vegetation is present Plantago lanceolata, seed, dormancy, germination, nitrate, gap detection