Seed dormancy,germination and soil seed bank of Lamiophlomis rotata and Marmoritis complanatum (Labiatae), two endemic species from Himalaya–Hengduan Mountains

Seed dormancy and germination characteristics are important factors determining plant reproductive success. In this study, we aimed to explore the characteristics of seed dormancy and germination of two endemic Labiatae species (Lamiophlomis rotata and Marmoritis complanatum) in the Himalaya–Hengduan Mountains. Germination was first tested in the light using freshly matured seeds at 25/15 and 15/5°C, and then again after dry after-ripening. Dried seeds were incubated in the light at a range of constant temperatures (1–35°C). The effects of dark and GA₃ on germination were tested at several different temperatures. Base temperature (T_b) and thermal times for 50% final germination (θ₅₀) were calculated. Seeds were also buried at the collection site to test seed persistence in the soil. Increased final germination after dry after-ripening indicated that the seeds of the two species exhibited non-deep physiological dormancy; however, they exhibited different germination characteristics and soil seed bank types. In L. rotata, GA₃ only promoted germination at 5°C, producing no significant effect at other temperatures. Dark conditions decreased germination significantly at all temperatures. T_b and θ₅₀ values were 0.6 and 82.7°C d. The soil seed bank of this species was classified as persistent. In M. complanatum, GA₃ significantly promoted germination at all temperatures except 15°C. Dark conditions depressed germination significantly at warmer temperatures (20 and 25°C) but had no effect at lower temperatures. T_b and θ₅₀ values were 0.1 and 92.3°C d. The soil seed bank was classified as transient. Our results suggest that the seed dormancy and germination of the two co-existing species share some commonalities but there are also species-specific adaptations to the harsh alpine environment.     

Response of Amaranthus retroflexus L. seeds to gibberellic acid, ethylene and abscisic acid depending on duration of stratification and burial

Freshly harvested, dormant seeds of Amaranthus retroflexus were unable to germinate at 25 and 35 °C. To release their dormancy at the above temperatures, the seeds were stratified at a constant temperature (4 °C) under laboratory conditions or at fluctuating temperatures in soil or by outdoor burial in soil. Fully dormant, or seeds stratified or buried (2006/2007 and 2007/2008) for various periods were treated with exogenous gibberellic acid (GA₃), ethephon and abscisic acid (ABA). Likewise, the effects of these regulators, applied during stratification, on seed germination were determined. The results indicate that A. retroflexus seed dormancy can be released either by stratification or by autumn–winter burial. The effect of GA₃ and ethylene, liberated from ethephon, applied after various periods of stratification or during stratification, depends on dormancy level. GA₃ did not affect or only slightly stimulated the germination of non-stratified, fully dormant seeds at 25 and 35 °C respectively. Ethylene increased germination at both temperatures. Seed response to GA₃ and ethylene at 25 °C was increased when dormancy was partially removed by stratification at constant or fluctuating temperatures or autumn–winter burial. The response to GA₃ and ethylene increased with increasing time of stratification. The presence of GA₃ and ethephon during stratification may stimulate germination at 35 °C. Thus, both GA₃ and ethylene can partially substitute the requirement for stratification or autumn–winter burial. Both hormones may also stimulate germination of secondary dormant seeds, exhumed in September. The response to ABA decreased in parallel with an increasing time of stratification and burial up to May 2007 or March 2008. Endogenous GA_n, ethylene and ABA may be involved in the control of dormancy state and germination of A. retroflexus. It is possible that releasing dormancy by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones.     

Variation in seed dormancy and germination among populations of Silybum marianum (Asteraceae)

Milk thistle (Silybum marianum) is a medicinal plant; however, lack of consistency in past dormancy studies has hindered propagation of this species from seeds. We tested the germination responses of freshly harvested and after-ripened (stored for 2 and 7 months; 25°C at 50% relative humidity) seeds from three populations (P1, P2 and P3) in Iran at varying constant or alternating temperatures, with or without GA₃ and in light and continuous darkness. No germination occurred in freshly harvested seeds incubated at any condition without GA₃ application, indicating that all the seeds were dormant. Seeds from P1 and P2, which developed under relatively dry, warm conditions, germinated over a wider range of temperatures after 2 months of dry storage, indicating type 6 of non-deep physiological dormancy (PD). Seeds from P3, which developed under relatively wet, cool conditions, incubated at constant temperatures (especially on GA₃), exhibited an increase in maximum temperature for germination, indicating type 1 of non-deep PD. Light improved germination of after-ripened seeds, and GA₃ application substituted for the light requirement for germination. This is the first report that environmental conditions during seed development may be correlated with differences in the type of non-deep PD. We conclude that milk thistle seeds are positively photoblastic and photodormant and the germination responses of after-ripened seeds from different populations are different under darkness. Therefore, the impacts of genetic differences and maternal effects on the induction of dormancy during seed development should be considered in attempts to domesticate this medicinal plant.     

Responses of dormant heather (Calluna vulgaris) seeds to light, temperature, chemical and advancement treatments

Two seed lots of Calluna vulgaris were obtainedfrom English Nature (seed of Cornish provenance) (EN) and John ChambersWildflower Seeds (JCWS). In laboratory tests, under continuous light untreatedseeds of both seed lots were partially dormant at temperatures between14–35 °C, but JCWS seeds were more deeply dormant thanENseeds. The optimum temperature for germination for both lots was ca 18°C. Germination of EN seeds was much lower in the dark than inthe light at all temperatures; JCWS seeds did not germinate in the dark. In thelight at 22 °C, dormancy of both seed lots was broken whenseeds were incubated in GA_4/7 solution(2×10^–4 M). Dormancy ofJCWSseeds at 22 °C in the light was broken when seeds wereincubated in four different smoke solutions but more so when used incombinationwith GA_4/7. Soaking seeds for 4h insmoke/GA_4/7solutions before sowing improved both the speed andpercentage germination in pot experiments on a mist bench in the glasshouse byat least 10-fold. Soaking with GA_4/7 alone produced a 5-fold increasein germination but seedlings were more etiolated than with thesmoke/GA_4/7 mixtures. A seed advancement treatment modified from thatused commercially on sugar beet seeds also promoted germination in bothlaboratory and glasshouse tests. This entailed soaking seeds in 0.2% thiramsuspension for 4h followed by incubation in excess solution at 22°C for 4 days. This treatment was not as effective as thesmoke/GA_4/7 seed soaks.     

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.

     

Abscisic acid regulates seed germination of Vellozia species in response to temperature

• The relationship between the phytohormones, gibberellin (GA) and abscisic acid (ABA) and light and temperature on seed germination is still not well understood. We aimed to investigate the role of the ABA and GA on seed germination of Vellozia caruncularis, V. intermedia and V. alutacea in response to light/dark conditions on different temperature.
• Seeds were incubated in GA (GA₃ or GA₄) or ABA and their respective biosynthesis inhibitors (paclobutrazol – PAC, and fluridone – FLU) solutions at two contrasting temperatures (25 and 40 °C). Furthermore, endogenous concentrations of active GAs and those of ABA were measured in seeds of V. intermedia and V. alutacea during imbibition/germination.
• Exogenous ABA inhibited the germination of Vellozia species under all conditions tested. GA, FLU and FLU + GA₃ stimulated germination in the dark at 25 °C (GA₄ being more effective than GA₃). PAC reduced seed germination in V. caruncularis and V. alutacea, but did not affect germination of V. intermedia at 40 °C either under light or dark conditions. During imbibition in the dark, levels of active GAs decreased in the seeds of V. intermedia, but were not altered in those of V. alutacea. Incubation at 40 °C decreased ABA levels during imbibition in both V. caruncularis and V. alutacea.
• We conclude that the seeds of Vellozia species studied here require light or high temperature to germinate and ABA has a major role in the regulation of Vellozia seed germination in response to light and temperature.

     

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

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

Factors affecting the dormancy and germination of bleeding heart [Lamprocapnos spectabilis (L.) Fukuhara] seeds

• Information on the optimal conditions to promote the germination of Lamprocapnos spectabilis (L.) Fukuhara seeds is limited; consequently, this study was conducted to establish the requirements to break seed dormancy and promote germination.
• The selected seeds had morphophysiological dormancy and had not begun embryo development. To study the dormancy breaking and embryo development processes, seeds were subjected to constant or changing temperature treatments during moist stratification.
• High temperature and humidity resulted in vigorous embryo growth, with the longest embryos occurring after 1 month of incubation at 20 °C. At 4 °C, the seeds required incubation period of at least 3 months to germinate. Embryo growth and germination were higher with changing high and low temperatures than under a constant temperature, and changing temperatures also considerably changed the endogenous hormone levels, embryo development and germination. Bioactive gibberellin (GA) content was higher in seeds incubated at 20 °C for 1 month, then at 4 °C for 2 months. The content of endogenous abscisic acid in seeds subjected to the same treatment decreased by 97.6% compared with that of the untreated seeds.
• Embryo growth and seed germination require changing high and low temperatures; however, exogenous GA₃ could substitute for high temperatures, as it also causes accelerated germination. In this study, the seeds of L. spectabilis were identified as an intermediate simple type, a sub‐level of morphophysiologically dormant seeds.

     

Effects of temperature,light and gibberellic acid on the germination of seeds of 43 species native to Western Australia

Abstract. Species native to the southwest of Western Australia, representing a range of plant families, life-history strategies, fire-response syndromes, seed-store types and seed weights, were tested for viability using tetrazolium chloride and for germination under combinations of constant temperatures of 15 °C or 23 °C, constantly dark or 12 h diurnal whitelight conditions, and with, or without, addition of gibberellic acid (GA₃, 50 mg/l). Species previously known to require a heat-shock treatment to overcome dormancy due to an impervious testa were pre-treated prior to imposition of temperature, light and GA₃ conditions. The test environmental conditions related to differences between winter and autumn temperatures and surface and buried seed germination positions of post-fire habitats. The viability of the selection of native Western Australian species ranged from 0 to 100 %, averaging 71 %. For all taxa, no combination of temperature, light and gibberellic acid treatment induced all viable seeds to germinate. The greatest percentage germination achieved in any combination of treatments averaged 71 % of all viable seeds for all species. Larger seeds (> 10 mg seed weight) tended to have greater viability percentages, but no overall patterns of viability or germinability were attributable to seed storage syndrome, strategy of fire recovery response or life-form type. Germination of most of the dominant tree representatives (Eucalyptus calophylla, E. diversicolor, E. erythrocorys, E. gomphocephala, and E. patens) was indifferent to the trial conditions of temperature, light and GA₃. However, Eucalyptus marginata showed reduced germination in the light, which was overcome with GA₃. GA₃ also overcame the inhibition resulting from exposure to light in some understorey species (e.g. Allocasuarina campestris, Regelia ciliata, Xanthorrhoea gracilis and X. preissii). Germination of many hard-seeded, understorey shrub and herbaceous perennial species, especially those with small (< 10 mg) seeds (e.g. Bossiaea ornata, B. aquifolium and Acacia drummondii ssp. candolleana) was greater at the lower trial temperature and in the dark. Some large (> 10 mg) seeded, understorey species (e.g. Acacia extensa, Kennedia coccinea, K. prostrata, Hovea trisperma and Hardenbergia comptoniana) germinated in high percentages in both temperatures, but maximum germination percentages still tended to be at 15 °C. Large-seeded species were less sensitive to exposure to light compared to the smaller seeded species. The largest seeded species tested, Paraserianthes lophantha, germinated best in the warmer incubation temperature and in the light. The ecological significance of the tests would be that species which have seed dormancy mechanisms capable of delaying germination until the cool temperature, winter rainy period of this mediterranean-type climate would be more likely to survive than if germination followed summer rain showers or the first, intermittent rains of autumn. Burial of seeds becomes more important if germination occurs when rains first begin as this period has less available soil moisture and temperatures are high. Also survival of seedlings could be enhanced if germination of seed was restricted to the positions protected from high light, higher temperatures and lower soil moisture by the presence of a forest canopy. Therefore, seeds which have an ability to sense the presence of a previous fire in the habitat, conditions in light environment and appropriate temperature level have an adaptive advantage to time emergence to situations of time and space where survival is maximized. Variation in viability and germination percentages were apparent in some cases where more than one seed collection of available for testing, indicating that further aspects, such as seed age, maturity at collection, storage conditions and depth of seed dormancy, remain to be considered.     

Nitrate,abscisic acid and gibberellin interactions on the thermoinhibition of lettuce seed germination

Germination of lettuce seeds has obvious thermoinhibition, but the mechanism for thermoinhibition of seed germination is poorly understood. Here, we investigated the interactions of nitrate, abscisic acid (ABA) and gibberellin on seed germination at high temperatures to understand further the mechanism for thermoinhibition of seed germination. Our results showed that lettuce (Lactuca sativa L. ‘Jianye Xianfeng No. 1’) seeds exhibited notable thermoinhibiton of germination at ≥17°C in darkness, and at ≥23°C in light, but the thermoinhibited seeds did not exhibit secondary dormancy. Thermoinhibition of seed germination at 23 or 25°C in light was notably decreased by 5 and 10 mM nitrate, and the stimulatory effects were markedly prevented by nitric oxide (NO) scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. The sensitivity of seed germination to exogenous ABA increased with increasing temperature. Thermoinhibition of seed germination was markedly decreased by fluridone (an inhibitor of ABA biosynthesis) and GA₃, and was increased by diniconazole (an inhibitor of the ABA-catabolizing enzyme ABA 8′-hydroxylase) and paclobutrazol (an inhibitor of GA biosynthetic pathway). The effect of fluridone in decreasing thermoinhibition of seed germination was obviously antagonized by paclobutrazol, and that of GA₃ was notably added to by fluridone, and that of nitrate was antagonized by paclobutrazol, diniconazole and ABA and was added to by GA₃ and fluridone. Our data show that thermoinhibition of lettuce seed germination is decreased by nitrate in a NO-dependent manner, which is antagonized by ABA, diniconazole and paclobutrazol and added by fluridone.     

The effect of temperature on the germination-promoting activities of cytokinin and gibberellin applied to celery seeds (Apium graveolens)

Celery seeds (Apium graveolens L. cv. Lathom Blanching) made dormant by high temperature pretreatment (28–40°C) during imbibition in the dark, germinated at 22°C in the light after treatment with benzyladenine (BA). This BA-induced promotion of germination increased with increasing pre-treatment temperature from 32 to 38°C. whether BA was given before or after pretreatment. A mixture of gibberellins A₄ and A₇ (GA_4/7) given before a 4 day high temperature pretreatment at 32°C partially inhibited the germination-promoting activity of GA_4/7 given after. It is suggested that gibberellin induces the formation of a thermola-bile product which is necessary for germination, the precursor of which has a limited source.     

The germination characteristics of <Emphasis Type="Italic">Scrophularia marilandica</Emphasis> L. (Scrophulariaceae) seeds

Investigations on seeds of Scrophularia marilandica L. were undertaken to determine their germination requirements. Seeds were collected from three naturally occurring sites and one greenhouse-grown population in London, Ontario in September and October of 1997. Some were set to germinate immediately after collection; others were stored in or on soil outside and/or under controlled laboratory conditions before testing. Germination was assessed under two light/temperature regimes (35°C 14 h light, 20°C 10 h dark and 25°C 14 h light, 10°C 10 h dark), in continuous darkness, and in the presence of two germination-promoting chemicals (GA₃ and KNO₃). Fresh seeds germinated best at 35/20°C, while stored seeds germinated best at 25/10°C. No differences in percent germination were found among three seed-maturity stages. All chemical treatments, except 0.01 M KNO_3, increased percent germination. Significant differences were found both among and within sites for most chemical treatments, but exposure to 3 × 10⁻⁴ M GA₃ caused almost every seed to germinate. When compared to the control, both the gibberellic acid and the soil-storage treatments contributed to faster germination. Exposure of seeds to naturally prevailing conditions on the soil surface followed by testing under the 25/10°C regime produced the highest percent germination. No seeds germinated in the dark. In summary, seeds of S. marilandica exhibit physiological dormancy, which can be alleviated by exposure to light, after-ripening and/or cold stratification. It is probable that the differences in germination response among sites can be attributed to differences in environmental conditions during seed production. These experiments indicate that the seeds of S. marilandica must be buried shortly after dispersal in order to form a persistent seed bank.     

Germination responses to GA3 and short‐time chilling of three endemic species: Tripleurospermum pichleri,Cirsium leucopsis and Senecio olympicus (Asteraceae)

In the present study, the germination characteristics of three endemic species from Turkey, Tripleurospermum pichleri (Boiss.) Bornm., Cirsium leucopsis D.C and Senecio olympicus Boiss. (Asteraceae), were investigated. Germination was studied for fresh seeds, for seeds subjected to short‐time chilling (15 days, moist +4°C), to GA₃ (100, 150 and 250 ppm) and a combination of chilling and GA₃; in all cases seeds were incubated either at 20/10°C day/night with light daytime or at 20°C in darkness with daily short‐time dim light (DSDL). In C. leucopsis seeds, all GA₃ treatments enhanced the final germination percentages. The mean germination time (MGT) of C. leucopsis was lower under DSDL than with photoperiod. The chilling treatment with GA₃ in DSDL significantly increased germination in S. olympicus seeds (from 45 to 87%). Germination increased to 55% in T. pichleri by chilling under photoperiod compared with 32% by chilling followed by DSDL. In conclusion, these three co‐existing endemic Asteraceae species have different germination behaviours; something that should be taken into account for ex situ propagation. However, an efficient way to germinate all species is to use 250 ppm GA₃ and 20/10°C with photoperiod.     

Seed ecology of the geophyte Conopodium majus (Apiaceae), indicator species of ancient woodland understories and oligotrophic meadows

• Conopodium majus is a geophyte with pseudomonocotyly, distributed in Atlantic Europe. It is an indicator of two declining European habitats: ancient woodland understories and oligotrophic hay meadows. Attempts to reintroduce it by seed have been hindered by scarce seedling emergence and limited knowledge of its seed biology.
• Micro‐CT scanning was used to assess pseudomonocotyly. Embryo growth and germination were studied in the laboratory and the field, using dissection and image analysis. The effects of temperature, light, nitrate and GA₃ on germination were tested. Seed desiccation tolerance was investigated by storage at different RHs and by drying seeds at different stages of embryo growth.
• Seeds possess morphological but not physiological dormancy. Embryo growth and germination were promoted by temperatures between 0 and 5 °C, arrested above 10 °C, and indifferent to alternating temperatures, light, nitrate and GA₃. Pseudomonocotyly appears to result from cotyledon fusion. While seeds tolerated drying to 15% RH and storage for 1 year at 20 °C, viability was lost when storage was at 60% RH. Seeds imbibed at 5 °C for 84 days had significant internal embryo growth but were still able to tolerate drying to 15% RH.
• Reproduction by seed in C. majus follows a strategy shared by geophytes adapted to deciduous temperate forests. The evolution of fused cotyledons may enable the radicle and the hypocotyl to reach deeper into the soil where a tuber can develop. The embryo is capable of growth within the seed at low temperatures so that germination is timed for early spring.

     

Effect of gibberellic acid- and water-based pre-soaking treatments under different temperatures and photoperiods on the seed germination of Allium stracheyi Baker: An endangered alpine species of Central Himalaya,India

Allium stracheyi Baker (Alliaceae, 2600–3000 m asl), an endangered species of Central Himalaya, India, has low seed germination in its natural habitat. This study is an attempt to improve seed germination by determining the seed viability with a low mean germination time (MGT) and germination index (GI) under optimum temperature, light, and pre-soaking treatments. The seeds were pre-soaked in hot water (80°C), cold water (10°C), and gibberellic acid (GA₃ at 50 and 100 mg/l) for 24 h and subjected to light (12 h light and 12 h dark) and continuous dark (24 h) conditions with different temperature regimes (10, 15, 20, 25, and 30°C). The viability varied between 66.0% and 69.67% and declined rapidly after 12 months of storage. Our studies suggest that the 100 mg/l GA₃ treatment was beneficial for seed germination and seedling growth. Pre-soaking in a 100 mg/l GA₃ solution and incubation at 20°C under light conditions enhanced the germination significantly (p < 0.05) and resulted in the highest (97.3%) germination with the lowest MGT = 5.7 days, with GI = 8.11. The recommendations of this study support the conservation of alpine A. stracheyi via simple and cost-effective techniques for optimal seed germination.     

Germination response of black and yellow seed coated canola (<Emphasis Type="Italic">Brassica napus</Emphasis>) lines to chemical treatments under cold temperature conditions

Seed quality is a key critical component to produce well established and vigorous seedlings under cool soil (<10°C) conditions experienced in Western Canada. A simple, relatively quick germination assay is required to separate small differences in seed germination which can have a significant impact on seedling growth. It has long been established that phytohormones regulate seed germination: abscisic acid inhibits germination whereas gibberellins enhance germination. We investigated the effects of ABA, GA, ethylene and inhibitors of these phytohormones alone and in combination on the germination rate of a black and a yellow seed canola (Brassica napus) imbibed at 8°C. The effects of either saline solutions, osmotic solutions, fusicoccin or testa on the germination of canola seeds imbibed at 8°C were also investigated. This temperature is representative of the soil temperatures experienced in the early spring of Western Canada. The two canola seed lines, especially the yellow seed line, were very sensitive to increasing concentration of saline solutions at 8°C, but not at 23°C; however, iso-osmotic solutions that reduced water potential were more inhibitory. The seed coat (testa) including the endosperm was a major factor affecting the germination rate of the yellow seed line at 8°C, however, GA₄₊₇ overcame the inhibitory effect of the testa, whereas ABA exacerbated it. Fusicoccin was more stimulatory to germination than GA₄₊₇, however, unlike GA₄₊₇, it was unable to overcome the inhibitory effect of paclobutrazol, a GA biosynthesis inhibitor. Fluridone, an ABA biosynthesis inhibitor, was unable to overcome the inhibitory effects of a saline solution suggesting that the inhibitory effect was not due to elevated ABA levels. Ethylene, a stimulator of germination did not appear to be involved in the germination of these two lines. Controlled deterioration at 35°C, 85% RH could be either partially or completely overcome by exogenous GA₄₊₇. This study demonstrated the effect of hormones, salinity and testa on the germination of canola seeds under less than ideal environmental conditions.     

The Mechanism of Low Temperature Dormancy in Mature Seeds of Syringa Species

The mechanism of seed dormancy at low temperatures (15-9°C) was investigated in the seeds of Syringa josikaea, S. reflexa and S. vulgaris. Low temperature dormancy in Syringa species was mainly imposed by endosperm embedding the radicle. Different degrees of embryo dormancy may occur in S. reflexa seeds. In most cases the low temperature dormancy was broken completely by removing the endosperm around the radicle. The endosperm did not seem to contain significant quantities of germination inhibitors, and the results indicate that it prevents germination mainly due to its mechanical resistance. The mechanical resistance of endosperm did not change during chilling or during induction of dormancy by high temperature incubation. The strength of the endosperm decreased rapidly in non-dormant seeds before visible germination. Similar changes were not observed in dormant seeds. Generally, the strength of the endosperm was lower in the non- (or less) dormant species S. josikaea and S. vulgaris than in the more dormant S. reflexa seeds. The growth potential of the embryos, measured as their ability to germinate in osmotic solutions (mannitol or polyethylen glycol 4000), was increased by chilling and by GA₃-treatment. The growth potential of untreated S. josikaea and S. vulgaris embryos was generally higher than that of S. reflexa embryos. Acid ethyl acetate fractions of methanol extracts from embryos of all three species contained substances with GA³-like activity in the lettuce hypocotyl test. The activity was found at Rf 0.9–1.0 on paper chromatograms run in distilled water. No significant changes in the activity were detected during chilling or prior to visible germination.     

Low-temperature stratification strategies and growth regulators for rapid induction of Paris polyphylla var. yunnanensis seed germination

The seeds of Paris polyphylla var. yunnanensis are deeply dormant, and they remain dormant for 18 months or longer in their natural environment. Periodic exposure of the seeds to a low-temperature of 4 °C broke the dormancy in about 16 weeks (112 days). The most effective temperature stratification scheme was an interval of 14 days at 4 °C and 14 days at 22 °C. Both GA₃ and ethephon significantly enhanced the germination rate during the stratification treatment. The seed coat, particularly the mesophyll outer layer of the seed coat, strongly inhibited the germination. With removal of the seed coat and exposure of the uncoated seeds to 600 mg/l GA₃ for 48 h before the temperature stratification of 14 days at 4 °C and 14 days at 22 °C for 112 days, a germination percentage as high as 95.3% of the seeds was attained in about 160 days.     

Thermodormancy in Celery Seeds and its Removal by Cytokinins and Gibberellins

Imbibition of celery (Apium graveolens L.) seeds at 32°C for up to 96 h lowered the upper temperature limit for germination. If this high temperature treatment was given in the light, these seeds germinated slightly earlier than those treated in the dark although the final percentage germination was similar for both treatments. The inhibitory effect of the high temperature treatment was completely removed by allowing the seeds to imbibe in a mixture of the gibberellins A₄ and A₇ (GA_4/7) and partially removed by the cytokinin N⁶-benzylaminopurine (BA). GA_4/7 was less effective when added before rather than after the high temperature treatment, whereas the opposite was true of BA. At constant temperatures more GA_4/7 was required to promote germination as the temperature was raised but addition of BA reduced the concentration of GA_4/7 required. A model is proposed for the control of celery seed germination by light and temperature through the action of endogenous cytokinins and gibberellins.     

Heat shock effects on seed germination of five Brazilian savanna species

Fire is considered an important factor in influencing the physiognomy, dynamics and composition of Neotropical savannas. Species of diverse physiognomies exhibit different responses to fire, such as population persistence and seed mortality, according to the fire frequency to which they are submitted. The aim of this study is to investigate the effects of heat shocks on seed germination of Anadenanthera macrocarpa (Benth.) Brenan, Dalbergia miscolobium Benth., Aristolochia galeata Mart. & Zucc., Kielmeyera coriacea (Spreng.) Mart. and Guazuma ulmifolia Lam., which are native species of the Brazilian savanna. The temperatures and exposure times to which the seeds were submitted were established according to data obtained in the field during a prescribed fire: 60 °C (10, 20 and 40 min), 80 °C (5, 10 and 20 min) and 100 °C (2, 5 and 10 min). Untreated seeds were used as controls. Seeds of A. galeata and K. coriacea showed high tolerance to most heat treatments, and seeds of A. macrocarpa showed a significant reduction in germination percentage after treatments of 80 °C and 100 °C. Treatments of 100 °C for 10 min reduced germination percentage for all species except G. ulmifolia, which has dormant seeds. For this species, germination was accelerated by heat treatments. The high temperatures applied did not interfere with the time to 50% germination (T₅₀) of the tolerant seeds. Seeds of the savanna species K. coriacea and A. galeata were more tolerant to heat shocks than seeds of the forest species A. macrocarpa. Guazuma ulmifolia, the forest species with seeds that germinate after heat shock, also occurs in savanna physiognomies. Overall, the high temperatures applied did not affect the germination rate of the tolerant seeds.