Secondary Dormant Seeds Research Articles

Global DNA methylation and cellular 5-methylcytosine and H4 acetylated patterns in primary and secondary dormant seeds of Capsella bursa-pastoris (L.) Medik. (shepherd\u2019s purse)

Despite the importance of dormancy and dormancy cycling for plants’ fitness and life cycle phenology, a comprehensive characterization of the global and cellular epigenetic patterns across space and time in different seed dormancy states is lacking. Using Capsella bursa-pastoris (L.) Medik. (shepherd’s purse) seeds with primary and secondary dormancy, we investigated the dynamics of global genomic DNA methylation and explored the spatio-temporal distribution of 5-methylcytosine (5-mC) and histone H4 acetylated (H4Ac) epigenetic marks. Seeds were imbibed at 30 °C in a light regime to maintain primary dormancy, or in darkness to induce secondary dormancy. An ELISA-based method was used to quantify DNA methylation, in relation to total genomic cytosines. Immunolocalization of 5-mC and H4Ac within whole seeds (i.e., including testa) was assessed with reference to embryo anatomy. Global DNA methylation levels were highest in prolonged (14 days) imbibed primary dormant seeds, with more 5-mC marked nuclei present only in specific parts of the seed (e.g., SAM and cotyledons). In secondary dormant seeds, global methylation levels and 5-mC signal where higher at 3 and 7 days than 1 or 14 days. With respect to acetylation, seeds had fewer H4Ac marked nuclei (e.g., SAM) in deeper dormant states, for both types of dormancy. However, the RAM still showed signal after 14 days of imbibition under dormancy-inducing conditions, suggesting a central role for the radicle/RAM in the response to perceived ambient changes and the adjustment of the seed dormancy state. Thus, we show that seed dormancy involves extensive cellular remodeling of DNA methylation and H4 acetylation.

Temperature Regulation of Primary and Secondary Seed Dormancy in Rosa canina L.: Findings from Proteomic Analysis.

Temperature is a key environmental factor restricting seed germination. Rose (Rosa canina L.) seeds are characterized by physical/physiological dormancy, which is broken during warm, followed by cold stratification. Exposing pretreated seeds to 20 °C resulted in the induction of secondary dormancy. The aim of this study was to identify and functionally characterize the proteins associated with dormancy control of rose seeds. Proteins from primary dormant, after warm and cold stratification (nondormant), and secondary dormant seeds were analyzed using 2-D electrophoresis. Proteins that varied in abundance were identified by mass spectrometry. Results showed that cold stratifications affected the variability of the highest number of spots, and there were more common spots with secondary dormancy than with warm stratification. The increase of mitochondrial proteins and actin during dormancy breaking suggests changes in cell functioning and seed preparation to germination. Secondary dormant seeds were characterized by low levels of legumin, metabolic enzymes, and actin, suggesting the consumption of storage materials, a decrease in metabolic activity, and cell elongation. Breaking the dormancy of rose seeds increased the abundance of cellular and metabolic proteins that promote germination. Induction of secondary dormancy caused a decrease in these proteins and germination arrest.

Dormancy cycling is accompanied by changes in ABA sensitivity in Polygonum aviculare seeds.

Polygonum aviculare seeds show high levels of primary dormancy (PD). Low winter temperatures alleviate dormancy and high spring temperatures induce seeds into secondary dormancy (SD), naturally establishing stable seedbanks cycling through years. The objective of this work was to elucidate the mechanism(s) involved in PD expression and release, and in SD induction in these seeds, and the extent to which abscisic acid (ABA) and gibberellins (GAs) are part of these mechanisms. Quantification of endogenous ABA both prior to and during incubation, and sensitivity to ABA and GAs, were assessed in seeds with contrasting dormancy. Expression analysis was performed for candidate genes involved in hormone metabolism and signaling. It was found that endogenous ABA content does not explain either dormancy release or dormancy induction; moreover, it does not seem to play a role in dormancy maintenance. However, dormancy modifications were commonly accompanied by changes in ABA sensitivity. Concomitantly, induction into SD, but not PD, was characterized by a increased PaABI-5 and PaPYL transcription, and a rise in GA sensitivity as a possible counterbalance effect. These results suggest that dormancy cycling in this species is related to changes in embryo sensitivity to ABA; however, this sensitivity appears to be controlled by different molecular mechanisms in primary and secondary dormant seeds.

Increased BnaMFT-transcript level is associated with secondary dormancy in oilseed rape (Brassica napus L.)

Brassica napus cultivars have little or no primary dormancy; however, they are prone to secondary dormancy induction. Secondary dormant seeds can produce volunteer plants, which can result in genetic contamination, reduced quality and biosafety issues. However, information regarding the molecular mechanism underlying secondary dormancy is limited. The MOTHER OF FT AND TFL1 (MFT) gene, which is evolutionarily conserved in the plant kingdom, acts in a complex gene network in the seed dormancy or germination processes. In this study, we identified four B. napus genes that share high homology with AtMFT, named as BnaMFT. Analyses of cis-acting elements showed that BnaMFT promoters contain multiple seed-specific regulatory elements, and various stress- and hormone-responsive elements. Further experiments validated that BnaMFTs were specifically expressed during seed maturation and in the dry seed, with peaks at 35–42 days after pollination. BnaMFTs were not sufficient for primary dormancy; however, they were significantly enhanced by secondary dormancy induction with PEG6000 treatment. Moreover, BnaMFT transcripts were elevated by treatment with abscisic acid (ABA), which is known to be accumulated during secondary dormancy. These results collectively suggest that increased BnaMFT transcription levels are associated with secondary dormancy induction in an ABA-dependent manner in B. napus.

Roles of abscisic acid and gibberellins in maintaining primary and secondary dormancy of Korean pine seeds

Primary dormancy of seeds of Korean pine (Pinus koraiensis Sieb. et Zucc.) after dispersal in the autumn and the induction of secondary dormancy the first summer following seed dispersal limit the regeneration of mixed broadleaved Korean pine forests in Northeast China. This study was to determine how changes in the levels of abscisic acid (ABA) and gibberellic acid (GA) maintain primary and secondary dormancy of Korean pine seeds under germination conditions. We transferred seeds with one of five primary dormancy states or three secondary dormancy states to germination conditions and measured changes in the levels of ABA, GA1+3 (GA1 and GA3) and GA4+7 (GA4 and GA7) in the seed coat, megagametophyte and embryo during incubation. Seed coat ABA levels in primary dormant seeds (PDS) and ABA levels in various parts of secondary dormant seeds (SDS) gradually declined during incubation but were still higher than in seeds for which dormancy was progressively released. GA4+7 and GA1+3 levels in embryos greatly decreased 35% and 24%, respectively, during incubation of SDS, and thus, the ratio of ABA to GA4+7 in embryos and megagametophytes significantly increased. The ratio of ABA to GA1+3 in various parts of SDS increased slightly during incubation. In contrast, in seeds for which secondary dormancy was already released, GA4+7 and GA1+3 levels in the embryo, GA4+7/ABA ratio in the embryo and seed coat, and the GA1+3/ABA in the embryo and megagametophyte significantly increased during incubation. There was no trend in the changes in the levels of ABA, GA4+7 or GA1+3 in embryos and megagametophytes of PDS or the levels of GA4+7 or GA1+3 in megagametophytes of SDS during incubation. The results suggest that high ABA levels in the seed coat maintain primary dormancy of Korean pine seeds. Maintenance of secondary dormancy involves a reduction of GA4+7, GA1+3, GA4+7/ABA, and GA1+3/ABA and the retention of high ABA levels.

Water stress, temperature regimes and light control induction, and loss of secondary dormancy in Brassica napus L. seeds

AbstractThis study investigated the induction and loss of dormancy in oilseed rape (Brassica napus). Twenty genotypes were preliminary screened; from these, two genotypes, RGS003 and Hayola 308, which possess high potential for dormancy induction (HSD) and medium potential to induce secondary dormancy (MSD), were selected. The stratification of seeds at alternating temperatures of 5–30°C (in dark) significantly relieved secondary dormancy, but dormancy was not fully released. The ψb(50) values were −1.05 and −1.06 MPa for the MSD and the HSD before dormancy induction. After inducing dormancy, the ψb(50) values for the MSD and the HSD were increased to −0.59 and −0.01 on day 0 stratification at 20°C. The hydrothermal time (θHT) value was low for one-day stratification for HSD in comparison with other stratification treatments. Water stress can induce dormancy (if the seeds have the genetic potential for secondary dormancy) and warm stratification (in dark) can only reduce the intensity of dormancy. The seeds with a high potential of dormancy induction can overcome dormancy at alternating temperatures and in the presence of light. It can, therefore, be concluded that a portion of seeds can enter the cycle of dormancy ↔ non-dormancy. The secondary dormant seeds of B. napus cannot become non-dormant in darkness, but the level of dormancy may change from maximum (after water stress) to minimum (after warm stratification). It seems that the dormancy imposed by the conditions of deep burial (darkness in combination with water stress and more constant temperatures) might be more important to seed persistence than secondary dormancy induction and release. The dormancy cycle is an important pre-requisite in order to sense the depth of burial and the best time for seed germination.

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 GAn, 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.

Water content: a key factor of the induction of secondary dormancy in barley grains as related to ABA metabolism

Primary dormant barley (Hordeum vulgare) grains germinate at 10-15°C but not at 30°C, and there exist a positive correlation between embryo ABA content after 24 h on water and the depth of dormancy. Incubation at 30°C results in a progressive loss of the ability to germinate at 15°C. This induction of a secondary dormancy is optimal after 3 days and requires an embryo water content higher than 0.50 g H₂O g⁻¹ DW, this corresponding with activation of the cell cycle. There exists no correlation between ABA content after 3 days at 30°C and the induction of secondary dormancy. However, at high water content (1.60-1.87 g H₂O g⁻¹ DW), secondary dormancy is associated with an high embryo ABA content after transfer to 15°C, resulting from an increase in HvNCED1 and HvNCED2 expression and a decrease in HvABA8'OH-1. Such changes are not observed at 0.45 g H₂O g⁻¹ DW. Incubation at 30°C also results in an increase in expression of genes involved in GA catabolism (HvGA2ox1, HvGA2ox3 and HvGA2ox5) and synthesis (HvGA3ox2, HvGA20ox1 and HvGA20ox3). The HvGA3ox2/HvGA2ox3 transcript ratio remains low (0.27-0.37) at 30°C and after transfer to 15°C in secondary dormant seeds, but it is higher than two when secondary dormancy is not induced. Changes in HvExpA11 expression indicate that GA signaling decreases when a secondary dormancy is expressed. Our results clearly indicate that expression of genes involved in ABA and GA metabolism differs in primary and secondary dormancies and furthermore, their expression is related to embryo water content.

Dormant and non-dormant Castanea sativa Mill. buds require different polyvinylpyrrolidone concentrations for optimal RNA isolation

Recovering RNA of high quality and sufficient quantity is a prerequisite for ensuring proper representation of expressed genes in a cDNA library. Like Arabidopsis seeds, Castanea sativa buds contain polyphenols, polysaccharides and lipids that often interface with RNA, restricting its yield and quality. The hot borate RNA isolation described for Arabidopsis seeds was used to extract RNA from C. sativa buds. The optimal concentration of polyvinylpyrrolidone in the extraction buffer differed between dormant (14.3 mg/ml) and non-dormant buds (4.3 mg/ml) for RNA yield. Furthermore, differences were found in RNA integrity and purity between different dormancy stages in buds. In the case of secondary dormant and non-dormant Arabidopsis seeds differences were found attending to the RNA yield but not integrity or purity. The optimal concentration for secondary dormant seeds was 10.0 and 14.3 mg/ml for non-dormant seeds. RNA of C. sativa buds proved suitable for RT-PCR and the construction of cDNA libraries. The different optima for dormant and non-dormant tissues likely reflect a different chemical composition. Unexpectedly, the total phenol content in dormant and non-dormant tissues did not explain these different requirements for polyvinylpyrrolidone. The reasons for this are discussed. Optimization of the PVP concentration is recommended as a standard operating procedure for the development of RNA extraction protocols.

The effect of stratification temperatures on the level of dormancy in primary and secondary dormant seeds of two Carex 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.

The release of secondary dormancy by ethylene inAmaranthus caudatusL. seeds

AbstractNeither ethylene nor 1-aminocyclopropane-1-carboxylic acid (ACC) was able to prevent the induction of secondary dormancy ofAmaranthus caudatusat 45°C. Both ethylene (4.5 × 10-9–4.5 × 10-7M) and ACC (10-3–10-2M) removed secondary dormancy at 25°C, although ethylene was much more effective. The presence of ethylene for only 10 h was sufficient to remove secondary dormancy in almost all seeds. Incubation of secondary dormant seeds for up to 5 d at 25°C did not change sensitivity to ethylene. The breaking of secondary dormancy by ethylene was prevented by 2,5-norbornadiene (NBD; 1.5 × 10-5–3 × 10-4M), indicating the physiological action of ethylene. Abscisic acid (ABA; 10-4–10-3M) increased the requirement for exogenous ethylene. It is suggested that secondary dormancy inA. caudatusseeds might be related to insufficient ethylene production associated with an insufficient amount of ACC.

Induction of secondary dormancy in Amaranthus caudatus seeds

Nondormant A. caudatus seeds germinated in the darkat temperatures between 20 and 35° but not at 45 °C.Incubation at this temperature for at least 10 h inhibited seedgermination over the temperature range 20 to 35 °C,temperatures previously suitable for germination. Thus incubation at 45°C induced secondary dormancy. Mechanical or chemicalscarification or exposure to pure oxygen caused complete or almost completegermination of dormant seeds although more slowly in comparison to nondormantseeds. Secondary dormant scarified seeds required a lower concentration of ABAthan nondormant seeds to inhibit germination. The high temperature, whichinduced dormancy, 45 °C, caused the seed coat to be partiallyresponsible for secondary dormancy. Involvement of ABA (synthesis orsensitivity) in the induction and/or maintenance of this dormancy should beconsidered.

Temperature‐dependent germination traits in oilseed rape associated with 5′‐anchored simple sequence repeat PCR polymorphisms

An experiment was conducted to test the hypothesis that phenotypes differing in germination rate and the presence or absence of secondary dormancy at low temperature were not genetically different. Seed of oilseed rape was germinated at 4, 10 and 19 degrees C, where selections were made in the percentile ranges 1-10 (early), 45-55 (intermediate) and 91-100 (late). Secondary dormancy occurred only in the late selections at the two lower temperatures. Thermal weighting of curves of cumulative germination on time gave circumstantial evidence that early percentiles were similar at all three temperatures and that seeds with secondary dormancy came largely from later percentiles above the 50th. To test for genetic differentiation between phenotypes, 5'-anchored simple sequence repeat primers were used to generate DNA marker profiles of seedlings raised from seed from each category. Principal coordinate analysis, and more detailed comparisons using the most discriminating markers, confirmed that the early germinators at the three temperatures were not associated with different banding profiles, but seeds entering secondary dormancy, particularly at 10 degrees C, were genetically distinct from germinators at the same temperature. Secondary dormant seeds at low temperature appear to originate mainly from the late germinating seed at higher temperature. Effects of temperature history and the requirement for alternating temperatures to break secondary dormancy were quantified. The results confirm the existence of genetically discrete sub-populations differing in ecologically significant traits.

Changes in protein composition in cellular membranes of various parts of secondary dormant cucumber seeds treated with ethanol

AbstractSecondary seed dormancy in cucumber (Cucumis sativus L.) cv.Poinsett 76 could be broken with ethanol. Breakage of dormancy was accompanied by significant changes in the composition of membrane proteins in various seed parts. Compared to the intracellular membranes, a change in the protein composition of plasma membranes from the embryonic axis and cotyledons was evident early in the time-course of ethanol-induced germination of dormant seeds. While few alterations in the membrane protein composition in response to ethanol treatment occurred both in dormant and nondormant seeds, a number of changes occurred exclusively in ethanol-treated dormant seeds. Notably, a 14 kD protein in the plasma membrane from the embryonic axis and a 35 kD protein in the microsomal membranes from the perisperm-endosperm envelope disappeared in the ethanol-treated dormant seeds, but not in the ethanol-treated nondormant seeds. Also marked decrease in the content of a 23 kD protein in the plasma membrane from the cotyledons was observed in ethanol-treated dormant seeds only. It is suggested that the particular changes in cellular membrane proteins, which occurred in ethanol-treated dormant seeds much before the first visible indication of germination, might be related to dormancy breaking rather than to the germination process.

Annual cycles of germinability and differences between primary and secondary dormancy in buried seeds of Echinochloa crus‐galli

The seasonal changes in percentage of dormant seeds of Echinochloa crus‐galli in the field were recorded for 4 years. The lots of seeds were wrapped in nylon fabric, buried 20 cm under the grass sward and exhumed at monthly intervals. The proportion of seeds germinating under light conditions at a constant temperature of 25 °C fluctuated between 0% and 96%, with maxima in May–July and minima in September–November. Small between‐year differences in the course of summer dormancy induction and its winter termination were probably caused by variation of weather conditions.Attributes of dormancy innate to seeds after maturation (primary dormancy) and dormancy induced in buried seeds during the summer (secondary dormancy) were compared by investigating the rate of dormancy termination during storage of (a) dry seeds at 25 °C, (b) imbibed seeds at 5°C and (c) in seeds buried under field conditions during October–June. Percentage of germination increased faster in secondary than primary dormant seeds at both constant 25 °C and 5 °C. The seeds with primary and secondary dormancy also differed in the response to `germination pre‐treatment', a 10‐day exposure of imbibed seeds at 25 °C that causes germination of the non‐dormant fraction of seed materials. After this treatment the time to resuming germination in primary dormant seeds was substantially increased, whereas the secondary dormant seeds were much less affected. Annual variation in the proportion of germinable seeds explains the low efficiency of autumn soil cultivation for decreasing reserves of E. crus‐galli seeds in the soil.

Dormancy/Nondormancy Cycles in Spores of the Liverwort Sphaerocarpos texanus

The presence of a dormancy/nondormancy cycle was investigated in spores of the winter, annual, liverwort, Sphaerocarpos texanus. To test for loss of dormancy, mature laboratoryderived spores were held at three daily alternating incubation temperature regimes (thermoperiods) of 35/20, 30/15, and 25/15?C. For 22 intervals (varying from one week to 91 weeks), spores were transferred from each of these three thermoperiods onto a wet substrate in a germination chamber held at 16/10?C. Loss of dormancy, indicated by the spores ability to germinate, increased as the length of time spores kept in the incubation thermoperiods increased. Loss of dormancy in spores held at 35/20?C increased faster than spores held at 30/15 and 25/15?C. Spores held at each of the three thermoperiods germinated best when transferred to 16/100C and failed to germinate when transferred to 35/20 and 30/15?C. Spores subjected to simulated seasonal temperature changes were induced back into secondary dormancy when subjected to low temperatures. The loss of dormancy and subsequent induction of dormancy suggest that S. texanus has a dormancy/nondormancy spore cycle similar to that found in seeds of obligate winter annuals. The occurrence of dormancy/nondormancy cycles in seeds of winter annual seed plants has been extensively documented (Baskin & Baskin 1985, 1989, 1998; Egley 1995 and ref. therein). These annual plants can tolerate the low temperatures of winter, but not the high temperatures and drought conditions of summer. They produce seeds that are innately dormant (seeds not germinating under any set of natural conditions), which prevents germination at a time when the chance of seedling survival is low (summer). Exposure to high temperatures in summer are required for these seeds to lose their dormancy. By autumn, seeds are nondormant and the mild autumn temperatures allow for seed germination when the habitat favors completion of the life cycle. Low winter temperatures induce seeds that did not germinate into dormancy again (secondary dormancy). Secondary dormancy prevents germination in spring when temperatures are mild, but insufficient time is available for life cycle completion. For innate and induced secondary dormant seeds to germinate, they must first become nondormant (through physiological and/or biochemical changes). Dormancy appears to be rare in bryophytes. No evidence of spore dormancy was found by Mogensen (1981) in over 60 tested species. A similar observation of a lack of spore dormancy was observed for 20 bryophytes species by Longton and Schuster (1983). Nevertheless, dormancy in spores has been reported in a few bryophyte species and was discussed by During (1979). Spores of Riccia nigrosquamata germinated a few at a time reaching 2% after 12 weeks (Berrie 1975). Optimum germination was obtained if spores were placed first in dry storage (Thompson 1941) or, placed in 250C in water in the dark (Steiner 1969) in Riella affinis and Sphaerocarpos donnellii, respectively. Several environmental factors have been found to influence spore germination including light (Hoffman 1964; Krupa 1964) and temperature (Krupa 1964; Longton & Greene 1979). Further experiments are needed to determine whether bryophyte spores that fail to germinate are innately dormant or whether they are nondormant, but fail to germinate due to unfavorable environmental conditions. Coupling such studies within the context of the timing of a species life cycle and the prevailing environmental conditions will provide insight into the roles that dormancy and germination patterns play in maintaining a species population. The liverwort, Sphaerocarpos texanus, is a winter annual that germinates in autumn, and the gametophore senesces in early spring. Freshly-matured spores collected from senescing plants in the field or from laboratory-grown plants failed to germinate when sown in simulated autumn temperatures. Since the life cycle of S. texanus is similar to that of winter annual seed plants, this study was initiated and designed to determine if spores of S. texanus have a similar dormancy/nondormancy cycle as the seeds of winter annual seed plants. Specifically, I tested for 1) temperature dependence and rate of loss of dormancy, 2) temperature conditions for germination, and 3) presence of secondary dor-

Induction of secondary dormancy in rape seeds ( Brassica napus L.) by prolonged imbibition under conditions of water stress or oxygen deficiency in darkness

Prolonged imbibition under conditions of water stress or oxygen deficiency can lead to the induction of secondary dormancy in rape seeds. During imbibition in darkness seeds develop light sensitivity. The percentage of seeds not germinating in the dark depends on various factors prevailing during and after the stress treatment, as well as on the intrinsic susceptibility of the seeds to these factors. In experiments on the effect of water stress it was found that the percentage of secondary dormant seeds could be increased in the range of 0–75%, by greater water suction and duration of imbibition of the seeds. Additionally, there were indications that increasing treatment temperature resulted in increasing percentages of dormant seeds. However, fewer secondary dormant seeds were produced when treatment temperature and test temperature differed greatly from each other. Experiments on the effect of oxygen deficiency also revealed a positive effect of treatment duration. The general degree of dormancy induction, however, in this series of experiments was low. At 20 °C almost no dormancy induction was observed, whilst at 12 °C there were between 0 and 30% dormant seeds. Genotypic differences previously assessed in model experiments with buried seeds were confirmed in both studies. Oilseed rape cultivars Bienvenu and Liglandor were shown to be able to build up high percentages of secondary dormant seeds, whereas cultivars Jet Neuf and Falcon had a rather low susceptibility towards the dormancy imposing factors tested. Cultivar Rubin was intermediate in response. There was often much variation between experiments with the same experimental design. The reasons for this are unclear.

Seed Germination Ecophysiology of Meadow Salsify (Tragopogon pratensis) and Western Salsify (T. dubius)

To understand persistence strategies of meadow and western salsify, ecophysiological characteristics of their seed germination were studied. Anaerobiosis (immersion in deoxygenated water) induced secondary dormancy in seeds of both species. Dormancy could be induced in 86% of meadow salsify seeds and in 65% of western salsify seeds by a 1-d anaerobiosis treatment. The induced dormancy was gradually released during storage of air-dried secondary dormant meadow salsify seeds, and the rate of this release was influenced by storage temperature; 30 C was more effective than 10 or 20 C in releasing secondary dormancy. These results suggest that the two species may rely on induced dormancy as an option in their persistence strategy. The optimum temperature for germination of nondormant seeds of both species was 15 C. Maximum germination percentages for both species were established within 4 to 6 d of incubation at 15 C and within 14 to 28 d of incubation at 25 C. Nondormant seeds did not germinate below 10 or above 30 C. Stratification (at 5 C for 2 to 10 wk) stimulated germination of secondary dormant seeds of meadow salsify. This stratification requirement can be important in preventing germination of dormant salsify seeds in the fall, thereby avoiding high seedling mortality in the winter. Light (red and far-red) had no effect on germination of seeds in secondary dormancy. In a separate study, seeds of both species were planted in pots at depths of 2 to 14 cm and seedling emergence was observed. Maximum emergence occurred when seeds were buried 2 cm deep. Seeds planted 8 cm or deeper germinated but did not emerge.

Changes in germination and respiratory potential of embryos of dormant Grand Rapids lettuce seeds during long-term imbibed storage, and related changes in the endosperm

Grand Rapids lettuce (Lactuca sativa L.) seeds were stored in an imbibed state for up to two years. Embryos dissected from stored seeds showed a progressive loss with time in their ability to germinate on polyethylene glycol (PEG) solutions. Little germination of dissected embryos from one-month imbibed seeds occurred on-6 bar PEG but only after four months of storage did the dissected embryos fail to germinate on-4 bar PEG. After two years storage 30% of dissected embryos still were able to germinate on-2 bar PEG. This loss of germination potential, which may be a symptom of the development of an embryo dormancy, could be reversed by N(6)-benzyladenine (BA) and red light (R) applied together or separately to dissected embryos. Two weeks of chilling of 12-month imbibed seeds restored sensitivity to R and a 48-h BA pretreatment prior to R resulted in germination rates similar to those of seeds emerging from primary dormancy. There was loss of embryo control of endo-β-mannanase activity after two weeks of storage even though the endosperms themselves retained their capacity for enzyme synthesis for six more weeks. Eventually, then, endo-β-mannanase synthesis is not possible because of inherent changes in both the embryo and endosperm, although each tissue undergoes changes at its own rate. Oxygen uptake by embryos dissected from two-month imbibed seeds did not increase to the same extent as embryos dissected from freshly imbibed seeds. In intact seeds germinating from a skotodormant state, oxygen uptake increased at a time coincident with radicle protrusion, but did not achieve the levels of uptake of those seeds germinating from a primary dormant state. The decline in uptake of oxygen by secondary dormant seeds is the result of a lowered respiratory capability of the embryo itself, rather than of changes in permeability of the surrounding structures.

Control of the germination of secondary dormant cocklebur seeds by various germination stimulants

Control of the germination of secondary dormant cocklebur seeds by various germination stimulants