Physiology of Oil Seeds: IV. Role of Endogenous Ethylene and Inhibitory Regulators during Natural and Induced Afterripening of Dormant Virginia-type Peanut Seeds

To further elucidate the regulation of dormancy release, we followed the natural afterripening of Virginia-type peanut (Arachis hypogaea L.) seeds from about the 5th to 40th week after harvest. Seeds were kept at low temperature (3 ± 2 C) until just prior to testing for germination, ethylene production, and internal ethylene concentration. Germination tended to fluctuate but did not increase significantly during the first 30 weeks; internal ethylene concentrations and ethylene production remained comparatively low during this time. When the seeds were placed at room temperature during the 30th to 40th weeks after harvest, there was a large increase in germination, 49% and 47% for apical and basal seeds, respectively. The data confirm our previous suggestion that production rates of 2.0 to 3.0 nanoliters per gram fresh weight per hour are necessary to provide internal ethylene concentrations at activation levels which cause a substantial increase of germination. Activation levels internally must be more than 0.4 microliter per liter and 0.9 microliter per liter for some apical and basal seeds, respectively, since dormant-imbibed seeds containing these concentrations did not germinate. Abscisic acid inhibited germination and ethylene production of afterripened seeds. Kinetin reversed the effects of ABA and this was correlated with its ability to stimulate ethylene production by the seeds. Ethylene also reversed the effects of abscisic acid. Carbon dioxide did not compete with ethylene action in this system. The data indicate that ethylene and an inhibitor, possibly abscisic acid, interact to control dormant peanut seed germination. The inability of CO₂ to inhibit competitively the action of ethylene on dormancy release, as it does other ethylene effects, suggests that the primary site of action of ethylene in peanut seeds is different from the site for other plant responses to ethylene.     

Physiology of Oil Seeds: II. Dormancy Release in Virginia-type Peanut Seeds by Plant Growth Regulators

Germination, ethylene production, and carbon dioxide production by dormant Virginia-type peanuts were determined during treatments with plant growth regulators. Kinetin, benzylaminopurine, and 2-chloroethylphosphonic acid induced extensive germination above the water controls. Benzylaminopurine and 2-chloroethylphosphonic acid increased the germination of the more dormant basal seeds to a larger extent above the controls than the less dormant apical seeds. Coumarin induced a slight stimulation of germination while abscisic acid, 2,4-dichlorophenoxyacetic acid, and succinic acid 2,2-dimethylhydrazide did not stimulate germination above the controls. In addition to stimulating germination, the cytokinins also stimulated ethylene production by the seeds. In the case of benzylaminopurine, where the more dormant basal seeds were stimulated to germinate above the control to a larger extent than the less dormant apical seeds, correspondingly more ethylene production was induced in the basal seeds. However, the opposite was true of kinetin for both germination and ethylene production. When germination was extensively stimulated by the cytokinins, maximal ethylene and carbon dioxide evolution occurred at 24 and 72 hours, respectively. Abscisic acid inhibited ethylene production and germinaton of the seeds while carbon dioxide evolution was comparatively high. The crucial physiological event for germination of dormant peanut seeds was enhancement of ethylene production by the seeds.     

Physiology of oil seeds: I. Regulation of dormancy in virginia-type peanut seeds

The germination and ethylene production by dormant Virginia-type peanut seeds were observed in relation to phytohormone treatments that could conceivably release the dormancy of these seeds. A comparison was made between the effects of these treatments on the less dormant apical seeds and the more dormant basal seeds. Indole-3-acetic acid did not stimulate ethylene production by, or germination of, the dormant seeds to any extent. Gibberellic acid at 5 × 10⁻⁴ M stimulated ethylene production by apical seeds to 17 millimicroliters per hour and germination to only 40% above the control. The more dormant basal seeds were affected even less by gibberellic acid than the seeds. Ethylene gas at 8 microliters per liter stimulated germination to 85% above the control for both apical and basal seeds. At this ethylene concentration the physiology of the more dormant basal seeds was altered, so that they behaved in a manner similar to the inherently less dormant apical seeds. 2-Chloroethylphosphonic acid at 10⁻³ and 5 × 10⁻⁴ M provided results similar to ethylene gas. Both apical and basal seeds germinated 100% at 48 hours. Among the phytohormones tested in this study, ethylene gas produced the greatest germination at low concentrations, and it appears must directly related to initiating the reactions required for converting the quiescent cells to an active state of growth.     

Breaking the apple embryo dormancy by nitric oxide involves the stimulation of ethylene production

Mature seeds of apple (Mallus domestica Borb. cv. Antonówka) are dormant and do not germinate unless their dormancy is removed by several weeks of moist-cold treatment. We investigated the effect of short-term (3 h) nitric oxide (NO) pretreatment on breaking of apple embryonic dormancy expressed as inhibition of germination and morphological abnormalities of young seedlings. Imbibition of embryos isolated from dormant apple seeds with sodium nitroprusside (SNP) or S-nitroso,N-acetyl penicillamine (SNAP) as NO donors resulted in enhanced germination. Moreover, NO treatment removed morphological abnormalities of seedlings developing from dormant embryo. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-teramethylimidazoline-1-oxyl-3 oxide (cPTIO) removed the above effects. NO-mediated breaking of embryonic dormancy correlated well with enhanced ethylene production. Inhibitor of ethylene synthesis (AOA) reversed the stimulatory effect of NO donors on embryo germination. Additionally SNP reduced embryo sensitivity to exogenously applied ABA ensuing dormancy breakage. We can conclude that NO acts as a regulatory factor included in the control of apple embryonic dormancy breakage by stimulation of ethylene biosynthesis.     

Release of heat pretreatment-induced dormancy in lettuce seeds by ethylene or cytokinin in relation to the production of ethylene and the synthesis of 1-aminocyclopropane-1-carboxylic acid during germination

The germination of lettuce (Lactuca sativa L.) seeds was greatly reduced when the seeds were heated at 97°C for 30 h prior to imbibition. This dormancy was effectively released when ethylene (1–100 ppm) or benzyladenine (BA) (0.005–0.05 mM) was applied during the imbibition period. Ethylene was not required during the early part of imbibition, but was essential during the period immediately prior to radicle protrusion. Treatment with 1-aminocyclopropane-1-carboxylic acid (ACC) (0.1–10 mM) stimulated germination, but was not as effective as ethylene or cytokinin treatment. During the germination of nondormant lettuce seeds, ethylene production increased rapidly and reached a peak at 24 h, which coincided with the emergence of the radicle, and then declined; the level of ACC increased as ethylene production rate increased, but remained at a high level after radicle protrusion. In heat-pretreated dormant lettuce seeds, the increases in percent germination, ethylene production, and ACC levels were all delayed and lower than those of nondormant seeds, and these increases were accelerated by treatment with ethylene or cytokinin.     

Action of ferric and aluminium ions on the dormancy breakage of <Emphasis Type="Italic">Stylosanthes humilis</Emphasis> seeds

Dormancy of scarified seeds of Stylosanthes humilis was broken by acidic Al³⁺ and Fe³⁺ solutions. Fe⁺³-stimulated seeds exhibited a high activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and produced great amounts of ethylene, which showed correlated with the germination process. In addition, specific inhibitors of ethylene biosynthesis and action largely depressed the Fe³⁺-stimulated germination, leading to the conclusion that the ion broke dormancy by triggering ethylene production by the seeds. By contrast, inhibitors of ethylene biosynthesis and action did not impair germination of Al³⁺-stimulated dormant seeds. Moreover, ethylene production and activity of ACC oxidase of Al³⁺-treated seeds was substantially decreased by inhibitors of ethylene biosynthesis, but germination kept large. Together these data suggest that ethylene biosynthesis was not required in the chain of events triggered by Al³⁺ leading to dormancy breakage. Methyl viologen (MV), a reactive oxygen species-generating compound, broke dormancy of seeds to the same extent as Al³⁺ did. Germination of both Al³⁺- and MV-stimulated dormant seeds was inhibited by sodium selenate, an antioxidant compound; selenate, however had no effect on germination of Fe³⁺-stimulated seeds. Together these data indicate that the mechanisms underlying the germination of Al³⁺- and Fe³⁺-treated seeds are not the same.     

Ethylene in seed dormancy and germination

The role of ethylene in the release of primary and secondary dormancy and the germination of non-dormant seeds under normal and stressed conditions is considered. In many species, exogenous ethylene, or ethephon – an ethylene-releasing compound - stimulates seed germination that may be inhibited because of embryo or coat dormancy, adverse environmental conditions or inhibitors (e.g. abscisic acid, jasmonate). Ethylene can either act alone, or synergistically or additively with other factors. The immediate precursor of ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), may also improve seed germination, but usually less effectively. Dormant or non-dormant inhibited seeds have a lower ethylene production ability, and ACC and ACC oxidase activity than non-dormant, uninhibited seeds. Aminoethoxyvinyl-glycine (AVG) partially or markedly inhibits ethylene biosynthesis in dormant or non-dormant seeds, but does not affect seed germination. Ethylene binding is required in seeds of many species for dormancy release or germination under optimal or adverse conditions. There are examples where induction of seed germination by some stimulators requires ethylene action. However, the mechanism of ethylene action is almost unknown.
The evidence presented here shows that ethylene performs a relatively vital role in dormancy release and seed germination of most plant species studied.     

Dormancy in Cereal Seeds: I. THE EFFECTS OF OXYGEN AND RESPIRATORY INHIBITORS

A wide spectrum of respiratory inhibitors has been found tostimulate the breaking of dormancy in barley. These includecarbon monoxide, cyanide, azide, hydrogen sulphide, sodium sulphide,hydroxylamine, diethyldithiocarbamate (DIECA), fluoride, iodoacetate,malonate, monofluoroacetate, and 2,4-dinitrophenol (DNP). Inrice, only the first six of these have been shown to be effective.Apart from CO, all the above inhibitors were tested on winteroats, but in this material only cyanide, azide, and hydroxylaminewere found to increase the germination of dormant seeds. Allthe terminal-oxidase inhibitors except CO were tested on perennialryegrass, but in this case only cyanide was found to break dormancy. As compared with air, an atmosphere of 96 per cent oxygen appliedto barley during the first 24 h after the seeds have been setto germinate stimulates the breaking of dormancy. When appliedat later stages, this high oxygen tension inhibits the germinationof dormant seeds although it has no effect on nondormant seeds.Paradoxically, the stimulatory effects of respiratory inhibitorsapplied during the initial stages of germination are relatedto their ability to inhibit oxygen uptake. Thus cyanide, azide,malonate, and monofluoroacetate, while stimulating the breakingof dormancy in barley, also inhibit oxygen uptake. In rice,cyanide and azide had similar effects, but fluoride, which hadno effect on dormancy, also had no effect on the oxygen uptakeof dormant seeds. These results are compatible with the hypothesis that some oxidationreaction is necessary for germination. This oxidation is notpart of the normal respiratory pathway, and does not proceedsatisfactorily in dormant seeds. It may be stimulated, however,by increasing the oxygen tension or by reducing normal respiratorycompetition with respiratory inhibitors.     

Bleach-induced Germination and Breakage of Dormancy of Seeds of Alectra vogelii

The effects of solutions of sodium hypochlorite and calcium hypochlorite (bleach) on dormancy and germination of Alectra vogelii seeds were investigated. Dry (non-pretreated) and 10-day water-pretreated seeds were exposed to various bleach concentrations, as well as to the host (Vigna unguiculata) root exudate. The 15-month-old Alectra seeds used were partially dormant in that only 40% or less of the 10-day pretreated seeds could be stimulated to germinate by the standard host root exudate. Comparable percentage germination of nonpretreated seeds was achieved with certain concentrations of bleach and the halogens, chlorine and bromine. Bleach and the halogens served not only as germination stimulant for pretreated Alectra seeds but appropriate concentrations induced also high percentage (70–90%) germination indicating a breakage of seed dormancy as well. The activity of the bleach in stimulating high percentage germination could be significantly reduced when the optimally bleach-treated seeds were rinsed daily with water during the germination period. Vigna root exudate, but not bleach, was shown to act as root stimulant for Alectra seedlings. The possible mechanism of the bleach effect on germination and dormancy of Alectra seeds is discussed.     

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

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

Breaking dormancy of <Emphasis Type="Italic">Stylosanthes humilis</Emphasis> seeds with low pH solutions

An acidic condition (low pH) of the germination media promoted dormancy breakage of scarified seeds of Townsville stylo (Stylosanthes humilis H.B.K.), an annual tropical forage legume, whether produced by either an unbuffered (HCl-KOH) or a buffered (phthalate, McIlvaine) medium. Except for aminooxyacetic acid, all ethylene biosynthesis inhibitors tested and supplied with the low pH solutions decreased germination to variable extents. Low pH-stimulated dormant seeds produced ethylene 4-fold as much than untreated seeds. Production of ethylene by seeds treated with high pH solutions, which did not affect their dormant state, was also very low.     

Abscisic Acid-like Inhibitors and Cytokinins during After-ripening of Dormant Peanut Seeds (Arachis hypogaea)

Changes in abscisic acid-like inhibitors and cytokinins were studied during dry storage after ripening of dormant peanut seeds. Decrease in ABA-like inhibitors and increase in cytokinin levels was found as the seeds lose their dormancy. Dormancy of peanut seeds is associated with presence of a high level of ABA-like inhibitors and a low level of cytokinins.     

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.     

Contribution of the Pentose Phosphate Pathway and Glycolytic Pathway to Dormancy Breakage and Germination of Peanut (Arachis hypogaea L.) Seeds

Levels of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADPH oxidoreductase, E.C. 1.1.1.49 [EC] ) 6-phosphogluconate dehydrogenase(6-phospho-D-gluconate : NADP⁺ oxidoreduc tase, E.C. 1.1.1.44 [EC] )and aldolase (fructose 1, 6-diphosphate, D-glyceraldehyde, 3-phosphatelyase, E.C. 4.1.2.13 [EC] ) were assayed in the seeds of geneticallydormant and non-dormant pure lines of groundnut. In dormantlines cotyledons showed increased levels of activity of G-6-PDHand 6-PGDH during dry storage after-ripening. While the embryonicaxis did not exhibit detectable levels of enzyme activitiesimmediately after harvest, the activity started after a lapseof time during dry storage. When seeds of dormant lines wereincubated with kinetin (6-furfurylaminopurine) a distinct increasein the levels of both the enzymes was observed. The levels ofaldolase activity gradually decreased in the cotyledons andincreased in the embryonic axis of both control and kinetintreated seeds during the period of after-ripening. Comparedto control, kinetin treatment increased the aldolase activityin the embryonic axis and decreased it in the cotyledons. In non-dormant lines the activity of both the enzymes of PPpathway increased sharply both in the cotyledons and embryonicaxis while aldolase activity decreased in the cotyledons andincreased in the embryonic axis during germination i.e., from24 h to 96 h of germination. Abscisic acid caused inhibitionof enzyme activities to a large extent. Key words: PP pathway, dormancy breakage, germination, peanut     

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.     

A meta-analysis of the effects of frugivory (endozoochory) on seed germination: role of seed size and kind of dormancy

Heretofore, no study has determined how germination of ingested seeds is affected by the kind (class) of dormancy nor by seed dormancy x seed size interaction. Thus, we aimed to determine the effects of seed size, kind of dormancy and their interaction on germination of defecated seeds using a meta-analysis. We collected data for 366 plant species in 97 plant families from 76 publications. In general, gut passage significantly increased germination percentage of defecated seeds by 5% compared with that of control seeds. Germination percentages of non-dormant, physiologically dormant, and morphologically/morphophysiologically dormant seeds (all water-permeable) significantly decreased after gut passage by 40, 18, and 14%, respectively, compared with control seeds (non-gut-passed). Changes in germination percentage of seeds with physical dormancy (water-impermeable) were positive, and gut passage increased germination by 69% compared with control seeds. Germination of small seeds decreased 8% after gut passage, whereas germination of both medium and large seeds increased by 18%. However, changes in germination percentage differed between categories of seed size in each class of dormancy. In physically dormant seeds, germination of all seed sizes improved after gut passage, and the magnitude of increase was higher for large than for medium and small seeds. Thus, gut passage increased germination of medium-size water-permeable seeds (physiologically dormant and morphologically/morphophysiologically dormant) more than it did for large and small seeds. However, gut-passage decreased or did not change the germination percentage of non-dormant seeds. Seed size and kind of dormancy should be included in studies on the effect of gut passage on germination.     

Seed Treatment Optimizes Benefits of Seed Bank Storage for Restoration‐Ready Seeds: The Feasibility of Prestorage Dormancy Alleviation for Mine‐Site Revegetation

Dormant seeds of 18 species from 9 families covering a diverse range of seed dormancy syndromes and life histories from the southwest Australian biodiversity hotspot were assessed for germinability following storage at 15–25°C for 36 months. A total of 10 species with physical dormancy (PY) and 8 with either physiological dormancy (PD) or morphophysiological dormancy (MPD) were assessed as part of the study. Prior to storage, germination from dormant seeds was 1–27%, rising to 41–100% following specific dormancy‐breaking treatments. When seed dormancy was removed prior to storage for 36 months seeds from all species were found to maintain a nondormant state and germinate to a similar level to that observed at the beginning of the experiment (44–100%). Likewise, seeds that did not receive a prestorage dormancy‐breaking treatment maintained a dormant state (0–50% germination) and subsequently responded well to a dormancy‐breaking treatment immediately prior to germination assessment (49–99%). There were minimal differences in response to dormancy‐breaking treatments before and after 36 months storage (average 4–6% difference) and in the germination responses observed between both storage environments assessed (15°C/15% eRH or 15–25°C air dried). Based on these findings, storing seeds in a nondormant state does not alter germinability and this approach provides significant benefits to current seed‐based restoration programs through reduction of double handling and improved seed use efficiency.     

葡萄属种子发育的物候、萌发行为及其对冷层积的反应

以3个起源和分布中心的不同葡萄种及其杂交种作为实验材料, 观察了开花、果实和种子成熟的物候, 研究了成熟种子的基本特性和最初萌发率以及冷层积对成熟种子萌发的影响。结果表明, 葡萄种子具有休眠特性, 生理成熟时都具有分化完全的胚, 在休眠解除过程中胚的形态不发生变化; 美洲种休眠最深, 欧亚种休眠最浅, 其它种类的葡萄种子的休眠程度介于美洲种和欧亚种之间; 不同种葡萄花期、果实和种子成熟过程的物候存在差异, 果实成熟与种子成熟不同步, 其间隔时间越短, 种子休眠程度越深。冷层积60天能有效地解除东亚种、欧亚种、欧山杂种和蘡欧杂种的种子休眠, 但对解除美洲种、美洲种种间杂种和欧美杂种的种子休眠效果较差。