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.     

Effect of a plant-derived smoke extract, N6-benzyladenine and gibberellic acid on the thermodormancy of lettuce seeds

The effects of a plant-derived smoke extract, BA and GA₃ on the thermodormancy of Grand Rapids lettuce seeds were studied. Thermodormant lettuce seeds treated either with BA, GA₃ or smoke extract alone did not germinate. Combinations of BA with smoke extract and BA with GA₃ were most effective in overcoming induced thermodormancy. GA₃ plus smoke did not break the induced thermodormancy. The effects of the different treatments on germination were concentration dependent. BA was most effective at 10^–5 to 10^–3 M in combination with smoke dilutions 1:5,000 to 1:1,000,000 in overcoming thermodormancy.Abbreviations BA N⁶-benzyladenine; - GA₃ gibberellic acid; - SM smoke extract     

Tolerance of kikuyu grass to long term salt stress is associated with induction of antioxidant defences

Primary dormancy in A. retroflexus seeds wascompletely broken by dry storage or ethylene treatment and partially removedwith GA₃. Norbornadiene counteracted the dormancy breaking action ofethylene and GA₃. The GA₃ effect was lowered bycobaltous ions. ABA increased the ethylene requirement in primary dormant seeds.Dormant seeds had a similar or different ability to produce ethylene and ACCoxidase in vivo activity than did non-dormant seeds,depending on the period of incubation. Dormant seeds contained less endogenousACC than non-dormant seeds. Thus, ethylene seems to play an essential role inthe release of primary dormancy in A. retroflexus seeds.Ethylene also participates in the release of dormancy achieved by GA₃treatment. The results indicate that both ethylene biosynthesis and action isinvolved in the control of primary dormancy in Amaranthusretroflexus seeds.     

Stimulatory effect of ethephon,ACC, gibberellin A3 and A4+7 on germination of methyl jasmonate inhibited Amaranthus caudatus L. seeds

Methyl jasmonate (JA-Me) inhibited or retarded germination of Amaranthus caudatus seeds in darkness at 24°C, Ethephon, ACC and gibberellins (GA₃ or GA₄₊₇) partially or completely reversed this inhibition depending on the concentration of JA-Me applied. Both ethephon and the gibberellins were more effective than ACC. Both GA₃ and GA₄₊₇ enhanced the stimulatory effect of ethephon or ACC on germination of seeds inhibited by JA-Me.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - JA jasmonic acid - JA-Me methyl jasmonate     

Changes in endogenous abscisic acid levels during dormancy release and maintenance of mature seeds: studies with the Cape Verde Islands ecotype,the dormant model of<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis>

Mature seeds of the Cape Verde Islands (Cvi) ecotype of Arabidopsis thaliana (L.) Heynh. show a very marked dormancy. Dormant (D) seeds completely fail to germinate in conditions that are favourable for germination whereas non-dormant (ND) seeds germinate easily. Cvi seed dormancy is alleviated by after-ripening, stratification, and also by nitrate or fluridone treatment. Addition of gibberellins to D seeds does not suppress dormancy efficiently, suggesting that gibberellins are not directly involved in the breaking of dormancy. Dormancy expression of Cvi seeds is strongly dependent on temperature: D seeds do not germinate at warm temperatures (20–27°C) but do so easily at a low temperature (13°C) or when a fluridone treatment is given to D seeds sown at high temperature. To investigate the role of abscisic acid (ABA) in dormancy release and maintenance, we measured the ABA content in both ND and D seeds imbibed using various dormancy-breaking conditions. It was found that dry D seeds contained higher amounts of ABA than dry ND after-ripened seeds. During early imbibition in standard conditions, there was a decrease in ABA content in both seeds, the rate of which was slower in D seeds. Three days after sowing, the ABA content in D seeds increased specifically and then remained at a high level. When imbibed with fluridone, nitrate or stratified, the ABA content of D seeds decreased and reached a level very near to that of ND seeds. In contrast, gibberellic acid (GA₃) treatment caused a transient increase in ABA content. When D seeds were sown at low optimal temperature their ABA content also decreased to the level observed in ND seeds. The present study indicates that Cvi D and ND seeds can be easily distinguished by their ability to synthesize ABA following imbibition. Treatments used here to break dormancy reduced the ABA level in imbibed D seeds to the level observed in ND seeds, with the exception of GA₃ treatment, which was active in promoting germination only when ABA synthesis was inhibited.Abbreviations ABA Abscisic acid - Cvi Cape Verde Islands - D Dormant - GA Gibberellin - GA₃ Gibberellic acid - ND Non dormant     

Effects of gibberellic acid,ethephon, and 1-aminocyclopropane-1-carboxylic acid on germination ofAmaranthus caudatus seeds inhibited by paclobutrazol

The specific inhibitor of gibberellin biosynthesis, (2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol), inhibited germination ofAmaranthus caudatus L. seeds. Addition of gibberellic acid (GA₃), 2-chloroethylphosphonic acid (ethephon), or 1-aminocyclopropane-1-carboxylic acid (ACC) effectively antagonized inhibition. Ethephon was found to be the most efficient antagonist. The transfer of seeds after 1 day's incubation in paclobutrazol to solutions of GA₃ or ethephon reversed the inhibition, the effect increasing with increasing concentration of GA₃ or ethephon. Seeds incubated in paclobutrazol for 5 days decreased sensitivity to GA₃ and ethephon.     

Endogenous ethylene and reversing methyl jasmonate inhibition of Amaranthus caudatus seed germination by benzyladenine or gibberellin

BA at 10^–5 M, GA₃ at 3×10^–4 M or GA₄₊₇ at 3×10^–5 M partially or largely reversed the inhibition of Amaranthus caudatus seed germination due to JA-Me. BA or GA₃ did not affect ethylene production and ACC oxidase activity in vivo in the presence of JA-Me before radicle protrusion. However, both increased ethylene production after 72 h of incubation, when the reversal of the JA-Me inhibition of seed germination was observed. AVG at 3×10^–4 M decreased ethylene production when it was applied simultaneously with BA and JA-Me or GA₃ and JA-Me, but it had no effect on seed germination. NBD almost completely reversed the stimulatory effect of BA, GA₃ or GA₄₊₇ on the germination of seeds in the presence of JA-Me. Exogenous ethylene reversed the inhibitory effect of NBD. The results indicate that action of endogenous ethylene is involved in the response of JA-Me inhibited seeds to BA or GA_s.     

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.     

Involvement of phytohormones in germination of dormant and non-dormant oat (Avena sativa L.) seeds

Oat seeds are susceptible to high temperature dormancy. Dormant grainsdo not germinate at 30 °C unless afterripened, dry, for severalweeks. Isolated embryos of dormant grains do germinate, especially ifGA₃ is added to the germination medium. ABA inhibits germinationproportionally to the concentration applied and GA₃ can overcome theABA inhibitory effect. Measurements of endogenous ABA and several GAs revealedthat the initial levels of ABA in dormant and non-dormant grains were quitesimilar. But, endogenous ABA in non-dormant seeds almost disappeared within thefirst 16 h of imbibition, while the amount in dormant grains haddecreased by less than 24%. The level of GA₁₉ in non-dormant seedswas higher, and GA₁₉ appears to be converted to GA₂₀ within the first 16h. The GA₂₀ was converted to GA₁ at leastduring the first 48 h of the germination process. Bothphytohormones thus appear to be involved in the germination process ofnon-dormant seeds. ABA first declines, while GA₁ is producedduring the first 16 h of imbibition to allow proper germination.Indormant grains the level of ABA remained high enough to prevent germinationduring at least a week and precursor GAs were not converted to GA₁.     

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.     

The responsiveness of Avena fatua aleurone protoplasts to gibberellic acid

The sensitivity to gibberellic acid (GA₃) of aleurone protoplasts isolated from a single harvest of an inbred line of Avena fatua seed that had been after-ripened over anhydrous CaCl₂ at 25±2°C and 4±2°C for three years was assessed. Protoplasts isolated from aleurones of seed stored at 25°C produced substantially more -amylase in response to 10^–7 M GA₃ than those isolated from aleurones of seed stored at 4°C. The apparent difference in responsiveness does not appear to be due to a change in the duration of the lag phase between addition of GA₃ and the production of -amylase. The dose response of aleurone protoplasts to GA₃, measured as -amylase production, is complex and appears to have three phases. Protoplasts from seed stored at both temperatures respond appreciably to 10^–14 M GA₃. With increasing concentrations of GA₃, up to 10^–9 M, -amylase production increases similarly in protoplasts from both lots of seed, reaching a level approximately 2.7–3.8 times greater than when no GA₃ is applied. GA₃-induced -amylase production increases markedly as the concentration is raised from 10^–9 M to 10^–6 M, and the response then appears to be saturated. Over this part of the response curve protoplasts from the two seed lots differ markedly in their responsiveness to GA₃. Those from seed stored at 25°C produce considerably more -amylase, >130-fold higher than the minus GA₃ control, than those from seed stored at 4°C, <35-fold higher than the minus GA₃ control. This apparent difference in the responsiveness of aleurone protoplasts to GA₃ could be correlated with the loss of embryo dormancy in seed stored at 25°C. Seed stored at 4°C retained the dormancy characteristics present immediately after harvesting.     

Gibberellin induces endopeptidase activity in detached cotyledons of Pisum sativum

Endopeptidase activity in cotyledons of 5-day seedlings of Pisum sativum increased rapidly during germination. However, the increase of the activity in detached cotyledons was depressed. We examined whether a growth regulator can be substituted for the embryonic axis on the development of endopeptidase activity. As monitored by an assay with azoalbumin, the development of endopeptidase activity from crude extracts of detached cotyledons appeared to be slightly accelerated by incubation with 10^–5 M GA₃. However, the pattern after gelatin-polyacrylamide gel suggested that the activity induced in detached cotyledons during a 5-d incubation at 10^–7 M GA₃ was the same as that in attached ones during germination for 5 days and an even greater increase in activity was obtained with 10^–5 M GA₃. These results suggest that GA₃ from the embryonic axis induces endopeptidase activity in attached cotyledons at the first stage of germination.Abbreviations ABA abscisic acid - IAA indole-3-acetic acid - GA gibberellic acid     

Membrene turnover in imbibed and dormant embryos of the wild oat (Avena fatua L.)

A comparative study of protein synthesis has been carried out with embryos excised from dormant (D) and non-dormant (ND) caryopses of the wild oat. Although D embryos imbibed in water or ND embryos imbibed in abscisic acid do not germinate, they incorporate [¹⁴C]leucine into TCA-insoluble material for the first 48 h as readily as embryos that do germinate (ND embryos imbibed in water, or D embryos imbibed in gibberellic acid). Pulsechase experiments with [¹⁴]leucine show that in both D and ND embryos the proteins associated with the membranes undergo turnover. The rates of decay of incorporated radioactivity are similar in both dormant and germinating embryos up to 98 h following embryo excision. Fractionation of the membrane proteins in SDS-polyacrylamide gels indicates that the different polypeptides have different rates of turnover. It is concluded that membrane proteins in imbibed D embryos are in a state of constant turnover, and that this is a part of the replacement processes necessary to maintain the integrity of hydrated cells. The continuation of such synthetic events could account for long term survival of dormant Avena fatua in the imbibed state.Abbreviations CCRSE cytochrome relative stain equivalents - D dormant - ND nondormant - ABA abscisic acid - GA gibberellic acid GA₃     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

Overwintering and growth regulator treatment effects on celery (Apium graveolens L.) flowering and seed production

Celery (Apium graveolens L.) plants cv. Jason overwintered in a polythene tunnel flowered earlier and grew taller than similar plants given a 10-week cold-treatment at 5°C prior to transplanting in the same tunnel in mid-February. However, there was no significant difference in the yield of seeds obtained from both treatments, plants grown at a density of 4m^-2 yielded less seeds than those at 2m^-2, though the yield per unit area was slightly higher from the high density treatment. Treatment with 100 mgl^-1 GA₃ applied twice just prior to flowering and during anthesis increased flower stalk, flower pedicel and stamen length but delayed flower opening and seed ripening and decreased seed set and seed yield. Treatment with a mixture of 1000 mgl^-1 GA₄ and GA₇ plus 1000 mgl^-1 ethephon on three occasions during seed ripening decreased seed yield and reduced seed germination though those seeds capable of germinating were less dormant than seeds from untreated plants. The size distribution of seeds was unaffected by any treatment other than the preseeding spray with GA₃ which reduced the percentage of medium-size seeds.     

Interactions of phytochrome and exogenous gibberellic acid on germination of Lamium amplexicaule L. seeds

Summary In henbit (Lamium amplexicaule L.) seeds, micromolar levels of gibberellic acid (GA₃) introduced at the onset of imbibition markedly stimulated germination. Sensitivity to GA₃ stimulation is rapidly lost if the seeds are pretreated in darkness at 15° for 1 day or more on a water substrate. Loss in responsiveness to GA₃ is hastened by pretreatment in far-red or by dark pretreatment at higher (25°) temperatures and is reduced by pretreatments at 5°. A portion of the lost responsiveness to GA₃ at 15° or higher is attributable to onset of a secondary dormancy, but the results of far-red treatment at 5°, where secondary dormancy is not a factor, clearly indicate that some phytochrome in the far-red absorbing form increased the effectiveness of gibberellic acid action.Abbreviations GA gibberellin - GA₃ gibberellic acid - R red light - FR far-red - P_r red-absorbing form of phytochrome - P_fr far-red-absorbing form of phytochrome Mention of a trade name or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable     

The Role of Hormones in Controlling the Mechanically Induced Dormancy of Suaeda spp.

Seeds of several taxa in the genus Suaeda with a mechanically resistant testa were stimulated to germinate with applications: of 1 × 10^?4M gibberellic acid (GAg). The barley endosperm bioassay indicated that the non-dormant taxa, S. macrocarpa, had high initial gibberellin-like activity while the dormant varieties, S. flexilis and S. vulgaris, had not. Soaking for 2 days increased the gibberellin-like activity of the dormant taxa. At 20 days hormonal activity was repressed, and few seeds germinated. The barley endosperm bioassay also revealed an inhibitor activity in the dormant varieties. The wheat coleoptile bioassay and UV irradiation of thin layer chromatograms indicated that an inhibitor occurred at the Rf of cochromatographed ABA and that it had similar inhibitory characteristics.     

Responsiveness of Orobanche ramosa L. Seeds to GR 24 as Related to Temperature, Oxygen Availability and Water Potential During Preconditioning and Subsequent Germination

Broomrape (Orobanche ramosa L.) is a root holoparasite responsible for important yield losses in numerous crops, particularly in the Mediterranean area. In this paper, the effects of temperature, oxygen concentration and water potential of the medium on broomrape seed germination were investigated. Seeds became able to germinate in the presence of a strigol analogue (GR 24) only after a preincubation period for at least 3 days at 20 °C. Their responsiveness to GR 24 increased with increasing duration of their preconditioning at 20 °C, and was optimal after 2–3 weeks. The preconditioning treatment was effective at temperatures ranging from 10 to 30 °C. At the optimal temperature (20 °C), it required at least 1% oxygen in the atmosphere and remained effective at a water potential of the medium of –2 MPa. A too prolonged preincubation of seeds at sub- or supraoptimal temperatures (5 and 30 °C) resulted in induction of a secondary dormancy. Seeds preconditioned for 14 days at 20 °C germinated in the presence of 1 mg L^–1 GR 24 at temperatures ranging from 10 to 25 °C, and the thermal optimum was the same (20 °C) than that of preconditioning. At 20 °C, seeds were able to germinate in the presence of GR 24 under atmospheres containing at least 3% oxygen and at a water potential of the medium as low as –3 MPa. The differences observed in the effects of environmental factors on preconditioning efficiency and germination of preconditioned seeds suggest that both processes involve different mechanisms. The results obtained might also help to better understand the regulation of O. ramosa spread in temperate areas.     

Necessity of gibberellin for stimulatory effect of KAR1 on germination of dormant Avena fatua L. caryopses

Avena fatua L. florets (caryopses enclosed by lemma and palea) were partially dormant at 10–20 °C and did not germinate at temperatures outside this range. After-ripening florets at 25 °C for 12 weeks completely removed dormancy. Caryopses (florets without lemma and palea) were able to germinate totally at 20 °C. Karrikinolide (KAR₁) and gibberellic acid (GA₃) applied at 10–25 °C partially or markedly induced germination of dormant florets and caryopses, respectively. Both florets and caryopses were more sensitive to KAR₁ than to GA₃. To obtain similar effects, 1,000 to 10,000 times lower concentrations of KAR₁ than GA₃ were required. After-ripening with time gradually increased sensitivity of caryopses to these regulators. Likewise, after-ripened, non-dormant caryopses were sensitive to KAR₁ and GA₃. Inhibitors of gibberellin biosynthesis, ancymidol, paclobutrazol and flurprimidol inhibited the effect of KAR₁. This inhibition was reversed by GA₃. Caryopses pre-incubated in water with ancymidol or paclobutrazol in the presence or absence of KAR₁ germinated completely but with different rates after transfer to GA₃. KAR₁ probably requires gibberellin biosynthesis to stimulate germination of dormant Avena fatua L. caryopses. Both KAR₁ and GA₃ increased α-amylase, β-amylase and dehydrogenases activities during imbibition before visible germination occurred.