Factors affecting the induction and release of secondary dormancy in Kalanchoë seeds

Abstract. Several short daily R irradiations are required from the first day of incubation on water to induce germination of Kalanchoë seeds. When the same light treatment is given after a prolonged dark incubation period at 20°C, secondary dormancy prevents germination. Factors controlling the induction and breaking of secondary dormancy have been investigated. The induction of secondary dormancy is very temperature dependent. Locally puncturing the seed coat strongly delays it. Secondary dormancy is not induced in the presence of GA₃ during the first 10 d of dark incubation, although this growth substance cannot induce dark germination. Prolonged or cyclic daily R irradiations can relieve secondary dormancy of seeds kept on water, even after a dark period of 20 d. A 24 h treatment at 4°C restores responsiveness to short R exposures of slightly secondarily dormant seeds. The synergism between GA₃ and Pfr in non-dormant Kalanchoë seeds, leading to high effectiveness of even one short FR irradiation, still occurs in seeds made secondarily dormant before transfer to GA₃, but more R or FR irradiations, in combination with GA₃, are required for the release of secondary dormancy. A combination of red light and 6-benzyl-aminopurine is ineffective in removing dormancy.     

Free and conjugated gibberellins in dormancy and germination of apple seeds

Halińska, A. and Lewak, St. 1987. Free and conjugated gibberellins in dormancy and germination of apple seeds.
The presence of gibberellin A₄ (GA₄) was confirmed in partly stratified seeds of apple ( Malus domestica Borb., cv. Antonówka) by mass spectrometry of the methyl ester. Levels of free and conjugated gibberellins A₄₊₇ and A₉ changed during drying of mature seeds, during cold and warm stratification, as well as during germination of dormant and non-dormant embryos. The temporary rise in GA₄₊₇ during cold stratification and during the culture of dormant embryos as well as the lack of it under conditions of warm stratification, allowed us to postulate a role for GA₄₊₇ in the removal of dormancy. In addition, GA₉ was absent in dormant embryos and increased during cold stratification and during the culture of non-dormant embryos. This suggests the involvement of GA₉, in induction of normal development of the seedling. The equivalence between changes in free and conjugated GAs suggests that formation and hydrolysis of conjugates are involved in the control of the physiologically active levels of free GA₄₊₇ and GA₉.     

Secondary dormancy in Avena fatua: Effect of temperature and after-ripening

To evaluate the effect of after-ripening on secondary dormancy induction in pure genetic lines of Avena fatua L., seed samples were periodically removed from standard conditions of storage and the caryopses then subjected to anoxia. Anoxia did not induce secondary dormancy in SH430, a line characterized by no primary dormancy at harvest maturity; secondary dormancy was induced in caryopses of other lines that had been after-ripened to over-come primary dormancy ranging in duration from a few days (CS40, CS166) to several months (AN51, AN127). Germination response to low GA₃ concentrations indicated that secondary dormancy in CS40 and CS166 was less intense than in AN51 and AN127. The longer the period of dry after-ripening prior to anoxia treatment, the lower the intensity of secondary dormancy induced. After a period of dry after-ripening, which was characteristic for each line, anoxia became an ineffective dormancy-inducing treatment. Caryopses selected for their response to dormancy induction by anoxia were subjected to temperatures from 5 to 35°C to investigate the effect of low (5 to 18°C) and high (20 to 35°C) temperatures on both thermo- and secondary dormancy induction. SH430 was not responsive to any treatment, while CS40, CS166 and AN51 were induced into a thermo-dormancy at temperatures above 20°C and CS166 and AN51 were induced into secondary dormancy by anoxia at temperatures from 5 to 35°C. The effect of anoxia on secondary dormancy induction in a range of pure genetic lines is discussed with reference to primary dormancy, after-ripening and temperature.     

Effects of temperature and drying rate during dehydration of celery seeds on germination, leakage and response to gibberellin and cytokinin

A high temperature treatment of 32°C which prevents dehydration injury in celery seeds imbibed for 3 days at 17°C and then dried at 20°C, reduced leakage during rehydration, compared with seeds not given the high temperature treatment. Treatments which would normally release celery seeds from dormancy, such as low temperature imbibition or gibberellin (GA_4/7) and benzyladenine (BA) applications had little effect on the germination of seeds exhibiting desiccation injury. However, GA_4/7 did induce splitting of the seed coat and swelling of the endosperm, and this effect was enhanced by BA. It is suggested that in celery seeds high temperature prevents irreversible embryo damage, including membrane damage, caused by drying.     

Antagonistic effects of abscisic acid and gibberellic acid on the breaking of dormancy of Fagus sylvatica seeds

Study of the factors involved in the dormancy of Fagus sylvatica seeds shows that such dormancy is due partly to the seed coats and partly to endogenous factors. Seed coat removal accelerates both the release from dormancy and the effects of the other treatments that abolish it. The dormancy of these seeds is eliminated by cold treatment at 4°C over a period longer than 8 weeks, and exogenous application of abscisic acid (ABA) reverses the effects of low temperature, the seeds remaining in an ungerminated state. Additionally, ABA reduces protein synthesis but slightly increases RNA synthesis, which suggests its involvement in the synthesis of RNAs related to this process. In vitro translation of the RNAs isolated from these seeds shows that ABA delays the disappearance of at least 2 polypeptides (of ca 22 and 24 kDa), which are abundant in dormant seeds and under conditions that prevent the release from dormancy, but which disappear under treatments that abolish it. Exogenous application of gibberellic acid (GA₃) proved to be efficient in breaking the dormancy of these seeds and in substituting for cold treatment as well as in antagonizing the effects of ABA on the synthesis of both DNA and proteins. GA₃ also accelerates the disappearance of the two polypeptides abundant in dormant seeds and in ABA-treated seeds. These findings suggest that both ABA and GA₃ could be involved in the regulation of nucleic acid and protein metabolism during dormancy, acting antagonistically in these processes and, specifically, in the regulation of the synthesis of the two proteins that appear to play a role in the maintenance of dormancy in these seeds.     

Role of endogenous gibberellins in germination of melon (Cucumis melo) seeds

The effect of the plant growth retardants ancymidol. mefluidide and uniconazole on germination of two melon accessions differing in their ability to germinate at 14°C was examined. The accessions were the cold sensitive Noy Yizre'el and the cold tolerant Persia 202. The three growth retardants were able to delay the germination of intact Noy Yizre'el seeds, but did not affect that of intact Persia 202 seeds. On the other hand germination of decoated seeds of both accessions was unaffected by these inhibitors at normal oxygen concentration, but was inhibited at 5% oxygen. When gibberellin-like activity was measured by a dwarf rice biological assay following HPLC fractionation, it was found that seeds of Persia 202 contained much more gibberellin-like activity than Noy Yizre'el seeds. Among the extracted compounds several endogenous gibberellins were identified by combined gas chromatography-mass spectrometry (GC-MS). They included GA₄, GA₂₀, GA₁ and GA₃ in Noy Yizre'el and GA₃₄, GA₂₀, GA₁ and GA₈ in Persia 202. It is suggested that the better germination of intact Persia 202 seeds, compared to Noy Yizre'el seeds at low temperature and low oxygen concentration, is due to a higher endogenous level of GA and a better seed coat permeability to oxygen.     

Gibberellin status and responsiveness in shoots of tall and dwarf genotypes of diploid rye (Secale cereale)

The recessive dwarfing alleles of rye ( Secale cereale L.), ct1 and ct2 , caused a 35–55% reduction in the length of leaf 2 compared with corresponding tall lines grown at both 10°C and 20°C. The dwarf lines were 45–50% as responsive to applied GA₃ as the tall lines at 20°C but the absolute GA-responsiveness of the dwarfs was greater at 10°C than at 20°C. There was no significant difference in the contents of GA₁₉, GA₂₀, GA₂₉, GA₁, GA₃ and GA₈ in the leaf extension zone of tall and dwarf seedlings grown at 20°C. It was concluded that the mechanism whereby GA homeostasis is maintained is functional in both tall and dwarf lines despite marked differences in leaf extension rate. The recessive rye mutations may cause loss of function late in the GA-cell elongation pathway or, alternatively, indirectly affect GA-responsiveness in vegetative tissues. The genetic and physiological evidence indicates that ct1 and ct2 are unrelated to the GA-insensitive Rht genes in hexaploid bread wheat.     

Effects of sodium chloride on prevention of thermodormancy, ethylene and protein synthesis and respiration in Grand Rapids lettuce seeds

Grand Rapids lettuce ( Lactuca saliva L.) seeds entered into a state of secondary dormancy (thermodormancy) when they were imbibed at 40°C for 72 h. The effect of 40°C in inducing thermodormancy was largely reduced by imbibing seeds at 40°C in solutions of polyethylene glycol (PEG), mannitol and NaCl. Despite similar water potentials of solutions, NaCl pretreatment was more effective. Subsequent germination in the dark at 25°C of saline, high-temperature-pretreated seeds required only gibberellic acid (GA₃), as was the case with nonthermodormant seeds. Thermodormancy reduced total respiratory capacity (V_T) and increased the ratio of alternate pathway (V_alt) to cytochrome pathway (V_cyt) respiration. This was prevented by saline pretreatment. Ethylene production and protein synthesis were depressed in thermodormant seeds, and this was partly alleviated by saline pretreatment. The patterns of protein synthesis in saline- and nonsaline-freated seeds at 40°C were similar, differing only in that the saline treated seeds produced in addition a 78 kDa polypeptide. The pattern of protein synthesis at 40°C differed significantly from that at 25°C.     

Priming relieves dormancy in lettuce seeds independently of changes in osmotic constituents

The relationship was studied between germination and dormancy of lettuce seeds ( Lactuca sativa L. cv. Musette) and both soluble amino nitrogen metabolism and osmotic potential. Germination at 15°C in darkness coincided with a rise in the levels of free amino acids and total soluble amino nitrogen compounds and in the activity of glutamine synthetase (GS, EC nr. 6.3.1.2). In further experiments GS activity was used as indicator of soluble amino nitrogen metabolism. GS activity increased after the start of growth indicated by an increasing intolerance to desiccation. At 30°C seeds did not germinate, unless dormancy was broken beforehand during incubation at 2° or 15°C (priming). The alleviation of dormancy occurred much earlier than the rise in the activity of GS. Priming at 15°C in polyethylene glycol instead of water retarded the breaking of dormancy and at –1.28 MPa even stimulated the induction of secondary dormancy, but did not prevent a continued rise in the activity of GS. GS activity was also not reduced during induction of secondary dormancy by dehydration of primed seeds, which antagonized the beneficial effect of priming. Psychrometric measurements showed that osmotic potential (Ψ_π) of the seeds remained constant during prolonged priming in polyethylene glycol at 15°C. During incubation in water, Ψ_π increased both prior to and after the moment of germination to less negative values. It is concluded that changes in the level of dormancy in lettuce seeds occur independently of soluble amino nitrogen metabolism and of changes in Ψ_π.     

Embryonal dormancy in apple seeds is controlled by free and conjugated gibberellin levels in the embryonic axis and cotyledons

Halińska, A., Sińska, I. and Lewak, St. 1987. Embryonal dormancy in apple seeds is controlled by free and conjugated gibberellin levels in the embryonic axis and cotyledons.
Free and conjugated gibberellins (GAs) A₄₊₇ and A₉ were determined in embryonic axes and in cotyledons of seeds of apple ( Malus domestica Borb., cv. Antonó wka) during breaking of dormancy under cold stratification. In both organs, the maximum level of free GA₄₊₇ was found at day 30 of stratification, but the concentration was 700 times higher in axes than in cotyledons. Comparison of changes in free and conjugated GA₄₊₇ levels during stratification allow us to suggest that the accumulation of free hormone in axes is, at the most, to 40% due to release from conjugates already present in the axis; that maximally 20% is derived from hydrolysis of cotyledonary conjugates translocated to axes; and that at least 40% originate from the novo biosynthesis of the hormone. Free and conjugated GA₉ levels were similarly altered in axes and in cotyledons, markedly increasing at the end of afterripening. Both release of the free hormone from conjugates and biosynthesis of GA₉, appeared to be involved in that increase; no translocation of free or bound GA₉, between axes and cotyledons was demonstrated.     

Differences in the nature of thermodormancy and far-red dormancy in lettuce seeds

Lettuce seeds cv. Noran germinate at 23°C in light as well as in darkness. However dormancy can be induced either by a long exposure (24 h) to far-red radiation or by an exposure of 48–72 h to a temperature of 37°C. The difference in response of these two types of dormant seeds to conditions inducing germination indicate that in both types P_fr is inactivated, but that a dark process required for immediate action of P_fr does not proceed at 37°C as it does during far-red radiation.     

Gibberellin-biosynthesis and -sensitivity mediated stimulation of seed germination of Sisymbrium officinale by red light and nitrate

Light stimulated seed germination of Sisymbrium officinale (L.) Scop, (hedge mustard) by means of two different mechanisms. Light effect I was absolutely dependent on the simultaneous presence of nitrate. Without nitrate, red (R) irradiated seeds did not escape from the antagonizing action of far-red (FR) irradiation. The data indicated that nitrate acted as a cofactor at the level of the FR absorbing form of phy-tochrome (P_fr). The combined action of R and nitrate could be replaced by addition of the gibberellins 4 and 7 (GA,₄₊₇). This action could be inhibited by the growth re-tardant tetcyclacis, an inhibitor to GA biosynthesis in cell free systems and intact plants. The action of tetcyclacis was fully neutralized by GA₄₊₇. It is concluded that the combination of R and nitrate stimulated GA biosynthesis. Omission of nitrate from the incubation medium enabled the study of light effects apart from G A biosynthesis. In such conditions R stimulated the sensitivity to GA₄₊₇, (light effect II). The two light effects could also be distinguished by their different reactions to the temperature of a pre-treatment in water and darkness. The sensitivity to R and nitrate was subject to breaking and induction of dormancy. Both processes were stimulated at rising temperatures. Due to a different optimum, breaking of dormancy prevailed at lower temperatures and induction of secondary dormancy at more elevated temperatures. The sensitivity to GA₄₊₇ rose and fell in a comparable way during dark incubation at a broad range of temperatures. The capacity of light to stimulate GA₄₊₇, action did not diminish at higher temperatures, it even tended to rise. The study indicated that seed germination is regulated by an increase in both the levels of GAs and the sensitivity to GAs.     

The effects of phosphate supply on uptake of potassium ions, 2,4-D and atrazine by wheat and maize

The germination percentage of peach [ Prunus persica (L.) Batsch cv. Halford] seeds at 20°C was low (< 20%) after incubation at 5°C for as long as 35 days, but then increased considerably (> 40%) when the seeds were maintained at 5°C for longer than 42 days. Four zones of gibberellin-like activity were found in partially purified seed extracts. Gibberellin-like activity remained low in seeds incubated at 5°C for as long as 28 days, but increased significantly in three of these zones after 35 days, and in the fourth zone after 49 days. The increase in gibberellin-like activity was evident prior to the transfer of the seeds to 20°C. Moreover, seeds maintained at 5°C germinated at this temperature after 63 days. For seeds incubated and germinated at 20°C, both the germination percentage and the gibberellin-like activity remained low throughout the experimental period. Application of the growth retardant paclobutrazol to seeds after 28 days of a 49 day total incubation period at 5°C did not substantially reduce seed germination, although the increase in gibberellin-like activity was prevented. Seeds did, however, require a longer time to germinate after transfer to 20°C and were dwarfed in appearance. Application of GA₃ to seeds prior to stratification increased the percentage germination of seeds only when they had been incubated at 5°C for at least 35 days. The major changes in gibberellin-like activity are, therefore, associated not so much with the processes which allow germination to take place in peach, but more with those processes which allow normal growth and development of the seedling.     

Ethylene-dependent action of gibberellin in seed germination of Amaranthus caudatus

The role of gibberellins in the germination of seeds of Amaranthus caudatus L. was examined. Tetcyclacis (BAS 106), an inhibitor of gibberellin biosynthesis, inhibited germination of the seeds. The inhibition caused by BAS 106 was antagonised by gibberellin A₄₊₇ (GA₄₊₇). Ethephon (2-chloroethylphosphonic acid) and 1-aminocyclopropane-1-carboxylic acid (ACC) could replace GA₄₊₇. Ethephon and ACC counteracted also the side effects of BAS 106 that are not reversible by GA₄₊₇. The rate of seed germination was not increased by gibberellin in the presence of aminoethoxyvinylglycine (AVG). AVG increased the effect of BAS 106. GA₄₊₇ could not reverse the effect of BAS 106 when AVG was simultaneously applied. The results indicate that the biosynthesis of endogenous gibberellins may be required for germination of A. caudatus seeds and that main physiological effects of GA₄₊₇ on seed germination may depend on ethylene biosynthesis.     

Germination of annual celery (Apium graveolens) seeds: Inhibition by paclobutrazol and its reversal by gibberellins and benzyladenine

Plants of annual celery ( Apium graveolens L.) were treated with paclobutrazol during anthesis. Seeds collected from the treated plants showed a marked reduction in germination in light and failed to germinate in the dark. Application of GA_4/7 to the imbibition solution reversed the inhibitory effect of paclobutrazol while gibberellic acid (GA₃) was ineffective. Benzyladenine (BA) interaction with GA_4/7 was light and concentration-dependent. At relatively low concentrations in the dark there was a synergistic effect, but at higher concentrations, especially in the light, BA, antagonized the GA_4/7 effect. Seedlings emerging from the seeds from paclobutrazol-treated plants were only slightly shortened. It is suggested that paclobutrazol applied to the mother plants inhibited the biosynthesis of endogenous GAs, which normally enable the germination of annual seeds under unfavorable conditions. Exogenously applied GA_4/7 fulfills the function of the absent endogenous GAs.     

Induction of secondary dormancy in sunflower seeds by high temperature. Possible involvement of ethylene biosynthesis

High temperature (45°C) inhibits seed germinition and seedling sunflower ( Helianthus annuus L. cv. Mirasol). Treatment of imbibed seeds at 45°C for more than 48 h induces a secondary dormancy, which is associated with progressive decrease of germination ability at optimal temperature (25°C) as well as with abnormal seedling growth. Ethylene (55μl l⁻¹) and 2-chloroethylphosphonic acid (ethephon) (2.5 m M ) improve germination of thermodormant seeds at 25°C. but the abnormal growth of the seedlings remains. O₂-enriched atmosphere and dry storage improve germination and normal seedling growth. The induction of thermodormancy in sunflower seeds seems associated with loss of their ability to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. Possible effects of high temperature on membranes and ethylene forming enzyme (EFE) are discussed.     

Changes in sensitivity of Amaranthus retrof/exus L. seeds to ethylene during preincubation. I. Constant temperatures

Abstract. Germination responses of redroot pigweed ( Amaranthus retroflexus L.) seeds to ethylene were determined at 25, 30, 35, or 40° C after preincubation at various temperatures (15–35° C) for different periods (0.5–32 d). After 7 d preincubation, seeds showed a log-linear germination response to ethylene concentration in most of the temperature treatments. Sensitivity to ethylene increased with longer preincubation; response thresholds of 0.03−0.09 cm³ m⁻³ were observed after 32 d, compared to 0.18−1.6 cm³ m⁻³ after 7 d of preincubation. Preincubation at 15 or 20° C generally enhanced germinability, whereas 25 or 30° C produced secondary dormancy, which was readily broken with ethylene. Temperature during preincubation also significantly influenced the slope of the dose-response curve. The responses of preincubated redroot pigweed seeds to ethylene suggested that, in the field, seeds would probably not lose their sensitivity to this gas during prolonged burial in soil.     

Gibberellin levels and cold-induced floral stalk elongation in tulip

To investigate the role of gibberellins (GAs) in the cold requirement of tulip ( Tulipa gesneriana L. cv. Apeldoorn), bulbs were dry-stored at 5°C or at 17°C for 12 weeks prior to planting at 20°C. Only precooled bulbs showed rapid sprout growth and developed a full-grown flower. Endogenous GA levels were measured in sprouts and basal plates at the time of planting and in the second week after planting, by combined gas chromatography-mass spectrometry using deuterated internal standards. GA₄ was the major gibberellin. while GA₁, GA₉ and GA₃₄ were present in lower amounts. At the time of planting, sprouts from non-cooled bulbs contained significantly more GA₄ and GA₁, per sprout than those from precooled bulbs. Hence, there is no direct correlation between rapid sprout growth after planting and high GA levels at planting. In the second week after planting, floral stalks of precooled bulbs contained 2 to 3 times more GA₄ and its metabolite GA₃₄ per floral stalk and per g fresh weight than those of non-cooled bulbs. The results are discussed with regard to the role of gibberellins in the cold-induced floral stalk elongation of tulip.