The Effect of Oxygen on Endogenous Cytokinin Levels and Germination of Leucadendron daphnoides Seed

The soybean callus assay was used to study the effect of high oxygen tensions on the cytokinin levels of Leucadendron daphnoides Meisn. seed, where dormancy is apparently due to the restricting effect of the seed coat on oxygen diffusion to the embryo. High oxygen tensions led to a six-fold increase in germination compared to seed incubated in air and resulted in significant increases in butanol soluble cytokinins prior to visible germination. It is suggested that the primary effect of oxygen is to increase the rate of respiration and thus, to provide the energy required for the synthesis of butanol soluble cytokinins which leads to cotyledon expansion and subsequent radicle elongation. Present indications are that untreated seeds remain dormant due to low concentrations of butanol soluble cytokinins in their embryos.     

Changes in Endogenous Cytokinins of Lettuce Seed during Germination

Using the soybean callus bioassay it has been shown that dormant lettuce seeds (Lactuca sativa L. cv. Grand Rapids) contain large amounts of water soluble cytokinins and small amounts of butanol soluble ones. When the seeds are irradiated with red light, or imbibed with 5 mg/1 gibberellic acid in the dark, the total cytokinin content of the seeds decreases, the level of water soluble cytokinins decreases, and the level of the butanol soluble cytokinins increases. Far-red light does not reverse this effect completely although cytokinin activity in the butanol extracts decreases following such irradiation. It is proposed that the interconversion of cytokinins initiated by red light, or gibberellic acid in the dark, is one of the primary events leading to radicle elongation in light-sensitive lettuce seed.     

Development of seed coat-imposed dormancy during seed maturation in Cynoglossum officinale

The relationship between seed phenolics and appearance of seed coat–imposed dormancy during seed development in Cynoglossum officinale L. was studied. Up to 24 days after anthesis, seeds failed to germinate upon imbibition in Petri dishes at 25°C. At 44 days after anthesis, seeds were fully germinable; removal of seed coats did not improve their germination or O₂ uptake. At 72 days after anthesis, mature seeds at the base of the cyme did not germinate unless their coats were removed. Removal of seed coat also stimulated O₂ uptake at this harvest date. The methanol-soluble phenolic content of the seeds increased during the early stages of seed development, in both the seed coat and the embryo. As seed development continued, the methanol-soluble phenolic content of the embryo stabilized, but that of the seed coat declined. This decline was associated with an increase in the thioglycolic acid–soluble phenolics, presumably lignins, in the seed coat. These results suggest that polymerization of methanol–soluble phenolics into lignins in the seed coat during later stages of seed development renders the seed coat of C. officinale impermeable to 03, and thus keeps the seed dormant.     

Combinational dormancy in seeds of Sicyos angulatus (Cucurbitaceae,tribe Sicyeae)

Seeds with a water‐impermeable seed coat and a physiologically dormant embryo are classified as having combinational dormancy. Seeds of Sicyos angulatus (burcucumber) have been clearly shown to have a water‐impermeable seed coat (physical dormancy [PY]). The primary aim of the present study was to confirm (or not) that physiological dormancy (PD) is also present in seeds of S. angulatus. The highest germination of scarified fresh (38%) and 3‐month dry‐stored (36%) seeds occurred at 35/20°C. The rate (speed) of germination was faster in scarified dry‐stored seeds than in scarified fresh seeds. Removal of the seed coat, but leaving the membrane surrounding the embryo intact, increased germination of both fresh and dry‐stored seeds to > 85% at 35/20°C. Germination (80–100%) of excised embryos (both seed coat and membrane removed) occurred at 15/6, 25/15 and 35/20°C and reached 95–100% after 4 days of incubation at 25/15 and 35/20°C. Dry storage (after‐ripening) caused an increase in the germination percentage of scarified and of decoated seeds at 25/15°C and in both germination percentage and rate of excised embryos at 15/6°C. Eight weeks of cold stratification resulted in a significant increase in the germination of scarified seeds at 25/15 and 35/20°C and of decoated seeds at 15/6 and 25/15°C. Based on the results of our study and on information reported in the literature, we conclude that seeds of S. angulatus not only have PY, but also non‐deep PD, that is, combinational dormancy (PY + PD).     

Hormonal Control of Germination under Saline Conditions of Three Halophytic Taxa in the Genus Suaeda

The interaction between hormones and salinity on seed germination of three halophytic taxa in the genus Suaeda: S. maritima (L.) Dum. var. flexilis Focke and var. macrocarpa Moq., and S. depressa (Pursh) Wats, was studied. Exogenous applications of kinetin and gibberellic acid (GA3) were applied in order to determine if either of these growth-promoting hormones would promote germination in the two dormant taxa, Suaeda depressa and S. maritima var. flexilis and to see if osmotically induced dormancy by NaCl could be alleviated. Our results indicate that gibberellic acid is capable of breaking dormancy in these species with dormant seeds, but kinetin proved to be ineffective. A seed dormancy that was induced by osmotic stress could also be alleviated by treatments with gibberellic acid. Endogenous concentrations of both cytokinins and gibberellins were measured in seeds exposed to osmotic stress (0.85 M NaCl), and we found a reduction in cytokinin activity in these three taxa. Gibberellin-like activity was reduced in S. depressa when seeds were soaked in 0.85 M NaCl.     

A Seed Coat Bedding Assay to Genetically Explore In Vitro How the Endosperm Controls Seed Germination in Arabidopsis thaliana

The Arabidopsis endosperm consists of a single cell layer surrounding the mature embryo and playing an essential role to prevent the germination of dormant seeds or that of nondormant seeds irradiated by a far red (FR) light pulse. In order to further gain insight into the molecular genetic mechanisms underlying the germination repressive activity exerted by the endosperm, a "seed coat bedding" assay (SCBA) was devised. The SCBA is a dissection procedure physically separating seed coats and embryos from seeds, which allows monitoring the growth of embryos on an underlying layer of seed coats. Remarkably, the SCBA reconstitutes the germination repressive activities of the seed coat in the context of seed dormancy and FR-dependent control of seed germination. Since the SCBA allows the combinatorial use of dormant, nondormant and genetically modified seed coat and embryonic materials, the genetic pathways controlling germination and specifically operating in the endosperm and embryo can be dissected. Here we detail the procedure to assemble a SCBA.     

种子物理休眠研究进展

种子物理休眠是由种皮不透水层引起的一种休眠类型,是植物在长期系统发育进程中获得的一种适应环境变化的特性。该文简述了种子物理休眠的定义与概念;从不透水层、种皮的特殊水孔器结构以及胚的形态特异性等方面,综述了物理休眠种子的形态特征、物理休眠与综合休眠的解除方法以及物理休眠的可能解除机制;利用Angiosperm Phylogeny Group Ⅲ(APG Ⅲ)系统分析了种子物理休眠的植物在系统发育中的位置;最后提出了今后种子物理休眠有待研究的主要问题。     

Gibberellin requirement for Arabidopsis seed germination is determined both by testa characteristics and embryonic abscisic acid

The mechanisms imposing a gibberellin (GA) requirement to promote the germination of dormant and non-dormant Arabidopsis seeds were analyzed using the GA-deficient mutant ga1, several seed coat pigmentation and structure mutants, and the abscisic acid (ABA)-deficient mutant aba1. Testa mutants, which exhibit reduced seed dormancy, were not resistant to GA biosynthesis inhibitors such as tetcyclacis and paclobutrazol, contrarily to what was found before for other non-dormant mutants in Arabidopsis. However, testa mutants were more sensitive to exogenous GAs than the wild-types in the presence of the inhibitors or when transferred to a GA-deficient background. The germination capacity of the ga1-1 mutant could be integrally restored, without the help of exogenous GAs, by removing the envelopes or by transferring the mutation to a tt background (tt4 and ttg1). The double mutants still required light and chilling for dormancy breaking, which may indicate that both agents can have an effect independently of GA biosynthesis. The ABA biosynthesis inhibitor norflurazon was partially efficient in releasing the dormancy of wild-type and mutant seeds. These results suggest that GAs are required to overcome the germination constraints imposed both by the seed coat and ABA-related embryo dormancy.     

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.     

Some reflections on the relationship between endogenous hormones and light-mediated seed dormancy

The relationships between phytochrome and endogenous hormones in the light-mediated control of seed dormancy are discussed. It is concluded that gibberellins are primarily involved in post-dormancy metabolic processes leading to embryo growth and radicle emergence, such as food reserve mobilisation and endosperm softening. Evidence is considered that germination inhibitors, particularly abscisic acid, are involved in the establishment and maintenance of primary dormancy. The role of cytokinins not fully elucidated but there is considerable evidence to suggest that phytochrome control may involve cytokinin effects on transmembrane ion fluxes. In terms of hormonal control, phytochrome mediated dormancy is a complex phenomenon. There is a need for molecular studies of processes controlled by phytochrome, GAs, CKs and ABA during dormancy and germination to unravel the complexities of the dormancy mechanisms. Such studies would be facilitated by the availability of CK-deficient mutants of classical light-sensitive species.     

Embryo Dormancy in Seeds of Acer platanoides

On dispersal the seed of Acer platanoides contained a dormant embryo, from which dormancy could be removed by chilling but not by hormone application. Dormany was deep in the embryonic axis, particularly in the radicle, but less so in the cotyledons. Under the storage conditions employed the dormancy pattern was modified so that during the subsequent incubation at 20°C, cotyledon expansion of isolated embryos from stored fruits was more rapid than that of isolated embryos from newly dispersed fruits. In addition, the dormancy of isolated embryos from fruits stored for several weeks could be broken by incubation in kinetin. It was concluded that embryo dormany in this seed can be divided into two phases, an initial phase when the embryo responds to chilling but not to cytokinins, and a later phase when cytokinins also become effective. The relationship between this transition and the apparent gradual reduction in the depth of embryo dormancy during storage is also discussed. The chilling period necessary for dormancy breaking was reduced by kinetin and to a lesser extent by gibberellin, indicating a close relationship between the concentrations of these hormones and the chilling requirement.     

Investigating seed dormancy in cotton (Gossypium hirsutum L.): understanding the physiological changes in embryo during after‐ripening and germination

• The dormancy of seeds of upland cotton can be broken during dry after‐ripening, but the mechanism of its dormancy release remains unclear.
• Freshly harvested cotton seeds were subjected to after‐ripening for 180 days. Cotton seeds from different days of after‐ripening (DAR) were sampled for dynamic physiological determination and germination tests. The intact seeds and isolated embryos were germinated to assess effects of the seed coat on embryo germination. Content of H₂O₂ and phytohormones and activities of antioxidant enzymes and glucose‐6‐phosphate dehydrogenase were measured during after‐ripening and germination.
• Germination of intact seeds increased from 7% upon harvest to 96% at 30 DAR, while embryo germination improved from an initial rate of 82% to 100% after 14 DAR. Based on T₅₀ (time when 50% of seeds germinate) and germination index, the intact seed and isolated embryo needed 30 and 21 DAR, respectively, to acquire relatively stable germination. The content of H₂O₂ increased during after‐ripening and continued to increase within the first few hours of imbibition, along with a decrease in abscisic acid (ABA) content. A noticeable increase was observed in gibberellic acid content during germination when ABA content decreased to a lower level. Coat removal treatment accelerated embryo absorption of water, which further improved the accumulation of H₂O₂ and changed peroxidase content during germination.
• For cotton seed, the alleviation of coat‐imposed dormancy required 30 days of after‐ripening, accompanied by rapid dormancy release (within 21 DAR) in naked embryos. H₂O₂ acted as a core link between the response to environmental changes and induction of other physiological changes for breaking seed dormancy.

     

The Role of Water Uptake in the Transition of Recalcitrant Seeds from Dormancy to Germination

In recalcitrant seeds of horse chestnut (Aesculus hippocastanum L.) maintaining a high water content during winter, dormancy is determined by the presence and influence of the seed coat, while the axial organs of the embryos excised from these seeds are not dormant. Such axial organs were capable for active water uptake and rapid fresh weight increase, so that their fresh weights exceeded those in intact seeds at the time of radicle protrusion. Fructose plays an essential role in the water uptake as a major osmotically active compound. ABA interferes with the water uptake by the axial organs and thus delays the commencement of their growth. The manifestation of seed response to ABA during the entire dormancy period indicates the presence of active ABA receptors and the pathways of its signal transduction. The content of endogenous ABA in the embryo axes doubled in the middle of dormancy period, which coincided with a partial suppression of water uptake by the axes. During seed dormancy release and imbibition before radicle protrusion, the level of endogenous ABA in axes declined gradually. Application of exogenous ABA can imitate dormancy by limiting water absorption by axial organs. Fusicoccin A (FC A) treatment neutralized completely this ABA effect. Endogenous FC-like ligands were detected in the seed axial organs during dormancy release and germination. Apparently, endogenous FC stimulates water uptake via the activation of plasmalemmal H⁺-ATPase, acidification of cell walls, their loosening, and turgor pressure reduction. FC can evidently counteract the ABA-induced suppression of water uptake by controlling the activity of H⁺-ATPase. It is likely that, in dormant intact recalcitrant seeds, axial organs, maintaining a high water content, are competent to elevate their water content and to start their preparation for germination under the influence of FC when coat-imposed dormancy becomes weaker.     

A smoke-induced alteration of the sub-testa cuticle in seeds of the post-fire recruiter, Emmenanthe penduliflora Benth. (Hydrophyllaceae)

Emmenanthe penduliflora is an obligate fire-recruiter and demonstrates a prolonged seed dormancy followed by germination closely cued to the immediate post-fire environment. This study investigated, at the ultrastructure level, the causal factor(s) associated with seed dormancy and the stimulation of germination after exposure to smoke. The seed coat was responsible for the proximal regulation of dormancy - a waxy cuticular layer situated between the testa and endosperm was the primary barrier to the diffusion of water and small diameter solutes. The sub-testa cuticle in dormant seeds was partially permeable, as indicated by the presence of permeate channels. A short exposure to dry smoke (3 min) promoted a significant increase in seed germination (dormant 80.3% smoke-treated 793%). Exposure to smoke also resulted in two major changes to the morphology of the seed. First, smoke treatment produced an intense chemical scarification at the seed surface; the external cuticle was plasticized to form numerous small spheres on the seed surface. Second, smoke significantly altered the permeability of the internal (sub-testa) cuticle. A significant increase in both the number and size of permeate channels in the sub-testa cuticle indicated that these modifications were directly associated with the breaking of seed dormancy. The observed changes at both the internal (sub-testa) and external cuticles are consistent with the hypothesis that volatiles in smoke exert a surfactant-like reaction to break seed dormancy in E. penduliflora.     

Changes in Cytokinin Activity during Seed Germination in Rice (Oryza sativa L.)

Cytokinin activity was determined in dry mature rice seeds,in endosperm and embryo tissues 24, 48 and 72 over imbibitionand in radicles 96 h after germination. Cytokinins with chromatographicproperties similar to zeatin, zeatin riboside, zeatin glucosideand zeatin riboside glucoside were datected in embryo and endosperm,but only the latter two were detected in mature seeds. Cytokininactivity was low during early toges of germination. Qualitativeand quantitative changes in cytokinins were observed in bothembryo and endosperm. The presence of higher cytokinin activityin the endosperm than in the embryo during the first 24 h aftergermination suggests that the endosperm may supply cytokininsuntil the embryo is able to synthaize its own cytokinins. Thepossible significance of high cytokinin glucoside activity inthe embryo early during germination and high cytokinin activityin the radicle during the later stages is discussed. Oryza sativa L., rice, cytokinin, germination, seed     

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 effect of light on endogenous cytokinin levels in seeds of Rumex obtusifolius

Summary Exposure of Rumex obtusifolius seed to red light resulted in a rapid increase in extractable cytokinins. This increase can be reversed if the red light treatment is immediately followed by far-red irradiation. In germinating seed hardly any cytokinin activity could be detected in the butanol and aqueous extracts. An increase in the amount of activity present in the petroleum ether extracts of these seeds was, however, observed.The present findings suggest that cytokinin activity in light-sensitive seeds could be under the control of phytochrome.     

Embryo Dormancy and Light Requirements in the Germination of Helipterum craspedioides (Compositae), an Arid-zone Annual

Seeds of Helipterum craspedioides require light for germination,and this may be given while the seed is not fully imbibed. Removalof the seed coat, or leaching, does not bring about germinationin the dark. Gibberellin enhances dark germination to some extent,whether or not the seed coat is removed. The light requirementtherefore resides in the embryo. Fresh seed is dormant in lightor dark. Excised embryos of these dormant seeds grow slowly,rarely giving mature plants, but a larger number of normal plantsis produced after gibberellin treatment. Dormancy is lost afterhigh temperature storage, but not stratification.     

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.     

Seed yield,seed size and germination behaviour in the annualPogogyne abramsii

Summary Models of the evolution of seed dormancy reveal that dormancy is favoured either when opportunities for establishment vary over time and when there is wide variation in the probability of success, or when the probability of success is limited by frequency dependence. Empirical evidence supporting the temporal heterogeneity hypothesis exists, but there is scant evidence for dormancy being favoured by frequency dependent competition among seedlings. We test the hypothesis that the intensity of between-sib competition should favour a positive relationship between maternal fecundity and seed dormancy. This hypothesis is supported for the rare, vernal pool annual,Pogogyne abramsii: the proportion of dormant offspring was significantly higher among high fecundity mothers than among other mothers. Dormancy inP. abramsii is controlled by the seed coat, a maternal tissue, so delaying germination favours the inclusive fitness of mothers by reducing the potential for competition among siblings. Seed weight and time to first germination varied significantly amongP. abramsii plants and mean seed weight increased linearly with plant biomass. Seed weight and seed number are independently regulated by plant size. Overall, seed weight varied 10-fold and variability in seed weight within mothers was not explained by plant biomass, seed yield or mean seed weight. GerminableP. abramsii seeds were significantly heavier than dormant seeds, and germinable seeds heavier than 0.20 mg germinated more rapidly than those smaller than 0.20 mg.