The control of seed germination in Trollius ledebouri A model of seed dormancy

Treatment of Trollius ledebouri seeds with gibberellins A₄+A₇ promotes germination. The efficacy of the treatment is dependent upon the duration of imbibition in distilled water prior to GA₄₊₇ application. Presoaking increases both the final percentage germination attained and also its rate of achievement. No presoaking effect is exhibited by seeds induced to germinate by testa removal in the absence of GA₄₊₇. Active washing of Trollius seeds enhances the presoaking effect and the eluent from washed seeds is inhibitory to germination. The results support the hypothesis that the presoaking effect exhibited by Trollius is the result of the leaching of a germination inhibitor from the seeds which is antagonistic to GA₄₊₇. Additionally, treatment of Trollius seeds with the gibberellin-biosynthesis inhibitor (2-chloroethyl)-trimethylammonium chloride (CCC) prior to testa removal retards germination. The inhibitory effect of CCC on germination is overcome by GA₄₊₇. Although CCC inhibits embryo growth during the presoaking of intact seeds, it does not affect the increased sensitivity of presoaked seeds to GA₄₊₇. Therefore, although endogenous gibberellins may be involved in the germination process, they do not contribute to the presoaking phenomenon. The expansion of isolated endosperm tissue is not affected by CCC. However, the chemical markedly inhibits endosperm expansion in intact seeds and implicates the embryo as both the site of production of the germination inhibitor and of gibberellin. These results are discussed in relation to previous studies and a model is presented to account for the characteristics of germination in Trollius.Abbreviations GA gibberellin - CCC (2-chloroethyl)-trimethylammonium chloride     

Seeds of tomato (Lycopersicon esculentum Mill.) which develop in a fully hydrated environment in the fruit switch from a developmental to a germinative mode without a requirement for desiccation

Seed water content is high during early development of tomato seeds (10–30 d after pollination (DAP)), declines at 35 DAP, then increases slightly during fruit ripening (following 50 DAP). The seed does not undergo maturation drying. Protein content during seed development peaks at 35 DAP in the embryo, while in the endosperm it exhibits a triphasic accumulation pattern. Peaks in endosperm protein deposition correspond to changes in endosperm morphology (i.e. formation of the hard endosperm) and are largely the consequence of increases in storage proteins. Storage-protein deposition commences at 20 DAP in the embryo and endosperm; both tissues accumulate identical proteins. Embryo maturation is complete by 40 DAP, when maximum embryo protein content, size and seed dry weight are attained. Seeds are tolerant of premature drying (fast and slow drying) from 40 DAP.Thirty-and 35-DAP seeds when removed from the fruit tissue and imbibed on water, complete germination by 120 h after isolation. Only seeds which have developed to 35 DAP produce viable seedlings. The inability of isolated 30-DAP seed to form viable seedlings appears to be related to a lack of stored nutrients, since the germinability of excised embryos (20 DAP and onwards) placed on Murashige and Skoog (1962, Physiol. Plant. 15, 473–497) medium is high. The switch from a developmental to germinative mode in the excised 30- and 35-DAP imbibed seeds is reflected in the pattern of in-vivo protein synthesis. Developmental and germinative proteins are present in the embryo and endosperm of the 30- and 35-DAP seeds 12 h after their isolation from the fruit. The mature seed (60 DAP) exhibits germinative protein synthesis from the earliest time of imbibition. The fruit environment prevents precocious germination of developing seeds, since the switch from development to germination requires only their removal from the fruit tissue.Abbreviations DAP days after pollination - kDa kilodaltons - SP1-4 storage proteins 1–4 - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis - HASI hours after seed isolation - MS medium Murashige and Skoog (1962) medium This work is supported by National Science and Engineering Research Council of Canada grant A2210 to J.D.B.     

A structural study of germination in celery (Apium graveolens L.) seed with emphasis on endosperm breakdown

Germination of celery seed occurred after 6 d of imbibition in light. During this time the embryo enlarged at the expense of the adjacent endosperm cells and at the time of germination was 2–3 times as long as in the dry seed. Breakdown of the endosperm cells near the root cap preceeded radicle emergence. None of these changes occurred in darkness.Endosperm digestion began adjacent to the embryo and spread radially. In degrading cells, the aleurone grains often became larger and fewer in number. The cell walls were modified and appeared to undergo partial degradation. Ultimately the cells seemed to lose their contents. In cells adjacent to the root cap, similar changes occurred except there was a transient appearance of starch grains. Radial progression of endosperm breakdown also occurred in isolated endosperm treated with gibberellin A₄₊₇.The results indicate that (1) the stimulus for breakdown of celery endosperm emanates from the embryo in response to light; (2) the stimulus may be a gibberellin because changes in endosperm cells and the sequence of endosperm digestion during germination resemble the responses of isolated endosperm to gibberellin; and (3) the radial progression of endosperm breakdown during germination may be the result of a sequential response of cells to a uniformly applied stimulus rather than the result of gradual embryo expansion.     

黄精种子萌发过程发育解剖学研究

采用石蜡切片技术对成熟黄精种子形态及萌发过程中的形态学变化及解剖结构特征进行了研究,以阐明黄精种子繁殖的生物学机制。结果显示:(1)成熟的黄精种子由外而内依次为种皮、胚乳和胚等3部分组成。其中种皮由一层木质化的细胞组成;胚乳占据种子的大部分结构,胚乳细胞含有大量淀粉,细胞壁增厚;胚处于棒型胚阶段。(2)黄精种子在萌发过程中棒型胚靠近种脐端分化为吸器、子叶联结和子叶鞘,靠近种孔的部位分化出胚根、胚轴和胚芽。(3)黄精种子萌发首先由子叶联结伸长将胚芽和胚根原基推出种孔,紧接着下胚轴膨大形成初生小根茎,吸器留在种子中分解吸收胚乳中的营养物质。(4)通过子叶联结连通吸器和初生小根茎,将胚乳中的营养物质由吸器-子叶联结这个通路转移到初生小根茎中,为初生根茎上胚芽和胚根的进一步分化提供物质保障。(5)黄精种子自然条件下萌发率较低,而且当年不出土。研究表明,黄精种子的繁殖生物学特性是其生态适应的一种重要机制。     

Gibberellins and pea seed development

The gibberellin (GA)-biosynthesis mutations, lh ⁱ , ls and Ie ⁵⁸³⁹ have been used to investigate the role(s) of the GAs in seed development of the garden pea (Pisum sativum L.). Seeds homozygous for lh ⁱ possess reduced GA levels, are more likely to abort during development, and weigh less at harvest, compared with wild-type seeds due to expression of the lh ⁱ mutation in the embryo and/ or endosperm. Compared with wild-type seeds, the lh ⁱ mutation reduces endogenous GA₁ and gibberellic acid (GA₃) levels in the embryo/endosperm a few days after anthesis and fertilizing lh ⁱ plants with wild-type pollen dramatically increases GA₁ and GA₃ levels in the embryo/ endosperm and restores normal seed development. By contrast, the ls and le ⁵⁸³⁹ mutations do not appear to reduce GA levels in the embryo/endosperm of seeds a few days after anthesis, and do not affect embryo or endosperm development. However, both the ls and lh ⁱ mutations substantially reduce endogenous GA levels in embryos at contact point (the first day the liquid endosperm disappears). Levels of GAs in seeds from crosses involving the ls and lh ⁱ mutations suggest that GAs are synthesised in both the embryo/endosperm and testa and that the expression of ls depends on the tissue and developmental stage examined. These results suggest that GAs (possibly GA₁ and/or GA₃) play an important role early in pea seed development by regulating the development of the embryo and/or endosperm. By contrast, the high GA levels found in wild-type seeds at contact point (and beyond) do not appear to have a physiological role in seed development.Abbreviations GA_n gibberellin A_n - DAA days after anthesis - WT wild-type We thank Noel Davies, Katherine McPherson and Peter Bobbi for technical assistance, Professor L. Mander (ANU, Canberra) for dideuterated GA standards, and the Australian Research Council and Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN, Japan), for financial support.     

Gibberellins regulate seed germination in tomato by endosperm weakening: a study with gibberellin-deficient mutants

The germination of seeds of tomato [Lycopersicon esculentum (L.) Mill.] cv. Moneymaker has been compared with that of seeds of the gibberellin-deficient dwarf-mutant line ga-1, induced in the same genetic background. Germination of tomato seeds was absolutely dependent on the presence of either endogenous or exogenous gibberellins (GAs). Gibberellin A₄₊₇ was 1000-fold more active than commercial gibberellic acid in inducing germination of the ga-1 seeds. Red light, a preincubation at 2°C, and ethylene did not stimulate germination of ga-1 seeds in the absence of GA₄₊₇; however, fusicoccin did stimulate germination independently. Removal of the endosperm and testa layers opposite the radicle tip caused germination of ga-1 seeds in water. The seedlings and plants that develop from the detipped ga-1 seeds exhibited the extreme dwarfy phenotype that is normal to this genotype. Measurements of the mechanical resistance of the surrounding layers showed that the major action of GAs was directed to the weakening of the endosperm cells around the radicle tip. In wild-type seeds this weakening occurred in water before radicle protrusion. In ga-1 seeds a similar event was dependent on GA₄₊₇, while fusicoccin also had some activity. Simultaneous incubation of de-embryonated endosperms and isolated axes showed that wild-type embryos contain and endosperm-weakening factor that is absent in ga-1 axes and is probably a GA. Thus, an endogenous GA facilitates germination in tomato seeds by weakening the mechanical restraint of the endosperm cells to permit radicle protrusion.Abbreviations GA(s) gibberellin(s) - GA₃ gibberellic acid     

Transient expression of storage-protein genes during early embryogenesis ofVicia faba: synthesis and metabolization of vicilin and legumin in the embryo,suspensor and endosperm

Seed storage proteins are thought to be accumulated exclusively in the cell-expansion phase of embryogenesis and metabolized during germination and seedling growth. Here we show by a sensitive immunohistological technique that the two Vicia faba L. storage proteins vicilin and legumin are accumulated in substantial amounts in the suspensor and coenocytic endosperm and to a lesser extent in the mid-globular embryo. Both proteins appear and disappear at precise stages specific for each tissue. In the endosperm the accumulation starts around 12 d after pollination (DAP). After a maximum attained at 14–15 DAP, storage proteins are degraded within about 4 d. Accumulation is restricted to that part of the endosperm which covers the embryo and displays the highest levels of endoploidy (maximum 96n). In all other parts of the endosperm, storage proteins do not appear to accumulate, although storage-protein-specific mRNA synthesis takes place. In the suspensor, storage proteins are already observed at 6 DAP and disappear very quickly at approximately 10 DAP. Low amounts of legumin and vicilin are also detectable in the mid-globular embryo, but disappear completely as the embryo enters the heart stage. We conclude that storage proteins of Vicia faba accumulated transiently during early seed development are used as nutritive reserves for the growing embryo.Abbreviation DAP days after pollination Dedicated to Prof. Rigomar Rieger in the occasion of his 65th birthdayThis research was supported by the Ministry of Science and Research, Land Sachsen-Anhalt, Germany. U.W. acknowledges additional support by the Fonds der Chemischen Industrie.     

Developing tomato seeds when removed from the fruit produce multiple forms of germinative and post-germinative endo-β-mannanase. Responses to desiccation, abscisic acid and osmoticum

Several isoforms of endo-1,4-D-mannanase (EC3.2.1.78) are produced in the endosperm and embryo of tomato (Lycopersicon esculentum Mill.) seed prior to the completion of germination. Other isoforms appear in the embryo and in the lateral endosperm following germination. This occurs in seeds removed from the fruit prior to completion of development at 45 d after pollination and placed directly on water, or following drying. Hence desiccation is not required to induce either germination- or post-germination-related mannanase activity. Incubating seeds in abscisic acid or osmoticum results in a reduction of both germination and total mannanase activity, but the isoforms that are produced in the embryo and micropylar region of the endosperm are identical to those produced in water-imbibed seeds prior to germination. Incubation of seeds in a high concentration of abscisic acid prevents all enzyme production. Only after the completion of germination does mannanase increase in the lateral regions of the endosperm. In contrast, mannanase is produced in the micropylar region regardless of whether the seed germinates or not. The isoforms produced in the two regions of the endosperm are different, those in the lateral endosperm being more similar to those produced in the cotyledons and axes of the embryo. Embryos and endosperms dissected prior to completion of germination and incubated separately produce far fewer isoforms than when these parts are together in the intact seed.Abbreviations ABA cis-abscisic acid - DAP days after pollination - GA gibberellin - IEF isoelectric focusing - PEG polyethyleneglycol - pI isoelectric point This work was supported by Natural Sciences and Engineering Council of Canada grant A2210. B.V. received a fellowship from the Deutscher Akademischer Austauschdienst for her research at the University of Guelph. We are grateful to Dr. H.W.M. Hilhorst, Wageningen, for his critical comments.     

Gibberellins in developing fruits of Pisum sativum cv. Alaska: Studies on their role in pod growth and seed development

Gibberellins A₁, A₈, A₂₀ and A₂₉ were identified by capillary gas chromatography-mass spectrometry in the pods and seeds from 5-d-old pollinated ovaries of pea (Pisum sativum cv. Alaska). These gibberellins were also identified in 4-d-old non-developing, parthenocarpic and pollinated ovaries. The level of gibberellin A₁ within these ovary types was correlated with pod size. Gibberellin A₁, applied to emasculated ovaries cultured in vitro, was three to five times more active than gibberellin A₂₀. Using pollinated ovary explants cultured in vitro, the effects of inhibitors of gibberellin biosynthesis on pod growth and seed development were examined. The inhibitors retarded pod growth during the first 7 d after anthesis, and this inhibition was reversed by simultaneous application of gibberellin A₃. In contrast, the inhibitors, when supplied to 4-d-old pollinated ovaries for 16 d, had little effect on seed fresh weight although they reduced the levels of endogenous gibberellins A₂₀ and A₂₉ in the enlarging seeds to almost zero. Paclobutrazol, which was one of the inhibitors used, is xylem-mobile and it efficiently reduced the level of seed gibberellins without being taken up into the seed. In intact fruits the pod may therefore be a source of precursors for gibberellin biosynthesis in the seed. Overall, the results indicate that gibberellin A₁, present in parthenocarpic and pollinated fruits early in development, regulates pod growth. In contrast the high levels of gibberellins A₂₀ and A₂₉, which accumulate during seed enlargement, appear to be unnecessary for normal seed development or for subsequent germination.Abbreviations GA_(a) gibberellin A_n - GC-MS combined gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - PFK perfluorokerosene - PVP polyvinylpyrrolidone     

Distribution of Cadmium,Lead, Nickel,and Strontium in Imbibing Maize Caryopses

Seed germination is tolerant to heavy metals apparently because the seed coat is impermeable to metal ions. However, it is not clear whether the seed coat is a universal barrier for all metals. In addition, depending on their physical and chemical properties, a distribution of various metals may differ within an imbibing caryopsis, and therefore they produce dissimilar effects on seed germination. The toxic effects of Cd(NO₃)₂, Pb(NO₃)₂, Ni(NO₃)₂, and Sr(NO₃)₂ were estimated from the germination rates of maize (Zea mays L.) caryopses following two-day incubation with these salts. The distribution of heavy metals and Sr was studied by histochemical methods based on the formation of colored complexes with dithizone (Cd and Pb), dimethylglyoxyme (Ni), and sodium rhodizonate (Sr). Although the metals under study did not affect maize radicle protrusion, they inhibited seed germination in the following order: Cd > Ni ≈ Pb > Sr. Cd and Pb accumulated mainly in the seed coat cells, but Sr and Ni in the embryo cells and in the cells of endosperm (Sr) and scutellum (Ni). Although Cd was found only in the seed coat, it was the strongest inhibitor of seed germination. Apparently, due to high toxicity, Cd exerted its inhibitory effect at the concentrations too low for histochemical assay. In spite of easy translocation across the seed coat of imbibing caryopses, Sr did not considerably inhibit radicle protrusion and seed germination, apparently because of its low toxicity and predominant localization in the apoplast of embryo and endosperm cells.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 635–640.Original Russian Text Copyright © 2005 by Seregin, Kozhevnikova.     

Changes in the patterns of phosphatase isozymes during the germination of rice seed

Crude phosphatase preparations from both the embryo and endosperm of germinating rice seeds were resolved into several components by Sephadex G-100 column chromatography. The substrate specificity of each isozyme was broad, but their spectra were more or less different. One of these iso-phosphatases in the embryo increased prominently during seed germination and decreased at the stage of senescence. Phosphatase activities for several phosphate esters were increased during germination, but to different extents. The rate increased more in the embryo than in the endosperm. Especially in the embryo, gibberellin A₃ promoted an increase in the activity of some phosphatases as well as an increase in the content of free phosphate.     

Posttranslational regulation of pyruvate, orthophosphate dikinase in developing rice (Oryza sativa) seeds

Pyruvate, orthophosphate dikinase (PPDK; E.C.2.7.9.1) is most well known as a photosynthetic enzyme in C₄ plants. The enzyme is also ubiquitous in C₃ plant tissues, although a precise non-photosynthetic C₃ function(s) is yet to be validated, owing largely to its low abundance in most C₃ organs. The single C₃ organ type where PPDK is in high abundance, and, therefore, where its function is most amenable to elucidation, are the developing seeds of graminaceous cereals. In this report, we suggest a non-photosynthetic function for C₃ PPDK by characterizing its abundance and posttranslational regulation in developing Oryza sativa (rice) seeds. Using primarily an immunoblot-based approach, we show that PPDK is a massively expressed protein during the early syncitial-endosperm/-cellularization stage of seed development. As seed development progresses from this early stage, the enzyme undergoes a rapid, posttranslational down-regulation in activity and amount via regulatory threonyl-phosphorylation (PPDK inactivation) and protein degradation. Immunoblot analysis of separated seed tissue fractions (pericarp, embryo + aleurone, seed embryo) revealed that regulatory phosphorylation of PPDK occurs in the non-green seed embryo and green outer pericarp layer, but not in the endosperm + aleurone layer. The modestly abundant pool of inactive PPDK (phosphorylated + dephosphorylated) that was found to persist in mature rice seeds was shown to remain largely unchanged (inactive) upon seed germination, suggesting that PPDK in rice seeds function in developmental rather than in post-developmental processes. These and related observations lead us to postulate a putative function for the enzyme that aligns its PEP to pyruvate-forming reaction with biosynthetic processes that are specific to early cereal seed development.     

Degradation of the endosperm cell walls of Lactuca sativa L., cv. Grand Rapids

The timing of mobilisation of lipid, sucrose, raffinose and phytate in lettuce seeds (achenes) (cv. Grand Rapids) has been examined. These reserves (33%, 1.5%, 0.7%, 1.4% of achene dry weight, respectively) are stored mostly in the cotyledons. Except for a slight degradation of raffinose and increase in sucrose, there is no detectable reserve mobilisation during germination. The endosperm (8% of seed dry weight), which has thick, mannan-containing cell walls (carbohydrate, 3,4% of seed dry weight), is completely degraded within about 15h following germination. Mannanase activity increases about 100-fold during the same period and arises in all regions of the endosperm. Also during this period sucrose and raffinose are degraded and fructose and glucose accumulate in the embryo. The endosperm hydrolysis products are taken up by the embryo, and are probably used as an additional reserve to support early seedling growth. However, endosperm cell-wall carbohydrates, such as mannose, are not found as free sugars. Lipid and phytate are degraded in a later, second phase of mobilisation. Low levels of sucrose are present in the embryo, mostly in the cotyledons, and large amounts of fractose and glucose (14% of seedling dry weight at 3 days after sowing) accumulate in the hypocotyl and radicle. It is suggested that sucrose, produced in the cotyledons by gluco-neogenesis, is translocated to the axis and converted there to fructose and glucose.     

A dual rôle for the endosperm and its galactomannan reserves in the germinative physiology of fenugreek (Trigonella foenum-graecum L.), an endospermic leguminous seed

Some 30% of the reserve material in the fenugreek seed is galactomannan localised in the endosperm; the remainder is mainly protein and lipid in the cotyledons of the embryo. The importance of galactomannan to the germinative physiology of fenugreek has been investigated by comparing intact and endosperm-free seeds. From a purely nutritional point of view the galactomannan's rôle is not qualitatively different from that of the food reserves in the embryo. Nevertheless, due to its spatial location and its hydrophilic properties, the galactomannan is the molecular basis of a mechanism whereby the endosperm imbibes a large quantity of water during seed hydration and is able to buffer the germinating embryo against desiccation during subsequent periods of drought-stress. The galactomannan is clearly a dual-purpose polysaccharide, regulating water-balance during germination and serving as a substrate reserve for the developing seedling following germination. The relative importance of these two rôles is discussed.     

Changes in germination, growth and soluble sugar contents of Sorghum bicolor (L.) Moench seeds under various abiotic stresses

The effect of various abiotic stresses on germination rate, growth and soluble sugar content in Sorghum bicolor (L.) Moench cv. CSH 6 seed embryos and endosperm during early germination was investigated. Under stress conditions germination, water potential and tissue water content decreased markedly. Subsequently, this reduction resulted in marked decreases in fresh weight both in embryos and endosperm. Conversely, a substantial increase in dry weight was observed. Furthermore, a considerable increase in the sugar contents in both embryo and endosperm was detected. The fructose level was always higher than glucose and sucrose in response to various stresses. However, as compared to the control the level of glucose and sucrose was higher in embryos and endosperm after stress treatments. Based upon these results a possible physiological role of sugars in the germination of sorghum seeds is discussed.     

The role of gibberellins A1 and A3 in fruit growth of Pisum sativum L. and the identification of gibberellins A4 and A7 in young seeds

Gibberellins A₁ and A₃ are the major physiologically active gibberellins (GAs) present in young fruit of pea (Pisum sativum L.). The relative importance of these GAs in controlling fruit growth and their biosynthetic origins were investigated in cv. Alaska. In addition, the non-13-hydroxylated active GAs, GA₄ and GA₇, were identified for the first time in young seeds harvested 4 d after anthesis, although they are minor components and are not expected to play major physiological roles. The GA₁ content is maximal in seeds and pods at 6 d after anthesis, the time of highest growth-rate of the pod (Garcia-Martinez et al. 1991, Planta 184: 53–60), whereas gibberellic acid (GA₃), which is present at high levels in seeds 4–8 d after anthesis, has very low abundance in pods. Gibberellins A₁₉, A₂₀ and A₂₉ are most concentrated in seeds at, or shortly after, anthesis and their abundance declines rapidly with development, concomitant with the sharp increase in GA₁ and GA₃ content. Application of GA₁ or GA₃ to the leaf subtending an emasculated flower stimulated parthenocarpic fruit development. Measurement of the GA content of the pods at 4 d after anthesis indicated that only 0.002–0.5% of the applied GA was transported to the fruit, depending on dose. There was a linear relationship between GA₁ content and pod weight up to about 2 ng · (g FW)⁻¹, whereas no such correlation existed for GA₃ content. The concentration of endogenous GA₁ in pods from pollinated ovaries is just sufficient to give the maximum growth response. It is concluded that GA₁, but not GA₃, controls pod growth in pea; GA₃ may be involved in early seed development. The distribution of GAs within the seeds at 4 d post anthesis was also investigated. Most of the GA₁, GA₈, GA₁₉, GA₂₀ and GA₂₉ was present in the testa, whereas GA₃ was distributed equally between testa and endosperm and GA₄ was localised mainly in the endosperm. Of the GAs analysed, only GA₃ and GA₂₀ were detected in the embryo. Metabolism experiments with intact tissues and cell-free fractions indicated compartmentation of GA biosynthesis within the seed. Using ¹⁴C-labelled GA₁₂, GA₉, 2,3-didehydroGA₉ and GA₂₀ as substrates, the testa was shown to contain 13-hydroxylase and 20-oxidase activities, the endosperm, 3β-hydroxylase and 20-oxidase activities. Both tissues also produced 16,17-dihydrodiols. However, GA₁ and GA₃ were not obtained as products and it is unlikely that they are formed via the early 13-hydroxylation pathway. [¹⁴C]gibberellin A₁₂, applied to the inside surface of pods in situ, was metabolised to GA₁₉, GA₂₀, GA₂₉, GA₂₉-catabolite, GA₈₁ and GA₉₇, but GA₁ was not detected. Gibberellin A₂₀ was metabolised by this tissue to GA₂₉ and GA₂₉-catabolite. Received: 23 July 1996 / Accepted: 2 September 1996     

GERMINATION OF LIGHT-INHIBITED SEED OF NEMOPHILA INSIGNIS

Germination of Nemophila insignis seed is inhibited by light over a wide range of temperatures. At low temperatures the light intensity required for inhibition is higher. At 25 C there is little germination (in darkness as well as in light); at 27.5 C germination is inhibited altogether. Virtually complete germination in light is obtained when the endosperm directly covering the radicle is removed. This operation also improves germination in darkness at 25 C. Mechanical scarification performed elsewhere in the seed is without effect. As with Phacelia tanacetifolia, Nemophila seed apparently fails to germinate in light because the endosperm restrains the expansive growth of the embryo. The embryo of dark-imbibed seed develops a force which enables it to overcome the physical resistance of the endosperm. Light inhibits the process which leads to generation of “expansive force.” Gibberellic acid at 5 × 10^–4 m stimulates germination in light and in the dark. Abscisic acid at 10^-4 m inhibits germination; at 10^-6 m it inhibits only root growth. The inhibition of germination or root growth caused by abscisic acid cannot be reversed by gibberellic acid. Eighty per cent oxygen under certain conditions promotes germination. The rate of O₂ uptake is enhanced in oxygen-enriched atmosphere; however, there is no corresponding increase in the rate of CO₂ output. Thus high oxygen tension enhances an oxidative process other than respiration. This reaction is favorable to seed germination.     

Changes in low-molecular-weight thiol-disulphide redox couples are part of bread wheat seed germination and early seedling growth

The tripeptide antioxidant glutathione (γ-l-glutamyl-l-cysteinyl-glycine; GSH) essentially contributes to thiol-disulphide conversions, which are involved in the control of seed development, germination, and seedling establishment. However, the relative contribution of GSH metabolism in different seed structures is not fully understood. We studied the GSH/glutathione disulphide (GSSG) redox couple and associated low-molecular-weight (LMW) thiols and disulphides related to GSH metabolism in bread wheat (Triticum aestivum L.) seeds, focussing on redox changes in the embryo and endosperm during germination. In dry seeds, GSH was the predominant LMW thiol and, 15?h after the onset of imbibition, embryos of non-germinated seeds contained 12 times more LMW thiols than the endosperm. In germinated seeds, the embryo contained 17 and 11 times more LMW thiols than the endosperm after 15 and 48?h, respectively. This resulted in the embryo having significantly more reducing half-cell reduction potentials of GSH/GSSG and cysteine (Cys)/cystine (CySS) redox couples (E_GSSG/2GSH and E_CySS/2Cys, respectively). Upon seed germination and early seedling growth, Cys and CySS concentrations significantly increased in both embryo and endosperm, progressively contributing to the cellular LMW thiol-disulphide redox environment (E_{thiol-disulphide}). The changes in E_CySS/2Cys could be related to the mobilisation of storage proteins in the endosperm during early seedling growth. We suggest that E_GSSG/2GSH and E_CySS/2Cys can be used as markers of the physiological and developmental stage of embryo and endosperm. We also present a model of interaction between LMW thiols and disulphides with hydrogen peroxide (H₂O₂) in redox regulation of bread wheat germination and early seedling growth.