Dormancy in Seeds of Charlock: I. DEVELOPMENTAL ANATOMY OF THE SEED

The anatomy of the embryo, endosperm, and testa throughout thedevelopment of charlock seeds is described. In mature seedsthe embryos are morphologically fully differentiated. Each embryoconsists of a shoot meristem with two large cotyledons but noleaf primordia, and a root meristem with a root cap. The embryois surrounded by a single layer of aleurone-like cells derivedfrom the endosperm and enclosed within the testa. Mucilagesand phenols which could greatly retard the diffusion of oxygeninto the tissues of the embryo are found in the testa. Dormancy,which appears to be induced by a shortage of oxygen, is probablyassociated with the presence of these covering layers.     

THE DEVELOPMENT OF THE SEED OF ABUTILON THEOPHRASTI. II. SEED COAT

Winter , Dorothy M. (Iowa State U., Ames.) The development of the seed of Abutilon theophrasti. II. Seat coat. Amer. Jour. Bot. 47(3) : 157—162. Illus. 1960.–The integuments of Abutilon theophrasti Medic. undergo a rapid increase in size, predominantly by anticlinal cell divisions during the first 3 days after fertilization. Within 7 days, the outer epidermis of the inner integument becomes thick walled. At maturity this compact, lignified, and cutinized palisade layer accounts for more than half the thickness of the seed coat. During early growth, the palisade cells form a continuous layer in the micropylar region. In the chalazal region the palisade layer is discontinuous in a slit-shaped region, 60 × 740 microns. The shape of this discontinuity constitutes a major difference between dormant-seeded Abutilon and non-dormant Gossypium seeds. Exterior to the palisade layer is the outer integument which consists of a small-celled layer and a large-celled layer sparsely covered with unicellular, lignified hairs. Interior to the palisade is the thick mesophyll of the inner integument which is largely digested during seed growth and leaves only 2 pigmented cell layers in most regions at maturity. The inner epidermis is small-celled, pigmented and cutinized and adheres tightly to the endosperm. Seed coat impermeability increases with seed maturity. Even immature seeds will germinate, if scarified, indicating a lack of embryo dormancy.     

Dormancy in Seeds of Charlock: IV. INTERRELATIONSHIPS OF GROWTH, OXYGEN SUPPLY AND CONCENTRATION OF INHIBITOR

Respiration measurements were made over a period of 24 h at25 °C on seeds and excised embryos maintained in Warburgflasks with partial pressures of oxygen ranging from 0 to 1atm. In the initial phase (0 to 4 h), the rate of oxygen uptake(Q_O2 of excised embryos increased linearly with external oxygenconcentration (C_O from 0 to 0.1 atm O₂ from 0.1 to 0.2 atm O₂the relation was curvilinear, and from 0.2 to 1.0 atm O₂ uptakewas independent of concentration. In later stages the relationbetween Q_O2 and C_o changed, and from 20 to 24 h the rate ofoxygen uptake increased with concentration to 1.0 atm O₂. Thechanges with time were associated with increase in rate of respiration,increase in cell size and cell number, and the oxidation offats. The decline in concentration of oxygen from the surfaceto the centre of embryos was calculated to be relatively smallat each external oxygen concentration. Althugh the rate of diffusionfailed to keep pace with consumption, the main parameters whichdetermined the internal oxygen status of the embryos were thesurface concentrations and the permeability of the seed coat.The resistance of the seed coat to diffusion of oxygen was foundto be very high, the coefficient of diffusion being about 10^–7mm² s^–1. The concentration of oxygen and in air were estimatedto be approximately 0.04 and 0.02 atm O₂, respectively. Sincea smaller concentration of oxygen (0.012 atm O₂) in the tissueswas found to be sufficient for growth, the dormancy of the seedswas not due to lack of oxygen. Dormancy appeared to be due tothe activity of growth-inhibiting substances, the concentrationof which increased with decrease in oxygen supply; below 0.1atm O₂ their rate of production increased with decrease in theoxygen concentration of the tissues. They accumulated withinthe testas of dormant seeds and prevented cell elongation. Extractsof the inhibitory substances were partially purifield by partitioningthe aqueous fraction with ether and separating chromatographically.The active principle(s) was not abscisic acid ((+)—AbscisinII, ‘Dormin’) or the mustard oil, allylisothiocyanate.     

Dormancy in Seeds of Charlock: III. OCCURRENCE AND MODE OF ACTION OF AN INHIBITOR ASSOCIATED WITH DORMANCY

The dormancy of charlock seeds appears to be associated withthe presence of an inhibitor which accumulates within the seed.This inhibitor diffuses into the external solution when seedsare placed in water under germination conditions or controlledexperimental conditions. A small quantity of inhibitor diffusesfrom the seed coats but most arises from the embryo. The increasein temperature was measured by comparing the relative growth-ratesof the radicles of excised charlock embryos in water and intest solutions of diffusate. The results suggest that the rateof accumulation of inhibitor in the external solution is controlledby diffusion, and that there is continuous production of inhibitorin the tissues of the embryo which have a low oxgyen tension.The critical concentration of the inhibitor which completelyprevents cell elongation is rapidly attained in seeds whichare dormant. The inhibitor is unlikely to be a mustard oil,such as allylisothio-cyanate.     

Dormancy in Rice Seed II: THE INFLUENCE OF COVERING STRUCTURES

Most of the dormancy in rice seed can be accounted for by theinhibitory influence of the husk, and most of the residual dormancyafter dehusking can be attributed to the inhibitory influenceof other covering structures—either the pericarp or testa,or both. It is shown that the rate of water absorption is thesame in dormant and non-dormant seeds and that dormant seedsare capable of absorbing sufficient water for germination. Thecovering structures therefore do not cause dormancy by restrictingthe entry of water. Removal of a small area of the husk breaks the dormancy of alarge proportion of the seeds; but for some seeds this treatmentis ineffective whereas removal of the entire husk would breakdormancy. The site of the excision of a small area of the huskcan alter the effectiveness of the treatment: removal of a portionof husk immediately over the embryo is no more effective thanexcising a similar portion nearby, but the removal of part ofthe husk some distance from the embryo is not as effective.Sealing the perforations with paraffin wax has little effectexcept when carried out as soon as possible after the excisionis made, and then only in positions distant from the embryo. Attempts to extract a water-soluble or ether-soluble germinationinhibitor from the husk and other parts of dormant seed or todemonstrate the presence of inhibitors by indirect methods havenot been successful. Nor has it been found possible to extracta water-soluble germination stimulator from seed which has brokendormancy. The implications of these results are discussed.     

白沙蒿种子萌发特性的研究 II. 环境因素的影响

白沙蒿(Artemisia sphaerocephala Kraseh．)是中国西北部沙漠的流动及固定沙丘上广泛分布的优势种灌木。白沙蒿种子为需光种子,种子在光下萌发而在黑暗中受到抑制。种子萌发的适宜温度为25℃,在10℃和30℃萌发速率和萌发率都很低,萌发在5℃受到抑制。种子在沙中被埋越深,其萌发速率和萌发率就越低。在沙平面下2cm或更深层沙土下出苗率为零。但是,当将种子上层沙土移走,只保留0.5cm沙土覆盖种子,这些种子的萌发率达到原先就位于沙土下0.5cm的种子的萌发率,但是后者的萌发速率较高。土壤水分含量越高,从1.7％到14.7％,其萌发就越快。土壤水分含量从19.4％起,种子的萌发受到了延迟,苗的发育受到了抑制.     

白沙蒿种子萌发特性的研究Ⅱ.环境因素的影响

白沙蒿（Artemisia sphaerocephala Krasch.）是中国西北部沙漠的流动及固定沙丘上广泛分布的优势种藻木。白沙蒿种子为需光种子,种子在光下萌发而在黑暗中受到抑制,种子萌发的适宜温度为25℃,在10℃和30℃萌发速率和萌发率都很低,萌发在℃受到抑制。种子在沙中被旱越深,其萌发速率和萌发率就越低。在沙平面下2cm或更深层沙土下出苗率为零。但是,当将种子上层沙土移走,只保留0.5cm沙土覆盖种子,这些种子的萌发率达到原先就位于沙土下0.5cm的种子的萌发率,但是后者的萌发速率较高,土壤水分含量越高,从1.7％至14.7％,其萌发就越快。土壤水分含量从19.4％起,种子的萌发受到了延迟,苗的发育受到了抑制。     

Dormancy in Cereal Seeds: II. THE NATURE OF THE GASEOUS EXCHANGE IN IMBIBED BARLEY AND RICE SEEDS

When barley seeds imbibe water, the O₂ uptake of non-dormantseeds is considerably less than that of dormant seeds for atleast the first 6 h, irrespective of the rate at which the seedshad previously lost dormancy. During the initial 6 h of imbibition, the CO₂ output of dormantbarley seeds is usually only slightly greater than and sometimesno different from that of nondormant seeds. The CO₂ output ofdormant seeds is reduced by about 66 percent by millimolar KCN,whereas that of non-dormant seeds is decreased by about 12–13per cent only. The CO₂ output of dormant barley in nitrogenis considerably less than the CO₂ output of non-dormant seedsunder the same conditions. Dormant rice seeds also show a higher initial O₂ uptake thannon-dormant seeds, though this is not generally as marked asin barley. Similarly, the initial CO₂ output of dormant seedsis distinctly greater than that of non-dormant seeds, but inmillimolar KCN it is depressed to a greater extent than in non-dormantseeds. In nitrogen, the CO₂ outputs of dormant and non-dormantseeds were found to be the same. Consequently, unlike barley,dormant rice seeds appear to be as capable of carrying out alcoholicfermentation under anaerobic conditions as nondormant seeds. In barley, increasing the O₂ tension from 21 per cent to 100per cent increased the oxygen uptake of dormant seeds more thanthat of non-dormant seeds (an increase of 53 per cent as against20–23 Per cent). In dormant seeds there was a concomitantincrease in CO₂ output (about 50 per cent), but the CO₂ outputof non-dormant seeds was hardly affected. High concentrations of CO₂ are inhibitory to the germinationof both dormant and non-dormant barley seeds. At a concentrationof 10 per cent, however, CO₂ is inhibitory only to dormant seeds,although at 2.5–5 per cent it is sometimes stimulatoryto the germination of dormant seeds. A 24–h treatmentwith appropriate concentrations of ethanol, lactic acid, oracetaldehyde is also stimulatory to the germination of dormantbarley seeds. Histochemical investigations in barley indicated the presenceof peroxidase, cytochrome oxidase, and -glycero-phosphate dehydrogenasein the embryo, aleurone layer, and in a layer associated withthe testa. A number of other redox enzymes were detected inthe embryo and aleurone layer only. No differences in distributionor intensity of activity were detected between dormant and nondormantseeds.     

MUCILAGE-PRODUCING CELLS IN THE SEED COAT OF PLANTAGO OVATA: DEVELOPMENTAL FINE STRUCTURE

As the ovule of Plantago ovata matures into a seed its epidermal cells are transformed from undifferentiated parenchyma to thin-walled containers of almost pure mucilage. During this process the volume of the cells increases 60–80 fold, and the protoplast degenerates to a remnant. Rapid cell expansion begins with pollination and is accompanied by an increase in the size of the nucleus and nucleolus, a change in the random arrangement of ribosomes, a decrease in the thickness of cell walls, and synthesis of starch. Deposition of mucilage inside vacuoles and between the plasma membrane and cell wall accompanies a marked increase in the number and size of Golgi vesicles. Histochemical evidence using the thiocarbohydrazide-osmium vapor method shows polysaccharide to be present within Golgi vesicles while they are still attached to the Golgi apparatus. Mucilage deposition is associated with further cell expansion, separation of the protoplast from the cell wall, fusion of vacuoles and extra protoplasmic space, and the disappearance of starch.     

新疆龙胆科种皮微形态学研究

中国新疆龙胆科5属18种种皮的扫描电镜观察展示了种皮微形态的高度多样性,这5个属可以分为二大类型,第一类包括龙胆属、扁蕾属和獐牙菜属,它们的种皮均具雕纹,此类型又可以分为3小类,每小类对应一个属,这3个属的雕纹类型互不相同;第二类包括假龙胆属和花锚属,它们的种皮均光滑。     

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

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

Studies on the Germination of the Seeds of Striga hermonthica: II. THE EFFECT OF THE STIMULATING SOLUTION ON SEED RESPIRATION

1. Data are presented which show the effect of the natural stimulatingsolution on the respiration of the seeds of Striga hermonthica.
2. The stimulating solution has been shown to enhance the aerobicrespiration of seeds exposed to it, compared with that of seedsto distilled water at the same temperature. This effect on therespiration of the seeds is quite independent of any previousmoisture-treatment of the seeds, and thus of germination.
3. Themaximum anaerobic rate of carbon-dioxide output of air-dryseedsafter treatment with the stimulating solution may be upto fifteentimes greater than that of seeds moistened with distilledwater.The anaerobic respiration of air-dry seeds after exposuretothe stimulating solution is greatly reduced by 0?025 M. sodiumfluoride and 0?001 M. sodium monoiodoacetate.
4. The stimulatingsolution has been found to have little effecton the anaerobicoutput of carbon dioxide from seeds which hadbeen moisture-treatedfor 6 days at 22? C. before treatmentwith the stimulating solution.
5. No correlation could be established between the effects ofthestimulating solution on germination and respiration.

     

EFFECT OF DESICCATION AND SCARIFICATION ON THE PERMEABILITY AND STRUCTURE OF THE SEED COAT OF CUSCUTA CAMPESTRIS

Changes occurring within the seed coat of Cuscuta campestris Yunkr. during desiccation were correlated with a decrease in germinability. Germination initially increased as the embryo matured, but then decreased as the testa dried. Completion of macrosclereid development signaled the onset of dormancy. Mechanical scarification with sandpaper or immersion in concentrated H₂SO₄ broke dormancy. The impermeable layer is a region above the light line and at the junction of the hypodermis and the palisade cell layers. This region appeared to be deposited during desiccation of the hypodermis. Overwintering or a period of cold storage also broke dormancy. The change in permeability was found throughout the testa rather than in a specific area.     

从胡卢巴种子子叶和种皮中制取甾体皂甙元的研究

豆科植物胡卢巴生产植物胶后的下脚料经脱脂、分离,将各成分分开后,分别采用适宜的方法加工利用,可得到甾体皂甙元、脂肪油和饲料添加剂。     

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

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

THE INFLUENCE OF THE ENVIRONMENT ON SEED AND SEEDLING MORTALITY

Maize grains were exposed to 16–20 combinations of soil moisture and soil temperature (1–30 C.) for varying periods of time. Grains germinated readily in the wettest soils at 30 and 20⁰ C, but germination was inhibited at the lower levels of soil moisture, and inhibited or delayed at the lower temperatures. Grains which failed to germinate in the soil were transferred to damp sand at 20⁰ C. and the residual viability of the samples was determined. Mortality of grain was highest at combinations of temperature and soil moisture which just did not allow of germination. Soil conditions even more unfavourable for germination gave greater residual viability in the samples. The viability of dormant grains was more rapidly destroyed at high than at low temperatures although the reduction in viability continued longer at low temperatures, probably because of the slower drying of the soil.
This mortality is interpreted in terms of pathogenic activity, and it is shown that mortality at high temperatures (20–35 ^o C.) is explicable on this general hypothesis. It is tentatively suggested that temperature and soil moisture conditions interact in selecting out and favouring the activity of specific populations of pathogens in the soil.