Phosphorus concentration in barley (Hordeum vulgare L.) seed: Influence on seedling growth and dry matter production

The effect of phosphorus (P) concentration in barley seed on seedling growth has not been much investigated. Consequently, two experiments were conducted in the greenhouse to determine the effect of P concentration in barley seed (Hordeum vulgare L., cv. Empress) on the seedlings grown in sand-filled boxes receiving a culture solution without P. Seeds were selected with three P concentrations: high-P (113.0 mmol P kg⁻¹), medium-P (80.7 mmol P kg⁻¹) and low-P (54.9 mmol P kg⁻¹). At 21 days after sowing, the shoot and root yield or shoot height was the least with seedlings from low-P seed. In the other experiment, high-P and low-P seeds were wetted with distilled water or with a solution of 25.8 cmol L⁻¹ of NaH₂PO₄ for 24 h, and then grown for 31 days. Solution P had been imbibed by seeds whether low or high in native P, but only the imbibed P held by low native P seed benefited seedling dry matter accumulation and shoot elongation. The lack of benefit from seed-imbibed P on seedlings grown from high-P barley seed was associated with low recovery of the imbibed P in those seedlings.     

The relationship between beta-mannosidase and endo-beta-mannanase activities in tomato seeds during and following germination: a comparison of seed populations and individual seeds

beta-Mannosidase and endo-beta-mannanase are involved in the mobilization of the mannan-containing cell walls of the tomato seed endosperm. The activities of both enzymes increase in a similar temporal manner in the micropylar and lateral endosperm during and following germination. This increase in enzyme activities in the micropylar endosperm is not markedly reduced in seeds imbibed in abscisic acid although, in the lateral endosperm, endo-beta-mannanase activity is more suppressed by this inhibitor than is the activity of beta-mannosidase. Gibberellin-deficient (gib-1) mutants of tomato do not germinate unless imbibed in gibberellin; low beta-mannosidase activity, and no endo-beta-mannanase activity is present in seeds imbibed in water, but both enzymes increase strongly in activity in the seeds imbibed in the growth regulator. For production of full activity of both beta-mannosidase and endo-beta-mannanase in the endosperm, this tissue must be in contact with the embryo for at least the first 6 h of imbibition, which is indicative of a stimulus diffusing from the embryo to the endosperm during this time. These results suggest some correlation between the activities of beta-mannosidase and endo-beta-mannanase, particularly in the micropylar endosperm, in populations of tomato seeds imbibed in water, abscisic acid and gibberellin. However, when individual micropylar endosperm parts are used to examine the effect of the growth regulators and of imbibition in water on the production of the two enzymes, it is apparent that within these individual seed parts there may be large differences in the amount of enzyme activity present. Micropylar endosperms with high endo-beta-mannanase activity do not necessarily have high beta-mannosidase activity, and vice versa, which is indicative of a lack of co-ordination of the activities of these two enzymes within individuals of a population.     

Proteomic analysis of seed dormancy in Arabidopsis

The mechanisms controlling seed dormancy in Arabidopsis (Arabidopsis thaliana) have been characterized by proteomics using the dormant (D) accession Cvi originating from the Cape Verde Islands. Comparative studies carried out with freshly harvested dormant and after-ripened non-dormant (ND) seeds revealed a specific differential accumulation of 32 proteins. The data suggested that proteins associated with metabolic functions potentially involved in germination can accumulate during after-ripening in the dry state leading to dormancy release. Exogenous application of abscisic acid (ABA) to ND seeds strongly impeded their germination, which physiologically mimicked the behavior of D imbibed seeds. This application resulted in an alteration of the accumulation pattern of 71 proteins. There was a strong down-accumulation of a major part (90%) of these proteins, which were involved mainly in energetic and protein metabolisms. This feature suggested that exogenous ABA triggers proteolytic mechanisms in imbibed seeds. An analysis of de novo protein synthesis by two-dimensional gel electrophoresis in the presence of [(35)S]-methionine disclosed that exogenous ABA does not impede protein biosynthesis during imbibition. Furthermore, imbibed D seeds proved competent for de novo protein synthesis, demonstrating that impediment of protein translation was not the cause of the observed block of seed germination. However, the two-dimensional protein profiles were markedly different from those obtained with the ND seeds imbibed in ABA. Altogether, the data showed that the mechanisms blocking germination of the ND seeds by ABA application are different from those preventing germination of the D seeds imbibed in basal medium.     

Protein synthesis in the embryo ofPinus thunbergii seed III.

Dissociability of the monomer ribosomes prepared from dry and imbibed pine (Pinus thunbergii) seed embryos was analyzed in sucrose density gradient containing a high salt buffer. Abnormal dissociation into the subunits was observed with the ribosome preparation from dry seed embryos when compared with that from imbibed seed embryos, i.e. each subunit peak was broader and localized at a lower site in sucrose density gradient. This indicates some change(s) in ribosomes during imbibition of seeds. These ribosomal changes also progressedin vitro. That is, after incubation of ribosome preparation from dry seed embryos in a high salt buffer for 5 min at 30 C or in a low salt buffer for 15 hr at 0 C, complete dissociation into the normal subunits was observed. No difference was found between polyacrylamide gel electrophoresis patterns of ribosomal RNA from dry and imbibed seed embryos. These results suggest some alteration in the protein components of ribosome during imbibition of pine seeds. This paper is dedicated to Prof. Shyogo Sawamura, Utsunomiya University on his retirement in March, 1979.     

Presence in Dry Pea Cotyledons of Soluble Succinate Dehydrogenase That Is Assembled into the Mitochondrial Inner Membrane during Seed Imbibition

SUCCINATE DEHYDROGENASE (SUCCINATE: phenazine methosulfate oxidoreductase, EC 1.3.99.1) activity in crude mitochondrial fraction from pea (var. Alaska) cotyledons increased during seed imbibition to reach a maximum after about 12 hours. The increase was not inhibited by either cycloheximide or d(-)threo-chloramphenicol. The postmicrosomal fraction from dry cotyledons, but not that from fully imbibed ones, contained a soluble form of succinate dehydrogenase. The soluble enzyme was partially purified by ammonium sulfate fractionation and diethylaminoethyl-cellulose and Sepharose 6B column chromatography. The enzyme showed no succinate-coenzyme Q oxidoreductase activity and had a molecular mass of about 100,000 daltons. The soluble enzyme seemed to differ only slightly from succinate dehydrogenase solubilized from the mitochondrial inner membrane from fully imbibed cotyledons by a detergent. It is proposed that the soluble succinate dehydrogenase is associated with an inert mitochondrial inner membrane in dry cotyledons to form an active one during seed imbibition.     

The roles of inorganic nitrogen salts in maintaining phytochrome- and gibberellin A3-mediated germination control in skotodormant lettuce seeds

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A₃ (GA₃). Inorganic nitrogen salts in the imbibition solutions reduced seed skotodormancy. Ten-day DS seeds, imbibed in 25 mm salt solutions followed by terminal R, germinated 99% if imbibed in NH₄NO₃, 70% if imbibed in KNO₃ or NH₄Cl, and 55% if imbibed in NaNO₃. Seeds imbibed in higher salt concentrations germinated fully upon terminal R treatment. Seeds imbibed in 25 mm NH₄Cl or in 50 mm NH₄NO₃ germinated completely upon GA₃ treatment. Osmotic effects of imbibition media accounted for only part of the effect, since seeds imbibed in 50 mm CaCl₂ or NaCl germinated poorly following R or GA₃ treatment. Seeds imbibed in 500 mm polyethylene glycol (PEG) 1000 or mannitol solutions for 10 days still exhibited skotodormancy. Treatments of R or GA₃ did not stimulate germination in seeds imbibed in mannitol, but germination was complete if seeds were given 1-h acid immersion plus a water rinse before the terminal R or GA₃ treatment. Seeds imbibed in 50–500 mm PEG during 10-day DS germinated significantly better in response to terminal R. Terminal GA₃ significantly improved germination only in seeds imbibed at 500 mm PEG. P_fr appeared to function in mannitol-imbibed seed only after an acid treatment. Seed exposure to inorganic nitrogen salts during the 10-day DS maintained seed sensitivity to terminal R or GA₃ treatment. The depth of seed skotodormancy was related to the availability of inorganic nitrogen and also involved the levels of P_fr or endogenous GA₃.Abbreviations FR far red - DS dark storage - R red - GA₃ gibberellin A₃ - PEG polyethylene glycol - SHAM salicylhydroxamic acid - ANOVA analysis of variance - GLM general linear model - LSD least squares difference - P_fr far-red absorbing form of phytochrome     

苦马豆和披针叶黄华种子硬实特性与活力关系

将苦马豆和披针叶黄华种子在恒温25℃下吸胀,每24 h取出吸胀种子,16 d后未吸胀的种子为硬实种子（H）,硬实种子用硫酸处理后恒温吸胀24 h,与非硬实种子进行发芽试验和各项活力指标测定。结果显示每日内吸胀的种子数量随时间推移以一定比列下降,苦马豆非硬实种子第3天后吸胀率下降到1%,第13～16天突然上升后又下降到1%,披针叶黄华非硬实种子第3天后下降到1%,第9、10天突然上升后又下降到1%。两种豆类都显示出硬实种子发芽率、发芽指数、活力指数、脱氢酶活性、呼吸速率和超氧化物歧化酶（SOD）活性均高于非硬实种子,而电导率、浸出液可溶性糖和丙二醛（MDA）含量低于非硬实种子,缓慢吸胀的硬实种子活力指标高于快速吸胀的硬实种子,这表明硬实种子活力高于非硬实种子,硬实种子吸胀过程中存在吸胀损伤。而在非硬实种子中,根据以上活力指标判断,晚吸胀的种子比早吸胀的种子活力高。     

Disappearance of desiccation tolerance of imbibed crop seeds is not associated with the decline of oligosaccharides

The relationship between sugar content and loss of desiccation tolerance of hydrated crop seeds (tomato, okra, snow pea, mung bean, and cucumber) was evaluated by imbibing seeds with or without ABA, followed by dehydration and germination. During the process of hydration, but before the seeds lost desiccation tolerance, monosaccharide content increased only slightly, sucrose increased in snow peas, mung bean and cucumber, but maintained its original level in other species and the oligosaccharides declined dramatically. At the time of losing desiccation tolerance, the sucrose content of imbibed seeds was 2-3 times higher than the original level in most species. Positive significant correlation coefficients (r) were found in many, but not all crop seeds between desiccation tolerance and the oligosaccharide mass, or oligo/sucrose ratio. The ratio of oligo/sucrose in intact seeds at the time of losing desiccation tolerance, however, was not a fixed value and varied among species. Oligosaccharides declined significantly in different seed parts of imbibed cucumber seeds while sucrose increased to a higher level in the radicle than in the hypocotyl. Radicles were far more sensitive to desiccation than hypocotyls. The same observation was found for cucumber seeds imbibed in 100 M ABA, yet desiccation tolerance was largely maintained in hypocotyls and cotyledons. It is concluded that sucrose and oligosaccharides are not the determinants of the loss of desiccation tolerance in hydrated seeds.Imbibed seeds did not show any differences between seed parts in their ability to resynthesize sugars during the process of slow dehydration. Differences in sensitivity to desiccation among seed parts were not due to differences in the initial water content or to the rate of water content increase among seed parts. Physiological regulation of the loss of desiccation tolerance in crop seeds during hydration is discussed.     

Freezing and desiccation tolerance of imbibed canola seed

Depending on the environmental conditions, imbibed seeds survive subzero temperatures either by supercooling or by tolerating freezing-induced desiccation. We investigated what the predominant survival mechanism is in freezing canola ( Brassica napus cv. Quest) and concluded that it depends on the cooling rate. Seeds cooled at 3°C h⁻¹ or faster supercooled, whereas seeds cooled over a 4-day period to −12°C and then cooled at 3°C h⁻¹ to−40°C did not display low temperature exotherms. Both differential thermal analysis and nuclear magnetic resonance (NMR) spectroscopy confirmed that imbibed canola seeds undergo freezing-induced desiccation at slow cooling rates. The freezing tolerance of imbibed canola seed (LT₅₀) was determined by slowly cooling to −12°C for 48 h, followed with cooling at 3°C h⁻¹ to −40°C, or by holding at a constant −6°C (LD₅₀). For both tests, the loss in freezing tolerance of imbibed seeds was a function of time and temperature of imbibition. Freezing tolerance was rapidly lost after radicle emergence. Seeds imbibed in 100 μ M abscisic acid (ABA), particularly at 2°C, lost freezing tolerance at a slower rate compared with water-imbibed seeds. Seeds imbibed in water either at 23°C for 16 h, or 8°C for 6 days, or 2°C for 6 days were not germinable after storage at −6°C for 10 days. Seeds imbibed in ABA at 23°C for 24 h, or 8°C for 8 days, or 2°C for 15 days were highly germinable after 40 days at a constant −6°C. Desiccation injury induced at a high temperature (60°C), as with injury induced by freezing, was found to be a function of imbibition temperature and time.     

The Importance of Genetically Determined Seed Coat Characteristics to Seed Quality in Grain Legumes

Comparisons of five pairs of isogenk lines of peas, differingonly in the A gene for seed coat colour showed that white seeds(genotype aa) imbibed more rapidly than coloured seeds (AA),suffered greater imbibition damage revealed by dead tissue onthe cotyledons, and higher solute leakage. Seed-coat pigmentationwas closely associated with slow water uptake, since when expressionof the A gene was suppressed by the recessive pollens gene,the resulting white seeds {palpal AA) imbibed rapidly. The slowwater uptake by coloured seeds was not due to the restrictionof water entry by the seed coat since the differences in imbibitionrate were maintained when a portion of the seed coat was removedand seeds were imbibed with the exposed cotyledon in contactwith moist filter paper. Imbibition of similarly treated seedsby immersion in polyethylene glycol solutions (1–4%) whichincreased the seed/solution wettability, had little effect onthe water uptake of coloured seeds compared to imbibition inwater whereas that of white seeds increased in the first 10mins imbibition. Poor wettability of the inner surface of colouredseed coats did not therefore explain the slow imbibition ofthese seeds. The white seed coats loosened rapidly during imbibitionwhilst the coloured seed coats remained closely associated withthe cotyledons suggesting that the adherence of the seed coatto the cotyledons and therefore the ease of access of waterbetween the testa and cotyledons determines the rate of imbibition.The rapid water uptake by white-coated seeds and the subsequentimbibition damage may explain the high incidence of infectionof these seeds by the soil-bome fungus Pythhan after 2 d insoil. Improved seed quality and emergence may therefore be achievedby breeding for seed coat characteristics leading to reducedrates of imbibition Pisum sativum, isogenic lines, A gene, seed coat colour, imbibition, imbibition damage, wettability, pollens gene, seed quality, grain legumes     

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.     

林线树种太白红杉种子萌发的生理生态特性

 太白红杉(Larix chinensis)是太白山的高山林线树种。通过在人工气候室内的试验,研究了太白红杉种子在6种不同的光照与温度组合处理条件下的萌发特性。结果表明：在恒温和变温两种条件下,交替光照对于种子吸胀后的脱落酸（ABA）和赤霉素 (GA)有刺激作用。在恒温条件下,持续光照对于种子吸胀后的生长素 (IAA)有刺激作用,而变温条件下交替光照对生长素有刺激作用。细胞分裂素（CTK）的变化情况与IAA相反。光照条件相同时,恒温条件下的植物激素含量要高于变温条件下的含量,说明恒温对于各种激素有刺激作用。在25 ℃环境下种子的萌发率高于在12 ℃环境下的萌发率,说明温度对于种子的萌发有重要作用。太白红杉种子的萌发受交替光照（12 h光照/12 h黑暗）的刺激;恒温（25 ℃）条件下的种子萌发率高于变温（12 ℃/25 ℃）条件下的种子萌发率。实验结果反映了内源激素在太白红杉种子萌发过程中起着重要作用。     

On the role of abscisic acid in seed dormancy of red rice

Abscisic acid (ABA) is commonly assumed to be the primary effector of seed dormancy, but conclusive evidence for this role is lacking. This paper reports on the relationships occurring in red rice between ABA and seed dormancy. Content of free ABA in dry and imbibed caryopses, both dormant and after-ripened, the effects of inhibitors, and the ability of applied ABA to revert dormancy breakage were considered. The results indicate: (i) no direct correlation of ABA content with the dormancy status of the seed, either dry or imbibed; (ii) different sensitivity to ABA of non-dormant seed and seed that was forced to germinate by fluridone; and (iii) an inability of exogenous ABA to reinstate dormancy in fluridone-treated seed, even though applied at a pH which favoured high ABA accumulation. These considerations suggest that ABA is involved in regulating the first steps of germination, but unidentified developmental effectors that are specific to dormancy appear to stimulate ABA synthesis and to enforce the responsiveness to this phytohormone. These primary effectors appear physiologically to modulate dormancy and via ABA they effect the growth of the embryo. Therefore, it is suggested that ABA plays a key role in integrating the dormancy-specific developmental signals with the control of growth.     

Regulation of hormone metabolism in Arabidopsis seeds: phytochrome regulation of abscisic acid metabolism and abscisic acid regulation of gibberellin metabolism

In a wide range of plant species, seed germination is regulated antagonistically by two plant hormones, abscisic acid (ABA) and gibberellin (GA). In the present study, we have revealed that ABA metabolism (both biosynthesis and inactivation) was phytochrome-regulated in an opposite fashion to GA metabolism during photoreversible seed germination in Arabidopsis. Endogenous ABA levels were decreased by irradiation with a red (R) light pulse in dark-imbibed seeds pre-treated with a far-red (FR) light pulse, and the reduction in ABA levels in response to R light was inhibited in a phytochrome B (PHYB)-deficient mutant. Expression of an ABA biosynthesis gene, AtNCED6, and the inactivation gene, CYP707A2, was regulated in a photoreversible manner, suggesting a key role for the genes in PHYB-mediated regulation of ABA metabolism. Abscisic acid-deficient mutants such as nced6-1, aba2-2 and aao3-4 exhibited an enhanced ability to germinate relative to wild type when imbibed in the dark after irradiation with an FR light pulse. In addition, the ability to synthesize GA was improved in the aba2-2 mutant compared with wild type during dark-imbibition after an FR light pulse. Activation of GA biosynthesis in the aba2-2 mutant was also observed during seed development. These data indicate that ABA is involved in the suppression of GA biosynthesis in both imbibed and developing seeds. Spatial expression patterns of the AtABA2 and AAO3 genes, responsible for last two steps of ABA biosynthesis, were distinct from that of the GA biosynthesis gene, AtGA3ox2, in both imbibed and developing seeds, suggesting that biosynthesis of ABA and GA in seeds occurs in different cell types.     

Germination response of black and yellow seed coated canola (<Emphasis Type="Italic">Brassica napus</Emphasis>) lines to chemical treatments under cold temperature conditions

Seed quality is a key critical component to produce well established and vigorous seedlings under cool soil (<10°C) conditions experienced in Western Canada. A simple, relatively quick germination assay is required to separate small differences in seed germination which can have a significant impact on seedling growth. It has long been established that phytohormones regulate seed germination: abscisic acid inhibits germination whereas gibberellins enhance germination. We investigated the effects of ABA, GA, ethylene and inhibitors of these phytohormones alone and in combination on the germination rate of a black and a yellow seed canola (Brassica napus) imbibed at 8°C. The effects of either saline solutions, osmotic solutions, fusicoccin or testa on the germination of canola seeds imbibed at 8°C were also investigated. This temperature is representative of the soil temperatures experienced in the early spring of Western Canada. The two canola seed lines, especially the yellow seed line, were very sensitive to increasing concentration of saline solutions at 8°C, but not at 23°C; however, iso-osmotic solutions that reduced water potential were more inhibitory. The seed coat (testa) including the endosperm was a major factor affecting the germination rate of the yellow seed line at 8°C, however, GA₄₊₇ overcame the inhibitory effect of the testa, whereas ABA exacerbated it. Fusicoccin was more stimulatory to germination than GA₄₊₇, however, unlike GA₄₊₇, it was unable to overcome the inhibitory effect of paclobutrazol, a GA biosynthesis inhibitor. Fluridone, an ABA biosynthesis inhibitor, was unable to overcome the inhibitory effects of a saline solution suggesting that the inhibitory effect was not due to elevated ABA levels. Ethylene, a stimulator of germination did not appear to be involved in the germination of these two lines. Controlled deterioration at 35°C, 85% RH could be either partially or completely overcome by exogenous GA₄₊₇. This study demonstrated the effect of hormones, salinity and testa on the germination of canola seeds under less than ideal environmental conditions.     

Importance of methionine biosynthesis for Arabidopsis seed germination and seedling growth

Proteomics of Arabidopsis seeds revealed the differential accumulation during germination of two housekeeping enzymes. The first corresponded to methionine synthase that catalyses the last step in the plant methionine biosynthetic pathway. This protein was present at low level in dry mature seeds, and its level was increased strongly at 1-day imbibition, prior to radicle emergence. Its level was not increased further at 2-day imbibition, coincident with radicle emergence. However, its level in 1-day imbibed seeds strongly decreased upon subsequent drying of the imbibed seeds back to the original water content of the dry mature seeds. The second enzyme corresponded to S -adenosylmethionine synthetase that catalyses the synthesis of S -adenosylmethionine from methionine and ATP. In this case, this enzyme was detected in the form of two isozymes with different p I and M r. Both proteins were absent in dry mature seeds and in 1-day imbibed seeds, but specifically accumulated at the moment of radicle protrusion. Arabidopsis seed germination was strongly delayed in the presence of dl -propargylglycine, a specific inhibitor of methionine synthesis. Furthermore, this compound totally inhibited seedling growth. These phenotypic effects were largely alleviated upon methionine supplementation in the germination medium. The results indicated that methionine synthase and S -adenosylmethionine synthetase are fundamental components controlling metabolism in the transition from a quiescent to a highly active state during seed germination. Moreover, the observed temporal patterns of accumulation of these proteins are consistent with an essential role of endogenous ethylene in Arabidopsis only after radicle protrusion.     

Biological control of Pythium damping‐off by seed‐treatment with pseudomonas putida: Relationship with ethanol production by pea and soybean seeds

The relationship between biocontrol activity of Pseudomonas putida strain N1R against Pythium ultimum on pea and soybean seeds and the reduction in ethanol evolution by imbibed seeds was investigated under different treatment conditions, including temperature and numbers of seed‐applied cells of the bacterium. Treatment with strain N1R increased emergence at all temperatures, except for soybean at 12 °C and reduced ethanol concentration in the spermosphere of imbibed seeds at several temperatures. The concentration of bacterial cells in the seed treatment suspension also significantly affected biocontrol efficiency and reduced ethanol production, especially in pea seeds. In contrast, the duration (0–7 h) of submergence of seeds in bacterial suspension had little effect on biocontrol activity of N1R, although submergence of soybean seeds reduced their emergence even in the absence of the pathogen or biocontrol agent. Competition for seed‐derived compounds, including ethanol, is suggested to be one possible mechanism of biocontrol of Pythium by strain N1R, which is not known to produce antifungal antibiotics.     

Dissociation of ribosomes and seed germination

Ribosomes from rice embryos (Oryza sativa) were dissociated into ribosomal subunits in vitro by systematic reduction of the Mg²⁺ concentration. Ribosomes from imbibed (28 C) embryos were more easily dissociated than those from nonimbibed embryos. This was not observed with ribosomes from either imbibed, nonviable embryos, or from embryos imbibed at 0 C. Ribosomes from embryos which had been imbided and subsequently dehydrated resembled ribosomes from nonimbibed embryos in their resistance to dissociation. The change in the resistance to dissociation was essentially complete after the first 20 minutes of imbibition at 28 C, and accompanied activation in vivo of protein synthesis as determined by amino acid incorporation in vitro. Ribosomes from either imbibed or nonimbibed embryos could be dissociated into subunits by 0.5 m KCl. These subunits were separated by density gradient centrifugation, and, if recombined, were active for polyphenylalanine synthesis in vitro. The individual subunits prepared from nonimbibed embryos could be replaced by the corresponding subunit fraction from imbibed embryos without loss of capacity to support polyphenylalanine synthesis. The change in the ease of dissociation of ribosomes appears to be a physiological process, and its possible relationship to the initiation of protein synthesis during seed germination is discussed.     

Long-term storage of dormant Grand Rapids lettuce seeds in the imbibed state: physiological and metabolic changes

Grand Rapids lettuce (Lactuca sativa L.) seeds retained their viability for up to ten months when maintained in the fully imbibed state on moist filter paper at 25°C in darkness. The ability of red light (R) to promote their germination was essentially lost within one week from the start of imbibition; sensitivity to gibberellic acid was retained for two weeks longer. Seeds which did not respond to either treatment had entered the state of secondary dormancy (skotodormancy). This could be relieved at all times by a combination of benzyladenine and R applied to the intact seed, or by isolation of the embryo and incubation on water. Protein synthesis increased initially following imbibition but declined after 72 h to a constant low level. Respiration declined over the first week of storage in the imbibed state to a much-reduced steady level. Cotyledonary lipid declined between four and ten months of storage but the axial lipid remained unchanged. Sucrose in the embryo increased after five months, but no changes in glucose, galactose, fructose or mannose were found. The total N content of the cotyledons declined over the first three months of storage in the imbibed state, with a concomitant rise in axial N; the latter declined slowly thereafter. Basal α-galactosidase (EC 3.2.1.22) activity decreased over seven months, but the phytochrome-induced component could not be raised by a 15-min R treatment even after one month. Germination induced by R and benzyladenine was achieved at later times without a rise in α-galactosidase levels.     

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

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