Photosensibilité des graines depinus banksiana Lamb.

Germination ofPinus banksiana seeds is controlled by the photoreversible phytochrome reaction. The seeds, even unimbibed, are sensitive to red light. At 660 nm, the energy required to promote germination to the same order of magnitude is much higher for unimbibed seeds than for the imbibed ones. In both cases it is possible to reverse the effect of a single red light irradiation by applying far red light (730 nm).     

Phytochrome-mediated changes in the ATP content of Kalanchoë blossfeldiana seeds

One short red (R) irradiation increases the ATP content of Kalanchoë blossfeldiana Poelln. cv Feuerblüte seeds before onset of germination. Phytochrome control is demonstrated by the full R/far-red light (FR) reversibility of the effect in water imbibed seeds. In seeds imbibed in the presence of gibberellin A₃ (GA₃, one short R exposure already increases the ATP content when given 2h after start of imbibition, showing phytochrome control at the energy-metabolic level when one R pulse cannot yet induce germination. After longer imbibition periods in the presence of GA₃, one short FR irradiation also increases the ATP content of ungerminated Kalanchoë seeds. The time course of the ATP levels after a R or FR germination inducing irradiation shows an initial increase that clearly preceeds germination. A second increase starts about 15 h after irradiation and is most probably the consequence of the germination itself. The results suggest that, in Kalanchoë seeds, the increase in ATP levels, induced by irradiation(s) and preceding germination, is a phytochrome-mediated process, supplying energy, required for germination.     

A histone methyltransferase inhibits seed germination by increasing PIF1 mRNA expression in imbibed seeds

Phytochrome‐interacting factor 1 (PIF1) inhibits light‐dependent seed germination. The specific function of PIF1 in seed germination is partly due to its high level of expression in imbibed seeds, but the associated regulatory factors have not been identified. Here we show that mutation of the early flowering in short days (EFS) gene, encoding an H3K4 and H3K36 methyltransferase, decreases the level of H3K36me2 and H3K36me3 but not H3K4me3 at the PIF1 locus, reduces the targeting of RNA polymerase II to the PIF1 locus, and reduces mRNA expression of PIF1 in imbibed seeds. Consistently, the efs mutant geminated even under the phyB_off condition, and had an expression profile of PIF1 target genes similar to that of the pif1 mutant. Introduction of an EFS transgene into the efs mutant restored the level of H3K36me2 and H3K36me3 at the PIF1 locus, the high‐level expression of PIF1 mRNA, the expression pattern of PIF1 target genes, and the light‐dependent germination of these seeds. Introduction of a PIF1 transgene into the efs mutant also restored the expression pattern of PIF1 target genes and light‐dependent germination in imbibed seeds, but did not restore the flowering phenotype. Taken together, our results indicate that EFS is necessary for high‐level expression of PIF1 mRNA in imbibed seeds.     

Roles of Soluble Sugars in Protecting 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). Soluble sugars in the imbibition solutions influenced the depth of skotodormancy. Ten-day DS seeds, imbibed in 50–500 mm sucrose or 100–500 mm glucose and given terminal GA₃ germinated completely and germinated about 80% when imbibed in 100 mm galactose, mannose, lactose, or maltose. In contrast, terminal R applied to 10-day DS seeds caused only 20–50% germination. If given R at day 0 and imbibed for 10 days in darkness in 500 mm sucrose or glucose, seeds washed free of exogenous glucose or sucrose then germinated about 50% in darkness in water. These seeds responded to terminal R or GA₃ with complete germination. When seeds were given FR at day 0, germination responses following terminal R or GA₃ were significantly lower when the duration of DS was increased from 7–10 day DS to 15 days. In 10-day DS seeds given initial FR and imbibed in either solutions of 50 or 100 mm sucrose and KNO₃, either terminal R or GA₃ treatment gave complete or near complete germination. It is concluded that seed exposure to certain soluble sugars and/or nitrate during a 10-day DS protected certain substrates and thereby extended the sensitivity of the seeds to terminal R or GA₃ treatment. The study provides substantial evidence for nonhormonal factors associated with light and GA action in the control of seed skotodormancy. Received October 30, 1996; accepted April 22, 1997     

Changes in germination and respiratory potential of embryos of dormant Grand Rapids lettuce seeds during long-term imbibed storage,and related changes in the endosperm

Grand Rapids lettuce (Lactuca sativa L.) seeds were stored in an imbibed state for up to two years. Embryos dissected from stored seeds showed a progressive loss with time in their ability to germinate on polyethylene glycol (PEG) solutions. Little germination of dissected embryos from one-month imbibed seeds occurred on-6 bar PEG but only after four months of storage did the dissected embryos fail to germinate on-4 bar PEG. After two years storage 30% of dissected embryos still were able to germinate on-2 bar PEG. This loss of germination potential, which may be a symptom of the development of an embryo dormancy, could be reversed by N⁶-benzyladenine (BA) and red light (R) applied together or separately to dissected embryos. Two weeks of chilling of 12-month imbibed seeds restored sensitivity to R and a 48-h BA pretreatment prior to R resulted in germination rates similar to those of seeds emerging from primary dormancy. There was loss of embryo control of endo--mannanase activity after two weeks of storage even though the endosperms themselves retained their capacity for enzyme synthesis for six more weeks. Eventually, then, endo--mannanase synthesis is not possible because of inherent changes in both the embryo and endosperm, although each tissue undergoes changes at its own rate. Oxygen uptake by embryos dissected from two-month imbibed seeds did not increase to the same extent as embryos dissected from freshly imbibed seeds. In intact seeds germinating from a skotodormant state, oxygen uptake increased at a time coincident with radicle protrusion, but did not achieve the levels of uptake of those seeds germinating from a primary dormant state. The decline in uptake of oxygen by secondary dormant seeds is the result of a lowered respiratory capability of the embryo itself, rather than of changes in permeability of the surrounding structures.Abbreviations BA N⁶-benzyladenine - Pfr active (far-redabsorbing) form of phytochrome - R red light - PEG polyethylene glycol     

Profile of Plant Hormones and their Metabolites in Germinated and Ungerminated Canola (Brassica napus) Seeds Imbibed at 8°C in either GA4+7, ABA, or a Saline Solution

Abscisic acid (ABA) and gibberellins (GAs) are two major phytohormones that regulate seed germination in response to internal and external factors. In this study we used HPLC-ESI/MS/MS to investigate hormone profiles in canola (Brassica napus) seeds that were 25, 50, and 75% germinated and their ungerminated counterparts imbibed at 8°C in either water, 25 μM GA₄₊₇, a 80 mM saline solution, or 50 μM ABA, respectively. During germination, ABA levels declined while GA₄ levels increased. Higher ABA levels appeared in ungerminated seeds compared to germinated seeds. GA₄ levels were lower in seeds imbibed in the saline solution compared to seeds imbibed in water. Ungerminated seeds imbibed in ABA had lower GA₄ levels compared to ungerminated seeds imbibed in water; however, the levels of GA₄ were similar for germinated seeds imbibed in either water or ABA. The ABA metabolites PA and DPA increased in seeds imbibed in either water, the saline solution, or ABA, but decreased in GA₄₊₇-imbibed seeds. In addition, ABA inhibited GA₄ accumulation, whereas GA had no effect on ABA accumulation but altered the ABA catabolism pathway. Information from our studies strongly supports the concept that the balance of ABA and GA is a major factor controlling germination.     

Osmoconditioning prevents the onset of microtubular cytoskeleton and activation of cell cycle and is detrimental for germination of Jatropha curcas L. seeds

• Jatropha curcas is an oilseed crop renowned for its tolerance to a diverse range of environmental stresses. In Brazil, this species is grown in semiarid regions where crop establishment requires a better understanding of the mechanisms underlying appropriate seed, seedling and plant behaviour under water restriction conditions. In this context, the objective of this study was to investigate the physiological and cytological profiles of J. curcas seeds in response to imbibition in water (control) and in polyethylene glycol solution (osmoticum).
• Seed germinability and reactivation of cell cycle events were assessed by means of different germination parameters and immunohistochemical detection of tubulin and microtubules, i.e. tubulin accumulation and microtubular cytoskeleton configurations in water imbibed seeds (control) and in seeds imbibed in the osmoticum.
• Immunohistochemical analysis revealed increasing accumulation of tubulin and appearance of microtubular cytoskeleton in seed embryo radicles imbibed in water from 48 h onwards. Mitotic microtubules were only visible in seeds imbibed in water, after radicle protrusion, as an indication of cell cycle reactivation and cell proliferation, with subsequent root development. Imbibition in osmoticum prevented accumulation of microtubules, i.e. activation of cell cycle, therefore germination could not be resumed.
• Osmoconditioned seeds were able to survive re‐drying and could resume germination after re‐imbibition in water, however, with lower germination performance, possibly due to acquisition of secondary dormancy. This study provides important insights into understanding of the physiological aspects of J. curcas seed germination in response to water restriction conditions.

     

Stimulation of Empress Tree Seed Germination by Liquid Smoke

The germination of Empress tree (Paulownia tomentosa Steud.) seeds is phytochrome-controlled. Liquid smoke could not induce germination in darkness but red light irradiation of liquid smoke imbibed seeds induced a high percentage of germination. Maximum germination was achieved at liquid smoke concentration of 0.1% (v/v) when present during the imbibition phase or during the phase of phytochrome activity. The light requirement of these seeds could be completely substituted by exogenously applied gibberellins. In the presence of liquid smoke, optimal concentrations of GA₃, GA₄, and GA₉ necessary for inducing germination were several times lower than in the controls, while that of GA₇ was equally active when applied at a concentration one order of magnitude lower. The inhibitory effect of the applied growth retardants was strongly reduced and liquid smoke, in the presence of retardants, allowed light-induced germination, if applied simultaneously or after retardants treatment.     

Mode of phytochrome B action in the photoregulation of seed germination in Arabidopsis thaliana

Arabidopsis thaliana seeds imbibed for a short duration show phytochrome B (PhyB)-specific photo-induction of germination. Using this system, the relationship was determined between the amount of PhyB in seeds and photon energy required for PhyB-specific germination in two transgenic Arabidopsis lines transformed with either the Arabidopsis PhyB cDNA (ABO) or the rice PhyB cDNA (RBO). Immunochemical detection of PhyB apoprotein (PHYB) showed that the expression level of PHYB in ABO seeds was at least two times higher than that in the wild-type seeds, but in RBO seeds the PHYB level was indistinguishable from that in wild-type seeds. The photon fluence required for induction and photoreversible inhibition of germination was examined using the Okazaki large spectrograph. At the wavelengths of 400–710 nm, the ABO seeds required significantly less photon fluence than wild-type seeds for induction of germination, whereas the RBO seeds required similar fluence to wild-type seeds. A critical threshold wavelength for either induction or inhibition of germination of ABO seeds shifted towards the longer wavelengths relative to wild-type seeds. By assuming that PhyA and PhyB are similar in their photochemical parameters, amounts of P_fr at each wavelength were calculated. The photon fluence required for 50% germination was equivalent to the fluence generating a P_fr/P_tot ratio of 0.21–0.43 in wild-type seeds, and of 0.035–0.056 in ABO seeds. These results indicate that PhyB-specific seed germination is not strictly a function of the P_fr/P_tot ratio, but is probably a function of the absolute P_fr concentration.     

Light and temperature action in germination of seeds of the empress tree (Paulownia tomentosa)

Seeds of the empress tree ( Paulownia tomentosa Steud.) were imbibed for two weeks in darkness at constant temperatures (18, 23 or 28°C), and then irradiated with red light for 5 min. Germination was poor if it took place at the same temperature as imbibition, but a high percentage was achieved if the seeds were exposed to higher or lower temperatures before they were irradiated. Maximum germination was obtained when the difference between pretreatment and imbibition was about 10°C. The effect increased with the duration of the pretreatment and was optimal at 24 h. The effect decreased as the time lapse between temperature pretreatment and red light irradiation increased, and it was lost after two days. If pretreatment was shorter than 24 h (12 h). a high percent of germination was obtained by alternating pretreatment and imbibition temperatures. The germination of seeds imbibed in 40% heavy water was also stimulated by temperature pretreatments. Light and temperature also exhibited an interactive effect in the germination of seeds that were imbibed in darkness for only 3 days. For each of the germination phases there was a temperature at which the time needed for 50% germination was the shortest, namely 35°C during imbibition, 37.5°C in the period of P_fr activity. and 32.5°C during radicle protrusion. The data obtained are shortly discussed in relation to the domestication of empress tree in Southern Europe.     

Field fate of Amaranthus patulus seeds subjected to leaf-canopy inhibition of germination

Summary The germination of seeds of Amaranthus patulus Bertol., is known to be sensitive to leaf-transmitted light. Seeds were enclosed in transparent polyester-mesh envelopes and placed horizontally in 10-cm deep soil or on the soil surface, beneath a closed vegetation cover in the field. Changes in the numbers of firm intact seeds and of germinable seeds were traced for up to 3 years by periodical retrievals and germination tests. Rapid loss of germinable seeds, mainly due to germination, was observed in the buried seed population, in which only 20% of seeds maintained their germinability after 1 year, and a negligible number after 3 years. In contrast, the seeds placed on the soil surface maintained germinability relatively well: over 80% of seeds remained germinable after 1 year and a low percentage still preserved their germinability after 3 years. Assuming exponential decay in germinability, the decay rates on and in the soil were calculated from the data of the 1-year experiment to be 0.21 and 0.84 year^-1 respectively. The fate of seeds that were exposed to canopy light on the soil for a month and then buried was shown to be almost the same as that of the seeds which had been continuously in 10-cm deep soil. Correspondingly, the possibility of the induction of secondary (induced) dormancy by exposure to canopy light was excluded in a laboratory experiment, in which it was found that the imbibed seeds suffering leaf-canopy inhibition of germination exuded some diffusible germination inhibitor responsible for apparent dormancy. Estimation of numbers of A. patulus in the seed bank of an early successional field showed that 3,500 seeds/m² remained in the soil to the depth of 10 cm after 3 years' exclusion of the species following the production of 700,000 seeds/m², by a population explosively established after experimental induction of secondary succession.     

After-ripening and phytochrome action in seeds of dormant lines of wild oat (Avena fatua)

The effects of a short exposure to red, far-red or alternate red/far-red light on the germination of seeds after-ripened for different periods of time were studied in dormant lines of wild oat ( Avena fatua L.). Three stages were distinguishable in the after-ripening period in the response of germination to light. Seeds stayed dormant and showed no response to light during stage I. Phytochrome-mediated germination was observed in seeds during stage II. The phytochrome action disappeared during stage III, i.e. seeds fully germinated following treatments of all light qualities. When the seeds were imbibed in polyethylene glycol solutions, dark germination was reduced and phytochrome again had an effect, which suggested the involvement of phytochrome in water uptake of the seed.     

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。     

Photomanipulation of phytochrome in lettuce seeds

Seeds of lettuce (Lactuca sativa L. cv. Grand Rapids) were imbibed and given either short irradiation with red or far red light prior to drying or dried under continuous red or far red light. Seeds treated with either short or continuous red germinate in darkness, whereas seeds treated with either short or continuous far red require a short exposure to red light, after a period of imbibition, to stimulate germination. Irradiation of dry red seeds with far red light immediately before sowing results in a marked inhibition of germination. This result was predicted since far red-absorbing form phytochrome can be photoconverted to the intermediate P650 (absorbance maximum 650 nm) in freeze-dried tissue. A similar far red treatment to continuous red seeds is less effective and it is concluded that in these seeds a proportion of total phytochrome is blocked as intermediates between red-absorbing and far red-absorbing form phytochrome, which only form the far red-absorbing form of phytochrome on imbibition. The inhibition of dry short red seeds by far red light can be reversed by an irradiation with short red light given immediately before sowing, confirming that P650 can be photoconverted back to the far red-absorbing form of phytochrome. The results are discussed in relation to seed maturation (dehydration) on the parent plant.     

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     

Dormancy of<Emphasis Type="Italic"> Arabidopsis</Emphasis> seeds and barley grains can be broken by nitric oxide

Seeds of Arabidopsis thaliana (L.) Heynh. and grains of barley (Hordeum vulgare L.) were used to characterize the affects of nitric oxide (NO) on seed dormancy. Seeds of the C24 and Col-1 ecotypes of Arabidopsis are almost completely dormant when freshly harvested, but dormancy was broken by stratification for 3 days at 4°C or by imbibition of seeds with the NO donor sodium nitroprusside (SNP). This effect of SNP on dormancy of Arabidopsis seeds was concentration dependent. SNP concentrations as low as 25 M reduced dormancy and stimulated germination, but SNP at 250 M or more impaired seedling development, including root growth, and inhibited germination. Dormancy was also reduced when Arabidopsis seeds were exposed to gasses that are generated by solutions of SNP. Nitrate and nitrite, two other oxides of nitrogen, reduced the dormancy of Arabidopsis seeds, but much higher concentrations of these were required compared to SNP. Furthermore, the kinetics of germination were slower for seeds imbibed with either nitrate or nitrite than for seeds imbibed with SNP. Although seeds imbibed with SNP had reduced dormancy, seeds imbibed with SNP and abscisic acid (ABA) remained strongly dormant. This may indicate that the effects of ABA action on germination are downstream of NO action. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide (cPTIO) strengthened dormancy of unstratified and briefly stratified Arabidopsis seeds. Dormancy of three cultivars of barley was also reduced by SNP. Furthermore, dormancy in barley grain was strengthened by imbibition of grain with cPTIO. The data presented here support the conclusion that NO is a potent dormancy breaking agent for seeds and grains. Experiments with the NO scavenger suggest that NO is an endogenous regulator of seed dormancy.Abbreviations ABA Abscisic acid - cPTIO 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3 oxide - GA Gibberellin - SNP Sodium nitroprusside - NO_x Gaseous oxides of nitrogen     

Secondary dormancy (skotodormancy) in seeds of lettuce (Lactuca sativa cv. Grand Rapids) and its release by light, gibberellic acid and benzyladenine

Lettuce seeds (Lactuca sativa L. cv. Grand Rapids) imbibed in darkness at supra-optimal temperatures (23 ± 1°C) develop a secondary dormancy, termed skotodormancy. The seeds first lose their ability to be promoted to germinate by gibberellic acid, and then lose their ability to be promoted by red light. A combination of red light and gibberellic acid will break skotodormancy for longer than either alone, but red light and benzyladenine together are much more effective. Desiccation of skotodormant seeds does not diminish their dormancy. Embryos dissected from skotodormant seeds will germinate, and are as capable of radicle expansion in the osmoticum polyethylene glycol as are newly-imbibed seeds. Hence skotodormancy is a whole seed dormancy and does not reside within the embryo as an inherent block to germination processes, but as an inability to respond to the stimulation of red light or to hormone.     

Identification of defence proteins from the seed exudates of Cicer arietinum L. and its effect on the growth of Fusarium oxysporum f.sp. ciceri

Germinating seeds tend to release a variety of proteins into their surrounding surfaces; some of which have an inhibitory action against plant pathogens. The aim of this study was to investigate and identify defence proteins present in the exudates from water-imbibed and chitosan-imbibed (0.1% w/v) seeds of chickpea (Cicer arietinum L). Chickpea seeds imbibed in chitosan released a higher amount of proteins in the exudate when compared to the seeds imbibed in water. The obtained exudates were analysed in regard to specific protein activities by enzymatic assays and SDS-PAGE analysis. Results showed that the exude obtained from chickpea seeds imbibed in chitosan solution exhibited a new isoform of chitinase, chitosanase and protease inhibitors. These exudates also have an “in vitro” inhibitory effect on the growth of the fungus, Fusarium oxysporum f.sp. ciceri. Our results suggest that seed exudates protect seeds during their germination from soil pathogens.     

Endogenous abscisic Acid levels in germinating and nongerminating lettuce seed

The concentrations of abscisic acid in Grand Rapids lettuce (Lactuca sativa L.) seeds imbibed under conditions which promote or inhibit germination were determined by electron capture-gas chromatography. The concentration of abscisic acid in dry seeds was 12 to 14 nanograms per 100 milligrams. During 24-hour imbibition, the abscisic acid content diminished more rapidly during conditions which allow germination (25 C in light) than in conditions which inhibited germination (35 C in light or darkness at 25 C). A decrease in endogenous levels of abscisic acid was not always correlated with germination.     

Evidence of a role for tyrosine dephosphorylation in the control of postgermination arrest of development by abscisic acid in Arabidopsis thaliana L

In the present paper evidence is presented indicating that tyrosine dephosphorylation is a key regulatory mechanism in postgermination arrest of Arabidopsis thaliana L. seed development mediated by abscisic acid (ABA). By using phenylarsine oxide (PAO), an inhibitor of tyrosine phosphatases, the sensitivity to the inhibitory effect of ABA on seed germination is enhanced. Consistent with this finding, we demonstrate that the ABA-responsive gene, RAB18, is hyperinduced in seeds imbibed in ABA plus PAO, compared with seeds imbibed only with ABA.