Showing papers in "Seed Science Research in 2000"
TL;DR: Low phytate mutants, that are now available for some key staple food crops such as maize and barley, offer potential benefits in such areas as the sustainability of lands used to grow crops, the mineral nutrition of humans and animals, and reduction in pollution of waterways.
Abstract: A very important mineral storage compound in seeds is phytate, a mixed cation salt of phytic acid (myo-inositol hexakis phosphoric acid). This compound is important for several reasons: (1) It is vital for seed/grain development and successful seedling growth. (2) It is often considered to be an antinutritional substance in human diets, but it may have a positivenutritional role as an anti-oxidant and an anti-cancer agent. (3) It represents a very significant amount of phosphorus being extracted from soilsand subsequently removed with the crop. (4) It plays a role in eutrophication of waterways. A key part of this review is an estimate of the annualtonnage of phosphorus and phytic acid sequestered in up to 4.1 billion metric tonnes of crop seeds and fruits globally each year. We estimate thatnearly 35 million metric tonnes of phytic acid, containing 9.9 million metric tonnes of P, is combined with about 12.5 and 3.9 million metric tonnes of K and Mg respectively, to form each year over 51 million metric tonnes of phytate. The amount of P inthis phytate is equal to nearly 65÷ of the elemental P sold world wide for use in mineral fertilizers. Dry cereal grains account for 69÷ of the total crop seed/fruit production but account for 77÷ of the total phytic acid stored each year. Low phytate mutants, that are now available for some key staple food crops such as maize and barley, offer potential benefits in such areas as the sustainability of lands used to grow crops, the mineral nutrition of humans and animals, and reduction in pollution of waterways.
463 citations
TL;DR: Germination became less dependent on light with increasing seed mass, and analysis of direct correlation and phylogenetically independent contrasts suggests that light response and seed mass coevolved.
Abstract: Germination in light and darkness was compared after cold stratification of seeds of 54 species known or suspected to accumulate persistent seed banks. Germination became less dependent on light with increasing seed mass. This pattern was clear in a direct correlation of individual species data (P <0.0001) as well as when considering phylogenetically independent contrasts (P <0.001). The latter analysis suggests that light response and seed mass coevolved.
442 citations
TL;DR: In this article, the effect of sunflower seed priming on seed vigour was investigated and the results indicated that the CAT isoform pattern might be used as an indicato of the priming treatment that promotes germination.
Abstract: Priming treatment of sunflower (Helianthus annuus L., cv Briosol) seeds for 7 days at 15°C with a polyethylene glycol solution at −2.0 MPa strongly improved their subsequent germination at 15°C on water. This stimulatory effect of priming remained after drying back the seeds at 20°C for 3 days. Malondialdehyde (MDA) and activities of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) were measured in control unprimed seeds, primed seeds, seeds primed then dried, and after 3 and 6 hours of imbibition of controland dried primed seeds in order to determine whether the cell antioxidant systems were involved in the beneficial effect of priming. The osmotreatment resulted in a strong increase in SOD and CAT activities but did not markedly affect MDA and GR activity. Following the 3 days of drying, MDA increased and the enzyme activities became similar to those measured in dry unprimed seeds, although the stimulatory effect of priming on germination remained. Imbibition of control dry seeds was associated with an increase in MDA and a decrease in CAT and GR activities, whereas reimbibition of dried primed seeds resulted in a decrease in MDA and an increase in SOD, CAT and GR activities. Isoform patterns on native gels showed no difference between treated (priming with or without subsequent drying) and control seeds for SOD (7 isoforms) and GR (5 isoforms), but the osmotreatment did induce a second isoform of CAT. The results obtained indicate that the CAT isoform pattern might be used as an indicato of the priming treatment that promotes germination. Involvement of theantioxidant systems in seed vigour is discussed
188 citations
TL;DR: The biological models and approaches that may shed additional light on the role of ethylene during seed germination are presented.
Abstract: In seed formation the role of ethylene has received little attention. The data available on zygotic embryogenesis suggest an association of the ethylene biosynthetic pathway and seed maturation. Over the course of dicot embryogenesis, ACC-oxidase mRNA can be expressed in the cotyledons and embryonic axis. However, as maturation proceeds, cotyledonary ACC-oxidase expression disappears. In some seeds that develop primary dormancy, ethylene synthesis can be among the prerequisites for breaking dormancy. Moreover, the persistence of dormancy may be related to the difficulty of the embryonic axis to produce the necessary ethylene levels or to low tissue sensitivity. The use of inhibitors of ethylene biosynthesis or its action has provided data implicating an ethylene requirement for seed dormancy or germination in some species. However, the role of ethylene in germination remains controversial. Some authors hold that gas production is a consequence of the germination process, while others contend that ethylene production is a requirement for germination. Furthermore, among seeds that require ethylene, some are extremely sensitive to the gas, while others require relatively high levels to trigger germination. Recent studies with Xanthium pennsylvanicum seeds suggest that � cyanoalanine-synthase is involved in ethylene-dependent germination. In addition, regulation of the partitioning of S-adenosyl-L-methionine (AdoMet) between the ethylene vs polyamine biosynthetic pathways may be a way of controlling germination in some seeds. Such regulation may also apply to the reversal of seed thermoinhibition, which can occur when polyamine synthesis is inhibited, thereby strongly channelling AdoMet towards ethylene. The biological models and approaches that may shed additional light on the role of ethylene during seed germination are presented.
176 citations
TL;DR: A new major gene (Phs) was identified as controlling the difference between two red-grained cultivars with widely different dormancies, and appeared to exert its effect in the embryo of the grain, in contrast to R gene expression in maternal testa tissue.
Abstract: Wheat grain dormancy is a multigenic trait controlled both by R genes conferring red testa pigmentation and by other genes, at least one of which has a major effect in the embryo. Enhanced grain dormancy and red test colour are inherited as pleiotropic effects of dominant R alleles at triplicate loci in hexaploid wheat. However, polymorphism for R genes cannot account for the wide variation in dormancy observed among different redgrained varieties. A variety of different dominant R alleles all have equivalent effects on dormancy when introgressed into white-grained wheats, although the latter vary in dormancy both in the absence and in the presence of dominant R alleles. As a result, certain redgrained genotypes can exhibit intermediate dormancy, similar to that of some white-grained genotypes with different genetic backgrounds. A new major gene (Phs) was identified as controlling the difference between two red-grained cultivars with widely different dormancies. The Phs gene appeared to exert its effect in the embryo of the grain, in contrast to R gene expression in maternal testa tissue. Discrete genetic functions thus underlie physiologically distinct mechanisms of coatimposed dormancy and embryo-imposed dormancy in wheat
171 citations
TL;DR: There is little evidence in the scientific literature to support assumptions that dormancy-break in seeds (and fruits) with water-impermeable coats occurs via soil-microbial action and/or abrasion by soil particles, which do not make good evolutionary sense.
Abstract: Books and review articles in various areas of ecology and seed and plant biology continue to report that dormancy-break in seeds (and fruits) with water-impermeable coats (i.e. physical dormancy) occurs via soil-microbial action and/or abrasion by soil particles. However, there is little evidence in the scientific literature to support these assumptions, which, in fact, do not make good evolutionary sense for two related reasons. First, several types of anatomically specialized water-restriction structures have evolved as part of the seed or fruit coat of taxa with physical dormancy. These structures act as ‘signal detectors’ of physical-environmental changes that cause seeds (and fruits) to become water-permeable only at these sites, in seasons and habitats in which there is a good chance that some seedlings will become established. Second, seed (and fruit) coat breakdown by microbial action or by abrasion likely would occur in seasons and habitats in which seedlings could not survive, thus lowering the fitness (λ) of the plant taxa in question.
125 citations
TL;DR: In this paper, seeds from two populations of the bunchgrass Elymus elymoides were incubated under seven temperature regimes following dry storage at 10, 20 and 30°C for intervals from 0 to 16 weeks.
Abstract: Hydrothermal time (HTT) describes progress toward seed germination under various combinations of incubation water potential ( ) and temperature (T). To examine changes in HTT parameters during dormancy loss, seeds from two populations of the bunchgrass Elymus elymoides were incubated under seven temperature regimes following dry storage at 10, 20 and 30°C for intervals from 0 to 16 weeks. Fully after-ripened seeds were primed for 1 week at a range of s. Data on germination rate during priming were used to obtain a HTT equation for each seed population, while data obtained following transfer to water were used to calculate HTT accumulation during priming. HTT equations accurately predicted germination time course curves if mean base water potential, b(50), was allowed to vary with temperature. b(50) values increased linearly with temperature, explaining why germination rate does not increase with temperature in this species. b(50) showed a linear decrease as a function of thermal time in storage. Slopes for the T × b(50) relationship did not change during after-ripening. This thermal after-ripening time model was characterized by a single base temperature and a constant slope across temperatures for each collection. Because the difference between initial and final b(50)s was uniform across tempera-tures, the thermal after-ripening requirement was also a constant. When seeds were primed for 1 week at −4 to −20 MPa, accumulation of HTT was a uniform 20% of the total HTT requirement. When primed at 0 to −4 MPa, HTT accumulation decreased linearly with decreasing priming potential, and a hydrothermal priming time model using a constant minimum priming potential adequately described priming effects. Use of these simple HTT relationships will facilitate modelling of germination phenology in the field.
106 citations
TL;DR: The range of temperatures and the concentrations of oxygen which were effective in priming were very similar to those which allowed solubilization of the B-subunit of 11-S globulin, supporting the robustness of this protein marker for optimization of sugarbeet seed priming.
Abstract: The relationship between initial mobilization of 11-S globulin storage protein (leading to solubilization of its Bsubunit) and germination performance after priming was studied in sugarbeet (Beta vulgaris L.) seeds. Priming was conducted for 2 d either in water (hydropriming) or in ‐2.0 MPa polyethylene glycol-8000 (osmopriming), at various temperatures ranging from 5 to 40°C and oxygen concentrations in the atmosphere ranging from 0 to 21%. For both types of pre-treatments, the range of temperatures and the concentrations of oxygen which were effective in priming were very similar to those which allowed solubilization of the B-subunit of 11-S globulin, supporting the robustness of this protein marker for optimization of sugarbeet seed priming. Furthermore, the temperature and oxygen dependence of priming efficiency closely paralleled that for germination of the untreated seeds, reinforcing the finding that the beneficial effect of priming corresponded to the advancement of germination sensu stricto (i.e. phase II of the germination process). For priming times longer than 2 d, particularly for osmopriming, there was a very dramatic decrease in germination of the treated seeds. For instance, following a 14 d osmopriming at 25°C as much as 60% of the pre-treated seed population failed to germinate when transferred to water. This loss in germination performance quite closely paralleled degradation of LEA (late embryogenesis abundant) proteins, notably a heat-stable seed-specific protein of about 60 kDa and a seed-specific biotinylated LEA protein.
97 citations
TL;DR: A consideration of the available evidence suggests that the first seeds were desiccation-sensitive, but tolerance evolved early and probably a number of times, independently.
Abstract: There is a substantial literature on the basic physiology and response to desiccation of recalcitrant seeds, but little is known about their ecology and even less of their evolutionary status. It is difficult to assess the response of early land plants to dehydration, but it is likely that desiccation tolerance in vegetative tissue arose concomitantly with the invasion of the land. Similarly, from the fossil record it is not possible to assess the desiccation response of early seeds, and furthermore, it is difficult to see phylogenetic relationships among species producing recalcitrant seeds. A consideration of the available evidence, however, suggests that the first seeds were desiccation-sensitive, but tolerance evolved early and probably a number of times, independently. The desiccation sensitivity and short life span (generally shorter than the interval between flowering) of recalcitrant seeds have implications in terms of regeneration ecology. A long-term soil seed bank as such does not exist; rather the seeds germinate and form a seedling bank. However, there is a wide range in post-shedding physiology among recalcitrant seed species, and although species producing recalcitrant seeds are common in the humid tropics, they do occur in habitats with more marked seasonal variation. Here regeneration strategies may be more specialized.
81 citations
TL;DR: Results indicate a functional R1 allele is not absolutely required for dormancy in wheat, but does enhance its expression in caryopses with dormant (sensitive) embryos.
Abstract: Red wheats (Triticum aestivum L.) are generally more dormant and sprout resistant than white wheats. Whether this is caused by pleiotropic effectsof the red grain colour genes (R) on dormancy and coat colour, or to tight linkage between R and dormancy genes has not been fully resolved. To directly determine the effect of the R1 allele on dormancy, mutations were induced with sodium azide in a pure line selection of the red genotype (R1R1r2r2r3r3) Chinese Spring wheat. Two white mutants (CSW01, CSW02) were recovered from M 3 caryopses derived from approximately 20,000 M 2 plants. Both mutants were shown to be allelic to a domesticwhite genotype (r1r1r2r2r3r3). Except for seed coat colour, CSW01 and CSW02 are morphologically indistinguishable from the wild type and are presumed to be near isogenic lines of Chinese Spring. Freshly harvested grainsproduced under four different environments were evaluated for post-harvest dormancy. In all environments, intact caryopses of all three isolines exhibited high temperature dormancy typical of cereal species, although the red wild type consistently exhibited greater dormancy than the white mutant isolines. Dormancy was dissipated by afterripening in dry storage at 37 ° C in a similar manner for the red and white isolines. Excised embryos of the three isolines exhibited similar levels of dormancy and sensitivities to exogenous abscisic acid. These results indicate a functional R1 allele is not absolutely required for dormancy in wheat, but does enhance its expression in caryopses with dormant (sensitive) embryos
80 citations
TL;DR: It is the interaction of these structural motifs combined with the molecular regulatory mechanisms that ensure the appropriate timing and positioning of hydrolase production and endosperm reserve mobilization that ensure an extended release of nutrients to fuel early seedling growth.
Abstract: Cereal grain germination and early seedling growth involve the co-ordinated action of endosperm and embryo tissues to mobilize the storage reserves of the starchy endosperm. This mobilization is accomplished by hydrolases secreted from the aleurone and scutellar tissues. The breakdown products are then transported to the growing seedling by the scutellum. This resource-harvesting system is regulated at multiple levels. One well-defined aspect of control is brought about by the hormone gibberellin (GA). Gibberellin is released from the embryo upon imbibition and activates the aleurone cells. The secretory apparatus of the aleurone then proliferates, supporting increased hydrolase synthesis and secretion to degrade the starchy endosperm. The molecules that regulate this response to GA are now being increasingly characterized. Elements such as cGMP, calcium, calmodulin and protein kinases are well known as regulators in other eukaryotic cell types and are emerging as key control factors in the aleurone hormone response. However, superimposed upon this molecular regulatory system is another level of control, the structural pattern of tissues and stored macro- molecules that was laid down during grain development. It is the interaction of these structural motifs combined with the molecular regulatory mechanisms that ensure the appropriate timing and positioning of hydrolase production and endosperm reserve mobilization. This integrated control system ensures an extended release of nutrients to fuel early seedling growth.
TL;DR: The results obtained are consistent with the concept that oligosaccharides may well be involved in the protection of membranes during dehydration.
Abstract: Germination and carbohydrate metabolism were studied in fresh developing pea (Pisum sativum L., cv Baccara) seeds and after artificial drying at 25°C and various relative humidities (20, 75 and 99% RH) to investigate whether the occurrence of desiccation tolerance was related to sucrose, raffinose and stachyose contents. Seeds became completely tolerant to fast drying at 25°C and 20% RH a few days after the end of reserve accumulation, i.e. when their moisture content dropped to approx. 50% (fresh weight basis). This acquisition of desiccation tolerance was associated with an accumulation of raffinose and stachyose, the latter being more abundant in the embryonic axis than in the cotyledons. The (raffinose+stachyose)/sucrose ratio increased during seed development and reached 1.1 in the axis and 0.2 in the cotyledons just before the onset of desiccation tolerance. When the natural acquisition of desiccation tolerance occurred on the mother plant, artificial drying of isolated seeds induced an increase in oligosaccharide content in the cotyledons. Immature seeds, the moisture content of which was higher than about 60% (fresh weight basis), did not tolerate fast drying (25°C and 20 or 75% RH). Such drying did not result in the synthesis of stachyose and induced an increase in electrolyte leakage, a decrease in the ability of seeds to convert 1-aminocyclopropane 1-carboxylic acid (ACC) to ethylene and an increase in ethane synthesis, thus indicating a deterioration of cell membrane properties and lipid peroxidation. In contrast, immature seeds tolerated drying either in the pods or at 25°C and 99% RH, and such drying induced a decrease in sucrose content, an increase in oligosaccharide content and a (raffinose+stachyose)/sucrose ratio higher than around 1. Soluble sugar contents of dried immature seeds depended on the conditions of dehydration. In cotyledons, the (raffinose+stachyose)/sucrose ratio reached 0.61 when seeds were dried at 25°C and 99% RH, whereas it was as low as 0.15 when drying was performed at 25°C and 20% RH. All the results obtained are consistent with the concept that oligosaccharides may well be involved in the protection of membranes during dehydration.
TL;DR: Dormancy-breaking requirements and types of dormancy were determined for seeds of Lonicera fragrantissima and L. japonica and about half of the seeds of the two species has nondeep simple MPD, and the other half has morphological dormancy (MD).
Abstract: Dormancy-breaking requirements and types of dormancy were determined for seeds of Lonicera fragrantissima Lindl. & Paxt., L. japonica Thunb., L. maackii (Rupr.) Maxim. and L. morrowii A. Gray. Seeds of all four species have underdeveloped spatulate embryos that are about 20–40%fully developed (elongated) when dispersed. Embryos in freshly matured, intact seeds grew better at 25/15°C than at 5°C. Gibberellic acid (GA3) (tested only in the light) was more effective in breaking dormancy in L. maackii and L. morrowii than in L. fragrantissima and L. japonica. Warm- followed by cold stratification was required to break dormancy in seeds of L. fragrantissima, whereas seeds of L. japonica required cold stratification only. Thus, seeds of L. fragrantissima have deep simple morphophysiological dormancy (MPD) and those of L. japonica nondeep simple MPD. About 50%of the seeds of L. maackii required warm- or cold stratification only to come out of dormancy and 50% of those of L. morrowii required warm stratification only, whereas the other 50% did not require stratification to germinate. Thus, about half of the seeds of the two species has nondeep simple MPD, and the other half has morphological dormancy (MD). In these laboratory tests, seeds of L. japonica, L. maackii, and L. morrowii generally germinated to significantly higher percentages in light than in darkness; seeds of L. fragrantissima were not tested in darkness. Peaks of germination for seeds of L. fragrantissima, L. japonica, L. maackii and L. morrowii sown on a soil surface and covered with Quercus leaves under near-natural temperature conditions shortly after seed maturity and dispersal in late June 1997, late November 1997, early November 1996 and late June 1998, respectively, occurred in early March 1998, late February 1998, late March 1997 and early October 1998, respectively. The germination phenologies of seeds of the same species and seed lots buried in soil were similar to those of seeds under leaf litter. High percentages of seeds of all four species germinated both under litter (78–96%) and beneath the soil surface (78–97%). These germination patterns correspond closely with the requirements for embryo growth and dormancy break in the four Lonicera species.
TL;DR: LS concentrations that broke dormancy of intact grains partially inhibited germination of nondormant, fully afterripened grains and storing grains in containers of flooded soil increased sensitivity to both LS and pricking.
Abstract: A commercial liquid smoke (LS) flavoring product (Reese Hickory brand) in aqueous dilutions of 4–5% (v/v) broke dormancy (40–100% germination) of both intact and dehulled red rice (Oryza sativa) To elicit this response in intact grains, the glumes had to be pricked with a dissecting needle following contact with LS and water rinsing Germination percentages rose with increasing contact time (up to 7 d), and grains first imbibed with water before transfer to LS showed similar germination percentages as those first incubated with LS before transfer to water LS dilutions were highly acidic (about pH 3) Dormancy-breaking activity of LS was pH-dependent and was reduced at pH values of 5–7 Buffer controls remained dormant irrespective of pH value Maximum activity varied primarily among different bottles of LS and only to a minor extent between red rice seed lots Dry-afterripening at 30°C for up to 14 d increased the sensitivity of unpricked, intact grains to LS compared with water controls LS concentrations that broke dormancy of intact grains partially inhibited germination of nondormant, fully afterripened grains Storing grains in containers of flooded soil increased sensitivity to both LS and pricking Nitrite levels in LS were negligible and insufficient to account for the bioactivity of LS
TL;DR: Addition of the compost activator resulted in a 20-fold increase of microbial activity in the pretreatment mixture, indicating that enhanced microbial growth resulted in higher and more predictable germination percentages.
Abstract: Germination of native tree and shrub species from seed can be unpredictable. Germination of Rosa corymbifera ‘Laxa’ was 2% under normal commercial conditions. This was obtained in the presence of the natural microflora found on the seeds. The microflora originated on the hips and the seeds become inoculated during extraction. Exclusion of microbes from such pretreatments resulted in no germination. Inoculation of surface sterilized seeds with members of the natural microflora resulted in 3% germination. The addition of Garotta TM , a commercial compost activator, to the commercial pretreatment increased germination to 95%. This high germination percentage was sustained over a 5 year period using seeds from the same stock bushes. Addition of the compost activator resulted in a 20-fold increase of microbial activity in the pretreatment mixture, indicating that enhanced microbial growth resulted in higher and more predictable germination percentages.
TL;DR: The results suggest that, although damage resulting in viability loss was clearly taking place, the limitation to desiccation tolerance did not result from inadequate free radical scavenging, and this may account for their different response to that of the seeds of other more sensitive recalcitrant species studied.
Abstract: Recalcitrant seeds are shed moist from the plant and do not survive desiccation to the low moisture contents required for prolonged storage. It has been widely hypothesised that during desiccation of these seeds a stress induced metabolic imbalance develops that leads to free radical mediated damage and viability loss. We investigated this hypothesis in a comparison of two sympatric species of Acer during late seed development and post-harvest desiccation: A. platanoides (Norway maple) has orthodox seeds and A. pseudoplatanus (sycamore) has recalcitrant seeds. In both species, respiration rates declined to similar levels at shedding, and the extent of defences against free radicals appears no less in sycamore than that in Norway maple. During drying there was no evidence for the accumulation of a stable free radical, increased lipid peroxidation or decline in free radical scavenging enzymes in either species. In addition, there was a very similar, large increase in total tocopherol in both species. This increase in sycamore was largely of alpha-tocopherol, whereas in Norway maple the increase was largely from its precursor, gamma-tocopherol. Arguably this suggests a similar mechanism in both species, but increased oxidative stress in sycamore. In general, the results suggest that, although damage resulting in viability loss was clearly taking place, the limitation to desiccation tolerance did not result from inadequate free radical scavenging. Soluble carbohydrates and dehydrin-like proteins were also measured during late seed development and drying in sycamore and Norway maple. The greater concentrations of sucrose, raffinose and stachyose and amounts of dehydrins in the radicles and cotyledons of Norway maple compared with those in sycamore was consistent with greater desiccation tolerance in the former. Sycamore seeds are dormant and at the tolerant end of the continuum of desiccation sensitivity among recalcitrant species, and this may account for their different response to that of the seeds of other more sensitive recalcitrant species studied.
TL;DR: It is concluded that a comparison of the timing of the onset of degeneration may well be diagnostic of its cause, and if fungal activity is curtailed, then the hydrated storage lifespan of A. marina seeds can be considerably extended.
Abstract: Three questions are considered in the context of the possible effects of seed-associated mycoflora, typified by Fusarium moniliforme, during hydrated storage of recalcitrant propagules of the tropical species, Avicennia marina. These pertain to storage lifespan, whether seed susceptibility to fungal attack changes and the possibility of discriminating ultrastructurally between inherent deteriorative changes and those that are fungallyinduced. The data indicate unequivocally that if fungal activity is curtailed, then the hydrated storage lifespan of A. marina seeds can be considerably extended. When inoculated immediately with F. moniliforme, newly harvested seeds were extremely susceptible to the adverse effects of the fungus, while seeds that had been wet-stored for 4 d showed a considerably heightened resilience to the effects of the fungus when inoculated at that stage. The enhanced resilience, although declining, persisted in seeds stored hydrated for up to 10 d prior to inoculation, being lost after 12 d. After 14 d of hydrated storage, seeds became more susceptible to the effects of the fungus than those in the newly harvested condition. The resilience of seeds that had been stored in the short-term was associated with ultrastructural changes indicative of enhanced metabolic activity associated with the onset of germination. However, with the sustained stress imposed by wet-storage conditions, the seeds became increasingly badly affected by the fungus. While it was not possible to discriminate with certainty among deteriorative subcellular events ascribable to inherent deterioration or the effects of the fungus, it is concluded that a comparison of the timing of the onset of degeneration may well be diagnostic of its cause.
TL;DR: Dormancy could be removed in a large proportion of the population by the application of a single heat shock to rehydrated seeds for 4 h at 36°C, with subsequent return to 26°C for germination, and desiccation induced seed dormancy rather than viability loss.
Abstract: The effects of desiccation and temperature on the germination capability of Carica papaya L. were investigated for seeds extracted from three commercial fruit batches. More than 50% of freshly isolated, cleaned (sarcotesta removed) but undried seeds germinated at 26°C. However, desiccation to approx. 20%seed RH reduced germination at this temperature to < 10%. A substantial increase in germination at alternating temperatures (33/19°C) indicated that desiccation induced seed dormancy rather than viability loss. Dormancy could be removed in a large proportion of the population by the application of a single heat shock to rehydrated seeds for 4 h at 36°C, with subsequent return to 26°C for germination. Longer (days) and shorter (minutes) periods of heat shock were less effective for releasing dormancy. Heat shock was generally applied 5 or 14 d after rehydration had started, but the treatment was equally effective after imbibition for only 1 d. Light was always applied during both imbibition and the post-heat shock treatment, but was not essential during the actual heat shock treatment. Rehydration and post-heat shock temperature treatments in the range of 16°C to 36°C revealed the same optima of 26°C. Dormancy was re-imposed in heat-shocked seeds when they were subsequently dried to seed relative humidities of 25 to 75%(5 to 11% moisture content [fresh weight basis]), but this state could be removed by a further heat shock. The longer heat-shocked seeds were held on agar-water at 26°C prior to re-drying, the greater the level of desiccation intolerance.
TL;DR: In this article, Orobanche aegyptiaca seeds were incubated at water potentials of 0 to 1.33 MPa and at constant temperatures from 5 to 29°C.
Abstract: Non-dormant seeds of Orobanche aegyptiaca were incubated at water potentials of 0 to –1.33 MPa and at constant temperatures from 5 to 29°C. Effects of water potential and temperature on final germination were modelled. In general, germination increased with increased temperature from 5 to 20°C and decreased above 26°C. Maximum germination occurred at 20–26°C and 0 MPa. Germination was reduced as the water potential decreased. Water potential also affected the temperature range over which high germination was observed; at 0 MPa high germination occurred over 9° (17–26°C) compared with 3° at -1.25 MPa (17–20°C). The optimum germination temperature also tended to decrease with a decrease in water potential. Final germination could be accounted for by seed-to-seed variation in the population assuming that each seed had a minimum temperature for germination and a maximum temperature above which it would not germinate. Seed-to-seed variation in these characteristics was assumed to be normally distributed, and it was further assumed that the two characteristics were independent. Effects of water potential on these temperature requirements were quantified, and the resulting empirical model accounted for final germination with reasonable accuracy (R2= 0.96).
TL;DR: The results do not support the hypothesis that a dormancy model can be applied universally to all seed lots of Cenchrus ciliaris, and methods of predicting the period of after-ripening required to achieve desired levels of dormancy for reseeding degraded rangelands are discussed.
Abstract: Seeds of Cenchrus ciliaris L. were produced under different hydro–photo–thermal environments with and without fertilizer. Dormancy loss of spikelets and extracted caryopses was tested during dry after-ripening at 40°C and 43% equilibrium relative humidity. Caryopses had higher initial germination and lost their dormancy faster than spikelets. Dormancy of both caryopses and spikelets generally decreased with an increase of maturation temperature and fertility, whereas dormancy increased if water stress was imposed during maturation. The latter effect was smaller when the mother plants were exposed to water stress after caryopses were fully formed than when water stress cycles were applied throughout maturation. Daylength extension (to 14 or 18 h d-1) by artificial light increased dormancy of both caryopses and spikelets. The effect of long days declined when plants were exposed to natural daylight for more than 10 h d-1. The after-ripening curves were consistent with the hypothesis that dormancy periods of individual seeds are normally distributed within each seed lot. Rates of loss of dormancy were quantified by the slopes of these curves. In a given experiment, these rates were identical for caryopses but not always for spikelets that matured in diverse environments. Even for caryopses, however, the slopes varied between experiments. Therefore, the results do not support the hypothesis that a dormancy model can be applied universally to all seed lots of Cenchrus ciliaris. Methods of predicting the period of after-ripening required to achieve desired levels of dormancy for reseeding degraded rangelands are discussed.
TL;DR: The simulations revealed a highly significant correlation between the duration of seed development and that of the wet season, and the mean number of dry months that seeds have to withstand after shedding was significantly correlated with the parameters used to quantify seed desiccation sensitivity.
Abstract: A broad variability for seed desiccation sensitivity, as quantified by the water content and the water activity at which half of the initial viability is lost, has been previously observed within nine African coffee species. In order to investigate if these different degrees of desiccation sensitivity correspond to an adaptive trait, additional data, such as the duration of seed development and seed water content at maturity, were measured for these species, and the relationships between these parameters and some climatic characteristics of their specific native environments were investigated. Since flowering in all coffee species occurs only a few days after the main rainfall marking the end of the dry season, simulations could be made, based on the continuous sequences of rainfall data compiled in databases of nine climatic stations, chosen for their appropriate location in the collecting areas. The simulations revealed a highly significant correlation between the duration of seed development and that of the wet season. Consequently, mature seeds are shed at the beginning of the following dry season. Moreover, the mean number of dry months that seeds have to withstand after shedding was significantly correlated with the parameters used to quantify seed desiccation sensitivity. By contrast, seed moisture content at maturity was not correlated with the level of seed desiccation tolerance. All these results are discussed on the basis of more detailed descriptions of the natural habitats of the coffee species studied.
TL;DR: In marked contrast, and as expected from results with pooled seed samples, the distributions of total B-subunit content were superimposed when comparing untreated and primed seed populations.
Abstract: Priming of sugarbeet (Beta vulgaris L.) seeds induces increased solubilization of the basic B-subunit of 11-S globulin (a major seed storage protein in sugarbeet). Using a sensitive single-seed ELISA, the soluble and total B-subunit contents of individual untreated and primed sugarbeet seeds were measured. With the untreated seeds, there was a 160-fold range of the soluble B-subunit content among individual seeds. The individual primed seeds also exhibited large variations in their soluble B-subunit content, yet only over a five-fold range. Furthermore, the frequency distributions of soluble B-subunit content were markedly different for the primed and untreated seed populations; the primed seed population exhibited a substantially higher median than that for the untreated seed population. In marked contrast, and as expected from results with pooled seed samples, the distributions of total B-subunit content were superimposed when comparing untreated and primed seed populations.
TL;DR: It is suggested that the feature of continuous protein synthesis activity with only a temporary interruption in active cell cycling in A. marina root primordia helps to explain both the rapidity in seedling establishment and the extreme vulnerability to desiccation.
Abstract: An autoradiographic study was made of leucine and thymidine incorporation into the meristematic root primordia and hypocotyl tips of seeds of the recalcitrant mangrove species, Avicennia marina. The investigations show that although there is a temporary reduction of protein synthesis at shedding, root primordia and surrounding hypocotyl cells of the axis never wholly cease incorporation of [ 3 H]leucine and regain preshedding levels of activity within a day. Precursor studies using methyl-[ 3 H]thymidine show that, at shedding, there is a temporary cessation of incorporation into root meristem nuclei that lasts no longer than 48 h and, within a day, pre-shedding levels are regained in the meristem nuclei. Analysis of DNA fragmentation patterns in root tips at the time of shedding, and their ability to repair radiation-induced DNA damage, indicate that DNA repair processes are markedly compromised in these cells if water loss reaches 22%. Protein synthesis and DNA replication are reduced by more than half by a water loss of 18% and 16%, respectively. DNA replication does not fully recover on rehydration after only 8% water loss. DNA fragmentation to nucleosomes indicates a programme of cell death at a water loss of 10%. We suggest that the feature of continuous protein synthesis activity with only a temporary interruption in active cell cycling in A. marina root primordia helps to explain both the rapidity in seedling establishment and the extreme vulnerability to desiccation.
TL;DR: The versatility and the sensitivity of the method, together with the feasibility to investigate germination rate in individual seeds within a population, suggest that image analysis techniques have high potential in seed biology studies.
Abstract: An image analysis system is described to study the process of imbibition in white cabbage (Brassica oleracea L.) seeds. The system consists of two components: 1) an environmental system where seeds germinate; 2) a computer imaging system, composed of a Charged Coupled Device (CCD)-imaging sensor, an image frame grabber, a computer and a video monitor. The measurement software can determine area, perimeter, width, length and eccentricity as descriptors of changes in seed size during swelling. The captured images of imbibing seeds allowed estimation of timing of ‘visible germination’ occurrence without any subjective evaluation. Using the area parameter, the imbibition process can be described graphically by a series of curves similar to the triphasic pattern of water uptake, with extension and rate depending on the degree of seed viability and germination medium conditions. Imbibitional osmotic stress, provided by -1.5 MPa mannitol, induced a large delay in germination and, upon stress removal, a more uniform radicle emergence. The versatility and the sensitivity of the method, together with the feasibility to investigate germination rate in individual seeds within a population, suggest that image analysis techniques have high potential in seed biology studies.
TL;DR: In this article, the authors used the Richards function in a factorial germination experiment with new-crop meadowfoam (Limnanthes alba Benth) and found that the results provided insights into possible dormancy mechanisms.
Abstract: While investigating germinability in the new-crop meadowfoam (Limnanthes alba Benth.), it was desired to gain maximum information despite minimal seed material and prior knowledge. Extended use of the Richards function in a factorial germination experiment proved very powerful. The functions yielded estimates of four correlated coefficients, requiring multivariate analysis of variance (MANOVA) to provide valid F-tests among germination profiles. These germination functions provided more rigorous discrimination among treatments than the univariate final germination level. The functions for the imbibants KNO3 and GA4+7, in darkness at 10/5°C, were best, being characterized by high mean absolute germination rate, relatively short duration, and high final germination (upper asymptote). The results provided insights into possible dormancy mechanisms.
TL;DR: A spectrum of severe ultrastructural degeneration, including indirect evidence of the collapse of the nucleoskeleton and extensive cell lysis, accompanied viability decline of the seeds to 4%, indicating a chemical basis for degeneration of hydrated recalcitrant seeds.
Abstract: Recalcitrant seeds of Trichilia dregeana were stored at 16 or 25°C, either at the water content at which they were shed or partially dried. Although having been exposed to a short period (approx. 6 h) at temperatures up to 30°C prior to storage, seeds at the original water content maintained viability for several weeks at 16°C. However, storage of undried seeds at 25°C was deleterious within 8 d, indicating a chemical basis for degeneration of hydrated recalcitrant seeds. Seeds that had been mildly dehydrated to the relatively high axis level of 1.68 g H 2 O g —1 dry mass, while maintaining full germinability immediately after drying, exhibited only 4% viability after 8 d in storage at 16°C and had completely lost viability after the same storage period at 25°C. Ultrastructural features characterizing hydrated seeds included indications of enhanced activity associated with initial exposure to the elevated temperature as well as some signs of stress. However, over an effective 15 d storage period at 16°C, ultrastructural features showed the cells to have retained little damage and to have been in an enhanced state of activity commensurate with ongoing development towards germination. After a longer storage period, however, signs of damage, including indirect evidence for disarray of the cytoskeleton in some axis cells, became apparent in line with the declining seed viability. Immediately following dehydration from an average axis water content of 1.97 to 1.68 H 2 O g g —1 (the sub-imbibed condition), some ultrastructural abnormalities were apparent, but the seeds remained 100% germinable. However, within the 8 d storage period in this sub-imbibed condition, a spectrum of severe ultrastructural degeneration, including indirect evidence of the collapse of the nucleoskeleton and extensive cell lysis, accompanied viability decline of the seeds to 4%.
TL;DR: For both species, changes in the thermal range permissive for germination as a result of dormancy modifications explained to a large extent the timing of the emergence periods observed in the field.
Abstract: Experiments on the facultative winter annuals Carduus acanthoides and Anagallis arvensis were performed: (i) to determine thermal conditions that induce or release dormancy, (ii) to investigate to what extent changes in dormancy level resulting from those thermal conditions explain the seasonal pattern of emergence of these species, and (iii) to estimate required thermal time and base temperature for the germination of non-dormant seeds. Carduus acanthoides required high temperatures followed by decreasing temperatures for dormancy release; however, low winter temperatures did not induce secondary dormancy as expected for a winter annual. To the contrary, low temperatures stimulated dormancy release in the long term. In A. arvensis, dormancy relief was enhanced by dry storage at 25°C, and the response to low temperature was different depending on moisture conditions. Prolonged exposure to moist-chilling increased the dormancy level of the population, while dry storage at 4°C relieved dormancy. For both species, changes in the thermal range permissive for germination as a result of dormancy modifications explained to a large extent the timing of the emergence periods observed in the field. In neither species did base temperature for germination change with the dormancy level of the population. Thermal time required forgermination of C. acanthoides varied with dormancy, while for A. arvensis seeds it was constant.
TL;DR: Small increases in the frequency of some rare or very rare microhabitat types were vital in making grazed rough grassland more suitable for emergence and survival of L. urens, which was particularly favoured by moss and an increase in NNDs.
Abstract: In Britain, Lobelia urens (L.) (the heath lobelia) occurs in rough grassland, is rare and only found in southern England, where it is at the northern limit of its range. Emergence and survival of L. urens was investigated at six locations in two geographically distinct sites experiencing spring, autumn or no grazing in two consecutive years. Four factors were evaluated qualitatively, as a means of characterizing microhabitats for germination and survival: all permutations of higher plant cover, bryophytes, plant litter and surface depressions. The potential effect of adjacent plants on recruitment was also assessed using the nearest neighbour distance (NND). Grazing created depressions, removed plant litter and increased the proportion of sites with higher plant cover. It also resulted in a more open sward with higher NNDs. None of these changes stimulated recruitment. Instead, small increases in the frequency of some rare or very rare microhabitat types were vital in making grazed rough grassland more suitable for emergence. Reduced litter loads and a greater quantity of moss were two key responses. Overall, survival of L. urens was less than 1% and was particularly favoured by moss and an increase in NNDs. Shading from higher plants, with or without plant litter, decreased emergence, but the precise role of litter was complex and most probably related to its quantity. In an experimental seed bed, only empty depressions favoured emergence. The microhabitat relationships of L. urens were unusually consistent among locations and consecutive years. L. urens requires high soil surface temperatures but also adequate water for large scale recruitment, and such conditions are encouraged by grazing. The particular problems of experimental design and statistical analysis of data from recruitment experiments are also discussed.
TL;DR: The micromorphology of the cork-oak (Quercus suber) acorn was examined to evaluate the efficiency of the pericarp as a barrier to water loss and a marked reduction in the mean rate of water loss was recorded for intact acorns compared with those from which thepericarp had been removed.
Abstract: Abstract The aim of the present study was to examine the micromorphology of the cork-oak (Quercus suber) acorn and to evaluate the efficiency of the pericarp as a barrier to water loss. When the desiccation curve obtained at 25 ± 1°C and 62% RH for physiologically mature acorns collected from the tree was compared with that corresponding to fully ripe acorns which had been shed, the latter showed a slightly lower rate of water loss. A marked reduction in the mean rate of water loss was recorded for intact acorns compared with those from which the pericarp had been removed. The external surface and transverse sections of the pericarp from both mature and fully ripe acorns were examined by scanning electron microscopy. Two zones presenting morphological and micromorphological differences were identified: an area including the point of attachment to the cupule and an apical zone covering the embryo. The microstructure of the pericarp showed an external thick cuticle and a single external palisade layer of closely packed cells with no intercellular spaces. Underlying this layer, there was a further parenchymatous layer of poorly differentiated, roughly isodiametric cells. The pericarp in the cupular zone consisted of only this undifferentiated layer between the two epidermal cell layers and contained vascular bundles with many xylem elements.
TL;DR: C cucumber seeds from the top fruit and from the peduncular fruit segments were delayed in reaching maximum quality compared with seeds from other positions, and this was correlated with a slower decline of chlorophyll fluorescence.
Abstract: The performance of cucumber (Cucumis sativus L.) seeds in relation to ovary and ovule position was monitored during seed production. Seeds from three (first, seventh and tenth nodes) fruit positions and three (stylar, intermediate and peduncular) ovule positions were harvested serially during development and characterized with physiological and cellular markers. Seed moisture contents declined to 30%approx. 35 d after pollination (DAP) and remained constant thereafter. At 42 DAP the maximum dry weight was acquired in seeds except those from peduncular segments, whose dry weight accumulation lagged behind. The onset of germinability and desiccation tolerance occurred before the attainment of maximum dry weight, whereas seed performance, as shown by laboratory germination and greenhouse emergence, was largely improved after the completion of seed development. Cell cycle activities (DNA per nucleus and -tubulin content) ceased by 28 DAP, whereas the degradation of seed chlorophyll continued during the entire maturation period. Seeds from the top fruit and from the peduncular fruit segments were delayed in reaching maximum quality compared with seeds from other positions, and this was correlated with a slower decline of chlorophyll fluorescence