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 β-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.

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.

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.

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).

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.

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.

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.

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.

Seed storage proteins were analysed in 27 varieties of alfalfa (Medicago sativaL.); these included five subspecies (glomerata, caerulea, falcata, hemicycla, praefalcata), seven of the nine sources of Medicago germplasm introduced into North America and a sample of additional cultivars. The protein patterns were remarkably consistent for all of these taxa. One-dimensional and two-dimensional gel electrophoresis revealed only minor differences in polypeptide composition within each of the three major classes of storage protein (7S globulin, 11S globulin, 2S albumin). The slight variations that were found provided no information on either parentage or evolutionary relationships amongst these particular taxa. Nonetheless, persistent and reproducible heterogeneity of some minor polypeptides of 11S globulin (medicagin) may be useful under other circumstances for cultivar identification in alfalfa. Both subfamilies (I and II) of the 11S globulin were strongly expressed in all of the cultivars and subspecies examined. It was concluded that this structural divergence within the 11S storage protein family predated the evolution of the M. sativa L. species complex. Most of the variability in storage proteins was quantitative. However, even this variability was reduced when data were standardized with respect to seed dry weights. The consistent similarities in qualitative and quantitative expression of seed storage proteins amongst all of these taxa suggest a high degree of uniformity in both seed physiology and genetics within the alfalfa species complex.

The decline of embryo moisture content from approx. 82 to 53% in 1997 and 56% in 1998 in recalcitrant seeds of Acer saccharinum during maturation was accompanied by decreased mitotic activity in the meristems and an increase in the percentage of cells in the G1 phase of the cell cycle. DNA synthesis and mitosis in the root apex ceased at approx. 53% embryo moisture content, and 67% of the cells were arrested in the G1 phase. During post-maturation drying, cell division in the shoot apex and embryonic leaves continued as long as the embryo moisture content was higher than 50 and 45%, respectively. Mitotic activity in the drying embryo may be controlled by its moisture level. Increased proliferation of the root, shoot and leaf meristems of the mature seeds was already recorded at 24 h of germination, before the root protruded through the seed coat. However, the increase in the embryo moisture content was small – from 56 to 59%. In the 3 d seedlings (10–15 mm long) the mitotic index reached 8% in the root apex and 12% in the shoot apex with leaves. Placing mature seeds in moist conditions might be necessary for the switch from proliferation decline towards its activation. Thus, in contrast with orthodox seeds, the preservation of cell division capacity and increased mitotic activity may be essential for rapid germination immediately after shedding in mature Acer seeds.

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

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 β-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.

The viability of Norway maple seeds collected 21 d before mass maturity (68%moisture content, wet basis) and at mass maturity (56% moisture content) was reduced from 52–85% to 0–7% if dried rapidly (at 10–12% r.h. and 15–17°C for 3 d, then 3 d over silica gel) to 4–5% moisture content. Moist storage of the fruits at 15°C improved the ability of the seeds to tolerate rapid desiccation considerably: 10 and 21 d of moist storage enabled seeds collected at mass maturity or 21 d earlier, respectively, to attain maximum desiccation tolerance to 4–5% moisture content. Moist storage and/or subsequent desiccation affected stachyose, sucrose, and to a lesser extent raffinose, concentrations. The oligosaccharide:total sugar ratio showed a similar pattern in relation to ability to germinate after desiccation to 4–5% moisture content among seeds collected on both dates: desiccation tolerance developed from nil to maximal in these seed populations between threshold oligosaccharide:total sugar values of just less than 0.3 and about 0.4.

The endosperm of dormant wild oat (Avena fatua L.) grains contains a number of mRNAs, including low levels of transcripts encoding avenin (AV10), puroindoline-like (AV1) and serine proteinase inhibitor-like (Z1) proteins. During dry afterripening, a period of warm dry storage that relieves dormancy, the abundance of these intact transcripts gradually declined to undetectable levels in pooled grain samples. In contrast, the three mRNAs did not decline in grains stored for the same period at −20°C, and levels of mRNA decay products derived from these transcripts did not change during afterripening. Disappearance of the three intact mRNAs was not a general phenomenon, since abundance of more than 5000 other seed mRNAs did not change during this period. Additional experiments were conducted to test the idea that the presence of these transcripts was positively correlated with seed dormancy status. First, the rate of intact Z1 mRNA decline was similar to the rate of dormancy relief during 9 months of afterripening. Second, cultivated oat (A. sativa) grains (which are not dormant at maturity) contained much lower levels of these mRNAs than did highly dormant wild oat grains. Third, levels of full-length AV1 and Z1 mRNAs remained constant in dormant grains during 6 d of imbibition. However, Northern analyses of individual grains from a partially afterripened seed lot showed that amounts of intact AV1 or Z1 mRNAs were highly variable in germinating and dormant grains and were not correlated with dormancy status. Even though loss of these transcripts occurred under the same environmental conditions that promote afterripening, a causal relationship between the two processes seems unlikely. The apparently specific mechanism(s) by which these mRNAs are degraded in endosperm tissue may represent a novel post-transcriptional process for regulating levels of certain transcripts in dry grains.

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

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.

The relationship between initial mobilization of 11-S globulin storage protein (leading to solubilization of its B-subunit) 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.

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.