The basic B-subunit of the seed storage protein 11-S globulin (an 11-S-legumin type protein) is the major polypeptide in soluble protein extracts from primed sugarbeet (Beta vulgaris L.) seeds. In contrast, only a small amount of this protein is present in corresponding extracts from untreated dry mature seeds. Here, and as for all 11-S globulins described so far, the B-chain is linked to other polypeptide(s), corresponding most presumably to an acidic A-chain, through the formation of disulphide bridge(s). Polyacrylamide gel electrophoretic analyses of total and soluble protein extracts from untreated and primed seeds strongly indicate that this priming-induced solubilization of the B-chain resulted from an endoproteolytic attack on the A-chain. Microscopical immunolocalization showed a uniform distribution of the 11-S globulin B-chain over the protein bodies of the embryonic cells from the untreated seeds. For the primed seeds, however, the B-subunit of 11-S globulin diffused out of the protein bodies and invaded the cytosolic compartment. This phenomenon occurred independently of the manner of priming, being observed with hydroprimed and osmoprimed seeds, as well as with sugarbeet seeds that had been primed by a prehydration treatment. Quantitative analyses of the amounts of soluble 11-S globulin B-chain have enabled the priming of sugarbeet seeds to be optimized.

How some plant cells can survive a period of desiccation to live again when other cells will die, was the theme of the recent Workshop in the mountain retreat of Franschhoek, near Cape Town, South Africa. Sixty-two biochemists and cell physiologists from around the world assembled to discuss these problems for the second time in three years.

Maize (Zea mays L., cv. B73 × Mo17), wheat (Triticum aestivum L., cv. Clark), and soybean (Glycine max L. Merrill, cv. Elgin 87) were grown in the field and seed samples were collected at frequent intervals for the determination of seed fresh and dry weight and water potential of excised embryos (maize and wheat) and axes (soybean). Seed water concentration declined during seed development and the concentration at physiological maturity (PM, maximum seed dry weight) was highest in soybean (550–590 g kg−1 FW), lowest in maize (326–377 g kg−1) and intermediate in wheat (437 g kg−1). The embryo/axis water potential was relatively constant during much of seed filling before decreasing rapidly as the seeds approached PM and there was little variation among species (soybean −1.52 to −1.63 MPa, maize −1.61 to −1.99 MPa and wheat −1.66 MPa). Seed water concentration when 10% of the seeds germinated (radicle ≥3 mm) was highest in soybean (514 g kg−1), lowest in maize (332 g kg−1) and intermediate in wheat (345 g kg−1) while the water potential of the embryo/axis varied from −2.07 to −2.20 MPa across the three species. There was little variation in the water potential of the embryo/axis among species at the end of seed growth (PM) or at the beginning of germination. This similarity is consistent with the suggestion that the water status of critical seed structures may play a regulatory role in seed growth and germination.

Sunflower (Helianthus annuus L.) seeds show more germination at high temperatures (25–30°C) than at temperatures below 20°C. Osmopriming with polyethylene glycol-6000 for 3–5 days at 15°C strongly increases germination at suboptimal temperatures. This stimulatory effect of priming persists after seed redrying and during subsequent storage at 20°C (55% RH) for at least 14 weeks. However, primed seeds deteriorate faster than untreated seeds during accelerated aging (45°C, 100% RH). The longer the priming treatment, the higher is the amount of germination but at the same time the higher is the sensitivity of seeds to accelerated aging. Priming enhances the respiratory activity of seeds transferred onto water and their ability to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. These effects remain after drying the seeds and are maintained in part during dry storage, whereas they disappear during accelerated aging. These results suggest that ACC-dependent ethylene production might be a good indicator of seed vigour; it increases with duration of priming and decreases very early during aging, well before significant loss of seed viability. Decrease in ACC conversion to ethylene indicates that aging is probably associated with membrane deterioration since in vivo ACC oxidase activity depends on membrane properties. However, no increase in electrolyte leakage is observed during aging.

By means of direct manipulation, this study showed that the testa protects the germinating seed and germinant from decay (an advantage in moist soils) and desiccation (an advantage in periodically dry soils) in four species, all of which retain their testas until the cotyledons are lifted above the soil. Furthermore, the testa prevents early spreading of the cotyledons (an advantage when elongating through soil and litter). Almost all germinants of Banksia hookeriana and Leucadendron eucalyptifolium, with their testas removed, died and rotted away after being sown on the soil surface or at a range of depths. Hakea cyclocarpa and H. strumosa showed only minor losses compared with the intact germinants. All naked germinants were smaller than the controls at any depth, except among H. cyclocarpa. Absence of the testa did not differentially affect the length of the hypocotyl in the dark or light, but lateral spread of the cotyledons was accelerated, especially in the dark. Removal of a small fraction of the testa was sufficient to cause general mortality of B. hookeriana germinants but not of H. cyclocarpa or H. strumosa. After imbibition, water content of the naked embryos of B. hookeriana reverted to the air-dry state within 24 h of sowing in air-dry sand, while intact seeds were still moist even after 7 days of drying. Compared with intact seeds, drying periods of 3–7 days decreased the amount of germination and root growth of naked embryos after rewetting, and increased microbial infection of the survivors. Removal of the testa from germinants was sufficient to cause general mortality after only 6 h in air-dry sand. In contrast, most intact germinants with short radicles recovered after drying for 2 days.

Links between germination, DNA replication and β-tubulin accumulation were studied with cabbage (Brassica oleracea L.) seeds, by using flow cytometric analysis of nuclear DNA content and immunodetection of β-tubulin levels. The seeds were incubated in water or 0.1–500 mM hydroxyurea solutions. Radicle tips isolated from dry cabbage seeds revealed most 2C (Go or G1 stage) and some 4C (G2 stage) signals of nuclear DNA contents and a constitutive level of β-tubulin. The onset of DNA replication in the radicle tip was observed between 12 and 24 h of imbibition in water and was preceded by an increase of β-tubulin levels. Incubation of the seeds in 1 mM hydroxyurea retarded DNA replication, whereas an arrest of DNA replication occurred upon incubation in 10 mM hydroxyurea or higher concentrations. The arrest of DNA replication and cell division did not block radicle protrusion and allowed some extension of the radicle. This demonstrated that DNA replication is not a prerequisite for radicle protrusion and initial extension. However, further seedling development, including root growth and root hair development, was dependent on DNA replication. Accumulation of β-tubulin was not affected by hydroxyurea. Thus, it can be deduced that both DNA replication and β-tubulin accumulation are two parallel and independent cell cycle events during seed germination.

Soluble carbohydrates are one of multiple components required for the acquisition of desiccation tolerance during seed development and maturation. Sucrose and the raffinose series of oligosaccharides have been extensively studied in relation to seed desiccation tolerance. These galactosyl sucrose oligosaccharides are present in viable tissues of many edible seeds, especially the legumes, and contribute to flatulence after ingestion. A reduction in oligosaccharides of the raffinose series is desired by nutritionists but, if present at less than a threshold level, this this may result in reduced desiccation tolerance and storability of seeds. Some seeds that have very low amounts of raffinose and stachyose accumulate galactosyl cyclitols and small amounts of free cyclitols. Galactosyl cyclitols present in various seeds include galactosyl or methyl derivatives of myo-inositol, D-pinitol, D-chiro-inositol, D-ononitol, and scyllo-inositol. Castor bean seeds accumulate galactinol and buck-wheat seeds accumulate fagopyritol in addition to sucrose. Fagopyritol accumulation is associated with the acquisition of desiccation tolerance in buckwheat seeds. The galactosyl sucrose and galactosyl cyclitol soluble carbohydrates in maturing seeds have been proposed to have various roles in desiccation tolerance including non-toxic and non-reducing forms of seed storage products and intracellular osmotic agents contributing to the structural stability of organelles, membranes, enzymes and proteins, other macromolecules, and the glassy state. Ether derivatives of cyclitols may form liquid crystals. Methyl ether derivatives of cyclitols may have roles as cryoprotectants, desiccation protectants, and hydroxyl radical scavengers.

Changes in germination ability and longevity were monitored during seed development in three japonica rice cultivars and one indica rice cultivar sown on three different dates, 14 October 1993, 24 November 1993 and 5 January 1994 at Los Baños, Philippines. Germinability of the seeds varied among cultivars in the early stages of development, but it was generally similar across sowing dates. The estimates of potential longevity (determined by storage at 40°C and 15% moisture and quantified by the values of the seed lot constant K1 of the viability equation) differed among cultivars and sowing dates. While the maximum potential longevity attained across different sowing dates was similar in the japonica cultivars Ju ku and Chianan 8, it was significantly higher in the first sowing than in the second or third sowings in cv. Akihikari. In the indica cultivar IR 58, maximum potential longevity attained in the second and third sowings was significantly greater than that in the first sowing. The mean potential longevity, averaged over cultivars and sowing dates, was marginally higher in the first sowing (K1=3.81) than in the second (3.65) and third (3.63); and averaged over cultivars and harvest times it was greatest at 34.8 days after flowering (DAF) in the first sowing, 31.8 DAF in the second and 28.3 DAF in the third. These results suggest that sowing in mid-October to allow seed ripening to coincide with the cooler and drier segment of the Los Baños dry season, and harvesting 35 DAF can improve the potential longevity of some japonica cultivars.

Electron paramagnetic resonance studies of high vigour (99% viable) and low vigour (38% viable) dry embryos of rice seeds (Oryza sativa L.) stored in a natural (warm and humid) environment were carried out. Loss in viability due to hot and humid conditions was found to be correlated with a decrease in free radical levels. The free radicals could be carbon-based and derived from quinones. Presence of the active oxygen scavenging enzyme, superoxide dismutase (EC 1.15.1.1) in embryos isolated from dry seeds showed a positive corrrelation with the state of vigour or viability. Anodic peroxidase (EC 1.11.1.7) activity in imbibed seeds also declined with the decline in vigour and viability. It is concluded that the deterioration of cells in the embryonic axis depends on the balance between free radical accumulation and the activity of active oxygen-scavenging enzymes which constitutes the active oxygen scavenging system (AOSS) during early imbibition. During prolonged storage under hot and humid conditions, cumulative effects of macromolecular damage due to oxidative chain products, compounded with the loss of enzyme activity, result in the final catastrophe, the death of the embryo.

Sugarbeet (Beta vulgaris) seeds which had undergone an advancement treatment germinated more quickly and showed greater germination in a standard test at 9°C than seeds which had been steeped in water. Both treatments included the fungicide, thiram (tetramethyl thiuram disulphide). Advanced seeds also emerged more quickly and showed higher establishment in the field. The levels of extractable high molecular weight (HMW) RNA and DNA from advanced, thiram-steeped and untreated seeds were compared. Advanced seeds contained significantly (P > 0.05) more HMW RNA and DNA than thiram-steeped or untreated seeds. Extractable DNA levels were higher from thiram-steeped seeds than untreated seeds but not significantly so. Cell-cycle studies on advanced seeds demonstrated an increase from 2% to 24% of root-tip nuclei with DNA contents of 4C and above. This indicates that DNA replication occurred during the advancement treatment. There was no evidence of cell division occurring, indicating that: (1) advanced seeds were arrested post-S-phase and (2) the period following DNA synthesis in sugarbeet seeds is desiccation tolerant.

Controlled aging experiments were carried out in order to evaluate the changes in seed longevity occurring following the attainment of maximum seed dry weight in foxglove (Digitalis purpurea L.). Seeds harvested in three successive years were stored at ~5% moisture content (fresh weight basis) and 50°C. Seed longevity was greater the later the seeds were harvested. When the data were modelled using the predictive viability model of Ellis and Roberts, these increases were attributable to increases in both the theoretical initial viability (in probits) of the seed-lot, Ki, and in the standard deviation of the normal distribution of seed deaths in time, σ. Furthermore, there was a positive relationship between Ki and σ which was independent of the year of harvest (r = 0.9016, for 11 d.f., P < 0.001). These results do not support the predictive model which relies on the assumption that σ will not differ between seed lots of the same species stored under identical conditions. When seeds were stored at a range of moisture contents (between 5 and 10%) at 50°C, increases in σ during seed development were manifest as changes in the negative logarithmic relationship between σ and moisture content; below an upper limit, the regressions for seed lots at different stages of maturity were significantly different (P < 0.05) but could be constrained to either a common intercept or to a common slope (P < 0.05). It is proposed that it was the inherent variation in individual seed lifespans which increased during seed development. These results raise important concerns regarding the use of predetermined ‘species constants’ to predict the longevity of seed lots during long-term storage in seed banks.

Proso millet (Panicum miliaceum L.) is both a crop and a weed in many parts of the world. The weedy biotypes exhibit a wide range of seed colours, but the strains with the most persistent seeds are distinguished by darker seeds. This paper compares the seed biology of a range of biotypes from Canada and demonstrates that darker seeds have heavier seed coats, imbibe and germinate more slowly, and suffer less imbibition damage (measured as electrolyte leakage). It is concluded that all these attributes contribute to the increased persistence in the soil of the dark-seeded weedy biotypes. Imbibition damage is widely implicated in poor emergence and low vigour of crop seeds, but has not previously been considered in the context of weed seed persistence.

Plants undergo a series of physiological, biochemical and molecular changes in response to adverse environmental conditions or stresses such as drought, low temperature or high salt. Several genes and their corresponding proteins have been described that may play a role in withstanding water-deficit-related stresses or full desiccation. In particular, sugars and late-embryogenesis-abundant (LEA) proteins have received the most attention. Plant responses to water-deficit and desiccation have been well-characterized at the molecular level; however, pinpointing the precise roles of the gene products in protecting cells under conditions of water deficit remains a challenging task. While few plants are capable of withstanding full desiccation, most seeds undergo this event as a pre-programmed and final stage in their development. These are the so-called ‘orthodox’ seeds. In contrast to seeds of orthodox species, those of recalcitrant species do not acquire desiccation tolerance during their development and are shed from the parent plant at relatively high water contents. The essential components of desiccation tolerance of seeds are likely to involve the ability to effect repair upon subsequent rehydration as well as the ability to accumulate protective substances that limit the amount of damage which otherwise would be caused by water loss. Studies have begun to examine whether the desiccation sensitivity of recalcitrant seeds is at least partially the result of an insufficient accumulation of LEA-type proteins, or whether other factors (including a lack of protective sugars) are more important. This review assesses some of these studies as well as recent research to understand gene and protein function using transgenic host plant systems.

It is proposed that desiccation tolerance in the embryo of seeds depends upon the capacity to repair damage to genomic DNA when the desiccated embryo is rehydrated. From a study of imbibed and hydrated embryos of rye (Secale cereale) and wild oat (Avena fatua) evidence is provided that it is neither the extent of water uptake by the cells, the ensuing stability of the DNA to desiccation, nor the onset of S-phase DNA synthesis in the first cell cycle of germination that determines whether the desiccated embryo will survive. It is shown that when α- and β-polymerases of DNA repair are inhibited by aphidicolin and dideoxythymidine-5'-triphosphate, respectively, a γ-irradiation-induced DNA fragmentation cannot be fully repaired. It is shown that in hydrated embryos, at a stage when desiccation tolerance is lost, embryo cells still repair irradiation-induced damage, but this repaired DNA is unstable to desiccation and cannot be rerepaired when water is again made available. The failure to re-repair on rehydration appears to be critical to embryo survival and successful germination.

Mean seed dry mass values were determined for 27 species of trees and shrubs in Amazonian caatinga (a forest-type especially short of nitrogen) and for 11 species in adjacent much taller forest on less poor soil. The tall trees (> 15 m) of caatinga have smaller seeds than the tall trees in adjacent forest on less infertile soil (both overall and in six taxonomically controlled comparisons), and than the tall trees in lowland rainforests elsewhere. The smaller seed size is interpreted in terms of a major advantage of keeping up seed number outweighing the marginal advantages of larger seed size. For trees of caatinga and adjacent forest considered together, there is a significantly greater concentration of P and Mg, and almost significantly greater concentration of N, in the embryo-cum-endosperm fraction of smaller-seeded species, but the content per seed of N, P and Mg is smaller in smaller seeds. The mean contribution of the seed coat (including endocarp for pyrenes) was 17% for dry mass, 3% for content of P, 10% for N and Mg, 15% for K, and 30% for Ca.

It has been shown that endosperm weakening, a necessary step in the light induction of germination in Datura ferox seeds, is related to the hydrolysis of cell wall mannose polysaccharides. It is demonstrated that the activities of two enzymes involved in mannan degradation (β-mannanase and β-mannosidase) are strongly promoted in the micropylar endosperm, after Pfr formation but long before radicle protrusion. An increase in enzyme activities occurred in dissected seed parts at similar rates to those in whole seeds, but only when at least a portion of the embryonic axis was present during incubation. De-embryonation of seed parts prevented the Pfr-induced increase in hydrolase activities. Exogenous GA3 promoted mannan-degrading enzyme activities both in the micropylar region of far red-treated whole seeds and those of isolated and deembryonated endosperm sections. The response to Pfr and GA3 was far more pronounced in the micropylar portion than in the rest of the endosperm. Pfr promotion of hydrolase activities and germination was blocked by paclobutrazol, an inhibitor of gibberellin synthesis. A close correlation was found between β-mannanase activity measured 45 h after Pfr formation and germination scored 24 h later. These results support the hypothesis that in D. ferox, endosperm softening is promoted by Pfr via the production in the embryonic axis of a factor, probably a gibberellin, which moves to the endosperm where it stimulates cell wall-mannan hydrolytic enzyme activities.

Pretreatment of moist seeds at certain temperatures can reduce seed dormancy, but such observations represent the net effect of pretreatment. Seeds of Sitka spruce (Picea sitchensis [Bong.] Carr.) were raised to 30% moisture content and pretreated at five different temperatures between 5 and 30°C for up to 24 weeks (168 d). Subsequent ability to germinate at 10°C and viability were then determined in order to investigate the effects of pretreatment on seed dormancy and survival. There was a curvilinear, negative semi-logarithmic relationship between seed longevity and pretreatment temperature, such that Q10 for loss in viability increased from 2.6 between 10 and 20°C to 2.8 between 20 and 30°C. Simple multiplicative models combining cumulative normal frequency distributions for each of loss in viability and loss in dormancy were able to describe the changes in ability to germinate at 10°C, after pretreatment at 5, 10 and 30°C. However, in order to quantify the changes in ability to germinate observed at 10°C after pretreatment at 15°C, it was necessary also to invoke a model of dormancy reimposition, while for the results at 20°C it was necessary to postulate both dormancy reimposition and the further loss of this reimposed dormancy. It is concluded that moist seeds of Sitka spruce held at 15 and 20°C cycle between the dormant and non-dormant condition.