Secondary dormancy is predominantly associated with seed behaviour in soil seed banks. Periodic changes in secondary dormancy may explain seasonal emergence of weedy species. Temperature and possibly soil water potential appear to be the predominant factors that determine the annual cycling of dormancy. Dormancy cycling is paralleled by modulations of germination responsiveness to germination stimulants, such as light and nitrate and to the width of the germination temperature window. Membranes have been proposed to be the primary target for the perception of temperature. It is hypothesized that alterations in properties of cellular membranes are involved in the regulation of dormancy. Possible mechanisms are discussed.

The technique of radioimmunoassay following sample resolution by HPLC was used to assay the amounts of the cytokinins zeatin (Z), zeatin riboside (ZR) and isopentenyladenine (IPA), the combined amounts of gibberellins1+3 (GA1+3), and the amounts of indole acetic acid (IAA) and abscisic acid (ABA) during germination in grains of sorghum. Concentrations of GA1+3 were low throughout germination and did not appear to be related to the time of germination. In the mature, non-germinated grain, the concentration of each of the other plant growth regulators was much higher in the smaller component comprised of the embryonic axis and scutellum than in the much larger endosperm tissue. During the germination period studied (64 h), these concentrations declined, with a peak in the amount of the cytokinin IPA and a small peak in Z+ZR (24 h) in the embryo following the first visible signs of root protrusion and coincident with a large enhancement in amylase activity. The high concentration of ABA in the embryo tissue prior to germination was noteworthy. It is suggested that the interaction of ABA and the cytokinins IPA and Z+ZR may play a significant role in controlling sorghum germination.

The control of seed germination by red and far-red light is one of the earliest documented phytochrome-mediated processes Phytochrome is now known to be a small family of photoreceptors whose apoproteins are encoded by different genes Phytochrome B (phyB) is present in dry seeds and affects germination of dark imbibed seeds but other phytochromes could also be involved Phytochrome A (phyA) appears after several hours of imbibition and mediates very-low-fluence responses PhyB and other phytochromes different from phyA mediate the classical low-fluence responses The phytochrome involved in high-irradiance responses of seed germination (inhibition of germination under continuous far-red) has not been unequivocally established, although phyA is the most likely candidate Phytochrome can affect embryo growth capacity and/or the constraint imposed by the tissues surrounding the embryo At least in some species, gibberellins participate in the signalling process In the field, phyA has been implicated in the perception of light during soil cultivations, and phyB would be involved in the perception of red/far-red ratios associated with the presence of gaps in the canopy This review describes recent advances in phytochrome research, particularly those derived from the analysis of germination in specific mutants, and their connection with traditional observations on phytochrome control of seed germination

Wheat and barley seeds were subjected to accelerated aging conditions at 12% moisture content and 35°C temperature over 28 days of storage, and to heat-shock treatment carried out by 4 h of incubation at 40°C following 16 h of imbibition at 20°C. Heat-treated, aged seeds showed altered germination behaviour and increasing leakage electroconductivity as well as a reduced incorporation of [35S]-methionine into embryo proteins, in comparison with the corresponding untreated seeds. Two-dimensional electrophoresis of labelled proteins from embryos gave evidence of further quantitative and qualitative changes: (a) ‘normal’ germination protein synthesis in wheat and barley was slightly modified by age, but following heat-shock treatment a general reduction of most of the control polypeptides occurred; (b) heat-shock response resulted in the production of several HSPs with different MW and pl, but a uniform general decline in their synthesis was not observed. Highly labelled HSPs (e.g., those with MW 66.7 or 89.3–66.7 kDa in wheat or barley, respectively) did not change over the entire period of aging, while some smaller HSPs with MW >29 kDa either decreased in intensity or disappeared. Conversely, specific low molecular weight HSPs (MW 17–14.2 kDa) were synthesized more in extremely aged embryos of both species. The hypothesis that these polypeptides may be used as biochemical markers in monitoring both vigour loss and heat-shock induced tolerance in aged seeds is discussed.

In the range of water contents studied (0.1–0.4 g H2O g dw−1), Coffea arabica seeds were less sensitive to desiccation than C. costatifructa, C. racemosa and C. sessiliflora seeds. At 0.20 g H2O g dw−1, 53% of C. arabica seeds germinated after direct immersion in LN (rapid cooling, 200°C min−1), but none of them developed into normal seedlings. By contrast, in C. costatifructa, C. racemosa and C. sessiliflora, when seeds were dehydrated to the optimal water content (0.19, 0.28 and 0.31 g H2O g dw−1, respectively), the percentages of seeds which developed into normal seedlings after LN exposure were 26, 78 and 31% of the desiccation control, respectively. Normal seedlings could be recovered from cryopreserved C. arabica seeds only if they were desiccated to 0.20 g H2O g dw−1 and precooled slowly to −50°C prior to immersion in LN. Precooling seeds at 2°C min−1 allowed 25% of seeds to develop into normal seedlings. The thawing rate had no effect on the survival of cryopreserved C. arabica seeds. In all cryopreservation experiments, the total germination did not reflect the percentage of seeds which developed into normal seedlings. Examination of excised embryos indicated a partial explanation of this difference since only the shoot apex was destroyed in abnormal embryos, whereas the hypocotyl and radicle were normal.

Submerging Phaseolus vulgaris cv. Top Crop seed in air-saturated water for 16 h markedly depresses subsequent germination. This is termed soaking injury. The postulate by Norton (1986) that soak-injured seeds merely run out of available energy was investigated. Soaking in air-saturated water reduced the total respiration (VT; in terms of O2 uptake) of excised axes and cotyledons resulting in lower ATP levels. The contribution of both the mitochondrial cytochrome respiratory pathway (VCYT) and the alternative respiratory pathway (VALT) to the total respiration (VT) was significantly lower in these axes. However, the adenylate energy charge (AEC) values for both axes and cotyledons excised from soak-injured seeds did not drop below 0.6, the threshold value indicating death of plant material. Differences between the ultrastructure of mitochondria in the radicle tips of axes from unsoaked and water-soaked seeds were observed. However, the capacity to operate the Krebs cycle was similar in all axes and soak-injured seeds were still capable of germinating when the testas were removed or the seeds dried. This means that axes contained a sufficient amount of carbohydrates and potential energy to germinate after soaking. Moreover, soak-injured seeds produced ethanol at a rate which was five times higher than that of unsoaked control seeds, both during the soaking and post-soaking periods. Together with the low rate of oxygen consumption by soak-injured seeds during the same soaking and incubation time, it appears that soak-injured seeds lack the potential to switch from an anaerobic to an aerobic respiration pattern causing low ATP levels in these axes. It is speculated that the latter seems to be the result of limited pyruvate supply, probably due to accelerated and prolonged fermentation or futile combustion through other pathways or both.

Artificial aging of wheat grains (Triticum aestivum cv. Mercia) for 3 d resulted in the gradual loss of seed vigour without loss of viability, but the latter decreased rapidly over the next 24 h. Loss of vigour was expressed as a delay in germination i.e. an increasing ‘lag’ period with aging before the onset of radicle emergence, but the subsequent rate of seedling growth was not affected. There was an overall decrease in the incorporation of 35S-methionine into protein in aged embryos, and associated with the aging-induced delay in the completion of germination (i.e. start of radicle emergence) there was a delay in the production of certain polypeptides. In embryos of both non-aged and aged seeds these polypeptides appeared just prior to radicle emergence but several of these were never synthesized in embryos from non-viable seeds. The synthesis of mRNA coding for in vitro translated polypeptides, or groups of polypeptides, was similarly delayed in lowvigour embryos. Furthermore, some polypeptides were identified which were induced by the accelerated aging treatment. It is concluded that a reduced level of protein and mRNA synthesis in the embryos of low-vigour seeds may result in the delay in the accumulation of specific of mRNAs and polypeptides which are needed for the normal completion of germination, i.e. radicle emergence.

Seed development is a series of events involving cell division, followed by cell differentiation and storage activity In legume cotyledons, cell differentiation starts in certain regions and gradually spreads to other parts, thereby building up developmental gradients The entire process appears to be subject to metabolic control The high hexose state of the premature legume embryo as controlled by seed coat-specific invertases favours cell division Differentiation is initiated when hexose decreases and sucrose increases Seed development occurs in a close interaction with seed metabolism and transport processes Movement of photoassimilates from the sieve tubes to the unloading region of the maternal seed tissue is symplasmic and controlled by plasmodesmal passage Sucrose uptake into Vicia faba cotyledons is mediated by a H+-sucrose symporter located in the outer epidermis which generates transfer cells Formation of the sucrose uptake system is induced during the early to mid-cotyledon stage by tissue contact with the maternal seed coat and is controlled by carbohydrate availability In contrast, a hexose transporter gene is also expressed in epidermal cells covering younger, mitotically active regions of the cotyledons The sucrose uptake system apparently generates the high sucrose state immediately preceding the storage phase Sucrose specifically induces storage-associated differentiation processes indicating a specific sucrose-dependent signalling pathway operating in maturing cotyledons Moreover, the mode of sucrose uptake — apoplasmic movement into the epidermal cells with subsequent symplasmic transfer to the storage parenchyma cells — appears to control coordinated cotyledon development Unlike sucrose, amino acid transport into legume cotyledons is passive during early development but at later stages when large amounts of storage proteins are synthesized an additional active uptake system is established to ensure a sufficient supply

Flavonoids are secondary metabolites that are present at high levels in most plant seeds and grains. These compounds appear to play vital roles in defence against pathogens and predators and contribute to physiological functions such as seed maturation and dormancy. At the same time, particular subclasses of flavonoids, such as the proanthocyanidins (condensed tannins), negatively impact the use of seeds and grains in animal feed and can add undesirable qualities to food products for human consumption. This article presents an overview of investigations into the physiological and agronomic aspects of seed and grain flavonoids as well as a review of molecular genetic studies, particularly in maize, Arabidopsis and soybean, where mutants deficient in flavonoid biosynthesis provide useful tools for stydying the metabolic machinery underlying the accumulation of these compounds in reproductive structures.

The dormancy pattern of buried seeds of Datura ferox was studied by testing germination responses to red light (R), far-red light (FR) or darkness after exhumation at regular time intervals. The seeds were deeply dormant at the time of burial in winter. After three months (spring), exhumed seeds began to respond to an R pulse. Summer temperatures did not induce secondary dormancy. During winter of the second year, seed germination after an R pulse increased further; however, germination after an FR pulse or in darkness also increased and reached values close to R. Changes in seed germination after an R or an FR pulse and in darkness were studied in a factorial design experiment, combining different depths of burial (0.5, 5 and 10 cm), presence of vegetation (with or without vegetation canopies), and date of exhumation (4 or 7 months after burial). The seeds placed at 0.5 cm depth did not germinate in any light treatment. A very high sensitivity to light, indicated by the promotion of germination by an FR pulse compared to darkness, was exhibited only in seeds buried at 5 cm in a soil without vegetation. A typical R/FR reversible response was observed in seeds exhumed at 5 and 10 cm in almost all situations of burial; except in a soil without vegetation, at 10 cm depth, in late summer when even dark controls showed high germination. It is concluded that buried seeds of D. ferox are released from dormancy in two steps, the first step during the spring of the first year and the second step during the subsequent winter. This process is not obviously controlled by summer temperatures. The acquisition of a very high sensitivity to light depends on vegetation cover and depth of burial, and thus it is predicted to be affected by agricultural practices.

Variation in dormancy level was tested in seeds of four species, each collected from three populations in 1994 and 1995 (experiment 1). Germination was tested in light and darkness on recently-harvested seeds and on those after-ripened in dry storage for one year. In addition, seeds from each of eight individual plants within each of eight populations were tested for germination when recently harvested and after warm stratification or cold stratification followed by a drying period (experiment 2). Seeds from the two years differed in dormancy level in Silene noctiflora, Sinapis arvensis and Spergula arvensis. Germination percentage differed significantly among populations in Sinapis arvensis and Spergula arvensis in both experiments and in Thlaspi arvense in experiment 2. Furthermore, dormancy level in seeds from different mother plants also varied in the three species tested in experiment 2. Variations at the three levels tested (year, population and mother plant) indicate that these species have a random pattern of variation in dormancy level. It is concluded that variation in seed dormancy among mother plants, populations and years must be taken into account when testing the germination characteristics of a species and also when attempting to model weed seed bank dynamics.

Prechilling seeds of Sitka spruce (Picea sitchensis [Bong.] Carr.) at 4°C with 30% moisture content for 12–14 weeks (84–98 d) removed conditional dormancy (i.e. they were then able to germinate at 10°C). The non-dormant status was preserved after redrying to 6% moisture content. However, conditional dormancy was gradually reimposed during subsequent air-dry storage at 4°C and 6% seed moisture content in all five seed lots tested. Further investigations with one seed lot showed that reimposition was reversed by a second prechill treatment, but was reimposed again during subsequent air-dry storage. The trend of dormancy reimposition within seed lots over time was quantified by negative exponential relations between ability to germinate at 10°C and duration of air-dry storage. The progress of dormancy reimposition was influenced by seed storage moisture content and was most rapid at 4–10%. At higher moisture contents (15 and 20%) the rate of the reimposition of conditional dormancy was much reduced, while at moisture contents of 25 and 30% further loss in dormancy occurred. Thus it is clear that dormancy reimposition occurred during storage at low water potential rather than solely during desiccation from high to low water potential.

Plants respond to sub-optimal conditions by the synthesis of specific ‘stress’ proteins, and these are thought to play a role in stress tolerance. Some of these proteins accumulate during late seed development, arguably to protect against damage during post-maturation drying and subsequent imbibition, prior to germination. Seed vigour is also determined during this late stage of seed development. High vigour seeds are those that can withstand the desiccation required for storage and successfully germinate under sub-optimal conditions to establish healthy seedlings. If stress proteins are involved in tolerating stress conditions, then they are likely to be important determinants of seed vigour. In this work the relationship between seed vigour (measured by seed germination performance following rapid aging, or under water stress) in Brassica oleracea var. capitata and the content of two classes of stress protein (dehydrins and a low molecular weight heat shock protein HSP17.6) at maturity was examined. Dehydrins did not show a positive relationship with seed performance. However, the protein HSP17.6 showed a positive correlation with seed performance, and a treatment that reduced the amount of this protein in the seed also caused a reduction in subsequent seed performance.

Polyclonal antibodies to purified recombinant barley haemoglobin (Hb) have been raised in rabbits and used to investigate its expression in monocotyledonous plants. Very little or no Hb expression was observed in dry barley seeds but germination resulted in the expression of Hb which peaked at 2–3 days after imbibition. Hb expression was also observed in maize, wheat, wild oat and Echinochloa crus-galli seeds during germination. Dissection of tissues from the barley seedlings showed that most of the haemoglobin was expressed in the root and seed coat (aleurone layer), with very little in the coleoptile. Imbibition of half-seeds or excised embryos resulted in the expression of haemoglobin. ATP measurements of barley embryos showed that ATP levels quickly increase after imbibition. α-Amylase activity was also determined in embryos to correlate Hb expression with a well-characterized germination response. The results demonstrate that Hb expression is a normal consequence of germination.

The loss of percentage germination and vigour of non-sprouted and sprouted seeds of winter wheat was determined during storage for 80 days at 11 different combinations of moisture content and temperature. Before storage the initial percentage germination and vigour were reduced according to the level of sprouting. During storage the percentage germination and vigour decreased. The rate of germination loss increased with increasing seed moisture content and/or increasing temperature. The species-specific constants KE, CW, CH and CQ of the viability equation were not different for non-sprouted and sprouted seeds. Likewise, the relation between the percentage of normal and abnormal seedlings was independent of sprouting. The rate of vigour loss was also independent of the storage conditions and of sprouting.

Embryos taken from late maturation phase seeds of Phaseolus vulgaris cv. Seminole prior to seed desiccation (35–45 DAA) can be induced to germinate in the absence of water by exogenous ethylene. NMR imaging of proton relaxation within the embryo shows changes in water status in putative (and not fully differentiated) vascular tissues of the hypocotyl within 3 h of ethylene administration. Difference imaging revealed that the change was progressive in the hypocotyl towards the radicle tip and was accompanied by changes in water status in the cotyledons. Water within plumular leaves was also affected. Increase in diameter of the hypocotyl–radicle axis (as estimated by pixel counts) was detectable from 3 h. Longitudinal radicle growth was detectable by NMR imaging at 18 h. Visible germination under the conditions used was apparent after 20 h. Changes in water status detected by this technique are an indication of changes in activity (concentration) or motion of water molecules or both. The data are consistent with a mode of action of ethylene in stimulating a redistribution of water within embryo structures from cotyledons to axis via the cotyledonary node and allowing the axis access to water sufficient to support germination. This supports the hypothesis that in vivo, quiescence at this developmental stage is induced and maintained by sequestration of water within the cotyledons.

The spatial variation of the soil seed bank in the steppe of Larrea divaricata and Stipa spp. was evaluated in relation to the spatial pattern of the established vegetation, over two years. Plant distribution was expected to affect the spatial pattern of the soil seed bank. It was hypothesized that patches of bare soil are seed-limited, while soil beneath the external crown of vegetation patches contains greater numbers of seeds. The soil seed bank of the steppe of Larrea divaricata and Stipa spp. displayed a patchy distribution associated with the spatial pattern of above-ground vegetation. Seed banks in areas of bare soil showed lower numbers of seeds than those beneath vegetation. The soil seed bank was dominated by annuals, both in open areas of soil and under vegetation canopies. The relative composition of the soil seed bank varied spatially. Annuals (dicots and grasses) were more abundant in grass-shrub patches (GSP) and grass patches (GP) than in incipient grass-shrub patches (IGSP) and bare soil (BS). Seeds of perennial grasses were more abundant under GP, less abundant under grass-shrub mixed canopies (IGSP and GSP) and practically absent in BS. In addition, seeds of perennial dicots were more frequent in grass-shrub mixed canopies, scarcely represented in GP and absent in BS. The concentration of seeds of perennial grasses in GP and GSP indicates that these are essential for the maintenance of the size of grass populations. Sustainable management should be focused to maintain the structure and function of these types of vegetation patches.