The induction of desiccation tolerance in germinated seeds by incubation in PEG for several days is reported. The induction coincided with an increased sucrose content and the formation of heat-soluble proteins. Germinated seeds can serve as a convenient model system in studies of desiccation tolerance. This finding may have important implications for the agricultural industry.

The emphasis of modern dormancy research is almost entirely on the form of dormancy that is acquired during seed development, primary dormancy. Abscisic acid (ABA) appears to be intimately involved in its regulation. The action of abscisic acid has also been implied in many other developmental processes. The coincidence of developmental events, such as dehydration and completion of maturation, with the acquisition of primary dormancy suggests that dormancy is influenced by these processes. Germinability, both during development and after maturation, is sometimes directly correlated with ABA content. The lack of such a correlation may be explained by assuming a decisive role for the responsiveness to ABA or other overriding factors. ABA has been detected in all seed components. The different seed tissues may all contribute, to various extents, to the degree of whole seed dormancy. It is concluded that ABA action in dormancy regulation is not restricted to the embryo but is also located in endospermic tissue. In addition, a role of ABA in the morphological development of germination modifying seed tissues is proposed. The mechanism for ABA action appears to be associated with cell wall properties.

Various sources of germination heterogeneity of carrot (Daucus carota L.) seeds were investigated: germination conditions (temperature, oxygen tension), size of seeds, position of the umbels on the mother plants, and pollination conditions of male-sterile plants in hybrid seed production. All seeds tested germinated over a large range of temperatures (5–35°C). However, low temperatures (5–10°C) and temperatures above 30°C reduced germination. Seeds were also sensitive to oxygen deprivation, but their sensitivity to hypoxia depended on the cultivar. The germination responses of seeds to temperature and oxygen depended on their size, particularly at sub- and supra-optimal temperatures. The largest seeds (1.8–2.1 mm) of commercial lots usually germinated better at 5°C and were more sensitive to oxygen deprivation than the smallest ones (1.2–1.8 mm). Experiments performed with open pollinated plants showed that seeds produced by umbels of first and second orders germinated faster and at higher percentages than those collected on third-order umbels. Evidence for the involvement of pollination conditions in the germination quality of carrot seeds was given by pollination by honey bees of male-sterile plants cultivated at various distances from the pollen donors. The longer the distance of the mother plants from the pollinating plants, the lower the seed yield, the heavier the mean seed weight, the more difficult the germination of seeds, and the higher their sensitivity to oxygen deprivation.

It has been proposed that sucrose and raffinose play a role in the storability of maize seeds. The levels of these sugars in the embryos and the glassy state were compared in maize seeds of contrasting storage stability to determine the relationship between sugar composition, glassy state and the storability of the seed. Sucrose was the predominant sugar but its content was not correlated with good storage. The content of raffinose as a mass fraction of total sugars and the magnitude of the glassy state showed positive correlations with storage stability. It is suggested that in maize seeds dry storage stability is a reflection not of the total soluble sugars, but of the mixture of sucrose with raffinose. Apparently raffinose amplifies the magnitude of the glass signal and this in turn is associated with enhanced storability.

Three seedlots of Agathis australis (Lambert) Steud. (kauri pine) were stored hermetically at moisture contents of 1.5–15.5% and at temperatures from −20° to 42°C; and one seedlot of Agathis macrophylla was stored at 7%, 9% or 13% moisture content and at 26°, 6° or −13°C. Seeds of A. australis showed orthodox storage behaviour; except that viability was progressively reduced with drying below about 5% moisture content, and, depending on seedlot, subsequent storage longevities were reduced at all temperatures. Seeds of A. macrophylla again showed essentially orthodox characteristics at 9 and 13% moisture content; but drying to 7% moisture content reduced viability significantly, and of those seeds surviving, a further proportion died as a result of transfer to −13°C, and the remainder survived for 2500 d with little further viability loss. Seeds of neither species conformed fully to orthodox characterisics, nor to those of the intermediate category. The results are discussed in relation to possible causes of this ‘sub-orthodox’ behaviour, especially current seed-harvesting methods, and the limits put upon ex situ conservation of these species in seed banks by their predicted comparatively short storage lives.

Dehydrin and QP47, proteins present in mature pea seeds (Pisum sativum), have been proposed to play protective roles during desiccation. To identify possible relationships between these proteins and desiccation tolerance, their tissue locations and patterns of synthesis and degradation have been examined during germination. Tissue locations were determined by immunocytochemistry using polyclonal antibodies raised against a conserved dehydrin amino acid sequence and against purified QP47. In embryonic axis and cotyledon cells, QP47 and dehydrin were distributed uniformly with no apparent nuclear or organellar specificity. Both proteins were present in 24 h-imbibed axes that had not initiated radicle growth but were completely absent from 24 h-imbibed axes that had begun to grow. The amounts of QP47 and dehydrin in embryonic axes decreased with time after the start of imbibition and were undetectable by 48 h. When germination was prevented by polyethylene glycol (PEG) or abscisic acid (ABA), both proteins remained at their original amounts. Thus, both QP47 and dehydrin disappeared coincidently with the beginning of growth and not simply as a function of the time after imbibition. QP47 persisted in cotyledons until at least 31 days into seedling growth, whereas dehydrin was not detectable in cotyledons after 7 days. Dehydrin, but not QP47, could be re-induced in pea shoots and cotyledons by dehydration. The timing of degradation of both proteins was correlated with the loss of desiccation tolerance during germination of pea axes.

An LEA-like protein has been isolated and characterized from pea (Pisum sativum) embryos. It is the most prevalent protein in a homogenate of pea axes heated for 10 min at 80°C and then centifuged for 10 min at 17000 g (80°C supernatant fraction). It has a molecular mass of 11 kDa and is very rich in hydrophilic amino acids, notably aspartate, glutamate and glycine. The protein is not recognized by an antibody to the group II dehydrin C-terminal consensus sequence. Antibodies to the isolated protein recognize a number of larger proteins in the 80°C supernatant fraction of pea and other legumes (chick pea, soybean and bean) as well as gramineous seeds (wheat, maize and barley). The protein undergoes a substantial increase in α-helical content at high ionic strength but does not dimerize.

In the present study, the efficiency of two cryopreservation strategies for oil palm (Elaeis guineensis Jacq.) was compared. If extracted from rehydrated kernels, 65% of the embryos desiccated to around 0.3 g H2O/g DW developed into plantlets after cryopreservation. In contrast, only 25% of embryos (0.12 g H2O/g DW) extracted from cryopreserved dry kernels developed into plantlets. However, this value was increased to 63% if kernels were partially rehydrated before freezing until the water content of embryos reached 0.3 g H2O/g DW. This study emphasizes the importance of partial rehydration of oil palm embryos before cryopreservation.

At the time of protrusion, radicles of lettuce seeds became susceptible to desiccation damage. Concomitant with radicle elongation, both the reducing sugar content and the amount of lipid-aldehydes increased. The role of the hydroxy- and lipid-aldehydes in browning reactions that occurred during desiccation and re-imbibition was analysed. It was concluded that both types of aldehydes reacted with amino-groups during desiccation and that the increase in lipid-aldehydes during germination could be due to the augmented reducing sugar content. Therefore, the high reducing sugar content in radicles of germinated lettuce seeds may be the primary source of hydroxyl radicals, and thereby, a principal cause of desiccation damage.

The conditions of osmopriming of maize seeds have been established. At an osmotic potential of–1.7 MPa, seed germination is inhibited and seeds osmoprimed for up to 3 weeks show an improved germinability when the osmotic agent is removed. This improvement also applies to seeds that lost vigour because of improper storage conditions. At the biochemical level, embryo axes from osmoprimed seeds can incorporate precursors into DNA, RNA and proteins although at a low level; after removing the osmotic agent, all 3 types of macromolecules are synthesized at much higher levels. No evidence of DNA replication or cell division was found during osmopriming; however, mitotic figures appear several hours earlier in germinated, osmoprimed root tissues compared with the time at which they appear in non-osmoprimed tissues. The behaviour of osmoprimed maize is compared with that of osmoprimed seeds from other plant species.

After 110 years of hermetic sealed storage at temperatures between 10 and 15°C and at a moisture content of 3.12%, Hordeum vulgare showed 90% germination and Avena sativa 81%. In addition, some seeds of Agrostemma githago, Lolium temulentum, Sinapis alba, Sinapis arvensis, and Vaccaria hispanica were still capable of germinating. This demonstrates that ultra-dry, long-term seed storage under ambient temperature conditions can successfully be achieved with the intention of cutting down risks and costs in germplasm conservation.

The effect of two priming treatments in PEG solutions on nuclear replication stages of pepper (Capsicum annuum L.) embryos were evaluated using flow cytometry. Priming at −1.1 MPa for 10 d induced almost 40% of nuclei in the embryo root tips to enter the synthetic phase of nuclear division, while no DNA synthesis occurred during 6 d at−1.5 MPa and in less than 10% of the nuclei at 10 d. Both priming treatments effectively increased seed performance. A decrease of 2.7 d in mean germination time (MGT) was observed after priming in PEG solution at −1.1 MPa, while the treatment at−1.5 MPa lowered the MGT by 2.1 d. Primed and untreated seeds were subjected to controlled deterioration treatments by exposure to 45°C after equilibration at 75% RH. Both osmotic treatments considerably lowered seed tolerance to adverse storage conditions as compared with untreated seeds. However, seeds in which DNA replication was induced by priming were more sensitive to controlled deterioration than seeds in which priming did not induce nuclei to enter the synthetic phase. This could be a consequence of a higher DNA content, which is a more vulnerable target for mutation inducing factors. Alternatively the increased sensitivity could be a consequence of their more advanced progress in germinative events, making seeds less resistant to deteriorative factors imposed during storage. On the whole, from the present results it appears that the nucleic acid DNA content of the embryo only plays an additive role in influencing seed storability.

The ecophysiology of germination in Pinus brutia and P. halepensis was studied in seeds collected from different areas of Greece. In regard to the temperature range of germination, both P. halepensis and the southern provenance (Lasithi, Crete) of the East Mediterranean pine, P. brutia, follow a typical Mediterranean pattern. In the latter species, dramatic differences in the degree of dormancy were noted among the three provenances investigated; in all seed lots however, 20°C was clearly the optimal temperature for germination. Stratification resulted in a considerable promotion of P. brutia seed germination. Nevertheless, the inductive effect of stratification was shown to differ among the three provenances used, escalating from a simple increase of germination rate (in the southern seed lot from Lasithi, Crete) through a broadening of the temperature range of germination (in the intermediate lot from Thasos Island) to, finally, a dramatic release from a particularly deep dormancy (in the northern lot from Soufli). These deeply dormant seeds of the latter provenance displayed an absolute stratification requirement; prolonged illumination or seed coat scarification could not substitute for the promotive effect of prechilling. A considerable interaction between far-red light and stratification was revealed in the dormant seeds of P. brutia (Soufli provenance); far-red pulses during stratification could either cancel or diminish the germination promotion induced by low temperatures. The differences observed in the germination behaviour among the various P. brutia provenances may be attributed to a variable ecophysiological strategy in regard to the temporal pattern of seedling emergence and establishment. According to the variants of this strategy, seed germination is timed to occur during either spring (in regions with relatively cold and moist climates), or autumn and early winter (in southern, mild and dry areas) or both (in intermediate conditions).

Imbibed embryos of after-ripened and secondarily thermo- and FR-dormant Agrostemma githago seeds were investigated as to their ability to synthesize DNA, RNA and proteins with the aim of finding characteristic differences connected with the induction and maintenance of developmental arrest. A gradual decrease in DNA synthesis was observed during the induction of thermodormancy. However, DNA synthesis was stimulated up to that of embryos of 30–h-imbibed after-ripened seeds within 24 h approximately after transferring the thermodormant seeds into temperatures which normally allow germination. DNA synthesis of embryos of FR-dormant seeds remained constant at a relatively low level during 7 d FR and another 7 d dark incubation. RNA synthesis decreased to different extents during induction of thermo- and FR-dormancy when it was arrested at a relatively low level in seeds transferred to temperatures which normally allow germination. Processes leading to an increase in RNA synthesis such as in embryos of after-ripened seeds appeared to be quantitatively and/or qualitatively repressed. Interestingly, protein synthesis was extremely depressed during induction of thermodormancy whereas it was slightly stimulated during induction of FR-dormancy. Nevertheless two-dimensional protein PAGE revealed several polypeptides which were new, increased, decreased or not synthesized predominantly in axes of thermo- and FR-dormant seeds in comparison to germinating after-ripened seeds. It is suggested that a connection exists between these polypeptides and the repression of germination. After transferring seconarily dormant seeds to temperatures which normally allow germination, a temporary stimulation of protein synthesis could be observed in both cases.

Excised axes from cotton (Gossypium hirsutum L. cv. M-8, a double haploid) were imbibed in vitro for 3,6,9,12,18 and 24 h. Data for length, fresh and dry weight and percentage water showed that excised axes undergo a triphasic pattern of growth: an initial burst, a short lag phase and then protracted rapid growth. Analysis of protein content with SDS-PAGE, and of activities of amino-, carboxy- and endopeptidases provided data indicating a direct correlation between major axis elongation growth between 12 and 24 h and enzymatic breakdown of storage proteins (especially those of 48 kDa) by carboxy- and endopeptidase. Data for axes imbibed at 30°C and 0–5°C for 12, 18 and 24 h indicated that a reduction in length and percentage moisture in the cold occurred in parallel with reductions in carboxy- and endopeptidase activities but not in ami-nopeptidase activity. The tentative conclusion is that carboxy- and endopeptidase are probably synthesized de novo in the axis during imbibition and are important in initial breakdown of storage proteins for axial elongation growth.

There is a carboxypeptidase in mung bean seeds that is localized in the protein bodies, the same vacuoles in which seed storage proteins are sequestered. This carboxypeptidase, called carboxypeptidase I (or Cpase I) has been purified by a series of ion-exchange and gel filtration columns. The pure enzyme consists of a single polypeptide chain with a MW of 41700 by SDS-PAGE or 42000 by size-exclusion HPLC. It has a pl of 4.36 and is a serine carboxypeptidase as shown by its inactivation by phenylmethylsulfonyl fluoride, and its resistance to other proteolytic inhibitory reagents. A survey of its activity with Cbz-dipeptides shows preference for C-terminal amino acids that are large, hydrophobic residues, and a small aliphatic amino acid such as alanine, but not glycine, at the penultimate amino acid residue. Cpase I can convert a trypsin inhibitor of the mung bean to its proteolytic intermediate lacking four amino acid residues at its C-terminus. This proteolytic intermediate is detected in the mung bean cotyledons during early growth. Levels of both the activity and immunological cross-reacting forms of this enzyme start high and decrease during early growth.

The interactions between water content and temperature on freezing and desiccation damage were examined for Zizania (wild rice) embryos at several stages of maturity. The water content of excised embryos was manipulated by flash drying at 35°C or room temperature to between 2.5 g H2O/g dw and 0.05 g/g. Embryos were then exposed to temperatures ranging from 5 to −50°C. Viability following the drying and cooling treatments was assayed by leakage of electrolytes and germination in culture. Viability of embryos decreased when embryos were dried below a critical water content. The critical water content was greatest for the least mature embryos. Critical water contents were also temperature dependent and increased with decreasing temperature. Even though the critical water content varied with developmental status and temperature, the water activity corresponding to the critical water content appeared to be constant at 0.90. The most mature embryos survived temperatures as low as −50°C while the least mature embryos survived only to −18°C. These trends were predicted by ‘phase diagrams’ based on the physical properties of water in embryos at different stages of maturity (Vertucci et al., 1994a). Our results confirm the earlier prediction that long term preservation of Zizania grains is possible at −20°C and the degree of success will be related to the maturity status of the embryos.

Ecophysiological aspects of seed germination were investigated in the widely distributed Mediterranean-endemic, aromatic plants thyme (Coridothymus capitatus), savory (Satureja thymbra) and oregano (Origanum vulgare subsp. hirtum). Thyme seed germination is light indifferent, whereas oregano seeds have an absolute light requirement; their germination can be promoted even by green safelight or far-red light. In savory, a portion of the seeds germinates in the dark and germination can be either increased or decreased by the appropriate illumination. All three species show more germination at a relatively low temperature range, a Mediterranean characteristic, with an optimum around 15–20°C. The rate of germination is considerably higher in the tiny seeds of thyme and oregano than in the larger seeds of savory. In the latter species, germination is also dependent upon the age of the seeds; old seeds germinate to a higher percentage than fresh ones, as already observed by Theophrastus, possibly as a result of the volatilization of the essential oils present on the nutlet coat. Seeds of thyme, savory and oregano are dispersed within the persistent fruiting calyces and the seeds eventually germinate within it. Essential oils in the calyx strongly inhibit germination of the enclosed seeds; germination is much suppressed in thyme and to a lesser extent in savory and oregano. This diaspore dormancy caused by essential oils apparently is overcome under natural conditions by leaching of the inhibitors with rainwater. It is suggested that this dormancy operates as an adaptation strategy that delays germination by acting as a rain gauge. In this way, seed germination and subsequent seedling establishment are prevented during the early phase of the rainy period, which is usually interrupted by drought spells in the Mediterranean climate.

Seed germination of the hemiparasitic angiosperm Striga hermonthica is elicited by compounds present in the root exudates of the host plant. A variety of chemicals can substitute for the host-derived signal although the exact mechanism through which these act is unknown. In the present study, thidiazuron (TDZ), a cytokinin-active urea derivative was found to stimulate germination. This thidiazuron-induced germination was shown to be mediated by endogenous ethylene, by a similar mechanism to germination induced by host-root exudate and the synthetic stimulant GR-24. Comparing the effects of TDZ, host-root exudate, GR-24 and the ethylene precursor 1-aminocyclopropane-1-car-boxylic (ACC) suggests that ethylene production increases as a result of increased ACC synthesis and is consistent with a model for Striga seed germination in which host-derived signals and other stimulants act by eliciting the synthesis of ethylene via a stimulation of ACC synthase activity. All germination stimulants tested trigger a rapid increase in O2 uptake by conditioned seeds. The patterns of O2 uptake following TDZ or GR-24 treatments show similarities and three distinct phases of respiration are apparent. In contrast, stimulation of O2 uptake by ethylene results in a different pattern, with no distinct phases obvious following the initial burst of activity.