The origin and significance of free radicals is described in the broad setting of animal and plant biology and in the specific context of seeds and seed viability. Evidence is given that free radicals play a central if not causal role in promoting molecular damage under the widest range of environmental stresses and induced ageing in mature plant tissuesIn ungerminated seeds, the evidence is less certain. The reason, we argue, is that many attempts to measure free radical processes in seeds are set not against gradients of damage or ageing (as they are in most other biological tissues) but against one of only two options, either seed germinability or mortality. Because free radical reactions differ quantitatively and qualitatively in living and dead tissues attempts to correlate radicalmediated damage with the viability of a population of seeds may be unreliable particularly when measurements are made at uncertain intervals after death. Despite this, the evidence that seeds, uniquely, are exempt from the ravages of oxygen assault is weak. Instead it ismore probable that oxygen plays a central role in seed mortality and may have significance in the evolution of seed persistence.

Development of the highly desiccation-sensitive (recalcitrant) seeds of primarily one species, Avicennia marina, is reviewed and compared with the ontogeny of desiccation-tolerant (orthodox) seeds. A. marina seeds undergo no maturation drying and remain metabolically active throughout development, which grades almost imperceptibly into germination. While PGR control of histodifferentiation is essentially similar to that characterizing desiccation-tolerant seeds, the phase of growth and reserve deposition is characterized by exceedingly high cytokinin levels which, it is proposed, promote a sink for assimilate import. While some starch accumulation does occur, the predominant reserves are soluble sugars which are readily available for the immediate onset of seedling establishment upon shedding. ABA levels are negligible in the embryo tissues during seed maturation, but increase in the pericarp, which imposes a constraint upon germination until these outer coverings are sloughed or otherwise removed. The pattern of proteins synthesized remains qualitatively similar throughout seed development in A. marina, and no LEA proteins are produced. This suggests both that seedling establishment is independent of maturation proteins and that the absence of LEAs and desiccation sensitivity might be causally related. The study on A. marina reveals that for this recalcitrant seed-type, germination per se cannot be defined: rather, it is considered as the continuation of development temporarily constrained by the pericarp ABA levels. This leads to a reexamination of the role of rehydration as key event sensu stricto, in the germination processes in desiccation-tolerant (orthodox) seeds.

The mechanisms and regulation of storage lipid formation in seeds are reviewed. Seed storage lipids are ultimately derived from simple carbon compounds, such as sucrose, which are imported into seeds from source tissues, such as leaves or pods. The partitioning of this important carbon towards storage lipid, carbohydrate or protein is regulated by a complex interaction between genetically-determined endogenous factors and external environmental influences. Storage lipids are synthesized from fatty acids, derived from acetyl-CoA, and glycerol 3-phosphate. Fatty acid biosynthesis and probably acetyl-CoA formation, occurs within the plastids to produce oleoyl-CoA. Further elaboration of oleoyl-CoA to produce polyunsaturates, hydroxylates or very long chain acyl-CoAs occurs on the endoplasmic reticulum, as does the esterification of acyl-CoAs to glycerol-3-phosphate to produce the final triacylglycerol storage oil. The temporal and hormonal regulation of storage lipid accumulation in seeds is discussed.

Seeds of Oryza sativa L. (desiccation tolerant, orthodox) completed each of five distinct stages of development approximately 7 days earlier than seeds of Porteresia coarctata (Roxb.) Tateoka (desiccation intolerant, recalcitrant), despite the fact that O. sativa plants matured under cooler conditions. Isolated embryos of O. sativa were more sensitive to rapid desiccation at 6 days post anthesis (DPA) compared with naked caryopses. More than 90% of the latter were desiccation tolerant at 8 DPA and at all stages tested the germination capacity and/or rate of germination was greater following drying. In contrast, drying resulted in a complete loss of viability in seeds of P. coarctata at all stages tested. In both species, abscisic acid (ABA) began to accumulate in embryos mid-way through development. At the respective peaks, the concentration of ABA in P. coarctata was twice that recorded in O. sativa confirming that ABA levels per se do not determine desiccation tolerance in seeds. The possibility that desiccation tolerance is linked to sensitivity to ABA is discussed.

Osmopriming of leek seeds (Allium porrum L.) in PEG 6000 solution (−10 bars) at 15°C for 14 days leads to reductions in both the spread of germination and mean time to germination, especially in low-vigour seed lots. In vivo methyl [3H]-thymidine pulse-labelling studies have demonstrated constant and low levels of DNA synthesis in leek embryo tissue during the osmopriming treatment. DNA synthesis during osmopriming was not inhibited by aphidicolin, an inhibitor of nuclear DNA replication. Replicative and repair-type DNA synthesis was investigated using BND-cellulose chromatography and these studies revealed that about 30% of the DNA synthesis after 1 day of priming was of a repair-type. DNA repair-type synthesis contributed to approximately 20% of the [3H]thymidine incorporated into DNA during the rest of the priming period in embryo tissue from both high-vigour and low-vigour seed lots. After 1 day of germination following priming, enhanced levels of both replicative and repair-type DNA synthesis were demonstrated. The replicative type of DNA synthesis detected in leek embyros during the priming period was not inhibited by aphidicolin and appears to represent a significant level of mitochondrial DNA synthesis. DNA synthesis could be detected in both nuclei and mitochondria of leek embryo tissue during the osmopriming treatment in the absence of any detectable cell division.

The effect of rate of dehydration was assessed for embryonic axes from mature seeds of Camellia sinensis and the desiccation sensitivity of axes of different developmental stages was estimated using electrolyte leakage. Rapidly (flash) dried excised axes suffered desiccation damage at lower water contents (0.4 g H2O (g DW)−1) than axes dried more slowly in the whole seed (0.9 g H2O (g DW)−1). It is possible that flash drying of isolated axes imposes a stasis on deteriorative reactions that does not occur during slower dehydration. Differential scanning calorimetry (DSC) of the axes indicated that the enthalpy of the melting and the amount of non-freezable water were similar, irrespective of the drying rate.

Very immature axes that had completed morphogenesis and histodifferentiation only were more sensitive to desiccation (damage at 0.7 g H2O (g DW)−1) than mature axes or axes that were in the growth and reserve accumulation phase (damage at 0.4 g H2O (g DW)−1). As axes developed from maturity to germination, their threshold desiccation sensitivity increased to a higher level (1.3−1.4 g H2O (g DW)−1). For the very immature axes, enthalpy of the melting of tissue water was much lower, and the level of non-freezable water considerably higher, than for any other developmental stage studied.

There were no marked correlations between desiccation sensitivity and thermal properties of water. Desiccation sensitivity appears to be related more to the degree of metabolic activity evidenced by ultrastructural characteristics than to the physical properties of water.

Germinating seeds of many species, especially those of tropical or subtropical origin, suffer chilling injury when exposed to low but non-freezing temperatures, resulting in poor seedling establishment and reduction in yield. The severity of injury depends upon several factors such as (i) the species or the cultivars involved, (ii) the initial water content of the seed, (iii) the temperature to which seed is exposed, (iv) the duration of chilling exposure and (v) the period during the course of germination when the chilling exposure takes place. The period of imbibition by seeds is the period of sensitivity to the stresses but especially to chilling temperatures. Imbibitional chilling injury is defined as sensitivity to a combination of low seed-water content and imbibition at cold temperature. The mechanism of chilling injury in seeds is different from chilling injury of hydrated tissues and is discussed in relation to the involvement of membranes, respiration, rate of hydration and the concept of discreet moisture levels. Furthermore, the mechanistic and applied aspects of pre-sowing seed treatments for improved germination performance under chilling stress are discussed with an emphasis on hydration-dehydration, osmotic priming and growth regulator treatments.

Seeds/grains of four species were imbibed for up to 90 minutes half embedded in agar. The agar was then freeze-dried and treated with a chromatographic reagent to detect the sites of potassium leakage from the imbibing seeds/grains. Soybean (Glycine max cv. Marathon) and pea (Pisum sativum cv. Little Marvel) leaked K across the entire surface of their testas. Aged pea seeds leaked much more extensively than seeds of a fresher lot of the same cultivar. Squash (Cucurbita maxima cv. Warted Hubbard) seeds leaked extensively from the flat sides of the seeds and at the hilum, but only slightly at the margins. Maize (Zea mays cv. Golden Beauty) leaked most extensively across the endosperm-only side (i.e. the side opposite the embryo) of the kernel, and kernels leaked more at the tip-cap end than at the broad end. Energy dispersive X-ray analysis determined that K was present in the testas/pericarps before imbibition in all species studied, and that the peak-to-background ratios of K were lower after the seeds/grains had been exposed to water. Neutron activation analysis verified that K was leaked out of the seeds/grains and absorbed into the agar. Seeds from all species studied showed varying amounts of seed-to-seed variation. These variations can be attributed in part to differences in testa/pericarp structure and condition.

Desiccation tolerance is one of the most fundamental properties of seeds. It is acquired late in seed development and is considered necessary for the completion of the plant's life cycle, as an adaptive strategy to enable seed survival during storage or environmental stress, and to ensure better dissemination of the species. The role of water status in desiccated tissues and problems related to testing tolerance in seeds are reviewed. The molecular mechanisms of desiccation tolerance has received extensive consideration only during this past decade. There is a general consensus that desiccation tolerance involves the protection of cellular membranes from the deleterious effect of water removal and the resultant necessity to maintain the bilayer structure in the absence of an aqueous environment. Therefore, some aspects of desiccation-induced membrane injury are described. Several strategies for coping with cellular desiccation have been identified the presence of high amounts of non-reducing sugars, the efficiency of free radical-scavenging systems and the expression of desiccation- and/or ABA-regulated genes. These molecular mechanisms allowing cellular protection are reviewed together with their respective role in dessication tolerance. It is concluded that desiccation tolerance is not likely to be ascribed to a single mechanism but rather to a multifactorial property in which each component is equally critical.

Five distinct groups of storage proteins are synthesized by embryos during development in the fruit at 35 days after pollination (35 DAP); none of them is synthesized in germinating embryos 48 h after isolation (48 HASI) of the seeds from the fruit. Endo-β-mannanase, a marker of germination and seedling growth, is produced in the isolated seeds, but not in the developing seed in situ 35 DAP. When seeds at this stage are removed from the fruit, but remain in intimate contact with the locular and sheath tissue, they continue to synthesize storage proteins for at least 48 HASI. Removal of the sheath from seeds isolated 35 DAP stops storage protein synthesis, and increases endo-β-mannanase activity. Both abscisic acid (ABA) and osmoticum at physiological concentrations maintain the synthesis of storage proteins in seeds isolated 35 DAP for at least 48 HASI, although osmoticum is more effective in preventing their eventual germination. Thus the effects of the sheath and the locular tissue are mimicked by both ABA and osmoticum in relation to the maintenance of storage protein synthesis in seeds 35 DAP and in the suppression of endo-β-mannanase activity for at least 48 HASI.

X-radiography is a simple, rapid and non-destructive method for analysing the morphology of embryos in dry, mature seeds of Arabidopsis thaliana. In wild type seeds, the cotyledons, hypocotyl and radicle tip can be readily distinguished. In seeds of the mutant types knolle, keule, and the double mutant keulelgnom, aberrations in embryo morphology can be visualized. X-radiography may therefore be useful in the isolation of embryo mutants from Arabidopsis seed samples.

A protein named QP47 has been purified from quiescent nuclei of root meristems of Pisum sativum, and used to prepare a polyclonal antibody. Immunolocalization of this protein with fluorescent probes revealed a nuclear distribution of thread-like structures. However, the relationship between the distribution of QP47 immunofluorescence and the structural organization of the chromatin required further investigation. The decrease in content of this protein in the nuclei of embryo cells seems to be correlated with the transition from quiescence to proliferation. QP47 degradation seems to depend upon an increase in the state of its phosphorylation. This protein is not present in normally proliferating cells, or in cells whose cell proliferation has been arrested by starvation or differentiation. It is hypothesized that QP47 may be required specifically during the quiescent period for specific structural organization of the chromatin.

Marrow (Cucurbita pepo L.) seed quality was monitored during seed development and maturation in 2 years. Mass maturity (end of the seed-filling phase) was attained 61–63 d and 54 d after anthesis in 1989 and 1990, respectively, when seed moisture contents had declined to 40–48% (wet basis). Considerable dormancy was encountered during standard germination tests, but was overcome by decoating the seeds. The ability of dried, decoated seeds to germinate normally in standard tests reached near maximal values shortly after mass maturity; these values were more or less maintained in seeds from subsequent harvests. Maximum seed longevity in air-dry storage was detected in seeds harvested 24 d (1989) and 26–31 d (1990) after mass maturity. Seedling dry weights 15 d after sowing were greatest for seeds harvested 2–22 d (basal fruits) or 14 d (apical fruits) after mass maturity in 1989, and were positively correlated (P<0.01) with times from seedling emergence to seedling harvest. Consequently in the subsequent year the hypothesis that these differences in seedling dry weight were solely due to differences in times from sowing to emergence was tested (and confirmed). Seedling relative growth rates did not differ with seed harvest date (P>0.25) in 1990, but absolute seedling size did (P<0.005); seeds harvested 21–31 d after mass maturity had the greatest seedling weight and also growth rate (in absolute terms) at any one time after sowing. Decline in seed quality (when assessed by both potential seed longevity and seedling growth) was not detected until the final harvest interval in 1990 (85–90 d after anthesis, 31–36 d after mass maturity). These results for marrow contradict both aspects of the general hypothesis that seed quality is maximal at the end of the seed-filling phase and that viability and vigour begin to decline thereafter.

Morphological changes in tomato (Lycopersicon esculentum Mill. cv. Moneymaker) seed during osmopriming and imbibition (‘hydropriming’) were followed using X-ray photographs. Embryo, endosperm and free space areas were measured. Both osmopriming and hydropriming resulted in free space development (+ 8.1% and + 10.8% of the whole seed planar area, respectively), almost all at the cost of the endosperm area. Planar dimensions of whole seeds were relatively constant and the dimension perpendicular to the planar surface, the thickness, could account for the volume increase of primed seeds reported in the literature. In dead seeds, only a small amount of free space developed while the planar area of the seed remained the same. In the imbibing viable seeds no deterioration of endosperm could be detected until the moment of root protrusion.

Seeds which were osmoprimed directly after harvest, i.e. in the fresh state, did not showthe induction of any free space, while free space developed normally after dehydration and a second priming treatment. Apparently, a dehydration step prior to the priming treatments is required for the development of free space in osmo- or hydroprimed seed.

X-ray photographs of hydroprimed seeds showed that the radicle tip adhered tightly to theendosperm cap. This results in various forms of damage to the root tips upon redrying. Priming did not introduce cotyledon abnormalities.

Freshly harvested oat (Avena sativa L.) seeds are considered to be dormant because they are unable to germinate at relatively high temperatures (above 20°–25°C). This primary dormancy results partly from the structures surrounding or adjacent to the embryo (pericarp, testa and endosperm) and partly lies in the embryo itself. Incubation of imbibed dormant seeds at 30°C induces a thermodormancy which is expressed as germination in a narrower temperature range. Induction of thermodormancy is apparent after 3–8 hours at 30°C, is optimal after about 1–2 days at this temperature, but diminishes with longer treatment times. Although whole grains become more dormant as a result of such treatment, the embryos themselves become less dormant, therefore the tissues covering or adjacent to the embryos must have become more inhibiting or the embryos have become more sensitive to their inhibitory action. The germination and sensitivity to ABA or oxygen of embryos isolated from such seeds are no longer so dependent on temperature. The effect of high temperature on primary dormancy and induction of thermodormancy is discussed with reference to sensitivity to ABA and temperature.

Barriers to water permeability in prickly sida seeds were characterized using solvents and water-soluble salts. The seeds imbibed water when barriers in the chalazal area broke down. The major barrier was in the distal region of the palisade layer bordered internally by the light line. Soaking dormant seeds in certain organic solvents overcame the barrier and enabled the seeds to imbibe water. The most active were non-polar solvents with dielectric constants between 3.7 and 18.5, and with a high ability to hydrate and swell cellulosic materials. We concluded that the most active solvents (pyridine, diethylamine) overcame the major barrier, entered the proximal portion of the palisade layer and caused the swelling of cellulosic cell components. The cellular expansion created stresses that broke thin walls of the underlying subpalisade cells in the chalazal area. The breakage resulted in separation of the cell layers and rapid entry of water into the seed. Highly non-polar or highly polar solvents had little or no activity. Some solvents overcame the barrier but did not cause swelling and disruption of the cell layers. We propose that the natural breakdown of impermeability in prickly sida seeds was due to the gradual seepage of water past the barrier in the distal portion of the coat in the chalazal area and into the inner portions where swelling of hygroscopic components triggered the breakage of subpalisade cell walls resulting in cell layer separation.

Two different pre-emergence treatments preventing radicle protrusion by limiting the extent of imbibition were tested on conifer seeds. Osmotic priming in solutions of polyethylene glycol (PEG) which prevented radicle protrusion during prolonged (3 weeks) treatment resulted in variable percentage germination, mean germination time, and coefficient of uniformity of germination among different seedlots when placed on water. All three germination parameters were either unchanged or inferior in seeds subjected to the various PEG concentrations, treatment temperatures and treatment durations when compared with prechilled controls. In contrast, white spruce and jack pine seeds invigorated at 30% moisture content and 15°C in Gore-Tex membrane tubes consistently germinated faster than prechilled control seeds when subsequently placed on water. Tube invigoration affected neither the ability of seeds to complete germination nor the uniformity of germination. The decrease in mean germination time as treatment duration progressed was always linear and plateaued at approximately 4 days after the tube-invigorated seeds were placed on water regardless of species or seedlot. On average, tube invigoration decreased white spruce mean germination time by 0.13±0.02, and black spruce by 0.23±0.03 day per day of invigoration for up to 20 days of treatment. Jack pine mean germination time decreased 0.2±0.04 day per day of invigoration for up to 14 days of treatment.

DNA synthesis following the addition of water to excised embryos of non-dormant rye and to embryos of non-dormant and dormant genetic lines of Avena fatua has been examined. All the samples exhibit an early unscheduled DNA synthesis, have similar responses to DNA polymerase inhibitors and a similar increase in DNA ligase function for the first 24 h of imbibition, irrespective of whether they germinate or remain dormant. A β-polymerase-mediated DNA repair activity is indicated immediately upon imbibition with the stable incorporation of 3H-methyl-thymidine into high molecular weight DNA. Following γ-irradiation of dry or imbibed embryos, inhibitor results suggest the appearance of an additional α- or δ-polymerase activity in the ensuing repair. Abscisic acid permits the early unirradiated repair synthesis, but like aphidicolin, it inhibits replicative DNA synthesis and partly inhibits the post-irradiation incorporation of thymidine. DNA synthesis takes place continuously throughout dormancy of imbibed embryos: precursor incorporation into DNA cannot be chased in the short term and occurs in the absence of an S-phase or endoreduplication of nuclear DNA. It is proposed that dormant imbibed embryos maintain the integrity of the genome by a continuous but slow replacement of DNA steered by a non-amplifying and modified form of replicative DNA synthesis and that abscisic acid may play a determining role in this process.

Ageing of wheat seeds (Triticum aestivum cv. Galahad, 1986 harvest) during storage is accompanied by an increase in the proportion of abnormal seedlings produced when aged seed lots germinate. The response of aleurone layers from normally and abnormally germinated seeds to heat shock has been investigated. [14C]-amino acid incorporation into aleurone layers from 3-d normally and abnormally germinated seeds diminished after 4 h at 42°C but the aleurone layers were able to recover significantly during a 5-h period at 20°C following heat shock. [35S]-methionine pulse-labelling of aleurone proteins showed that α-amylase isoenzyme synthesis was abolished upon heat shock in aleurone layers for both normally and abnormally germinated seeds and that only aleurone layers from normally germinated seeds were capable of recommencing substantial α-amylase synthesis during the recovery period. Heat shock caused aleurone layers from abnormally germinated seeds to synthesize polypeptides not synthesized by aleurone layers from normally germinated seeds under the same conditions. The synthesis of certain polypeptides by aleurone layers from abnormally germinated seeds was enhanced at 20°C, 42°C and during recovery compared with normally germinated seeds. These results are discussed in terms of the control of gene expression during heat shock and recovery during seed ageing.