This review examines the presence of free and bound PAs during the life cycle of seeds. Although data are still rather scarce, PAs, present, but with an unknown function during embryogenesis, are accumulated in relatively high quantities in mature dry seeds of some species. PA concentration can be altered by the process of stratification, and PAs are synthesized in growing organs after germination, but it is also possible that the reserve organs synthesize and export some types of PA to the axis. This metabolism and sink-source relationship depend on the germination conditions and on the greater or lesser presence of enzymes related to synthesis (ADC, ODC, LDC and AdoMetDC) and degradation (DAO and PAO) of free PAs. The biochemical mechanism binding the PAs to a series of compounds of low and high molecular weight is currently unknown in seeds. The present study also considers intra- and intercellular compartmentation of free and bound PAs in growing axes and the physiological implications of the ethylene-production pathway in relation to seed behaviour.

DNA polymerase 2 holoenzyme is an α-type polymerase present as a major activity in 24-h germinated maize embryo axes. At this time, several of the polypeptides that compose this multisubunit enzyme are phosphorylated. During the germination of artificially deteriorated maize embryo axes, total DNA polymerase activity decreases by around 50% when compared with activity in non-deteriorated axes. DNA polymerase 2 appears to be damaged by the deterioration treatment, greatly reducing its activity. This decrease in activity is related to protein degradation during germination since the amount of both the holoenzyme and the different polymerase 2 subunits is reduced as imbibition progresses. The decrease in amount is more dramatic for some of the subunits than for others, i.e., the amount of the 90-kDa catalytic subunit is reduced by about half, whereas all other subunits totally disappear at different times during germination. The phosphorylation state of the different polypeptides is also greatly affected.

Seeds of Celosia argentea L. displayed an absolute requirement for light for germination. Germination could be induced by far-red light as well as red light, and therefore, the effect of red light was not completely reversible by far-red light. A considerable proportion of the seed population was sensitive to red light within the range 0.01–0.1 μmol m−2, which suggested that phytochrome was operating in the very low fluence response mode. Seeds showed a gradual increase in germination in response to red light with an increasing duration of previous dark imbibition. Interruption of the dark imbibition period with very low fluence (10−4−10–1 μmol m−2) or low fluence (1–103 μmol m−2) red light prevented germination subsequent to a terminal, saturating red light irradiation. Increasing responsiveness to red light with increase in imbibition temperature and inhibition of development of photosensitivity by cycloheximide suggested that phytochrome synthesis occurred during the dark imbibition period. An absolute requirement for light for seed germination, inhibition of seed germination on interrupting the dark imbibition period with very low fluence or low fluence red light, and the recovery from photoinhibition of seeds in darkness are consistent with the hypothesis that PhyA is the principal phytochrome involved in the photoregulation of seed germination in C. argentea.

The germination of intact, moist-chilled white spruce (Picea glauca [Moench.] Voss.) seeds and liquid–nitrogen–decoated (LN2), unchilled seeds is significantly more than of intact, unchilled seeds and intact, unchilled seeds exposed to LN2. The testa is largely responsible for imposition of dormancy in these seeds, although the megagametophyte and/or nucellus also play a role. One or both of these tissues undergoes significant weakening during moist chilling. Excised embryos from dormant and nondormant white spruce seeds elongate when placed on solid Murashige and Skoog minimal organics medium supplemented with a carbon source (sucrose, glucose or galactose), but elongate very little on medium without sugar. They are not killed by rapid imbibition on unsupplemented media. Thus, embryos of white spruce do not exhibit innate dormancy, but are dependent on a carbon source for elongation, and have dormancy imposed on them by their surrounding structures.

Features of chemicals which break seed dormancy and the mechanisms of their action are discussed with reference to results obtained with red rice (Oryza sativa) and other relevant research. The significance of chemical structure, including the nature and location of functional groups, is considered, and the fact that similar structures can elicit different biochemical responses is illustrated with examples. As yet it is unclear whether changes in cell pH, highly correlated with the uptake of, and metabolism to, weak acids, and which some consider a possible trigger or marker in dormancy breaking, are causative, consequential or merely correlative. Uptake and metabolism of dormancy-breaking compounds is, or can be, very rapid and the importance of establishing the time course of dormancy-breaking, as opposed to germination events, is emphasized.

Many research questions focused upon seed dormancy and germination remain unanswered due to problems created by inappropriate assumptions concerning the design and implementation of experiments. Lack of attention to the genetic background and growing conditions required for seed production and subsequent storage may compromise an investigator's ability to pool data or assimilate data trends due to variability in seed performance from year to year. Seed aging events during the afterripening process can be mistaken for those involved in the transition from the dormant to nondormant state. After imbibition, inaccurate criteria for and scoring of the germination event may lead to confusion of seed vs. seedling physiology and biochemistry. In addition, a sufficient number and proper spacing of data points during timecourse experiments are required to properly stage a sequence of events. Studies comparing dormant vs. nondormant seeds must be supplemented with kinetic experiments examining the effect of a dormancy-breaking treatment upon the transition between the dormant and nondormant states. In comparisons of the relative efficacy of various dormancy-breaking chemical treatments, the physical properties of substances under evaluation (e.g. lipophilicity) should be considered. In addition, since many dormancy-breaking chemicals are toxic to vegetative growth, there is a danger of introducing pharmacological artifacts that confuse interpretation of the results obtained. These toxic properties also necessitate the use of viability tests during construction of dose-response curves to avoid confusing seed death with dormancy. Seed scientists must also avail themselves of the extensive literature on developmental arrest in other organisms, which may generate fresh ideas for experimentation.

Changes in seed dry weight, moisture content and germinability of three japonica cultivars of rice during development and maturation in the dry season (November–May) field multiplication at Los Baños, Philippines were compared with those in a cooler controlled environment (24°/18°C). Under field conditions, maximum dry weight accumulation, which indicates mass maturity, was attained 18–21 days after flowering in all cultivars. In the cooler environment however, mass maturity was achieved 21 days after flowering in Fujisaka 5 and Minehikare, and 28 days in Shuang cheng nuo. The moisture content of the seeds at mass maturity varied between 28 and 35% among cultivars in the two environments. Although the ability of seeds to germinate differed in early harvests (7 and 14 days) among cultivars and across two environments, germination of the mature seeds was similar in all. There were no significant differences in potential longevity of the seeds harvested at 28, 35 and 42 days after flowering within each cultivar, which was determined by the germination of seeds that had been hermetically stored with 15% moisture content at 35°C. Differences among cultivars within each seed production environment were also not significant. However, potential longevity of the seeds harvested from the cooler environment was higher than those from the field (P<0.05), which confirmed that japonica cultivars which evolved under temperate regions are sensitive to warmer seed production environments. However, the magnitude of differences in potential seed longevity between the two environments was not high. This indicates that the seed production environment during the dry season at Los Baños is not harsh for japonica cultivars.

Freshly harvested tomato (Lycopersicon esculentum Mill, cv. Moneymaker) seeds were osmotically primed for 8 d in −1.0 MPa PEG-6000 solution and dried to about 6% water content for storage. Such so–called ‘fresh PEG priming’ enhanced seed germination and improved seedling performance as compared with the untreated control. Fresh PEG priming neither alleviated seed dormancy nor promoted DNA replication as was the case when seeds were dried upon harvest and subsequently primed in PEG (normal PEG priming). However, the addition of 10 μM GA4+7 to the osmotic priming solution triggered replicative DNA synthesis of fresh-priming seeds and further enhanced the germination process. After 5 months of storage in ambient temperature conditions, fresh PEG-primed seeds maintained more positive effects gained from priming, whereas, normal PEG-primed seeds had lost the promoting effects on germination. Normal PEG-primed seeds were much more susceptible to controlled deterioration than fresh PEG-primed seeds. It is suggested that the advancement of germination is negatively correlated with seed storability. The mechanisms of seed priming in relation to nuclear replication activities and physical changes are discussed.

Bromus tectorum L. is an invasive winter annual grass with seeds that lose dormancy through the process of dry after-ripening. This paper proposes a model for after-ripening of B. tectorum seeds based on the concept of hydrothermal time. Seed germination time course curves are modelled using five parameters: a hydrothermal time constant, the fraction of viable seeds in the population, base temperature, mean base water potential and the standard deviation of base water potentials in the population. It is considered that only mean base water potential varies as a function of storage duration and incubation temperature following after-ripening. All other parameters are held constant throughout after-ripening and at all incubation temperatures. Data for model development are from seed germination studies carried out at four water potentials (0, −0.5, −1.0 and −1.5 MPa) at each of two constant incubation temperatures (15 and 25°C) following different storage intervals including recently harvested, partially after-ripened (stored for 4, 9 or 16 weeks at 20°C) and fully after-ripened (stored for 14 weeks at 40°C). The model was fitted using a repeated probit regression method, and for the two seed populations studied gave R2 values of 0.898 and 0.829. Germination time course curves predicted by the model generally had a good fit when compared with observed curves at the incubation temperature/water potential treatment combinations for different after-ripening intervals. Changes in germination time course curves during after-ripening of B. tectorum can largely be explained by decreases in the mean base water potential. The simplicity and good fit of the model give it considerable potential for extension to simulation of after-ripening under field conditions.

The role of free radical-induced damage as a cause of loss of vigour in seeds is by no means resolved. In this contribution, the effects of environmental treatments known to reduce viability rapidly were compared with the effects of long-term, low-temperature storage on germination, hypocotyl growth and free radical accumulation and lipid peroxidation in soybean seeds. Accelerated aging was achieved by incubating seeds at 35°C and 1% relative humidity over H2SO4 for up to 69 days in the light and in darkness. In contrast, seeds under long-term storage were maintained at 5°C and 6% moisture content in darkness for up to 6 years. At 35°C there were rapid and significant reductions in rates of seed germination and hypocotyl extension. Loss of viability and declining vigour were associated with increases in lipid peroxidation and free radical build-up but the latter, surprisingly, was largely confined to the testa rather than the cotyledon. Exposure to light greatly enhanced lipid peroxidation and increased organic free radical accumulation in the translucent testas of seeds, but not in the cotyledons. Similar responses to light were recorded in testas detached from seeds. These results show that in soybean the testa is a significant locus of free radical degenerative events induced by high temperature combined with low moisture.

Storage of tomato seeds under deteriorating conditions reduced the rate of germination, the uniformity of germination, total germination and the proportion of normal seedlings. Pre-storage humidification and hydropriming of seeds resulted in a significant increase in resistance to deterioration. In contrast, osmoprimed seeds were more deterioration sensitive. Humidification or hydropriming for one day did not allow nuclei to enter the S phase of the cell cycle. Osmopriming strongly increased the percentage of nuclei with replicated DNA in the embryonic root tip, indicating initiation of the cell cycle and progression towards the G2 phase. The interaction between the prestorage treatments, cell cycle progression and deterioration resistance, is discussed. Based on the changes in moisture content equilibrium and cell cycle activity it is hypothesized that the beneficial effects of pre-storage humidification and hydropriming were related to metabolic activities induced by the partial hydration and that the adverse effects of osmopriming were caused by a decrease in DNA repair activity due to progression in the cell cycle.

Endosperms of fenugreek and carob which have been leached following their isolation exhibit increased activity of endo-β-mannanase, part of which is released into the surrounding incubation medium. This activity was suppressed by the addition to the endosperms of abscisic acid or endosperm/seed coat leachate. The leachate from carob inhibited the increase of endo-β-mannanase activity in fenugreek and that from fenugreek inhibited enzyme activity in carob but more weakly; neither of these leachates inhibited the production of α-amylase in wheat endosperm. Polyethylene glycol-induced stress on non-leached endosperms inhibited endo-β-mannanase production, but not when the stress was applied following endosperm leaching. However, the stress did reduce the amount of enzyme released into the surrounding incubation medium. Several isoenzymes of mannanase could be detected on isoelectric focusing activity gels in the endosperms of both fenugreek and carob, but their pis differed between the two species. When activity was suppressed by ABA or leachate, all pl forms declined equally.

Among the forage legumes somatic embryogenesis has been most studied in alfalfa (Medicago sativa L.) reflecting inherent breeding problems and the high demand for seeds of this species. Current methods of somatic embryogenesis are reviewed with emphasis on each of the 3 stages involved: the acquisition of competence, the induction of somatic embryogenesis, and somatic embryo development and maturation. The role of plant growth regulators is particularly stressed. Artificial seed production, the development of reduced-generation synthetics or hybrids and genetic transformation of alfalfa are considered. Research on somatic embryogenesis and plant transformation in a wide range of other forage legumes is examined but it is suggested that the application of artificial seed technology is unlikely in forage legumes other than alfalfa.

The rate of oxygen consumption by cabbage seeds increased on imbibition and there was a further sharp increase on germination. This was delayed in artificially aged seeds of low vigour. The increases in oxygen consumption reflect the increased oxidation of carbohydrates via respiratory pathways. The activities of key regulatory enzymes of glycolysis and the oxidative pentose phosphate pathway were measured in aged and unaged seed lots of cabbage. Differences in the activities of glucose 6-phosphate dehydrogenase and pyrophosphate:fructose 6-phosphate 1-phosphotransferase were correlated with the rate of germination (T50) in seed lots with large differences in seed vigour induced experimentally by artificial aging. However, the activities of these enzymes could not be used to distinguish between untreated seed lots which had smaller vigour differences apparent only under stress. The enzymes are presumably not controlling and determining seed vigour, but merely reflecting actual seed performance under the current conditions.

The presence of dehydrins could not be demonstrated in axes of mature, undried recalcitrant seeds of the tropical wetland species Avicennia marina, Barringtonia racemosa, Bruguiera exaristata, Bruguiera cylindrica, Bruguiera gymnorrhiza, Ceriops tagal, Rhizophora apiculata, Rhizophora mucronata and Rhizophora stylosa, but were present in the temperate species Acer saccharinum, Aesculus hippocastanum, Araucaria angustifolia, Camellia sinensis, Castanea sativa, Poncirus trifoliata and Zizania palustris. They were also present in axes of Castanospermum australe (of tropical origin) seeds which underwent development in a temperate climate, and were produced in response to drying in axes of Barringtonia racemosa but not Avicennia marina. The presence of dehydrins was associated with high abscisic acid contents. These proteins may provide protection against low temperatures in temperate seeds and against water loss to which the seeds may be naturally exposed. The presence of dehydrins was unrelated to the evolutionary status of the families studied.

Protein storage vacuoles (PSVs) from radicles and cotyledons of dry cotton seeds were isolated by differential centrifugation following homogenization in glycerol. Protein complement analysis of isolated PSVs with one dimensional SDS-PAGE gels revealed similar major storage proteins, viz. 53 and 48 kDa, with differences in lower molecular mass proteins. Radicle PSVs have apparently more 35-kDa and less 22-kDa storage protein than do cotyledon PSVs. The mineral composition of whole radicles, cotyledons and isolated PSVs from radicles and cotyledons was determined by atomic absorption spectroscopy and colorimetric elemental analyses. The concentration of calcium (Ca), magnesium (Mg), potassium (K) and phosphate (P) was lower in isolated PSVs from radicles than from cotyledons, resulting in a marked difference in the Mg/Ca and (Mg+Ca)/K ratios in PSVs from these two sources. Analysis of radicle and cotyledon tissue from dry seeds for mineral distribution with EDX and scanning electron microscopy revealed major concentrations of Mg, K and P in PSVs. These observations indicate that PSVs in radicles are similar in protein and mineral composition to PSVs in cotyledons. PSVs in radicles have the potential function as storage organelles to provide minerals and nutrients for radicle growth during imbibition and germination.

The effects of temperature on dormancy loss, germination and viability were investigated in seeds of Aesculus hippocastanum L. harvested over a 4-year period. Release from embryo dormancy in freshly harvested seeds was manifest in two phases of morphological growth: initially, when the seed lot was only partially released, axis emergence resulted primarily from cotyledonary petiole extension without radicle extension; subsequently, when the seed lot was totally released, axis emergence of all seeds was followed immediately by extension to >1 cm through growth of the radicle. Germination (axis emergence and radicle extension) at 16°C was a function of pre-treatment period at 2–11°C. The rate of dormancy loss (probit germination d−1) increased linearly below a ceiling temperature for the chilling response; this temperature was estimated to vary from 13°C to 16°C for two seed lots harvested in separate years. Dormancy periods for individual seeds within both seed lot populations can be described by cumulative normal distributions; the predicted standard deviation of chilling units below the ceiling temperature (i.e. thermal time) was 186°C d. Visible germination occurred during the process of stratification at 2°C, starting after 21–25 weeks. By contrast, three years of hy-drated seed storage at 16°C, which was a non-permissive temperature for dormancy loss, resulted in little pre-emergence of the axis during stratification; approximately one third of the seeds remained germinable. The implications of these quantitative analyses of the physiological processes in recalcitrant seeds for the development of improved storage methods are discussed.

Predispersal seed predation may have implications for plant population dynamics and the evolution of plant traits, but assessing the level of seed predation for an individual plant is not always straightforward. Seeds of Lotus corniculatus (Fabaceae) are often only partially eaten by a weevil seed predator, Apion loti. Samples of these seeds were tested for viability and subsequent seedling vigour. A large proportion of these damaged seeds were viable, and the resulting seedlings almost as vigorous as those from undamaged seeds. The ubiquity of these findings, and their ecological significance, is discussed.