Oxygen is a major factor of seed germination since it allows resumption of respiration and subsequent metabolism reactivation during seed imbibition, thus leading to the production of reducing power and ATP. Most studies carried out in the 60s to 85s indicate that oxygen requirement depends on the species and is modulated by environmental factors. They have also demonstrated that the covering structures mainly inhibit germination by limiting oxygen supply to the embryo during imbibition through enzymatic oxidation of phenolic compounds by polyphenol oxidases (catechol oxidase and laccase) and peroxidases. Recent use of oxygen-sensitive microsensors has allowed to better characterize the oxygen diffusion in the seed and determine the oxygen content at the level of embryo below the covering structures. Here, I will also highlight the major data obtained over the last 30 years indicating the key role of oxygen in the molecular networks regulating seed germination and dormancy through (1) the hormonal balance (ethylene, ABA and GA), the hormone-signalling pathway and, in particular, the ABA sensitivity, (2) the emerging role of mitochondria in ROS production in hypoxia and (3) the involvement of the N-degron pathway in the turnover of proteins involved in seed tolerance to hypoxia.

As in other cultivated species, dormancy can be seen as a problem in cereal production, either due to its short duration or to its long persistence. Indeed, cereal crops lacking enough dormancy at harvest can be exposed to pre-harvest sprouting damage, while a long-lasting dormancy can interfere with processes that rely on rapid germination, such as malting or the emergence of a uniform crop. Because the ancestors of cereal species evolved under very diverse environments worldwide, different mechanisms have arisen as a way of sensing an appropriate germination environment (a crucial factor for winter or summer annuals such as cereals). In addition, different species (and even different varieties within the same species) display diverse grain morphology, allowing some structures to impose dormancy in some cereals but not in others. As in seeds from many other species, the antagonism between the plant hormones abscisic acid and gibberellins is instrumental in cereal grains for the inception, expression, release and re-induction of dormancy. However, the way in which this antagonism operates is different for the various species and involves different molecular steps as regulatory sites. Environmental signals (i.e. temperature, light quality and quantity, oxygen levels) can modulate this hormonal control of dormancy differently, depending on the species. The practical implications of knowledge accumulated in this field are discussed.

Methods of evaluation of seed quality providing accurate prediction of seed performance under field conditions are needed by the seed industry. Various physiological tests, i.e. germination tests in suboptimal conditions of temperature, oxygenation and water potential of the medium, or accelerated ageing and controlled deterioration allow sensitive differentiation between seed lots. A better understanding of the biochemical, cellular and molecular mechanisms involved in the acquisition of seed vigour during seed development, in the germination process and in seed deterioration during ageing could suggest various markers of seed quality. Among these markers, electrolyte leakage and ethylene production during imbibition, cell-cycle markers (DNA replication, β-tubulin), soluble sugar metabolism (in particular, changes in the raffinose family of oligosaccharides), proteins (11S globulin B-subunit, late embryogenesis abundant protein, heat-shock protein) and the efficiency of reactive oxygen species scavenging through antioxidant defence systems (e.g. catalase activity) have potential for the evaluation of the state of seed maturity, seed performance and effectiveness of seed priming treatments. Use of global approaches such as transcriptomic, proteomic or metabolomic analysis could also result in the identification of new markers.

Dormant barley grains cannot germinate at 30°C and this inability to germinate is imposed mostly by the glumellae which have been suggested to limit oxygen supply to the embryo. Hypoxia imposed either artificially or by the glumellae to embryos from dormant grains, increases embryo sensitivity to abscisic acid (ABA) and promotes the accumulation of ABA during the first hours after imbibition. Expression of candidate genes involved in ABA synthesis (HvNCED), catabolism (HvABA8OH1) and signalling (HvABI5, HvVP1 and HvPKABA) was analysed in embryos isolated from dormant whole or de-hulled grains incubated in air or in hypoxia (5% oxygen). The presence of the glumellae enhanced the expression of genes involved in ABA metabolism and signalling with respect to that observed in de-hulled grains incubated in air. These results suggest that at least part of the observed physiological responses to the presence of the glumellae are regulated at the level of gene expression. However, hypoxia imposed on dormant de-hulled grains did not mimic the presence of the glumellae in terms of expression of candidate genes. Hypoxia mimics the presence of the glumellae in terms of dormancy maintenance and ABA accumulation and sensitivity, but its effects appear to operate through different mechanisms.

Seeds vary widely in the sensitivity of germination to oxygen (O2) partial pressure, depending upon the species, temperature, dormancy state and physiological status of the seeds. Most analyses of the O2 sensitivity of germination have focused on final germination percentages and estimated the O2 percentage in air that is required to reduce germination to a given percentage (usually 50%). In contrast, we have applied a population-based threshold model utilizing time courses of germination to quantify three parameters related to seed germination sensitivity to O2 availability: the median base (or threshold) O2 percentage, the standard deviation of O2 thresholds among seeds in the population, and an oxygen–time constant that relates O2 percentage to germination timing. The model fits germination responses accurately across a wide range of O2 concentrations. The response to O2 was logarithmic in all cases, with the O2 percentage required for 50% germination ranging from 21% to as low as 0.005%, depending upon the species, the temperature and the seed dormancy level. Modelling indicated that some seeds can adapt to low O2 percentages and shift their thresholds to lower values over time. Lower temperatures decreased the minimum O2 threshold, as did after-ripening. Seed priming generally reduced the oxygen–time constant and increased the standard deviation of germination responses, but had relatively little effect on the O2 sensitivity per se. The population-based threshold model can be used to quantify the O2 sensitivity of seed germination and to predict germination rates and percentages when O2 availability is limiting.

Relationships between desiccation tolerance and dry matter, water and sugar contents were studied throughout the development of oil palm (Elaeis guineensis Jacq.) zygotic embryos and in immature embryos cultured on a sucrose-enriched medium. Embryo dry weight during in planta development increased between 80 and 140 d after pollination (DAP) and was then stable until maturity. Embryos underwent dehydration until 120 DAP, but their moisture content remained high at maturity (c. 2 g H2O g-1 DW). Desiccation tolerance was acquired between 83 and 104 DAP, and was positively correlated with embryo age and dry weight, and negatively correlated with initial water content during this period. Sucrose, the main soluble sugar present throughout embryo development, accounted for an average of 24% of the dry weight. Glucose and fructose contents decreased to less than 1 mg g-1 DW in embryos at maturity. At 117 DAP, as embryos became tolerant to desiccation, the monosaccharides/sucrose ratio fell to 0.015 and raffinose was detected. Stachyose appeared later in 147-day-old embryos and accumulated until shedding. In vitro culture of immature embryos in the presence of high sucrose concentrations (350 and 700 mM) resulted in an increase in their dry weight and a decrease in their water content, and induced the acquisition of desiccation tolerance. Under these conditions, sucrose accumulated in embryos to 30–40% on a dry weight basis, but neither raffinose nor stachyose was detected. Acquisition of desiccation tolerance by oil palm immature embryos was associated both in planta and in vitro with an accumulation of dry matter, a reduction of moisture content, and a fall in the monosaccharides/sucrose ratio. In planta, survival to dehydration was also related with the deposition of oligosaccharides whereas in vitro, it was related with high sucrose accumulation. The role of sugars in the acquisition of desiccation tolerance in oil palm embryos is discussed.

Priming treatment of sunflower (Helianthus annuus L., cv Briosol) seeds for 7 days at 15°C with a polyethylene glycol solution at −2.0 MPa strongly improved their subsequent germination at 15°C on water. This stimulatory effect of priming remained after drying back the seeds at 20°C for 3 days. Malondialdehyde (MDA) and activities of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR) were measured in control unprimed seeds, primed seeds, seeds primed then dried, and after 3 and 6 hours of imbibition of controland dried primed seeds in order to determine whether the cell antioxidant systems were involved in the beneficial effect of priming. The osmotreatment resulted in a strong increase in SOD and CAT activities but did not markedly affect MDA and GR activity. Following the 3 days of drying, MDA increased and the enzyme activities became similar to those measured in dry unprimed seeds, although the stimulatory effect of priming on germination remained. Imbibition of control dry seeds was associated with an increase in MDA and a decrease in CAT and GR activities, whereas reimbibition of dried primed seeds resulted in a decrease in MDA and an increase in SOD, CAT and GR activities. Isoform patterns on native gels showed no difference between treated (priming with or without subsequent drying) and control seeds for SOD (7 isoforms) and GR (5 isoforms), but the osmotreatment did induce a second isoform of CAT. The results obtained indicate that the CAT isoform pattern might be used as an indicato of the priming treatment that promotes germination. Involvement of theantioxidant systems in seed vigour is discussed

Germination and carbohydrate metabolism were studied in fresh developing pea (Pisum sativum L., cv Baccara) seeds and after artificial drying at 25°C and various relative humidities (20, 75 and 99% RH) to investigate whether the occurrence of desiccation tolerance was related to sucrose, raffinose and stachyose contents. Seeds became completely tolerant to fast drying at 25°C and 20% RH a few days after the end of reserve accumulation, i.e. when their moisture content dropped to approx. 50% (fresh weight basis). This acquisition of desiccation tolerance was associated with an accumulation of raffinose and stachyose, the latter being more abundant in the embryonic axis than in the cotyledons. The (raffinose+stachyose)/sucrose ratio increased during seed development and reached 1.1 in the axis and 0.2 in the cotyledons just before the onset of desiccation tolerance. When the natural acquisition of desiccation tolerance occurred on the mother plant, artificial drying of isolated seeds induced an increase in oligosaccharide content in the cotyledons. Immature seeds, the moisture content of which was higher than about 60% (fresh weight basis), did not tolerate fast drying (25°C and 20 or 75% RH). Such drying did not result in the synthesis of stachyose and induced an increase in electrolyte leakage, a decrease in the ability of seeds to convert 1-aminocyclopropane 1-carboxylic acid (ACC) to ethylene and an increase in ethane synthesis, thus indicating a deterioration of cell membrane properties and lipid peroxidation. In contrast, immature seeds tolerated drying either in the pods or at 25°C and 99% RH, and such drying induced a decrease in sucrose content, an increase in oligosaccharide content and a (raffinose+stachyose)/sucrose ratio higher than around 1. Soluble sugar contents of dried immature seeds depended on the conditions of dehydration. In cotyledons, the (raffinose+stachyose)/sucrose ratio reached 0.61 when seeds were dried at 25°C and 99% RH, whereas it was as low as 0.15 when drying was performed at 25°C and 20% RH. All the results obtained are consistent with the concept that oligosaccharides may well be involved in the protection of membranes during dehydration.

The relationship between initial mobilization of 11-S globulin storage protein (leading to solubilization of its B-subunit) and germination performance after priming was studied in sugarbeet (Beta vulgaris L.) seeds. Priming was conducted for 2 d either in water (hydropriming) or in –2.0 MPa polyethylene glycol-8000 (osmopriming), at various temperatures ranging from 5 to 40°C and oxygen concentrations in the atmosphere ranging from 0 to 21%. For both types of pre-treatments, the range of temperatures and the concentrations of oxygen which were effective in priming were very similar to those which allowed solubilization of the B-subunit of 11-S globulin, supporting the robustness of this protein marker for optimization of sugarbeet seed priming. Furthermore, the temperature and oxygen dependence of priming efficiency closely paralleled that for germination of the untreated seeds, reinforcing the finding that the beneficial effect of priming corresponded to the advancement of germination sensu stricto (i.e. phase II of the germination process). For priming times longer than 2 d, particularly for osmopriming, there was a very dramatic decrease in germination of the treated seeds. For instance, following a 14 d osmopriming at 25°C as much as 60% of the pre-treated seed population failed to germinate when transferred to water. This loss in germination performance quite closely paralleled degradation of LEA (late embryogenesis abundant) proteins, notably a heat-stable seed-specific protein of about 60 kDa and a seed-specific biotinylated LEA protein.

The temperate grass species, red fescue (Festuca rubra ssp. litoralis), perennial ryegrass (Lolium perenne) and Kentucky bluegrass (Poa pratensis), are often sown in mixtures for turfgrass. Differences in germination response to environmental conditions often result in different establishment success. The three species were compared in a study of imbibition at reduced water potential (Ψ) and in a study of the effects of Ψ and temperature on germination. In all three species, imbibition rate and extent were reduced with decreasing Ψ, causing a prolonged lag-phase before germination, particularly in Kentucky bluegrass. In perennial ryegrass and Kentucky bluegrass, water content at radicle emergence decreased non-linearly with decreasing Ψ, with a lower asymptote expressing the critical water content for germination. Seeds were germinated in factorial combinations of two temperatures (10 and 25°C) and nine Ψ levels (–2 to 0 MPa). Kentucky bluegrass required a longer thermal time to germinate than the other species and exhibited conditional dormancy at 25°C, which affected the use of the thermal time model. The effects of Ψ were described by a hydrotime model. Kentucky bluegrass had higher base water potential (Ψb) and required a longer hydrotime for germination than the other species. However, Ψb increased with temperature in Kentucky bluegrass and red fescue, but decreased in perennial ryegrass, and the base temperatures also changed with Ψ. The interacting effects of Ψ and temperature affected the ability of the hydrothermal time model to predict germination performance across all temperature and Ψ conditions.

The aim of this study was to investigate whether there was a relationship between growth of sunflower seedlings at 15°C in the dark and activities of enzymes involved in scavenging of reactive oxygen species (ROS), especially superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), or in production of free radicals, namely lipoxygenase (LOX). Untreated control seeds were compared with seeds exposed to accelerated ageing (5 d at 45°C and 100% relative humidity), osmopriming (7 d at 15°C with a polyethylene glycol (PEG) solution at –2 MPa) and accelerated ageing followed by priming. Accelerated ageing decreased seed germinability and slowed down hypocotyl growth, whereas priming resulted in an increase in germination rate and enhanced seedling development. Osmopriming of aged seeds almost completely restored the initial rate of germination and seedling growth. The activity of all the enzymes studied increased during seed germination and seedling development, except that of SOD. Seed imbibition or radicle protrusion were related mainly with an increase in CAT activity and, to a lesser extent, in GR activity. Increase of LOX activity was clearly associated with the onset of hypocotyl elongation. However, in all cases, malondialdehyde measurements did not reveal intense lipid peroxidation. Priming induced a marked stimulation of CAT and GR during seed imbibition or very early during seedling development, as compared to the control seedlings and particularly to the seedlings generated by aged seeds. Hydrogen peroxide (H2O2) contents of seeds and seedlings were closely correlated to the activities of CAT and GR and to the kinetics of seedling development. The results obtained establish a clear relationship between sunflower seed vigour and ROS scavenging.