Active oxygen species (AOS) are involved in various aspects of seed physiology. Their generation, which occurs during seed desiccation, germination and ageing, may lead to oxidative stress and cellular damage, resulting in seed deterioration. However, cells are endowed with detoxifying enzymes and antioxidant compounds that scavenge AOS and participate in seed survival. The detoxifying mechanisms play a key role in acquisition of desiccation tolerance of developing seeds, completion of seed germination and seed storability. However, AOS must also be regarded as molecules intervening in cellular signalling. They are involved in growth processes occurring at early embryogenesis during seed development, and participate in the mechanisms underlying radicle protrusion during seed germination. AOS might also have a regulatory function in the changes in gene expression during seed development, dormancy and germination. Their interplay with other molecules, particularly with hormones such as abscisic acid, suggests that they should be considered as key components of an integrated signalling network involved in many aspects of seed physiology.

The proposal is made that seed scientists need an internationally acceptable hierarchical system of classification for seed dormancy. Further, we suggest that a modified version of the scheme of the Russian seed physiologist Marianna G. Nikolaeva be adopted. The modified system includes three hierarchical layers – class, level and type; thus, a class may contain levels and types, and a level may contain only types. The system includes five classes of dormancy: physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY) and combinational dormancy (PY + PD). The most extensive classification schemes are for PD, which contains three levels and five types (in the non-deep level), and MPD, which contains eight levels but no types. PY is not subdivided at all but probably should be, for reasons given. Justifications are presented for not including mechanical dormancy or chemical dormancy in the modified scheme. PD (non-deep level) is the most common kind of dormancy, and occurs in gymnosperms (Coniferales, Gnetales) and in all major clades of angiosperms. Since, first, this is the class and level of dormancy in seeds of wild populations of Arabidopsis thaliana and, secondly, Type 1 (to which seeds of A. thaliana belong) is also common, and geographically and phylogenetically widespread, it seems that biochemical, molecular and genetic studies on seed dormancy in this model species might have rather broad application in explaining the basic mechanism(s) of physiological dormancy in seeds.

A critical analysis of different criteria used in investigating seed diversity was made. It is concluded that the most promising way to understand the evolution of seeds is to study not only their morphological features, as used by most workers in the field, but also the physiological diversity of seeds in connection with the taxonomic system of plants, together with their ecological and geographical peculiarities.

Soybean (Glycine max (L.) Merrill) seeds accumulate sucrose, raffinose, stachyose and lesser amounts of galactopinitol A, galactopinitol B and fagopyritol B1 in axis and cotyledon tissues as part of the seed maturation process. Somatic embryos appear to be deficient in D-pinitol and galactopinitols, indicating a lack of synthesis by embryo tissues in vitro. Isolated immature soybean zygotic embryos were fed myo-inositol, D-pinitol, D-chiro-inositol and sucrose, individually and in combination, to evaluate the role of substrate availability on galactosyl cyclitol accumulation during precocious maturation. Feeding myo-inositol transiently doubled galactinol accumulation with little effect on other soluble carbohydrates. Feeding D-pinitol increased free D-pinitol 8-fold, galactopinitol A 4.5-fold and galactopinitol B 4.2-fold. Stachyose concentration was 2-fold higher in cotyledons after feeding D-pinitol than after feeding D-chiro-inositol. Feeding D-chiro-inositol increased fagopyritol B1 17-fold in the axis and 7-fold in the cotyledons, but did not increase other soluble carbohydrates. Feeding D-pinitol and D-chiro-inositol together reduced uptake of D-chiro-inositol and steady-state accumulation of galactinol and galactopinitols by 50%, compared to feeding D-pinitol alone. Increasing sucrose concentration from 0 to 200 mM had no effect. Recombinant soybean galactinol synthase, heterologously expressed in Escherichia coli, catalysed the synthesis of fagopyritol B1 and galactinol, but not galactopinitols. These results were consistent with the following interpretations: D-pinitol and D-chiro-inositol were transported from maternal tissues and not synthesized in the embryo, D-chiro-inositol uptake into embryos may be reduced by D-pinitol, fagopyritol B1 was synthesized by galactinol synthase while galactopinitols were not, and fagopyritol B1 and galactopinitols accumulated in response to the supply of free D-chiro-inositol and D-pinitol to embryos.

Mineral nutrient stores in cereal grains are mainly phytate, a salt of the phosphorus-rich compound phytic acid. Quantitative measures of total phosphorus, phytic acid-phosphorus, potassium, magnesium, calcium, iron, manganese and zinc were obtained for whole grains, embryos and rest-of-grain portions of cv. Kaybonnet rice (wild type) (Oryza sativa L.) and a low phytic acid (lpa1–1) mutant strain with a 45% reduction in phytic acid. P, K and Mg were present in higher amounts than Ca, Mn, Fe and Zn in both grain types. Whole-grain amounts of total P, Ca, Mn and phytic acid-phosphorus were lower in whole lpa1–1 grains than in wild-type grains; K, Mg and Fe amounts were similar, and Zn was higher. Embryos, which comprise 3.5% or less of grain dry weight, were comparatively rich in all measured elements. The lpa1–1 mutation influenced the phytic acid content of the embryo more than that of the aleurone layer. Aleurone-layer cells of wild-type grains had many phosphorus-rich globoids 2μm or larger in diameter, whereas lpa1–1 grains contained more of the smaller globoids. The reduction in globoid size was consistent with the reduction in phytate. Energy-dispersive X-ray analysis of both aleurone-layer cells and sections of globoids in aleurone-layer cells revealed that P, K and Mg were the main mineral nutrient elements present in both grain types; traces of Ca, Mn, Fe or Zn were present. Starchy endosperm cells contained virtually no P, K or Mg, whereas scutellum cells were rich in these elements.

Endo-β-mannanase (EC 3.2.1.78), an enzyme that mobilizes the endosperm cell walls of the lettuce (Lactuca sativa L.) seed, increases in activity in the micropylar and lateral regions of this tissue following the completion of germination. Its complementary DNA (cDNA) sequence (LsMan1) was determined using the polymerase chain reaction (PCR) with degenerate primers. The 3‘-end of the cDNA sequence was obtained by 3‘-end rapid amplification of cDNA ends (RACE), and the 5‘-end sequence by genome walking and 5‘-end RACE. The predicted amino acid sequence from the cDNA has a high identity with endo-β-mannanases present in other species (e.g. 67% identity with coffee β-1,4-mannan endohydrolase, 62% identity with tomato fruit endo-β-mannanase). Southern blot analysis suggests the presence of several members of an endo-β-mannanase gene family in the lettuce genome. Several isoforms of the enzyme, including three major ones, were detected by isoelectric focusing. Based on Northern blot analysis, accumulation of the endo-β-mannanase mRNA occurred only after lettuce seeds had germinated, and increased thereafter, although enzyme activity persisted after transcription declined.

A novel technique allows long-term monitoring of real-time oxygen consumption during seed germination in an open system. Most current techniques used to detect oxygen consumption by seeds measure the decrease in oxygen concentration in a closed chamber. This is not ideal for long-term experiments because the chamber must be replenished with air periodically, subjecting the seeds to abrupt changes in oxygen concentration. The current technique employs an open system, in which seeds are submerged in a continuously aerated aqueous environment. Oxygen electrodes are used to measure the steady-state concentration of oxygen in the solution, which is a function of both the rate of oxygen consumption by the seed and the rate of aeration from the atmosphere. The rate of aeration is directly dependent on the oxygen concentration of the bathing solution; therefore, previous calibration of the system allows the direct conversion of steady-state oxygen concentrations into oxygen consumption rates. Because oxygen is not limiting, the experimental design described here can monitor the same sample non-intrusively every minute for more than 24 h, allowing for greater precision than hourly readings often reported with current techniques. Multiple treatments and/or replicates can be run simultaneously, allowing sensitive comparison of various seed treatments or seed types. To illustrate its potential application, the technique was used to follow the rehydration and pre-emergence phases of germination of cucumber (Cucumis sativum), pea (Pisum sativum) and mustard (Brassica juncea) seeds, detect the inhibitory effects of surface sterilization techniques on seed respiration of cucumber, and follow the interaction of a bacterial biocontrol agent with germinating cucumber and pea seeds.

The effects of preconditioning temperature and preconditioning period on the sensitivity of parasitic weed seeds to the synthetic germination stimulant GR24 were studied under laboratory and field conditions. The temperature during preconditioning of Orobanche cumana and Striga hermonthica seeds strongly affected the responsiveness of the seeds to the applied germination stimulant. Preconditioning at an optimal temperature (21°C for O. cumana and 30°C for S. hermonthica) rapidly released dormancy and increased the sensitivity to GR24 by several orders of magnitude. After reaching maximum sensitivity, prolonged preconditioning rapidly induced secondary dormancy, i.e. decreased sensitivity of O. cumana and S. hermonthica to GR24. The rapid change in sensitivity of preconditioned seeds to germination stimulants during prolonged preconditioning was particularly visible at low concentrations of GR24. GR24 at higher concentrations (0.1 and 1 mg l1) usually induced high germination of both species, regardless of the preconditioning period. The striking similarities between the response of parasitic weed seeds to GR24, described here, and results in the literature on non-parasitic wild plant seeds are discussed. Our results show that parasitic weed seeds are highly sensitive to the germination stimulant for a short period of time only, and then enter into secondary dormancy relatively quickly. The similar germination pattern of S. hermonthica seeds preconditioned for prolonged periods of time under laboratory and field conditions suggests that the mechanism observed is of ecological significance.

Changes in population hydrotime parameters were determined during stratification in Polygonum aviculare L. seeds in order to model dormancy loss. Seeds buried in pots were stored at three temperatures (1.6, 7 and 12°C) for 110 d and were exhumed at regular intervals during the storage period. Exhumed seeds were incubated at different water potentials at 15°C and germination time courses were analysed to determine hydrotime parameters. The population mean base water potential (Ψb(50)) decreased concomitantly with seed dormancy, while the hydrotime constant (θH) and the standard deviation of base water potential (θΨb) displayed only minor changes. Based on these results, a model for simulating P. aviculare seed dormancy loss in relation to low temperature was developed. The model employs Ψb(50) as an index of mean seed population dormancy status. While Ψb(50) was allowed to vary as seeds were released from dormancy, θH and θΨb were held constant. Changes in Ψb(50) were related to the time and temperature, using a previously developed thermal stratification time index (Stt), which quantifies the accumulation of thermal time units below a threshold temperature required for dormancy loss to occur. Therefore, Ψb(50) varied in relation to the accumulation of Stt according to time and temperature. Model performance showed acceptable prediction of timing and percentage of germination of seeds buried in irrigated plots, but did not accurately predict germination of seeds exhumed from rain-fed plots. Thus, environmental factors other than temperature could also be involved in the regulation of dormancy status of buried seeds under field conditions.

The impact of rehydration on a multicellular organism was studied in lettuce (Lactuca sativa L.) embryos, using cryo-scanning electron microscopy (cryo-SEM). Naked embryos were sensitive to imbibitional stress, whereas embryos with an intact, thick-walled endosperm were not. Imbibitional injury to naked embryos was mainly confined to the outer 2–3 cell layers of the axis. These cells failed to swell and appeared poorly hydrated. Deeper layers were not affected even after extended periods of cold rehydration. The proportion of damaged cells (6–7% of total) roughly corresponded with the additional K+ that gradually leached from the embryos. Damaged embryos were able to survive the loss of their surface layers and form adventitious roots. The swelling of inner tissues caused the dead surface layers to rupture into patches. Plasma membranes in dried embryos showed normal bilayer structure with a homogeneous distribution of intra-membrane particles (IMPs), also after non-injurious rehydration. Imbibitionally damaged plasma membranes showed many irregularities, such as globular insertions, that probably resulted from malfusions in the ruptured membrane, but the IMPs were still randomly distributed.

Dormancy and desiccation tolerance were studied in Fagus sylvatica seeds subsequent to 0–26 weeks of either moist prechilling (5°C) or warm (15°C) pretreatment at a restricted water content of 34% (fresh weight basis), i.e. c. 5% below full imbibition. After the pretreatments, seeds were either fully imbibed at 5 or 15°C, or dried for 7 d to around 13% moisture content and then fully imbibed at 5°C. Most of the dormancy was released after 5 weeks of prechilling, with nearly 70% of the seeds germinating at 15°C, and dormancy was fully released after 11 weeks of prechilling. Most of the seed desiccation tolerance, measured as the final germination percentage for dried seeds, was lost between 11 and 13 weeks of prechilling. A further decline in desiccation tolerance was recorded from 13 to 19 weeks, with a moderate recovery after 26 weeks. Germination was compared with the expression of a previously described abscisic-acid-responsive GRPF1 mRNA for a glycine-rich protein in F. sylvatica. There was a clear decline in accumulation of the transcript in seeds during prechilling, but not during the warm pretreatment. The amount of GRPF1 expression correlated significantly (P < 0.001) with dormancy status.

Senna multijuga and Plathymenia reticulata are tropical tree species native to the Brazilian Atlantic Forest and the Brazilian Cerrado, respectively. Seed-coat dormancy variation was evaluated within and among natural populations of these two species. Scarified and non-scarified seeds from different plants within populations were germinated at 28°C, and the percentage of germinated seeds was estimated for both species. Mean germination percentages of non-scarified seeds tended to be higher for P. reticulata populations (40 and 62%) than for S. multijuga populations (9 and 35%). After scarification, germination percentages increased significantly in both species, with all populations showing mean values above 84%. The level of seed dormancy, evaluated through the experiment with non-scarified seeds, differed significantly within and among populations of both species (P < 0.05). The values of the coefficient of genotypic determination were high for populations of both species (b = 0.85). Although this coefficient is an overestimation, since it includes non-genetic maternal effects, its high values suggest that a considerable part of the phenotypic variation in seed dormancy in S. multijuga and P. reticulata is of genetic origin. Variation in seed dormancy can be an important factor for increasing genetic diversity in populations of these species, making them able to respond to environmental changes.

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.

An early difference in net protein phosphorylation activity occurred in dormant and after-ripened wheat (Triticum aestivum L.) grains within the first hour of imbibition at 30°C. Embryos from dry, dormant and after-ripened caryopses exhibited a high degree of protein phosphorylation, particularly of four proteins (68, 54, 51 and 42 kDa). Upon hydration, protein phosphorylation activity in dormant embryos decreased within 1 h and reached minimal phosphorylation activity by 5 h. Protein phosphorylation activity in after-ripened (germinable) embryos was not suppressed and remained high during germination. If the hydrated dormant embryos were dried, protein phosphorylation activity was restored. Application of the protein phosphatase inhibitor, okadaic acid, partially overcame dormancy and slowed the decrease in phosphorylation activity. These results suggested that reduction of protein phosphorylation activity slowed the rate of germination.

Saline soils contain multiple types of salt, each of which may exert a different effect on seed germination and seedling growth of plants. The effects of five types of salt on the initial growth of Haloxylon ammodendron, a shrub found on both saline and non-saline areas in deserts of China, were investigated. Seeds were incubated at 20°;C in the dark in a solution (0 to –5.1 MPa) of a salt (NaCl, MgCl2, CaCl2, Na2SO4 or MgSO4) or polyethylene glycol (PEG)-6000, or in a salt (NaCl or MgCl2) or PEG solution containing a low concentration of CaCl2. Seed germination, seedling growth and cation (Na+, Mg2+, Ca2+ and K+) contents of seedlings were examined. Each salt had a different effect on seed germination, seedling growth and influx and outflux of cations in the seedlings. In both NaCl and MgCl2 treatments, the addition of low concentrations of CaCl2 favoured seed germination and seedling growth, and reduced K+ outflux from seedlings, but caused no appreciable decrease in the influx of Na+ or Mg2+ into seedlings. Marked abnormalities in seedlings were found only in treatments with Mg2+ salts, but these effects were completely alleviated by a low concentration of Ca2+ (Ca2+/Mg2+ = 0.012). The different responses of the initial growth in H. ammodendron to different isotonic salt solutions were attributed to differences among salt components in membrane permeability, toxicity and effects on functions of the plasma membrane and/or the cell wall.

The changes in water status of germinating and non-viable soybean (Glycine max L. Merr.) seeds were characterized by nuclear magnetic resonance (NMR) spectroscopy. There were distinct changes in water status between viable and non-viable soybean seeds. In dry seeds, there were only two components, bound and bulk water, as revealed by component analysis of NMR (T2) data. On the contrary, a three-component water proton system (bound, bulk and free water) was observed in both germinating and non-viable soybeans during Phase I of hydration. The bulk water component of non-viable seeds disappeared completely during the lag phase (Phase II) of hydration, resulting in a two-component water proton system. In contrast, the three-component water proton system in Phase II was observed in the germinating seeds. Rapid hydration (Phase III), following Phase II, was observed in germinating soybean seeds only. Due to reorganization of water protons, there was a concomitant increase in bulk and free water, but a decrease in bound water. The physical state of water in these seeds (analysed by NMR spectroscopy) and the measurements of tissue leachate conductivity suggest that non-viable soybean seeds were more affected by the disorganized cell structure in the seed membrane system. The present study also provides evidence that physical reorganization of water is essential in germinating soybean seeds during hydration.

Seed germination and emergence are influenced by the position of seeds in the soil bank profile. Mediterranean grasslands are heavily dependent on seed banks, as these systems are mainly composed of annual species. Seed bank germination experiments in a greenhouse were conducted to analyse the role played by burial depth on seed bank dynamics in annual Mediterranean grasslands. Specifically, they addressed two objectives: (1) to assess the ability of seeds in the shallow layer of the soil bank to emerge when they are buried at different depths, and (2) to ascertain the ability of seeds from deep layers to germinate and emerge to the surface. The study also produced a depth profile of species and seeds. The results show that: (1) all species (100%) and the majority of viable seeds (98.9%) are situated in the first centimetre, with a significant fall in the number of species and seeds in the soil bank as depth increases; (2) for the majority of species (92%) and seeds (85.4%) in the shallow bank, the emergence percentage declines significantly with burial depth; and (3) seeds that are present in deep layers need to rise to the surface in order to produce seedlings. In conclusion, the function of the seed bank in Mediterranean grasslands depends on the number of species and seeds in it, but also on the seed position in the profile and vertical movements that enable them to reach the surface.

The aim of this study was to establish associations of kernel weight and colour with ecogeographic factors and molecular markers, based on ten wild barley [Hordeum spontaneum (C. Koch) Thell.] populations sampled in Israel across a southward transect of increasing aridity. Kernel weight and colour category were scored using barley kernels (naked caryopsis). Small kernel sizes (0.011 g kernel–1) and dark kernels were found in xeric populations. A higher variation of kernel weight was observed in xeric populations. A higher proportion of variation occurred within, rather than among, populations. Water, temperature and soil factors were associated with kernel size variation. Among 18 simple sequence repeats (SSRs) investigated, HVM14, HVM36, HVM43, BMS64 and BMS90 were associated with kernel weight, and HVM68 with kernel colour. The results indicated that high phenotypic variation and genetic diversity are related to ecological stress, and that the association of phenotypic traits with molecular markers, based on natural plant populations, should be interpreted cautiously due to the high chance of spurious associations between traits and molecular markers.

Exposure to smoke is required for the germination of seeds from dormant genotypes of Nicotiana attenuata, a post-fire annual of the Great Basin Desert. Germination can be elicited by GA1,3,4,7 treatments and inhibited by the GA biosynthesis inhibitor, paclobutrazol (PAC), abscisic acid (ABA) and terpenes leached from unburned litter of the plant’s natural habitat. We analysed the endogenous GA and ABA dynamics during the 22 h after imbibition, when smoke-treated dormant seeds commit to germination. Extractable GA1+3 pools decreased in all seeds, but the decrease was more dramatic within 2 h of smoke exposure, which was followed by an increase between hours 2 and 4. Extractable ABA pools increased shortly after imbibition and remained stable in control, water-treated seeds, but decreased sharply in smoke-treated seeds. PAC completely inhibited smoke-induced germination when seeds were treated for 12 h after smoke exposure, consistent with the requirement of de novo GA synthesis for germination. Smoke treatment in the dark did not result in germination, whereas GA3 treatment did, a result consistent with phytochrome-mediated GA biosynthesis. Smoke exposure dramatically increased the sensitivity of seeds to exogenous GA3 treatments in both the light and dark, and light exposure increased this sensitivity an additional tenfold. Taken together, these results suggest that while germination requires endogenous GA synthesis, the effects of smoke treatment increase GA sensitivity, which is correlated with a decrease in endogenous ABA pools.