The control of seed germination by red and far-red light is one of the earliest documented phytochrome-mediated processes Phytochrome is now known to be a small family of photoreceptors whose apoproteins are encoded by different genes Phytochrome B (phyB) is present in dry seeds and affects germination of dark imbibed seeds but other phytochromes could also be involved Phytochrome A (phyA) appears after several hours of imbibition and mediates very-low-fluence responses PhyB and other phytochromes different from phyA mediate the classical low-fluence responses The phytochrome involved in high-irradiance responses of seed germination (inhibition of germination under continuous far-red) has not been unequivocally established, although phyA is the most likely candidate Phytochrome can affect embryo growth capacity and/or the constraint imposed by the tissues surrounding the embryo At least in some species, gibberellins participate in the signalling process In the field, phyA has been implicated in the perception of light during soil cultivations, and phyB would be involved in the perception of red/far-red ratios associated with the presence of gaps in the canopy This review describes recent advances in phytochrome research, particularly those derived from the analysis of germination in specific mutants, and their connection with traditional observations on phytochrome control of seed germination

Seed development is a series of events involving cell division, followed by cell differentiation and storage activity In legume cotyledons, cell differentiation starts in certain regions and gradually spreads to other parts, thereby building up developmental gradients The entire process appears to be subject to metabolic control The high hexose state of the premature legume embryo as controlled by seed coat-specific invertases favours cell division Differentiation is initiated when hexose decreases and sucrose increases Seed development occurs in a close interaction with seed metabolism and transport processes Movement of photoassimilates from the sieve tubes to the unloading region of the maternal seed tissue is symplasmic and controlled by plasmodesmal passage Sucrose uptake into Vicia faba cotyledons is mediated by a H+-sucrose symporter located in the outer epidermis which generates transfer cells Formation of the sucrose uptake system is induced during the early to mid-cotyledon stage by tissue contact with the maternal seed coat and is controlled by carbohydrate availability In contrast, a hexose transporter gene is also expressed in epidermal cells covering younger, mitotically active regions of the cotyledons The sucrose uptake system apparently generates the high sucrose state immediately preceding the storage phase Sucrose specifically induces storage-associated differentiation processes indicating a specific sucrose-dependent signalling pathway operating in maturing cotyledons Moreover, the mode of sucrose uptake — apoplasmic movement into the epidermal cells with subsequent symplasmic transfer to the storage parenchyma cells — appears to control coordinated cotyledon development Unlike sucrose, amino acid transport into legume cotyledons is passive during early development but at later stages when large amounts of storage proteins are synthesized an additional active uptake system is established to ensure a sufficient supply

Plants respond to sub-optimal conditions by the synthesis of specific ‘stress’ proteins, and these are thought to play a role in stress tolerance. Some of these proteins accumulate during late seed development, arguably to protect against damage during post-maturation drying and subsequent imbibition, prior to germination. Seed vigour is also determined during this late stage of seed development. High vigour seeds are those that can withstand the desiccation required for storage and successfully germinate under sub-optimal conditions to establish healthy seedlings. If stress proteins are involved in tolerating stress conditions, then they are likely to be important determinants of seed vigour. In this work the relationship between seed vigour (measured by seed germination performance following rapid aging, or under water stress) in Brassica oleracea var. capitata and the content of two classes of stress protein (dehydrins and a low molecular weight heat shock protein HSP17.6) at maturity was examined. Dehydrins did not show a positive relationship with seed performance. However, the protein HSP17.6 showed a positive correlation with seed performance, and a treatment that reduced the amount of this protein in the seed also caused a reduction in subsequent seed performance.

Embryos taken from late maturation phase seeds of Phaseolus vulgaris cv. Seminole prior to seed desiccation (35–45 DAA) can be induced to germinate in the absence of water by exogenous ethylene. NMR imaging of proton relaxation within the embryo shows changes in water status in putative (and not fully differentiated) vascular tissues of the hypocotyl within 3 h of ethylene administration. Difference imaging revealed that the change was progressive in the hypocotyl towards the radicle tip and was accompanied by changes in water status in the cotyledons. Water within plumular leaves was also affected. Increase in diameter of the hypocotyl–radicle axis (as estimated by pixel counts) was detectable from 3 h. Longitudinal radicle growth was detectable by NMR imaging at 18 h. Visible germination under the conditions used was apparent after 20 h. Changes in water status detected by this technique are an indication of changes in activity (concentration) or motion of water molecules or both. The data are consistent with a mode of action of ethylene in stimulating a redistribution of water within embryo structures from cotyledons to axis via the cotyledonary node and allowing the axis access to water sufficient to support germination. This supports the hypothesis that in vivo, quiescence at this developmental stage is induced and maintained by sequestration of water within the cotyledons.

To understand the cellular processes related to iron transport and sequestration within the developing pea seed (Pisum sativum), total iron and ferritin iron were analysed in seed coat and embryo tissues of the iron-hyperaccumulating pea mutant, Sparkle [dgl, dgl], and its wild-type parent, cv. Sparkle. For plants grown hydroponically with 2 μM Fe, embryo Fe concentrations averaged 65 μg g−1 dry weight in mature wild-type seeds and 163 μg g−1 dry weight in mature dgl seeds; iron concentrations were also higher in dgl seed coats. Extracted and electrophoretically separated seed proteins were probed with a polyclonal antibody raised against pea seed ferritin. In both genotypes, ferritin was detected in the embryo, but not in the seed coat. Ferritin iron accounted for 92% of the total iron in mature wild-type embryos, but only 42% of the total iron in mature dgl embryos. Radiotracer studies using 59Fe were used to characterize the movement of iron within the seed coat. Unequal distribution of 59Fe in opposing sections taken from the two hemispheres of the seed coat demonstrated that iron was symplastically phloem unloaded. These results suggest that iron resides transiently within the nonvascular seed coat cells and that all cells at the inner surface of the seed coat may be involved in the release of iron to the embryo apoplast. However, the form of iron resident within the seed coat and/or taken up by the embryo is presently unknown.

Untreated (control) tomato seeds germinated at temperatures ranging from 15 to 35°C. There was a positive linear relationship between the rate of germination (expressed as the reciprocal of time to obtain 50% germination) and temperature up to the thermal optimum. At higher temperatures, the negative relationship was also linear. The thermal optimum for seed germination was 27–28°C and the lower and maximal temperatures were around 9 and 40°C, respectively. Germination was strongly reduced in atmospheres containing less than 10% oxygen. After pretreatment at 15°C in a polyethylene glycol-8000 solution at –1.0 MPa, seeds germinated faster in a wider range of temperatures, and this stimulatory effect remained after drying back the seeds. Such a pretreatment did not change the optimal and maximal temperatures of germination, but decreased the lower temperature to about 6°C. The beneficial effect of osmoconditioning increased as the duration of the treatment increased and was maximal after 5–7 days. Osmotic pretreatment also reduced the sensitivity of seeds to oxygen deprivation; 50% of the seeds germinated within 7 days in 5% oxygen. The range of temperatures and the concentrations of oxygen which were effective in osmoconditioning were very similar to those which allowed the germination of untreated seeds. In particular, the optimal temperature (27–28°C) and oxygen concentrations (more than 10%) for osmoconditioning and germination were the same. These results confirm that the beneficial effect of osmotic pretreatment (priming) corresponds to the realization of germination sensu stricto (phase II of the germination process).

On the basis of embryo and seed colour, morphology, accumulation of fresh and dry weight, seed development of Chinese cabbage (Brassica rapa subsp. pekinensis) can be divided into 10 discrete but contiguous stages. Precocious germination (PG) occurs in the seeds of a Chinese cabbage mutant on the parent plant mainly during the maturation period (from stage 5 to 8), with either the radicle or the cotyledon protruding out of the testa. All plants of the mutant line produce some PG seeds, but among the seeds, only 18% (maximum) germinate precociously. The developing mutant seeds have higher water contents and lower dry weights than those of the wild-type and are less tolerant of desiccation. The testa structure of Chinese cabbage seed is similar to that of other Brassica species. In developing seeds, it consists of the epidermis, subepidermis, palisade layer and pigmented layers, while in the mature dry seeds, all the cell layers except the palisade layer are crushed into non-cell structures and are stacked on the outer and inner faces of the palisade layer; the aleurone layer is fused to the testa. An alteration in testa structure of the mutant seeds (both non-germinated and precociously-germinated seeds) is probably related to PG. The secondary cell wall materials are less, or not deposited on the radial and basal walls of the palisade cells on the adaxial side of the testa, which could result in a weaker mechanical restraint, thus leading to PG.

Seed longevity of plant species is an important topic in restoration management, and little is known about the effects of environmental conditions on seed survival and longevity under natural conditions. Therefore, the effect of groundwater level on the survival of seeds in the soil seed bank of a natural grassland community was investigated. Large soil cores, mesocosms, were sampled from a grassland site and transferred to two basins under a glass roof. The mesocosms were subjected to different groundwater-level treatments (high and low, respectively 5 and 30 cm below the soil surface) for nearly three years. After that period the soil seed bank of the mesocosms was sampled. In total 15 789 seeds of 38 taxa emerged from the experiment. Significant differences between the number of viable seeds that emerged in the two treatments were found for several species. More seeds of Glyceria fluitans, Cardamine pratensis and Myosotis palustris germinated in the high water-level treatment, whereas fewer seeds of Juncus spp., Cerastium fontanum and Stellaria media were found in this treatment than in the low water-level treatment. The experiment showed that the anoxic conditions prevailing in the high water-level treatment were beneficial to the survival of seeds of species of wet grassland communities. Species of dry grasslands, although represented by only two species, survived better under aerobic conditions.