Peptide transport plays a major role in the nitrogen nutrition of the cereal embryo during germination. During germination, enzymatic hydrolysis of storage proteins in the cereal grain endosperm forms a reservoir of small peptides and amino acids which are translocated across the scutellum to supply organic nitrogen to the growing embryo. Uptake of these solutes by the scutellum, a modified cotyledon which functions in nutrient transport, is mediated by carriers localized to the plasma membrane of the scutellar epithelium. To date the peptide transporter HvPTR1 is by far the best characterized example of a nutrient transporter involved in reserve mobilization during germination. Peptide transport in the barley scutellum has been relatively well-characterized biochemically, and in recent years the barley scutellar peptide transporter HvPTR1 has been cloned and characterized at the molecular level. Here, we review the physiological role and importance of peptide transport in germination, focusing on recent characterization of the barley peptide transporter HvPTR1. In barley, the uptake of small peptides by the scutellum appears to be mediated by a proton-coupled peptide transport system capable of handling peptides 2–4 amino acid residues in length.

An Escherichia coli phytase gene was introduced into Arabidopsisplants using an embryo-specific promoter and a signal peptide for vacuolar targeting. Three independent transgenic lines were analysed. Phytase activity in dry seeds was observed in transgenic lines, whereas no activity was detected in control, untransformed seeds. Transgenic seeds expressing the phytase gene had lower levels of phytic acid than the controls. Concomitant with the decrease in phytic acid was an increase in free phosphate. These results indicated that embryo-expressed phytase can reduce the levels of phytic acid stored during development.

The mature seeds of durian (Durio zibethinus L.) are recalcitrant, with the cotyledons storing both starch (amylose, 78% dry weight) and some protein (7%), but having few triglyceride reserves (<1%). Reserve accumulation occurs during the latter third of the 110-d seed developmental period prior to fruit shed. The storage proteins of durian seeds consist of two immunologically related unglycosylated albumins, light and heavy zibethinin (20.2, 21.4 kDa), that account for 37–46% of the total protein during development. Light zibethinin is a single polypeptide, most likely having internal sulphydryl linkages, whereas heavy zibethenin is composed of two sulphydryl-linked subunits. The zibethinins are deposited in swellings of the endoplasmic reticulum (ER) which then either expand or coalesce to form protein storage vacuoles. This mode of protein body formation in durian varies from both the ER–Golgi complex–vesicle–vacuole pathway characteristic of many dicotyledonous seeds and the direct ER to protein storage vacuole pathway involved in deposition of certain prolamins in cereal endosperm cells. Protein storage vacuoles in durian seed storage parenchyma never become fully filled, but contain only diffuse peripheral clumps of zibethinin protein. There is a 15% loss in starch, a precipitous loss of total protein, and a concomitant loss of the zibethinin storage proteins during the 5 d prior to fruit shed, suggesting that both reserve mobilization and germination has begun.

The thickened cell walls of endosperm cells of asparagus (Asparagus officinalis) seeds are composed of glucomannans. During development, the extensive thickening of the cell wall is specific to the lateral region of the endosperm and not to the micropylar region. The micropylar endosperm may thus be predisposed to facilitate germination. Following germination there is a progressive mobilization of the reserves in the cells of the lateral endosperm in a wave-like manner away from the haustorial cotyledon, accompanied by the loss of cytoplasm and the crushing of the lateral endosperm cells. Although the internal reserves are mobilized from the micropylar endosperm cells, the cells themselves remain intact throughout the period of lateral endosperm mobilization and may form a living barrier to prevent the loss of soluble hydrolysis products of the lateral endosperm to the surrounding environment. The location and timing of endo-β-mannanase production and the increase in activity of β-mannoside mannohydrolase in seeds of germinated asparagus were followed. Endo-β-mannanase activity increases greatly in the endosperm until the mid-point of mobilization, and is about 45 times higher than in the embryo on a per seed part basis. Unlike endo-β-mannanase, which is extractable in low-salt buffer, β-mannoside mannohydrolase requires high salt (0.5 M NaCl) for extraction. This enzyme continually increases in activity in both the endosperm and embryo following germination, with the majority of the activity being concentrated in the embryo when considered on a per seed part basis.

This study examined the hypothesis that desiccation sensitivities of recalcitrant and intermediate seeds can be categorized into discrete levels of critical water potential. The equilibrium dehydration method was used to determine the critical water potential (CWP) below which desiccation damage started to occur. The CWP values of Bruguiera cylindrica, Lansium domesticum, Litchi chinensis and Lumitzera racemosa are approximately –4 MPa. The CWP values of Andira inermis, Avicennia alba, Castanea sinensis (from New Zealand), Citrus aurantifolia, Ginkgo biloba, Nephelium lappaceum and Theobroma cacao (immature axis) are approximately –8 MPa. The CWP values of Acer pseudoplatanus, Castanea sinensis (from China), Quercus rubra and Theobroma cacao(mature axis) are approximately –12 MPa. The CWP values of Artocarpus heterophyllus and Hevea brasiliensis are approximately -23 MPa, while the CWP values of Acer platanoides, Azadirachta indica, Carica papaya and Coffea arabica are approximately –73 MPa. Together with data available in earlier literature, these CWP values suggest that there are five discrete levels of critical water potential among desiccation-sensitive seed tissues. These data support the hypothesis that discrete levels of desiccation sensitivity occur among recalcitrant and intermediate seeds, and suggest that specific damaging and protective mechanisms exist at certain hydration levels.

A developmentally regulated, novel β-amylase gene (ESDbamy) from Hordeum vulgare L. has 76% amino acid similarity with a previously described barley β-amylase. However, the 3' -end sequences of these β-amylases were different. The glycine-rich repeats, which are signature sequences of endosperm-type β-amylases, are absent in this novel ESDbamy gene. ESDbamy is highly expressed in the caryopsis during early development; however, it is not expressed in other tissues. Early seed development-specific (ESD) β-amylase mRNA levels and enzyme activities were higher in a shrunken endosperm mutant, seg8, than in the wild type. Seg8 has defective endosperm development, reduced starch synthesis and elevated monosaccharide concentrations

The relationship between seed mass variation and germinability in ten Abutilon theophrasti Medic. plants from a single agricultural population was examined under controlled environmental conditions. Seeds were collected in the autumn of 1995 and dry-stored in paper bags at 4°C until the start of trials in the spring of 1998. For each plant, between 549 and 1000 randomly selected seeds were separated into seven mass fractions based on individual seed mass, with classes ranging from <6.0 to >11.0mg. Subjecting the seeds to a cold stratification (4°C) period for 7d followed by a 21-d alternating day/night (25/14°C) temperature and 14-h photoperiod regime, resulted in 75% overall germination, 24% dormancy and 1% non-viable seeds. The majority of the seeds germinated within 7d of being exposed to the alternating temperature/light regimes. There was a significant (P < 0.001) difference in mean seed mass between the ten plants, with mean mass ranging from 8.8 to 9.6mg. For nine of the ten plants, the greatest proportion of seeds occurred in the 9.0–9.9mg mass fraction, while the lowest proportion of seeds was generally found within the 6.0–6.9mg mass fraction. Most seeds (96%) having a mass below 6.0mg were non-viable and, of the viable seeds, none germinated. There was a significant (P < 0.0001) relationship between seed mass and total germination, but not rate of germination. Germination peaked for seed mass fractions comprising the greatest proportion of total seeds (8.0–9.9mg) and was lowest for seeds with high or low mass. There was a weak trend of lower germinability for heavier seeds (>10.0mg) compared with lighter seeds. Maternal source had a significant effect (P < 0.05) on total germination and germination rate. Findings from this study suggest that intrapopulation variation in Abutilon theophrasti seed mass and its influence on germinability of seeds may play a significant role in maintaining a variable germination pattern and persistent seed bank in this troublesome annual agricultural weed

Previous studies on several floras have shown that species with small, rounded seeds tend to accumulate persistent seed banks in the soil, while species with larger, less compact seeds do not. The suggested underlying mechanism is predation. Small seeds experience less predation and are more likely to become buried, which itself offers significant protection from predation by vertebrates. The relationship between seed size and shape and persistence in the soil was examined for the flora of the Arasbaran Protected Area in northwest Iran. Seed size was related to persistence in the soil in Iran in the same way as in most other floras examined, but seed shape was not. It is suggested that predation prevents persistence of large seeds in most floras. Where large-seeded persistent species predominate, for example in Australia and (to a lesser extent) in New Zealand, other factors may interfere with the relationship between seed size and predation.