To elucidate the molecular aspects of cereal endosperm development, a good understanding of the histo-differentiation process is necessary. Reviewing the extensive history of lightand electron-microscopic investigations has revealed similarities between cereals with respect to cell ontogeny. Thus, using barley as a model and taking into account many developmental mutants in this and other species, a model for endosperm differentiation is proposed. Based on this model, molecular studies on endosperm gene-regulation are discussed and the available techniques for further investigations are considered.

Somatic embryo quality is an important aspect of artificial or synthetic seed production and is dependent on proper morphological development and deposition of sufficient storage reserves to support early growth of the seedling. The abilities of a somatic embryo to survive desiccation and to establish a rapidly growing seedling were used as estimates of quality and were dependent on a number of factors during the elongation and maturation phases of somatic embryo development. Embryo quality was significantly enhanced when the sucrose content of the elongation and maturation media was increased to 50 g I−1 from 30 g I−1. This increased dry weight to >2 mg embryo−1 suggesting greater deposition of storage reserves. Higher sucrose concentrations were inhibitory and reduced survival after desiccation and seedling vigour. An optimum number of embryos can be supported on one Petri plate; the number was controlled by the plating density of the embryogenic cell clusters spread on the elongation medium. This optimum was dependent to some extent on the sucrose content of the medium, indicating that sucrose had a nutritive effect on embryo development and not solely an osmotic effect. Light intensity was another critical factor. Intensities > 75 μmol m−2 s−1 photosynthetic photon flux density (PPFD) at the elongation stage of development blocked globular-stage somatic embryo development. Maturing somatic embryos were far less sensitive to high light intensities and development was more rapid at 150 μmol m−2 s−1 PPFD. Light quality was manipulated by covering plates with filters that selectively transmitted certain regions of the visible spectrum. All filter treatments reduced embryo yield and quality compared with embryos developing under fullspectrum light; varying the light spectrum does not seem to be a viable method of enhancing somatic embryo quality.

Nuclear DNA of wheat embryos was probed with a synthetic oligonucleotide (C3TA3C3TA3) to assess changes in the length of telomeres during storage and germination. Chromosomal DNA of dry embryos, especially those derived from long-term-stored caryopses, was considerably depleted of telomeric repeats, but, shortly after the start of imbibition, the chromosomal DNA became rich in sequences hybridizing to the telomeric probe. Simultaneously, newly synthesized telomere-related sequences appeared in an extrachromosomal form that could be separated from bulk nuclear DNA by velocity and isopycnic centrifugations. It is concluded that, in addition to typical repair events, rapid synthesis of telomeric repeats occurs in wheat embryos at the onset of germination.

A survey of Pisum genotypes for seed trypsin inhibitors revealed a tenfold range in the extent of inhibition. Approximately 90% of trypsin inhibitory activity was associated with two albumin fractions in selected variant lines. The differences among extreme variants were consistent in three environments, between two sources of trypsin tested and whether expressed on a unit protein or dry weight basis.

A study of the appearance of trypsin inhibitors during seed development in selected highand low-inhibitor lines showed differences in the accumulation pattern of active inhibitors. An endogenous protease was identified in Pisum seed protein preparations, whose in vitro trypsin-like activity was predominant in protein from early stages of seed development, when little or no trypsin inhibitor was present. However, there was no correlation between the amount of this protease and the extent of trypsin inhibitory activity in lines that varied for inhibitor content.

Seeds of watermelon (Citrullus lanatus cv. Sugar Baby) germinated optimally in the dark, throughout the range 20–40°C. Germination was inhibited by continuous (c) irradiation with farred (FR), blue or white incandescent light; short pulses were ineffective. Intermittent FR could fully substitute for cFR; the effect of alternating red and FR pulses during 30-min dark cycles depended upon the timing of the illuminations within each cycle. It is concluded that germination in watermelon is controlled by the low-energy reaction of phytochrome. However, continuous and intermittent red light resulted in partial reductions in germinability. Opening the seed coat at the radicle end enhanced germination in the dark and reduced photosensitivity towards FR. The outer, lignified part of the testa exerted a mechanically restrictive force upon the expanding radicle; this force was estimated to be equivalent to 0.3 MPa. The kinetics, at 25°C in the dark, of the time course of germination and the escape from the inhibitory actions of cFR and osmoticum (0.5m mannitol) were all sigmoid curves, which, upon transformation to normal distributions, had different means, but statistically similar variances. The cFR-irreversible activation of germination by phytochrome and the mannitol-irreversible onset of radicle elongation preceded radicle protrusion (mean at 32 h, 25°C) by 8 and 6 h, respectively. From the results and data on imbition, it is concluded that activation of germination in watermelon takes place during the second, stationary phase of imbibition (20–30 h after sowing at 25°C).

Seed germination of watermelon (Citrullus lanatus cv. Sugar Baby) was fully suppressed by intermittent far-red (FR) irradiation (1 min every 30 min). When the intervening dark period was increased, a linearly increasing final germination percentage was obtained. However, a 4-day intermittent FR treatment induced phytochrome-controlled dormancy and the longer the dark interval the deeper was the dormancy of the non-germinated seeds. When seeds were soaked in a fully inhibitory osmotic solution, no dormancy was imposed. However, a single FR pulse at the time of transfer induced partial, secondary dormancy; the kinetics of the imposition of dormancy followed a negative exponential curve (half-life 1.5 days; 3 days for the cultivar Crimson Sweet). Seeds osmotreated for 10 days in darkness and subsequently dehydrated (with and without a final FR pulse) acquired germination characteristics similar to those in light-requiring and dark-germinating achenes, respectively, of the lettuce cultivar Grand Rapids. In the light-requiring osmomanipulated seed population, the induction of germination was brought about by the low-energy reaction of phytochrome, chilling, dry storage and decoating. The transformation through osmomanipulation of the dark-germinating watermelon seeds (the inhibition of which required prolonged exposure to light) to positively or negatively photosensitive seeds (that responded to brief light pulses), might be attributed to the slow relaxation of existing meta-Fa and meta-Rb phytochrome intermediates to Pfr upon hydration.

Norway maple (Acer platanoides L.) seeds were harvested at different stages of seed development and maturation in 1989–91. As maturation drying progressed, the seed populations showed increasing desiccation tolerance: at 67–69% moisture content, no seeds survived desiccation below 10% moisture content; maturation drying to 55–57% moisture content (values corresponding with the end of the seed-filling phase) improved desiccation tolerance, but nevertheless most seeds were unable to withstand desiccation to 5–7% moisture content; further maturation drying to 27–28% moisture content enabled the seeds to survive considerable desiccation, no loss in viability occurring in seeds dried to 3% moisture content. This considerable improvement in desiccation tolerance after the end of the seed-filling phase was correlated (P<0.05) with the progress of maturation drying and may be associated with the increase in the potential longevity of seeds of other species that occurs during seed development subsequent to seed filling.