The development of techniques for somatic embryogenesis in conifers has led to rapid advances in the ability to culture conifer tissue in vitro. Somatic embryogenesis now provides a means to clonally propagate commercially valuable conifers of several species and offers an in vitro experimental system for studying embryogenesis. As a result of these developments, the conifer zygotic and somatic embryo system has recently attracted the attention not only of the cell biologist but also of biochemists and molecular biologists. In this review I have attempted to consolidate the recent information on ultrastructure, biochemical and molecular aspects of conifer zygotic and somatic embryogenesis. In the first section, salient features of zygotic embryogenesis are highlighted followed by biochemical and ultrastructural characterization of development with reference to storage reserve deposition. In the second section, detailed characterization of seed storage proteins, changes in gene expression with emphasis on storage protein genes and Lea genes during zygotic and somatic embryogenesis are described. Effect of culture treatments such as abscisic acid, osmoticum and desiccation on storage protein gene expression in somatic embryos is discussed. Based on these studies, comparisons are presented between angiosperm and gymnosperm embryogeny regarding the unique morphological and developmental patterns of conifer embryogenesis. Finally, a brief discussion of recent progress in biochemical and molecular characterization of conifer seed germination is presented.

Stachyose, raffinose and related flatulence-producing oligosaccharides (α-galactosyl derivatives of sucrose) are associated with desiccation tolerance and storability of seed germplasm. Orthodox seeds of species with a sucrose-to-oligosaccharide ratio of <1.0 have storability half-viability periods >10 years while those >1.0 have a storability half-viability period <10 years. Seeds vary in their composition of oligosaccharides and some accumulate α-galactosyl derivatives of cyclitols. Known and proposed pathways for biosynthesis of soluble oligosaccharides, cyclitols and galactosyl derivatives of cyclitols are presented. Axes, cotyledons, embryos or seeds of 19 species in 7 families (all orthodox seeds) were analysed for sucrose, galactosyl derivatives of sucrose, cyclitols and galactosyl derivatives of cyclitols by high resolution gas chromatography. Sucrose and myo-inositol are universally present and galactinol is present in seeds accumulating stachyose series oligosaccharides. Seeds of some species of Leguminosae accumulate mostly stachyose series oligosaccharides, whereas seeds of other species accumulate varying levels of galactosyl derivatives of cyclitols in addition. Castor bean (Euphorbiaceae) seeds accumulate galactinol and buckwheat (Polygonaceae) embryos accumulate galacto-chiro-inositol instead of the stachyose series oligosaccharides. The mass ratio of sucrose:non-sucrose is related to storability and is applicable to seeds accumulating cyclitol derivatives. Galactinol and galacto-chiro-inositol are proposed to function in the same role as raffinose and stachyose in facilitating desiccation tolerance and storability.

A study was conducted to assess the effect of the supply of phosphorus and the major cations on the mineral nutrient composition of whole mature wheat grains and the electron-dense globoid crystals found in the aleurone cells. Ears of wheat plants, growing in the field, were detached after anthesis. The ears were cultured to maturity in a sterile control solution which contained all essential elements. Other ears were cultured in solutions which contained no phoshorus, no calcium, no magnesium, combinations of these (−P, −Ca; −P, −Mg) or a solution which contained additional potassium but no phosphorus, calcium or magnesium. Ears cultured in the control solution produced grains with higher concentrations of P, Mg, K and Ca than grains from ears which matured on plants in the field. All the −P treatments produced grains with lower concentrations of P and higher concentrations of Mg, K and Ca than the plants grown in the field. The appearance and elemental composition of the globoid crystals in the aleurone cells support the hypotheses that the size of globoid crystals is regulated, in part, by the amount of phosphorus deposited in the grain and also by the relative proportions of K, Mg and Ca in the tissue. The culture of individual ears of cereal plants is a useful technique by which to study the transfer to and deposition of mineral nutrients in cereal grains.

Secondary dormancy (thermodormancy) in Agrostemma githago L. seeds, caused by imbibing the seeds for 4 d at 35°C in the dark, is broken under the influence of cyanide (10 mM), azide (1 mM or BAP (0.44 mM). Both respiration inhibitors (cyanide and azide) and BAP increase the synthesis of five specific soluble proteins in the embryo axes and cotyledons. The activity of β-cyanoalanine synthase (β-CAS, EC 4.4.1.9.), which is able to detoxify cyanide, decreased in response to cyanide and azide. The relevance of these findings to the breaking of thermodormancy is discussed.

A nuclear protein (QP47) is synthesized during the last stage of seed maturation when the embryo cells start to dehydrate and enter a condition of metabolic quiescence. This protein is localized in the nucleoplasm surrounding the chromosomes. The correlation existing between the synthesis of QP47 and arrest of cell proliferation, suggests that the presence of this protein in the nucleus could influence its metabolic activities. This hypothesis is supported by the fact that degradation of this protein precedes resumption of cell proliferation during the early stage of radicle elongation.

Electroconductivity tests are currently used for seed vigour assessment. They are rapid and simple and after further development, they may yet provide the seed industry with a non-destructive alternative to the standard germination test, which is thoroughly destructive to the sample, time consuming, and expensive. Seed injury, a result of soaking required by the electroconductivity test, was evaluated using high quality Zea mays L. seeds responding to successive imbibition/dryback cycles. If the soaking time is brief, injury to the seeds should be minimal, thus permitting successive tests on the same sample. We tested 5 imbibition/dryback cycles (C) and 5 imbibition periods, or cycle durations (CD) of 2, 4, 6, 7 and 8 h. Dryback periods lasted 5–7 d at room temperature. Seeds were permitted to dry back to 10% moisture. Electroconductivity readings were obtained at the end of each CD for each C. Each treatment (C × CD) sample, n=100 seeds, was germinated at 25°C for 7 d; radicle lengths were measured after 3 d. The experiment was repeated yielding a total of 50 observations. Viability and vigour losses were measured in response to successive C and increasing CD. Five cycles of 6 h each resulted in only a 10% loss of viability, but a 20% loss of relative vigour, confirming that vigour is more sensitive to the testing procedure. Cycles had the greatest effect on loss of seed quality since 45% of the readily leachable electrolytes were lost from the seeds during the first soaking period. There was no interaction between C and CD.