Variant maturity in seed structures of Pinus albicaulis (Engelm.) and Pinus sibirica (Du Tour): key to a soil seed bank, unusual among conifers?

Abstract

The seeds of Cembrae pines are dispersed by nutcrackers (Genus Nucifraga), which cache seeds in soil during autumn. The dispersal and establishment of seedlings via this mutualistic relationship is highly successful. On the other hand, irregular quality of seed crops and lack of detailed knowledge on germination process of Cembrae pine seeds hamper effective seedling production in the nursery. Therefore we studied basic structures and maturity of whitebark pine (Pinus albicaulis Engelm.) and Siberian stone pine (Pinus sibirica Du Tour) seeds, as well as structural changes during a multi-step treatment of whitebark pine seeds, using field emission scanning electron microscopy, transmission electron microscopy and light microscopy. The most striking differences compared to many other conifer seeds were the solid surface structures, early structural differentiation of the embryo, clustering of the thin-walled megagametophyte cells, and great accumulation of starch in both the untreated and treated seeds. Protein bodies of the embryo were in early developmental stages, whereas in the megagametophyte their stages varied. The number, form and size of lipid bodies also varied within different parts of the seed, and lipids dissolved easily. Our results indicated that despite maturity of the seed coat and advanced differentiation of the embryo, the embryo and the megagametophyte were still immature. These morphological features and a notable proportion of storage reserves remaining in unstable form may, however, be advantageous for maintaining viability and reaching maturity within a soil seed bank. Well-controlled pre-treatment simulating natural conditions should result in improved germination.

Seed germination of Pinus koraiensis in response to light regimes caused by shading and seed positions

“…(median seed mass: 214 mg), which elucidated that seed morphological traits might not be an adaptive indicator in predicting the light requirement of seed germination. It was also reported that the mechanical restraint of the thick seed coat was diminished as the P. sibirica embryo became truly mature by undergoing natural cold stratification from winter to early spring (Tillman-Sutela et al, 2008), which was also true for P. koraiensis. The light requirement for the germination of P. koraiensis might be created after seed prechilling, as has been reported for P. strobus (Toole et al, 1962).…”

Pine embryogenesis

“…However, the suspensor remnants have a further role during germination: when moistened, they swell and facilitate protrusion of primary root through the micropyle. 6,16 There are no reports concerning gymnosperm seeds, but it has been suggested that in angiosperms the continued growth of the suspensor may be inhibited by the embryo proper during early stages of development. 4 When suspensors die, subordinate embryos loose their fixed position within the Scots pine seed as well as their source of nutrition, which may lead to starvation if subordinate embryos are not capable of utilizing nutrients from the CC fluid.…”

“…2,7 The cell wall, plasma membrane and nuclear envelope of the starch-rich cells break down with the release of cell debris and nucleic acids into the CC (Vuosku et al unpublished), where the remnants form a malleable zone between the ESR and the developing embryo 6 (Fig. 1E).…”

“…2 The suspensor holds the embryo in a fixed position within the seed, anchors the embryo to the maternal tissue and provides nutrition and growth regulators for the developing embryo during early stages of embryogenesis. 4 The suspensors of the subordinate embryos deteriorate during later stages of development, leaving remnants of tissues at the micropylar end of the corrosion cavity (CC), 5 but the dried and coiled suspensor of the dominant embryo survives, attached to both the root cap and the nucellar cap until germination 5,6 (Fig. 1C).…”

“…8 The continuation of bidirectional metabolism between the embryo and the megagametophyte during germination is ensured by the mucilaginous remnants of the ESR cells that balance the moisture content of the mature Scots pine seed. 6,9 The diploid maternal cells of the nucellar layers face destruction during early embryogenesis 7,10 and end up being used as nutrition for the surrounding tissues of the pine seed. The length of this process varies depending on environmental factors, such as photoperiod and temperature.…”