Structures contributing to the completion of conifer seed germination

Abstract

The variety of interpretations of the origin and role of the tissues surrounding the emerging radicle of conifer seeds prompted us to study changes during germination. The structures contributing to the opening of the seed coat and protrusion of the radicle in pine and spruce seeds were examined using light microscopy and field emission scanning electron microscopy. The opening of the seed coat was mainly mechanical and primarily affected by the enlargement of the imbibed endotesta cells lining the micropylar canal. The hypertrophied nucellar apex, swelling remnants and mucous substances in the micropyle further enhanced the testa splitting which was reinforced by the straightening of the folded nucellar cap. The expansion of the imbibed megagametophyte and the embryo were conducive to the seed coat opening only at a later stage. This was followed almost immediately by the appearance of the root cap showing an early geotropic response typical for taproots and assuring an immediate anchoring of the emerged embryo to the ground. The translucent tissue composed of elongating cell rows preceded and surrounded the protruding radicle and derived mainly from two origins: the micropylar end of the megagametophyte and the root cap. Observations about the origin of the sheath clarify the reasons for previous interpretations about the putative role of the massive embryo root cap in testa splitting and germination. The protective and adhesive function of the sheath is concluded to be essential to the conifer embryos, which present the epigeal type of germination.

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

“…The basic structures of whitebark pine and Siberian stone pine seeds were similar, but differed considerably from the mature structures of the wind-dispersed conifer seeds that we have studied previously (Tillman-Sutela and Kauppi 1995a, b, 1998, 2000. Our results confirmed observations of a hard but permeable seed coat in both whitebark pine (Leadem 1986;Farmer Jr 1997) and Siberian stone pine seeds.…”

“…The microscopy revealed that the megagametophyte tissue of whitebark and Siberian stone pine seeds differed clearly from mature structures in many other conifer seeds (Krasowski and Owens 1993;Owens et al 1993;TillmanSutela and Kauppi 1995a, b, 1998, 2000Gifford 1997, 1999). The most striking difference was the grouping of the megagametophyte cells into clusters of several cells with very thin and frequently discontinuous cell walls as observed in TEM studies.…”

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.…”

“…15 In the Scots pine, a major part of the megagametophyte cells stay alive, and the megagametophyte remains a viable and metabolically active tissue from the early phases of embryo development until early germination 7 and even until the completion of germination. 16 The megagametophyte develops from a haploid megaspore before the actual fertilization of the plant egg occurs. 17 When somatic embryos, VEIDase activity increases at the early stages of embryo development that coincide with massive cell death during shape remodeling.…”

Survival strategies of Silene tatarica (Caryophyllaceae) in riparian and ruderal habitats

“…Furthermore, a micropylar collar can protect a very young seedling against cold and desiccation, which is common during seed germination in extreme weather conditions. This has also been shown in Picea glauca (Tillman-Sutela and Kauppi 2000). It is possible that a germinated seed can remain for a while in a dormant stage inside the micropylar collar.…”