Structures restricting passage of water in the mature seeds of yellow-cedar (Chamaecyparis nootkatensis)

Abstract

Anatomical structures of seed surface layers and their role in impeding passage of water were studied for mature yellow-cedar (Chamaecyparis nootkatensis D. Don) seeds. The structures of the seed coat, nucellar layers, and megagametophyte of both dry and moistened, sectioned seeds were examined with a field emission scanning electron microscope. The anatomical details of resin-embedded seeds were studied by light or fluorescence microscopy using stained and unstained sections. The permeability of the structures exterior to the megagametophyte was analyzed by placing seeds in a Methylene Blue solution and examining them under a stereomicroscope. Results proved that the seed coat proper had only a minor effect on restricting passage of water. Penetration of staining solution was efficiently directed by the wing and epicuticular wax layer covering it, and by the large, impermeable nucellar cap. These structures, typical for yellow-cedar, essentially differed from those studied in Picea and Pinus seeds. Still, the most effective barrier to the penetration of water was in the junction formed by the megaspore membranes and the strong cuticle of the megagametophyte. These structures together with the phenolic nucellar tissues probably contribute to physiological dormancy in yellow-cedar seeds. Consequently, studies of the localization of dormancy should be focused on these layers rather than on the seed coat.Key words: conifer, seed coat, anatomy, scanning electron microscopy, imbibition.