RESUMO
⢠Hair roots of Woollsia pungens are shown to have thick-walled epidermal cells, a feature found in a small number of other species within the Epacridaceae. Hair roots otherwise had a structure typical of the Ericales. ⢠Ultrastructural, immunocytochemical and histochemical techniques were used to investigate the structure and composition of these thick-walled epidermal cells. ⢠The thick walls were multilamellate with a helicoidal arrangement of microfibrils typical of a secondary cellulosic wall. Staining techniques revealed a relatively high abundance of ß-glucans; these were not ß 1-3 linked and there was no detectable protein. Galactose side-chains were abundant but not mannose or glucose side-chains. The wall contained a pH-dependent net negative charge. Although apparently rich in COOH groups the thick wall did not react, or only minimally, with the monoclonal antibodies JIM5 and JIM7, testing for nonesterified and methyl-esterified pectins, respectively; this contrasted with the strong positive reaction in the cortical and stelar cells. In epidermal cells colonized by mycorrhizal fungi the thick wall had additional layers of spongy appearance with many interconnected, irregular patches containing dispersed material. Colonized cells retained their integrity longer than noncolonized cells. ⢠The thick wall might be important in long-term survival of infected cells and the low levels of pectin might control mycorrhizal endophyte infection.
RESUMO
BACKGROUND AND AIMS: Seeds of east Australian Grevillea species generally recruit post-fire; previous work showed that the seed coat was the controller of dormancy in Grevillea linearifolia. Former studies on seed development in Grevillea have concentrated on embryology, with little information that would allow testing of hypotheses about the breaking of dormancy by fire-related cues. Our aim was to investigate structural and chemical characteristics of the seed coat that may be related to dormancy for three Grevillea species. METHODS: Seeds of Grevillea linearifolia, Grevillea buxifolia and Grevillea sericea were investigated using gross dissection, thin sectioning and histochemical staining. Water movement across the seed coat was tested for by determining the water content of embryos from imbibed and dry seeds of G. sericea. Penetration of intact seeds by Lucifer Yellow was used to test for internal barriers to diffusion of high-molecular-weight compounds. KEY RESULTS: Two integuments were present in the seed coat: an outer testa, with exo-, meso- and endotestal (palisade) layers, and an inner tegmen of unlignified sclerenchyma. A hypostase at the chalazal end was a region of structural difference in the seed coat, and differed slightly among the three species. An internal cuticle was found on each side of the sclerenchyma layer. The embryos of imbibed seeds had a water content six times that of dry seeds. Barriers to diffusion of Lucifer Yellow existed at the exotestal and the endotestal/hypostase layers. CONCLUSIONS: Several potential mechanisms of seed coat dormancy were identified. The embryo appeared to be completely surrounded by outer and inner barriers to diffusion of high-molecular-weight compounds. Phenolic compounds present in the exotesta could interfere with gas exchange. The sclerenchyma layer, together with strengthening in the endotestal and exotestal cells, could act as a mechanical constraint.