CONSERVATION OF CELL ORDER IN DESICCATED MESOPHYLL OF SELAGINELLA-LEPIDOPHYLLA ([HOOK AND GREV] SPRING)

Understanding of the basis of desiccation tolerance in mature plant ti ssues that survive extreme dehydration requires knowledge of the degre e of cellular order in the dry stale. Generally, aqueous fixatives hav e been used in ultrastructural studies of such material, and these are known to be inadequate in the preservation of dry material. Cryoprese rvation provides a more assured level of fixation fidelity than aqueou s fixatives, particularly with dry material. Using freeze substitution and electron microscopy, we examined the ultrastructure of dry mesoph yll cells of Selaginella lepidophylla ([Hook and Grev.] Spring). In th is material the cells were condensed and had highly folded walls. The plasmalemma was bounded on both sides by layers of granular material, and the membrane was in close and continuous apposition to the walls. The conformation and position of organelles and their structure appear ed to be influenced by being compacted within the shrunken cells, but the ultrastructural integrity of all organelles and cellular membranes , including mitochondria, chloroplasts and vacuoles, was maintained in the dry state. These cells had numerous small vacuoles clustered in a ggregates, and the tonoplast membranes appeared to be coated on the in ternal side by a fine granular layer. The vacuoles contained osmiophil ic material of varying degrees of condensation and had embedment holes suggesting the presence of salt crystals within the vacuoles. The gen eral conclusions from these studies are that a critical level of cell order is maintained in the dry state in these desiccation-tolerant pla nts, and a high degree of effective packing and shape filling of cellu lar constituents with the compaction forces of dehydration underlies t his conservation of cell order. (C) 1997 Annals of Botany Company.