The Ultrastructure of Ion-Secreting and Non-Secreting Salt Glands ofLimonium platyphyllum

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Abstract
The ultrastructure of salt glands in developing leaves of Limonium platyphyllum is described prior to exposure to 3% NaCl solution (with non-secreting glands) and after 4.5 and 18 h exposure to the salt solution. It is shown that in most glands, the transition to active chloride transport was accompanied by the displacement of vacuoles toward the cell periphery and by the establishment of plasmalemma contact sites with the tonoplast which appeared similar to gap junctions in animal epithelial cells. No evidence for the exocytosis of vacuoles was found. It is suggested that gland vacuoles may have a primary role in chloride secretion and that the tonoplast may be functionally asymmetrical, so that the free part facing the hyaloplasm bears ion pumps, whereas highly permeable ion channels are active along the zone of contact with the plasmalemma. It follows that the active step in chloride transport in Limonium glands is the influx of ions into the vacuoles. Within the inner cup cells of the gland, vacuoles come into contact with the plasmalemma only at sites where the cell wall is adjacent to secretory and accessory cells. Such an asymmetry appears to ensure the directed flux of ions into this cell wall. Wall protuberances in the gland cells are rudimentary and presumably not involved directly in NaCl secretion. The nucleolus is activated during secretion and the frequency of free ribosomes is significantly increased, which is suggestive of their involvement in the synthesis of membrane transport proteins. The ultrastructure of about one-third of the glands remained unchanged in salt-treated leaves.