SALT EFFECTS ON MEMBRANES OF THE HYPODERMIS AND MESOPHYLL-CELLS OF AVICENNIA GERMINANS (AVICENNIACEAE) - A FREEZE-FRACTURE STUDY

Freeze-fracture electron microscopy was used to investigate intramembr anous particle (IMP) densities and particle distributions in the plasm a membrane and tonoplast of the cells of secreting and nonsecreting le aves of Avicennia germinans (L.) Steam. Intramembranous particle densi ties of the protoplasmic (P) and exoplasmic (E) face of the plasma mem brane and tonoplast were significantly higher in hypodermal cells of s ecreting leaves than of nonsecreting leaves. In contrast, no significa nt differences in the frequency of intramembranous particles were foun d in any membrane faces of secreting or nonsecreting mesophyll cells. However, particle densities were higher in the plasma membrane and ton oplast of the mesophyll cells, compared to the hypodermal cells, with the exception of the P-face of hypodermal plasma membranes of secretin g tissue, which had the highest particle density measured. Particle di stributions were dispersed and no discernible patterns such as paracry stalline arrays or other multi-IMP structures were observed. Results s upport the hypothesis that secretion is coupled to changes in membrane ultrastructure, and the possibility that salt secretion is an active process driven by integral membrane proteins such as the H+/ATPase. Ad ditionally, the hypodermal cells of the leaf may function as storage r eservoirs for salt as well as water, suggesting a regulatory role in s alt secretion.