Zanichellia palustris is a submerged halophyte which occurs in waters of low salinity (5%o to 10%o). It is not capable of surviving at high salinity. Reasons for its inability to tolerate hypersaline conditions is important to our understanding of its biology. in the present study, leaf ultrastructure of plants growing at normal salinity was compared with plants growing at high salinity so as to assess the effects of hypersalinity. Attention was focused on ultrstructural changes occurring in leaf blade epidermal cells.

Plants were grown in seawater at two salinities : 10%o (control) and 20%o (high salinity). Pieces of mature leaf blades from both treatments were harvested and prepared for Transmission Electron Microscopy (TEM) following standard procedures. in addition, the overall distribution and concentration of chlorine (Cl) in the leaves was ascertained since it is the most abundant anion in seawater and is important in considerations of salt tolerance in submerged halophytes. Cl was determined by means of a nuclear microprobe. Pieces of sectioned leaves were rapidly frozen, freeze dried and carbon coated. Elemental distribution maps of Cl were obtained using the true elemental imaging system (Dynamic Analysis). Two complementary techniques, Proton Induced X-Ray Emission (PLXE) and Proton Back Scattering (BS) were performed simultaneously using the GeoPIXE suite of programmes. Sample thickness and composition of major elements were obtained from the BS spectra.

The ultrastructural morphology of leaf blade epidermal cells, which possess transfer cell characteristics differed at each salinity. While the cytoplasm was well preserved in epidermal cells of plants at the control salinity : 10%o (Fig 1), considerable cytoplasmic damage occurred in epidermal cells of plants at high salinity, in the form of breakdown of membranes such as the plasmamembrane and tonoplast, and senescence of various organelles including chloroplasts and mitochondria (Fig. 2).