Ruppia maritima L. is a submerged halophyte which is capable of growing in a wide range of salinities. In a previous study, the effects of varying salinity on the ultrastructure of leaf blade epidermal cells of R. maritima were investigated. The ultrastructural morphology of the cells, including organelle number and distribution, differed at different salinities. However, salinity effects on the endoplasmic reticulum (ER) system could not be assessed since this cytoplasmic component was not readily discernible in material processed in the conventional manner for transmission electron microscopy (TEM). In the present study, selective staining of the ER was achieved, and the response of this membrane system to different salinities ascertained.

Leaf material from plants growing at two salinities (5‰and 20‰) was examined. The zinc iodide-osmium tetroxide (ZIO) impregnation technique was used to stain the ER, and its morphology and distribution determined in leaf blade epidermal cells exposed to the different salinities. Leaf samples were fixed in glutaraldehyde, post fixed in an aqueous mixture of ZIO, dehydrated, and embedded in resin. Controls were processed in exactly the same way, but without ZIO post fixation. Sections with dark gold to purple interference colours were cut and examined unstained with a Phillips 301 TEM.