Tonoplast fine structure and osmotic regulation in Porphyra umbilicalis

In Porphyra, an intertidal red alga, the fine structure of the tonoplast was studied by freeze-fracture electron microscopy. It was shown that density and size of intramembraneous particles on the protoplasmic fracture face vary with external osmotic potential. The frequency of particles grouped in size classes (calculated per cell) increases with increasing osmotic stress and shows a maximum in 3 to 4 x artificial seawater medium ASP₁₂. It is concluded that the intensity of tonoplast transport, which probably is enhanced with increasing osmotic stress from 1 to 4 x media, is most likely correlated with a change in membrane fine structure of the tonoplast.