Ultrastructure and crassulacean acid metabolism in Mesembryanthemum crystallinum leaves during normal and NaCl-induced ageing

Abstract

NaCl-treated young plants of Mesembryanthemum crystallinum undergo specific cytological changes which are accompanied by distinct alterations in CO₂ gas exchange reactions. Prior to salt treatment mesophyll cells are quite normal. Only lomasome-like structures extrude from the cytoplasm into the vacuole. CO₂ gas exchange is that of a Calvin-plant. Later on NaCl-treated plants differ from control plants in several respects. They show the CO₂ gas exchange reactions typical for CAM-plants. Electron microscopy clearly shows the formation of new vacuole-like spaces just under the chloroplasts between plasmalemma and cell wall, which are mostly filled with structures similar to “Hecht'sche Fäden” of plasmolyzed cells. In many cases a large amount of vesicles and membranes can be observed in the central vacuole, at least some of which are in connection with cytoplasm and even chloroplasts. Chloroplasts themselves sometimes seem to be damaged by high concentrations of NaCl. While having a more distinct crassulacean acid metabolism, old NaCl-treated plants exhibit features of aged cells: cytoplasm becomes empty and vacuolized, stoma thylacoids are reduced. Not only the described changes in fine structure but also the altered CO₂ gas exchange reactions take place when glycophytic control plants grow older, thus indicating that NaCl may accelerate the normal ageing process in Mesembryanthemum crystallinum.