Water Stress-induced Ultrastructural Changes of Leaf tissues and Cells in the Ice Plant, Mesembryanthemum crystallinum L.

A facultative halophyte ice plant, Mesembryanthemum crystallinum L., switchs over from C_3 photosynthesis to Crassulacean acid metabolism (CAM) under environmental stress, such as water stress (osmotic stress or ionic stress). The ultrastructural changes of tissues and cells during the metabolic switch were examined in common ice plants cultured hydroponically with 400 mM NaCl. To define the stress-induced metabolic switch, the concentration of malate was measured in well expanded green leaves kept in the light and dark. The concentration was significantly higher in leaves kept in the dark than in the light, indicating CAM induction by water stress in the ice plant. The leaves of stress-induced CAM plants and unstressed plants were fixed chemically by conventional methods, and ultrathin sections were examined with a light microscope and an electron microscope. In contrast with those found in unstressed plants, in stress-induced CAM plants, epidermal bladder cells were well developed, mesophyll cells changed to small and round in shape, and the intercellular spaces became remarkably narrow. These changes may be caused directly by waterstress. Furthermore, in mesophyll cells, the chloroplasts contained conspicuously swelled thylakoids and a few small starch grains. These structural changes in chloroplasts may reflect the metabolic switch induced by water stress