Stuructural Changes and Intra- and Extracellular Ion Movements in Motor Cells during Leaf Closure of Insectivorous Venus Flytrap

To examine the regulatory mechanism of leaf closure in the insectivorous plant Venus flytrap, light and electron microscope studies on the distribution of motor cells, the ultrastructure of those cells, and the intra- and extracellular movement of various ions during leaf closure were performed in a specified region near the midrib at the middle of the blade. Light microscope observation revealed that one or two well-arrayed hypodermal cell layers were present beneath the epidermis. In hypodermal and epidermal cells, during leaf closure, the cell volume measured by the montage method decreased significantly on the adaxial side and increased significantly on the abaxial side, indicating their major contribution as motor cells to leaf closure. Electron microscope observation showed that motor cells of hypodermal and epidermal cell layers contained electron-dense materials located along the inner surface of tonoplasts. On the adaxial side, electron-dense materials found as flattened sheets changed shape to numerous small globules during leaf closure. An inverse change in electron-dense materials in the structure was found on the abaxial side. Since the structural change of electrondense materials is correlated with the volume change of motor cells during leaf closure, they may play a significant role turgor variation in those cells. Quantitative X-ray microanalysis of cryosections showed that, during leaf closure, the concentration of K decreased significantly in the vacuolar electron-dense material, vacuolar lumen, and cell walls on the adaxial side, while it increased on the abaxial side. In the blade region near the midrib, K ions released from the vacuolar electron-dense material in the adaxial cells may cause the movement of water to the abaxial region, and eventually turgor movement by the swelling of abaxial motor cells. The reciprocal Ca movement between adaxial and abaxial motor cells during leaf closure indicates the possible role of Ca ions in regulating the turgor pressure.原