Development of secretory cells and crystal cells in Eichhornia crassipes ramet shoot apex

The distribution and development of secretory cells and crystal cells in young shoot apexes of water hyacinth were investigated through morphological and cytological analysis. The density of secretory cells and crystal cells were high in parenchyma tissues around the vascular bundles of shoot apexes. Three developmental stages of the secretory cells can be distinguished under transmission electron microscopy. Firstly, a large number of electron-dense vesicles formed in the cytoplasm, then fused with the tonoplast and released into the vacuole in the form of electron-dense droplets. As these droplets fused together, a large mass of dark material completely filled the vacuole. To this end, a secretion storage vacuole (SSV) formed. Secondly, an active secretion stage accompanied with degradation of the large electron-dense masses through an ill-defined autophagic process at periphery and in the limited internal regions of the SSV. Finally, after most storage substances were withdrawn, the materials remaining in the spent SSV consisted of an electron-dense network structure. The distribution and development of crystal cells in shoot apical tissue of water hyacinth were also studied by light and electron microscopy. Crystals initially formed at one site in the vacuole, where tube-like membrane structures formed crystal chambers. The chamber enlarged as the crystal grew in bidirectional manner and formed needle-shaped raphides. Most of these crystals finally occurred as raphide bundles, and the others appeared as block-like rhombohedral crystals in the vacuole. These results suggest that the formation of both secretory cells and crystal cells are involved in the metamorphosis of vacuoles and a role for vacuoles in water hyacinth rapid growth and tolerance.