Histological and Scanning Electron Observations on Embryogenic and Non-embryogenic Calli of Aromatic Thai Rice (Oryza sativa L. cv. Khao Daw Mali 105)
Two types of callus proliferation namely, embryogenic (E) and non-embryogenic (NE) calli were obtained by culturing mature rice (Oryza sativa L. cv. Khao Daw Mali 105) seeds in modified MS medium supplemented with 2 mg/l 2,4-D, 10 mM proline and 3% sucrose. Both light and scanning electron micrographs were employed to distinguish the surface details as well as cell shape of E and NE calli. Light micrographs of E callus was compact, nodular, knobs, white or creamy. NE callus was soft, friable, unorganized light-stained cells, translucent, watery and light yellow. Scanning electron micrographs indicated distinct morphology of cells shape. NE cell shape was long-like tubular and loosely arranged cells. E callus comprised nodular or knobby, quite deep embedded and tightly packed cell was similar to typical E callus. Histological observations of 5 μm of callus paraffin sections revealed that E callus occurred on the surface as well as in the deeper regions of the callus. The abundance of vessel tracts in the callus which was consisted of a high content of vessel elements, rounded and cytoplasmically dense cells with totipotency. This may imply that nutrients are easily transported via these vessel tracts for the nourishment of growing somatic embryos or leafy structure. The number of vessel elements appeared to be a crucial factor to determine the E and NE calli development.