ULTRASTRUCTURE OF SUNFLOWER PROTOPLAST-DERIVED CALLUSES DIFFERING IN THEIR REGENERATIVE POTENTIAL

The ultrastructural properties of microcalluses derived from mesophyll protoplasts of commercial sunflower (Helianthus annuus L.) cultivars were investigated by light and electron microscopy. Two culture regime s were chosen : Regime A giving rise to callus formation but of little embryogenic potential and regime B resulting in higher embryogenicity . Bipolar colonies that developed during early stages of regime A were found to be composed of mostly degenerated structures. No differentia tion or embryonal organization as suggested by the compactness and sha pe of the microcalluses could be observed. Amorphous calluses obtained at later stages of the same regime consisted of small groups of meris tematic as well as vacuolated cells. Incomplete cellular divisions occ urred in almost all colonies grown under the regime A, causing most pr obably the lack of further callus organization. In contrast calluses o f irregular shape, cultivated under regime B, mostly lacked incomplete cell partitioning but showed the formation of organized regions. Thes e structural investigations can give us a tool to identify and charact erize the quality of embryogenic calluses.