ULTRASTRUCTURAL CHANGES OF CHLOROPLAST DURING CALLUS INDUCTION IN LEAF EXPLANTS OF STEVIA REBAUDIANA BERTONI

Ultrastructural changes of chloroplasts were observed during callus induction in the leaf explants of Stevia rebaudiana Bertoni. Transfer of the leaf explants into the culture medium brings about the gradual degeneration of the chloroplast lamellar structures. The process is frequently accompanied by the occurrence of chloroplast gemma-like extension (here called plastid bud)and the formation of proplastids; presumably the newly proplastids originate from the gemma-like extensions, while the chloroplasts themselves ultimately disintegrate completely and then disappear. The changes in the ultrastructure of the chloroplasts parallel with the highly vacuolated mesophyll cells from a dedifferentiation to a less differentiated, meristematic state. Subsequent growth results in the reformation of large central vacuoles in the cells both within and on the periphery of callus. Transforming the meristematic callus cells into the highly vacuolated parenchymatous cells, the plastid lamellar structures are reformed in the cells close to the periphery of the callus, while the plastids in the cells from the interior, which are often devoid of lamellar structures, are filled with starch grains.