EARLY CELLULAR EVENTS DURING ORGANOGENESIS AND SOMATIC EMBRYOGENESIS INDUCED ON IMMATURE ZYGOTIC EMBRYOS OF SUNFLOWER (HELIANTHUS-ANNUUS)

Both organogenesis and somatic embryogenesis can be induced in 6-mm im mature zygotic embryos of sunflower, depending on the sucrose concentr ation of the induction medium; shoots were induced at a concentration of 3%, while embryos were formed at 12%. Both morphogenic events were direct, without intervening callus formation, and occurred in the lowe r third of the hypocotyl. Shoots and somatic embryos had a multicellul ar origin. They were initiated by periclinal cell divisions in the epi dermis and the outer cell layers of the hypocotyl cortex within the Is t day of culture. The two types of morphogenic events could already be distinguished on the 2nd day of culture, in that proteins and lipids started to accumulate in the somatic embryogenic tissues but not in th e shoot tissues. This system provides a model that can be used to stud y early biochemical and molecular events taking place in explants at t he onset of organogenesis and somatic embryogenesis, as these two proc esses occur in a predictable manner and location.