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 immature zygotic embryos of sunflower, depending on the sucrose concentration 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 lower third of the hypocotyl. Shoots and somatic embryos had a multicellular origin. They were initiated by periclinal cell divisions in the epidermis and the outer cell layers of the hypocotyl cortex within the Ist 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 the shoot tissues. This system provides a model that can be used to study early biochemical and molecular events taking place in explants at the onset of organogenesis and somatic embryogenesis, as these two processes occur in a predictable manner and location.