ABSTRACT

Depending on the culture conditions, somatic embryos or adventitious shoots can be induced alternatively or simultaneously on immature zygotic embryos of sunflower (Helianthus annuus L.). Their relative number is function of the sucrose concentration of the medium. This culture system is well characterised: the induction of shoots or embryos can be predicted with precision in space and time. The cells involved in the morphogenic reaction are localised in a particular region of the zygotic embryo. They are competent for either development. This potential is expressed as a function of their position relative to the medium and thus, to a putative gradient. According to our present working hypothesis, somatic embryogenesis would occur above a certain threshold value within this gradient, caulogenesis when the values experienced by the competent cells are below this value. The components of this gradient are so far unknown but the various classes of plant growth regulators are likely candidates. Their effect on the morphogenic response has been studied in detail. Changes in their concentration have been recorded during the early phases of the culture. It has become clear from these experiments that the externally added factors, i.e. sucrose and a single phytohormone, cytokinin, condition profound changes in the quantitative distribution pattern of internally synthesised auxin and cytokinins.