INDUCTION OF EMBRYOGENESIS VERSUS CAULOGENESIS ON IN-VITRO CULTURED SUNFLOWER (HELIANTHUS-ANNUUS L.) IMMATURE ZYGOTIC EMBRYOS - ROLE OF PLANT-GROWTH REGULATORS

Somatic embryos or shoots can be induced in vitro on immature zygotic embryos of sunflower (Helianthus annuus L.) depending on the sucrose c oncentration of the culture medium, On a medium containing 3% sucrose only shoots are induced while on a medium containing 12% sucrose somat ic embryos are formed. Although these two different reactions are not induced by changes in the concentration of externally supplied growth regulators, the induction of both morphogenic events is probably depen ding on internal hormone regulation, in response to the medium composi tion. We have studied the effect of the external supply of some growth regulators known to induce organogenesis, such as cytokinins, or soma tic embryogenesis, such as auxins and abscisic acid (ABA). We have als o studied the effect of auxin polar transport inhibitors on the develo pmental response. Addition of auxins or abscisic acid to the organogen ic medium (3% sucrose) induced somatic embryogenesis whereas addition of auxin polar transport inhibitors led to shoot formation under other wise embryogenic conditions (12% sucrose). Moreover, among the cytokin ins tested, zeatin had an unexpected embryogenic effect independently of the sugar concentration of the medium. Possible relevance of the di fferent hormones, for the morphogenic response and their interactions, is discussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reser ved.