Induction of adventitious shoots or somatic embryos on in vitro cultured zygotic embryos of Helianthus annuus: Variation of endogenous hormone levels

The induction of both shoots and somatic embryos from the same cells of imm ature zygotic embryos (IZE) of sunflower (Helianthus annuus L.) is dependen t on the sugar concentration of the induction medium. With the same concent ration of benzyladenine (BAP), shoots are induced at a sucrose concentratio n of 3 %, while somatic embryos will develop at 12 % sucrose. To understand the role played by endogenous hormones, we have investigated the temporal variation of their levels during culture of IZE on caulogenic or embryogeni c medium. The BAP taken up from the medium reaches a maximum concentration in the explants at 6-15 h of culture. When the IZE were cultured under caul ogenic conditions, the endogenous concentration of BAP was 2-fold higher th an in an IZE cultured on embryogenic medium. Inversely, the concentration o f indolyl acetic acid (IAA) induced by the exogenously added BAP was 4-fold higher in explants cultured on embryogenic than in those cultured on caulo genic medium. The IAA concentration peaked in the sample taken at 24 h, the time when the responding cells become determined towards one or the other morphogenic reaction. The IAA/BAP ratio was higher in the IZE induced to em bryogenesis compared to those that gave rise to shoots. The effect of absci sic acid (ABA) on the morphogenic process appears to be indirect and can be explained by a modification of the IAA content. (C) 1999 Editions scientif iques et medicales Elsevier SAS.