Endogenous hormone levels in explants and in embryogenic and non-embryogenic cultures of carrot

Carrot (Daucus carota L. F1 hybrid Starca) excised hypocotyls were cultured on Murashige and Skoog medium with and without 2,4-dichlorophenoxy acetic acid (2,4-D) to determine the effect of this plant growth regulator on thei r further development and their endogenous hormone levels, Culture in the a bsence of 2,4-D stimulated root development at one end of the hypocotyl seg ments and increased the endogenous bevels of free indole-3-acetic acid (IAA ), zeatin/zeatin riboside and N-6(Delta (2)-isopentenyl) adenine/N-6(Delta (2)-isopentenyl) adenosine, as determined by radioimmunoassay. On the other hand, the presence of 2,4-D in the culture medium promoted callus inductio n and proliferation, together with abscisic acid (ABA) accumulation, in the hypocotyl segments during the first weeks of culture. When the callus segm ents generated in the hypocotyl sections cultured in the presence of 2,4-D were cultivated further, the development of two callus types was observed, one composed of preglobular and globular embryos and the other translucent, watery and lacking any sign of organisation. The embryos of the first type germinated when callus segments were transferred to regeneration condition s, while no change was observed when the second type was induced to regener ate, Higher levels of free IAA and ABA were obtained in the embryogenic cal li when compared to the non-embryogenic, while no differences were observed among callus types in the other hormones evaluated. The possible role of t he different plant hormones during induction of somatic embryogenesis is di scussed.