FAILURE TO ESTABLISH A FUNCTIONAL SHOOT MERISTEM MAY BE A CAUSE OF CONVERSION FAILURE IN SOMATIC EMBRYOS OF DAUCUS-CAROTA(APIACEAE)

A carrot cell culture line was shown to be highly embryogenic, but pla ntlet recovery (conversion) was low (about 14%). The majority of somat ic embryos that did not convert showed pronounced vacuolation in the a pical notch, leading to their inability to form primary leaves and the reby convert. Comparisons of developing meristems in the shoot apical notch of converting somatic and zygotic embryos revealed similarities in cytoplasmic density and meristem organization between the two popul ations. Abscisic acid (ABA) was shown to significantly increase conver sion in somatic embryos of globular, torpedo, and preplantlet stages ( 62%, 62.5% and 40%, respectively). Somatic embryos that were treated w ith 50 muM ABA showed retention of the highly cytoplasmic cells in the apical notch. Histological analysis showed a resemblance between shoo t apices of converting somatic embryos and ABA-treated somatic embryos . ABA may be an induction agent for meristematic organization, or perh aps may cause cells of the apical notch to extend competence for deter mination as meristematic cells.