A stage-specific block is produced in carrot somatic embryos by 1,2-benzisoxazole 3-acetic acid.

1,2-Benz-isoxazol-3-acetic acid is slightly toxic to unorganized growing carrot cells and completely prevents somatic embryogenesis beyond the globular stage. Its action is time dependent: if added early, it affects the determination of the early stages; if added later, after determination has occurred, it has practically no effect. The blocked forms can evolve, in the presence of the compound, creating secondary embryos that, in turn, are arrested at the globular stage. Since the secondary embryos adhere to the primary ones, irregular polyembryonic masses of typical appearance are created. The internal structures of the blocked globular embryos were examined and were shown to be enlarged, vacuolized and detached from the central cells. The protoderm also displayed an altered appearance. These data, when considered together with the effects of the compound on rooting and callus formation, suggest that benz-isoxazol acetic acid acts as a weak auxin.