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Induction of shoots or of somatic embryos is the key step for gaining the morphogenetic potential in sunflower (Helianthus annuus L.), species known as recalcitrant to in vitro regeneration. In the immature zygotic embryo derived tissues or in other juvenile tissues resulted from seedlings, the acquisition of the competence for regeneration can be achieved directly by cytokinin treatment or by preconditioning the explants on cytokinin containing medium. In this paper is presented a new type of explant for sunflower in vitro culture, consisting of the apex with primordial leaves, resulted from ungerminated mature zygotic embryo, in which a specific morphogenetic response was triggered by the exogenously applied auxins. Among the auxins tested, indole-3-acetic acid, indole-3-butyric acid and 1-naphthaleneacetic acid are inducers of an organogenetic response, apical/axillary shoots and adventitious buds being regenerated while 2,4dichlorophenoxyacetic acid, 3,6-dichloro-2-methoxybenzoic acid and 4-amino-3,5,6-trichloropicolinic acid led to somatic embryo formation. Among the auxins tested only 4-amino-3,5,6-trichloropicolinic acid sustains the embryos development up to mature stage. A high amount of sucrose (120 g L) supplied during the auxin treatment promotes the maturation of the embryos directly on the induction medium for all tested auxins with embryogenic effect. These findings show that regardless of the type of morphogenetic response aimed in sunflower meristematic tissues resulted from mature embryos, the presence of auxins is mandatory.

The effect of auxins in inducing organogenesis or somatic embryogenesis in mature sunflower zygotic embryo derived apex