The morphological behavior of explants (hypocotyls, epicotyls, roots and node and internode segments) of annatto (B. orellana L.) was evaluated by means of in vitro morphogenic responses regarding the duration of exposition to light throughout germination period; the responses to hypoxia in semi-solid medium, and the response gradients of hypocotyls and inverted hypocotyls explants as compared to the normal physiological polarity cultured ones. The maintenance of the cultures for 30 days in the absence of light, throughout the germination period, enabled higher rates of morphogenic responses. Hypocotyls placed on the surface of the culture medium displayed direct organogenesis; unlike the others that presented varied responses dependent on the depth of culture in semi-solid medium. Regarding the morphogenic potential of hypocotyledonary explants, either related to the in vitro responsiveness or genotype-specificity, it was observed that treatments kept under light regime displayed satisfactory responses. However, different responses were observed along the explants, and these were specific for each genotype. For all hybrids, the use of inverted hypocotyl segments was more efficient in inducing adventitious organogenesis as compared to the normal oriented explants. For root cultures, the utilization of liquid medium under agitation was superior as compared to the semi-solid counterpart; however, the induction of adventitious organogenesis demanded the incorporation of growth regulators (GR). After 45 days, in cultured root explants in the semi-solid medium; in the absence of light, differentiation of shoot buds and elongated shoots were scarce and with certain abnormality, but non-hyperhydric. The use of liquid medium, regardless the light regime, led to differentiation of hyperhydric shoots. As for the use of adult explants, the formation of shoot-buds occurred more frequently in the cut surfaces. BAP-induced shoot-buds, despite the lower number, were more vigorous. The histological analyses revealed that the initial regeneration responses started from an intense cellular proliferation in the pericycle and vascular tissues areas, mainly associated with the secondary phloem. At 30 days of culture in MS medium supplemented with GR, a regeneration of shoots was observed, when internode segments (INS) were used as explant source, whereas only callusing was obtained in the control treatment (MS0). The shoots presented, in average, 0,1 cm in length and, in most of the treatments, they were formed in the whole surface of the explant. The majority of treatments benefited organogenic responses, but greater averages of responsive explants were obtained in BAP-added media, whilst those with 2iP were less responsive. The morphogenic responses occurred generally by means of direct pathway, except for BAP-supplemented media. The initial cellular division processes that culminated with morphogenesis were coincident with an intense proliferation from cambium, extending to secondary phloem region. When NS were used as source of explants average shoot length varied among treatments. Similarly, to those shoots derived from SIN cultures, those from SN had leaf senescence, requiring the addition of STS to the elongation and maintenance media.
