Regeneration of Bulblets on <i>Hippeastrum</i> Bulb Segments Excised from Various Parts of a Parent Bulb

Studies of in vitro tissue culture were carried out to make a comparison of the responses in adventitious bulblet formation between tissues excised from different parts of parent bulbs. The culture medium used was White′s supplemented with 2% sucrose and 0.8% agar. No exogenous growth regulators were supplemented. All cultures were maintained at 25°C under dark. A preliminary test showed that 25°C and 30°C were best for the culture, and that illumination was not necessarily required for bulblet formation itself, though it hastened the growth of the bulblets formed. The test also demonstrated that the cultures showed no seasonal response with respect to regeneration.<br>1. Longitudinal position on a scale. Segments containing both tissues of scale base and basal plate produced bulblets from the proximal end of the scale and roots from the basal plate tissue at a high rate. Segments of the extreme base of scale also produced bulblets and the bulblets rooted later. In these cases, a higher regenerative rate was found for outer scale segments than for inner ones. Segments whose proximal end had been located 2mm above the junction of the basal plate produced bulblets at the rate of only 3% and no regeneration was found from those 3mm and more above the junction. Explants of bulb plate tissue alone regenerated neither roots nor bulblets.<br>2. Position around a scale base. For all circular scales (leaf bases), the leaf-blade-extended-side is thicker than the opposite side. As successive leaf blades extend 180° apart around a bulb, the thicker part of a scale and the thinner part of the next scale are arranged alternately. In the culture of segments of a single scale base, either with or without basal plate tissue, no difference in regenerative rate was shown between those excised from the thicker and thinner parts of a scale. In segments, each containing two thin and thick scale base fragments attached to basal plate tissue, bulblets developed at a higher rate and more rapidly from the position between the scale fragments than from either of the scale surface exposed sides. In a comparison of the two scale fragments with respect to regeneration from the surface exposed sides, the thinner one showed a clear tendency to produce bulblets at a higher rate than the thicker one. Lower regenerative ability of the thicker part, which was not clarified by the culture of a single fragment, appeared to be due to competition of the two segments.<br>3. Dorsiventral polarity of regeneration. Although some explants with basal plate tissue planted with the adaxial surface upwards regenerated bulblets from the adaxial side of the scale base, most explants regenerated bulblets from basal abaxial areas alone, regardless of the orientation of segments When segments of a scale base with basal plate tissue were bisected parallel to their two surfaces and cultured, more than 60% of the adaxial halves which were planted erectly or with the adaxial surface upwards regenerated bulblets at their proximal end. When segments having both surfaces were cultured in a liquid medium by a reciprocating and a rotary shaker, 17% and 10% of the segments, respectivly, produced bulblets on the adaxial side. These results suggest that some substance involved in the regenerative response and transmittable across the tissue may be more richly concentrated in the abaxial side than the adaxial side of the scal base.