Plant regeneration in Spanish cedar, Cedrela odorata L., using zygotic embryo explants from mature seed and improvement of embryogenic nodule initiation by heat shock

Conditions for induction of embryogenic nodules and subsequent somatic embryogenesis in the tropical hardwood Cedrela odorata are described. Embryo explants from ungerminated mature seeds were placed on Driver and Kuniyuki Walnut (DKW) medium using 0.8% w/v agar as the gelling agent, plus benzyladenine (BA) and 2,4-dichlorophenoxyacetic acid (2,4-D). Various phytohormone combinations were tested, from which 5 µM BA + 50 µM 2,4-D were chosen as the standard regime based on a maximum frequency of embryogenic nodule occurrence of 20-25% on this medium. Nodules, when excised from the cotyledons and placed on growth-regulator-free medium, produced both plantlets and secondary embryogenic tissue. With extended culture on growth-regulator-free DKW medium, plantlets developed roots and could be transplanted into pots for further growth. The frequency of nodule initiation could be improved by either orienting the cotyledon explants with their lower (abaxial) surface in contact with the medium or imposing a pre-excision period of heat shock. The treatments together were additive. An optimum heat-shock temperature (47°C) and range of exposure times (8-12 h) were defined.