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S.A. Merkle and B.A. Watson-Pauley

Bigleaf magnolia (Magnolia macrophylla Michx.) cultures were initiated from immature seeds on an induction medium containing 9.0 μm 2,4-D, 1.1μm BA, and 1 g casein hydrolysate/liter. After 2 months on induction medium, one culture produced adventive embryos. Clumps of embryos transferred to liquid induction medium proliferated as nodules, which grew in diameter, but failed to produce embryos while maintained in induction medium. Nodules transferred to basal medium produced clumps of somatic embryos, which continued to produce repetitive embryos with monthly transfer to fresh basal medium. Individual embryos transferred to basal medium lacking casein hydrolysate germinated and leaves expanded. Plantlets derived from these embryos were transferred to potting mix and acclimatized to greenhouse conditions. Chemical names used: (2,4-dichlorophenoxy)acetic acid (2,4-D); N -(phenylmethyl)-lH-purin-6-amine (BA).

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S.A. Merkle and B.A. Watson-Pauley

Low conversion rates of somatic embryos and poor early growth of somatic embryo-derived plantlets of some forest trees may be related as much to prolonged maintenance in vitro as to basic developmental problems with the embryos. We tested ex vitro conversion as an alternative method for producing the rare North American pyramid magnolia (Magnolia pyramidata Bartram) plantlets from somatic embryos. Tissue cultures were initiated from immature seed explants of pyramid magnolia. Immature seeds collected from each of three trees formed proembryogenic masses (PEMs) following 7 to 10 weeks of continuous culture on semisolid medium containing 9.0 μm 2,4-D, 1.1 μm BA, and 1 g casein hydrolysate/liter. PEMs transferred to semisolid medium without plant growth regulators produced somatic embryos that germinated following transfer to the same medium without casein hydrolysate. Conversion frequency to plantlets was higher and plantlets were more vigorous when germinants were transferred directly to potting mix and grown in a humidifying chamber instead of being maintained in plantlet development medium in test tubes. Chemical names used: 2,4-dichlorophenoxyacetic acid (2,4-D); N-(phenylmethyl)-1H-purine-6-amine (BA).