Ex Vitro Conversion of Pyramid Magnolia Somatic Embryos

Abstract

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).