EX-VITRO CONVERSION OF PYRAMID MAGNOLIA SOMATIC EMBRYOS

Low conversion rates of somatic embryos and poor early growth of somat ic embryo-derived plantlets of some forest trees may be related as muc h to prolonged maintenance in vitro as to basic developmental problems with the embryos. We tested ex vitro conversion as an alternative met hod for producing the rare North American pyramid magnolia (Magnolia p yramidata Bartram) plantlets from somatic embryos. Tissue cultures wer e 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 medi um containing 9.0 mum 2,4-D, 1.1 mum BA, and 1 g casein hydrolysate/li ter. PEMs transferred to semisolid medium without plant growth regulat ors produced somatic embryos that germinated following transfer to the same medium without casein hydrolysate. Conversion frequency to plant lets 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 tu bes. Chemical names used: 2,4-dichlorophenoxyacetic acid (2,4-D); N-(p henylmethyl)-1H-purine-6-amine (BA).