TISSUE CULTURE PROPAGATION OF PAPAYA

A procedure for the rapid tissue culture propagation of papaya (Carica papaya L.l has been developed as part of a project to ,obtain virus tolerance in this crop. The success of this technique is dependent upon 1) time of year, 2) the nature of the primary explant, and 3) the hormone component of the growth medium. This propagation method should enable one to multiply disease-tolerant varieties useful to the industry of South Florida. The papaya disease caused by distortion ringspot virus has effectively limited the production of this crop in South Florida for several years (1). A breeding program at Homestead under the direction of Prof. R. A. Conover has been in existence since 1975 (2) and efforts have been directed toward introducing tolerance to the virus from plant accessions from many tropical countries. At the same time, attention has been focused on obtaining lligh yielding papaya lines that bear fruit of good quality. In order to preserve these desirable characteristics, and still retain tolerance to distortion ringspot, a reliable method for the clonal propagation of papaya is required. However, papayas have traditionally been propagated by seed, and considerable variability occurs as a result of open pollination. In some parts of the world, multibranched papayas are favored, and excised branches are routinely rooted. The papaya that is grown in Florida is generally non-branching. Even after the decapitation of the plant to induce a branching habit, this papaya does not lend itself to clonal propagation. Nor has grafting been very successful. Tissue culture has been developed as an alternative to conventional propagation methods, and has been successfully adapted to many tropical plants. Therefore, a tissue culture unit was established at Homestead during the summer of 1976 in order to formulate in vitro systems for the preservation of breeding lines and for the rapid clonal propagation of papaya plants. There have been 2 attempts to define a tissue culture propagation system for papaya. One of these (3) obtained proliferating plants from tissues isolated froln 5-6 cm seedlings on a simple medium with kinetin. However, this formulation has been ineffective for tissues from mature papaya plants whose characteristics are known. The other system (4) was formulated for use with explants from mature papaya tissues, and caused rapid callus induction from petiole segments. Embryo and shoot formation was induced from this callus by a lnore complex medium. This system is not ideal for the plant breeder or propagator because papaya callus is heterogeneous in origin, and the cells l1ave different ploidy levels. Materials and Methods Papaya plants (Carica papaya L.) of any state of developUlent have been used. However, after some trial and error only the apical regions of the plants were retained. The Auxin conc