ANATOMY OF SHOOTS AND TUMORS OF IN-VITRO HABITUATED RHODODENDRON MONTEGO (ERICACEAE) CULTURES WITH TISSUE PROLIFERATION

Tissue proliferation (TP) is characterized primarily by the formation of galls or tumors at the crown of container-grown rhododendrons that were initially propagated in vitro. In the cultivar 'Montego', TP-like symptoms are first observed in vitro as shoot clusters with small lea ves and nodal tumors. In addition, unlike the normal in vitro non-TP ( TP-) shoots, in vitro TP (TP+) shoots proliferate rapidly without the presence of the plant growth th regulator cytokinin in the tissue cult ure medium. Comparisons of the anatomy of TP+ and TP-shoot tips showed that TP+ shoots had a less developed vascular system, longer cells in the pith and cortex, and altered internodal elongation at the shoot a pex, In addition, TP+ axillary buds were abnormal in that they were di splaced onto the stem above the leaf axil, and a small group of prolif erating cells replaced the shell zone at the base of the bud. Initiati on of tumor formation began with the expansion of this region of cell proliferation (RCP) and shoot growth from the abnormal axillary bud (t umor bud). Organization of the tumor bud and extension of the RCP char acterized the further development of two types of tumors. In polar sho ot tumors, shoot growth continued from the persistent tumor bud and th e tumor at the base of the shoot remained small in size. The RCP exten ds downward to the vascular junction of the subtending leaf and the st em of the TP+ shoot. In nonpolar tumors, continuous de novo meristem f ormation led to the development of large tumors with or without shoots . The RCP is present throughout the tumor and is associated with de no vo meristem formation. Comparisons to the anatomy of other tumor-like structures showed that TP tumors of Rhododendron 'Montego' are most si milar to tobacco genetic tumors.