DISTRIBUTION AND STAINING PROPERTIES OF CYMBIDIUM MOSAIC VIRUS IN BUDS OF CATTLEYA ORCHIDS
A comparison of propagation methods used for Cymbidium and Cattleya orchids reveals differences in the handling and response of the two genera to meristeming techniques. These differences may relate to the success of culturing virus-free plants. Morel (1964) described excision of a 0.1 mm apical tissue piece from Cymbidium infected with cymbidium mosaic virus (CyMV) which included the apical meristem and two leaf primordia. Plants derived from this small excision were virus-free. Cattleyas have been vegetatively propagated by culturing 5 to 8 mm axillary buds excised from young stems (Champagnat and Morel, 1969).
Failure to achieve virus-free plantlets in Cattleya may result from the large size of the tissue piece excised. The larger the tissue piece, the greater the probability of including infected cells.
Rutkowski (1971) presented results that suggest an explanation of the origin of tissue capable of proliferation. He concludes that Cattleya and Cymbidium give rise to new tissue in a different manner. Excised Cymbidium tips give rise immediately to protocorm-like structures designated pseudo-bulbs. Many cattleyas do not form a protocorm-like structure but give rise directly to a plantlet (Rutkowski, 1971). According to Rutkowski (1971), proliferation does not occur extensively from the outer layers of the base of the plantlets or from the outer formed leaves. Rutkowski concluded that the inner areas of Cattleya tissue contain the least differentiated cells and those most capable of meristemic activity. It is possible that these excised pieces contain intercalary meristem not included in tissue from outer parts of the plant. Epidermal cells on the inner leaf scales may have also been included.
Meristem tissue pieces including only the meristematic dome cultured from a rapidly proliferating virus-infected Cattleya mericlone showed little survival when explants less than 0.5 mm were cultured on a synthetic medium (Ishii, 1972). A marked increase in survival resulted when tissue pieces were 0.5 to 1.0 mm, but the plants obtained from such tissue remained virus-infected. Tissue pieces 0.2 to 0.5 mm