CHANGES IN MORPHOLOGY AND MEASUREMENT OF CYTOKININ LEVELS DURING THE DEVELOPMENT OF WITCHES-BROOMS ON COCOA

The anatomy of diseased stems from brooms induced on cocoa (Theobroma cacao) by the fungus Crinipellis perniciosa was examined using the sca nning electron microscope, and the amounts of selected cytokinins were measured by immunoassay. Although the diseased stems were wider in di ameter with a lower dry weight:fresh weight ratio than stems of health y plants, the overall internal organization of tissues remained unchan ged, and all the cell types were present. However, the vascular tissue was less differentiated in diseased stems since xylem vessels were ab sent and the ratio of phloem fibres and sieve tubes to phloem parenchy ma was reduced. Cell sizes in diseased stems were larger for all tissu es but cell numbers were unchanged except for xylem where fewer cells were seen. Cytokinins (zeatin, zeatin riboside, isopentenyl adenine an d isopentenyl adenosine) were measured in healthy plants and diseased stems at specific stages of disease development. Of these cytokinins o nly zeatin riboside was present in significantly greater amounts in di seased tissue. The likely role of plant growth regulator balance in st em enlargement and proliferation in witches' broom disease of cocoa is discussed.