Effects of auxin, cytokinin and ethylene treatments on the endogenous ethylene and auxin-to-cytokinins ratio related to direct root tip conversion ofCatasetum fimbriatum Lindl. (Orchidaceae) into buds

The present investigation attempts to correlate the effect of the exogenous auxins, cytokinins and ethylene with the endogenous ethylene and auxin-to- cytokinin ratio, and with the direct root-to-bud conversion in Catasetum fi mbriatum. Incubation of root tips with IAA and NAA strongly inhibited both root elongation and bud formation. On the other hand, treatment with IBA re sulted in a conspicuous stimulation of longitudinal root growth, while bud formation was retarded. High levels of exogenous zeatin, iP and BA enhanced bud formation, and inhibited root growth. Ethylene production was measured in IAA, IBA and zeatin-treated roots. All growth regulators elicited ethyl ene evolution, which decreased considerably along the period of incubation. Addition of AVG to the media reduced ethylene evolution. Endogenous auxin, cytokinins, and ABA contents were determined by means of HPLC and ELISA me thods in root tips of C. fimbriatum pre-incubated with ethylene (CEPA) and cytokinin (iP). Both exogenous substances stimulated bud formation and cond uced to an endogenous IAA/Cks ratio favourable to Cks. From these results i t is proposed that the stimulatory effect of exogenous Cks and ethylene on root-to-bud conversion of C. fimbriatum seems to be mediated by the establi shment of an endogenous auxin-to-cytokinins balance favourable to shoot for mation.