High cytokinin accumulation following root tip excision changes the endogenous auxin-to-cytokinin ratio during root-to-shoot conversion in Catasetum fimbriatum Lindl (Orchidaceae)

 Shoot initiation and development was observed in Catasetum (Orchidaceae) cultured on hormone-free media. Endogenous auxin and cytokinin contents were determined in excised root tips of two Catasetum fimbriatum genotypes incubated in a hormone-free medium. During the culture period, significant accumulation of all measured cytokinins was observed in the isolated root tips of both genotypes, reducing, by the 10th day of incubation, the auxin/cytokinin ratio tenfold in both genotypes. Root excision and the competence for shoot development in C. fimbriatum may be attributable to the establishment of an endogenous auxin/cytokinin ratio favoring cytokinins. Received: 20 August 1998 / Revision received: 30 January 1999 / Accepted: 15 February 1999