Endogenous hormonal levels and growth of dark-incubated shoots of Catasetum fimbriatum

Apical shoots and lateral buds of the epiphytic orchid Catasetum fimbriatum give rise to rootless etiolated stolons, when cultured in the presence of light and then transferred to the dark. The stolons are characterized by fast and continuous apical longitudinal growth. Measurements of four endogenous cytokinin, indole-3-acetic acid (IAA) and abscisic acid (ABA) levels in etiolated shoots and light-grown plants were low. However, after transfer of green plants to the dark, cytokinin levels increased 3- and 7-fold by 10 and 30 days of incubation, respectively. IAA levels also increased significantly, but the increase was not as great as for cytokinins. A similar trend was observed in the roots. A close relationship seems to exist between both cytokinin accumulation and the formation of etiolated stolons. Variations in ABA levels were practically inconspicuous. The presence of paclobutrazol in the medium, a potent inhibitor of gibberellin synthesis, strongly inhibited etiolated and non-etiolated longitudinal shoot growth, although no apparent effect was observed on apical meristem activity.