Factors influencing the growth of micropropagated shoots and in vitro flowering of gentian

About 70% of the shoots developed from nodal explants of Gentiana triflora flowered in vitro on double strength WPM medium containing 3% (w/v) sucrose, 0.5 mg/l BA after 12 weeks of culture in a growth room at 22degreesC with continuous illumination (PPFD = 60 mumol m(-2) s(-1)). The influences on in vitro shoot development and flowering of several factors including the position of the explant, requirements for sucrose, cytokinin or GA(3), variations of pH and photosynthetic photon flux density (PPFD) were investigated. In vitro flowering but not shoot development of G. triflora decreased notably with increased distance from the apex of the shoot, indicating the presence of a "floral gradient" in the micropropagated shoots. Conversely, as little as 0.01 mg l(-1) GA(3) in the medium promoted shoot development but even up to 0.2 mg l(-1) GA(3) did not induce in vitro flowering. Even though BA could substitute GA(3) for a high level of shoot development, it also promoted a high level of in vitro flowering at the PPFD of 60 mumol m(-2) s(-1). Sucrose was required for shoot development and flowering in vitro and higher levels of PPFD could not compensate effectively for the omission of the sugar from the medium. In general, the effects of different concentrations of BA in the medium or variations of pH on shoot development and flowering in vitro were found to be influenced by PPFD. A novel observation is that precocious flowering of micropropagated gentian shoots did not occur if they were first cultured for 5 weeks in the dark before transfer to the light condition.