Abstract
The role of benzyladenine (BA) in the differentiation of tracheary elements in Jerusalem artichoke (Helianthus tuberosus L.) tuber expiants was studied. For maximum differentiation of tracheary elements (25-30% of the cell population), treatment with optimal concentrations of benzyladenine (5-0 mg dm-3) in the presence of a-naphthaleneacetic acid [NAAI (1-0 mg dm~') for the first 6 d was as effective as its continued presence during the entire 14 d period of study. A majority of the differentiated tracheary element appeared between the 10th and 14th days of culture. It was further observed that concentrations of active cytokinins in the tissue were considerably reduced within 2 d after transfer from the BA-containing medium to a BA-free medium. This was shown in three different ways: (1) monitoring the amount of ethanol-soluble radioactivity at various times after transfer from [14C]-BA containing medium to BA-free medium: (2) bioassay of various cytokinin fractions from tissue extract separated by thin layer chromatography; (3) indirect assay of tissue cytokinin activity through its interaction with abscisic acid for the promotion of auxin-induced cell division in this tissue. Both gibberellic acid (5-0 mg dirr3) and abscisic acid (2-0 mg dm-3) effectively inhibited the differentiation of tracheary elements even if provided after 6 d of pre-incubation in a high tracheid inducing medium. However, the appearance of differentiated cells for the first 2 d after transfer was not significantly affected. A hypothetical scheme for the role of benzyladenine in the differentiation of tracheary elements in this tissue is discussed. It is suggested that during one or more critical cell divisions in the presence of optimal levels of benzyladenine, a proportion of cells are induced or committed for later differentiation into tracheary elements. The high concentrations of benzyladenine required during induction are not needed during the intervening cell divisions, nor for the actual differentiation of the tracheary elements.
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