ABSTRACT
A test system for root regeneration was developed that consists of stem slices of apple shoots (ca. 0.5 mm thick; fresh weight ca. 1 mg). Roots regenerated synchronously without intervening callus formation and without interference with compounds originating from other parts of the plant. Supply of indolebutyric acid (IBA) or indoleacetic acid (IAA) induced maximally an average of 8 or 4.5 roots per slice, respectively. After uptake of IBA, a high degree of conjugation resulted in a recovery of 2.5% as internal free IBA (ca. 2 times the medium concentration). Due to conversion of absorbed IBA into IAA a fraction of 0.4% was recovered as (physiologically active) free IAA. After incubation on medium with IAA, 0.5% of the absorbed hormone was recovered in the free acid form. No conversion of IAA into IBA was observed. Equimolar contents of internal free IAA after incubation on IBA or IAA resulted for IBA in a higher number of roots than for IAA. This means that IBA may also act via internal free IBA or may synergistically modify the action or endogenous synthesis of IAA.
ABSTRACT
Explants from flower stalks of Nicotiana tabacum L. were cultured on different cytokinins to induce flower bud formation. All cytokinins tested except zeatin and zeatin-riboside induced the same maximal number of flower buds. Benzyladenine, benzyladenosine, and dihydrozeatin were the most active compounds whereas isopentenyladenosine and isopentenyladenine acted at a 20-fold higher concentration. These data suggest that the active cytokinins bind to the same receptor with different affinities. The presence of benzyladenine in the medium was necessary only during the first 2 days of culture (initiation period). The equilibrium between benzyladenine and its conjugates (the riboside, glucoside, and nucleotides) after a 4-day pulse was independent of the benzyladenine concentration whether it was inductive or noninductive for bud formation. The level of all derivatives was proportional to the benzyladenine concentration in the medium. Isopentenyladenine was used as a competitive inhibitor of benzyladenine conjugation. Isopentenyladenine concentrations that were too low for bud formation led to a synergistic increase in bud number when applied together with benzyladenine. Isopentenyladenine decreased benzyladenine uptake and conjugation. In spite of the lower uptake, the concentration of free benzyladenine inside the explants was higher in the presence of isopentenyladenine than in its absence whereas the concentration of the 7-glucoside of benzyladenine was lower. It was concluded that the free cytokinin base is the main active compound.
ABSTRACT
The competence of pedicel explants of tobacco (Nicotiana tabacum L. cv Samsun) to regenerate flower buds in response to auxin was manipulated by preincubating excised tissues in the absence of auxin. When exposed to 1 micromolar 1-naphthaleneacetic acid, these tissues formed fewer buds than controls that were not preincubated. The number of buds eventually formed correlated with the 1-naphthaleneacetic acid concentration in the tissue 6 hours after the start of hormone application. The internal concentrations in pretreated explants were lower than in tissues that were not pretreated due to diminished uptake per milligram fresh weight and increased hormone conjugation. The change in the developmental state induced by auxin deprivation had a dual effect on bud regeneration: (a) the pretreatment caused fewer buds to be formed at any 1-naphthaleneacetic acid concentration tested, and (b) a higher auxin concentration in the medium was required to get a maximum bud number on precultured explants. An increase of the 1-naphthaleneacetic acid concentration in the medium led to an elevated hormone level in freshly cut as well as in preincubated tissues. It was concluded that the developmental state of the tissue directly affects the maximum number of buds that can be regenerated. Apart from that there is an indirect effect exerted via modulation of the ratio between external and internal auxin concentration. The change in this ratio can be compensated for by an adjustment of the auxin concentration in the medium.
