The use of cultivars of Raphanus sativus for cytokinin bioassay

Six cultivars of radish (<i>Raphanus sativus</i>) were tested for their usefulness in radish cytokinin bioassay by the method of <i>Letham</i> (1971). The best cultivar was found to be 'Sopel Lodu' which responds well to both zeatin and 2iP over a wide range of concentrations. The fresh weight of cotyledons increased at most by 71.5% (if treated with zeatin) or 101.0% (if treated with 2iP) compared to untreated cotyledons. This cultivar is also sensitive to the partially purified cytokinin-like fraction isolated from the pine (<i>Pinus silvestris</i>) cambial region. The cultivar 'Sopel Lodu' is therefore proposed to be a suitable plant for cytokinin bioassays.

Cytokinin metabolism in Narcissus bulbs: chilling promotes acetylation of zeatin riboside

The levels of the cytokinins zeatin, dihydrozeatin and N6-(isopent-2-enyl)adenine and the corresponding 9-ribosides were determined in whole Narcissus pseudonarcissus L. bulbs after onset of summer dormancy, and after storage of the dormant bulbs at 4 or 15°C under moist conditions. The cytokinin level was increased at both temperatures; the dominant cytokinin at 15°C was zeatin riboside but at 4°C the free base, zeatin, was the principal cytokinin. The metabolism of 3H-labelled zeatin, zeatin riboside and 6-benzylaminopurine was studied in scales and base plates excised from bulbs previously held at 4 or 15°C. The chilling treatment promoted conversion of [3H]zeatin riboside to O-acetylzeatin riboside (an unusual cytokinin metabolite) in the excised base plate and inhibited 9-riboside formation from 6-benzylaminopurine. Chilling also promoted formation of O-acetylzeatin riboside from [3H]zeatin in excised scale tissue. Endogenous O-acetylzeatin and O-acetylzeatin riboside were concentrated in the base plate. Relative to zeatin riboside, O-acetylzeatin riboside was degraded to adenosine and related compounds less rapidly in the leaf cluster. Since acetylation of the zeatin side chain enhances activity in cytokinin bioassays, O-acetylzeatin riboside may play a role in leaf development following chilling of Narcissus bulbs.

PLANT GROWTH REGULATOR ACTIVITY OF SOLUBLE HUMIC COMPLEXES

Auxin and cytokinin bioassays were performed to test the biological activity of soluble humic complexes (SHC). "Pea split stem curvature" (auxin test) and "cucumber" tests (cytokinin test) did not show any quantifiable biological activity of SHC. On the contrary, "cress test" and "senescence test" offered good evidence of a hormonal-like activity 100 times lower than that of IAA (10 mg IAA∙g−1 SHC) and 10 times lower than that of N6 BA (100 mg N6 BA∙g−1 SHC). At a high concentration of SHC, toxic effects were evident, indicating the presence of inhibitory substances which counteracted the hormone-like activity of humic complexes. Key words: Plant growth regulators, soluble humic complexes