The role of auxin and cytokinin in the control of morphogenesis in vitro in the plant families 'Rosaceae' and 'Ericaceae'
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The regulation of morphogenesis by exogenous auxin / cytokinin ratio was investigated in thirty ornamental species and cultivars of Rosaceae and Ericaceae. Shoot explants were cultured in vitro on nutrient medium containing 0 to 20 mg 1-1 indole-butyric acid (IBA), naphthalene-acetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), benzyladenine (BA), isopentenyladenine (2iP), or kinetin. Shoot, root and callus formation were recorded after four-week incubation periods. Exogenous cytokinin was necessary for shoot formation, and exogenous auxin for root formation, while callus formation occurred in the presence of exogenous auxin or cytokinin. BA induced the formation of many short shoots whereas 2iP induced fewer longer shoots probably due to 2iP oxidation and BA-stimulated ethene synthesis. Four different callus types formed in response to growth regulator applied, growth regulator concentration, light and temperature. Auxin-induced callus and root formation was promoted by darkness whereas cytokinin- promoted callus formation was promoted by light. Elimination of potassium from the medium increased root formation but decreased shoot and callus formation. Low medium pH promoted root and callus formation while a higher pH stimulated shoot formation. The presence of axillary buds and the shoot apex promoted root and callus formation but inhibited shoot formation. Cytokinin-enhanced respiration rate and ethene synthesis was related to caulogenesis, but auxin- promoted respiration rate and ethene synthesis was not correlated with rhizogenesis. Ethephon and ethene promoted simultaneous shoot and root formation but ethene was shown to be unlikely to act as an intermediate in all auxin- and cytokinin-induced differentiation. Repeated subculture of shoots resulted in an initial rise followed by a decline in caulogenesis, a decline in root formation and an increase in callus formation (apparently irreversible decline). Shoot length and leaf size decreased with repeated subculture but increased after auxin, gibberellin, or decreased illumination treatment. Auxin and cytokinin treatment could not override species and cultivar variation in morphogenesis.
Thesis, PhD Doctor of Philosophy
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