Differentiation of roots
Rhizoids are the most frequent organoids differentiated in ginseng callus cultures (Fig. 8). They occur in calli of different origin.
The formation of rhizoids is influenced by exogenous plant growth regulators and light/dark conditions. Plant growth factors affect especially the beginning of rhizogenesis. The shortest induction time for initiation of rhizoid formation has been observed under the influence of NAA or IBA with kinetin or IBA alone (Furuya et al., 1986, Cellarova et al., 1992). In the dark the rhizoids were formed continuously without any loss of morphogenetic potential during long term cultivation. Rhizoids elongated vigorously without branching in response to NAA. A combination of IBA and kinetin resulted in elongation as well as in the formation of branch roots and subsequent differentiation of the former rhizoids into callus that gave rise to new rhizoids.
Morphology of rhizoids differentiated in vitro was studied by Cellarova et al. (1992). Usually they are about 2 cm long and form white hairy protuberances that resemble the adventitious roots often differentiated in callus cultures of
any other plant species. Rhizoids arose from a small group of mitotically active cells present in callus cultures. Usually the conical apical part of a rhizoid comprises a centre of dividing cells. The epidermis is composed of a monolayer of small and closely connected cells, although a typical root cap has not been observed. The presence of thin walled cells around the apical part of rhizoids predicts their protective and absorptive function. Ground tissues in the cortex and a central cylinder are formed by iso-diametric parenchymatous cells and small intercellular spaces. Parenchymatous tissue sometimes contains storage elements such as starch grains. Conductive tissues in the central cylinder are reduced. They form a continual conductive system along the whole rhizoid and connect with tracheal elements in callus cultures.
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