EFFECT OF VARIOUS MEDIA ON GROWTH RESPONSES OF DIFFERENT CLONES OF CARROT EXPLANTS

The induction of growth in otherwise quiescent tissue explanted from carrot root has been investigated with reference to the effects of different kinds of growth-promoting substances added as supplements to a basal medium, singly and in combination. The effects of these media upon different clones of carrot explants are described. The idiosyncrasies of different clones of explants were detected by their responses measured by the incidence of cell division, the extent of cell enlargement, and by their nucleic acid content. The basal medium which contains salts, sugar, and vitamins supported only a minimal amount of growth; the basal medium supplemented with casein hydrolysate and coconut milk (10 per cent by volume) supported the highest level of growth obtained in any of the treatments tried. The active components of the coconut milk (AFcm) when refined required the further participation of either indole-3yl-acetic acid (IAA) or inositol, and were further stimulated by casein hydrolysate (CH). Thus the over-all stimulus of the coconut milk comprised two parts-now recognized as growth-promoting systems I and II, respectively. The effects of System I were mediated by appropriate combinations of inositol and the corresponding active growth-promoting factors (AF1) which were, in turn, represented by a purified factor previously isolated from Aesculus (AF2). System I induced both cell division and cell enlargement in balance, whereas System II stimulated internal cell division more than cell enlargement. The effects of System II were mediated by appropriate combinations of IAA and active growth-promoting factors (AF2), which were represented by the substance zeatin. The maximum growth of any given clone of carrot explants isolated from a given carrot root was only supported by exogenous requirements, over and above a basal nutrient medium, which meet its specific endogenous limitations. The paper shows how these limitations may be diagnosed, and discusses the over-all growth stimulus due to coconut milk in terms of the partial responses elicited by the known components of Systems I and II. © 1969 OXFORD UNIVERSITY PRESS.