Annals of Botany 33: 615-646, 1969
© 1969 Annals of Botany Company
RESEARCH-ARTICLE |
Endogenous Characteristics of Different Clones of Carrot Explants and their Exogenous Requirements for Growth
Laboratory of Cell Physiology and Growth, Cornell University Ithaca, N. Y.
Accepted: 9 December 1968
The first part of the paper focuses attention upon the different growth responses observed when standard carrot phloem explants are stimulated to grow by various supplements to a basal nutrient medium. These supplements were inositol and active fraction (AF1) which represent growth-promoting System I, and indole-3yl-acetic acid and another type of active fraction (AF2) which represent System II. AF1 was an isolate from Aesculus woerlitzensis, designated AFaesc; and AF2 was the substance zeatin. The over-all stimulus to growth due to whole coconut milk, which comprises effects due to both Systems I and II, exceeded the maximum response due to System I and System II separately or in combination. The growth of explants drawn from some 70 clones to the component parts of Systems I and II are presented and classified in terms of certain typical situations.
The second part of the paper is concerned with the relationships between the growth responses of the carrot tissue to the component parts of growth-promoting Systems I and II and that due to the whole complex of growth-promoting substances in coconut milk. When growth was measured in terms of fresh weight (mg per explant after 18 days) there was a direct relationship between the growth that occurred in response to whole coconut milk and that stimulated by System I and by inositol alone. However, the data also show that the response to coconut milk was inversely related to the partial response of the tissue to System II and to indole-3yl-acetic acid alone. The suggestion is that System I promotes both cell division and a measure of cell expansion in appropriate sequence, whereas System II preferentially promotes cell division. The indirect growth responses of explants from a wide selection of carrot clones as affected by coconut milk and the components of Systems I and II reflect the endogenous limitations and the exogenous requirements of the different clones of carrot tissue. These relationships are discussed and interpreted in terms of concepts already developed, and their significance is indicated for the interpretation of growth induction in otherwise mature, quiescent cells and for the bioassay of growth-promoting substances. Against this background the factors which progressively limit further growth of the tissue during its development are discussed.