Investigations on the growth and metabolism of cultured explants of Daucus carota : IV. Effects of iron, molybdenum and the components of growth promoting systems and their interactions.

The induction and maintenance of growth in small, standard explants of carrot root exposed to a trace-element-free basal nutrient medium (B (**)) have been investigated. The organic growth factors that induce the growth are complex as represented by a trace-element-limited coconut-milk preparation (CM(**)) or the component parts of distinct growth promoting systems mediated by inositol or by 3-indoleacetic acid (IAA). In System I inositol interacts with growth factors from Aesculus, of which a known example is an IAA-rhamnoseglucose compound; in System II IAA interacts with adenyl compounds, of which zeatin is a known example. But the organic growth-promoting substances also interact with trace elements; therefore, the effects of the component parts of the growth promoting systems, separately and in combination, have been investigated with respect to their interactions with Mo, with Fe and also with these trace elements in combination. The growth so induced has been measured in terms of the fresh weight of the explants, the number and average size of their cells, as well as their total content of protein and nucleic acids. The metabolic responses of the cultured explants to the combined effects of growth factors and of trace elements are also described in terms of the principal soluble nitrogenous compounds. Each basis is informative, and suitable graphical devices are adopted so that the interactions of the multivariate factors that affect the behavior of the cultured tissue may be seen with respect to the various parameters selected. The study shows the range of complexity to be understood before the exogenous factors that determine cell growth and metabolism are both known and controllable; it also shows the limitations when attention is directed, simplistically, to one parameter or to the controlling influence of any single factor, or class, of growth factors.