Investigations on the growth and metabolism of cultured explants of Daucus carota

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Abstract
The responses of carrot explants to various growth-promoting agents and to certain trace elements with which they interact have been investigated. A great range in the metabolic behavior of the tissue may be brought about in this way. The responses to the exogenously applied substances are described in terms of the growth of the carrot explants in fresh weight and number of cells and also in terms of their metabolism, as shown by the final content and composition of the non-protein N compounds, by the relations between protein and non-protein (alcohol-soluble) N and by the content of nucleic acid in the cultured tissue. The growth-promoting agents employed consisted of (1) the balanced complex of factors found in coconut milk, (2) an active isolate from Aesculus (AFaesc), which is one of a class of growth factors (AF1) that interact with inositol (AF1+inositol) and which in this sense comprise growth-promoting System I, (3) the substance zeatin (Zeat) which is typical of a class of active factors (AF2) that interact with indoleacetic acid (AF2+IAA) and which, therefore, function as a growth promoting complex termed System II in the culture of carrot tissue. The carrot explants stimulated by coconut milk grew better than those stimulated by the other combinations of growth factors and they converted their soluble N more effectively to protein. The growth, whether it was induced by coconut milk or by System I or II, and other specific effects attributable to the growth factors employed were markedly affected also by the elements iron and molybdenum. The carrot explants that had responded to coconut milk emphasized alanine in their soluble, non-protein, nitrogenous pool, whereas those subjected to the active components of System I or of System II as clearly emphasized glutamine as the prominent non-protein, nitrogen-rich compound. The partial effects due to the component parts of System I (AFaesc or inositol) and to the component parts of System II (Zeat. or IAA), as these interacted also with iron and molybdenum in an otherwise trace element free basal medium (B **), revealed a pattern of interlocking effects, due to trace elements and to growth factors, upon the metabolism (especially the nitrogen metabolism) of the aseptically cultured carrot explants. These effects show that the individual growth factors do not act alone and that their implications are far reaching. The interactions between growth promoting Systems I and II and their component parts, with each other, with various environmental factors, and especially with trace elements constitute a network, or a matrix, of parameters that will merit further investigation to reveal all that is required to control the growth and metabolism of carrot cells.