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

Summary

Earlier papers of this series relate to different growth-promoting substances and systems which, singly and in combination, have interacted with trace elements (Mn and Mo) and Fe to induce growth and to affect the metabolism of aseptic cultures of carrot. The solutes of cultured carrot cells (K⁺, Na⁺, Cl⁻, total solutes) are also affected. Two clones were grown in 9 combinations of growth factors and under 4 trace-element regimes (a complete complement including Fe, and this complement lacking either Mn or Mo, or both Mn and Mo), a total of 36 treatments under otherwise standardized experimental conditions. Under the treatments applied the number of cells varied over a 35fold range and their average size over a 7fold range; the concomitant effects on their solutes are expressed in terms of concentrations and of total content per cell. Both growth and the solutes accumulated were variously affected by carrot growth-promoting system I (mediated by inositol), by system II (mediated by IAA), and by coconut milk in the presence of Fe, with and without Mn, Mo, or Mn and Mo.

The greatest concentrations of total solutes occurred in tissue cultured in nutrient solutions which lacked the stimuli to rapid cell multiplication and were also limited by the trace elements Mn and Mo. Moreover, specific regulatory effects of the trace elements on solute content, not solely attributable to their effects on cell growth, have been noted. An imbalanced growth-factor regime (zeatin acting alone, i.e. without IAA) shifted the normal preference for K⁺ over Na⁺ strongly toward Na⁺, a trend which could also be induced by certain trace elements and more balanced growth-factor regimes, e.g. in a basal coconut milk medium lacking only Mn.

The data are interpreted in the context of views on the de-novo uptake of salts and solutes in cultured cells as they grow. These cells respond to a network, or matrix, of interacting factors by distinctive effects that are attributable to the component parts of the culture medium acting singly and in various combinations. These interactions (involving trace elements and exogenous growth factors) control growth (fresh weight, number and size of cells) and regulate the solutes (organic and inorganic; K⁺ vs. Na⁺; organic anions vs. Cl⁻) which the cells acquire as they grow and develop. The intensity of the response of the cultures to balanced, or imbalanced, growth factors creates the internal spaces accessible to solutes; and the metabolism, as it is also affected by growth factors and trace elements, determines how these spaces are to be filled at a given osmotic value. The evidence shows the range of factors that affect the accumulation of solutes in cells as they grow and is to be contrasted with conventional observations on mature cells held in steady states under conditions that preclude all growth and when only a single ionic species is followed over a very short interval of time.