The cytokinin content of root exudate of sunflowers increases during the exponential growth phase of the plants. The concentration of cytokinins drops, however, by a factor of ten when the plants have reached their final size. The reduced supply of cytokinins from the root to the shoot is regarded as one of the factors bringing about shoot senescence.

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Symphytum officinale L cells were grown in Erlenmeyer flasks at four different temperatures: 15, 20, 25, and 30°C. A mathematical model of the culture growth is presented. The intracellular and extracellular products are considered in separate equations. An interrelation between fresh weight, dry weight, and viability is considered in the balances. The model includes a description of the changes in time of wet and dry biomass, cell viability, substrate concentration and polysaccharide concentration, both intra‐ and extracellular. The model was tested by fitting the numerical results to the data obtained. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 57: 732–740, 1998