Thiamine requirement in relation to cytokinin in "normal" and "mutant" strains of tobacco callus.

High concentrations of kinetin (400-2,000 μg/l) permit continuous growth of tobacco callus cultures (Nicotiana tabacum, var. Wisconsin No. 38) in the absence of exogenous thiamine. On the optimum concentration (1,000 μg/l) the tissue has been maintained through 21 bimonthly passages without change in vigor or other growth characteristics.The effect of kinetin is general, not "mutagenic", because tissue returned to low-kinetin, thiamine-free medium failed to grow.Kinetin-thiamine interactions in "cytokinin mutant" strains which were grown without cytokinin in light and darkness suggest that the endogenous content of cytokinins may markedly affect the requirement for thiamine and possibly the tissue content of this vitamin and other growth factors.The viability of tissue on low-kinetin media in enhanced by thiamine, but the addition of this vitamin does not eliminate the requirement for a cytokinin.The great divergence in minimum kinetin concentrations required for growth of the tissue in the presence and absence of thiamine indicates that the growth promoting action of cytokinin must be different in the two cases.