Control of logarithmic growth rates of tobacco callus tissue by cytokinins.

The growth rates of tobacco callus tissues on media containing 10(-6) to 10 mum 6-(gamma,gamma-dimethylallylamino)purine (2iP) were measured. At concentrations of 10(-4) mum and above growth rates were exponential and dependent on cytokinin concentration. At 2iP concentrations of 10(-4) to 0.33 mum, the exponential rate was maintained for 4 to 5 doublings of fresh and dry weight. After this period a linear phase, resulting in approximately 1 doubling of weight, occurred. The growth of tissues on media containing higher than 0.33 mum 2iP was exponential for only about 15 days. At the end of this time, and well before they achieved half their final weight, they exhibited growth which was less rapid than logarithmic but more rapid than linear. Comparisons with zeatin, 6-benzylaminopurine and kinetin indicated that, although the maximum growth rates obtained with relatively high concentrations (0.1-1 mum) were similar, the naturally occurring cytokinins, 2iP and zeatin, promoted faster rates at lower concentrations (10(-3)-10(-2) mum) than did 6-benzylaminopurine and kinetin.