Modification of logarithmic growth rates of tobacco callus tissue by gibberellic Acid.

Logarithmic growth rates (either fresh or dry weight basis) of tobacco callus tissues grown on 10(-4) to 10(-1) mum cytokinin are increased if gibberellic acid (10(-3)-2 mum) is incorporated into the medium. At higher (1-10 mum) cytokinin concentrations gibberellic acid has little effect on growth rate but extends the duration of logarithmic growth. The gibberellic acid effect is noticeable only after one weight doubling, is dependent on concentration, and occurs when either glucose or sucrose is used as carbon source. The gibberellic acid response includes a decrease in percentage of dry weight relative to control tissues. The maximum dry weight yield, although achieved sooner than controls, does not differ appreciably from yields of tissue not treated with gibberellic acid.