An Agrobacterium-transformed root culture of Solanum tuberosum was grown in a 15-l bubble column. The specific respiration rate decreased by a factor of ten as the tissue grew over a as-day culture period. On days 5, 8, 13, and 21, respiration was shown to be independent of aeration rare over a range of 0.05-0.4 vvm (volume of air per volume of liquid min(-1)). Gas dispersion measured from argon tracer residence time distributions increased fourfold due to increased stagnation and channeling of gas through the bed of growing roots; however, introduction of an antifoam surfactant on day 20 greatly reduced dispersion with no accompanying change in respiration. Taken together, the gas dispersion and respiration studies suggest that the gas-liquid interface is not the dominant resistance to oxygen mass transfer. Liquid mixing time measured with a dye tracer increased from 1.45 +/- 0.45 min with no root tissue to 40.2 +/- 1.6 min with 180 g FW l(-1) of roots in the column. In addition, the oxygen uptake rate of growing tips (5.2 +/- 0.2 mm) of individual root segments of S. tuberosum measured in a stirred microcell (600 mu l) increased with the oxygen tension of the medium. Based on these results, the role of liquid mixing, gas-phase dispersion, and diffusion in the tissue in the scaleup of root culture is discussed.