Root meristem oxygen uptake, root tip extension rate, and specific growth rate are assessed as a function of dissolved oxygen level for three transformed root cultures. The influence of hydrodynamic boundary layer was considered for all measurements to permit correlation of oxygen-dependent kinetics with the concentration of oxygen at the surface of the root meristem. Oxygen uptake rate is shown to be saturated at ambient conditions, and a saturation level of approximately 300 mu mole O-2/(cm(3) tissue(.)hr) was observed for all three of these morphologically diverse root types. In nearly all cases, the observation of a minimum oxygen pressure, below which respiration, extension, or root growth would not occur, could be accounted for as a boundary layer mass transfer resistance. The critical oxygen pressure below which respiration declines is below saturated ambient oxygen conditions. In contrast, critical oxygen pressures for root tip extension were much higher; extension was nearly linear for the two thicker root types (Hyoscyamus muticus, henbain; Solanum tuberosum, potato) above ambient oxygen levels. The performance of the thinnest root, Brassica juncea (Indian mustard) was consistent with reduced internal limitations for oxygen transport. Extension rates did not correlate with biomass accumulation. The fastest growing henbain culture (mu = 0.44 day(-1)) displayed the slowest extension rate (0.16 mm/hr), and the slowest growing mustard culture (mu = 0.22 day(-1)) had the fastest tip extension rate (0.3 mm/hr). This apparent paradox is explained in terms of root branching patterns, where the root branching ratio is shown to be dependent upon the oxygen-limited mersitem extension rate. The implications of these observations on the performance of root culture in bioreactors is discussed.