Inhibitory role of root hairs on transport within root culture bioreactors.

An experimental system was developed to produce root cultures of Hyoscyamus muticus with and without the profuse root hairs. Growth in the presence of 7.6 microM pyrene butyric acid (PBA) and 2.2 mM phosphate virtually eliminated root hairs, whereas growth rate, general morphology and nutrient yields remained unchanged in well-mixed flask culture. These root cultures were used to demonstrate decreased flow resistance in a tubular reactor as a result of root hair removal. To assess the impact on bioreactor performance, hairy and hairless root cultures were grown in a highly characterized 15-L bubble column bioreactor. In the absence of root hairs, the mixing was greatly enhanced; mixing times became shorter for the hairless culture at roughly 100 g (fresh weight)/L. By the end of the 3-week culture period, the mixing time of the hairy culture was 29 times longer than that of the hairless culture. The growth rate of the hairless culture in the bioreactor was as much as 2.4 times greater than growth of the hairy culture under the same conditions. The improved reactor performance was reflected in greater biomass accumulation and respiratory activity. These results show that the root hairs-which facilitate nutrient uptake in a static soil environment-are detrimental to growth in a liquid environment as an effect of their stagnating fluid flow and limiting oxygen availability.