Root hairiness: effect on fluid flow and oxygen transfer in hairy root cultures

The effect of root hairiness on fluid flow and oxygen transfer in hairy roo t cultures was investigated using wild-type, transgenic and root-hair mutan ts of Arabidopsis thaliana. The root hair morphologies of the A. thaliana l ines were hairless, short hairs, moderately hairy (wild-type) and excessive ly hairy, and these morphologies were maintained after transformation of se edlings with Agrobacterium rhizogenes. Filtration experiments were used to determine the permeability of packed beds of roots; permeability declined s ignificantly with increasing root hairiness as well as with increasing biom ass density. Hairy roots of wild-type ii. thaliana grew fastest with a doub ling time of 6.9 days, but the hairless roots exhibited the highest specifi c oxygen uptake rate. In experiments using a gradientless packed bed reacto r with medium recirculation, the liquid velocity required to eliminate exte rnal mass transfer boundary layer effects increased with increasing root ha iriness, reflecting the greater tendency towards liquid stagnation near the surface of roots covered with hairs. External critical oxygen tensions als o increased with increasing root hairiness, ranging from 50% air saturation for hairless roots to ca. 150% air saturation for roots with excessive roo t hairs. These results are consistent with root hairs providing a significa nt additional resistance to oxygen transfer to the roots, indicating that v ery hairy roots are more likely than hairless roots to become oxygen-limite d in culture. This investigation demonstrates that root hairiness is an imp ortant biological parameter affecting the performance of root cultures and suggests that control over root hair formation, either by use of geneticall y modified plant lines or manipulation of culture conditions, is desirable in large-scale hairy root systems. (C) 2000 Elsevier Science B.V. All right s reserved.