DROUGHT-INDUCED CHANGES IN SOIL CONTACT AND HYDRAULIC CONDUCTIVITY FOR ROOTS OF OPUNTIA-FICUS-INDICA WITH AND WITHOUT RHIZOSHEATHS

Water movement between roots and soil can be limited by incomplete roo t-soil contact, such as that caused by air gaps due to root shrinkage, and can also be influenced by rhizosheaths, composed of soil particle s bound together by root exudates and root hairs. The possible occurre nce of air gaps between the roots and the soil and their consequences for the hydraulic conductivity of the root-soil pathway were therefore investigated for the cactus Opuntia ficus-indica, which has two disti nct root regions: a younger, distal region where rhizosheaths occur, a nd an older, proximal region where roots are bare. Resin-embedded sect ions of roots in soil were examined microscopically to determine root- soil contact for container-grown plants kept moist for 21 days, kept m oist and vibrated to eliminate air gaps, droughted for 21 days, or dro ughted and vibrated. During drought, roots shrank radially by 30% and root-soil contact in the bare root region of nonvibrated containers wa s reduced from 81% to 31%. For the sheathed region, the hydraulic cond uctivity of the rhizosheath was the least limiting factor and the root hydraulic conductivity was the most limiting; for the bare root regio n, the hydraulic conductivity of the soil was the least limiting facto r and the hydraulic conductivity of the root-soil air gap was the most limiting. The rhizosheath, by virtually eliminating root-soil air gap s, facilitated water uptake in moist soil. In the bare root region, th e extremely low hydraulic conductivity of the root-soil air gap during drought helped limit water loss from roots to a drier soil.