Soil sheaths incorporating aggregated soil particles surround young roots of many species, but the effects of such sheaths on water movement between roots and the soil are largely unknown. The quantity and location of root exudates associated with soil sheath formation and root water loss were therefore examined for Opuntia ficus-indica, which has prominent soil sheaths along the entire length of its young roots, except within 1.4 cm of the tip. The soil sheaths, which averaged 0.7 mm in thickness, were composed of soil particles and root hairs, both of which were covered with exuded mucilaginous material. As determined with a 14C pulse-labeling technique, 2% of newly fixed 14C-photosynthate was translocated into the roots at 3 d, 6% at 9 d, and 8% at 15 d after labeling. The fraction of insoluble 14C in the roots increased twofold from 3 d to 15 d. Over the same time period, 6%-9% of the 14C translocated to the roots was exuded into the soil. The soluble 14C compounds exuded into the soil were greater in the 3-cm segment at the root tip than elsewhere along the root, whereas mucilage was exuded relatively uniformly along roots 15 cm in length. The volumetric efflux of water increased for both sheathed and unsheathed roots as the soil water potential decreased from -0.1 MPa to -1.0 MPa. The efflux rate was greater for unsheathed roots than for sheathed roots, which were more turgid and had a higher water potential, especially at lower soil water potentials. During drying, soil particles in the sheaths aggregate more tightly, making the sheaths less permeable to water and possibly creating air gaps. The soil sheaths of O. ficus-indica thus reduce water loss from the roots to a drying soil.
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Huang et al. (1993) studied this question.