inglés al español Ciencias Agricultura biochemistry

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Our suggestion of a bimodal distribution of rhizosphere hydraulic properties is partly an analogy to the observations of McCully and co-workers (Vermeer and McCully, 1982; Wang et al., 1991; Watt et al., 1994), who observed bimorphic behaviour for grass roots, in particular corn roots. They distinguished between sheathed roots and bare roots. The formation of sheaths around roots (rhizosheaths) is closely linked to the presence of mucilage, and sheathed roots are reported to show higher water content compared with bare roots. As roots develop, rhizosheath develops around young roots about 2 cm behind the tips, and sheath is lost as root age increases. Loss of soil sheath coincides with the maturation of late metaxylem vessels (Wang et al., 1991).


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 along the entire length of its young roots, except within 1.4 cm of the tip. The soil sheaths, which average 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 [sup 14]C pulse-labeling technique, 2% of newly fixed [sup 14]C-photosynthate was translocated into the roots at 3d, 6% at 9 d, and 8% at 15 d after labeling. The fraction of insoluble [sup 14]C in the roots increased twofold from 3 d to 15 d. Over the same time period, 6%-9% of the [sup 14]C translocated to the roots was exuded into the soil. The soluble [sup 14]C 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 increase for both sheathed and unsheathed roots as the soil water potential decreased form -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.