HYDRAULIC CONDUCTANCES OF THE SOIL, THE ROOT SOIL AIR GAP, AND THE ROOT - CHANGES FOR DESERT SUCCULENTS IN DRYING SOIL

Water movement to and from a root depends on the soil hydraulic conductivity coefficient (L(soil)), the distance across any root-soil air gap, and the hydraulic conductivity coefficient of the root (L(P)). After analytical equations for the effective conductance of each part of the pathway are developed, the influences of soil drying on the soil water potential and L(soil) are described during a 30 d period for a loamy sand in the field. The influences of soil drying on L(P) for three desert succulents, Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica, are also described for a 30 d period. To quantify the effects of soil drying on the development of a root-soil air gap, diameters of 6-week-old roots of the three species were determined at constant water vapour potentials of -1.0 MPa and -10 MPa as well as with the water vapour potential decreasing at the same rate as soil drying during a 30 d period. The shrinkage observed for roots initially 2.0 mm in diameter averaged 19% during the 30 d period. The predominant limiting factor for water movement was L(P) of the root for the first 7 d of soil drying, the root-soil air gap for the next 13 d, and L(soil) thereafter. Compared with the ease of water uptake from a wet soil, the decrease in conductances during soil drying, especially the decrease in L(soil), caused the overall conductance to decrease by 3 x 10(3)-fold during the 30 d period for the three species considered, so relatively little water was lost to the dry soil. Such rectifier-like behaviour of water movement in the soil-root system resulted primarily from changes in L(soil) and, presumably, is a general phenomenon among plants, preventing water loss during drought but facilitating water uptake after rainfall.