Hydraulic Conductances of the Soil, the Root-Soil Air Gap, and the Root: Changes for Desert Succulents in Drying Soil

Abstract

Water movement to and from a root depends on the soil hydraulic conductivity coefficient (L^n), the distance across any rootsoil air gap, and the hydraulic conductivity coefficient of the root (Lp). 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 Kll are described during a 30 d period for a loamy sand in the field. The influences of soil drying on Lp 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 20mm in diameter averaged 19% during the 30d period. The predominant limiting factor for water movement was L r of the root for the first 7 d of soil drying, the root-soil air gap for the next 13 d, and L Kll thereafter. Compared with the ease of water uptake from a wet soil, the decrease in conductances during soil drying, especially the decrease in L Kil , caused the overall conductance to decrease by 3 x lO^-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^n and, presumably, is a general phenomenon among plants, preventing water loss during drought but facilitating water uptake after rainfall.

Arbuscular mycorrhizae and soil/plant water relations

“…In addition to the possibility of direct hyphal contributions to root water uptake (Hardie 1985;Faber et al 1991;Ruiz-Lozano and Azcón 1995), soil hyphae may increase soil-to-root contact in drying soils (e.g., Reid 1979;Fitter 1985;Davies et al 1992). Root and soil shrinkage creates gaps between the root and the soil, which can decrease water absorption (e.g., Nobel and Cui 1992). Root hairs can help prevent air gaps at the soil-root interface, as they grow into very small pores and effectively "glue" themselves to soil particles with exuded mucilages (Klepper 1990).…”

Do roots mind the gap?

“…Few studies have simultaneously measured gap formation, transpiration rate, and soil and plant water status. Important work has been performed by Nobel and co-workers (Nobel and Cui 1992;North and Nobel 1997). Nobel and Cui (1992) measured the hydraulic conductance of soil and roots of a desert succulent and they compared them to the conductance of an air-filled gap, assuming water flow only via the vapor phase.…”

“…Nobel and Cui (1992) solved this difficulty by separately measuring each component of the soil-plant continuum and then piecing them together as a flow in series. However, in such a way, rhizosphere processes and gradients in water potential towards the root, which are expected to become increasingly important as soil dries, cannot be included.…”

Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

“…The root system often represents the primary limitation on water movement along the soil-to-leaf continuum (Nobel & Cui 1992). Typically, 50% or more of the total resistance to water flow occurs below-ground despite the tendency for xylem conduit diameters to be largest in roots (Passioura 1984).…”