15.1.3 Soil–plant hydraulic conductivity

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As soil dries, hydraulic conductivity of the soil–plant system declines due to several factors. The first is root and/or soil shrinkage, causing root and soil to pull away from each other, thereby reducing the hydraulic (i.e. wet) linkage between root and soil. Because of this loss of contact between soil and root, the apparent Lp of the soil–plant system declines (Passioura 1988). This is especially a problem in clay soils that expand and contract.

The second factor is a build up of solutes at the root–soil interface. With transpiration, soil solutes may accumulate at root surfaces. An osmotic gradient which is the reverse to that for water uptake results, and Lp appears to decrease.


Figure 15.4 Water flux across a root increases linearly (above a threshold value) as the driving force (pressure applied) increases. The slope of the relationship is the root hydraulic conductivity (Lp). Water stress and exogenous abscisic acid (ABA) reduce Lp. These data were obtained from a root system immersed in dilute Hoagland solution and therefore the decline in Lp, in water-stressed roots is not the result of root/soil shrinkage. Similarly, the decline in Lp is not likely to result from solute accumulation at the root surface because the rate of water flux across the root, and hence the potential for solute accumulation at the root surface, is lower in water-stressed or ABA-treated roots (Based on Eamus et al. 1996)

The third factor is a real change in root Lp. Hydraulic conductance does vary, and can be measured using a pressure chamber and excised roots. Water is forced across a root and out of the cut end of a detopped root. A plot of exudation rate (Jv) as a function of ΔP (the driving force) yields a series of straight lines (Figure 15.4) where slope = Lp (similar in concept to whole plants; Equation 15.1). This methodology has the benefit that roots in Hoagland solution can be used and therefore any potential for conflicting effects of loss of hydraulic continuity between soil and root due to soil/root shrinkage is removed. Furthermore the impact of temperature and plant hormones and water stress can be assessed experi-mentally (Eamus et al. 1996). From this and other earlier work by Fiscus, Markhart and co-workers, root Lp is known to be reduced by low temperatures and is affected by drought stress and abscisic acid, the phytohormone mediating some plant responses to drought stress (Figure 15.4).

The fourth factor concerns xylem emboli. If xylem tension becomes large enough due to a large decline in Ψleaf which is not satisfied by root uptake due to extremely low Ψsoil, xylem water columns may ‘snap’ under tension, generating gas-filled cavities and releasing sound energy that can be detected with a sensitive micro-phone appressed to the stem (Section 5.1). Whole-plant Lp will decline as vessels or affected tracheids cease to conduct water. Water supply to foliage is reduced, causing Ψleaf and stomatal conductance (gs) to decrease accordingly.


Figure 15.5 Leaf xylem water potential (Ψleaf) declines (becomes more negative) as hydraulic conductivity decreases due to an accumulation of emboli in xylem elements. Plants from the humid tropics suffer greater cavitation (more air-filled xylem elements) as Ψleaf  decreases compared to those from the semi-arid subtropics. Eucalyptus camaldulensis taken from the humid environment of Petford in North Queensland is more vulnerable to loss of xylem continuity than E. camaldulensis taken from the semi-arid environment of Tennant Creek, 1000 km north of Alice Springs. Cross-sectional area of xylem conduits is a likely cause because provenances adapted to semi-arid) environments have a preponderance of narrower conduits (Based on Franks et al. 1995)

Some plants sustain transpiration by minimising cavitation (Figure 15.5). Species found in environments where drought is a recurrent feature must have a considerably smaller vulnerability to cavitation than species found in more mesic environments and this can be brought about by having smaller-diameter vessels or tracheids as well as smaller pores in the pit membranes of those conduits (Section 5.1).

Xylem emboli and foliar phenology are related. Evergreen conifers exhibit the smallest loss of Lp in winter due to cavitation, a result consistent with the necessity for meeting transpirational demands very early in spring. In deciduous hardwood trees, as the magnitude of the loss of Lp in winter due to emboli increases, leaf flushing gets progressively later. Similarly, diffuse porous hardwoods are relatively resistant to cavitation during late-season drought and the date of their leaf fall is later than in ring-porous or semi-porous species (Wang et al. 1992).