Box 1 Theory of hydraulic limitations
Coordinating stomatal and xylem functioning – an evolutionary perspective
Article first published online: 13 MAY 2004
Volume 162, Issue 3, pages 568–570, June 2004
How to Cite
Sperry, J. S. (2004), Coordinating stomatal and xylem functioning – an evolutionary perspective. New Phytologist, 162: 568–570. doi: 10.1111/j.1469-8137.2004.01072.x
If one could make a plant transpire at any rate E, the trajectory of E vs leaf water potential (Ψ) should look something like Fig. 1 (solid curve; Sperry et al., 2002). When E is zero, the leaf Ψ would equal the bulk soil Ψ (ignoring gravitational effects). As E is increased the Ψ will drop disproportionally because the hydraulic conductance of the flow path declines with increasingly negative Ψ. There are two well understood reasons for this decline: cavitation in the xylem, and soil drying in the rhizosphere between bulk soil and the root surface. Although there may be additional changes in hydraulic conductance with E, such as variable aquaporin activity in root or leaf membranes (Henzler et al., 1999), or variable KCl concentration in xylem sap (Zwieniecki et al., 2001), the Ψ-dependence of these factors is not well characterized, as opposed to the inevitable physical processes of rhizosphere drying and xylem cavitation.
The E vs Ψ trajectory cannot go to infinity, but has a maximum steady-state value of E: Ecrit (Fig. 1, open symbols) with an associated Ψcrit. Any higher steady-state rate of E is impossible, because the drop in pressure drives the remaining hydraulic conductance in the bulk soil-leaf pathway to zero, breaking apart the hydraulic continuum. The critical values of E and Ψ describe a physical boundary to gas exchange with respect to soil and plant hydraulics. Transpiration and plant Ψ must be regulated to stay within these physical limits or else canopy desiccation will occur. A drier soil will have a lower Ψ intercept and a flatter E vs Ψ trajectory with a lower Ecrit (Figure 1, dashed curve). Maximizing gas exchange while avoiding hydraulic failure means operating on the edge of dysfunction, and requires rapid stomatal control of E to prevent it from exceeding Ecrit.
Under many situations, xylem cavitation is more limiting than rhizosphere drying. In these cases, Ψcrit approximates the Ψ causing 100% loss of hydraulic conductance (Fig. 1, Ψcrit≈Ψ100). Brodribb & Holbrook estimate safety factors from hydraulic failure by measuring the leaf Ψ in detached leaves at full stomatal closure. This Ψ represents the lowest leaf Ψ permitted by stomatal regulation (vertical Ψmin≈Ψclosure line). The difference (Ψmin–Ψcrit) gives the safety margin in terms of leaf Ψ (Fig. 1, safety margin). Seedless vascular plants had broader safety margins than angiosperms from the same habitat, as shown diagrammatically in Fig. 1. The difference was primarily due to a less negative Ψclosure in the seedless vascular plants (Fig. 1P) vs angiosperms (Fig. 1A). The vulnerabilities to cavitation (and hence Ψcrit) did not differ systematically between the two groups. Note that Brodribb & Holbrook report safety margins slightly differently – as (Ψclosure–Ψ50), where Ψ50 is the leaf Ψ at 50% loss of conductivity.
- Issue published online: 13 MAY 2004
- Article first published online: 13 MAY 2004
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