Water transport from soil to leaves is critical to land-plant activity. By replacing the water lost to the atmosphere via transpiration, water transport prevents highly negative and damaging leaf water potentials from developing, and permits continued photosynthesis. The efficiency of water transport is quantified as the stem-specific xylem hydraulic conductivity (KS), the rate of water flow per unit cross-sectional xylem area and per unit gradient of pressure (kg m–1 s–1 MPa–1). It can also be quantified as the LA-specific xylem hydraulic conductivity (KL), the rate of water flow per unit of supported LA per unit pressure gradient (kg m–1 s–1 MPa–1). KL expresses the conductivity of a branch, relative to the transpiration demand of the foliage that the branch services. Xylem refers here to the conducting tissue in the stem and, for trees, equates to sapwood. KL equals KS, divided by the ratio between LA and sapwood cross-sectional area (see Section 2.8). KS is a function of the numbers and conductivity of individual xylem conduits and their interconnections via pit membranes, per unit of xylem cross-sectional area. Conductivity of an individual conduit increases with the fourth power of its lumen diameter (as can be modelled using Poiseuille’s law of flow in cylindrical tubes).
What and how to collect
The measurement may conveniently be made indoors on stem samples brought in from the field, provided they are kept cool and with their ends in water. The tested branches should be longer than the length of the longest conducting element in the xylem, so that the measurement includes the resistivity of both the conduits and the inter-conduit connections. Conduit length varies greatly across species, gymnosperms having short tracheids (usually less than 1 cm long), whereas in some angiosperms, vessel length can reach more than a metre (although it is usually 10–30 cm, see Point 1 of Special cases or extras in the present Section). Segments ~30 cm long are commonly used. KS typically declines towards the leaves because of tapering of vessel numbers and sizes. In most cases, measurements are made on stems collected from the outer canopy, and so they can be considered the minimum conductivity for the stem part of the water-transport system.
These methods have been developed largely for the stems of woody plants, for which the methods are simplest. Analogous methods have, however, been devised for herbaceous plants, leaves and roots. Relatively sophisticated types of apparatus for performing xylem-conductivity measurements have been described; however, in many cases, simple systems built from ordinary laboratory equipment can be used. In the simplest case, a known pressure head is applied to push a 10 mM KCl solution (use filtered, degassed water) through a stem with a known cross-sectional area and no (or with sealed-off) side branches. At the distal end of the segment, a collecting container catches emerging liquid, and after known time intervals, its volume is determined either directly or gravimetrically. Conductivity is then calculated as
KS = J × L × A–1 × ΔΨ–1,
where J is the rate of water flow through the stem (kg s–1), L is the length of the segment (m), A is the mean cross-sectional area of the xylem of the stem (m2) (to a first approximation, the average of the areas at the two ends of the segment), and ΔΨ is the pressure difference (MPa) between the upstream and downstream ends of the segment. If the stem segment is held vertically during the measurement, its length (in m) divided by 10 should be added to the applied pressure (MPa) (but not if the segment is horizontal). KS is typically reported in kg m–1 s–1 MPa–1. With measurements on terminal or subterminal branch segments, one can usually assume that A is the entire xylem cross-sectional area (all of the xylem being conductive); however, with larger branches or tree trunks, A would have to be the conducting xylem area, at a maximum, the sapwood area; however, often an even smaller area is actually conductive, which is not easy to determine for routine measurements.
Xylem conductance refers to the capacity of a vascular system, with whatever length and cross-sectional area it happens to have, to transport water under a unit pressure difference. Conductance can be calculated from the above equation by simply omitting its A and L terms.
Special cases or extras
(1) Length of vessels. This is needed to ensure that the length of stem segments used for conductance measurements exceeds that of their vessels. To determine the maximum length of the vessels, cut several stems in the field (see Fig. 7 for further details on the procedure). In the laboratory, from the upper end of one of these, remove a portion such that the segment that remains is likely to be a little longer than the length of its longest vessel. Proceed as indicated in Fig. 7.
References on theory, significance and large datasets: Zimmermann and Jeje (1981); Brodribb and Hill (2000); Meinzer et al. (2001); Zwieniecki et al. (2001); Tyree and Zimmermann (2002); Sperry (2003); Cavender-Bares et al. (2004); Maherali et al. (2004); Santiago et al. (2004); Holbrook and Zwieniecki (2005); Sperry et al. (2008a).
References on methods: Sperry et al. (1988); North and Noble (1992); Alder et al. (1996); Kocacinar and Sage (2003); Sack and Holbrook (2006).