Xylem Hydraulics and the Soil–Plant–Atmosphere Continuum: Opportunities and Unresolved Issues

about the hydraulic properties of the plant vascular system. The traditional view of plant hydraulics being domSoil and xylem are similar hydraulically. An unsaturated conductivinated by resistances of endodermis and root cortex ity curve for soil is called a vulnerability curve for xylem—but the (Boyer, 1985; Philip, 1966) is expanding to include well underlying physical basis is the same. Thus, any transport model that documented and surprisingly dynamic responses of xytreats unsaturated soil conductivity would benefit by also incorporating the analogous xylem vulnerability curves. This is especially the lem flow resistance to environment. The behavior of case for crop plants, which as a group have relatively vulnerable xylem. water in soil and xylem is strikingly identical, making it Although the cohesion–tension mechanism for xylem transport has possible to model xylem flow with the same quantitative withstood recent challenges, a number of gaps remain in our underprecision as soil flow. The result is an opportunity for standing of xylem hydraulics. These include the extent and mechanism SPAC models to incorporate more mechanistic and preof cavitation reversal and thus hysteresis in the vulnerability curve, dictive treatment of plant hydraulics and a better underthe structural basis for differences in air entry pressure ( cavitation standing of how the SPAC is influenced by drought pressure) for different xylem types, a quantitative model of xylem cycles. conductivity, and a mechanistic understanding of how stomata reguBeyond this immediate opportunity, there are several late plant water status. Improving the representation of xylem hydrauunresolved issues involving xylem transport. The validlics in models of crop water use is necessary to achieve a mechanistic link between soil water availability and canopy water use. An impority of the cohesion–tension mechanism with its seemtant additional knowledge gap concerns the hydraulics of the living ingly counterintuitive prediction of negative water prestissues of absorbing roots and transpiring leaves, which are more sure comes under perennial scrutiny. Xylem cavitation complex than in xylem and less amenable to mechanistic modeling and its reversal can be readily documented, but in many at present. cases, the underlying mechanisms and linkage to xylem structure are poorly known. Quantitative models linking xylem conductivity to structure have struggled to move A understanding of the hydraulic resistances in beyond the Hagen–Poiseuille equation, hampering the soil and plant is fundamental to any treatment of ability to quantify the role of pits and ionic effects in the soil–plant–atmosphere continuum (SPAC). Without determining the hydraulic conductance of xylem. The knowing these resistances and how they change with stomatal response to changes in xylem and plant conducsoil and plant water content, we cannot understand and tance is well characterized, but the mechanism remains predict the response of plant water use to environment. unclear. Finally, although outside the main focus of this Traditionally, the treatment of soil resistances in transreview, our understanding of the hydraulics of water port models has been much more mechanistically based transport across the living tissues of the root and leaf and complete than the corresponding description of remains limited. Although this pathway is infinitesimally plant hydraulics. Although complex, the physical nature short compared with the xylem flow path, it creates of flow through soil makes it more amenable to quantitasignificant resistance to flow, which cannot at present be tive treatment than flow through the plant. Neverthemodeled at the same mechanistic level as soil and xylem. less, plant resistance dominates the total hydraulic resisWith this review, we briefly document the importance tance of the continuum under moist soil conditions of xylem hydraulics and how it can be usefully incorpo(Boyer, 1985; Gardner, 1965). Even under dry condirated in SPAC models before addressing the unresolved tions when soil resistance increases (Jury et al., 1991), issues touched on in the preceding paragraph. A final the plant resistance also increases—thus continuing to section briefly considers analogous gaps in our knowlexert a major influence on water movement even during edge of the hydraulics of nonxylary tissues of plants. drought (Blizzard and Boyer, 1980; Nobel, 1994; Sperry et al., 1998). However, the basis for changes in plant XYLEM HYDRAULICS AND hydraulic resistance through drought cycles has typically SOIL–PLANT–ATMOSPHERE not been handled in a mechanistic manner in many CONTINUUM MODELS models of the continuum. The sophistication with which plant resistances are On a per-unit length and area basis, xylem hydraulic handled in SPAC models is improving as we learn more conductivity (volume flow rate per pressure gradient per cross-sectional area) is roughly eight orders of magnitude greater than the corresponding conductivity of Biol. Dep., Univ. of Utah, 257 S, 1400E, Salt Lake City, UT 84112. root cortex and endodermis (Sperry et al., 2002b; Steudle Financial support from USAID and NSF (IBN-0112213). Received and Peterson, 1998). This creates the impression that 23 July 2002. *Corresponding author ([email protected]). Published in Agron. J. 95:1362–1370 (2003).  American Society of Agronomy Abbreviations: cryo-SEM, freezing-stage scanning electron microscopy; SPAC, soil–plant–atmosphere continuum; , water potential. 677 S. Segoe Rd., Madison, WI 53711 USA