Water Transport in the Liana Bauhinia fassoglensis (Fabaceae)

Abstract

To determine the efficiency of xylem conductance in the liana (woody vine) Bauhinia fassoglensis Kotschy ex Schweinf., we measured hydraulic conductance per unit stem length (measured Kh), leaf-specific conductivity (LSC = Kh/distal leaf area), transpiration rate (E), xylem water potential (I), vessel number, and vessel diameter. The measured Kh was 49% (SE = 7%) of the predicted Kh from Poiseuille's law. The mean LSC for unbranched stem segments was 1.10 x 10-6 square meters per megapascal per second (SE = 0.07). LSCs were much lower (about 0.2) at branch junctions. At midday, with E at 7 x 10-6 meters per second, the measured drop in 'I was about 0.08 megapascal per meter along the stems and branches and about 0.27 megapascal in going from stem to leaf. In addition, there was a drop of about 0.20 megapascal at branch junctions as predicted by E/LSC. In diurnal measurements leaf ' never dropped below about -1.2 megapascal. For long (e.g. 16 meters) stems, the predicted midday drop in I through the xylem transport system might be great enough to have substantial physiological impact.draulic pathways is to measure Kh in isolated stem segments from throughout the shoot systems (8,11,12,27,28).Poiseuille's law for ideal capillaries can be used to relate vessel number and diameter to Kh by the following formula:predicted Kh = ir 2 d4/128 v (1) with K-in m4 MPa-' s-', i = dynamic viscosity of the fluid (MPa s), and the summation is over all conducting elements (diameter di) for the ith capillary in m.To determine how effective a stem is at supplying its leaves, we need to consider not only measured Kh and predicted Kh, but also the leaf area that the stem segment supplies and the E of the leaves (27). LSC is equal to measured Kh divided by the leaf area distal to the stem segment. LSCs have the advantage that they allow a theoretical basis for relating measured Kh to dp/dxs in shoot systems with long and complex transport pathways (19,(26)(27)(28): dp/dx = E/LSCThe long and slender stems of lianas (woody vines) are commonly thought to be extremely efficient in water transport due to their long and wide vessels (5, 6, 8, 15-17, 29, 30). However, although there is much evidence that liana vessels are wider than in closely related trees (5,8,10), there are few published data to indicate whether liana stems are particularly efficient at supplying their leaves with water.Xylem transport efficiency is often expressed as the measured hydraulic conductance per unit stem length (measured Kh2), which is defined as the xylem flow rate divided by the pressure gradient (dp/dx). When such measurements are made on intact plants, they are valid only when taken along unbranched portions of the plant axis that do not bear leaves. On branched systems and/or systems with different transport distances to each leaf, a single Kh measurement can be meaningless (18,26). Our approach to modeling the various hy-'

Analysis of anatomical and functional traits of xylem in Humulus lupulus L. stems

“…The values of measured Kh were on average only 14-26% of theoretically predicted Kh values based on Hagen-Poiseuille equation for ideal capillary. Similar results were found also in other studies (Ewers et al 1989, Martre et al 2000. There are three main factors that contribute to this discrepancy.…”

“…Leaf specific conductivity was determined as the ratio between measured Kh and leaf area distal from the segment (Ewers et al 1989). Huber value was calculated as ratio of conducting xylem area per supported leaf area (Tyree and Ewers 1991).…”

Xylem hydraulic characteristics of subtropical trees from contrasting habitats grown under identical environmental conditions

“…2), higher conductivities being associated with larger vessel diameters. This association is already well documented (Zimmermann, 1983 ;Calkin et al, 1985 ;Ewers et al, 1989) and is attributed to the Hagen-Poiseuille equation whereby conductance is related to the fourth power of the conduit radius (Zimmermann, 1983). The pattern of the response of hydraulic conductivites to environmental conditions seems to vary between species.…”

Hydraulic architecture of Monstera acuminata: evolutionary consequences of the hemiepiphytic growth form

“…Note the log scale of the y axis. Huber values increase exponentially from roots to stems ( Ewers et al (1989), and are based on leaf-specific conductivities (LSC) shown in Fig. 3.…”