A method for measuring hydraulic conductivity and embolism in xylem

Hydraulic conductivity of the xylem is computed as the quotient of mass fiow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of timedependent reductions in fiow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh, stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m~^ oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after fiow-impeding air emboli have been removed by repeated high-pressure (175 kPa) fiushes. Correlation between microbial contatnination and declining conductivity suggests that long-tenn (>4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (<4h) measurements may be caused by movements of air emboli in vessels and/or particulate matter. Key-words: xylem hydraulic conductivity; xylem cavitatioti; xylem embolism; sugar maple; Aeer saeehariim. Introduction Measurement of xylem hydraulic conductivity has formed the basis for studies on fiuid mechanics of xylem conduits (e.g. Giordano, Salleo & Wanderlingh, 1978; Siau, 1984; Jeje, 1985; Calkin, Gibson & Nobel, 1986), distribution and origin of resistances and pressure gradients in the water transport system (e.g. Dimond, 1966; Eiscus, Parsons & Albertc, 1973; Zimmcrmann, 1978; Tyree et al,, 1983; Ewers, 1985), pathogen modes of action (e.g. Dimond, 1970; Suhayda & Goodman, 1981; Van Alfen et aL, 1983; Newbanks, Bosch & Zimmermann, 1983), and methods of wood preservation (e.g. Anderson, Gortncr & Schmitz, 1941). Hydraulic conductivity is typically measured on excised xyletn samples, i.e. cither dowels split from the wood or lengths of the plant axis. These are attached to a hydraulic system for measuring the pressure difference (AP) of a fiuid (usually water) Correspondence: Dr John S. Sperry, Department of Botany, University of Vermont, Burlington, VT 05405 U.S.A. across the sample and the mass fiow rate {m) through the sample. The pressure-fiow relationship has been expressed in a variety of ways, the simplest being hydraulic conductivity (L), where L = ni/AP. Conductivity can be compared between samples of various lengths (/) by substituting pressure gradient {AP/I) for pressure difference; variation in crosssectional area of samples can be factored out by expressing conductivity per unit area ('specific conductivity'). Despite the simplicity and utility of such measurements, results have been ambiguous for two reasons. The first is that conductivity of a sample often declines with time. This decline, which can begin within the first minutes of measurement (Zimmermann, 1978), has been observed for xylem of several species under a variety of experimental conditions. Explanations include blockage by air coming out of fluid within the sample (Kelso, Gertjejansen & Hossfeld, 1963); narrowing of vessel diameters due to swelling of surrounding tissue (Jeje, 1986); eleetro-osmosis (Buckman, Schmitz & Gortner, 1935); particulate clogging (Krier, 1951); and swelling of intervascular pit tnembranes (Zimmermann, 1978). The second and less obvious problem with conductivity measurements is the potential presence of naturally occurring air-filled tracheids or vessels. These 'emboli' arise from water stress induced cavitation in the transpiration stream, and they cause variable degrees of 'embolism', or blockage of fiow (Sperry, 1986); winter freezing of xylem sap may also create emboli (Zimmermann & Brown, 1971). Unless this blockage is quantified, the interpretation of a conductivity measurement is limited. In this report, we describe a method for measuring xylem conductivity that prevents long-term decline, and allows the initial conductivity of a plant axis to be expressed as a percentage of the maximum value obtained after the removal of embolism. As a means of studying xylem cavitation the method has the advantage of being simple, and informative of its most immediate physiological consequence: increased resistance to water fiow. Although this method was developed primarily with sugar maple {Acer saccharum Marsh.), we have used it successfully with