The Cohesion Tension theory of sap ascent: current controversies

Transcription

1 Journal of Eperimental Botany, Vol. 48, No. 315, pp , October 1997 REVIEW ARTICLE Te Coesion Tension teory of sap ascent: current controersies Melin T. Tyree1 USDA Forest Serice, Aiken Forestry Sciences Laboratory, PO Bo 968, S. Burlington, VT 05402, USA Receied 12 February 1997; Accepted 21 May 1997 Abstract Key words: Coesion Tension teory, caitation, embolism, ylem pressure probe, pressure bomb. Introduction Two oter reiews of te current controersy oer te Coesion Tension (C T) teory ae been written (Canny, 1995; Milburn, 1996). Te first concentrates on a number of strange and unproen ideas. Te second proides an interesting, altoug limited, istorical oeriew of some of te current questions. Te present reiew is intended to put te Coesion Tension teory in a quantitatie and biopysical contet and reiews wat I beliee to be te strongest, quantitatie eidence aailable for te C T teory. Essential elements of te C T teory In recent years, te Coesion Tension (C T) teory of sap ascent in plants as come under question because of work publised by Professor Ulric Zimmermann and colleagues at te Uniersity of Würzburg, Germany. Te purpose of tis reiew is to (1) state te essential and testable elements of te C T teory, (2) summarize te negatie eidence for te C T teory, and (3) reiew critically te positie eidence for te C T teory and te eidence tat te Scolander Hammel pressure bomb measures ylem pressure potential (P ) correctly, because muc of te eidence for te C T teory depends on pressure bomb data. Muc of te current eidence negates te conclu- sions drawn by Zimmermann from studies using te ylem pressure probe (XPP), but it is not yet clear in eery instance wy te XPP results disagree wit tose of oter metods for estimating ylem pressure. Tere is no reason to reject te XPP as a useful new tool for studying ylem tensions in te range of 0 to 0.6 MPa. Additional researc is needed to test te C T teory wit bot te XPP and traditional metods. Te C T teory was proposed 103 years ago by Dion and Joly (1894), and some aspects of te C T teory were put on a quantitatie basis by an den Honert (1948) wit te introduction of te Om s law analogue of sap flow in te soil plant atmospere continuum. According to te C T teory, water ascends plants in a metastable state under tension, i.e. wit ylem pressure (P ) more negatie tan tat of te apour pressure of water. Te driing force is generated by surface tension at te eaporating surfaces of te leaf. Te tension is transmitted troug a continuous water column from te leaes to te root apices and trougout all parts of te apoplast in eery organ of te plant. Eaporation occurs predominantly from te cell walls of te substomatal cambers due to te muc lower water potential of te water apour in air. Te eaporation creates a curature in te water menisci of apoplastic water witin te cellulosic microfibril pores of cell walls. Surface tension forces lower P in te liquid directly beind te menisci (te air water interfaces). Tis creates a lower water potential, y, in adjacent regions, including adjoining cell walls and cell protoplasts. Te lowering of y is a direct consequence of P being one of te two major components of water potential in plants, te oter component being osmotic pressure, p: y=p p (1) Te energy for te eaporation process ultimately comes from te sun, wic proides te energy to oercome te latent eat of eaporation of te water molecules; i.e. te energy to break ydrogen bonds at te menisci. 1 To wom correspondence sould be addressed. Fa: MelTyree@AOL.COM

2 1754 Tyree Water in ylem conduits is said to be in a metastable condition wen P is below te apour pressure of water (P ), because te continuity of te water column, once broken, will not rejoin until P rises to alues aboe tat of P. Metastable conditions are maintained by te coesion of water to water and by adesion of water to walls of ylem conduits. Bot coesion and adesion of water are manifestations of ydrogen bonding. Altoug air/water interfaces can eist anywere along te pat of water moement, te small diameter of pores in cell walls and te capillary forces produced by surface tension witin suc pores preent te passage of air into conduits under normal circumstances. Howeer, wen P becomes su ciently negatie, air bubbles can be sucked into ylem conduits troug porous walls. Te tension (negatie P ) at te eaporating surface of leaes is ultimately transferred to te roots were it lowers y of te roots below te y of te soil water. Tis causes water uptake from te soil to te roots and from te roots to te leaes to replace water eaporated at te Fig. 1. Te Om s law analogy. Te total conductance is seen as surface of te leaes. resultant conductance (K) of te root, stem, and leaf in series and Van den Honert (1948) quantified te C T teory in a parallel. Water flow is drien by te di erences in water potential classic paper in wic e iewed te flow of water in te between te soil y and te water potential at te eaporating surface, soil y. On te rigt is te simplest Om s law analogy wit conductances plant as a catenary process, were eac catena element is eap in series. On te left is a more comple conductance catena were some iewed as a ydraulic conductance (analogous to an conductance elements are in series and some in parallel. electrical conductance) across wic water (analogous to electric current) flows. Tus, an den Honert proposed an Om s law analogue for water flow in plants. Te Te Scolander Hammel pressure bomb (Scolander Om s analogue leads to te following predictions: (1) et al., 1965) is one of te most frequently used tools for te driing force of sap ascent is a continuous decrease estimating P. Te C T teory does not depend on te in P in te direction of sap flow, (2) eaporatie flu accuracy of te pressure bomb, but muc of wat is density from leaes (E) is proportional to negatie of te known about te range of P tolerated by di erent species pressure gradient ( dp /d) at any point (cross-section) of plants depends on te pressure bomb. Typically, P along te transpiration stream. Tus at any gien point can be as low as 2 MPa (in crop plants) or to 4 MPa of a root, stem, or leaf ein, (species in arid zones) or een 10 MPa (California dp /d=ae/k +(fg d/d) (2a) caparral species). How te pressure bomb functions is discussed later. were A=leaf area supplied water by a stem segment wit ydraulic conductiity K and rg d/d is te graita- tional potential gradient were r=density of water, g= Negatie eidence for C T teory summarized acceleration due to graity and d/d=eigt gained, d, Using a ylem pressure probe ( XPP), Balling and per unit distance, d, traelled by water in te stem Zimmermann (1990) demonstrated tat te Scolander segment. Hammel pressure bomb (Scolander et al., 1965) does In te contet of stem segments of lengt (L) wit finite not measure ylem water tension correctly under many pressure drops across ends of te segment, circumstances. Te pressure bomb functions properly DP =LAE/K +rgd (2b) only if tere is direct pressure transmission from te air in te bomb to te ylem fluid. For eample, wen sap Figure 1 illustrates water flow troug a plant represented e u from te cut end of a leaf is blocked by a fluidby a linear catena of conductance elements near te centre filled pressure transducer on a leaf at y=0, tere sould and a branced catena of conductance elements on te be a 151 relationsip between gas pressure in te bomb left. Te number and arrangement of catena elements is and P measured by te transducer. But a 1:1 relationsip dictated primarily by te spatial precision desired in te was not found ( Balling and Zimmermann, 1990) in representation of water flow troug a plant; a plant can tobacco leaes, and tis finding as been confirmed be represented by anywere from one to tousands of independently in Tsuga canadensis by C Wei and MT conductance elements. Tyree (unpublised results). Te failure of pressure trans-

3 Coesion Tension teory 1755 mission in te positie pressure range in T. canadensis is Eidence in support of te pressure bomb and te related to te elasticity of woody stems and te compression of air bubbles in embolized traceids and does not C T teory Te Scolander Hammel pressure bomb inalidate te teoretical functioning of te pressure bomb in te normal mode of operation wen P <0. Te Scolander Hammel pressure bomb is a deice used More worrisome is an apparent failure of pressure to measure te equilibrium ylem pressure, P. For transmission in te negatie pressure range, i.e. P measte petiole protruding troug a rubble seal to te outside eample, a cut leaf is placed inside a pressure essel wit ured wit te XPP in indiidual essels did not increase as epected wit an increase of air pressure in te pressure air. Wen compressed gas is admitted into te pressure bomb. Professor Zimmermann concluded tat te presdirectly to te ylem fluid raising P. Wen P reaces a camber, te gas pressure is presumed to be transmitted sure bomb oerestimates tension (gies a P tat is too negatie). alue sligtly aboe zero (= atmosperic pressure) ten He as also sown tat ylem tension eceeding water begins to flow out of te essels at te cut end of 0.6 MPa is rarely obsered wit te XPP ( Zimmermann te petiole. Te gas pressure wen water first emerges is et al., 1993b). For eample, te XPP as been inserted called te balance pressure, P. Te negatie of P is B B into essels of many species in te morning and P alues equated to P prior to admitting compressed gas to te usually are positie (on an absolute pressure scale were camber. a acuum is 0 MPa). As te day progresses and transpirstated as follows: Wen a transpiring soot is ecised Te basic ypotesis of te pressure bomb can be ation increases, P falls to 0.2 to 0.6 MPa. Ten te essel punctured by te XPP caitates and P returns to from a plant, te negatie P prior to ecision is main- positie alues consistent wit embolized lumens filled tained after ecision in te ylem by surface tension at wit water apour and air. Wen many plants are probed pit membranes in essel walls. Te actual situation is at midday, most essels are found to be embolized, sligtly more complicated because a transpiring soot i.e. te XPP records absolute pressures >0 MPa. will ae gradients of P wereas te pressure bomb Zimmermann and colleagues ae proposed a number of measures an equilibrium P after te gradients of P ae disappeared. Let us focus on ow P canges near te mecanisms of sap transport in plants tat are consistent point of ecision, i.e. witin one essel lengt of te cut wit sap flow around embolized essels, wic tey beliee surface. Immediately after cutting, P temporarily is te normal state in plants (Zimmermann et al., 1993a, becomes zero in all seered essels. Ten P becomes b). Zimmermann does not reject te C T teory outrigt; negatie again as water is sucked into deydrated cells in e acknowledges tat tension-drien water moement is leaes by osmosis draining te essels until a meniscus is occurring wen e measures P alues in te range of 0 re-establised on pit membranes at te ends of remaining to 0.6 MPa wit te XPP, but e also postulates tat intact essels. As water flows into te liing cells teir y oter mecanisms must be at work wen y alues drop becomes sligtly more positie. At equilibrium two tings muc below 0.6 MPa. appen: (1) Any gradients in P originally between te Important questions can be raised and answered by cut surface of te stem or petiole and te eaporating eamining te current literature in te net section dealing surface of te leaes disappears. ( 2) An equilibrium is wit positie eidence for te C T teory. establised between te y of all liing cells and te y of ylem fluid (=P p ). So te equilibrium P could be ( 1) Does normal water transport occur wile essels are more negatie tan originally present at te point of embolized? If plants normally transpire wit most cutting if tere was a large pressure gradient from te cut essels embolized ten tis condition ougt to be point of te stem to te leaes, because te more deydidentifiable by measuring te ydraulic conductance rated leaf cells will tend to draw water from te less of stems in teir natie state and by quantitatie tests deydrated petiole or stem cells until te gradient disapof te Om s law analogue (Equations 2a, b). pears. Or te equilibrium P could be less negatie, if (2) Do most intact ylem conduits embolize at P tere was substantial reydration of liing cells wen te > 0.6 MPa or do tey embolize only because te cut essels were drained of water and only some of te ylem wall as been damaged by insertion of te essels refill wen te balance pressure, P, is establised. XPP? Tis question can be answered by looking at B Tus, equilibrium P in te ylem does not return to te te mecanism of caitation eents and by using P in te ylem at te time of cutting. cleer ways of inducing ylem tension. Regardless of ow muc te equilibrium P di ers (3) Is tere independent eidence of te epected pressure from te steady-state P in te transpiring soot, te transmission in a pressure bomb wen P <0 MPa? One way to answer tis question is to use a temperature corrected stem ygrometer on woody plants. pressure-bomb ypotesis can be stated as follows: Wen te soot is placed in a pressure bomb wit te cut end protruding toug a pressure seal to te outside

4 1756 Tyree air, te alue of P can be canged by applying air pressure P to te entire soot surfaces witin te bomb. a For eery unit increase in P, P will become less negatie a by one unit until te balance pressure (P ) is reaced B wen P =0 and water is balanced on te cut end. Tis can be epressed as: P =P P (3a) a B P =y +p (3b) Te second equality states tat P can be epressed in terms of y (=te ylem water potential ) minus p (= te ylem osmotic pressure). Equating Equation 3a to 3b yields: y =P p P (3c) a B In an enclosed air space adjacent to ylem water, te water potential of te apour pase (y ) sould equal y proided equilibrium of temperature and water-apour pressure is obtained. Tus we ae: y =P a p P B (4) Equation 4 and tus te alidity of te pressure bomb ae been confirmed on Tuja occidentalis soots oer te range of 0 to 2.1 MPa using temperature-corrected stem psycrometers to measure y under rigorously documented conditions of apour equilibrium (Dion and Tyree, 1984). In tese eperiments, a large T. occidentalis soot was enclosed in a pressure bomb and deydrated to a P =2.1 MPa. A temperature-corrected stem ygrometer was attaced to te cut basal surface of te soot B outside te pressure bomb. Dion and Tyree demonstrated tat apour equilibrium occurred wit a alf-time of 19 s. Termal equilibrium neer was obtained since te wood surface always was cooler tan te termocouple in te ygrometer used to sense y, but temperature stability was reaced wit a alf-time of 27 s. A termocouple toucing te surface of te wood was used to measure te stable temperature di erence between te wood surface and te sensing termocouple. Tis di erence could be used to make termodynamic corrections to y. Witout termal corrections, te data resembled Fig. 2a, and Fig. 2b after correction. Tese data proide strong eidence for te alidity of te pressure bomb ypotesis. More recently, Holbrook et al. ( 1995) ae used an elegant metod to create ylem tension mecanically and furter test te pressure-bomb ypotesis. Unfortunately, te eperiment as caused confusion because Holbrook et al. ( 1995) did not discuss te underlying pysics in su cient detail due to space limitations in te original publication. Tey ecised stem segments wit a single leaf at te midpoint of te segment, and mounted te midpoint of te stem on te rotating ais of a motor-drien saft (Fig. 3). Te rotation of te stem segment will produce a centrifugal tension at te Fig. 2. Eperimental demonstration of ow air pressure in a pressure bomb controls te P in te ylem of an ecised soot of Tuja occidentalis. Te -ais is te predicted P based on te current air pressure (P =te ariable) and te balance pressure (P =a constant in a B tis eperiment). Te y-ais is te water potential of te apour pase aboe te cut end of te soot. A temperature-corrected stem ygrometer was attaced to te cut end and te ygrometer camber approaced apour equilibrium wit a alf-time of 27 s and temperature stability was reaced wit a alf-time of 19 s. (A) Te relationsip before correction for te steady-state temperature di erence between te cut surface of te stem and te measuring termocouple of te ygrometer. (B) te relationsip after correction for te influence of te measured temperature di erences ony. Te dased line sows te 151 relation.

5 Coesion Tension teory 1757 fly o ends in a fine spray. Wen te motor is turned o te P in te midpoint will be presered by te y of te deydrated leaf cells. After centrifugation, te balance pressure of te ecised leaes correlated well wit te epected P (Fig. 3). Tis eperiment demonstrated tat ylem tensions created by centrifugation of soots are transmitted to leaes and measured correctly wit te pressure bomb. Tis eperiment also proed tat P alues down to 1.8 MPa could be sustained witout caitation because Equation 5 correctly predicts P only if te water columns are continuous in te centrifuged stem. Te centripetal force of te rotating mass of water molecules near te ends of te column are transmitted toward te ais of rotation by te coesion of water. Te alue of P is most negatie at te ais of rotation and rises continuously to atmosperic pressure at te ends of te stem segment. If te water columns ad broken, some of te mass toward te ends would ae been lost from te rotating system and P would ae been less negatie tan predicted by Equation 5. Fig. 3. Te Holbrook eperiment. (A) Te eperimental metod. A stem segment was mounted on a centrifuge motor. Centrifugation of te segment induced a P as gien in Equation 5. Te strobe ligt was used to measure angular elocity. (B) Results sow tat te rotational tension was transmitted to te attaced leaf; te -ais is te calculated rotation tension=p in Equation 5 and te y-ais is te balance pressure of te attaced leaf at te end of te eperiment. midpoint of te segment wic in pressure units will be equal to: P = 0.5s2r2 (5) were s=te fluid density, =te angular elocity of rotation, and r=distance between te ais of rotation and te end of te stem segment. Te negatie P at te midpoint of te segment will cause a deydration of te liing cells in te attaced leaf, and te water drawn out of te leaf will trael to te ends of te rotating segment were te centrifugal forces on te water will cause it to Hydraulic arcitecture and te Om s law analogue Recent studies of te ydraulic arcitecture of woody plants ae proided strong support for te C T teory. Te basic approac as been to obtain independent measurements of P (usually wit a pressure bomb), E (ia weigt loss or gas ecange metod), and K (wit a conductiity apparatus) and ten test te alidity of te Om s law analogue, Equation 2a, b. Testing te Om s law analogue on a large plant (tree or liana) requires te following steps: (1) Estimate te regression of K ersus stem diameter oer te range of diameters on te tree. (2) Make a ydraulic map of te tree in wic eac branc is made up of discrete segments, te segments being delineated by nodes (branc insertion points) so tat Equation 2b can be applied to eac segment. Keep a record of segment lengt (L) and diameter so tat K can be estimated from te regression in (1), and keep a record of leaf area attaced to eac segment. (3) Measure E for representatie leaes. (4) Compute DP for eac segment using Equation 2b. Ten, using te map, add te DP alues from te base of te tree to any soot ape. Benkert et al. (1995) measured P at te ape of seeral brances at di erent eigts,, in a large liana during te day wen E>0. Tey argued tat te pressure gradient, dp/d, sould be < rg d/d (Equation 2a), i.e. P sould decline more tan 0.01 MPa m 1 gain in eigt in ertical stems were d/d=1. Benkert et al. (1995) also argued tat te Om s law analogue was incorrect

6 1758 Tyree Fig. 4. Hydraulic conductiity of stems per unit pressure gradient, K, plotted ersus diameter of te stem (ecluding bark, -ais). Species represented are Tuja occidentalis, Acer saccarum, and Sce era morototoni. Bot aes are logaritmic. Adapted from Tyree et al. (1991). because te pressure gradient was > rg d/d. Tey Zimmermann also stated tat te pressure bomb underestimates P, i.e. gies alues tat are too negatie (inflat- pointed to similar eamples of P (measured wit a pressure bomb) ersus eigt in tall trees failing to satisfy ing dp/d). But studies ae confirmed equality in Equation 2a. Te implication of tis argument would Equation 2a, suggesting tat bot measures of K and of appear to be tat water transport must be drien by a P (measured wit te pressure bomb) are correct (Tyree, force in addition to dp/d. It will be seen later tat suc 1988; Ewers et al., 1989). reasoning is inalid because Equation 2a correctly predicts An eamination of case studies will proe te points dp/d along te patway of water transport, i.e. along just raised. Figure 4 sows K ersus stem diameter for a te ais of stems. Te error made by Benkert et al. (1995) gymnosperm, a temperate angiosperm, and a tropical is in using Equation 2a to predict te pressure gradient angiosperm. K aries oer seeral orders of magnitude between apices at di erent eigts wit no knowledge of for a gien species because large-diameter stems can K along te patway of water flow. It is erroneous to transport more water and ence are more conductie assume dp/d at soot apices can be predicted by dp/d tan small-diameter stems. Te species ariation is een witout knowing te functional dependence of K more remarkable; e.g. a Sce era stem can be as muc ersus. as 100 times more conductie tan a Tuja stem of te Zimmermann would predict tat te left side of same diameter. Equation 2a sould be larger tan first term of te rigt A parameter of more interest is leaf specific conductance, K =K /A. Tis parameter is useful because dp /d L side (AE/K ). He suggested tat ylem conduits usually are embolized wen P is more negatie tan 0.6 MPa. in a stem segment is inersely proportional to K at a L So K sould be lower at times of maimum E. Values gien E if graity is ignored in Equation 2a: of K are measured in a conductiity apparatus under dp /d=e/k (6) positie P, so few conduits will be embolized and ence L K sould be maimal (reducing te estimated AE/K ). Ranges of alues of K ersus stem diameter are sown L

7 Coesion Tension teory 1759 in Fig. 5. Again, K can di er by up to two orders of P profiles were calculated using alues of E at midday, L magnitude between species for stems at a gien diameter. K, and te ydraulic maps. L Also, K increases as a function of diameter, so dp /d Looking at te pattern of P at branc apices in te L is steepest in te smallest diameter brances. In some Tuja profile, it can be seen tat tere is no consistent cases dp /d can be as muc as 1 MPa m 1 as in Tuja pattern in declining P wit eigt up te tree. Te apices were K =10 5 kg s 1 m 1 MPa 1 for stem segments mark wit * are at eigts 4.1 and 7.5 m in te ydraulic L 2 4 mm in diameter and E=10 5 kg s 1 m 2 at noon. map, but ae P alues of 1.0 and 0.9 MPa, respectiely, giing a dp /d= MPa m 1 rater tan te L Te consequence of tis pattern of K ersus diameter can be seen in P profiles predicted by te Om s law required alue of 0.01 MPa m 1 needed to lift water analogue and te ydraulic map of representatie trees. against te force of graity. Tyree (1988) computed te Figure 6 sows plots of P ersus pat lengt,, from mean P of all small-diameter brances ersus in Tuja te base of te tree to representatie branc apices. Te and concluded tat tere is no consistent decline of P Fig. 5. Leaf specific ydraulic conductance K L =K /A, were A=leaf area apical of te stem segment on wic K was measured plotted ersus diameter of te stem (ecluding bark). Species are as in Fig. 5; bot aes are logaritmic. Adapted from Tyree et al. (1991).

8 1760 Tyree good agreement in young Acer saccarinum plants (Fig. 7). Te stark contrast between tese eamples and Zimmermann s predictions sould be noted. In eery case cited, K L, was measured wit positie pressures, so according to Zimmermann, all K L alues would be oerestimates of te alues tat apply during midday wen e beliees essels are embolized. Zimmermann also suggested tat te pressure bomb underestimates P. Hence, te Om s law analogue (Equation 2b) sould be iolated if e is correct. Since Equation 2b is quantitatiely correct, Zimmerman s obserations must not represent te normal status of plants, and te pressure bomb must be presumed more reliable tan suggested by is studies. Fig. 6. Profiles of ylem pressure potential, P, ersus distance water must trael from te base of te tree to a stem ape. All alues include te graitational potential gradient required to lift water up te tree. Te cures were calculated from te ydraulic maps and te representatie midday eaporatie flu densities (E) indicated for eac species in te grap. Adapted from Tyree et al. (1991). ersus (Tyree, 1988; Fig. 3). Neerteless, P declined wit increasing distance along eery patway suc tat dp /d always was < 0.01 MPa m 1 in all stem segments. Tyree (1988; Fig. 5) also found tat te Om s law analogue combined wit diurnal measurements of E proided good predictors of actual diurnal canges in P measured wit a pressure bomb. Te agreement between measurement and teory suggests tat tree conditions were met simultaneously: (1) te pressure bomb proides a correct estimate of P ; (2) te metod of measuring K yields alid results; and (3) te Om s law analogue is correct. Oter studies ae confirmed te correctness of te Om s law analogue. Ewers et al. (1989) studied te ydraulic arcitecture of a large woody ine (>20 m long) growing along te ground to aoid te e ects of graity on P. Tey measured K, E, and constructed L ydraulic maps. Tey used te Om s law analogue to compare predicted alues of dp /d wit alues meas- ured on bagged leaes using a pressure bomb. Te predicted gradients ( 0.083±0.033 MPa m 1) agreed wit te gradients measured during midday wit a pressure camber (0.076±0.016 MPa m 1). Measured and pre- dicted gradients also agreed wen E=0. Tsuda and Tyree (unpublised results) compared te predicted drop in P Te mecanism of caitation and te tensions tat seed caitations Finally, recent studies on te mecanism of caitation in ylem conduits ae confirmed tat caitations are airseeded. Te air-seeding ypotesis proides strong eid- ence for te eistence of large negatie P alues prior to caitation eents. A caitation eent in ylem conduits ultimately results in dysfunction. A caitation occurs wen a oid of su cient radius forms in water under tension. Te oid is gas filled (water apour and some air) and is inerently Fig. 7. Computed ersus measured DP =P P for Acer saccarinum L soil plants growing in pots. Eac point represents a di erent plant. Eaporatie flu density (E) was estimated from weigt of water lost from te potted plants and leaf area measured at te end of te eperiment. E was aried by controlling ligt intensity. Once a steadystate E was reaced, te balance pressure of a transpiring leaf was estimated (P ). Soil-water potential (P ) was estimated by te balance L soil pressure of nontranspiring plants. So te y-ais is te measured pressure drop from soil to leaf. At te end of te eperiment, te entire plant was arested and te ydraulic conductance of te root (K ), te R from soil to leaf (based on graimetric measures of E stems (K ) and te leaes (K ) was estimated wit a ig pressure S L flowmeter. Te predicted DP on te -ais was computed by applying and wole soot and root ydraulic conductances) to Equation 2b to eac conductance element (root, stem, and leaf ). alues of P measured wit a pressure bomb. Tey found Unpublised data of Makoto Tsuda and Melin T Tyree.

9 Coesion Tension teory 1761 unstable, i.e. surface-tension forces will make it collapse Air bubbles are rarely stable in ylem conduits because spontaneously unless te water is under su cient tension transpiration can draw P to alues <0. As P falls w w (negatie pressure) to make it epand. A digression towards zero te air bubble epands according to te follows to eplain wy tis is true. ideal gas law, but V can neer grow larger tan te Te cemical force driing te collapse is te energy olume of te conduit, so P can neer fall low enoug stored in ydrogen bonds, te intermolecular force to allow P P to balance 2t/r witout a decline in P. w w between adjacent water molecules. In ice, water is bound Once te bubble as epanded to fill te lumen, te to adjacent water molecules by 4 ydrogen bonds. In te conduit is dysfunctional and no longer capable of trans- liquid state, eac water molecule is bound by an aerage porting water. Fortunately for te plant, a dynamic of 3.8 ydrogen bonds at room temperature. In te liquid balance at te meniscus in cell walls ultimately is acieed. state, ydrogen bonds are forming and breaking all te Tis stability will be discussed first in te contet of a time permitting more motion of molecules tan in ice essel and its pit membranes. (Slatyer, 1968). Howeer, wen an interface between As te air bubble is drawn up to te surface of te pit water and air is formed, some of tose ydrogen bonds membrane in essel cell walls, te pores in te pit mem- are broken and te water molecules at te surface are at brane break te meniscus into may small menisci at te a iger energy state because of te broken bonds. Te opening of eac pore. As te meniscus is drawn troug force (N=Newtons) eerted at te interface as ydrogen te pores, te radius of curature of te meniscus, r, m bonds break and reform can be epressed in pressure falls toward te radius of te pores, r. Again, as long as p units (Pa), because pressure is dimensionally equal to r eceeds r, te necessary conditions for stability are m p energy (J=Joules) per unit olume of molecules, i.e. acieed, i.e. Jm 3=Nm m 3=Nm 2=Pa. Stable oids in water tend to form speres because speres ae te least P P =2t/r w m (8) surface area per unit olume; and tus, a sperical oid Usually, a dysfunctional conduit eentually will fill as te minimum number of broken ydrogen bonds per wit air at atmosperic pressure (as demanded by Henry s unit olume of oid. Te pressure tending to make a oid law), so P eentually approaces 0.1 MPa as gas di uses collapse is gien by 2t/r, were r is te radius of te troug water to te lumen and comes out of solution. sperical oid and t is te surface tension of water (= Wen P equals 0.1 MPa, te conduit is said to be fully Pa m at 25 C). embolized. As P rises and falls as dictated by te w For a oid to be stable, its collapse pressure (2t/r) must demands of transpiration, r adjusts at te pit-membrane m be balanced by a pressure di erence across its surface or pores to aciee stability. Wen te conduit is fully meniscus=p P, were P is te absolute pressure (= embolized, bot sides of Equation 8 can be epressed in w w P +te atmosperic pressure) of te water and P is te terms of ylem pressure potential, absolute pressure of te oid. P $ (P P )= 2t/r (9) w m P P =2t/r (7) w Te minimum P tat can be balanced by te meniscus P always is aboe absolute zero pressure (=perfect is gien wen r equals te radius of te largest pit- m acuum) since te oid usually is filled wit water apour membrane pore bordering te embolized conduit. If te and some air. Relatiely stable oids are common in daily largest pore is 0.1 or 0.05 mm, te minimum stable P is life, e.g. te air bubbles tat form in a cold glass of water 1.44 or 2.88 MPa, respectiely. So te porosity of fresly drawn from a tap. An entrapped air bubble is te pit-membrane is critical to preenting dysfunction of temporarily stable in a glass of water because P is a essels adjacent to embolized essels (Sperry and Tyree, w relatiely constant 0.1 MPa and P is determined by te 1988). Wen P falls below te critical alue, te air ideal gas law, P =nrt/v, were n=te number of moles bubble is sucked into an adjacent essel, seeding a new of air in te bubble, R=gas constant, T=absolute temperature, caitation. and V=te olume of te bubble. So te Consequently, te genetics tat determines pit morpo- tendency of te oid to collapse (2t/r) makes V decrease logy and pit-membrane porosity must be under strong wic causes P to increase according to te ideal gas selectie pressure. A safe pit-membrane will be one wit law because P is inersely proportional to V. Te rise in ery narrow pores and one tick enoug and tus strong P proides te restoring force across te meniscus needed enoug to sustain substantial pressure di erences witout for stability. But an air bubble in a glass of water is stable rupturing. only temporarily because according to Henry s law, te Te situation for traceids of conifers is di erent solubility of a gas in water increases wit te pressure of because air moement from an embolized traceid to an te gas. So te increased pressure eerted by 2t/r makes te gas in te bubble more soluble in water and it slowly collapses as te air dissoles, i.e. as n decreases. adjacent traceid is preented by te sealing (aspiration) of te torus against te oerarcing border of te pit. In most cases, te porosity of te margo tat supports te

10 1762 Tyree Fig. 9. Te first eperimental test of te air-seeding ypotesis. (A) Te eperimental apparatus. A willow branc is bent around in a large pressure bomb so tat bot cut surfaces are outside te bomb. Water Fig. 8. Four possible mecanisms of caitation induction. (1) Air- continually passes troug te stem segment under positie pressure seeding troug a pore occurs wen te pressure di erential across te from a water column to a cotton collector. Flow rate is estimated by meniscus is enoug to allow te meniscus to oercome surface tension measuring te weigt cange of te cotton collector oer known time and pass troug te pore. (2) Air-seeding troug a ydropobic interals. (B) Te results are sown as a ulnerability cure were te crack occurs wen a stable air bubble resides at te base of a y-ais is te per cent loss of ydraulic conductiity (PLC) and te - ydropobic crack in te wall of a ylem conduit. Wen te P ais is te negatie P or te air pressure in te bomb needed to cause becomes negatie enoug te bubble is sucked out of te crack. (3) te plotted PLC. Open symbols are for PLC induced by negatie P Homogeneous nucleation inoles te spontaneous generation of a oid and closed symbols are induced by positie pressure applied in te in a fluid. It is a random process requiring termal motion of te water pressure bomb. See tet for more details. molecules. Te ydrogen bonds at a specific locus are broken wen all water molecules randomly moe away from any locus at te same instant wit su cient energy to break all ydrogen bonds between water molecules. As te tension in te water increases te ydrogen bonds are stretced and weakened so te energy needed to break te ism into oter conduits. Embolisms are te natural con- bonds decreases making a omogeneous nucleation more likely. (4) sequence of foliar abscission, erbiory, wind damage, Hydropobic adesion failure is similar to omogeneous nucleation and oter mecanical fates tat migt befall a plant. It ecept tat ydrogen bonds are broken between water and a ydropobic patc in te wall were te energy of binding between water and wall is reduced. is now appropriate to question weter all emboli are seeded from adjacent conduits or weter anoter mecanism occurs in some or most of te cases. Four mecanisms for te nucleation of caitations in torus is too large to preent meniscus passage at pressure di erences eceeding 0.1 MPa (Sperry and Tyree, 1990). plants ae been proposed. Tese are illustrated in Fig. 8, But te margo is elastic, so a pressure di erence of only wic sows for eac mecanism te sequence of eents 0.03 MPa is su cient to deflect te torus into te sealed tat migt occur as P declines in te lumen of a conduit. position. Air bubbles pass between traceids wen te See Zimmermann (1983), Tyree et al. (1994), and Pickard pressure di erence becomes large enoug to rip out te (1981) for a detailed discussion of te four mecanisms. torus from its sealed position (Sperry and Tyree, 1990). Oter air-seeding mecanisms ae been proposed tat A biopysical understanding now eists of te stability apply to SCUBA diers ( Yount, 1989). Suc mecanisms of emboli and ow tey can be sucked into a water-filled also migt occur in plants wen gas solubility decreases conduit from a neigbouring embolized conduit. Plants in ylem water as it warms, but little is known about te always will ae some embolized conduits to seed embol- importance of tis mecanism in plants. Tis study is

11 Coesion Tension teory 1763 concerned only about wic mecanism occurs most min.) Wen P was increased gradually beyond te a frequently in plants. critical alue te stem conductance began to fall Eperiments can discriminate between te air-seeding (increased PLC). Wen P was gradually decreased, te a mecanism and te oter tree in Fig. 8 and Yount PLC stopped decreasing. Te ulnerability cure from ( 1989). All mecanisms predict caitation wen ylem tis eperiment was identical to tat found for similar fluid is under tension, but te air-seeding mecanism brances deydrated in te air. predicts tat air can be blown into essels wile te fluid Tis is te strongest eidence presented to date tat is under positie pressure. Te air-seeding mecanism te air-seeding mecanism eplains ow caitations occur, requires only a pressure di erential (P P ), were P toug tere is oter circumstantial eidence (Crombie a w a is te air pressure outside and P is te fluid pressure et al., 1985; Sperry and Tyree, 1988). Te air-seeding w inside (Fig. 9a). It makes no di erence if P is 0.1 and ypotesis can be iewed as proiding strong eidence a P is 3.0 or if P is 3.1 and P is 0.1. Eperiments ae for te eistence of large negatie P alues prior to w a w sown tat te same ulnerability cure results weter caitation eents. Te aspiration of te meniscus into a P is reduced by air deydration or P is increased in a caitating essel is drien by a pressure di erence= w a pressure bomb (Cocard et al., 1992). A ulnerability P P. If te pressure di erence is, say, 8 MPa wen a w cure is a plot of per cent loss ydraulic conductiity P =0.1 and P =8.1 MPa, it follows tat P must equal w a w (PLC) ersus te P required to cause te PLC by 8.1 MPa (P = 8.0) wen P =0.1 MPa. Sperry et al. a caitation eents. Te results of tis eperiment are sown (1996) compared te ulnerability cures of numerous in Fig. 9b (see also Jarbeau et al., 1995). species measured by te application of positie air pressure ersus cures measured by benc-top deydration Willow stem segments wit leaes were enclosed in a pressure camber wit cut ends protruding into te open were P alues were measured by te pressure bomb air. Water was passed continually troug te ylem metod. A 151 agreement was found for P or P from a under positie pressure. Wile stem conductance was 1.0 to 9.5 MPa ( Fig. 10). For tis agreement to eist, two being monitored, te gas pressure, P, in te pressure tings ae to be true: ( 1) Te air-seeding ypotesis a camber was increased gradually. Initially, te ydraulic must be te mecanism of caitation and (2) te pressure conductance of te stem segment did not decrease until bomb must measure alues of P correctly. For readers a critical pressure of 1 MPa was applied. ( Eac solid wo still may doubt te accuracy of te pressure bomb, circle in Fig. 9b represents te application of pressure for after all te arguments in tis paper, one oter elegant Fig. 10. Vulnerability cures were generated in two ways. (1) Caitations induced by negatie P in wic te pressure bomb was used to estimate P = P measured on leaes attaced to te deydrated stems. (2) Caitations induced by positie pressure, P, in an eperiment like tat in B a Fig. 9. Te ulnerability cures were eamined in te range of PLC and for eac PLC alue corresponding alues of P and P were a B recorded and plotted on te - and y-ais, respectiely. Tese data were compiled from ulnerability-cure data on 12 species. (modified from Sperry et al., 1996). Te slope is not significantly di erent from 1.0 (Student s t test, t =0.301, P>0.5) indicating tat negatie pressures are s measured correctly by te pressure bomb and tat caitations occurred by air-seeding.

12 1764 Tyree Fig. 11. Comparison of ulnerability cures obtained by di erent metods. Tey-ais is per cent loss ydraulic conductiity (PLC) induced by a pressure on te -ais: solid symbols, negatie pressure induced by centrifugation; open symbols, negatie pressure induced by air-deydration and measured on ecised leaes wit a pressure bomb; dotted symbols, PLC induced by positie pressure as in Fig. 9. and two indirect pieces of eidence. Te pressure bomb agrees wit ylem tension measured directly wit a tem- perature-corrected termocouple ygrometer and wit tensions created by centrifugation. Te pressure bomb is alidated indirectly because it as been used to confirm two ypoteses: ( 1) te air-seeding ypotesis and (2) te ypotesis tat water flow can be treated as an Om s law analogue. Bot ypoteses make quantitatie predic- tions about P and te pressure bomb measurements ae been used to confirm bot ypoteses. Te following must be true for te confirmation to occur: (1) te ypotesis must be true and (2) te pressure bomb must measure P correctly. More eperiments of te type described aboe must be repeated to confirm te accuracy of te pressure bomb wit oter plant species. Tis eperiment can be presented to sow tat ylem conduits can sustain substantial tensions witout caitation. Tension can be controlled independently and induced in stems by centrifugation ( Holbrook et al., 1995), so it follows tat centrifugation can be used to induce caitations and measurable loss of ydraulic conductiity. It as been sown tat ulnerability cures produced by centrifugation are te same as tose produced by benctop deydration (Pockman et al., 1995, Alder et al., 1997) (Fig. 11). Conclusions Te Scolander Hammel pressure bomb seems to measure P correctly. Tis as been confirmed by two direct

13 Coesion Tension teory 1765 study s tentatie conclusion is tat te XPP must be Dion MA, Tyree MT A new temperature corrected stem incorrect if it does not agree wit te pressure bomb. Te ygrometer and its calibration against te pressure bomb. Plant, Cell and Enironment 7, XPP does not measure P < 0.6 MPa in plants; te most Ewers FW, Fiser JB, Ciu ST Water transport in te likely eplanation is tat te XPP induces embolisms by liana Bauinia fassoglensis ( Fabaceae). Plant Pysiology air seeding wen P < 0.6 MPa. It is less clear wy te 91, XPP disagrees wit te pressure bomb in direct comparpressures Holbrook NM, Burns MJ, Field CB Negatie ylem in plants: A test of te balancing pressure tecnique. isons made by Balling and Zimmermann (1990), so tis Science 270, work sould be repeated in oter laboratories and wit Jarbeau JA, Ewers FW, Dais SD Te mecanism of additional species. Tere is no reason to reject te XPP water stress-induced embolism in two species of caparral outrigt as a useful metod to measure ylem-fluid pres- srubs. Plant, Cell and Enironment 126, sure in te range of 0 to 0.6 MPa. Furter adances in Milburn JA Sap ascent in ascular plants: callenges to XPP tecnology may een etend te useful range to te coesion teory ignore te significance of immature ylem and te recycling of Münc water. 78, more negatie pressures. Pickard WF Te ascent of sap in plants. Progress in Studies of ylem dysfunction due to caitation proides biopysics and molecular biology 37, te best eidence tat water is transported in plants in a Pockman WT, Sperry JS, O Leary JW Sustained and metastable state as proposed by te C T teory. Muc significant negatie water pressure in ylem. Nature 378, of wat is now known about ylem anatomy and eolu Scolander PF, Hammel HT, Bradstreet EA, Hemmingsen EA. tion is best eplained in terms of te function of ylem Sap pressure in ascular plants. Science 148, structures in te aoidance of caitations and readers Slatyer RO Plant water relationsips. London, New may consult Tyree et al. (1994) for details. Recent studies York: Academic Press. of te Om s law analogue for sap flow in plants ae Sperry JS, Saliendra NZ, Pockman WT, Cocard H, Cruiziat proided strong quantitatie support for te C T teory. P, Dais SD, Ewers FW, Tyree MT New eidence for large negatie ylem pressures and teir measurement by te pressure camber metod. Plant, Cell and Enironment Acknowledgements 19, Sperry JS, Tyree MT Mecanism of water stress-induced ylem embolism. Plant Pysiology 88, I tank te following for teir critical reiew of an earlier draft Sperry JS, Tyree MT Water-stress-induced ylem embolof tis manuscript: Peter Becker, Stepen Dais, Frederick ism in tree species of conifers. Plant, Cell and Enironment Meinzer, Jon Sperry, and Daid Yount. 13, Tyree MT A dynamic model for water flow in a single tree: eidence tat models must account for ydraulic References arcitecture. Tree Pysiology 4, Tyree MT, Dais SD, Cocard H Biopysical perspecties Alder NN, Pockman WT, Sperry JS, Nuismer S Use of of ylem eolution: Is tere a tradeo of ydraulic e ciency centrifugal force in te study of ylem caitation. Plant, Cell for ulnerability to dysfunction. IAWA Bulletin 15, and Enironment (in press). Tyree MT, Snyderman DA, Wilmot TR, Macado J-L Balling A, Zimmermann U Comparatie measurements Water relations and ydraulic arcitecture of a tropical tree of te ylem pressure of Nicotiana plants by means of te (Sce era morototoni). Plant Pysiology 96, pressure bomb and pressure probe. Planta 182, Yount DE Growt of bubbles from nuclei. In: Brubakk Benkert R, Zu JJ, Zimmermann G, Türk R, Bentrup FW, AO, Hemmingsen BB, Sundnes G, eds. Supersaturation and Zimmermann U Long-term ylem pressure measure- bubble formation in fluids and organisms. Trondeim: Tapir ments in te liana Tetrastigma oinierianum by means of te Publisers ylem pressure probe. Planta 196, an den Honert TH Water transport in plants as a Canny M A new teory for te ascent of sap: coesion catenary process. Discussions of te Faraday Society 3, supported by tissue pressure. Annals of Botany 75, Zimmermann MH Xylem structure and te ascent of sap. Cocard H, Cruiziat P, Tyree MT Use of positie Berlin: Springer-Verlag. pressures to establis ulnerability cures: furter support Zimmermann U, Haase A, Langbein D, Meinzer F. 1993a. for te air-seeding ypotesis and possible problems for Mecanism of long-distance water transport in plants: a pressure-olume analysis. Plant Pysiology 100, re-eamination of some paradigms in te ligt of new Crombie DS, Hipkins MF, Milburn JA Gas penetration eidence. Pilosopical Transactions of te Royal Society of of pit membranes in te ylem of Rododendron as te cause London 341, of acoustically detectable sap caitation. Australian Journal Zimmermann U, Benkert R, Scneider J, Rygol J, Zu JJ, Plant Pysiology 12, Zimmermann G. 1993b. Xylem pressure and transport in Dion HH, Joly J On te ascent of sap. Pilosopical iger plants and tall trees. In: Smit JAC, Gri ts H, eds. Transactions of te Royal Society London, Series B 186, Water deficits: plant responses from cell to community. Oford: BIOS Science Publisers,