IAWA Bulletin n.s., Vol. 6 (4),1985 283 XYLEM EMBOLISM IN THE PALM RHAPIS EXCELSA by John S. Sperry Harvard Forest, Harvard University, Petersham, Massachusells01366, U.S.A. Summary Xylem failure via gas embolism (cavitation) several species (Zimmermann & Milburn, 1982; was investigated in Rllapis excelsa (Palmae). Tyree et al.,1984a & b). Ilowever, there is little Embolism was detected using measurements of information about its distribution within a xylem now resistance in excised stems and plant, at what pressure potentials it occurs, to petioles: a decrease in resistance after the re­ what extent it affects xylem transport, and moval of now-impeding embolisms by a pres­ whether it is reversible. This investigation con­ sure treatment indicated their previous pres­ sidered these fundamental questions by asking ence in the axis. Results suggested that Rllapis the more specific question: how does embolism avoids serious damage from embolism in at affect the water transport system in a palm, least four ways. I) Xylem pressure potentials which has a fixed vascular capacity'? In dicoty­ reached embolism-inducing levels (c. -2.90 MPa) ledonous trees and other plants with secondary only during prolonged drought. 2) When em­ growth, embolised vessels can be replaced with bolism did occur, it was confined to leafxylem; new functional vessels as new wood is produccd. slem xylem, most critical to shoot survival, re­ In palms, however, the problcm of embolism is mained fully functional. This is due in part to especially severe because they lack the ability hydraulic architecture: 70 to 85% of shoot xy­ to renew vascular tissues by secondary growth. lem resistance is in the leaf, and thus xylem According to a hypothesis advanced by Zim­ pressures are much lower in leaves than stems. mermann and Sperry (1983; see also Zimmer­ 3) Even during prolonged drought, the amount mann, 1983), Rhapis and other palms suffer of embolism is probably limited by complete embolism only in their leaves, which can be stomatal closure, which occurred at xylem pres­ readily replaced, whereas the stem xylem re­ sure potentials of -3.20 ± 0.18 MPa. 4) Embo­ mains fully functional, insuring the continued lism is potentially reversible during prolonged growth of the plant. TIle exclusive occurrence rains, since embolism dissolved within 5 h at of embolism in leaf xylem is a consequence of zero pressure (atmospheric), and xylem pressure an observed hydraulic constriction in the xy­ potential can reach zero during extended rain. lem of the stem-to-Ieaf connection (Fig. I); this Key words: RIJapls. xylem embolism, xylem constriction is constituted primarily by the transport, cavitation, water relations, water protoxylem conncction between the discrete stress, xylem pressure potential. hydrolulic metaxylem systems of stcm and leaf (between architecture. B' and D, Fig. I). Xylem pressure potential is always much lower in leaves than stems be­ In traduction cause of the high resistance to water flow in Embolism in the xylem of plants refers to the conslriction. In times of water stress, pres­ gas-filled tracheids or vessels. It occurs as a re­ sure potentials in the leaf xylem reach embo­ sult of the negative pressure potential of xylem lism-inducing levels while the stem xylem re­ water falling below the cohesive capacity of mains at a safe value. Of course, palms lIlay water-filled tracheary elements. At the limiting avoid excessive damage from embolism simply negative pressure potential for a given vessel, because embolism does not occur over tile nor­ xylem water undergoes a rapid transition to the mal range of xylem pres.sure potentials; or, if equilibrium vapour phase, termed ·cavitation'. it does occur, the gas-lilled vessels may refill TIle resulting embolised vessd is initially vapour­ readily with water, restoring their conductivity. filled, but air soon enters by diffusion. Embo­ The first part of this study determincd the lism restricts the conducting system of plants importance of the constriction in the stem-to­ because an cmbolised vessel cannot conduct leaf connection in confining low xylem pres­ water. sure potentials to leaves. This involved describ­ Embolism is potentially a serious problem ing the 'hydraulic architecture' of the Rllap;s for plants, and is known 10 occur routinely in shoot; that is, the relationship between shoot 284 IAWA Bulletin n.s., Vol. 6 (4), 1985 1 bolism; 3) the relationship of the xylem pres­ I sure potential required to induce embolism to ,I A the range in nature, and to the value inducing I complete stomatal closure. This indicated the I Iikelyhood of embolism in nature. and the role ' ofstomata in limiting its occurrence. I.. ~ z ,~ - ..z ~ :> .. !l:l • i Materials and Methods !,. ~ l~ ;c a.. Plum material .. RlJapis exee/sa (lllunb.) Henry is a rhizoma­ • tous coryphoid palm native to eastern Asia. Palms used in the study grew in the Fairchild Tropical Garden in Miami. Florida; erect shoots were 2 to 3 m tall, each with roughly 10 to IS B' expanded leaves. For comparative purposes, I leaves were numbered beginning with the I , youngest-expanded one. Measurements of xy­ I I lem sap pressures and xylem flow resistance de­ I ,. scribed below concerned the four youngest­ II I ... ...z expanded leaves (nos. 1-4), and a senescing I 2 ~ I leaf. I II! I ,. H)'draulic arelliteeture I ..t; i I The Scholander pressure bomb (Scholander i BI et aI., 1965) was used to measure the difference , in xylem pressure potential between stem and ,I leaf. On a given palm, pressure potential of la­ I A t mina xylem at mid-day was measured in the ~"nventional fashion. and compared with near­ Fig. I. Diagrammatic representation of depar­ ly simultaneous estimates of pressure potential ture of single leaf-trace bundle in RlJapis excel­ in stem xylem. These estimates were pressure sa (c. 100 leaf-trace bundles per leaf). Solid bomb measurements of leaves that had been curves A-B-C-A and D-E indicate metaxy­ enclosed in plastic bags and foil-wrapped while lem (mx) vessels; dashed lines, narrow proto­ attached to the shoot. Enclosing the leaves xylem (px) tracheids. Transverse sections of stopped transpiration and allowed lamina and vascular bundles (A-E, right) correspond to po­ stem pressure potentials to equilibrate (Begg & sitions A-E in curves on left. Structure shown Turner, 1970). in diagram repeats itselfaxially, as indicated by The difference in pressure potential (~P) 2 positions marked A. 8-B' is where water measured in this way can be accounted for by flows from the stem mx into px which enters resistance to flow (R) in the xylem betwl.'Cn the the leaf at D. The high resistance of the px tra­ stem and the lamina according to the equation cheids connecting the discrete mx systems of derived by analogy with Ohm's law (Pickard, stem and leaf indicate a hydraulic constriction 1981): ~p = R Jv, where Jv is the volume flow (modified from Zimmermann & Sperry, 1983). rate of water in the xylem. Thus, at a given vol­ lime flow rate, the relative values of xylem re­ sistance in the stem, stem-to-Ieaf connection, petiole, and lamina xylem approximate their morphology, xylem anatomy, amI hydraulic relative contribution to the overall pressure po­ resistance of the xylem. The second part devel­ tential difference between lamina and stem oped and evaluated a method for detecting measured with the pressure bomb. By measur­ embolism using xylem flow resistance measure­ ing these resistances, the importance of the ments in petiole and stem segments. The study constriction at the leaf insertion in generating concluded by using this method to evaluate low xylem pressure potentials in the leaf rela­ how RlJapis copes with embolism. The hypoth­ tive to the stem was estimated. eses in the previous paragraph were investigated Flow resistance (R) was delined as: by determining: I) if, in fact, embolism is con­ ~p fined to the leaves as predicted by Zimmermann R=J;T and Sperry: 2) the potential reversibility of em- IAWA Bulletin n.s., Vol. 6 (4), 1985 285 where AP is the pressure difference, Jv is vol­ and increased resistance in axes. A second ex­ ume flow rate, and I is axis length. Although periment compared flow resistance as an indio this definition gives a unit length measure, in cator of embolism with the more established some cases, resistance was expressed by incor­ acoustic method pioneered by Milburn (1973a, porating axis length (e.g. Fig. 3). Shoots were b), and recently refined by Tyree and colleagues divided into lamina, petiole, leaf insertion (leaf (Tyree & Dixon, 1983; Tyree et al., 1984a, b; sheath and portion of subtending stem; this Dixon et aI., 1984). This latter method is based segment included the constriction in the stem· on considerable evidence that ultrasonic vibra­ to-leaf connection; Fig. I), and stem segments. tions produced by a dehydrating axis correspond Resistance in all but lamina segments was de­ to cavitation events. Experiments were conduct­ termined by measuring volume flow rate in a ed in the laboratory of Dr. Tyree at the Univer­ pipette attached to one end of the axis under a sity of Toronto. Finally, anatomical studies pressure gradient induced by a vacuum pump were made in an attempt to confirm the pres­ attached to the other end (Sperry, 1985). la· ence of embolism in vessels of axes with resis­ mina xylem resistance was measured using the tance above the base level, and to determine if pressure bomb (Tyree & Cheung, 1977). in fact the soaking or pressure treatments dis­ solve these embolisms. Detection ofembolism Embolism was indicated in petiole and stem Applicatio1l ofresistance met/rod segments by the presence of xylem flow resis­ Resistance above the base level was assessed tance (R) above the minimum, or 'base' resis­ in stem and petiole segments excised directly tance (RB) of the segment.