Plant, Cell and Environment {^992) 15, 471-477

Hydraulic architecture of sugarcane in relation to patterns ofwater use during plant development"^

F, C, MEINZER.' G, GOLDSTEIN,' 1 H, S, NEUFELD.'* D, A, GRANTZ' § & G, M, CRISOSTO'*

'Experiment Station, Hawaiian Sugar Planters' Association, P.O. Box 1057, Aiea, and'USDAIARS, Experiment Station,HSPA, P.O. Box 1057, Aiea, HI 96701, USA

ABSTRACT

Hydraulic conductance was measured on leaf andstem segments excised from sugarcane plants atdifferent stages of development. Maximum transpir-ation rates and leaf water potential (V'L) associatedwith maximum transpiration were also measured inintact plants as a function of plant size. Leaf specifichydraulic conductivity (LsJ and transpiration on a unitleaf area basis (E) were maximal in plants withapproximately 0-2 m^ leaf area and decreased withincreasing plant size. These changes in Eand L^c werenearly parallel, which prevented H\ in larger plantsfrom decreasing to levels associated with substantialloss in xylem conductivity caused by embolism for-mation. Coordination of changes in E and leafhydraulic properties was not mediated by decliningleaf water status, since V L increased with plant size.Hydraulic constrictions were present at nodes and inthe node-leaf sheath-leaf blade pathway. This patternof constrictions is in accord with the idea of plantsegmentation into regions differing in water transportefficiency and would tend to confine embolisms to therelatively expendable leaves at terminal positions inthe pathway, thereby preserving water transportthrough the stem.

Key-words: Saccharum spp.; sugarcane; hydraulic con-ductivity; xylem cavitation; transpiration; stomata.

Symbols and abbreviations: A, leaf surface area; E,transpiration per unit leaf area; L, hydraulic conductance;Lsc. leaf specific conductivity; TVD. top visible dewlap; V',water potential.

* Published as Paper No, 754 in Ihc journal series of theExperiment Station. HSPA,t Present address: Botany Department, University of Hawaii.Honolulu, HI 96822, USA,:|: Present address: Biology Department. Appalachian State Uni-versity, Boone. NC 28608. USA,§ Present address: University of California, Kearney AgriculturalCenter. Parlier. CA 93648. USA,

Correspondence: Frederick Meinzer, Hawaiian Sugar Planters'Association, P.O. Box tO57, Aiea, Ht 96701, USA.

INTRODUCTION

The intrinsic or absolute efficiency of xylem watertransport through a plant segment is often expressed asthe hydraulic conductance (L) per unit segment length

L = q/{AP/Ax) (1)

where q is the rate of water flow through the segmentand AP/Ax is the pressure gradient driving the flow.Recently, the hydraulic architecture of a number ofwoody species has been characterized from thestandpoint of the relative rather than the absoluteefficiency of the xylem in supplying water to differentparts of the plant (Tyree et al. 1983; Ewers & Zimmer-mann 1984a,b; Ewers, Fisher & Chiu 1989). Acommonly used measure of this relative efficiency ofwater transport is the leaf specific conductivity (P^c),which is defined as L divided by the leaf area (A ) distalto the segment, i.e.

,. = LIA. (2)

Lsc can be used to determine the magnitude of the //; situpressure gradient generated along a segment in supply-ing its attached leaves with water because

A/7A.V = (3)

where E is the prevailing transpiration rate on a unit leafarea basis. Thus. L^ and E provide additional informa-tion needed to interpret the functional significance ofdifferences in intrinsic xylem properties (e,g, L) withinindividual plants or atnong species. That is, at a given E,a segment with a larger E^^ value will always experiencea smaller pressure gradient than a segtnent with asmaller E^^ value. This comparison cannot be madedirectly using only E beeause the pressure gradientdepends on both /i and the total leaf area supplied by thesegment.

Studies of transpiration and xylem pressure gradientsin relation to hydraulic architecture assume additionalsignificance in the context of the vulnerability of xylemto loss in conductivity through cavitation and embolismformation (Tyree & Sperry 1989), Considerablevariation among species has been reported in themagnitude of xylem tension at which substantial cavi-tation- and embolism-induced lo.ss in conductivitybegins to occur (Tyree & Dixon 1986; Sperry. Tyree &

471

472 F. C. Meinzer et al.

Donnelly 1988). The magnitude of xyiem sap tensiondeveloped at a particular location in a plant is a functionof the xyiem water flux, the soil water potential and thesum of the resistances in the flow pathway to that point.It has recently been suggested that stomatal regulationof transpiration in many species results in operationallevels of xyiem tension just above the point of catastro-phic blockage from embolism formation (Tyree &Sperry 1988). However, there is little informationavailable concerning the possible coordination of tran-spiration with tissue hydraulic properties during plantdevelopment.

In growing sugarcane, we have observed that stomatalconductance and total apparent hydraulic conductanceof the soil/root/leaf pathway exhibit parallel changes asplant size increases (Meinzer & Grantz 1990). In ourprevious study, the apparent hydraulic conductance wasdetermined from measurements of transpiration andhydrostatic pressure differences using an Ohm's lawanalogy. The location and nature of the developmentalchanges in hydraulic conductance were not determined.Stomatal opening and closing responses following par-tial defoliation and root pruning, respectively, suggestedthat the coupling between vapour and liquid phaseconductance may have been mediated by a chemicalsignal originating in the roots. The objectives of thepresent study were to characterize the developmentalcourse of hydraulic conductances and associatedchanges in the hydraulic architecture of sugarcane and todetermine the relationship between these hydraulicparameters and patterns of regulation of water use andleaf water status during plant development. In contrastwith our previous studies, hydraulic properties wereassessed by direct measurement of water flow throughisolated plant segments subjected to a constant pressuredifference. An additional objective was to locatepossible hydraulic constrictions where the pressuregradient would abruptly become steeper at high rates oftranspirational water movement.

MATERIALS AND METHODS

Plant material

Sugarcane {Saccharum spp. hybrid cv H65-7052) plantswere grown from stem segments containing lateral buds.In the greenhouse, nodal segments containing singlebuds were planted in 11 dm- soil-filled pots and irrigatedtwice daily. Field-grown plants were obtained fromwell-irrigated commercial sugarcane fields near Wai-pahu, Oahu, Hawaii. The shoot architecture of sugar-cane consists of a single, erect stem with alternate,widely spaced, erect leaves. Leaf blades are typically100-200cm long and 2-8cm wide. Internodes may be upto 25 cm long. The reference point for enumerating nodeand internode position was the node at which the topvisible dewlap (TVD) leaf is attached. This was desig-nated as node zero and additional nodes were numbered

consecutively down the stalk. The TVD leaf is theyoungest non-elongating leaf whose blade is fullyexposed and not enclosed in the sheaths of older leaves.The area of the leaf blade attached to each node wasmeasured with an area meter (Delta-T Devices Ltd,Cambridge, UK).

Hydraulic conductance

Leaf segments for conductance measurements wereobtained by excising entire TVD leaf blades from field-and greenhouse-grown plants at the junction betweenthe blade and sheath. The remaining leaf blades wereexcised for measurement of total leaf area per plant. Thebases of the TVD leaf blades were recut under water andthe leaves were transported to the laboratory. In thelaboratory, the middle portion of the leaf was submer-ged in a 10 mol m"-' oxalic acid solution while a 20-cmsection was excised midway between the base and tip ofthe blade. Previous measurements using a compressedair technique (Ewers & Fisher 1989) indicated that 20cmwas roughly 2-5 times the average maximum xyiemvessel length. A 0-75 x 7cm segment of lamina was thenexcised from the midpoint of the section, midwaybetween the midrib and leaf margin with a sharp razorblade. This leaf segment remained submerged in theoxalic acid solution while it was inserted into a slit in arubber stopper and connected to the hydraulic conduc-tivity apparatus. Flow induced by a ()-0l25 MPa gravi-tational pressure difference was measured over twosuccessive 3-min intervals. The segments were thenflushed by applying a regulated pressure of 0-1 MPa for 2min. After flushing, the segments were again subjectedto the 0-0125 MPa gravitational pressure difference andthe conductance was measured as before. Conductancebefore flushing was usually within 10% of conductancedetermined after flushing.

Stem segments were obtained by first retnoving all ofthe leaves from a stalk then excising the stalk near thesoil surface. The leaves were saved for measurement ofthe total area and area associated with each node, ln thelaboratory, the desired internode or nodal segmentswere obtained by cutting the stalk under water. Thelength of the internode segments used was approxi-tnately 7-lOcm and was detertnined by the total inter-node length. Nodal segments were approximately8-lOcm long and an effort was tnade to maintain aconstant ratio of internode to node tissue in each nodalsegment used. Conductance measuretnents were tnadeas described for leaf segments except that rapid flowrates through stems permitted time intervals of 1 min tobe used.

Hydraulic conductance (L, mmol s~' m MPa^') wascalculated from the flow rate (mmol s"') of a degassed,filtered 10 mol m""* oxalic acid solution through leaf andstem segments, divided by the gravitational pressuregradient (MPa m~') along the segtnents (Eqn I). Thesemeasurements were made with an apparatus similar to

Hydraulic architecture of sugarcane 473

0.0 0.2 0.4 0.6

Leaf Area (m^ plant"

Figure 1. (A) Average transpiration tate per unit leaf area inrelation to plant size for sugarcane growing in the field (O) andin the greenhouse ( • ) . (B) L^ of the uppermost fully expanded(TVD) leaf in relation to plant size. L^^f. and corresponding leafareas represent averages over ()• 1 m- ranges of leaf area. Verticaland horizontal bars indicate SEs (/; = .5-11).

that described by Sperry, Donnelly & Tyree (1988). Theapparatus was configured to accommodate six samples.Flow rates were expressed as mmol s~' for comparisonwith conventional units used for transpiration. Thisfacilitated calculation of //) situ pressure gradients usingleaf specific conductivity (L^c) and transpirationmeasurements. L c of stem segments was calculatedfrom L and the total leaf area above the segmentaccording to Eqn 2. L^^. of TVD leaves was cotnputedfrom L of leaf segments according to Eqn 2 in which Awas the area of the leaf from which the segment wasobtained, and L was multiplied by the ratio of total leafwidth (excluding the tnidrib) to the segtnent width.

Determination of vulnerability of leaf xylem to cavi-tation and embolistn fortnation is described in detailelsewhere (Neufeld t'/o/. 1991). Briefly, leaf blades wereexcised at the ligule and allowed to dehydrate in thelaboratory for varying periods of lime. A 20-cni segmentwas excised approximately 30cm from the leaf tip fordetermination of leaf water potential (V^i) with apressure chamber (Saliendra, Meinzer & Grantz 1990).The rest of the leaf was then submerged in the oxalic acidsolution where a second 20-cm segment was cut at adistance greater than 20cni from either cut end.Hydraulic conductivity was measured in 0-75 X 7cmsegments excised frotn this segment as described above.The per cent loss in conductivity corresponding to agiven V-'L was detertnined fiotn the ratio of conductivitybefore flushing to that after (lushing for 2 min at apressure of 0-1 MPa to remove embolistns.

Transpiration and H^ gradients

Transpiration of greenhouse-grown plants wasmeasured gravimetrically and scaled to a unit leaf areabasis. The pots were sealed in plastic bags duringmeasuretnents and weighed frequently. Weights wererecorded to the nearest 0-lg and time intervals to thenearest second. Canopy transpiration of field-grownplants was determined as evaporative heat flux with theBowen ratio technique and was scaled to a unit leaf areabasis using the leaf area index as described previously(Meinzer & Grantz 1989, 1990).

Water potential of segments of the TVD leaf bladeexcised approximately 1 m from the ligule was measuredwith a pressure chamber on clear days between 1000 and1400h when E was maximal. In greenhouse-grownplants, stem water potential at the soil surface wasestimated from the water potential of a covered, non-transpiring leaf attached to the stem near the origin ofthe crown roots (Saliendra & Meinzer 1989). Since V oiifor these well-irrigated plants was near zero, the V' s,em atthis point also represents an estimate of the difference inV across the entire root systetn.

RESULTS

E decreased with increasing plant size above approxi-tnately 0-2m' total leaf area (Fig. lA). Therefore,transpiration on an entire plant basis did not increaselinearly with increasing leaf area; it tended to saturateabove about 0-3 m" leaf area per plant after exhibiting aninitial rapid increase (data not shown). This pattern wasnot caused by self shading because transpirationbehaved similarly in field-grown plants and in isolated,well-illuminated greenhouse-grown plants. Further-tnore, the shoot architecture of sugarcane would mini-tnize self-shading.

The Lsc of the uppermost fully expanded leaf exhi-bited a plant size dependence sitniiar to that of £and wasmaximal in plants with approximately 0-2m" leaf area(Fig. IB). The magnitudes of Lsc and £ given in Fig. 1suggested that substantial gradients in V' should developalong sugarcane leaves at maximal transpiration rates.The predicted gradient {E/L^^., Eqn 3) varied from 0-67MPa tn~' in plants with 0 2 m' leaf area to 0-47 MPa m~'in plants with {)-83m~ leaf area (Fig. 2A; solid symbols).This gradient would have remained constant withincreasing plant size if L c and E had varied with plantsize in a precisely parallel fashion (Fig. 1). If stomatalregulation were such that E remained constant at itsmaximum value of 4 mmol m~~s~' with increasing plantsize, while L c continued to exhibit the pattern shown inFig. 1, the predicted gradient in V''along the TVD leafwould have increased to 12 MPa tn ' in plants with0-83111 leaf area (Fig. 2A; open symbols).

Actual measurements of ^\ at tnaxitnum transpir-ation rates in TVD leaves of greenhouse- and field-grown plants indicated a tendency for V L to increase

474 F. C. t\/leinzer et al.

-2 ,00.0 0,2 0,4 0,6

Leaf Area (m^ plant"

0,8

Figure 2. (A) Relationship between plant size and predictedgradient in water potential (AVVA,<r) along the TVD leaf atmaximum transpiration rate. Water potential gradients werecaleulated from data presented in Fig, 1 using either theobserved relationship between leaf area. L c and transpiration(•) , or assuming that transpiration remained constant at itsmaximum value (4mmol m ' s ') with increasing leaf area (O),(B) Average minimum leaf water potential of well-irrigated,greenhouse- (•) and field-grown (O) sugareanc in relation toplant size. The dashed line represents the predicted minimumwater potential if maximum transpiration remained constantinstead of decreasing with plant size as shown in Fig, 1, Thedotted line indicates the water potential corresponding to 5()'Xiloss in hydraulic eonduetanee from eavitation and embolismformation.

with increasing plant size (Fig, 2B, solid line). This wasin agreement with the predicted decrease in the Vgradient along these leaves with increasing plant size (cf,Fig, 2A; solid symbols). If E had remained constantwhile L,c decreased (cf. Fig, 2A; open symbols), Vi,would have decreased with inereasing plant size (Fig,2B, dashed line). Under these conditions, Vi, in plantswith approximately 0-45 m^ leaf area would havedropped below the -1-3 MPa threshold correspondingto 50% loss in hydraulic conductivity from cavitationand embolism formation (Fig, 2B; dotted line).

Analysis of the trend in L c along the flow pathwaywithin the stem and from the stem to the leaf bladerevealed hydraulic constrictions at nodes and in the leafsheath and blade (Fig, 3), L j. of internode segments wasapproximately an order of magnitude higher than that ofadjacent nodal segments, L c dropped by an additionalorder of magnitude along the leaf sheath and blade.Thus, Lsc lower in more distal portions of the hydraulicpathway of individual leaves. ,

E of younger internodes near the top of the plant washigher than that of older internodes near the soil surface(Fig, 4A). E of nodal segments was considerably lowerthan that of internodes, and in contrast to internodes,tended to decrease with increasing distance from the soilsurface. When hydraulic properties were expressed onthe basis of leaf area supplied (L^c), internode conduc-tivity increased even more with height above theground, while nodal conductivity remained constant orincreased slightly. These patterns caused the predictedgradient in V sicm in a rapidly transpiring sugarcane plantto decline substantially with increasing height above theground (Fig, 5), The small magnitude of the stem ^gradient in comparison with the leaf ^^ gradient (Fig,2A) was attributable to the large hydraulie capacity ofthe internodes (Fig, 4),

LscS of stem, node/leaf sheath, and leaf bladesegments were used with tnaxitnum transpiration ratesmeasured in intact plants to calculate V at differentpoints along the stem/leaf pathway (Fig, 6), Using areference value of stem ^near the soil surface, obtainedfrom the Vof a basal, covered, non-transpiring leaf, thepredicted value of TVD leaf V was in close agreetnentwith tneasured values. Over 90% of the total drop in V ,

Blade6,8 ±

Internode/Node/Sheath11,4 ± 1,5

Node65,6 ± 21

Internode583 ± 120

Figure 3. Diagram showing L,e (±SE; « = 4) below and abovethe third node of a sugarcane plant, L,,s for the internode andinternode/node/internode are based on total leaf area above thethird internode, L s for internode/node/sheath and blade arebased on the area of the leaf attached to the node. Values are inmmol s ' m ' MPa ',

Hydraulic architecture of sugarcane 475

150

Position

Figure 4. Hydraulic conduetanee and L ^ of node and internodesegments in relation to their positioti along the stetii. Total leafarea distal to a given node or internode was used lor L^^computations.

from the soil surface alotig a 2-4m hydraulic pathway toa point 1 m along the TVD leaf, occurred within the leafitself.

DISCUSSION

E and L^c exhibit parallel changes with increasing leafarea in growing sugarcane. L,,. was tneasured by apply-ing a hydrostatic pressure to excised leaf segments, andpresumably reptesents a fixed physical property of theleaf vasculatuie. Transpiration, on the other hand, was

4 6 B 10

Internode Position

Figure 5. Predieted gradient in water potentialalong internodes as a function of their position below the nodeof TVD leaf attaehtnent. Gradients wete ealeulated fromeonduetivity measuretiients lor live plants with an average leafarea of 0-68 nr and an average tratispitation rate of 2()9 mmol

g. 1).

measured in entire intact plants and reflects a dynamicphysiological response of the stomata to changes asso-ciated with increases in plant size (Meinzer & Grantz1990). This coordination of stomatal regulation oftranspiration with leaf hydraulic properties duringdevelopment prevents \PL of well-irrigated plants fromfalling below the point at which xylem conductivitydeclines due to cavitation and etnbolisrn formation.Average trtinimum V ,, ranged from approxitnately - 1 • tMPa in plants with 0-1 m~ leaf area to —0-9 MPa in plantswith O-Sm" leaf area. The first detectable embolism-induced loss in L in the cultivar studied occurred at a '/'Lof about -1-0 to - M MPa, which increased to a 50%loss in conductivity at a '^\ of about —1-3 MPa, thoughsubtantial genotypic variation in the vulnerability ofsugarcane leaf xylem to cavitation exists (Neufeld et al.1991).

0.0

-0.2

DQ.

-0.4

-0.6

-O.B

-1.0

- 1 . 2

Stem• OOOO-OO 02.. Leaf Sheath

\Leaf

\

•

•

Blade

0.0 0.4 O.B 1.2 1.6 2.0 2.4

Distance From Soil (m)

2.B

Figure 6. Predicted (O) and measured (•) water potential ingreenhouse-grown sugarcane plants as a funetion of height abovethe soil surface. Average maximum transpiration rates of liveplants ranging in leaf area from ()• 18 to ()-23m" wete used withappropriate L s of stem, node/leaf sheath and leaf bladesegments to obtain predicted values of water potential.

It has been proposed that a principal consequence ofstotnatal limitation of transpiration in droughted plantsis the avoidance of catasttophic xyletrt dysfunctionresulting ftotn runaway etiibolistn formation (Tyree &Sperty 1988). Our results indicate that even in well-irri-gated sugatcane plants, stotnata pennit V i, to fall tolevels slightly above those associated with substantialloss in L caused by etnbolism fortnation. L c andstotnatal regulation of gas exchange were coordinatedwith changes in leaf area, root system size and therelative efficiency of the roots in supplying the leaveswith water during nortiial development independent ofsoil water supply. These dynatnic changes in stomatalregulation of transpiration and in leaf hydraulicproperties tnust be taken into account in the interpreta-tion of patterns of water use and hydraulic architectureof plants at different stages of development.

The results obtained heic indicate that approximatelyone-half of the total hydraulic resistance of the soil/root/

476 F. C. Meinzer et al.

leaf pathway was located in the soil and roots (Fig. 6).This is in agreement with the findings of Saliendra &Meinzer (1989), who used transpiration rates and in situmeasurements of ^ in plants of the same cultivar used inthe present study to partition total hydraulic resistanceinto a shoot and a soil/root component. In a contrastingsugarcane cultivar (Saliendra & Meinzer 1989), totalshoot hydraulic resistance was similar, but the soil/rootresistance comprised only 28% of the total resistance.

The pattern of decreasing L c along the internode/node/leaf sheath/leaf blade pathway (Fig. 3) supportsthe idea of plant segmentation into regions differing inwater transport efficiency and susceptibility of xylem tocavitation (Zimmermann 1983). Hydraulic constrictionsat nodes and in petioles of other species (Begg & Turner1970; Sperry 1986) result in abrupt decreases in themagnitude of the xylem pressure potential at thesepoints. This would tend to confine cavitation andembolism formation to the relatively expendable leavesand conserve water transport through the stem which iscritical for survival of meristematic zones.

The acropetal increase in L c of sugarcane steminternodes (Fig. 4) contrasts with the pattern observedin several woody species in which L c has been reportedto decrease acropetally along the trunk and branches(Ewers & Zimmerman 1984a,b; Salleo, Rosso & LoGullo 1982). These contrasting patterns of hydraulicarchitecture reflect differences in shoot architecture andgrowth habit between sugarcane and dicotyledonouswoody species. Sugarcane grows indeterminately, with-out branching. As stem elongation proceeds, a maxi-mum leaf area is attained, which is maintained by abalance between leaf production and leaf shedding.Thus, as the distance between the soil and the transpir-ing portion of the plant increases, acropetally increasinginternode conductivity would partially offset the expec-ted decline in total stem conductivity associated withincreasing stem length. This would cause the \f gradientalong the stem to decrease with increasing distance fromthe soil (Fig. 5) and allow stem ^ to remain relativelyconstant with increasing stem length.

The nature of the signal enabling sugarcane stomatato coordinate transpiration with changes in planthydraulic properties is uncertain. It is unlikely that thesignal is related to leaf water status or other leafproperties. Developmental patterns of stomatal conduc-tance that regulate transpiration as shown in Fig. 1 aredetermined by changes in the composition of the xylemsap arriving at the leaves rather than by alterations ininherent stomatal properties at the leaf level (Meinzer,Grantz & Smit 1991). It is conceivable that xylem sapcomposition at sites of leaf vascular development couldinfluence subsequent hydraulic properties of the matureleaf xylem. For example, the phytohormones auxin,cytokinin and gibberellin have been reported to influ-ence vascular differentiation and subsequent character-istics of mature xylem elements (Aloni 1987) and arealso constituents of xylem sap (Goodwin, GoUnow &

Letham 1978). L c of the root system in sugarcane alsoexhibits developmental variation similar to thatobserved for stomatal conductance, transpiration andL,e of the TVD leaf (Meinzer et al. 1991). The signalsresponsible for coordination of stomatal properties withroot and shoot hydraulic properties during developmentthus appear to originate outside the leaves, probablywithin the roots and presumably reflect developmentalvariation in the relative ability of the root system tosupply the shoot with water.

During moderate soil drying, VL in sugarcane remainsnearly constant as a result of parallel declines in stomataland root hydraulic conductance (Saliendra & Meinzer1989). This may be another manifestation of a root-based signal permitting sugarcane stomata to maintainy L above levels leading to catastrophic xylem failure.This coordination of gas exchange with physical andphysiological changes in hydraulic properties duringdevelopment merits further study.

ACKNOWLEDGMENTS

This work was funded in part by U.S. Department ofAgriculture/Agricultural Research Service cooperativeagreement 58-91H-8-143 with the Hawaiian SugarPlanters' Association.

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Received t7 June 1991; received in revised form 6 September 1991;accepted for publication 10 December 1991