Journal of Experimental Botany, Vol. 47, No. 296, pp. 387-401, March 1996Journal ofExperimentalBotany

Hydraulic and osmotic properties of oak roots

Ernst Steudle1 and Anatoli B. Meshcheryakov2

Lehrstuhl fOr Pflanzenokologie, Universita't Bayreuth, D-95440 Bayreuth, Germany

Received 9 May 1995; Accepted 27 October 1995

Abstract

Hydraulic and osmotic properties of root systems of2.5-8-months-old oak seedlings (Quercus robur andQ. petraea) were measured using the root pressureprobe. Root pressures of excised roots rangedbetween 0.05 and 0.15 MPa which was similar tovalues obtained for herbaceous species. Roothydraulic conductivity (Lpr; per unit of root surfacearea) was much larger in the presence of hydrostaticthan in the presence of osmotic pressure differencesdriving water flow across the roots. Differences wereas large as a factor of 20 to 470. Roots of the youngseedlings of Q. robur grew more rapidly than those ofQ. petraea and had a hydraulic conductivity which wassubstantially higher. Nitrogen nutrition affected rootgrowth of Q. robur more than that of Q. petraea, butdid not affect root LpT of either species. For O. robur,LpT decreased with root age (size) which is interpretedby an effect of suberization during the development offine roots. Root hydraulic conductance remained con-stant for both species. For Q. robur, this was due tothe fact that the overall decrease in LpT was compens-ated for by an increase in root surface area. Rootreflection coefficients (er,r) were low and rangedbetween <rsr=0.1 and 0.5 for solutes for which cellmembranes exhibit reflection coefficients of virtuallyunity (salts, sugars etc.). Solute permeability was smalland was usually not measurable with the technique.When root systems were attached to the root pressureprobe for longer periods of time (up to 10d), solutepermeability increased due to ageing effects which,however, did not cause a general leakiness of the rootsas LpT decreased. Hence, values were only used frommeasurements taken during the first day. Transportproperties of oak roots are compared with thoserecently obtained for spruce (Rudinger et al., 1994).They are discussed in terms of a composite transport

model of the root which explains low root <r,r at lowsolute permeability and reasonable root Lp^ The modelpredicts differences between osmotic and hydraulicwater flow and differences in the transport propertiesof roots of herbs and trees as found.

Key words: Composite transport, hydraulic conductivity,nitrogen nutrition, Quercus, reflection coefficient, roottransport, water relations.

Introduction

In order to meet the demands of transpiring shoots,plants have to be efficient in the acquisition of water fromthe soil. The hydraulic conductance of root systems hasto be sufficiently high, which requires both a reasonablehydraulic conductivity of roots (= hydraulic conductanceper unit root surface area) and a large root surface area.On the other hand, roots have to retain nutrient ions(solutes) once taken up into the xylem by active processes.This means that passive losses across the root cylinderhave to be low due to a low solute permeability. An'optimization' of root permeability (water versus solutes)is required (Steudle, 1994ft) which is related to the devel-opment of transport barriers, namely, of Casparian bandswhich prevent the backflow of solutes.

Root water transport also involves osmotic processes,i.e. interactions between water and ion transport. Theaccumulation of solutes in the root xylem creates osmoticforces which drive some water flow. From roots ofherbaceous plants, there is considerable evidence thatosmotic pressure differences across the root cylinder causewater flows which are substantially smaller than thosecaused by equivalent hydrostatic gradients (e.g. causedby transpiration). In part, this is due to root reflectioncoefficients smaller than unity, which would be the valuefor an ideal osmometer. In herbs, root reflection coeffi-cients range between air = 0.4 and 0.8 (Steudle, 1994a, b)

1 To whom correspondence should be addressed. Fax: +49 921 552564.1 Permanent address: Timiriazev Institute of Plant Physiology, Russian Academy of Sciences, Botanitcheskaya 35, Moscow 127276, Russia.

Oxford University Press 1996

388 Steudle and Meshcheryakov

for solutes for which root cell membranes would exhibitreflection coefficients of close to unity (salts, sucrose,mannitol, etc.). In spruce roots, o,T values are even smaller(cr5r = 0.2 to 0.3; Rudinger et ctl., 1994).

In roots of herbs and trees, large differences have beenfound between hydraulic and osmotic water flows.Gradients in hydrostatic pressure caused much higherwater flow than osmotic gradients. Hence, there are alsobig differences between osmotic and hydrostatic root LpT.In spruce, differences were as large as two to three ordersof magnitude (Rudinger et al., 1994). Low root reflectioncoefficients corresponded with big differences betweenosmotic and hydraulic water flow (osmotic and hydraulicLpT). At least qualitatively, the findings have beenexplained by the 'composite transport model of the root'(Steudle, 1992, 1994a, b). During radial uptake of water,the model considers parallel apoplastic and cell-to-cellpathways. An apoplastic bypass of water in the root ispossible if Casparian bands are somewhat permeable towater, but are virtually impermeable to ions or if regionslacking Casparian bands contribute to the overall radialwater flow (secondary root initials, the very tip of roots;Steudle, 1992, 1994a, b; Steudle et al., 1993). In terms ofthe model, a low root a,T as well as differences betweenosmotic and hydraulic water flow and between herbs andwoody species would be explainable. The model alsoexplains variable hydraulic resistances of roots whichhave often been reported in the literature (Fiscus, 1975;Weatherley, 1982; Passioura, 1988).

There are several reports on hydraulic properties oftree roots (e.g. Sands et al., 1982; Colombo and Asselstine,1989). Usually, the pressure chamber technique (Fiscus,1975) has been used to measure hydraulic conductancesof excised root systems. Effects of phosphorus nutritionand of root age on root hydraulics have been investigatedfor both herbaceous and woody plants (Radin andMatthews, 1989; Andersen et al., 1989). In other studies,effects of environmental factors such as drought, highaluminium, or mycorrhizae have been determined (Safiret al., 1972a, b; Sands and Theodorou, 1978; Kruger andSucoff, 1989; North and Nobel, 1991). These studiesshow that nutrients probably act on root hydraulics byaffecting root surface area (biomass) and that effects ofmycorrhizae on root hydraulic conductivity are small.Root age and suberization play an important role.However, despite these facts, a detailed knowledge ofmechanisms of water uptake into tree roots and of howpathways of water would be affected by environmentalfactors and root development is still missing. Properquantitative transport models are still lacking. Modelsshould also imply interactions between water and solute(nutrient) flows in the root which are crucial for anunderstanding of osmotic properties of roots.Measurements of relevant parameters are rare for tech-nical reasons. For example, there is, to date, no rigorous

analysis of water relations of tree roots at the cell androot level as they have been performed for herbaceousplants (Steudle and Jeschke, 1983; Steudle et al., 1987;Steudle and Brinckmann, 1989; Zhu and Steudle, 1991).At the level of entire root systems, there are two recentstudies of osmotic and hydraulic properties on spruce(Rudinger et al., 1994) and on some tropical tree species(Tyree et al., 1995). However, only the study on spruceprovides data of transport coefficients (LpT, atT). It wouldbe expected that there are differences between deciduousand coniferous trees. For example, it is known that rootpressures of conifers are much smaller than those ofbroad leaf trees (Kramer, 1983) which may be related todifferences in solute permeability, rates of ion pumping,or root reflection coefficients. Also, there are differencesin xylem structure which may affect the hydraulic conduct-ance of root systems.

In the present paper, hydraulic and osmotic propertiesof root systems of oak seedlings grown in sand culture inclimatic chambers were investigated. In order to providesome variation in the morphology and extension of rootsystems, two different mesotropic mid-European species{Quercus robur and Q. petraea) were used. At seedlingage, Q. robur grows faster than Q. petraea and preferswetter and deep-layered soil with a higher nutrient content(Levy et al., 1992). In order to vary growth rate, theseedlings were grown at a high and a low level of nitrogennutrition. Besides the root hydraulic and osmotic proper-ties, root biometry was carefully investigated.

Materials and methods

Materials

Acorns of two different species of oak (Q. robur and Q. petraea)were germinated for 4 weeks at 28 °C in a humid chamber onvermiculite soaked with tap water. When the seedlings hadattained an above-ground height of 100 mm, they weretransferred to sand culture (particle size: 1.5 mm) and grown ingrowth chambers (day/night rhythm: 15/9 h; 20/18°C).Seedlings were watered three times a week with nutrient solution(composition in mM: K 0.6, Mg 0.3, Ca 0.13, PO4 0.1, SO4 0.3;micronutrients in ^M: Mn 3.6, Cu 0.3, Zn 0.3, BO3 2.4,FeEDTA 140; Lutz and Breininger, 1986). Seedlings receiveddifferent nitrogen treatments (0.5, 1.0, and 5 mM N) which wassupplied as Ca(NO3)2, KNO3 and NH4NO3 in the molar ratioof 1:2:3 to the nutrient solution. When used in experiments,seedlings were between 100 and 400 mm tall and had developed4-30 leaves. Stem diameters at the shoot base were 2-8 mmdepending on seedling age. Root fresh weights ranged between2-100g. Root systems were 100-400 mm long.

Root surface area

Root surface area was measured after washing out the sandand subdividing the roots into primary (P) and three differentorders of lateral roots (orders 1 to 3). Usually, fourth orderlaterals were not present. Diameters of primary roots rangedbetween 4-8 mm, first order laterals between 2.5—4 mm, andsecond order laterals between 1 2.5 mm. Third order laterals

(fine roots) were smaller than 1 mm in diameter. Diameters andlengths of primary roots and of first and second order lateralswere measured using a caliper and a ruler. Surface areas of fineroots were determined using an image analysing system asdescribed earlier (Image Analysing Programme, SkyeInstruments Ltd., Llandrindod Wells, UK; Rudinger et al.,1994). For better contrast, fine roots were stained withtoluidine blue O.

Measurement of root pressure (PJ and of root hydraulicconductivity (LpJ

Prior to the hydraulic and osmotic experiments with excisedroot systems, seedlings were well watered by circulating nutrientsolution through the soil (sand) for 2-3 h (Fig. 1). Shoots wereenclosed in black plastic bags for 4 h to prevent transpirationand to establish a positive root pressure so that no air enteredthe xylem during excision. Shoots were excised with a razorblade leaving a stump of 10-70 mm above-ground. Stumpswere tightly connected to a root pressure probe using siliconeseals which were adapted in diameter to that of the stem(Fig. 1). Seals were 10 mm long to provide a tight connectionbetween probe and stem when compressed by the screw.Compared with herbaceous plants, the problem of an interrup-tion of tracheary elements was low with woody stems (Rudingeret al., 1994; Hallgren et al., 1994). Nevertheless, it was regularlytested whether or not sealing did cause artificially high axialresistances and if those resistances contributed significantly tothe overall resistance. Great care was taken to remove all airbubbles from the measuring system. Air bubbles would havedampened changes in root pressure and would have reducedthe sensitivity of the measuring system.

A solution of 0.1 mM CaCl2 was placed on top of the cut

Hydraulic properties of oak roots 389

root surface. It filled part of the equipment (Fig. 1). The restcontained silicone oil of low viscosity (Type AS4 from WackerGmbH, Munchen, FRG) so that a meniscus formed betweenoil and water in the measuring capillary. By keeping themeniscus at a certain position (stereo microscope) steady-stateroot pressures (PIO) were established at zero water flow acrossthe root. Water flows were induced either by changing root(xylem) pressure (PT) with the aid of the metal rod (Fig. 1;'hydrostatic experiments') or by exchanging the soil solutionwith a solution having a different osmotic pressure ( / ; 'osmoticexperiments'). Thus, a water potential difference driving waterflow across the root was created either by a change producedinside the probe (Pr) or in the medium (TT°). Characteristic 'rootpressure relaxations' were obtained. After a change, the systemtended to assume a new stationary state (water flow equilibrium).Rate constants (k^) or half-times (Tf/2) of pressure relaxationsrepresented a measure of the rate of water exchange betweenxylem and medium and were related to the hydraulic conduct-ance of the root system (Lr in m ; )s~1MPa~1 which is theinverse of its hydraulic resistance, Rr; Steudle, 1993, 1994a):

1/2

1 APT

' RrAVs(1)

The factor APJA Vs is the elastic coefficient of the system whichwas also measured with the root pressure probe by instantan-eously moving the meniscus in the measuring capillary of theprobe (A Vs) and recording the peak response in pressure on achart recorder (APt). Largely, APJA Vs is a property of the rootpressure probe (Steudle et al., 1987). It represents the inverseof the storage capacity of the equipment plus xylem for waterin MPa m~3. Provided that the axial (longitudinal) hydraulic

screw

metal rod

water

siliconeseal

oak rootin sand nutrient solution

with differentosmolality

Fig. 1. Experimental set-up for measuring root pressure and root hydraulic and osmotic properties. Oak roots were tightly connected to the rootpressure probe by a silicone seal after completely filling the equipment with water and silicone oil. Root pressures were measured with the aid of apressure transducer. During measurements, an oil/water meniscus in a measuring capillary served as a point of reference. By moving the meniscuswith a metal rod within the capillary (diameter: 250-625 ^m) defined changes of volume could be created in the system. Corresponding changes inroot pressure were measured with the pressure transducer so that the elastic coefficient of the measuring system could be evaluated as well. Whena steady-state root pressure was attained, water flows across the root were induced either by changing the hydrostatic pressure in the system (metalrod; 'hydrostatic experiments'; Fig. 3A) or by rapidly exchanging the soil solution ('osmotic experiments'; Fig. 3B). From the resulting relaxationsin root pressure back to a steady-state value, half-times of water exchange and root hydraulic conductivity (Lp,) were evaluated (Eq. (1)).

390 Steudle and Meshcheryakov

resistance of the root is much smaller than the radial, Lr isdirectly related to the radial hydraulic conductivity of the root(Lpr in m s"1 MPa~') by the surface area of the root (AT):

L=ArxLn (2)

It should be noted that the geometric surface should be differentfrom the 'effective root surface area'. For example, if there aresuberized or thickened roots which exhibit an LpT of virtuallyzero, the effective surface area would refer only to youngerroots or root zones still allowing radial flow of water. Inaddition, the very ends of root tips where xylem elements arenot yet matured should not contribute to the effective rootsurface area (Peterson and Steudle, 1993). At an extreme, onlythe fine roots or part of them would be employed in wateruptake. To account for the 'composite nature' of root hydraulicconductance, Eq. (2) is re-written as:

r = £ A\Lp\. (3)

Here, the superscript 'i' denotes the different root parts orzones. The real situation is, however, less complicated than itwould appear from Eq. (3), since fine roots contributed most(71-90%) to the geometric root surface area (Ar). Therefore,the total (geometric) root surface area was used in this paperto evaluate an 'overall LpT' which would have the meaning ofan averaged value. It would largely refer to fine roots whichare assumed to be the most active in the acquisition of water.

In osmotic experiments it was found that 7*7/2 w a s muchlarger than in hydrostatic. This could have resulted fromunstirred layers in the soil, i.e. from a long time intervalnecessary to completely exchange soil solutions. The possibilitywas tested by exchanging the nutrient solution for nutrientsolution containing an osmotic test solute and following thechanges in concentration with time at the outlet of the pot withthe root system (Fig. 1). It was proved that the half-time forexchanging soil solution was much smaller than the half-timesof water exchange measured in osmotic experiments (Fig. 2).In some experiments, hydrostatic A:WI(7*7/2) w a s estimated fromexperiments in which the water potential of the soil solutionwas raised by applying pneumatic pressure to the roots andfollowing the response in xylem (root) pressure using thepressure probe. During these experiments, the pot with the rootsystem was enclosed in a pressure chamber which allowedpressurization (chamber not shown in Fig. 1; RUdinger el al.,1994). With the aid of a tank with compressed air, it waspossible to raise pneumatic pressures within the chamber witha time constant which was somewhat smaller than thoseobtained during conventional root pressure relaxations.

Root reflection coefficients (csjIn osmotic experiments, Pr(i) curves should, in principle, bebiphasic. A rapid phase of water flow equilibration ('waterphase') should be followed by a slower 'solute phase' whichindicates a passive uptake of the osmotic solute (Steudle el al.,1987; Steudle, 1993, 1994a). However, with the oak rootsinvestigated here, solute phases were missing in most of theexperiments which indicated low solute permeability. Hence, noexact values of permeability coefficients could be given.Reflection coefficients of root (an) were determined frommaximum changes of root pressure (dPimMX or APrTnin) at agiven change in the osmotic concentration of the medium(osmotic pressure change, A-n°\ Steudle, 1993, 1994a):

15 -

0 -J-r

-zr i Kg'1 Nad

+27

2 1

Tone, t (min)

tl-n(4)

Fig. 2. Rate of exchange of soil solution in the pots used to grow oakseedlings and to perform osmotic experiments (volume: I I ) . In theexperiment shown, soil (sand) was soaked with nutrient solution whichwas then changed by nutrient solution plus an osmoticum (first arrow;+ 27 mOsmol kg"1 of NaCI). Increase in osmotic concentration at theoutlet of the pot was measured with time (Fig. 1). When concentrationwas constant, solution was changed back to the original one (secondarrow). Half-times required for the exchange of soil solution were 11 sand 7 s, respectively. This is much shorter than the half-times foundduring osmotic root pressure relaxations (Fig. 3B; Tables 2-4). Notethat the concentration difference measured at the outlet of the pot wassmaller than that in the solution because of a mixing of the solutionadded with the soil solution. For the calculation of changes of osmoticconcentrations, measured values were used.

It was generally valid that the elastic modulus of the xylem (e jwas much larger than the osmotic pressure of xylem sap (77,).Hence, the second term on the right side of Eq. (4) could betaken as unity to a good approximation. Because of the lowrate of permeation of solutes across the root cylinder, acorrection for the solute flow into the xylem during the timebetween application of solute and minimum (maximum) wasnot necessary.

Typical course of an experimentIn a typical experiment, roots were connected to the rootpressure probe. A steady-state root pressure was establishedafter a few to 20 h. Then the elastic coefficient of the system(APJAVS) was determined, followed by measurements ofhydrostatic and osmotic pressure relaxations. The root systemwas left on the pressure probe for several days, and measure-ments were repeated each day. At the conclusion of theexperiments with each root system, the stem was cut at the sealto ensure that the hydraulic resistance of the sealing area wasmuch smaller than that of the root. This was indicated by arapid drop of root pressure to zero upon cutting the xylem anda decrease of half-time of water exchange by at least a factorof five after the cut. Otherwise, the results from that root weredisregarded. The evaluation of root transport coefficients (Lpr,olr) according to Eqs. (1) and (4) was based on a two-compartment model of the root (root xylem and soil solutionseparated by an osmotic barrier). The model assumes that axialhydraulic resistances were much smaller than the radial. Thiswas tested by excising a single or a few roots while the rootsystem was still attached to the probe and measuring the effectof excision on the steady-state root pressure and on thehydraulic conductance of the root system (half-times of waterexchange).

Results

Root morphology and development

For both oak species, there was no visible infection ofroots by mycorrhiza. Under the conditions used forgrowing the plants, this would probably occur later duringroot development. Although roots of Q. robur differedslightly from those of Q. petraea, morphological differ-ences between species were small during the first 3 months.Later, Q. robur developed a longer tap root thanQ. petraea. Table 1 lists changes in root surface area andfresh weight for two different age classes and levels of Nnutrition (low: 0.5-1.0 mM, and high: 5.0 mM nitrogenin the soil solution). It should be noted that under theconditions used (sand culture; climatic chamber), a levelof 0.5-1 mM of nitrogen was fairly small, but was suffi-cient to provide the development of the seedlings(V. Stiller, personal communication). During 5-8 months,roots of young seedlings of Q. robur grew more rapidlythan those of Q. petraea. Root growth of Q. robur wasmore affected by nitrogen than that of the other species.It can be seen from Table 1 that between an earlier(2.5-4.2 and 2.9-4.3 months) and later (5.3-7.9 and4.7-5.5 months) age, root surface area and fresh weightincreased. However, increases were only significant forQ. robur (P = 0.05). For both species, fine roots (diameter

Hydraulic properties of oak roots 391

< 1 mm) contributed to most of the root surface area(71-90%; Table 1). The contribution of fine roots to theoverall root surface area tended to decline with root ageand increasing N nutrition, but effects were only signific-ant for Q. robur at low N, and for Q. petraea at a higherage. For evaluating 'averaged LpT values' according toEq. (1), the geometric (total) root surface was used.

Typical experiment

Q. robur and Q. petraea showed similar osmotic andhydrostatic responses. A typical experiment is shown inFig. 3 for a 5.7-month-old seedling of Q. robur.Measurements were carried out over a 10 d period. In thebeginning of the experiment, the root system was attachedto the root pressure probe. A steady-state root pressureof / ^ 0.083 MPa was attained after about 18 h. Thenthe elastic coefficient was measured (Fig. 3A, trace a).When root pressure was steady again, hydrostatic(Fig. 3A) and osmotic (Fig. 3B) pressure relaxations wereperformed using NaCl as the osmotic solute. Hydraulicand osmotic responses did not change with the time ofthe day. During the first day, osmotic relaxations weremonophasic, indicating that the solute permeability ofthe root was very small. However, as time proceeded,responses tended to show some measurable uptake of

Table 1. Effect of seedling age and nitrogen nutrition on the surface area and fresh weight of roots of seedlings o/Quercus robur (A)and Quercus petraea (B) grown on sand culture

Surface areas and fresh weights were measured separately for coarse roots (CR = primary roots and laterals of order 1 to 2) and fine roots (FR;laterals of order 3 with a diameter of < 1 mm). It can be seen that fine roots represented most of the root surface area. Roots from the two specieswere grouped into two similar age classes. Mean values are given ±SD; n = number of roots (trees). Although root area and root fresh weighttended to increase with increasing N nutrition and age, effects were not significant except for Q. robur at higher age (Mest; /> = 0.05; values incolumns followed by different letters differ significantly). Different from Q. petraea, Q. robur grew faster during the course of the experiment andgrowth was significantly affected by N treatment. This is also evident from the absolute values of root surface area and fresh weight gained after5-8 months. Percentages of contribution of fine roots to total root surface area tended to decrease with increasing age and N nutrition, but effectswere only significant for Q. robur at low N, and for Q. petraea at high age.

Seedlingage[month]

Nitrogenconcentrationof medium[mM]

(A) Q robur2.5-4.2

5.3-7.9

0.5 to 1.0

5.0

0.5 to 1.0

5.0

(B) Q petraea2.9-4.3

4.7-5.5

1.0

5.0

1.0

5.0

Numberof trees

7

2

5

2

2

3

5

4

Typeof roots

CRFRCRFRCRFRCRFR

CRFRCRFRCRFRCRFR

Root area[mm2]

2700±141034 540±244909 190±3 170

3699O±1389O17690±1435054 140±1142057280±5 160

191290±55040

5 350±123016880±1726012180±987048810±15 1008 830 ±3 620

42 140± 1080019280±873047210±1855O

Root areaP/oOftotal area]

9.7%90.3±6.3a20.0%80.0±0.6 ab22.7%77.3 ± 12.2 b23.4%76.6±3.6b

31.5%68.5 ± 24.5 ab18.0%82.0 ± 7.8 ab17.2%82.8±4.2 a29.5%70.5 ± 8.3 b

Root freshweight[g]

3.1 ±2.14.0±3.38.3±4.35.6±1.7

14.3± 10.56.4 ±1.8

40.0 ±5.220.7 ±4.6

2.4±0.31.6±1.66.6 ±3.95.1 ± 1.74.8 ±2.03.2 ±0.9

10.7±5.54.4± 1.5

Total rootarea[mm2]

3724O±25710a

46 180± 17070 ab

71 830± 19880 b

248 570 ±60 200 c

2223O±1849Oa

60990±24 530a

50970±13220a

66490±23270a

Total rootfresh weight[g]

7.1 ±5.3 a

13.9±6.0ab

20.7± 10.6 b

60.7±9.8c

4.0 ±2.0 a

11.7±5.3 a

8.0 ±2.7 a

15.1 ±6.2 a

392 Steudle and Meshcheryakov

Day 1

a.- o.io

0.06

Day 2T^-2.38 TVi-2.0 8

60 120 60 120

60 120 180

Day 10

a.- o.io-

0.06-

0

Tva- 7.8 8 I T i , - 8.4 8

f-

0 60 120 180 0

Time, t (s)

60 120

Fig. 3. Typical root pressure probe experiment with an excised root system of a seedling of Quercus robur (seedling no. 12) grown for 5.7 monthsin sand culture. The experiment lasted over 10 d. For the sake of clarity, hydrostatic (A) and osmotic (B) experiments are given separately. Afterconnecting the root to the pressure probe (Fig. 1) for 18 h, a steady-state root pressure of 0.083 MPa was established. In (A), several groups ofvertical peaks are shown (a,c,e,g,i) which are traces of measuring changes in root pressure in response to volume changes (determination of elasticcoefficient of measuring system). Groups of peaks are followed by hydrostatic root pressure relaxations (b,d,f,hj), Measurements were repeatedeach day up to day 10. Examples are shown for days 1, 2, 3, 5, and 10. It can be seen from Fig. 3A that the peaks per unit volume change becamelarger during the time of experiment (i.e. APT/4VS increased; Fig. 4C). Half-times of hydrostatic water exchange increased as well indicating adecrease of the root's water permeability (hydraulic conductivity, Lp^ Eq. (1); Fig. 4B). As for the hydrostatic experiments, examples are shown inFig. 3B for osmotic pressure relaxations (day 1, 2, and 5) using NaCl as the osmotic test solute. The example shown in Fig. 3B demonstrates thatpressure relaxation curves were monophasic on the first day (no solute flow detectable), but tended to become biphasic later. Compared withhydrostatic experiments, half-times of water exchange uere much longer (water permeability, Lp^ much smaller) in osmotic experiments whichindicated a different pathway for water. Reflection coefficients were estimated from maximum changes in root pressure and corresponding osmoticpressure changes of the medium. For NaCl, they ranged between 0.20 and 0.35. At the conclusion of the experiment, the root was cut at the sealto test for a possible interruption of xylem vessels by the sealing procedure (Fig. 3A, traces k and 1). Since TJ2 decreased by a factor of 4-5 afterexcision, it was proved that the seal did not rate limit water flow across the root. Note different scales on the time axes of Figs 3A and B.

Hydraulic properties of oak roots 393

I • +32 nOral Nri3

0.10

0.08

-32 nOralDay 1

O-035

a- 032

Ti- «»•

0 1 2 3 4 6 6 7 8 0 1 0 1 1 1 2

Day 2

0.10

008

0-1

+24 Oral Nad

i £ - 861*

-24 Oral Nad

Om- 0.22

0,-031

0 1 2 3 4 6 6 7 8 9 1 0 1 1 a

0.10

0 J

-MS mOmal N«a -17Day 5

o 1 3 4 S 6 a o io ii a

Time, t (h)

solute (Fig. 3B, traces c and e). This demonstrated thatroots changed their transport properties and became morepermeable to solutes. However, even under these condi-tions, solute permeability was still low and half-times ofsolute exchange long. By contrast to solute permeability,hydraulic conductance decreased during the measure-ments (half-times increased; Figs 3A, 4B). This indicated

that changes in solute permeability were not caused by ageneral leakiness, but were an effect specific to solutes.Since the elastic coefficient (APT/AVS) increased withincreasing age (Figs 3A, 4C), it is evident that the increasein TJ/2 was not caused by a decrease of the elasticcoefficient (Eq. (1)). On the contrary, the increase ofAPJAVS in part compensated for the decrease of root

394 Steudle and Meshcheryakov

Quercus robur

o teeJliu no. O• wading no. 13• Medina no. 16

0.20-

0.15-

0.10-

0.06-

100-

5 0 -

Days of experiment1 2 3 4 5

Days of experiment

Fig. 4. Changes in steady-state root pressure (/>„,, A), hydrostatic half-time of water exchange (T'n, B), elastic coefficient of measuring system{APJA V%, C), and root reflection coefficient ( a^ D) during the time of experiment for root systems of three different seedlings of Quercus robur(seedlings no. 12, 13, and 16; age: 5.7 to 7.2 months). Root pressure gradually decreased and T~l2 and elastic coefficients increased. Reflectioncoefficients either decreased or increased.

hydraulic conductance and reduced 77/2- Because of thechanges found for different parameters with the age ofexcised roots, initial (first day) values were used whichshould have corresponded best to those of the roots inthe intact plant.

Hydraulic conductivity

Mean root pressures did not change during root develop-ment for either species (Table 2) and ranged between afew tenths of a bar to 1.5 bar. Half-times of waterexchange following a change in root pressure (hydrostaticexperiments) were as short as a few seconds. Hydraulicconductivities calculated from hydrostatic TYI2 showedthat root Lpr of Q. robur was substantially smaller thanthat of Q. petraea. For the osmotic LpT, values appearedto be similar, but the evidence here was not as clear(Table 2). Root LpT did not change for Q. petraea for

either treatment (N nutrition and age), but decreasedsignificantly for Q. robur with increasing size (age) of theroot system (Table 2). This suggests that there was nodirect effect of N nutrition on Lpr. Rather, N nutritionappeared to affect Lpr by stimulating root growth anddevelopment. It is remarkable that decreases in root LpT

were not reflected by a decrease of root hydraulic conduct-ance because the decrease in root LpT was compensatedfor by the increase in root surface area (Tables 1, 2).

Cutting experiments revealed that the hydraulic resist-ance of roots was largely determined by radial ratherthan by axial hydraulic resistances (Fig. 5). Tables 3 and4 indicate large differences in the half-times of waterexchange (^"2) between osmotic and hydrostatic experi-ments for both species. Differences were as large as twoto three orders of magnitude. They were caused bycorresponding differences in root hydraulic conductivity

Hydraulic properties of oak roots 395

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and Lpr(O), respectively). As already indicatedby Table 2, the data presented in Table 3 for individualtrees show that for Q. robur, LpT(W) decreased withincreasing age. Again, they suggest that N nutrition didnot cause significant changes in root hydraulic conduc-tivity other than via its effect on root growth.

Pressure chamber experiments

Figure 6 shows a typical experiment in which responsesin root pressure were measured following the applicationof pneumatic pressure to the soil solution in a pressurechamber. It can be seen that responses in root pressurewere fast and similar to those obtained during conven-tional pressure relaxations (Fig. 3). However, there wasa difficulty with these experiments in that the time requiredto change the gas pressure in the chamber was not muchdifferent from T™/2. This was so because the volume ofthe chamber had to be rather large. Thus, the rise timeof root pressure was, to some extent, influenced byexperimental problems, and no data of LpT(H) could beevaluated from transient responses in root pressure duringpressure chamber experiments. Hence, half-times foundin these experiments should represent an upper limit of7^/2 of those roots. Figures 6 and 7 indicate that changesin steady-state root pressure were similar to the changesin air pressure applied to the root medium. This resultdiffers markedly from that obtained in osmotic experi-ments where steady-state responses in root pressure weremuch smaller than changes of the osmotic pressure of themedium (soil solution).

Reflection coefficient

Permeability of roots to test solutes ( K G , NaCl, glycerol,mannitol, sucrose) was low. Usually, there was no permea-tion detectable during the first 2 d. As root systems aged,permeability increased which could have been due tochanges in root structure creating leaks (see Discussion).Although root permeability to solutes appeared toincrease during the course of the experiment, there wasno such effect on the reflection coefficient (Fig. 4). Therewas a fairly high variability in the values of afr. Withinthe limits of accuracy, no differences between species andnutrition on asr could be detected. Nevertheless, the resultsshow that reflection coefficients were low. They rangedfrom asr = 0.1 to 0.5 for solutes for which cell membranesexhibit a, values of virtually unity (KG, NaCl, glycerol,mannitol, sucrose).

Discussion

The results show that hydraulic and osmotic propertiesof oak roots are similar to those of herbaceous speciesand to another woody species (spruce) which have beenmeasured in the past few years in great detail using the

396 Steudle and Meshcheryakov

8.

•xctjion ot latartl root

fi

0.06 -

oao-

-0.06-

out at m l

- 0.3 8

-330 nl -330 rtT w - 0.3 »

30 60 90-i-// / / "

120 30 90 30

Time, t (s)

Fig. 5. Effect of excising a 1.5 mm lateral root of an oak (Q. robur, seedling no. 16) root system grown in sand culture for 7 months on the root'shydraulic properties. Initial root pressure was 0.02 MPa (a, b) and rapidly declined to zero after excision of a lateral of second order (7"1/2 = 2.6s;(c)). Before excision, r " 2 was 4.5 s, and decreased to 1.2-1.5 s after the cut (e), indicating a considerable leak across the excised root and a highaxial hydraulic conductance within the root system. After the cut right at the seal, 7"7/2 was reduced further to 0.3 s (g), i.e. to a value which wasby a factor of 11 smaller than that of the closed root system.

Table 3. Initial (first day) values of hydraulic data (half-time of water exchange, 77/2, and hydraulic conductivity, LpJ and ofreflection coefficients (aK) of roots o/Quercus robur

Hydrauhc conductivities were referred to total root surface area. Depending on the force used to drive a water flow across the root (osmotic orhydrostatic pressure gradient), LpT and Tfc differed markedly (factor of 35-467). Reflection coefficients were low. Roots are ordered according toseedling age. Figures in brackets in the first column denote the nitrogen content of the medium (0.5, 1.0 or 5.0 mM). Mean values are given ±SDfor T~2 and Lp,. In hydrostatic experiments, n = 8-12, and in osmotic experiments, n = 2-4.

Rootno.

I d )2(5)

3(0.5)

4(1)5(0.5)6(5)

7(1)

8(1)

10(1)11(1)12(0.5)13(0.5)14(5)15(1)16(5)

18(0.5)

Rootage[d]

7581

85

8998

105

111

112

125159170170183194215

236

Half-time of waterexchange, T~2 [s]

Hydrostatic

4.8 ±1.77.0±2.7

5.4 ±1 3

3.8±0.76.3 ±3.52.2±0.6

3.3 ±1.0

3.0 ±1.0

4.5 ±1.52.7±0.62.4±0.72.2±0.73.7±0.62.6±0.75.9 ±0.9

29±0.5

Osmotic

55+1398±51

456 ±285

—517±443316±10

525 ±139

341 ±149

——995 ±435800 ±242——235±121

—

Hydraulic conductivity,Lprx 10s [ms~

Hydrostatic

4.8± 1.32.2 ±1.0

3.4±0.2

3.9±0.61 9± 1.22.7±0.8

3.9 ±1.2

l.4±0.5

2.1 ±0.73.1 ±0.81.4 ±0.31.2±0.4

0.53±0.09l.0±0.31.0±0.3

0.6 ±0.1

•'MPa"1]

Osmotic

0.040±0.0100.062 ±0.050

0.043 ±0.027

—0.010±0.006O.033±O.O23

0.030 ±0.021

0.011 ±0.005

——0.003 ±0.0010.004±0.001——0.025 ±0 017

—

Lpr(H)/Lpr(O)

12035

79

19082

130

127

——467300——

40

—

Osmoticsolute

MannitolMannitolNaClMannitolKNO3

NH4NO3

NaClKC1NaClGlycerolKC1NaClKC1

—NaClNaCl——KC1Mannitol—

m

0.190.220.170.430.28

0.320.210.180.140.500.180.250.35——0.310.30——0.120.30—

root pressure probe (see reviews: Steudle 1992, 1994a, b;ROdinger et ai, 1994). However, there are also substantialdifferences. Compared with spruce, differences refer tothe higher root pressure observed in oak. Compared withroots of herbaceous species, oak roots are much lesspermeable for both water and solutes.

Root pressure

In herbaceous and broad leaved woody species, steady-state values of root pressure have been found to rangebetween 0.05 and 0.5 MPa (Kramer, 1983). The valuesreported here for oak are within this range. For conifers,

Hydraulic properties of oak roots 397

Table 4. Initial (first day) values of hydraulic data (half-time of water exchange, T7/2, and hydraulic conductivity, Lpr/) and ofreflection coefficients (a,r) of roots o/Quercus petraea

Hydraulic conductivities were referred to total root surface area. Depending on the force used to drive a water flow across the root (osmotic orhydrostatic pressure gradient), Lp, and T^n differed markedly (factor of 20-390). Reflection coefficients were low. Roots are ordered according toseedling age. Figures in brackets in the first column denote the nitrogen content of the medium (1.0 or 5.0 mM). Mean values are given ±SD for77,2 a r |d LPz- ' n hydrostatic experiments, n = 8-12, and in osmotic experiments, n = 2-4.

Rootno.

17(1)19(5)20(5)21(5)22(1)23(5)24(1)

25(5)26(5)27(1)

28(1)29(1)

30(1)

31(5)

Rootage[d]

88110116120128141143

149151152

156156

163

166

Half-time of waterexchange, 77;2 [s]

Hydrostatic

8.0± 1.83.3±0.84.7±0.95.3 ±1.34.6±1.14.5±0.93.3 ±0.4

7.2 ±0.93.5 ±1.24.7±0.8

2.8±0.32.9±0.1

2.4±0.5

3.7 ±1.0

Osmotic

264 ±97—————608 ±308

——322 ±108

—750 ±209

636 ±246

844±515

Hydraulic conductivity,Z./»rxl08[ms-1

Hydrostatic

0.74 + 0.170.46±0.100.95 + 0.130.39 ±0.060.41 ±0.100.33±0.060.47 ±0.06

0.59±0.141.1 ±0.5

0.40 ±0.08

0.43 ±0.060.62 ±0.18

0.50±0.11

0.78 ±0.23

MPa"1]

Osmotic

O.O37±O.O15—————0.003 ±0.002

——0.006 ±0.002

—0.003±0.001

0.002 ±0.001

0.002 ±0.002

tpxayLp,(O)

20—————157

——

67

—207

250

390

Osmoticsolute

NaCl—————MannitolNaCl——KC1NaCl—NaClKC1KC1NaC1MannitolSucrose

a

m

0.21—————0.490.30——0.260.22—0.370.450.370.340.410.43

literature values are as low as some 10 cm of watercolumn (some kPa or 10~2 bar). This was also foundwith the root pressure probe (Rtldinger et al., 1994).Because leak rates (solute permeability) were low inspruce, low root pressures have been explained by a lowerrate of active pumping of nutrients and by the low rootreflection coefficient. Absolute values of root a,r aresimilar for spruce and oak. This suggests that higher rootpressures of oak are due to a higher rate of activepumping of nutrients. To some extent, our results suggestthat root pressure depended on the leak rate. When thesolute permeability increased after several days, rootpressure declined (Fig. 4). However, a decline of activepumping of nutrient ions may have occurred as well.

Hydraulic conductivity

Root hydraulic conductivity of Q. robur measured usinghydrostatic pressure gradients was substantially largerthan that of Q. petraea, but differences tended to becomesmaller at higher age (Table 2). This may point to differ-ences in root structure between the two species and,namely, to differences in the development of Casparianbands. The hydrostatic root Lpt of Q. robur was similarto that measured with the root pressure probe for youngroot systems of spruce (Lpr = 6.4 x 10~8 m s"1 MPa"1;Rildinger et al., 1994), but was smaller than the valuegiven by Sands et al. (1982) for loblolly pine(1.4xl0"7ms~1MPa~ I). The latter authors used thepressure chamber technique of Fiscus (1975). As for oak,

axial hydraulic resistances were substantially smaller thanradial. It should be noted that the data of Sands et al.(1982) refer to the range of high driving forces andvolume flows which should be equivalent to the LpT foundduring pressure relaxations as discussed by Rtldinger et al.(1994). Rildinger et al. (1994) measured LpT using differ-ent techniques, i.e. the root pressure probe (transientwater flows) or a pressure chamber (steady-state waterflow). Both techniques gave similar results. Summarizingdifferent values of hydrostatic root LpT of woody species,it appears that they are, on average, smaller by about afactor of 5 to 10 than those found for herbaceous species.The finding correlates with the fact that solute permeabil-ity of roots of woody species is also smaller than that ofherbs. Roots of woody species appear to be more 'tight'than those of herbs which may be related to differencesin the degree of suberization. With the pressure chambertechnique a dependence of LpT on the water flow throughthe root was found (RUdinger et al., 1994) which hasbeen reported in the literature many times and for differ-ent species (Fiscus, 1975; Sands et al., 1982; Colomboand Asselstine, 1989). In the present paper, driving forceswere too small to measure a pressure dependence. It is,however, remarkable that injecting pressure pulses intothe xylem at the root base during pressure relaxationsrevealed similar half-times of water exchange across theroot (and, hence, of Lpt) as during the application ofpressure steps to the soil. The finding provides furtherevidence that, at least to a first approximation, the two-

398 Steudle and Meshcheryakov

1 day

3.9 s

Time, t (min)

2 day

3 0 1 2 0

Time, t (min)

Fig. 6. Typical pressure chamber experiment with a root of Q. petraea. As in Fig 2, the experiment was started with relaxations while the rootsystem was at atmospheric pressure (a). Then a pneumatic pressure step of 0.04 MPa was applied (dashed line) which caused the stationary rootpressure to increase rapidly by approximately the same amount. Compared with the half-time of the rise in air pressure in the chamber, theresponse in root pressure was somewhat delayed. At the higher level of root pressure (b), half-times of relaxations of root pressure were similar tothose measured in the beginning. Then pressure was again decreased to atmospheric (c) and increased again in two steps (d) to check for thereversibility of processes. It can be seen that responses were fast. Half-times of water exchange across the root ( r ; / 2 ) were not affected bypressurizing the root system. Half-times of water exchange estimated from the response curves following a change of gas pressure (dP^) weresomewhat larger than those for the rise in air pressure in the chamber suggesting that Tfn was mainly limited by water exchange across the rootFor further explanation, see text.

compartment model may be used to evaluate root trans-port properties.

It should be noted that the values given for LpT inTables 2 to 4 refer to the overall (geometric) root surfacearea. If the 'effective surface area' would be only afraction of the geometric and water uptake would onlytake place in younger parts of fine roots, this wouldincrease the absolute value of LpT when referring to asmaller area. However, fine roots will, probably, take upwater across most of their surface area and, since fineroots contribute most of the surface area, the averagedroot Lp, given here probably represents a fairly goodmeasure of the hydraulic conductivity of fine roots. Thereduction of LpT observed for Q. robur at higher agewould be then simply due to a higher degree of suberiz-ation of fine roots.

Nitrogen nutrition did not significantly affect thehydraulic conductivity of roots of both Q. petraea and

Q. robur. However, since N nutrition increased rootgrowth of Q. robur, it can not be completely excludedthat nitrogen nutrition may have contributed to thereduction of LpT of this species at higher age. It can beconcluded that, for the young seedlings used, N nutritiondid not have a substantial ameliorative effect on root LpT,e.g. by increasing the hydraulic conductivity of root cellmembranes as it has been demonstrated for cotton roots(N and P nutrition; Radin and Matthews, 1989). Forroots of ectomycorrhizal Douglas fir seedlings and forseedlings of green ash, root LpT (per unit root length)increased with increasing P nutrition (Coleman et al.,1990; Andersen et aJ., 1989). However, for green ash theeffect was related to root growth and to biomass distribu-tion between shoot and root. Hence, direct effects ofnutrition on cell Lp (as for cotton roots) have beenexcluded as it is also indicated by our data for oak. Towork out the different effects of nutrition on root Lpt, it

• Root 14O 15* 28

Fig. 7. Effect of pneumatic pressure (AP^) applied to root systems ofthree different seedlings of Q. robur (seedlings 14, 15, and 28) on thesteady-state root pressure as measured with the root pressure probe. Itcan be seen that changes in pneumatic pressure caused a similar changein stationary root pressure (see also Fig. 6). Thus, a change of thehydrostatic pressure of soil solution caused a much larger effect on rootpressure than an equivalent change in osmotic pressure. Dotted linerepresents the 45° line.

is necessary to measure hydraulic conductivity at differentlevels (root cell, root zones and segments, fine roots,coarse roots, and root systems). Using pressure probesthis is, in principal, possible, but has not yet been donewith tree roots.

Differences between osmotic and hydraulic water flow

Besides the low root reflection coefficients (air), the mostobvious result of this paper is the large difference betweenosmotic and hydrostatic LpT. Differences were as large asa factor of 20 to 470. Certainly, such differences can notbe explained by effects of unstirred layers in the soil(which have been checked for; Fig. 2) or by unstirredlayer effects in the root cortex (Steudle and Frensch,1989). Large differences between hydrostatic and osmoticwater flow have also been found for roots of someherbaceous species (maize, onion, Steudle and Frensch,1989; Melchior and Steudle, 1993), although differenceswere smaller with the latter. For spruce, hydrostatic andosmotic Lpr differed by a factor of 380 which is similarto what was found for oak. Differences may be due to achange in the major pathway depending on the forceapplied. They have been explained by the existence of anapoplastic radial path for water in addition to the cell-to-cell path, which includes the symplastic and transcellu-lar path (Steudle and Jeschke, 1983). Hence, there aretwo parallel pathways and roots should exhibit transportproperties of a 'composite' osmotic barrier ('compositetransport model of root'; Steudle, 1992, 1994a, b; Steudle

Hydraulic properties of oak roots 399

et al., 1993). In the presence of hydrostatic gradients, theapoplastic path would be quite effective. However, in thepresence of osmotic gradients its contribution should besmall because the reflection coefficient of the apoplast isvery low. If Casparian bands were not completely tightfor water, there would be an apoplastic path aroundprotoplasts. A by-pass flow also may occur across second-ary root initials (Peterson et al., 1981), or across youngroot tips where Casparian bands are not yet developed(Steudle 1992, I994a,b). Measurements with 'modified'roots support this view (Peterson et al., 1993; Steudleet al., 1993). In terms of the composite transport model,the osmotic LpT should largely refer to the cell-to-cellpath. Depending on the Lp of root cells, the number ofcell layers which have to be crossed, and on the degreeof suberization, the osmotic LpT could become quite low.

Axial versus radial water flow

The other important question regarding proper transportmodels is whether or not the axial hydraulic resistance ofthe root would also contribute to the overall resistance.If contributions of axial resistances were considerable,the simple two-compartment model used to evaluatetransport coefficients would have to be disregarded.Pressure chamber experiments provide some evidence thatthe model is correct. Moreover, the experiments in whichsingle roots were excised while the root system wasattached to the probe (Fig. 5) showed that the axialhydraulic resistance of xylem vessels was much smallerthan that for radial transport across the root cylinder.Similarly, severing of the root system at the end of eachexperiment demonstrated that sealing the root did notaffect the hydraulic resistance in the area compressed bythe seal. In fact, it was much easier with the woody oakstems to connect the probe tightly to the root systemcompared with roots of herbs. All these findings stronglysupport the view that the radial transport of water acrossthe root cylinder rather than the axial movement withinthe root system were rate-limiting as assumed by the two-compartment model.

Solute permeability and reflection coefficients

According to the lower hydraulic conductivity of oakroots as compared with roots of herbs, one would expecta lower solute permeability of these roots (lower leakrate). This was found indicating, that tree roots are, ingeneral, more 'tight' than roots of herbaceous plants. Theresults resemble those obtained for spruce (Rudingeret al., 1994). Usually, no solute phases could be measuredwith oak roots just attached to the probe. Only afterabout 2-3 d on the probe were there indications ofbiphasic responses. This could have been due to altera-tions in root structure, physiology, or metabolism causingan increased leakiness. Increases in solute permeability

400 Steudle and Meshcheryakov

correlated with a decrease in root pressure over a periodof several days. Interestingly, Lpr did not increase, butdecreased, indicating that leakiness was specific to solutes.Estimates of root permeability coefficients (P,T) after2-10 d, ranged between 10~10 and 1 0 " 8 m s " ' which issimilar to the P, of cell membranes (Steudle, 1992, 1994a).For roots of herbaceous species, P,t values of 10"10 to10~8 m s"1 have been found for substances such as NaCl,KNO3, sucrose, and mannitol (Steudle et al., 1987;Steudle and Brinckmann, 1989; Steudle, 1992, 1994a).The data indicate that, despite the reasonable hydrostaticLpr and low reflection coefficients, root systems weresufficiently tight to allow only a minimal leakage ofnutrient ions from the stele (xylem) back into the soil.Because transport properties of oak roots changed signi-ficantly after prolonged times of measurement, it is con-cluded that measurements should not be performed forperiods of longer than 2 d.

Low reflection coefficients measured for different elec-trolytes and non-electrolytes appear to contradict thefinding of low solute permeability. Reflection coefficientsfor oak were smaller by a factor of 2-3 than those foundfor herbaceous species (aIr = 0.4-0.8; Steudle, 1992,1994a, b). This is so despite the fact that the permeabilityto solutes is even lower than that found for herbaceousspecies. Effects of unstirred layers which may haveresulted in an underestimation of reflection coefficientsshould have been small. In the experiments, it has beennecessary to wait for water flow equilibration for consider-able periods of time (Fig. 3; Steudle and Frensch, 1989).Again, the problem is solved by considering the root asa composite or patchy structure which consists of differentosmotic barriers arranged in parallel (Steudle, 1992,\994a,b; Steudle et al., 1993). Parallel elements are theapoplastic and cell-to-cell path whereby the latter incorp-orates a transcellular and symplastic component (Steudleand Jeschke, 1983). Different parallel pathways may alsobe brought about by the existence of different root zoneswhich should exhibit different transport properties (Lpr,P,T, asr). On young primary roots of maize, it has beenshown that, during root development, PtT would decreaseand a,r increase from the root apex toward the base(Frensch et al., 1996). Such systems with a 'composite'pattern of transport may exhibit properties similar tothose observed, i.e. a low a,r despite low P,T and largedifferences between osmotic and hydrostatic water flow(for details of the model, see cited literature).

The composite transport model predicts some variabil-ity of root Lp^ i.e. the finding of an increase of LpT withincreasing water flow across the root (Steudle, \994a,b).The significance of the composite structure and transportfor the plant is evident. High root LpT provides sufficientwater acquisition which may be adapted according to thedemand of the shoot (high root Lpx at high tensions inthe xylem). At the same time, roots do not leak ions

because of the low root PST which is largely a consequenceof the formation of apoplastic barriers (Casparian bands).Hence, the composite structure of the root may allow foran optimization of transport properties for both waterand solutes (nutrient ions). The low root asr is just aconsequence of this structure as discussed.

Conclusions

For root systems of Q. robur and Q. petraea, the samebasic principles apply in the measurement of root pressureand root permeability (water, solutes) as for roots ofherbaceous plants. However, the overall permeability oftree roots to water and solutes (nutrients) is much smallerthan that of herbs. The oak species better adapted towetter soil and higher nutrient levels (Q. robur) had ahigher hydraulic conductivity. During root growth anddevelopment, the root LpT of Q. robur decreased atconstant hydraulic conductance. At least for the youngseedlings used, there were no indications of a positiveeffect of N nutrition on root LpT. From the comparisonbetween oak and spruce roots it is concluded that,although deciduous and coniferous trees differ consider-ably in the absolute value of root pressure, basic transportproperties of root systems are similar. Differences betweenosmotic and hydraulic water flow and low root reflectioncoefficients of tree roots are most obvious. They areexplained by the composite transport model of the rootwhich appears to have general significance for plant roots.Although they would largely refer to fine roots, thepresent results for oak as well as those published recentlyfor spruce represent overall values. To work out the'hydraulic architecture of roots', completely, it is neces-sary to measure hydraulic properties of root cells, isolatedroots or root parts and root tips besides entire rootsystems (cell and root pressure probe). Anatomicalchanges during root development would have to befollowed besides the transport. Different from conven-tional techniques, the root pressure probe provides addi-tional information about solute transport besides thewater which, in turn, allows deeper insight into transportmechanisms and interactions between flows.

Acknowledgements

The authors are indebted to Burkhard Stumpf for experttechnical assistance. This work was supported by a grant fromEUROSILVA (Project no. 39473C) to ES.

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