Pugnaire F.I. Valladares F. Functional Plant Ecology

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clever way of measuring vulnerability curves using a centrifuge: the Cochard Cavitron. The

Cochard Cavitron has the advantage of a very fast and accurate technique. The vulnerability

curves of plants, in concert with their hydraulic architecture, can give considerable insight to

drought tolerance and water relations ‘‘strategies.’’

The vulnerability curves for a number of species are illustrated in Figure 6.11 (Tyree et al.

1994a). These curves represent the range of vulnerabilities observed thus far in more than 120

species; 50% loss K

occurs at P

values ranging from 0.7 to 11 MPa. Many plants

growing in areas with seasonal rainfall patterns (wet and dry periods) appear to be drought

evaders, and others are drought tolerators. The drought evaders evade drought (low P

and

high percent loss K

) by bearing deep roots and a highly conductive hydraulic system;

alternatively, they evade drought by their deciduousness. The other species frequently reach

very negative P

for part or all of the year and are shallow-rooted or grow in saline

environments. These species survive because they have a vascular system highly resistant to

cavitations.

When air bubbles are sucked into xylem conduits from adjacent embolized conduits, the

cavitation event is said to be air-seeded. Plants always have some embolized conduits to seed

embolism into adjacent conduits. Embolisms are the natural consequence of foliar abscission,

herbivory, wind damage, and other mechanical fates that might befall a plant. It is now

appropriate to ask if all emboli are seeded from adjacent conduits or if some other mechanism

occurs in some or most of the cases.

Four mechanisms for the nucleation of cavitations in plants have been proposed; these are

illustrated in Figure 6.12, which shows for each mechanism the sequence of events that might

occur as P

declines in the lumen of a conduit. Readers are referred to Zimmermann (1983),

Tyree et al. (1994a), and Pickard (1981) for a detailed discussion of the four mechanisms.

Other air-seeding mechanisms have been proposed that apply to SCUBA divers (Yount

1989), and such mechanisms might occur in plants when gas solubility decreases in xylem

water as it warms. However, little is known of the importance of this mechanism in plants. We

need to be concerned only about which mechanism occurs most frequently in plants.

−14 −12 −10 −8 −6 −4 −20

Percent loss conductivity

CJ R AT P

100

(MPa)

FIGURE 6.11 Vulnerability curves for various species. Y-axis is percent loss of hydraulic conductivity

induced by the xylem pressure potential, P

, shown on the x-axis. C, Ceanothus megacarpus;J,Juniperus

virginiana;R,Rhizophora mangle;A,Acer saccharm;T,Thuja occidentalis;P,Populus deltoids.

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200 Functional Plant Ecology

Experiments can discriminate between the air-seeding mechanism and the other three

mechanisms (Figure 6.12; Yount 1989). All mechanisms predict cavitation when xylem fluid is

under tension, but the air-seeding mechanism predicts that air can be blown into vessels while the

fluid is under positive pressure. The air-seeding mechanism requires only a pressure differential

P

), where P

is the air pressure outside and P

is the fluid pressure inside. It makes no

difference if P

is 0.1 and P

is 3.0 or if P

is 3.1 and P

is 0.1. Experiments have shown that

the same vulnerability curve results whether P

is reduced by air dehydration or P

is increased

in a pressure bomb (Cochard et al. 1992). A vulnerability curve (VC) is a plot of percent loss

Increasing tension

Wall

Air seeding through pore(1)

(2)

(3)

(4)

Homogeneous nucleation

Hydrophobic adhesion failure

Air seeding through hydrophobic crack

Air Water

FIGURE 6.12 Four possible mechanisms of cavitation induction. (1) Air seeding through a pore occurs

when the pressure differential across the meniscus is enough to allow the meniscus to overcome surface

tension and pass through the pore. (2) Air seeding through a hydrophobic crack occurs when a stable air

bubble resides at the base of a hydrophobic crack in the wall of a xylem conduit. When the P

becomes

negative enough, the bubble is sucked out of the crack. (3) Homogeneous nucleation involves the

spontaneous generation of a void in a fluid. It is a random process requiring thermal motion of the

water molecules. The hydrogen bonds at a specific locus are broken when all water molecules randomly

move away from any locus at the same instant with sufficient energy to break all hydrogen bonds

between water molecules. As the tension in the water increases, the hydrogen bonds are stretched and

weakened so that the energy needed to break the bonds decreases, making a homogeneous nucleation

more likely. (4) Hydrophobic adhesion failure is similar to homogeneous nucleation, except that

hydrogen bonds are broken between water and a hydrophobic patch in the wall where the energy of

binding between water and wall are reduced.

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Water Relations and Hydraulic Architecture 201

hydraulic conductivity (PLC) versus the P

required to cause the PLC by cavitation events. The

results of this experiment are illustrated in Figure 6.13b (see also Jarbeau et al. 1995).

Willow stem segments with leaves were enclosed in a pressure chamber with cut ends

protruding into the open air. Water was passed continually through the xylem under positive

pressure. While stem conductance was monitored, the gas pressure, P

, in the pressure

chamber was gradually increased. Initially, the hydraulic conductance of the stem segment

did not decrease until a critical pressure of 1 MPa was applied. (Each solid circle in Figure

6.13b represents the application of pressure for 30–40 min.) When P

was gradually increased

beyond the critical value, the stem conductance began to fall (increased PLC). When P

was

gradually decreased, the PLC stopped decreasing. The VC from this experiment was identical

to that found for similar branches dehydrated in the air.

HOW PLANTS DEAL WITH EMBOLISMS

Embolisms may be dissolved in plants if P

in the xylem becomes positive or close to positive

for adequate time periods (Tyree and Yang 1992, Lewis et al. 1994). Embolism disappears by

dissolution of air into the water surrounding the air bubble. The solubility of air in water is

proportional to the pressure of air adjacent to the water (Henry’s law). Water in plants tends

to be saturated with air at a concentration determined by the average atmospheric pressure of

gas surrounding plants. Thus, for air to dissolve from a bubble into water, the air in the

bubble has to be at a pressure in excess of atmospheric pressure. If the pressure of water (P

)

surrounding a bubble is equal to atmospheric pressure (P

), bubbles naturally dissolve

165 cm

Water column

Cotton collector

Pressure chamber

Gas in/outlet

(a)

Water potential (MPa)

–3

100

PLC

(b)

Applied pressure (MPa)

–2 –1

FIGURE 6.13 The first experimental test of the air-seeding hypothesis. (a) The experimental apparatus.

A willow branch is bent around in a large pressure bomb so that both cut surfaces are outside the bomb.

Water continually passes through the stem segment under positive pressure from a water column to a

cotton collector. Flow rate is estimated by measuring the weight change of the cotton collector over

known time intervals. (b) The results are shown as a vulnerability curve where the y-axis is the percent

loss of hydraulic conductivity (PLC) and the x-axis is the negative P

, or the air pressure in the bomb

needed to cause the plotted PLC. Closed circles indicate PLC measured as in Figure 6.13a. Open circles

indicate PLC induced by negative P

in bench-dehydrated shoots. Open triangles indicate PLC induced

by shoot dehydration in a pressure bomb but conductivity measured outside the pressure bomb on

subsamples (stem segments excised from dehydrated shoot).

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202 Functional Plant Ecology

because surface tension (t) of water raises the pressure of air in the bubble (P

) above P

.In

general, P

¼2 t=r þP

, where r is the radius of the bubble. According to the cohesion theory

of sap ascent, P

is drawn below P

during transpiration. Since 2 t=r of a dissolving bubble in

a vessel is usually less than 0.03 MPa, and since P

is in the range of 0.1 to 10 MPa during

transpiration, P

is usually P

and hence bubbles, once formed in vessels, rarely dissolve.

Repair (¼dissolution) occurs only when P

grows large via root pressure. One notable

exception to this generality has recently been found in Laurus nobilis shrubs that appear to

be able to refill embolized vessels even while P

is at 1 MPa (Salleo et al. 1996, Tyree et al.

1999); the mechanism involved has evaded explanation (see also Holbrook and Zwieniecki

1999, Holbrook et al. 2001).

Not all plants deal with embolisms by dissolving them. Embolism dissolution is pro-

bably most important in monocots, because they lack the ability to grow new vessels

by secondary growth. In dicots, however, stems can grow bigger in diameter by secondary

growth of the cambium. Cambium divides the phloem (bark in woody plants) from the xylem

(wood in woody plants). Cell division on the inner surface of the cambium ultimately results

in the growth and differentiation of new, water-filled xylem conduits. So some species simply

grow new conduits to replace conduits made dysfunctional by embolism.

FACTORS CONTROLLING THE RATE OF WATER UPTAKE AND MOVEMENT

Environmental conditions control the rate of water movement in plants. The dominant

environmental factor is net solar radiation, as mentioned in Section ‘‘Water Relations of

Whole Plants.’’ Water movement can be explained by the solar energy budget of plants. We

now take a more quantitative look at the solar energy budget of plants at both the leaf and

stand level.

LEAF-LEVEL ENERGY BUDGETS

Objects deep in space far away from solar radiation tend to be rather cold (approximately

5 K), whereas objects on earth tend to be rather warm (270–310 K). The reason for the

elevated temperature on earth is the warming effects of the sun’s solar radiation. The net

radiation absorbed by a leaf (R

,Wm

2

) can be arbitrarily divided into shortwave

and longwave radiation with a demarcation wavelength of 1 mm. Most shortwave radiation

comes from the sun and most longwave radiation comes from the earth. As a leaf absorbs

, the leaf temperature (T

) rises, which increases the loss of energy from the leaf by three

different mechanisms: black body radiation (B), sensible heat flux (H), and latent heat of

vaporization of water (lE, where E is the evaporative flux density of water due to trans-

piration [mol of water s

1

2

] and l is the heat required to evaporate a mole of water

[J mol

1

]). R

can also be converted to chemical energy by the process of photosynthesis and

energy storage, but these two factors are generally small for leaves and are ignored in the

following equation:

¼ B þ H þlE: (6:25)

The B in Equation 6.25 is the black body radiation. All objects emit radiation. Very

hot objects such as the sun emit mostly shortwave radiation, whereas cooler objects on

earth emit mostly longwave radiation. The amount of radiation emitted increases with

Kelvin temperature according to the Stefan–Boltzmann law: B ¼ e 

, where e is the

emisivity (0.95 for leaves) and s

is the Stefan–Boltzmann constant equal to 5.67  10

8

2

4

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Water Relations and Hydraulic Architecture 203

H in Equation 6.25 is sensible heat flux density, that is, the heat transfer by heat

diffusion between objects of different temperature. For leaves, the heat transfer is between

the leaf and the surrounding air. The rate of heat transfer is proportional to the difference in

temperature between the leaf and the air, so H ¼k (T

T

), where T

is the air tempera-

ture and k is the heat transfer coefficient. Heat transfer is more rapid and hence k is

large under windy conditions than in still air. However, the value of k is also determined

by leaf size, shape, and orientation with respect to the wind direction. Readers interested in

more detail should consult reports by Slatyer (1967) or Nobel (1991).

The factors determining E have already been given in Equation 6.8. If we combine

Equation 6.8 with the other equations for B and H, we get

¼ e 

þ k(T

T

) þ lg

]X

): (6:26)

Every term on the right side of Equation 6.25 is a function of leaf temperature. This gives

the clue about how the balance is achieved in the solar energy balance equation. At any

given R

, the value of T

rises or falls until the sum of B, H,andlE equals R

.AsR

increases or decreases, the value of T

increases or decreases to reestablish equality. In

practice, this equality is achieved with a leaf temperate near T

; T

is rarely less than 58K

below T

or more than 158K above T

. Some examples of leaf energy budgets are shown in

Figure 6.14. Once we know the value of T

and solve the equation, we can calculate the

value of E. If we sum the values of E for every leaf in a stand of plants, then we can calculate

stand-level water flow.

Equation 6.25 provides a good qualitative understanding of the dynamics of energy

balance for a community of plants (¼the stand level) because the evaporation rate from a

stand is the sum of the evaporation from all the leaves in the stand. However, Equation 6.25 is

of little practical value because it is not possible to obtain values of R

, T

k, and g

for

every leaf in a stand to compute the required sum.

STAND-LEVEL ENERGY BUDGETS

Another approach to energy budgets is to measure energy and matter flux in a region of air

above an entire stand of plants (Figure 6.15). A net radiometer can be used to measure net

radiation (R

) above the stand. R

is the total radiation balance (incoming longwave and

FIGURE 6.14 Solar energy budgets of leaves under different conditions. R

, net adsorbed short-

wave and longwave radiation; B, black body radiation from leaves; H, sensible heat flux; lE, latent

heat flux. Direction of arrow indicates direction of flux and length of arrow indicates relative magnitude.

Specific conditions: (left) high transpiration rate so that leaf temperature is below air temperature

because B þlE > R

; (middle) intermediate transpiration rates so that leaf temperature is above air

temperature; (right) during dew fall. Because B > R

at night, leaf temperature is below the dew point

temperature of the air.

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204 Functional Plant Ecology

shortwave radiation minus outgoing longwave and shortwave radiation, so at the leaf-level

¼R

B); the energy balance equation for this situation is

¼ G þ H þlE: (6:27)

The rate of heat storage in the soil G (W m

2

), can be significant because soil temperature

can change a few degrees Kelvin on a daily basis in the upper few centimeters of soil. Heat

storage rate is usually measured with soil-heat-flux sensor. The H and lE terms are similar to

those in Equation 6.25, but the equations define the flux densities between the heights marked

* and ** in Figure 6.15. The flux of heat and water vapor across the distance DZ is controlled

by vertical air convection (eddy) and the defining equations are

H ¼C

DT=DZ (6:28)

and

lE ¼ lK

D[H

O]=DZ, (6:29)

where K

is the eddy transfer coefficient C

is the heat capacity of air, r

is the air density, DT

is the difference in air temperature measured at the two levels in Figure 6.15 separated by a

height difference of DZ, and D[H

O] is the difference in water vapor concentration at the two

levels in Figure 6.15 separated by a height difference of DZ.

It is easy to measure DT and D[H

O], but difficult to assign a value for K

, which changes

dynamically with changes in wind velocity. The usual practice is to measure the ratio of

H=lE ¼b, which is called the Bowen ratio. From Equation 6.28 and Equation 6.29 it can be

seen that

b ¼

D[H

: (6:30)

∗

∗∗

∗∗∗

FIGURE 6.15 Points at which measurements are made for the solar energy budget of a uniform stand of

plants. Net radiation (R

) is measured at location marked*, air temperature and relative humidity are

measured at locations marked* and **, and rate of heat storage in soil is measured at***.

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Water Relations and Hydraulic Architecture 205

Estimates of stand water use are obtained by solving Equation 6.29 and Equation 6.30 for E

E ¼

G

l[b þ 1]

: (6:31)

An example of the total energy budget measured over a pasture is reproduced in Figure 6.16.

The data in Figure 6.16 shows again that the main factor determining E is the amount of solar

radiation, R

, as in Figure 6.4. However, the leaves in a stand of plants do have some control

over the value of E. As soils dry and leaf water potential falls, stomatal conductance falls.

This causes a reduction in E, an increase in leaf temperature, and thus an increase in H.

Very intensive monitoring of climatic data is needed to obtain data for a solution to

Equation 6.31. Temperature, R

, and relative humidity has to be measured every second at

several locations. Ecologists prefer to estimate E with a less complete data set. Fortunately,

the Penman–Monteith formula (Monteith 1964) permits a relatively accurate estimate of E

under some restricted circumstances. The Penman–Monteith formula is derived from energy

budget equations together with a number of approximations to turn nonlinear functions into

linear relations. After many obtuse steps in the derivations (Campbell 1981), a formula of the

following form results:

E ¼

G) þr

l e

þ g 1 þ

hi

, (6:32)

where e

is the rate of change of saturation vapor pressure with temperature at the current air

temperature, V

is the difference between the vapor pressure of air at saturation and the

current vapor pressure, g is the psychometric constant, g

is the aerodynamic conductance,

and g

is the canopy conductance of the stand. The problem with Equation 6.32 is that g

and

are both difficult to estimate and are not constant. The value of g

depends on wind speed

–200

200

400

Solar energy balance (W m

–2

)

600

800

510

Time of day

15 20 25

FIGURE 6.16 Solar energy budget values measured in a meadow. R

, net solar radiation of the

meadow; lE, latent heat flux; H, sensible heat flux; and G, rate of heat storage in soil. (Adapted from

Slatyer, R.O., Plant Water Relationships, Academic Press, New York, 1967.)

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206 Functional Plant Ecology

and roughness parameters that describe the unevenness at the boundary between the canopy

and the bulk air. Surface roughness affects air turbulence and hence the rate of energy transfer

at any given wind speed. Surface roughness changes as stands grow and is difficult to estimate

in terrain with hills or mountains. The value of g

is under biological control and difficult to

estimate from climatic data. In one study on an oak forest in France, E was estimated

independently by Granier sap flow sensors in individual oak trees. This permitted the

calculation of g

versus season, morphological state of the forest, and climate (Granier and

Bre

da 1996). The value of g

was found to be a function of global radiation, V

, leaf area per

unit ground area, which changes with season, and relative extractable water, which is a

measure of soil dryness. However, once all of these factors were taken into account, Equation

6.32 provided a reasonable estimate of half-hourly estimate of E over the entire summer.

PLANTS UNDER STRESS

ILTING

Wilting denotes the limp, flaccid, or drooping state of plants during drought. Wilting is most

evident in leaves that depend on cell turgor pressure to maintain their shape; hence, wilting

occurs when turgor pressure falls to zero. Many plants maintain leaf shape through rigid leaf

fiber cells. Wilting in these species is considered to commence at the turgor loss point. Wilted

plants generally have low E because stomata are closed and g

is very small when leaf water

potential C

falls during drought. Continued dehydration beyond the wilt point usually

causes permanent loss of hydraulic conductance due to cavitation in the xylem. Complete

loss of hydraulic conductance usually causes plant death, but there is a lot of species diversity

in the water potentials causing loss of hydraulic conductance (Figure 6.11). Some plants in

arid environments avoid drought either by short reproductive cycles confined to brief wet

periods or by bearing deep roots that can access deep sources of soil water (Kramer 1983).

As plants approach the wilt point there is a gradual loss in stomatal conductance and,

hence, a reduction in E and photosynthetic rate (Schulze and Hall 1982). Some species are

much more sensitive than others (Figure 6.17b). The short-term effects of decreased C

transpiration are less dramatic than long-term effects (Figure 6.17a). Long-term effects of

drought are mediated by hormone signals from roots that cause a medium-term decline in g

and by changes in root morphology, for example, loss of fine roots, suberization of root

surfaces, and formation of corky layers (Ginter-Whitehouse et al. 1983). The morphological

changes to roots cause a decrease in whole-plant hydraulic conductance (K

); therefore, C

becomes C

¼C

soil

E=K

. Very severe drought can further lower K

due to cavitation of

xylem vessels.

WATERLOGGING

Waterlogging denotes an environmental condition of soil water saturation or ponding of

water that can last for just a few hours or for many months. Plants absent from flood-prone

sites are damaged easily by waterlogging. On the other hand, plants that inhabit flood-

prone sites include species that can grow actively in flooded soils or species that survive

flooding in a quiescent or dormant state. Paradoxically, the most common sign of tobacco

roots with an excess of water is the development of a water deficit in the leaves (Kramer 1983,

Kramer and Jackson 1954), but flood-tolerant species are not so easily affected.

Flooding often affects root morphology and physiology. The wilting and defoliation that

is found on flooding can be traced to an increased resistance to water flux in the roots (Mees

and Weatherley 1957). In most flood-tolerant species, flooding induces morphological

changes in the roots. The modifications usually involve root thickening with an increase in

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Water Relations and Hydraulic Architecture 207

porosity. The increase in porosity increases the rate of oxygen diffusion to root tips and thus

permits continued aerobic metabolism in the inundated roots. In flood-sensitive species, root

and shoot growth are rapidly reduced on flooding, and root tips may be damaged. Growth of

roots can be renewed only from regions proximal to the stem. The physiological responses

and adaptations to waterlogging are numerous and beyond the scope of this chapter, but

interested readers may consult a report by Crawford (1982).

300

0.0

–1.6

–1.2

–0.8

–0.4

0.5 1.0 1.5 2.0

(b)

(a)

250

200

150

100

(mmol m

–2

–1

)

Leaf water potential (MPa)

Transpiration rate (mmol plant

–1

)

Leaf water potential (MPa)

–3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0

Short-ter

m changes in pla

nt water

status

<--Long-term changes in plant water status-->

FIGURE 6.17 Effects of drought on transpiration, leaf water potential, and stomatal conductance

(a) Short- and long-term effects of drought on leaf water potential and transpiration. The short-term

effects are dynamic changes in leaf water potential that might occur in the course of 1 day. The long-

term effects are associated with slow drying of soil over many days. The different lines show the short-term

relation between leaf water potential versus transpiration rate measured in well-watered plants (—) or

after several days without irrigation, for example, 5 days (), 10 days (), or 20 days (–  –  –). (b)

Short-term effects of leaf water potential on stomatal conductance. Species represented are: A, Acer

saccharum;C,Corylus avellana;E,Eucalyptus socialis;G,Glycine max;T,Triticum aestivum; and O,

Olea europeae. (Adapted from Schulze, E.D. and Hall, A.E., Encyclopedia of Plant Physiology New

Series, O.L. Lange, P.S. Nobel, C.B. Osmond, H. Ziegler, eds, Springer-Verlag, New York, 1982.)

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208 Functional Plant Ecology

REFERENCES

Bidwell, R.G.S., 1979. Plant Physiology. Macmillan, New York.

Campbell, G.S., 1981. Fundamentals of radiation and temperature relations. In: O.L. Lange, P.S.

Nobel, C.B. Osmond, and H. Ziegler, eds. Encyclopedia of Plant Physiology New Series,

Vol. 12A. Springer-Verlag, New York, pp. 11–40.

Chang, J.-H., 1968. Climate and Agriculture. Aldine Publishing, Chicago, IL.

Cochard, H., P. Cruiziat, and M.T. Tyree, 1992. Use of positive pressures to establish vulnerability

curves: Further support for the air-seeding hypothesis and possible problems for pressure-

volume analysis. Plant Physiology 100: 205–209.

Cochard H., G. Damour, C. Bodet, I. Tharwat, M. Poirier, and T. Ameglio, 2005. Evaluation of a new

centrifuge technique for rapid generation of xylem vulnerability curves. Physiologia Plantarum

124: 410–418.

Crawford, R.M.M., 1982. Physiological response to flooding. In: O.L. Lange, P.S. Nobel, C.B.

Osmond, and H. Ziegler, eds. Encyclopedia of Plant Physiology New Series, Vol. 12B.

Springer-Verlag, New York, pp. 453–477.

Davies, W.J. and J. Zhang, 1991. Root signals and the regulation of growth and development of plants

in drying soil. Annual Review of Plant Physiology and Plant Molecular Biology 42: 55–76.

Dawson, T.E., 1993. Hydraulic lift and water parasitism by plants: Implications for water balance,

performance, and plant-plant interactions. Oecologia 95: 565–574.

Dixon, H.H. and J. Joly, 1894. On the ascent of sap. Philosophical Transactions of the Royal Society

London, Series B, 186, 563–576.

Ewers, F.W. and M.H. Zimmermann, 1984a. The hydraulic architecture of balsam fir (Abies balsamea).

Physiologia Plantarum 60: 453–458.

Ewers, F.W. and M.H. Zimmermann, 1984b. The hydraulic architecture of eastern hemlock (Tsuga

canadiensis). Canadian Journal of Botany 62: 940–946.

Ewers, F.W., J.B. Fisher, and K. Fichtner, 1991. Water flux and xylem structure in vines. In: F.E. Putz

and H.A. Mooney, eds. The Biology of Vines. Cambridge University, Cambridge, pp. 127–160.

Frensch, J. and E. Steudle, 1989. Axial and radial hydraulic resistance to roots of maize (Zea mays L.)

Plant Physiology 91: 719–726.

Ginter-Whitehouse, D.L., T.M. Hinckley, and S.G. Pallardy, 1983. Spatial and temporal aspects of

water relations of three tree species with different vascular anatomy. Forest Science 29: 317–329.

Granier, A. and N. Bre

da, 1996. Modeling canopy conductance and stand transpiration of an oak forest

from sap flow measurements. Annales des Sciences Forestie

res 53: 537–546.

Green, P.B., R.O. Erickson, and J. Buggy, 1971. Metabolic and physical control of cell elongation rate—

in vivo studies of Nitella. Plant Physiology 47: 423–430.

Holbrook, N.M. and M.A. Zwieniecki, 1999. Embolism repair and xylem tension. Do we need a

miracle? Plant Physiology 120: 7–10.

Holbrook, N.M., E.T. Ahrens, M.J. Burns, and M.A. Zwieniecki, 2001. In vivo observation of cavitation

and embolism repair using magnetic resonance imaging. Plant Physiology 126: 27–31.

Jarbeau, J.A., F.W. Ewers, and S.D. Davis, 1995. The mechanism of water stress-induced embolism in

two species of chaparral shrubs. Plant Cell and Environment 126: 695–705.

Kramer, P.J., 1983. Water Relations of Plants. Academic Press, New York.

Kramer, P.J. and W.T. Jackson, 1954. Causes of injury to flooded tobacco plants. Plant Physiology 29:

241–245.

Lewis, A.M., V.D. Harnden, and M.T. Tyree, 1994. Collapse of water-stress emboli in the tracheids of

Thuja occidentalis L. Plant Physiology 106: 1639–1646.

Machado, J.-L. and M.T. Tyree, 1994. Patterns of hydraulic architecture and water relations of two

tropical canopy trees with contrasting leaf phonologies: Ochroma pyramidale and Pseudobombax

septenatum. Tree Physiology 14: 219–240.

Mees, G.C. and P.E. Weatherley, 1957. The mechanism of water absorption by roots. II. The role of

hydrostatic pressure gradients. Proceedings of the Royal society London, Series B. 147: 381–391.

Monteith, J.L., 1964. Evaporation and environment. Symposium of the society for Experimental

Biology 19: 205–234.

Nobel, P.S., 1991. Physiochemical and Environmental Plant Physiology. Academic Press, New York.

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