Hara: the effect of xylem structures - Tree Physiology

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doi:10.1093/treephys/tpq011 

Tree Physiology Advance Access published April 5, 2010 

Changes of hydraulic conductivity during dehydration and rehydration 

in Quercus serrata Thunb. and Betula platyphylla var. japonica Hara: 

the effect of xylem structures 

MAYUMI OGASA, 1,2 NAOKO MIKI 1 and KEN YOSHIKAWA 1 

1 Graduate School of Environmental Science, Okayama University, Tsushima-naka 1-1-1, Kita-ku, Okayama, 700-8530 Japan 

2 Corresponding author (dev421201@s.okayama-u.ac.jp) 

Received June 24, 2009; accepted January 28, 2010 

Summary Xylem cavitation and its recovery were studied 

in 1-year-old stems of ring-porous Quercus serrata Thunb. 

and diffuse-porous Betula platyphylla var. japonica Hara. 

The Q. serrata had 5–100 µm vessel diameter in the functional 

current xylem and 5–75 µm in nonconducting 1year-old 

xylem; B. platyphylla had a narrower range of vessel 

diameters of 5–55 µm and more than double the number of 

vessels in both functional growth rings. Although hydraulic 

conductivity of Q. serrata appeared to decrease after release 

of moderate water stress of a half loss of native hydraulic 

conductivity—about −2 MPa in xylem water potential—no 

significant recovery of hydraulic conductivity was observed, 

probably because of intraspecific variation in vessel diameter 

distribution, which induced variable vulnerability to cavitation. 

Furthermore, in terms of xylem anatomy, larger and 

more efficient vessels of the current xylem did not show obvious 

refilling. In B. platyphylla, after release of water stress, 

rapid (1 h) recoveries of both hydraulic conductivity and water 

potential were apparent after rewatering: so-called ‘novel 

refilling’. During that time, a high degree of vessel refilling 

was observed in both xylems. At 12 h after rewatering, embolized 

vessels of the current xylem had refilled completely, 

although about 20% of vessels were still embolized in 1-yearold 

xylem. This different pattern of vessel refilling in relation 

to xylem age for B. platyphylla might be attributable to structural 

faults in the 1-year-old xylem, such as pit degradation or 

perhaps xylem aging itself. Results show that Q. serrata performs 

water conduction using highly efficient large vessels 

instead of unclear vessel refilling. In contrast, B. platyphylla 

transports water via less efficient but numerous vessels. If 

cavitation occurs, B. platyphylla improves water conduction 

by increasing the degree of vessel refilling. 

Keywords: diffuse-porous, ring-porous, vessel diameter, 

vessel refilling, water stress, xylem embolism. 

© The Author 2010. Published by Oxford University Press. All rights reserved. 

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Introduction 

Hydraulic conductivity in plants is affected by embolism in 

conduits. According to the air seeding hypothesis (Tyree and 

Zimmermann 2002), which is the most convincing mechanism 

of cavitation by drought, cavitation occurs in a waterfilled 

conduit when air bubbles are sucked into the conduit 

via a pit membrane pore from an adjacent air-filled conduit 

at a given tension. The risk of cavitation varies along with 

the transpiration demand (Sperry 2000, Brodribb et al. 

2003, Bucci et al. 2003). Once plants lose their function of 

transporting water in their conduits, a decline in leaf physiological 

traits and dieback ensue, eventually leading to death 

(Field and Holbrook 1989, Kondoh et al. 2006). It is 

therefore important for plants to be protected from cavitation 

under diurnal environmental variation of soil moisture, 

light conditions, etc. (Cochard et al. 1999, Cochard 

et al. 2002, Lemoine et al. 2002, Lo Gullo et al. 2005). Nevertheless, 

cavitation can be characterized as a common event 

for plants because negative pressure experienced in the field 

is close to the threshold pressure of cavitation (Tyree and 

Sperry 1988); daily xylem dysfunction is not an unlikely 

phenomenon. 

Although plants cannot avoid water-stress-induced cavitation 

entirely as long as they transport water under tension 

(Sperry et al. 2003, Wheeler and Stroock 2008), some species 

can recover their decreased hydraulic conductivity 

(Zwieniecki and Holbrook 1998, Bucci et al. 2003, Brodribb 

and Holbrook 2004, Domec et al. 2006). To regain hydraulic 

conductivity that has once been impaired by diurnal cavitation, 

plants must refill their embolized conduits rapidly. 

Otherwise, hydraulic conductance will no longer be depressed 

by drought until new xylem can be produced (Sperry et al. 

2003). Recently, in addition to passive conduit refilling 

according to a mechanical process (Henry’s law;Yang 

and Tyree 1992), reports have described that refilling can occur 

under more negative water potential associated with os- 

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2 

mosis than the mechanically expected potential in intact 

woody plants (Tyree et al. 1999, Salleo et al. 2004, 2006). 

The conduit diameter can strongly affect the conductive efficiency 

of the xylem. According to Poiseuille’s law for ideal 

capillaries, the flow rate is proportional to the fourth power of 

the radius of the capillary (Tyree and Zimmermann 2002), 

which demonstrates that a slight increase in conduit diameter 

brings about a considerable increase in water flow. Hydraulic 

conductivity is determined by the extent of functional conduits—by 

their number and width—so that xylem recovery 

depends on the extent of conduit refilling. For example, even 

if the numbers of refilled conduits were equal, conduits with 

larger diameter would provide sharp recovery of hydraulic 

conductivity. Reportedly, the extent of conducting conduits 

of wood would determine its hydraulic conductivity so that 

ring-porous species bearing large vessels would yield efficient 

xylem conductivity, whereas diffuse-porous species 

have many small but less efficient vessels (Tyree and 

Zimmermann 2002, Hacke et al. 2006). Until now, many 

studies have described the water-conducting properties of 

woody plants merely in terms of water-stressed conditions 

(e.g., Hargrave et al. 1994, Hacke et al. 2006). Along with 

the results of those studies, incorporating the conditions after 

water-stress release will advance the evaluation of the conducting 

strategy because the recovery of hydraulic conductivity has 

actually been performed (Zwieniecki and Holbrook 1998, 

Bucci et al. 2003). Then, ring-porous trees might have less 

ability to refill vessels cavitated by drought because the remaining 

functional large vessels are still efficient. In contrast, 

diffuse-porous trees might require high ability of vessel refilling 

to regain lost cavitated vessels because the remaining functional 

vessels are too narrow and not sufficiently efficient to 

conduct water after water-stress release. 

In our study, we specifically investigated the effect of xylem 

structures on water-conducting properties during daily 

water stress and its subsequent release. We attempted to evaluate 

the extent of cavitation and xylem recovery in saplings 

of ring-porous Quercus serrata Thunb. and diffuse-porous 

Betula platyphylla var. japonica Hara. We discuss hydraulic 

properties not only under water stress but also after its release 

in these porous-wood trees. We determined the extent of hydraulic 

conductivity and functional or embolized vessels during 

dehydration and rehydration phases. 

Materials and methods 

Plant materials 

Experiments were conducted on potted 2- to 3-year-old 

plants of Q. serrata, ring-porous wood, and B. platyphylla 

var. japonica,diffuse-porous wood, during summer: August– 

October 2008. All these plants were grown in an experimental 

field at Okayama University (34°41′ N, 133°53′ E) and were 

well watered regularly until the experiments were conducted. 

OGASA ET AL. 

TREE PHYSIOLOGY 

The plants, which were of almost uniform size, were 1.45 ± 

0.09 m tall for Q. serrata and 1.68 ± 0.23 m tall for B. platyphylla. 

Stems holding two annual rings, located in the middle 

part of the main stem, were used for experiments. We confirmed 

that the stem segments that were studied had current 

and 1-year-old xylem by using a stereoscopic microscope 

(SZX-ILLB100; Olympus Optical Co., Ltd, Japan) to count 

annual rings in a cross section cut from the segments after 

the experiments. Only data obtained from the segments holding 

two annual rings were used for analyses. 

Dehydration experiments and hydraulic conductivity 

measurements 

Potted plants were dried by stopping irrigation for several 

days to a week and then transporting them to the laboratory 

for experimentation. To ensure that the target water potential 

was obtained, plants were dehydrated more slowly after 

transport to the laboratory. Plants’ water status was monitored 

by measuring the leaf water potential regularly using a pressure 

chamber (model 600; PMS Instrument Co., USA). When 

the leaf water potential was close to a target value, three randomly 

chosen leaves were enclosed for at least 24 h in a 

black plastic bag to remove the water potential gradient between 

the xylem and leaf. The whole plant was also enclosed 

for at least 2 h in a black plastic bag to stop transpiration and 

to stabilize the xylem tension. Subsequently, the xylem water 

potential, Ψxylem, was obtained by measuring the water potential 

on three previously enclosed leaves using a pressure 

chamber (bagged shoot method, e.g., Turner 1981); the mean 

was applied as plant Ψ xylem. 

At various values of stem Ψ xylem, a stem segment holding 

two annual rings was cut underwater from the middle 

height of the main stem of the plant. The segments were 0.4– 

0.8 cm in diameter and around 5 cm long. We considered that 

open vessels in the segment of ring-porous Q. serrata had less 

effect on the initial hydraulic conductivity (K init) because K init 

had not changed depending on the segment length in preliminary 

experiments. After cutting the segments, K init was 

measured immediately for perfusion with a 0.1-µm-filtered 

20 mM KCl solution. Measurements were performed using 

a conductivity apparatus (Sperry et al. 1988) under hydrostatic 

pressure of 30 cm hydraulic head for Q. serrata and 

60 cm for B. platyphylla. Axial flow, F, was recorded after 

it became stable. The values of K init were calculated as 

shown below. 

Kinit = F= ðΔP=LÞ ð1Þ 

Therein, ΔP signifies the pressure difference across the 

segment length L. After K init was determined, segments 

were flushed with the filtered KCl solution at 100 kPa 

for 20 min and remeasured for their maximum hydraulic 

conductivity, K max, from Eq. (1) at hydrostatic pressure. 

We had confirmed previously that segments reached K max 


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Figure 1. Relationship between loss of hydraulic conductivity and Ψxylem in A, Q. serrata and B, B. platyphylla. Each point represents one 

stem. The solid line shows the regression curve. 

after 20 min flushing at 100 kPa. The percentage loss of 

hydraulic conductivity (PLC) was then calculated as the 

following. 

PLC = ð1−Kinit=KmaxÞ × 100 ð2Þ 

Data were presented as relationships of PLC to Ψ xylem that 

induced the embolism, indicating vulnerability curves as in 

Figure 1 in which each plot referred to one value of one 

segment cut from a plant. The following sigmoid function 

was fitted to the experimental vulnerability curves (Pammenter 

and van der Willigen 1998). 

PLC = 100= 1 + exp a Ψxylem−Ψ50 

ð3Þ 

In that equation, a signifies the slope of the curve, and Ψ 50 

denotes the Ψ xylem value that yields 50 PLC. The specific 

hydraulic conductivity, K s, was obtained by dividing K init 

by the functional xylem cross-sectional area of the segment. 

For each species, 17 plants were used for dehydration 

experiments. 

LOSS OF XYLEM CONDUCTIVITY AND ITS RECOVERY 3 

Rehydration experiments 

After stopping irrigation, potted plants were dehydrated to 

−1.5 to −2.0 MPa for Q. serrata and −1.7 to −2.0 MPa for 

B. platyphylla in leaf water potential, which was sufficiently 

low to cause the predicted half loss of hydraulic conductivity 

against well-watered status by the vulnerability curve; leaves 

were not wilted completely for either species (Ranney et al. 

1991, Saito and Terashima 2004). These water-stressed potted 

plants were rewatered well to field capacity. They were 

kept in the laboratory for 1 and 12 h after rewatering to simulate 

overnight rehydration periods (Nardini et al. 2008). The 

photosynthetic photon flux density (PPFD) and air temperature 

were, respectively, 0–13 µmol m −2 s −1 and 26°C in the 

laboratory. Unlike the dehydration experiment, whole plants 

were not enclosed during rehydration. At the end of each rehydration 

period, Ψ xylem was obtained from three leaves that 

had been previously enclosed for 24 h; Kinit of a 5-cm-long 

stem segment cut from a plant was measured under the same 

hydrostatic pressure using the same apparatus described in 

the previous section. Specific hydraulic conductivity (K s), 

Figure 2. Specific hydraulic conductivity as measured in nondehydrated, dehydrated and rehydrated stem segments cut from Q. serrata (A) and 

B. platyphylla (B) 1 and 12 h after rewatering. Mean ± SD (n = 3). **P < 0.01 denotes a significant difference against nondehydrated segments 

for Dunnett’s test. 

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Figure 3. Xylem water potential as measured in nondehydrated, dehydrated and rehydrated stem segments cut from the plants 1 and 12 h after 

rewatering in Q. serrata (A) and B. platyphylla (B). Mean ± SD (n = 3). **P < 0.01 and ***P < 0.001 denote significant differences from 

nondehydrated segments for Dunnett’s test. 

obtained by dividing Kinit by the functional xylem crosssectional 

area, was used as an indicator of the recovery of 

hydraulic conductivity to rule out embolism in situ. Rehydration 

experiments were conducted on three plants in 

each 1- and 12-h rehydration period for both species. In 

addition, out of 17 plants used for dehydration experiments, 

three plants before dehydration and three plants after 

moderate dehydration, which were of the same 

dehydrated condition immediately before rewatering, were 

selected and used for a comparison of Ks or xylem water 

potential at different water statuses (see Figures 2 and 3). 

Xylem anatomy 

We analyzed the xylem anatomy on various plant water 

statuses—before and after dehydration (about 0 and −2 

MPa in Ψ xylem, respectively) and 1 and 12 h after rewatering—to 

compare the changes in functional and embolized 

vessel size distribution with water status. Segments for anatomical 

measurements were excised underwater from the 

same stems and a significantly closer position where hydraulic 

measurements were conducted. They were perfused 

with 0.1% wt/vol. of 0.1-µm-filtered safranin dye to distinguish 

functional vessels from embolized ones (Sperry et al. 

1994, Kondoh et al. 2006). Dye perfusion was performed 

in the same setting as that of hydraulic measurements under 

hydrostatic pressure of 30 cm hydraulic head for Q. 

OGASA ET AL. 

serrata and 60 cm for B. platyphylla. After dye perfusion, 

30-µm-thick cross sections were obtained from the middle 

part of dye-perfused segments using a sliding microtome 

and observed using a stereoscopic microscope. For each 

cross section, we randomly chose a 30° (inner-angle) sector 

of xylem that was delimited continuously by two radial 

rays from the pith to cambium and measured all 

functional and embolized vessel numbers and lumen areas 

within this sector for each growth ring. The stained functional 

vessel and unstained embolized vessel diameters 

were calculated as the diameter of a circle with an area 

equal to the measured lumen area. In addition to the 

vessel diameter distribution in the 5-µm size class for current 

and 1-year-old xylem, we calculated the percentage 

number of functional or embolized vessels to that of all 

vessels belonging to each size class to assess the dependence 

of vessel size on sensitivity to dehydration and rehydration. 

We measured the vessel diameter using three 

cross sections cut from each three plant-stem segment 

for each water status and from three additional segments, 

and analyzed the current and 1-year-old xylem separately. 

Theoretical specific hydraulic conductivity 

Based on Poiseuille’s law for ideal capillaries, theoretical Ks 

(kg m −1 MPa −1 s −1 ) was calculated for the only stained func- 

Table 1. Percentage of the functional vessel number on each xylem and the current xylem contribution to whole-xylem specific conductivity 

(K s), as calculated from the functional vessel diameter for well-watered plants of Q. serrata and B. platyphylla. Mean ± SD (n = 3 per species). 

Percentage of functional vessels Current xylem contribution in K s (%) 

Current xylem 1-year-old xylem 

Q. serrata 60.6 ± 8.7 21.5 ± 18.6 99.5 ± 0.5 

B. platyphylla 100.0 ± 0.0 100.0 ± 0.0 48.6 ± 0.0 




tional vessels in each xylem of the cross section, as shown 

below (Tyree and Ewers 1991). 

theoretical Ks ¼ ðπρ=128ηAwÞ∑ n 

i−1 

d 4 i 

ð4Þ 

In that equation, ρ represents the density of the solution 

(kg m −3 ), η is the dynamic viscosity of the solution 

(MPa s), A w signifies the xylem cross-sectional area (m 2 ), 

d denotes the diameter (m) of the ith vessel and n stands 

for the number of vessels in the section observed for xylem 

anatomy. Calculation of theoretical K s before dehydration 

was conducted on three cross sections analyzed for xylem 

anatomy to evaluate the contribution of current xylem hydraulic 

conductivity to the hydraulic conductivity of the 

whole xylem (see Table 1). Furthermore, theoretical K s obtained 

from three cross sections for each water status— 

after dehydration and two rehydration periods in addition 

to ‘before dehydration’—are shown for diffuse-porous B. 

platyphylla to examine the effects of xylem age on the 

extent of vessel refilling and consequently on the recovery 

of measured K s (see Figure 7). 

Statistical analyses 

One-way ANOVA was used to test the effect of different water 

status on hydraulic conductivity or xylem water potential. 

Dunnett’s test was applied for multiple comparisons among 

water status. Software (Statistica; StatSoft Inc. USA) was 

used for statistical analyses. 


Figure 4. Vessel diameter distribution in 5 μm classes in Q. serrata (A, B) and B. platyphylla (C, D). A and C show data for the current xylem, 

B and D show 1-year-old xylem. Class means ± SD (n = 15). Data in each graph represent mean ± SD of the vessel number (vessel mm −2 ) and 

diameter (µm). 

Results 

Dehydration vulnerability curves 

Although the data presented herein varied widely throughout 

xylem water potential, about 40% loss of hydraulic conductivity 

was recorded in the high xylem water potential in Q. 

serrata (Figure 1A). The percentage loss of hydraulic conductivity 

increased slightly with decreasing Ψxylem values. 

By contrast, B. platyphylla showed no loss of hydraulic 

conductivity of Ψ xylem values between 0 and −1.5 MPa 

(Figure 1B). At lower Ψ xylem values, the percentage loss of 

hydraulic conductivity then increased sharply. At Ψxylem of 

less than −2.0 MPa, the loss of hydraulic conductivity was 

raised more than 80%. 

Recovery of hydraulic conductivity and Ψ xylem 

Specific conductivity (K s)inQ. serrata was decreased by dehydration 

from 12.0 kg m −1 MPa −1 s −1 under well-watered 

conditions to 7.0 kg m −1 MPa −1 s −1 (Figure 2A) after rewatering, 

but no significant difference was found between them. 

In addition, the mean values of K s did not recover significantly 

in either 1- or 12-h rehydration periods. In contrast, B. platyphylla 

exhibited a significant loss of K s from 2.9 to 1.4 kg 

m −1 MPa −1 s −1 by dehydration but recovered rapidly to the 

value equal to that under well-watered conditions (1 h) 

(Figure 2B). Comparison of K s values among species shows 

that Q. serrata displayed higher hydraulic efficiency than B. 

platyphylla through water status. 




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Figure 5. Percentage of functional and embolized vessels in each of the 5-µm classes in Q. serrata. Gray and open bars, respectively, represent 

functional and embolized vessels: A–D are for the current xylem; E–H are for the 1-year-old xylem; A and E, before dehydration; B and F, after 

dehydration; C and G, 1 h after rewatering; D and H, 12 h after rewatering. Class mean ± 95% of the confidence interval (n = 3). 

Although no significant change of loss of hydraulic conductivity 

was observed by rewatering in Q. serrata, Ψ xylem 

was significantly improved 1 h after rewatering (Figure 3A). 

During the 12-h rehydration period, Ψ xylem was recovered 

completely to be equivalent to the nondehydrated value. 

Similarly, B. platyphylla showed considerable recovery of 

Ψxylem in 1 h of rehydration period and then recovered fully 

in 12 h. 

Xylem anatomy 

Results show that Q. serrata had widely differing vessel diameters 

of 5–100 and 5–75 µm in the current and 1-year-old 

xylem, respectively (Figures 4A and B). The percentage 

OGASA ET AL. 


number of functional vessels was 60.6 ± 8.7% in current xylem 

and 21.5 ± 18.6% in 1-year-old xylem, and the contribution 

of current xylem to whole-xylem specific hydraulic 

conductivity was 99.5 ± 0.5% (Table 1). In B. platyphylla, 

by contrast, the vessels showed a narrower range of diameters 

of 5–55 µm, and many more vessels were present in both 

aged xylem than Q. serrata had (Figures 4C and D). Furthermore, 

all vessels were functional in the whole xylem under a 

well-watered condition. The current and 1-year-old xylem 

showed similar water conduction (Table 1). 

During dehydration for Q. serrata, the percentage numbers 

of functional vessels in the respective size classes did not differ 

markedly for vessel diameters of 5–50 µm in the current 

xylem, but for vessel diameters larger than 50 µm, more ves- 



sels seemed to be embolized by dehydration (Figures 5B vs. 

A). After rewatering, embolized vessels in some size classes of 

current xylem were refilled 1 h after rewatering (Figure 5C). 

The refilling was clearer in smaller vessels for 12-h rehydration 

(Figure 5D). In 1-year-old xylem, most vessels had already 

embolized in the absence of water stress (Figure 5E), as reported 

previously (Sperry and Sullivan 1992, Cochard 

2006), and remained embolized, irrespective of rewatering 

(Figures 5F–H). 

In B. platyphylla, although all sizes of vessel were functional 

before dehydration in current and 1-year-old xylem 

(Figures 6A and E), 50% or more of vessels in each size class 

were embolized when water stress was induced (Figures 6B 

and F). Larger vessels were embolized by dehydration, espe- 


Figure 6. Percentage of functional and embolized vessels in each 5-µm class in B. platyphylla. Gray and open bars, respectively, represent 

functional and embolized vessels: A–D show data for current xylem; E–H are for 1-year-old xylem; A and E, before dehydration; B and F, after 

dehydration; C and G, 1 h after rewatering; D and H, 12 h after rewatering. Class mean ± 95% of the confidence interval (n = 3). 

cially in 1-year-old xylem. The rapid refilling of xylem vessels 

was observed during 1-h rehydration (Figures 6C and G). 

At 12 h after rewatering, most vessels in all classes had fully 

refilled in the current xylem (Figure 6D), although about 20% 

of vessels were still embolized in 1-year-old xylem 

(Figure 6H). At 1 h after rewatering, larger vessels of current 

xylem appeared to be embolized still; they were cases in 

which vessels of more than 50 µm diameter belonged to only 

one plant. 

Composition of Ks in B. platyphylla 

Theoretical specific hydraulic conductivity (K s) in current 

and 1-year-old xylem of B. platyphylla is portrayed in Fig- 



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Figure 7. Theoretical specific hydraulic conductivity in current (A) and 1-year-old (B) xylem for B. platyphylla as measured in nondehydrated, 

dehydrated and rehydrated stem segments 1 and 12 h after rewatering. Mean ± SD (n = 3). NS denotes a nonsignificant difference compared 

with nondehydrated segments (P < 0.05) for Dunnett’s test. 

ure 7. In current xylem, no significant loss of theoretical K s 

was observed; it was the same as that at 1 or 12 h after rewatering 

(Figure 7A). In 1-year-old xylem, loss of more than 

half of the mean theoretical K s was observed by dehydration 

and was apparently recovered by rewatering, but no change 

was found to be statistically significant (Figure 7B), probably 

because of large variation in the data. 

Discussion 

During plant dehydration, the percentage loss of hydraulic 

conductivity of ring-porous Q. serrata was scattered throughout 

a range of xylem water potential (Figure 1). These widely 

varied values among individuals of Q. serrata might result 

from intraspecific variation in xylem cavitation. Some reports 

have described that intraspecific variation exists in xylem 

cavitation and hydraulic mean vessel diameter (Sperry and 

Saliendra 1994, Matzner et al. 2001). Therefore, slight differences 

of the maximum functional vessel diameter might invite 

a great effect on the extent loss of hydraulic conductivity 

of plants, especially in big-vessel-bearing species. For Q. serrata, 

which shows variation in distribution of vessel diameter 

(Figures 4 and 5), it would be the case in which vulnerability 

curves might appear as various shapes among individuals, although 

intraspecific variation in structural pit properties remains 

unclear. Other studies have also shown vulnerability 

curves of ring-porous species with larger deviations than that 

of diffuse-porous species (e.g., Sperry and Sullivan 1992). 

Considering these facts, however, it can be inferred that 1year-old 

xylem almost loses its water transport function, even 

under high xylem water potential, thereby contributing to a 

high PLC and increasing PLC by small degrees with decreasing 

Ψxylem. This lack of a clear pattern of xylem recovery was 

observed after rewatering in Q. serrata (Figure 2), although the 

xylem water potential recovered completely with time (Figure 

3). By contrast, diffuse-porous B. platyphylla exhibited 

a critical xylem water potential at which a distinct loss of 

OGASA ET AL. 


hydraulic conductivity was shown (Figure 1). In addition, 

this study provided clear evidence that the loss of hydraulic 

conductivity by natural drought was recovered rapidly 

(1 h after rewatering), reverting to the values shown for the 

well-watered status despite incomplete repair of the xylem water 

potential through ‘novel refilling’, as reported by Hacke 

and Sperry (2003). Recent studies also showed asynchrony 

between the recovery of specific conductivity and recovery 

of the xylem water potential (Hacke and Sperry 2003, Nardini 

et al. 2008) and/or hysteresis between dehydration and 

rehydration courses (Hacke et al. 2001, Stiller et al. 2005). 

Rapid xylem recovery in the absence of root pressure was 

performed even under tension by operations such as osmosis 

because of starch-to-sugar conversion in wood parenchyma 

cells or because of increased concentration of inorganic 

ions in xylem sap, as reported in the field by Buccietal. 

(2003) or as demonstrated experimentally by Tyree et al. 

(1999) and Salleo et al. (2004, 2006). Our results also support 

their suggestions with respect to the actual Ψxylem during rehydration 

being sufficiently lower than Ψ xylem refilling 

(Ψ conduit refilling = −2T/r, where T is the surface tension of 

water and r is the conduit radius (m); Yang and Tyree 

1992), as calculated from mean refilled vessel diameter 

(−0.4 vs. −0.015 MPa in Q. serrata and −0.2 vs. −0.016 

MPa in B. platyphylla, at 25°C). Results of the present 

study show—for evaluation of the extent of vessel refilling—that 

the intensity of water stress was set at a level 

where leaves were not wilted completely for Q. serrata 

(Saito and Terashima 2004) and B. platyphylla (Ranney et 

al. 1991) and plants were dried naturally at the whole plant 

level to evaluate daily possible refilling under natural conditions. 

Salleo et al. (1996) described that the degree of hydraulic 

conductivity recovery would change depending on 

the intensity of the water stress to which plants were subjected: 

a limiting point would exist at which the cavitated 

vessel refilled rapidly. Even though plants can eventually 

resprout after severe water stress (Viagrosa et al. 2003), if 

such stress greatly exceeds the wilting point of leaves, un- 



like our case, then embolized vessels might be refilled only 

slightly—not only quickly but even overnight. 

The high degree of 1-h rapid refilling of the embolized 

vessels in B. platyphylla, which is observed in all size 

classes of both current and 1-year-old xylem, provided rapid 

recovery of the hydraulic conductivity (Figure 6). Embolized 

vessels of the current xylem were almost refilled 

by taking sufficient times of 12 h. It is notable that about 

20% or more of vessels of any size class in 1-year-old xylem 

had been embolized, which indicates that cavitation in 

1-year-old xylem vessels cannot be refilled during a single 

day (17.10 ± 13.67% (mean ± SD, n = 3) in PLC of B. 

platyphylla stems at 24 h after rewatering, data not shown). 

Therefore, although the extent of conducting vessels did 

not affect the xylem specific conductivity for different water 

status irrespective of xylem age (Figure 7), different refilling 

patterns between xylem ages might leave insufficient 

time for refilling, with some structural fault in 1-year-old 

xylem, such as pit degradation (Sperry et al. 1991) or perhaps 

aging itself, even though both aged vessels were fully 

conducted under well-watered conditions. In contrast, for Q. 

serrata, some vessel refilling in current xylem was apparent 

in each size class, especially more in small size classes than 

in larger size classes, while vessels of the 1-year-old xylem 

remained embolized after rewatering (Figure 5). This might 

result from greater sensibility of smaller vessels for refilling 

mechanically or priority of small vessels phenologically 

than that of larger vessels in relation to preparation for next 

year’s growth. 

Pits are pathways not only of water but also of air during 

drought-induced cavitation (Sperry and Tyree 1988, Tyree 

and Zimmermann 2002). In relation to the xylem structure, 

our results, showing that dependence of cavitation on vessel 

size was seemingly observed in either the studied species or 

other species (Hargrave et al. 1994), imply that the pit distribution 

depends on the vessel size. As reported by Wheeler et 

al. (2005) and Hacke et al. (2006), the total pit area per vessel 

determines the safety to cavitation interspecifically. Considering 

our cases of both species intraspecifically, larger vessel 

diameter might imply a higher risk of cavitation. 

Highly specific conductivity of Q. serrata was apparent 

throughout the experiments, even in the presence of much 

native embolism in current xylem and of nonconducting 1year-old 

xylem compared with B. platyphylla, indicating 

that Q. serrata holds ‘disposable’ xylem vessels and can 

compensate water conducting with its remaining efficient 

functional vessels. These results are explainable by the 

trade-off between the conducting efficiency and cavitation 

safety (Tyree and Zimmermann 2002). Large vessels provide 

dramatically high efficiency of water conduction but 

simultaneously provide less safety. Hacke et al. (2006) also 

explained that the remaining larger vessels under drought 

are more efficient for water conduction than smaller ones, 

even if some larger ones were sacrificed by cavitation in a 

so-called sacrificial strategy. Furthermore, such ‘disposable’ 

xylem vessels of Q. serrata can associate with stronger sto- 


matal regulation of transpiration in ring-porous species 

(Bush et al. 2008) in respect of preventing vessels from cavitation. 

Consequently, Q. serrata might retain a stable water 

supply under daily drought and have low vessel-filling ability. 

In less efficient and small-vessel-bearing species, on the 

other hand, they must meet the likely demand for water 

conduction by holding numerous safer vessels (vessel-packing 

strategy; Hacke et al. 2006). In fact, Ψxylem at which the 

cavitation for B. platyphylla started was very close to that 

level resulting in substantial or complete loss of conductivity 

(the midday minimum Ψ leaf of 3-year-old B. platyphylla 

was around −2.0 MPa in October 2007; data not shown). 

For that reason, recovery from cavitation might be necessary 

for sufficient water supply throughout the growth season: 

the high extent of vessel refilling observed here is 

explainable as a safety net to regain hydraulic efficiency 

that was once lost. In other words, B. platyphylla holds ‘reusable’ 

xylem vessels. 

This study revealed different hydraulic traits of Q. serrata 

and B. platyphylla. Irrespective of Q. serrata showing 

no marked vessel refilling and recovery of hydraulic conductivity, 

they might perform their water conduction using 

the remaining large vessels, which are still more efficient 

than smaller and inherently safer vessels. In contrast, B. 

platyphylla, which has smaller vessels, retained water conduction 

because of its capacity of rapid vessel refilling 

even though vessels lose their conductivity through water 

stress. As indicated for Q. serrata and B. platyphylla, if 

it were commonly appropriate that ring-porous and diffuse-porous 

species would hold ‘disposable’ and ‘reusable’ 

vessels in the xylem, respectively, then knowledge of this 

new aspect of xylem recovery after water-stress release 

might provide an advanced interpretation of a water-conducting 

strategy toward water stress for plants. 

Acknowledgments 

We are grateful to Dr. K. Sakamoto and Dr. M. Hirobe for their insightful 

comments. Comments from anonymous reviewers were 

very helpful for improving the final version. This study was supported 

by a Grant-in-Aid for JSPS Fellows (21.5030) and was conducted 

under the Cooperative Research Program of Arid Land 

Research Center, Tottori University. 

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