Tree Physiology 18, 333--339
© 1998 Heron Publishing----Victoria, Canada
Hydraulic conductivity in roots of ponderosa pine infected with
black-stain (Leptographium wageneri) or annosus (Heterobasidion
annosum) root disease
GLADWIN JOSEPH,1 RICK G. KELSEY2 and WALTER G. THIES2
1
Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA
2
USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR 97331, USA
Received March 14, 1997
Summary Roots from healthy and diseased mature ponderosa pine, Pinus ponderosa Laws., trees were excavated from a
site near Burns, Oregon. The diseased trees were infected with
black-stain root disease, Leptographium wageneri Kendrick,
or annosus root disease, Heterobasidion annosum (Fr.) Bref.,
or both. Axial hydraulic conductivity of the roots was measured
under a positive head pressure of 5 kPa, and the conducting
area was stained with safranin dye to determine specific conductivity (ks). In diseased roots, only 8--12% of the cross-sectional xylem area conducted water. Resin-soaked xylem
completely restricted water transport and accounted for 13-16% of the loss in conducting area. In roots with black-stain
root disease, 17% of the loss in conducting area was associated
with unstained xylem, possibly resulting from occlusions or
embolisms. Based on the entire cross-sectional area of infected
roots, the ks of roots infected with black-stain root disease was
4.6% of that for healthy roots, whereas the ks of roots infected
with annosus root disease was 2.6% of that for healthy roots.
Although these low values were partly the result of the presence
of a large number of diseased roots (72%) with no conducting
xylem, the ks of functional xylem of diseased roots was only
33% of that for healthy roots. The low ks values of functional
xylem in diseased roots may be caused by fungus induced
occlusions preceding cavitation and embolism of tracheids.
The ks of disease-free roots from diseased trees was only 70%
of that for healthy roots from healthy trees. The disease-free
roots had the same mean tracheid diameter and tissue density
as the healthy roots, suggesting that the lower ks in disease-free
roots of diseased trees may also have been caused by partial
xylary occlusions.
Keywords: annosus root disease, black-stain root disease,
Pinus ponderosa, root conductivity, specific conductivity,
wood density.
Introduction
Many of the root diseases that commonly infect ponderosa
pine, Pinus ponderosa Laws. interfere with axial water conductivity in the stem. Black-stain root disease, Leptographium
wageneri var. ponderosum (Harrington & Cobb) Harrington &
Cobb, is a vascular wilt fungus that does not decay the xylem
(Hansen et al. 1988). Black-stain root disease is thought to
inhibit water movement by blocking tracheids with hyphal
growth in the lumen and bordered pit pairs (Smith 1967), or by
nonresinous (tyloses) and resinous occlusions produced by
host cells in response to the infection (Hessburg and Hansen
1987). Annosus root disease, Heterobasidion annosum (Fr.)
Bref., unlike black-stain root disease, decays the xylem
(Schmitt et al. 1997) and creates nonconducting dry zones in
the sapwood of stems by allowing air to enter as bordered pits
are enzymatically broken down and water is withdrawn by
hydrostatic tension (Coutts 1976). Blue-stain fungi, Ophiostoma spp., which are typically found in the sapwood and
phloem of trees attacked by bark beetles (Harrington 1993),
disrupt water transport by cavitation and air entry into the
tracheids (Mathre 1964). Most studies on diseased conifers
have focused on the water relations of infected stems and many
of them provide only qualitative descriptions of how the disease affects water conductance.
Axial water conductivity in roots of conifers and hardwood
trees is several-fold higher than in stems or branches (Gartner
1995). This greater conductivity in roots generally parallels the
presence of wider and longer tracheids or vessels in roots than
in stems and branches (Zimmerman and Potter 1982, Gartner
1995, Sperry and Ikeda 1997). Although roots have higher
water conductivity, root tracheids may be more susceptible to
cavitation and embolism (Sperry and Saliendra 1994, Alder
et al. 1996, Sperry and Ikeda 1997) than tracheids in stems or
branches, making them more susceptible to pathogen-induced
embolism. However, little is known about the mechanism or
magnitude of decline in water conductivity of diseased conifer
roots.
In our study of ponderosa pine, roots from healthy and
diseased trees infected with black-stain root disease, annosus
root disease, or both, were excavated and their hydraulic conductivities measured. Our objectives were to determine the
magnitude of decline in specific conductivity as a result of the
disease, and to determine the extent and characteristics of
nonconducting xylem associated with diseased roots.
334
JOSEPH, KELSEY AND THIES
be perfused with dye. Specific conductivity (ks) was calculated
as (Tyree and Ewers 1991):
Material and methods
Plant material
The ponderosa pine trees were located 39 km NE of Burns,
Oregon (43°52′15″ N; 118°45′5″ W) in the Malheur National
Forest. The site is at 1690 m elevation, with a 6% slope, an
eastern aspect, and a mean annual precipitation of 63.5 cm.
The soil is a loamy clay type. Most trees in the overstory were
less than 100 years old. Two healthy and eight diseased trees
with a range of crown symptoms were pushed over with a
skidder. Roots of the diseased trees were infected with
L. wageneri and H. annosum and some also had blue-stain
fungus, Ophiostoma spp. Root segments, 0.3 to 3 cm in diameter and 15 to 25 cm in length, were selected with a range of root
disease symptoms. Roots with severe decay were not collected. Disease-free root segments from diseased trees were
collected from roots with no apparent symptoms of disease in
the portion of the root that was excavated. Seven to 15 roots
were sampled from each tree. Root segments were wrapped in
moist paper towels, sealed in plastic bags and stored on ice. In
the laboratory, root segments were stored at 4 °C for a maximum of 10 days during the measurements.
Hydraulic conductivity measurements
Root hydraulic conductivity was measured with an apparatus
that maintained a constant head pressure of 5 kPa. The system
consisted of a reservoir with the perfusion fluid connected to a
closed loop of flexible tubing (main stem) with four T-junctions and flexible tubing of appropriate diameter to attach to
the root segments with a tight seal. A clamp between the root
segment and the T-junction regulated the flow. Roots were
perfused with degassed distilled water containing prefiltered
(0.22 µm nylon membrane) 0.07% HCl (pH 2.6). The water
was acidified to inhibit microbial growth on the inner walls of
the tubing, which would otherwise cause rapid clogging of the
xylem (Sperry et al. 1988).
Hydraulic conductivity was measured on a root segment
(ranging from 2.7 to 8.2 cm in length) cut from each field
sample under distilled water immediately before the measurement. The thin bark around healthy roots was easily peeled.
When this occurred the sapwood was wrapped with Parafilm
before or after insertion into the tubing to prevent desiccation
and possible surface embolism. The bark was not easily removed from diseased roots, so it was shaved from the ends of
each segment to expose the sapwood before each segment was
inserted in the tubing. The clamps were opened for 10 min
allowing four root segments to equilibrate simultaneously with
the perfusion solution at 5 kPa of pressure. Preliminary measurements showed no change in flow rates from 10 min to 4 h
after pressurization. Clamps were then closed to three of the
four segments, and the fluid flowing through the unclamped
segment was collected in a preweighed dry beaker for 15 s and
weighed to the nearest 0.1 mg. This was repeated four or five
times before proceeding to the next root segment. After a
segment was measured it was placed in distilled water for
approximately 60 min to prevent desiccation while waiting to
ks =
wl
= kg s−1 m − 1 MPa −1,
tap
(1)
where w is the weight of the perfusion fluid (kg), l is the
segment length (m), t is the duration of perfusion (s), a is the
cross-sectional area of the root or conducting xylem (m2), and
p is the head pressure (MPa).
The cross-sectional area of conducting xylem was determined by perfusing the segments with dye in an apparatus
similar to that used for measuring hydraulic conductivity. The
dye solution was 0.2% (w/v) safranin in degassed distilled
water that had been passed through a 0.22 µm nylon membrane
filter. The dye reservoir was placed 1 m above the segments to
generate a head pressure of 10 kPa. The root segments were
inserted in the T-junctions with appropriately sized tubing and
the dye allowed to perfuse for approximately 5--10 min. Dye
emerged from the opposite ends of the segments in a few
minutes. Most of the conducting tracheids were stained in this
interval, and we assumed that they were functional. Several
segments were cut at their midpoint and visually checked to
insure that the dye had perfused through the root. Diseased root
segments with low conductivity were allowed to perfuse for
another 10--15 min to insure that most of the conducting
tracheids were stained. Increasing the dye perfusion time by
10--15 min for the diseased segments should not have altered
staining patterns, because there was no change in ks from
10 min to 4 h of pressurization in preliminary tests. Each root
segment was measured to the nearest 0.001 cm and then stored
at --35 °C in a sealed plastic bag for later measurements.
To measure the stained cross-sectional area, a disk (0.2-0.5 cm wide) was cut from the center of each segment and
dried at room temperature for 24 h. The disk images were
scanned into a computer (Sony, Cypress, CA) and the crosssectional areas of stained and unstained sections were measured with image analyzing software (NIH Image Version 1.52,
Rasband and Bright 1995).
Disease characterization
Nonconducting portions of each root disk not penetrated by
dye were sorted into categories described below, and their
relative cross-sectional areas measured with the aid of a microscope. Sections with the characteristic black-stain were identified as L. wageneri (Smith 1967). Sections with brown-stain or
decayed, yellow spongy tissue were identified as H. annosum
(Schmitt et al. 1997). Blue-stained sections with a wedge
shape and dry appearance were identified as Ophiostoma spp.
(Harrington 1993). A clear stain, of unknown origin, often
occurred on the periphery of the black-stain. Resinosis appeared as water-soaked zones, whereas nonconducting-unstained zones, which were nearly healthy in appearance, were
probably occluded or embolized. Nonconducting cores of a
few healthy roots also had embolized tracheids for reasons that
are unknown.
TREE PHYSIOLOGY VOLUME 18, 1998
AXIAL CONDUCTIVITY OF DISEASED ROOTS
Root tissue density
A disk 0.5--1 cm in width was removed from the mid-section
of each root to measure tissue density. The disk was dried at
102 °C for 16 h, weighed to the nearest 0.01 g and then
immersed in a beaker containing distilled water. The volume
of water displaced was measured to the nearest 0.01 g. Density
was calculated as the ratio of dry weight to dry volume. Dry
volume was used in place of wet volume because the roots had
been stored frozen and this might have affected the wet volume
measurements.
Anatomical measurements
Healthy roots, disease-free roots from diseased trees, and diseased roots with some conducting xylem were used to measure
tracheid diameters. Free-hand transverse sections were made
with a blade and mounted in ethyl glycol. Radial lumen diameters were measured on 40 to 126 tracheids from each cross
section with an occular micrometer at 40× magnification.
Eight to 16 sectors were randomly located over the entire cross
section, except near the root center where tracheids may have
been compressed. In each sector, a radial row of tracheids was
measured within a growth ring. The percentage of tracheids in
5-µm diameter classes was calculated, as well as the percentage of estimated hydraulic conductance contributed by tracheids in each diameter class. Tracheid hydraulic conductance
was assumed proportional to the diameter raised to the fourth
power as predicted by the Hagen-Poiseuille equation (Zimmerman 1983).
Statistical analysis
Separate analyses were conducted for roots classified in two
ways: (1) healthy roots, black-stain root disease roots, and
annosus root disease roots, and (2) healthy roots from healthy
trees, disease-free roots from diseased trees, or diseased roots
335
from diseased trees with some conducting tissue. Differences
among mean percent conducting xylem, ks, tissue density, and
tracheid diameter for the disease classes were analyzed as a
one-way ANOVA. Data were loge transformed when necessary
to ensure normal distribution and homogeneity of variance.
Back transformed means were presented when data were transformed. For each category of nonconducting xylem (Table 1),
the percentage cross-sectional area was rank transformed before analysis. Significantly different means were identified by
Fisher’s Protected Least Significant Difference (LSD) at α =
0.05. The relationships between mean tracheid diameter and ks,
and tissue density were analyzed by linear regression.
Results
In roots with black-stain root disease, nonconducting-unstained areas, which were presumably occluded or embolized,
occurred adjacent to actively conducting tissue (stained areas),
but did not show any particular pattern (Figure 1). Resinosis
associated with black-stain root disease often extended from
the periphery of the root toward the central core in a wedgeshaped pattern with a water-soaked appearance, rather than the
dry appearance of the wedge-shaped blue-stain. In one root
section, severe black-stain root disease and resinosis had completely destroyed xylem function. In trees infected with annosus root disease, several roots were completely soaked in resin
proximal to the infection and had no functional xylem. Some
healthy roots had a central core with little or no functional
xylem (Figure 1).
In healthy roots with no visible disease, close to 100% of the
xylem conducted water, whereas in diseased roots only 8--12%
of the xylem conducted water (Table 1). Based on entire
cross-sectional areas, the ks of diseased roots was 2.5--4.6% of
that for healthy roots and the tissue densities of diseased roots
were significantly lower than those of healthy roots (Table 1).
Figure 1. Diseased and
healthy root cross sections
of ponderosa pine perfused
with saffranin dye. Stained
xylem (d), black-stain (b),
annosus decay (a), nonconducting-unstained xylem
(possibly occluded or embolized) (e), resinosis (r), and
nonconducting core (p); ks =
specific conductivity (kg s −1
m −1 MPa −1), f = conducting
(functional) area (%).
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336
JOSEPH, KELSEY AND THIES
Table 1. Mean (± SE) % conducting xylem, specific conductivity (ks), tissue density, and characteristics of nonconducting xylem of healthy and
diseased (black-stain root disease and annosus root disease) ponderosa pine roots1--3.
Healthy roots (n = 53)
Conducting xylem (%)
ks (kg s −1 m −1 MPa −1)
Tissue density (g ml −1)
96.6 ± 1.7 a
7.32 + 0.52 -- 0.49 a
0.44 ± 0.02 a
Characteristics of nonconducting xylem (%)
Black-stain
0
Decay or brown-stain
0
Blue-stain
0
Resinosis
0
Clear-stain
0.3 ± 1.0 a
Unstained
0.05 ± 0.15 a
Nonconducting core
2.6 ± 0.8 a
1
2
3
Roots with black-stain root disease (n = 10)
Roots with annosus root disease (n = 22)
12.0 ± 3.8 b
0.34 + 0.20 -- 0.17 c
0.62 ± 0.04 b
7.5 ± 2.6 b
0.19 + 0.12 -- 0.11 c
0.53 ± 0.03 b
37.7 ± 4.1 a
0
5.3 ± 1.8 a
16.3 ± 8.7 b
11.2 ± 2.5 b
17.5 ± 3.4 b
0
4.2 ± 2.8 b
74.3 ± 4.0 a
0
12.8 ± 5.9 b
0.8 ± 1.7 a
0
0.2 ± 1.2 b
The ks was calculated using the entire cross-sectional area of the root.
Means (± SE) followed by the same letters are not significantly different at P < 0.05 (Fisher’s Protected LSD).
Means (± SE) of ks were back-transformed.
Twenty-three of the 32 diseased roots had no conducting
xylem. Nonconducting unstained tracheids accounted for
17% of the loss in functional xylem in roots with black-stain
root disease, whereas there were no nonconducting unstained
tracheids in roots with annosus root disease. The loss in xylem
conduction in roots with black-stain root disease was probably the result of occlusions or embolism. Many roots with
annosus root disease had large areas of advanced decay with
the typical yellow spongy appearance (Table 1). Resinosis
accounted for 12--16% of the nonconducting xylem in roots
with either disease. Blue-stain disease was associated with the
black-stain root disease and covered 5% of the cross-sectional
area.
Disease-free roots from diseased trees had a 30% lower ks
than healthy roots from healthy trees (Table 2), but they had a
similar area of conducting xylem. The ks of diseased roots was
48% of the ks of disease-free roots from diseased trees, and
only 33% of the ks of healthy roots from healthy trees. Because
the ks values in Table 2 were calculated based on the areas of
conducting xylem only, they are higher than the ks values in
Table 1. There were no significant differences in mean tracheid
diameters among the roots (Table 2), but the distribution of
tracheid diameters and estimated hydraulic conductance revealed differences between the diseased roots and the diseasefree and healthy roots (Figure 2). Both disease-free and healthy
roots had a higher frequency of larger diameter tracheids and
associated hydraulic conductance than the diseased roots.
There was no difference in tissue density between disease-free
roots from diseased trees and healthy roots, but tissue densities
of both root types were lower than the tissue density of diseased roots (Table 2).
Mean tracheid diameter was positively related to ks for both
disease-free and healthy roots (Figure 3). This relationship was
nonsignificant for the diseased roots (data not shown). The
relationship for disease-free roots from diseased trees had a
slightly smaller slope than that for the healthy roots (0.32
versus 0.47) indicating that ks of disease-free roots may be
restricted by xylary occlusions. Root densities were negatively
correlated with ks and mean tracheid diameters (Figure 4).
Both ks and tracheid diameters decreased rapidly as the tissue
density increased from 0.3 to 0.6 g ml −1. This relationship was
similar in disease-free, healthy, and diseased roots.
Table 2. Mean (± SE) % conducting xylem, specific conductivity (ks), tracheid diameter, and density of root tissue from healthy and diseased
(black-stain root disease and annosus root disease) ponderosa pine trees1--3.
Conducting xylem (%)
ks (kg s −1 m −1 MPa −1)
Tracheid diameter (µm)
Tissue density (g ml −1)
1
2
3
Healthy trees (n = 2)
Diseased trees (n = 9)
Healthy roots (n = 16)
Disease-free roots (n = 36)
Diseased roots (n = 9)
96.6 ± 2.1 a
9.59 + 1.40 -- 1.22 a
30.4 + 2.1 -- 1.9 a
0.43 ± 0.02 a
96.3 ± 3.2 a
6.62 + 0.66 -- 0.55 b
29.7 + 1.4 -- 1.3 a
0.45 ± 0.02 a
37.6 ± 4.2 b
3.23 + 0.64 -- 0.54 c
26.7 + 2.8 -- 2.5 a
0.54 ± 0.04 c
The ks was calculated using only the conducting areas of the root, whereas in Table 1 ks was calculated using the cross-sectional area of the entire
root. Only diseased roots with some conducting xylem were used to calculate the means for diseased roots.
Means (± SE) followed by the same letters are not significantly different at P < 0.05 (Fisher’s Protected LSD).
Means (± SE) of ks and tracheid diameters were back-transformed.
TREE PHYSIOLOGY VOLUME 18, 1998
AXIAL CONDUCTIVITY OF DISEASED ROOTS
337
Figure 3. Specific conductivity (ks) as a function of mean tracheid
diameter. For healthy roots, y = --3.53 + 0.471x, P < 0.04, r 2 = 0.28,
and n = 16. For disease-free roots from diseased trees, y = --2.57 +
0.32x, P < 0.001, r 2 = 0.68, and n = 36.
Figure 2. Percent tracheids or % calculated hydraulic conductance
versus 5-µm tracheid diameter class for diseased roots (A, n = 9),
disease-free roots from diseased trees (B, n = 36), and healthy roots
from healthy trees (C, n = 16). Vertical bars are one SE of the mean.
Discussion
In diseased roots, most of the tracheids could not conduct
water because they were structurally altered by decay, plugged
by nonresinous or resinous occlusions, or were embolized.
Tracheids associated with advanced decay as a result of annosus root disease were probably nonfunctional because they
were entirely embolized (Coutts 1976), although annosus root
disease may have also plugged some tracheids with hyphae. In
black-stain root disease roots, nonresinous occlusions or embolized tracheids probably prevented the xylem from conducting water in both the stained and unstained areas. Black-stain
root disease physically plugs tracheid lumens or bordered pits
with hyphae, gums, or tyloses (Smith 1967, Hessburg and
Hansen 1987). Resinous occlusions blocked about the same
proportion of tracheids in roots infected with annosus root as
in roots infected with black-stain root disease. Complete resinosis of the root, which inhibited water transport entirely, was
Figure 4. Specific conductivity (ks) (A), and mean tracheid diameter
(B) as a function of root tissue density (n = 59) for categories of roots.
A: y = 39.0 -- 102.4x + 72.4x 2, P < 0.001, r 2 = 0.35; B: y = 112.9 -283.8x + 216.5x 2, P < 0.001, r 2 = 0.55.
more common in roots infected with annosus root disease than
in roots infected with black-stain root disease. Pine species
have well-defined resin systems that respond rapidly to injury
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338
JOSEPH, KELSEY AND THIES
caused by pathogens or other factors (Nebeker et al. 1993).
Both constitutive and induced resin can flow into sapwood
surrounding the source of injury, thus isolating the injured
tissue, but also restricting the pathway for water transport.
Tracheids infected with blue-stain probably were disabled by
cavitation (Mathre 1964).
Specific conductivity (ks) of diseased roots, calculated on
the basis of the entire cross-sectional area of the root, was only
3--5% of that for healthy roots. For those diseased roots that
had conducting xylem, the ks was 33% of that for healthy roots.
In a similar study, the ks of symptomatic roots (calculated on
the basis of the entire cross-sectional area of the root) from
Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and grand
fir (Abies grandis (D. Don ex Lamb.) Lindl.) infected with a
complex of several diseases including annosus root disease and
black-stain root disease, was 20--70% of that for asymptomatic
roots (Baker et al. 1994). Thus, it is apparent that root disease
in conifers substantially reduces the ks of roots. The low ks
values of diseased ponderosa pine roots in our study was partly
because a large number (72%) of diseased roots had no conducting xylem, and perhaps indicates that the ponderosa pine
trees were at a more advanced stage of infection than the two
species studied by Baker et al. (1994).
Based on microscopic examination of cross sections of
diseased roots, we conclude that the low ks value of functional
xylem in diseased roots (only 33% of that for healthy roots)
was not caused by the loss of conductance in larger diameter
tracheids. However, diseased roots had a higher percentage of
small tracheids and higher tissue density than healthy roots
(Figure 2). Slower growth of roots of diseased trees, caused by
overall stress from an impaired root system, may result in
smaller tracheids and increase the percentage of latewood
tracheids that have smaller diameters and thicker cell walls
than earlywood tracheids (Gartner 1995), leading to reduced
axial and radial hydraulic conductivities (Kramer and Kozlowski 1979). In addition, the presence of partially occluded
tracheids and bordered pits probably caused resistance to water
flow. This explanation is supported by the finding that, in roots
with annosus root disease, most of the tracheids associated
with brown-stain showed some conductivity.
We do not know why the ks of healthy roots from healthy
trees was higher than that of disease-free roots from diseased
trees. Because tracheid diameter distributions and tissue densities were similar in disease-free roots and healthy roots
(Figure 2), the lower ks in disease-free roots suggests the
presence of occlusions, or some other resistance to water flow
in the tracheids that are conducting water. For black-stain root
disease, occlusions by gum-like tyloses have been observed
only in tracheids colonized by the fungus and in adjacent
uncolonized tracheids and bordered pits (Hessburg and Hansen 1987). These occlusions have not been observed in tracheids that are some distance from the stain.
The occurrence of partially occluded, but functional tracheids in diseased and disease-free roots suggests that plugging of tracheids may precede embolism and also predispose
the xylem to stress-induced embolisms. When root ks decreases as a result of plugging, steeper hydraulic gradients are
required to transport water through the roots, assuming similar
transpiration rates and soil water contents. The resulting increase in hydraulic tension over the air-seeding threshold
causes cavitation to occur, resulting in embolism of the tracheids (Zimmerman 1983). In contrast, embolism may precede
the physical plugging of vessels in Dutch elm (Ulmus americana L.) infected by Ceratocystis ulmi (Buism.) C. Moreau
(Newbanks et al. 1983), because the fungus degrades cell walls
allowing air to enter the tracheids (Dimond and Husain 1958).
Although fungal infections cause the formation of occluded or
embolized xylem in ponderosa pine, the mechanism allowing
air to enter these tracheids is not known.
Diseased roots reduce water conductivity and this can lead
to increased water stress in the trees, as indicated by decreased
stem water content (unpublished data). In conifers, the entire
sapwood functions in axial water transport (Lassoie et al.
1977), whereas in ring-porous hardwoods, such as Dutch elm,
90% of the fluid flow occurs through the outermost ring (Ellmore and Ewers 1985). Therefore, in conifers, fungal infections and any associated effects may have to extend over a large
cross-sectional area of the conducting sapwood before significant disruption of tree water balance occurs. In addition, most
conifers have well developed resin systems that can isolate the
fungal infection in roots by resinosis, which could restrict the
hydraulic damage. Although roots are more vulnerable to embolism than shoots (Sperry and Ikeda 1997), embolized tracheids in conifer roots may readily refill when water is
available, even in the absence of positive pressures (Borghetti
et al. 1991, Sperry et al. 1994). On the other hand, in hardwoods, embolized vessels refill only under positive pressures,
or they require new xylem production to restore hydraulic
conductance (Sperry et al. 1994). Consequently, the differences in hydraulic characteristics between conifers and hardwoods may result in a slower development of water stress in
diseased conifers than in diseased hardwoods.
In summary, the reduction in conducting area of diseased
roots by occlusions, embolisms, and resinosis, combined with
a large drop in ks of conducting xylem can significantly disrupt
root water transport, and may also decrease water uptake in
diseased trees. Additionally, disease-free roots of diseased
trees have reduced ks which may further affect the water
balance of such trees. The subsequent development of water
stress in diseased trees can affect their leader growth (unpublished data) and possibly weaken their defence system, making
them more susceptible to insect attack and colonizaton (Nebeker 1993).
Acknowledgments
We thank Max Ollieu (USDA Forest Service, Region Six, Forest
Insect and Disease) and Mark Loewen (USDA Forest Service, Burns
Ranger District) for arranging the excavation of trees, Dr. B. Gartner
for use of the image analyzing equipment, Dr. J. Zaerr and Dr. B. Yoder
for helpful reviews of the manuscript, and Dr. L. Ganio for assistance
with the statistics.
TREE PHYSIOLOGY VOLUME 18, 1998
AXIAL CONDUCTIVITY OF DISEASED ROOTS
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