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Micromorpholog
ical and chemic
al characteristics
of
Cengal (Neobalanocarpus heimif Heartwood Decayed
by Soft Rot Fungi
Yoon Soo Kim, Adya P. Singh, Andrew H. H. Wong,
Tae-Jin Eom, and Kwang Ho Lee.
\:{ q4l *E
H34& H2W iE€ 136f;ffi
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Reprinted from
Joumal of the Wood Science and Technologr
Vol. 34, No. 2 Q3A 2W
..= i.,,:,nghak 34(2)
: 68-77. 2006
Micromorphological and Chemic il Characteristics
of
Cengal
(Neobalanocarpus
heimiD Heartwood Decayed
by Soft Rot Fungi*'
Yoon Soo Kim*21, Adya P. Singh*3, Andrew H. H. Wong*.,
Tae-Jin Eom*s, and Kwang Ho Lee*'
ABSTRACT
The hearfwood
of cengal
(Neobalanocarpus
heimii) is known to have a high degree of decay
resistance
b1 virtue of its high extractive content.
After 30 years in ground contact an utility pole of this tropical
hardwood was found to be degraded only in the surface layers by cavity-forming soft rot fungi. The
present
work was undertaken l) to characterize the degradation of cengal heartwood
from the aspect ol
ultrastructure
and chemistry
and 2) to investigate the correlation between soft rot decay and its extractive
microdistribution in wood tissues. The chemical analysis of cengal heartwood
revealed the presence
of a
high amount of extractives as well as lignin. The wood contained
a relatively high amount of condensed
lignin and the guaiacyl units. Microscopic observations revealed that vessels,
fibers and parenchyma
cells
(both ray and axial parenchyma)
all contained extractives in their lumina, but in variable amounts. The
lumina of fibers and most axial parenchyma
were completely or almost completely filled with the
extractives. TEM micrographs showed that cell walls were also impregnated
with exfiactives and that pit
membranes connecting
parenchyma
cells were well coated and impregnated with extractives.
However,
fungal hyphae were present
in the extractive masses localized in cell lumina, and indications were that
the extractives did not completely inhibit fungal growth. The extent of cell wall degradation
varied with
tissue types. The fibers appeared to be more susceptible
to decay than vessels and parenchyma.
Middle
lamella was the only cell wall region which remained intact in all cell types which were severely
degraded. The microscopic observations suggested a close conelation between
extractive microdistribution
and the pattem and extent of cell wall degradation. In addition to the toxicity to fungi, the physical
consffaint of the extractive material present
in cengal heartwood cells is likely to have a profound effect
on the growth and path of invasion of colonizing fungi, thus confening protection
to wood by restricting
'l Received on May 31, 2005; accepted on September 20,2005.
This work was supported
by the grant from Regional
Research Centers Program of Korea Research Foundation
through Bio-Housing Research Institute
at Chonnam
National
University.
*2 Department
of Forest
Products and Technology,
Chonnam National University, Gwangiu 500-757,
Korea
*3 New Zealand Forest Research Institute Ltd. Private Bag 3020,
Rotoru4 New Zealand
*4 Faculty of Resource Science and Technology, Universiti Malaysia Sarawak
GfNIMAS), 94300 Kota Samarahan,
Sarawak, Maiaysia
15
Department
of Wood Science and Technology, Kyungpook National
University,
Daegu,
Korea
' Conesponding author : Yoon Soo Kim (kimys@chonnam.ac.k)
-68-
Micromorphological
and
Chemicai Characteristics
of Ceneal
(Neobalanocarpus
helnrri)
Heartu'ood Decayed
by Soft Rot Fungi
lungal
entry into cell walls. The presence
of relatively
high amount
of condensed
lignin is also likely to
be a factor in the resistance
of cengal
heartwood
to soft rot decay.
Ketwords:
cengal heartwood,
Neobalanocarpus
heimii, natural
durability,
soft rot, extractive
micro-
distribution,
microscopy
1. INTRODUCTION
Heartwoods
of many tree species,
particularly
those growing in the tropics,
are known to hat'e
a high degree
of decay resistance
by virtue of
their high extractive
content
(Schultz
and Nich-
trlas, 2000). Analytical and bioassay
studies
have
rhou'n many extractive components of such
rioclds
to be toxic to wood degrading
microor-
ganisms,
and therefore
potential exists to harvest
lhc'se
components
as future "wood preservatives"
r
Snrith et al., 1989).
The heartwood
of the Malaysian timber cengal
r \artbulanocarpus heimii) has high natural
Jurability
(Mohd. Dahlan
and Tam, 1985),
and
Ih-sidiomycete
resistance
property of its extractives
iu-r bssn ascertained
(Yamamoto and Hong 1988).
llrr\\'€\'er,
cengal heartwood is not completely
:ilrmllne
to fungal decay. Soft rot fungr, which
:r'long to Ascomycetes
and Deuteromycetes,
can
l;iusc substantial
decay of wood under conditions
'.rhere
competition
with wood decaying
Basiodio-
r:r\
cctes is not present or minimal. Soft rot
:.rngi
are widespread
in nature,
being tolerant
to
.: ri ide range of moisture, pH and temperature
I)aniel
and Nilsson,
1998).
The cengal
hearfwood
examined
in this study
.,-r. degraded
only in the surface layers
by soft
: ': tirngi after 30 years
in ground contact.
The
.i,,rk uas undertaken
to understand
why the
-:rgal heartwood
was so durable, with a focus
: ,'htain information
on the following features:
tirc content
and microdistribution
of extractives
.,::i I ) the rype and content of lignin.
2. MATERIALS
and
METHODS
2.1.
Cengal Wooden
Pole Sampling
The ground line surface
of a 30-yr-old square-
sawn (120 x 120 mm cross-sectional
area)
utility pole of cengal (Neobalanocarpus
heimii)
located in the rural area in Kuala Selangor,
Selangor, Malaysia, was found to be softened,
with suspected premature failure, by the na-
tional telecommunication company (Telekom
Malaysia
Berhad). The pole was excavated
and
a 500 mm long wooden pole sample at the
ground line position was probed for extent of
biodegradation.
The cross-section
of the pole at
this region revealed
that the wood material was
essentially heartwood, and that dark brown
discoloration
of surface degradation
(suspecting
fungal decay)
on all sides
of the wood extended
10-15 mm deep from the surface,
accompa-
nied with isolated
occuffence
of surface subter-
ranean
termite attacks
on the discolored zone.
The boundary between this discolored, degraded
zone and the adjacent sound heartwood was
rather abrupt.
2.2. Determination
of Extractives,
Lig-
nin
and Sugar
Content
For the chemical analysis,
well-known methods
were applied
to the woodmeal
of cengal
heart-
wood, such as extraction with ethanol-benzene,
hot water and loh NaOH. The wood meals of
sound and decayed cengal heartwood were
hydrolyzed with 3oh H:SO+ and the alditol-
acetate of neutral sugar was analyzed with gas
I
69
Yoon Soo Kim, Adya P. Singh, Andrew H. H. Wong, Tae-Jin Eom, and Kwang Ho Lee
chromatography. Lignin content was determined
by Klason method. The composition of lignin
was characteized by the oxidation of lignin in
an alkaline nitrobenzene solution (Chen, 1992).
FT-IR spectra were taken with KBr pellets
(lmg extractives-free wood powder in 300 mg
KBr) using a Perkin Elmar 282 IR-spectro-
photometer.
2.3,
Microscopy
Pieces of cengal heartwood obtained from
sound (nondecayed) and decayed regions from
the pole were sectioned with a sliding micro-
tome, and were examined with a confocal laser
scanning microscope for cellular diskibution of
the exhactive material. For transmission electron
microscopy (TEM), small pieces of wood taken
from moderately and heavily decayed regions
were dehydrated in acetone and embedded in
Spurr's low viscosity resin (Sprnr, 1969). Transverse
ultrathin sections were cut with a ultramicrotome
using a diamond knife. Sections were stained
with lYo potassium permanganate
(prepared in
0.1% sodium citrate) and then examined with a
JEOL IO1O
TEM.
3. RESULTS and DISCUSSION
Examination of hand-cut sections rmder polari-
zation microscopy indicated presence
of soft rot
cavities in wood cell walls (not illustrated). No
other form of microbial decay was present.
However, termites were found associated with
the degraded regions of the pole.
3.1. Chemical Characteristics
The chemical composition of sormd and decayed
cengal heartwood is summarized in Table l. It
is apparent that cengal heartwood contains high
amounts of ligrrin and extractives. The percentage
of extractive soluble in organic solvent (15%) is
much greater than the percentage of water
soluble extractives. In the degraded cengal
wood, the values of hot water (162%) and lo/o
alkali (56.1%o) soluble extractives are much
greater as compared to sound wood, suggesting
that a good proportion of low-molecular polysac-
charides is degraded.
In contrast, the amount of
organic solvent soluble extractives decreased
in
decayed cengal wood. The analysis with thin
layer chromatography showed that some spots
which were present in the organic solvent-
extractives of sound wood were absent in the
decayed wood (figures not shown). Chemical
analysis indicated some extractives (mainly phe-
nolics) were degraded over a long period of
exposure of the pole in ground contact. In com-
parison with sound wood, the decayed cengal
wood contains higher amormt of lignin and less
holocellulose. Only about half of the total
polysaccharides
remains in the highly degraded
cengal heartwood (Table 1), reflecting the extent
of polysaccharide losses due to degradation.
The lignin content of sormd (34.5%) (Table
l) cengal heartwood is much higher than that of
softwoods growing in temperate
zones and also
many hardwoods. The structural information
obtained on the lignin of sorud cengal heartwood
from the nitrobenzene oxidation products (Table
Table 1. Chemical composition
of sound and decayed cengal
heartwood
(%)
Hot water
Extractives l% alkaline
extractives EthanoVbenzene
extractives Holocellulose Lignin Ash
Sound
Decayed
3.2
16.2
LZ.)
56. I 15.0
9.7 6l .8
38.2 34.5
67.s 2.1
4.4
70-
Table 2. Lignin monomer
structures of sound and decayed cengal heartwood
(%)
p-Hydroxybenzaldehyde Vanillin Syringaldehyde Total
Sound
Decayed
1.5
0.2 7.0
2.1 12.r
3.7 20.6
6.0
Micromorphological
and Chemical Characteristics
of Cengal
(Neobalanocarpus
heimii)
Heartwood Decayed by Soft Rot Fungi
-l\-
a
i tq
Fig. 1 FTJR spectra of sound (S) and decayed (D) cengal heartwood.
I
ll
/i
i
ii
iti
r!
r- -'
."1
j
;
"ti
""
I i
".t;
::l/
I\ ilh-
\ ll;'
\r itu
- r_\ jv\
2) provides evidence of a much lower content
of p-hydroxylbenzaldehydes in this wood as
compared to the majority of hardwoods growing
in temperate zones. Lower amount of phenolic
aldehydes indicates
that cengal heafiwood contains
a
high proportion of condensed lignin. The alkaline
nitrobenzene oxidation gave a ratio of vanillin
to syringaldehyde
1:1.7, suggesting that cengal
hearfwood has a relatively high amount of
guaiacyl lignin as compared to most temperate
hardwoods. The reaction products of nitrobenzene
oxidation showed that lignin in cengal heartwood
has more condensed structure and more guaiacyl
lignin than that in the hardwoods growing in
temperate zones.
Chemical differences between sound and de-
cayed woods are illustrated in the FT-IR spectra.
The most notable differences in the IR spectra
between the sorurd and the decayed cengal heart-
wood samples
are: 1) A total loss of absorption
band 1730 cm-t. Thir might be due to hemi-
cellulose degradation
(Kuo et al., 1988). 2)
Increased
absorbance at 1620. 1420- 1270 and
1130 cm-t due to lisnin. A sisnificant increase
in the absorption around 1620 cm-' in the
decayed cengal wood reflects a high lignin and
phenolics content of this wood. 3) Decreased
absorbance at 1060,1040 and 890 cm-'in
degraded cengal wood might be due to the
degradation of cellulose. Additionally, X-ray
diffractograms indicated a loss in cellulose
crystallinity in the degraded wood (figures not
shown), suggesting that even cellulose crystallinity
suffers from soft rot attack.
3.2. Anatomical and Micromorpho-
logical Characteristics
The pattems of extractive microdisttibution in
degraded and sound cengal heartwood are shown
in Figs. 2 and 3. The bulk of extractives are
associated with rays and axial parenchyma
cells.
Rays are almost completely filled with extractives.
Although the amount of extractives in axial
parenchyma cells appears to be variable, these
cells also contain large masses of extractives
(Fig. 2). The lumina of a large proportion of
parenchyma cells are completely filled with ex-
7I
Yoon Soo Kim, Adya P. Singh, Andrew H. H. Wong, Tae-Jin Eom, and Kwang Ho Lee
Fig. 2. Transverse section of decayed heartwood
showing the dishibution of extractives and
the degradation in various tissues.
Ray cells
are almost completely filled with extractives.
Extractives are also widely present axial
parenchyma
cells. Fiber cell walls are heavily
degraded. V, vessel; R, ray; F, fibers and
A, axial parenchyma. Confocal micrograph.
Bar : 50 1lm.
Fig. 3. Transverse
section of sound heartwood fiber
cells. Fiber lumens are variously filled with
extractives. Extractives appear to be present
in the walls of all tissues. Confocal micro-
gaph. Bar : 25 pm.
._1
tractives, but in some cells the lumina are only
partially filled. Extractives are also widely present
in the lumina of fibers (Fig. 3). The fibre walls
are impregnated with extractives, although their
distribution in the cell walls appears patchy
(Fig. 3). In vessels, extractives appear to be
mainly associated
with cell walls, including the
walls of tyloses where present.
In order to obtain the information on the
pattem of extractive microdistribution at cellular
and subcellular levels in relation to the extent
and pattem of degradation of various tissues,
sections obtained from heartwood regions varying
in the extent of degradation were observed
under TEM. In the present work, wti focused
mainly on sections taken through moderately
degraded regions, which provided useful com-
parisons of the differences among cell types
with regard to their resistance/susceptibility
to
degradation.
Fig. 3 shows that vessel and parenchyma
cells are intact, while the cell wall of fiber is
heavily degraded,
leaving only the middle lamella.
Several fungal hyphae are present in the lumina
of parenchyma cells completely filled with
extractives.
Middle lamella and primary pit-field
regions are very dense; the secondary wall is
less dense and also inhomogeneous, some parts
appearing denser than others. In vessel, the
warty layer appears to be heavily impregnated
with extractives. The warty layer has separated
from the secondary wall. The vessel wall also
appears to have a high concentration
of extractives,
and appears striated. Fig. 4 also shows largely
intact vessel and parenchyma
cell walls..However,
localized degradation
has occurred only in small
parts of the double cell wall, where fungal
hyphae are traversing this common wall between
adjoining parenchyma cells. Fungal hyphae are
also present in the extractive masses,
which fill
the lumina of parenchyma cells, with a preference
for the peripheral regions.
Fig. 5 shows parenchyma-fiber
and fiber-frber
entry by fungal hyphae, resulting from direct
72'
Micromorphological
and Chemical
Characteristics of Cengal
(Neobalanocarpus
heimii) Hearwood Decayed by Soft Rot Fungi
Fig. 4. Transverse
section through
a degraded region
of heartwood.
Fiber
(F) is heavily
degraded
but
parenchyma (P) and vessel
(V) walls
are
intact.
The lumens
of parenchyma
cells are
completely filled with extractives
(E), which
appear
as a dense, compact mass, and
contains
fungal hyphae
(FH). Vessel and parenchyma
cell walls appear to be impregnated with ex-
tractives, and the exhactive material is closely
applied
to parenchyma
cell walls, including
that of the primary
pitfield region
(asterisk).
The thin warty layer, which has detached
from the vessel wall, appears very dense
(arrowhead).
TEM micrograph.
penetration of the shared walls. Much of the
secondary walls in fibers is degraded, and the
coarse granular material which remains is most
likely lignin residue. The middle lamella is not
degraded, and so is the innermost part of the
secondary wall, which sunounds extractive-
filled lumina. Parenchyma
cell walls are largely
intact, although extractive masses have separated
from the wall, and thus no longer form a tight
seal with it, but this may be a fiacture artifact.
Advanced decay is shown in Fig. 6. The walls
of both fibers and parenchyma are heavily
degraded. In the pmenchyma cell, fungal hyphae
are present in the extractive masses which fill
the cell lumen, being localized mainly in the
pqripheral regions near the degraded cell wall.
Fig. 7 also shows an advanced stage of
Fig. 5. Transverse section through a degraded
region
of heartwood.
Vessel (V) wall is intact, and
parenchyma
cell walls show localized de-
gradation. Extractives completely fill the hr-
mens of parenchlnna
cells and are also present
in the walls of these cells and vessels. A
thin, dense warty layer present in the vessel
is closely applied to the secondary wall
(anowheads). Fungal hyphae (FFI) are present
throughout the extractive masses
(E) contained
in the lumens of parenchyma cells (P) and
also traverse the common walls between
these cells (asterisks).
TEM micrograph.
decay. The secondary walls of both fibers and
parenchyma are completely degraded, although
the lumina of parenchyma cells are completely
filled with extractives. Only the middle lamella
remains intact. Fungal hyphae are mainly
located in peripheral regions of the extractive
masses
present in cell lumina. Fig. 8 shows a
region containing heavily degraded fibers.
Secondary
walls have largely disappeared except
in cell comers, where the 51 layer remains.
However, in one of the cell corners even this
wall layer is no longer present,
and only middle
lamella remains intact. The circulm bodies which
are present throughout the degraded regions of
the hber secondary
wall are reminiscent of soft
rot cavities.
The information presented
here using a range
of chemical and microscopic techniques suggests
-73-
Yoon Soo Kim, Adya P. Singh, Ardrew H. H. Wong, Tae-Jin Eom, and Kwang Ho Lee
Fig. 6. Transverse
section through
a degraded region
of heartwood. The arrowheads
indicate direct
hyphal penetration
of common
walls between
a parenchyma
cell (P) and a fiber (F) and
between two fibers. Much of the secondary
wall in fibers is degraded; the remaining coarse
granular material (GM) is most probably
lignin residue. The middle lamella is not
degraded and so is the innermost part of the
secondary
wall around the extractive-filled
lumens
(paired
asterisk). Parenchyma cell walls
are largely intact, although fungal hyphae
(FH) colonize the extractive
masses
present
within the cell lumen. Asterisks indicate
cracks within the extractive masses. TEM
micrograph.
that a combination of factors, including high
content of extractives and lignin, lignin structwe,
and the pattem of extractive microdistribution,
may have contributed to the long service life of
cengal heartwood pole, where only surface
decay due to soft rot was detected. It is well
known that woods which contain high amounts
of extractives and lignin have high natural
durability (Singh et al., 1987; Nilsson e/ a/.,
1988, 1992; Schultz et al., 1995;
Taylor et al.,
2002). Considerable information is also available
on the effect of lignin concentration in various
regions
of wood cell walls and their resistance/
susceptibility to microbial decay
(Kim and Singh,
2000; Singh et al., 2003). However. much less
is known about the relationship between micro-
Fig. 7. Transverse section through
a degraded
region
of heartwood. Fungal hyphae (arrowheads)
are present within peripheral regions of the
extractive mass which fills the lumen of a
parenchyma cell (P). The secondary
walls of
both parenchyrna
and fibers (F) are heavily
degraded.
TEM micrograph.
Fig. 8. Transverse section through
a degraded region
of heartwood. Atthough parenchyma cell
lumens contain abundant
extraotives
(E), the
secondary
walls of these cells are heavily
degraded.
FH, fungal hyphae. TEM micro-
graph.
distribution of extractives in wood cells and
their biodegradation, and therefore the discussion
will be focused mainly on this aspect.
It is evident liom the microscopic observations
that fibers were least resistant to soft rot decay.
Fg
Micromorphological
and Chemical Characteri$ics
of Cengal
(Neobalanocarpus
heimil Heartwood Decayed by Soft Rot Fungi
Fig. 9. Transverse
section
through a degraded region
of heartwood
containing fibers. The secondary
walls of fibers are heavily degraded,
except
cell corner regions
of the Sr layer, which
appear intact
(anowheads).
However,
in one
cell
comer even this wall is absent
(asterisk).
Middle
lamella is not degraded.
TEM micro-
graph.
Bar - 2 p^ for Figs. 3-8.
Parenchyma cells showed remarkably high degree
of resistance
and vessels appeared to be most
resistant. Judging by the observed pattem of
extractive microdistribution,
it is clear that cellular
location of extractives had an important influence
on the decay characteristics of various cengal
heartwood tissues.
It is apparent that cengal hearfwood paren-
chyma cells, which were highly resistant
to soft
rot decay, contained abundant extractives. Al-
though fi.mgal hyphae were present in the
extractive masses
filling the cell lumina, fungal
growth is likely to have greatly slowed down
due to the physical blockage of the cell lumina
by the extractive masses and the fungistatic
effect of extractives. Phenolic type extractives
are among the most potent chemicals which
inhibit fungal growth (Aloui er al., 2004) and
the chemical analysis showed that they were
abundant in cengal heartwood.
Extractive impregnation of the cell walls of
parenchyma
and vessels, including the membranes
of primary pit-field regions and the warty layer
in vessels,
may also be an important factor re-
stricting cell wall degradation
and also intercellular
movement of fungal hyphae. Pit membranes are
the cell wall regions most susceptible to fungal
decay, enabling hyphae to readily grow and
multiply within wood tissues. However, in the
parenchyma
of cengal heartwood, the membrane
of pit-field regions appeared to be highly
resistant to invasion by fungal hyphae because
these cell wall regions were completely covered
with extractives and were also impregnated with
this material
The distribution of extractives in fiber lumina
was more variable as compared to parenchyma
cells. The lumina of the majority of fibers were
incompletely filled with extractives; only small
proportions were completely filled, containing
aompact masses of extractives. This would in
part explain the greater susceptibility of fibers
to soft rot as compared to parenchyma cells.
Also, fiber walls appeared to be less heavily
impregnated with extractives as compared to
parenchyma cells and vessels,
judging by their
higher natural color, and fluorescence
under the
confocal microscope. The type and amount of
lignin present in fiber walls may be another
factor, although this was not determined at the
cell wall level in the present work.
A discussion of the relationship of total ex-
tractive and lignin content of cengal heartwood
to the high durability of this wood in ground
contact is also relevant. The analytical data
(Table 1 and 2) indicate that cengal heartwood
is highly rich in extractives,
phenolic extractives
being the likely major component. Additionally,
the lignin content of cengal wood (Table 2)
appears to be among the highest recorded for
hardwood species
(Fengal and Wegener, 1984),
particularly among tropical hardwoods (Wong,
1993; Nilsson et al., 1988). Lignin structure of
cengal heartwood would also be an important
-15-
Yoon Soo Kim, Adya P. Singh, Andrew H. H. Wong, Tae-Jin Eom, and Kwang Ho Lee
factor in the high durability of this tropical
hardwood against various wood decaying micro-
organisms. It is well known that both the
amount and type of lignin and extractives in
wood are important factors in relation to bio-
degradation of wood (Eriksson et al., 7990),
and the work presented
here shows that cengal
heartwood is rich in both lignin and extractives
containing high proportions of the type of lignin
(condensed)
and exffactives (phenolic type) which
are known to be highly resistant to degradation
by wood decay organisms.
Natural durable wood species are characteris-
tically rich both in lignin and extractives, and
the work presented
here on cengal heartwood in
addition to confirming this provides additional
new information on the characteristics
of lignin
and extractives
present in cengal hearfwood and
the microdistribution of these components at
cellular level, which forms the basis for under-
standing why such timbers are naturally durable.
The presence of surface decay in cengal hear-
twood by soft rot after several years in ground
contact was not surprising. The cengal hear-
twood surface at the ground line would have
been susceptible to prolonged leaching of water-
soluble extractives which may be one reason for
the presence
of soft rot attack. However, suffi-
ciently high amounts of extractives would still
have remained in the wood, as the TEM obser-
vations indicate, restricting decay to surface
layers. Soft rot fungi can adapt to a wide range
of environmental conditions, high degree of
tolerance to copper compounds and other toxic
preservatives
(Daniel and Nilsson, 1998). Never-
theless, slow rate of degradation suggests
that
cengal heartwood have high resistance also to
soft rot for the reasons discussed
above.
In conclusion, the work using a range of
chemical and microscopic techniques presented
here provides evidence that in addition to the
amount and toxicity of heartwood extractives,
the pattern of their cellular distribution, both
within lumina and cell walls (Kuo and Arganbright,
1980; Streit and Fengel, 1994; Kleist and
Schmitt, 1999; Kleist and Bauch, 2001), is an
important factor in relation to microbial decay
of wood. A combination of factors, including
high extractive content, the presence
of propor-
tionally higher phenolic extractives, the micro-
distribution of extractives, the high lignin content,
and the presence
of substantial amount of con-
densed lignin, is likely to have contributed to
the observed high decay resistance of cengal
heartwood.
REFERENCES
1. Aloui, F., N. Ayadi, F. Charrier, and B. Charier.
2004. Durability of European oak (Quercus
petraea
and Quercus robur) against white rot fungi
(Coriolus versicolor): relations
with phenol extrac-
tives. Holz Roh- und Werkstoff 62: 286-290.
2. Chen, C.-L. 1992. Nitrobenzene and cupric oxide
oxidations, In: Lin, S. Y. and Dence, C. W.
(Eds) Methods in Lignin Chemistry. Springer-
Verlag, Berlin.
3. Daniel, G. and T. Nilsson. 1998. Developments
in the study of soft rot and bacterial decay. In:
Bruce, A and Palfreyman, J.W. (Eds.) Forest
Products Biotechnology. Taylor and Francis Ltd.,
London.
4. Eriksson, K.-E. L., R. A. Blanchette, and P.
Ander. 1990. Microbial and Enzymatic Degradation
of Wood and Wood Components. Springer-
Verlag, Berlin.
5. Fengel, D. and G. Wegener.
1984. Wood: Chemistry,
Ultrastructure, Reactions. Walter de Gruyter,
Berlin.
6. Kim, Y. S. and A. P. Singh. 2000. Micromor-
phological characteristics of wood biodegradation
in wet environments:
a review. IAWA Joumal
2l: 135
- 155.
7. Kleist, C. and U. Schmitt. 1999. Evidence of
accessory compounds
in vessel
walls of Sapelli
heartwood (Entrandrophragma obtained
by transmission electron
microscopy. Holz Roh-
/o -
Micromorphological
and Chemical Characteristics of Cengal
(Neobalanocarpus
heinii) Heutwood
Decayed by Soft Rot
Fungi
und Werkstoff
57: 93-95.
8. Kleist, G. and J. Bauch. 2001. Cellular UV
microspectrophotometric
investigation on Sapelli
heartwood
(Dttran&ophragma
cylindricwn Sprague)
from natural
provenznces
in Africa. Holdorschung
55: ll7 -122.
9. Kuo, M. and D. F. Arganbright.
1980. Cellular
distribution
of extractives
in redwood
and
incense
cedar.
Holzforschung 34: l7 -22.
10. Kuo, M. I., J. F. McClelland, S. Luo, P.-L.
Chien, R. D. Walker, and
C.-Y. Hse. 1988. Appli-
cations of infrared photoacoustic
spectroscopy
for wood samples. Wood Fiber Science 20: 132
- t45.
11. Mohd.
Dahlan
J. and M. K. Tam. 1985. Natural
durability
of some
Malaysian
timbers by stake
tests. Malaysian
Forester
48: 154-159.
12. Nilsson, T., J. R. Obst,
and
G. Daniel. 1988. The
possible
significance of the lignin content and
lignin type on the performance
of CCA+reated
timber in ground
contact.
Intemational Research
Group on Wood Preservation, Document IRG/
wP/1357.
13. Nilsson, T., A. P. Singh, and G. Daniel. 1992.
Ultrastructure of the attack of Eusiderorylon
eaageri
wood
by tunneling bacteria- Holzforschung
46:361-367.
14. Schultz, T. P., W. B. Harms, T. H. Fisher, K. D.
Mcmurtrey, J. Minn, and D. D. Nicholas.
1995.
Durability of an angiosperm heartwood:
the
importance of extractives.
Holzforschung 49: 29
-34.
15. Schulta T. P. and D. D. Nicholas. 2000. Naturally
durable heartwood:
evidence for a proposed
dual
defensive function of the extractives. Phyto-
chemisfy 54: 47
- 52.
16. Singh, A. P., T. Nilsson, and
G. Daniel. 1987.
Ultrastructure
of the attack
of wood of two high
lignin tropical hardwood species,
Alstonia sch-
olaris and Homalium
foetidum, by tunneling
bacteria. Journal of Institute of Wood Science
ll: 26-42.
17.
Singh,
A. P., Y. S. Kim, S. G. Wi, K. H. Lee,
and
L J. Kim. 2003.
Evidence
of the
degradation
of middle lamella
in a waterlogged
archaeological
wood.
Holzforschung
57: l15-119.
18. Smith, A. L., C. L. Campbell,
D. B. Walker, and
J. W. Hanover. 1989. Extracts
from black
locust
as wood preservatives:
extraction
of decay
resis-
tance from black locust heartwood.
Holdorschune
43:293-296.
19. Spun, A. R. 1969.
A low viscosity embedding
medium for electron microscopy. Joumal of
Ultrastructure
Research 26: 3l -43.
20. Streit,
W. and
D. Fengel. 1994. Hearnvood
forma-
tion in Quebracho
colorado: Tannin distribution
and
penetration
of extractives
into the cell walls.
Holzforschung 48: 361
-367.
21. Taylor, A. M., B. L. Gartner, and
J. J. Monell.
2002.
Heartwood formation and natural
durability-
a review. Wood and Fiber Science 34: 587
-
6l l.
22. Wong, A. H. H., 1993.
Susceptibility
to soft rot
in untreated and copper-chrome-arsenic treated
Malaysian
hardwoods. Ph. D. Thesis, Univ.
Oxford.
23. Yamamoto, K. and L. T. Hong. 1988. Decay
resistance
of extractives
from cengal (Neobal-
anocarpus heimii). Joumal of Tropical Forest
Science l: 5l
-55.
-77 -
