Holzforschung 2016; aop
Urszula Zajączkowska* and Paweł Kozakiewicz
Interaction between secondary phloem and xylem
in gravitropic reaction of lateral branches of Tilia
cordata Mill. trees
DOI 10.1515/hf-2015-0230
Received October 25, 2015; accepted February 17, 2016; previously
published online xx
Abstract: The tension wood (TW) of Tilia cordata (lime tree)
does not contain gelatinous fibers. Based on anatomical
studies of secondary phloem (secPhl) and xylem by means
of microscopy, digital imaging, and biomechanical tests,
it was hypothesized that there is an interaction between
the phloem and xylem as a response of gravitropic forces
on lateral branches. The goal of the present study was
to check this hypothesis. The results demonstrated that
dilated phloem rays are longer and wider on the upper
side (US) of a branch compared to the lower side (LS) and
that the ratio of fiber/ray parenchyma in the phloem is
lower on the US of the branches. Bark strips consisting of
secPhl with periderm have higher elastic modulus (MOE)
on the US of branches. The results support the hypothesis
that the compression stress of ray parenchyma may cause
phloem fibers to stretch, which may result in the development of axial tensile stresses that are higher on the US of
branches. However, the wider rings of xylem formed on
the US of branches and the results of biomechanical tests
can be interpreted that a higher MOE of wood in the US of
lateral branch are the main factors responsible for gravitropic reaction of Tilia branches. TW can be considered as
a kind of biotensegrity.
Keywords: biotensegrity, dilating rays, eccentric growth,
gravitropic response, phloem fibers, ray parenchyma,
reaction wood, secondary phloem, tension wood, Tilia
cordata
*Corresponding author: Urszula Zajączkowska, Department
of Forest Botany, Warsaw University of Life Sciences, 159
Nowoursynowska Street, 02-776 Warsaw, Poland,
e-mail: [email protected]
Paweł Kozakiewicz: Division of Wood Science and Wood Protection,
Warsaw University of Life Sciences, 02-776 Warsaw, Poland
Introduction
The gravitropic adaptation of trees occurs via formation
of compression wood (CW) on the lower side (LS) and
tension wood (TW) on the upper side (US) of branches of
gymnosperms and angiosperms, respectively. These processes go along with special morphological and chemical changes in the cell walls (Wilson and Archer 1977;
Timell 1986; Altaner et al. 2007; Lehringer et al. 2008;
Nanayakkara et al. 2014; Sharma and Altaner 2014; Shirai
et al. 2016). The gelatinous layer (G-layer) in fibers of TW
is responsible for developing tensile stresses involved in
counteracting against gravity. Nevertheless, about half of
angiosperm tree species do not form xylem fibers containing a G-layer (Onaka 1949; Clair et al. 2006) and are still
able to develop tensile stresses enabling organ reorientation, respectively keeping branches in the proper position.
Higher stresses occur in the wider portion of the asymmetric annual rings of wood. However, tensile stresses
are also developed in normal wood during maturation
of cambial xylem derivatives (Clair et al. 2011) leading to
‘growth stresses’ in stems and branches. The most remarkable tree without formation of G-layer in TW fibers is Tilia
cordata Mill. (lime tree). Böhlmann (1971) proposed that
in Tilia species, the most important biomechanical reactions are produced by the secondary phloem (secPhl) with
its axial system consisting of layers of sieve tubes with
companion cells, axial parenchymal cells, and sclerenchymatous fibers, and the radial system, which is composed of ray phloem parenchyma cells. On average, the
annual growth increment of the secPhl in lime shoots is
0.5 mm, and its sieve tubes are considered to be functional
for up to 10 years (Holdheide 1951). Non-functional axial
phloem parenchyma undergoes sclerification (Esau 1939).
According to Böhlmann (1971), the main biomechanical
factor responsible for the properties of secPhl in lime tree
is its anatomical structure. Its axial system is interspersed
with zones of the radial elements consisting primarily of
axial sclerenchymatous thick-walled fibers and highly
hydrated thin-walled cells of the parenchyma of dilating
rays. The dilation of the secPhl rays occurs only in some
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2 U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata
rays that exhibit an increased frequency of anticlinal divisions (Esau et al. 1953). This process takes place during
the entire life of a tree; as a result, the rays adopt a typical
cone geometry, with the base at the peripheral zone of
the trunk and the apex in the cambial radial initial cell.
Böhlmann (1971) suggested that ray dilation generates
tensile stresses along the lateral branch and trunk that are
a direct result of fiber stretching due to anticlinal divisions
and growth in the radial parenchyma. Phloem dilation in
Tilia trees seems to be rapid and effective and needs low
energy expenditure. Ray dilation in the secPhl has been
observed in the stems of some tree species (Schneider
1955; Şen et al. 2011), lignified vines (Davis and Evert 1970;
Stevenson et al. 2005) as well as in the roots of woody
plants (Machado et al. 2005; Golwala and Patel 2009) and
cacti (Mauseth 1999; Niklas et al. 2002). The phloem dilation was also found in Sphenophyllum, a Devonian fossil
plant (Eggert and Gaunt 1973). The increased dilation of
the radial parenchyma may also be a response to specific
environmental stress conditions (Ying-Shan et al. 2005)
following the period of plant etiolation (Maynard and
Bassuk 1996) or of a microinjury caused by an aphid biting
into the plant’s tissue (Evert et al. 1968). Apart from Böhlmann’s (1971) paper, the dilation of the rays of T. cordata
has never been a subject of more detailed studies, though
the subject is interesting both from the viewpoint of tissue
differentiation processes and organ biomechanics, on the
one hand, and an interaction with environmental conditions, on the other hand.
This paper attempts to give a more detailed description of the structure of dilating rays and axial phloem with
respect to their possible interaction as part of the gravitropic response of lateral branches of T. cordata trees.
Anatomical studies of the phloem will be accompanied by
biomechanical tests of the longitudinal strips of secPhl with
periderm and supplemented by the microscopic analyses
and the measurement of physical and mechanical properties of wood. The expectation is that the applied methods
will provide new information, inspire new questions, and
reveal new aspects of the possible interaction between the
secPhl and xylem in gravitropic reaction of the branches.
fragments with a length of 20 cm each and discs with a width of 2 cm
were cut out. The transverse surfaces of discs were scanned, and the
boundaries of annual growth rings were marked at intervals of five
years. The shape of these ring boundaries was analyzed by the Fiji
software (Schindelin et al. 2012) to determine the circularity parameter Circ = 4π∗Area∗Perimeter∗m-2 and the ratio between the longest
and shortest diameter (aspect ratio, AR). Material for a microscopic
analysis of the secondary phloem (secPhl) and xylem was obtained
from the upper side (UP) and lower side (LS) of the cross-section
of lateral branch discs. Microscopic sections were obtained with a
sliding microtome (Reichert, Vienna, Austria), while cuts in three
directions were performed (transverse, radial, and tangential). The
sections were stained with phloroglucinol-HCl (Sigma-Aldrich) for
the presence of lignin or with safranine-astra blue (Sigma-Aldrich).
Microscopes: Olympus binocular and an Olympus BX-61 (Tokyo,
Japan) optical microscope under a bright field of transmitted light
and UV radiation. Structure of the xylem tissue was observed also by
scanning electron microscopy (FEI QUANTA 200, FEI Co., Hillsboro,
USA) at 25 kV. The following measurements of the phloem were performed with cell P software: (1) width of the secondary xylem (secXyl)
zone on a cross-section, (2) surface area of the phloem ray parenchyma zone, and (3) area of the axial phloem (sclerenchymatous
fibers, sieve tubes, companion cells, and axial parenchyma in total).
The WinCELL (Instruments Regent Inc., Ville de Quebec, Canada)
image analysis system served for measuring the lumen surface area
of the tracheary elements in the annual rings of the secXyl.
Material for biomechanical studies was collected from the lateral branches of 15 adult trees of T. cordata trees grown in the same
area. A single healthy branch was collected from each tree. The age
of each branch at its base at the trunk ranged from 17 to 44 years,
and its diameter was within the limits between 60 and 130 mm.
Three adjacent segments were collected; each segment was approximately 350 mm in length (beginning at ca. 500 mm from the base of
a branch) and did not have any apparent material flaws (Figure 1a). A
cross-section disc with a thickness of 20 mm was obtained from the
wider end of each segment to allow for the assessment of the following parameters:
–
average width of annual rings;
–
flattening: F = ⎡⎣(d max -d min )∗d max-1 ⎤⎦∗100%, where dmax and dmin:
maximal and minimal diameter, respectively;
–
eccentricity: E = ⎡⎣e/(d max ∗2-1 )⎤⎦∗100%, where e: distance of the
pith from the center of the cross-section disc ⎡⎣e = (d max ∗2-1 )-d min ⎦⎤ ;
Materials and methods
Branch no.
Five lateral branches of the small-leaved lime tree (Tilia cordata
Mill.) were collected from five trees aged 40–60 years old grown in
the Młynary Forest District and at the Arboretum of the Warsaw University of Life Sciences in Rogów, Poland. The age of the branches
(determined at the base of the branch), length of the tested branch
segments, and diameter at the base and top of the segments are
presented in Table 1. Branch segments were cut transversely into
Table 1: General characteristics of the examined small-leaved
lateral branches used for anatomical studies: the age of each
branch was determined at the base of the examined segment.
1
2
3
4
5
Age (years)
Length of segment (cm)
Base-top diameter
(cm)
12 14 35 37 24 60 60 140 140 100 5.5–3.2
6.2–3.4
17.2–5.0
16.5–5.2
13.2–4.0
Branches 1 and 2 were collected in the Młynary Forest District
and branches 3–5 were collected at the Arboretum of the Warsaw
University of Life Sciences in Rogów.
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U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata 3
Results and discussion
Anatomical studies on wood
Figure 1: Scheme of samples collection for physical and mechanical
properties of lateral branches of T. cordata.
(a) Three successive fragments (A, B, C) of lateral branches;
(b) schematic view on specimens for mechanical tests of wood and
secondary phloem with periderm.
–
bark width ratio: B = bUS ∗bLS-1 , where b is for bark collected from
the US or LS of the branch, respectively.
Subsequently, a heart board with a length along the grain of 300 mm
and a thickness of 20 mm and width similar to the vertical diameter
of the branch was collected from each segment according to the
scheme shown in Figure 1b. Two strips of bark (secPhl with periderm)
300 mm in length and 20 mm in width were also collected from each
segment.
Wood density (according to ISO 13061-2: 2014) and the elastic
modulus (MOE) during bending (according to ISO 13061-4: 2014) of
the heart boards were then determined. A constant 5 mm central
displacement of the bent beam was used. Following the assessment of the heart board MOE, two test pieces were cut out from each
board (one from the US and one from the LS the branch) having
square cross-section with a side 20 mm and a length along the grain
300 mm. The density and MOE during bending of the fresh wood
pieces were also measured (MC > fiber saturation point). Following
the mechanical tests, shrinkage was assessed (ISO 4858:1982) on
45 wood specimens with a square cross-section (20 × 2 0 mm2) and
with 60 mm length along the grain. Oar-shaped specimens were
prepared from the strips of secPhl with periderm (Figure 1b). The
specimens were stretched along the fibers for tensile MOE measurements (the specimens were similar in shape to those stipulated by
the ISO 13061-6:2014 standard).
The cross-section of each of the tested lateral branches
shows an eccentric pith located below the branch center
due to increased cambial activity expressed on the upper
side (US) in the secondary xylem (secXyl) layer (Figure 2a).
The secondary phloem (secPhl) zone was twice as wide on
the US as on the lower side (LS). Image analysis of transverse sections shows that the eccentric diameter growth
of branches is expressed by a reduced circularity of the
successive annual rings constituting wood boundary lines
measured in five-year intervals (Figure 3). This is also
emphasized by increased aspect ratio (AR) values: the
vertical diameter increases faster the than the horizontal diameter in the cross-sections. Remarkably, in all six
tested branches, the circularity of the external border of
the secPhl is slightly greater than that of the border of the
last growth ring of the xylem.
Microscopic structure of wood from the US and LS is
presented in Figure 4a, b. The diffuse-porous wood in lime
tree contains various types of tracheary elements such as
vessels, tracheids, and fibers that differ with respect to
their lumen. All of the elements are thin-walled. No cells
with gelatinous wall layers (G-layer) are visible in the
annual rings, neither on the US nor on the LS. The measured cell lumens for all types of tracheary elements are
presented in Figure 4c. A greater number of elements of
a
b
c
Figure 2: Lateral branch of T. cordata.
(a) Transverse section with eccentric pith; (b) Microscopic image of
secondary phloem with dilated (dilPhl-ray) and non-dilated phloem
rays (Phl-ray); Xyl xylem, Per periderm. (c) Transverse surface of
secPhl with bulges (black arrows) released by dilated ray parenchyma (dilRayP), when the phloem layer was cut off; Scale bar in:
a = 10 mm, b = 2.5 mm, c = 3.0 mm.
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4 U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata
2.000
1.100
Circularity
Aspect ratio
1.439
Circularity
1.150
1.500
1.398
1.183
1.000
Aspect ratio
1.421
1.342
1.000
0.900
0.500
0.800
6
11
16
21
Age (years)
24
Phloem
0
Figure 3: Changes in the circularity and aspect ratio of the cambium
with increasing age of a lateral branch.
the lumen with a surface area of <500 µm2 (mostly xylem
fibers) were observed on the US (Figure 4d), while a
greater number of larger tracheary elements (lumen area
>500 µm2, mostly tracheids and vessels) were found on
the LS (Figure 4e). The share of structures with a lumen
a
c
surface area of < 500 µm2 was about 10 times higher than
those with a greater surface area.
The increased cambial productivity of xylem on the US
is commonly observed in the tree species forming typical
tension wood (TW) with G-fibers, which is responsible for
the generation of tensile stress along the longitudinal axis
(Onaka 1949; Okuyama et al. 1994; Nishikubo et al. 2007).
The eccentric pith and wider zone of the secXyl on the US,
which occurs in tree species without formation of TW fibers
with a G-layer, and this can be considered as a specific type
of reaction of wood triggered by biomechanical conditions
such as gravity and wind. In the case of eccentric annual
rings and the wider rings on the US of branches, one has
to take into consideration the unequal distribution of axial
tensile stresses on both sides of the branches arising from
the maturation process in the course of normal wood formation (Almeras et al. 2005; Clair et al. 2011).
Anatomical studies on phloem
The average width of the secPhl zone was about 5 mm on
the US and about 3 mm on the LS (Figure 5a). On average,
d
e
b
Figure 4: Transverse sections of wood from the US (a) and LS (b) of lateral branches seen under SEM.
(c) Image of eccentric annual rings of wood seen on cross-sections under optical microscope. (d) Frequency of tracheary elements seen on
(c) of the lumen surface area < 500 µm2 and (e) with the surface area > 500 µm2. Scale bars = 100 µm apply to a and b; scale bar in c = 1 mm.
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U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata 5
Figure 5: Comparison of the secPhl structure on the US and LS of
lateral branches.
(a) The secPhl width and the share of axial phloem and phloem ray
parenchyma on the US and LS along the length of the lateral branch.
The results are given for the five successive (a–e) transverse sections
from the top (a) to the base (e) of the segments. Schemes of transverse (b, d) and tangential (c, e) sections of secPhl on the US (b, c)
and LS (d, e) of lateral branches. Scale bar = 2.5 mm applies to b and e.
the width of the Phl zone on the LS amounted to 59%
(SE = 5%) of the width of the Phl on the US of the branches.
This difference was smaller in younger branches (no. 1 and
2), for which the parameter amounted to 74% (SE = 5%).
The maximal width of the secPhl in individual branches
was noted in the oldest branch (no. 5) and amounted to
135 mm and 71 mm on the US and LS, respectively. The
different growth rates of the secPhl between the US and
LS of the branches were also reflected in anatomical differences in the tissue in these areas. Microscopic measurements of the transverse sections of secPhl tissues revealed
a varying percentage of the share of the surface area of
the axial phloem (consisting mostly of fibers) compared to
the share of the secPhl rays (Figure 5a). On the US of the
branches, the fiber share was smaller than on the LS with
ca. 50% (SE = 8%) and 63% (SE = 6%), respectively. In the
case of the older branches (no. 3 and 4), the share of fibers
with respect to rays was higher on the US and smaller on
the LS. In all cases, the dilating rays on the US had greater
tangential dimensions than the dilating rays on the LS
(Figure 5b–e).
Microscopic image of the secPhl with dilated and nondilated rays is shown on Figure 2b. Due to high hydration
level, the axial compression is probably generated in the
phloem ray parenchyma and released as bulges on transverse surfaces, when a phloem layer is cut off (Figure 2c).
Phloem rays may begin dilating after they have
been functional for different periods of time. It was also
observed that phloem rays are dilating immediately after
the formation of a ray cell from cambial ray, as well as in
cases, where the dilation started more than 10 years after
the secPhl was formed. The observation of the secPhl on
the cross-sections in a zone, where dilation had not yet
occurred, namely proximal to the cambium, revealed
tangential rows of orderly arranged parenchyma cells
between the axial parenchyma cells on both sides of the
ray (Figure 6a). A specific disturbance in the order of
the arranged parenchyma cells occurred in areas, where
intense anticlinal divisions began inside the ray and the
atrophy of the ordered parenchyma cells was observed
(Figure 6b). In addition, in many cases, the dilation
began in rays, in which parenchyma cells began to grow
intensely in a tangential manner (Figure 6c). Cells of this
type were not observed in rays without dilatation. Intense
growth and anticlinal division of parenchyma cells in the
dilated rays often cause the bundles of phloem ray sclerenchymatous fibers to divide into smaller fragments in the
peripheral regions near the periderm (Figure 6c). Observations of cell size in the dilating ray parenchyma in the
cross-section indicate a small difference between cells in
the central and peripheral part of the ray. The central zone
seems to show not only an intense cell division dynamics
but also intensive cellular growth. Closely packed phloem
ray parenchyma cells seen in radial plane are in regions
closer to the cambium. Two types of cells can be distinguished: radially elongated ray cells, which are in contact
with sclerenchymatous fibers and isodiametric ray cells
that are in contact with remaining axial phloem elements
(sieve tubes, companion cells, axial parenchyma). Interestingly, in the regions closer to the periderm, the loosely
arranged ellipsoid cells in configurations similar to a nondirectionally proliferating callus were frequently observed
in the radial plane (Figure 6e), whereas tightly packed
cells could be observed in the cross-sections.
Microscopic analysis of tangential sections stained by
phloroglucinol-HCl (specific for lignin) revealed that the
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6 U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata
a
b
e
c
d
f
Figure 6: Microscopic images of secPhl; transverse (a, b, c), tangential (d), and radial (e, f) sections.
(a) The region of non-dilated ray; axial parenchyma cells (axP) on both sides of the ray are linked by series of orderly arranged ray parenchyma cells (rayP); sieve tubes (ST). (b) Disturbance of the order of rayP (arrows) between the axP cells on both sides of the dilRay; the rayP
in tangential rows (dotted lines). (c) Separation of axPhl fiber bundles into smaller fragments (arrows) by rayP cells near the periderm (d)
RayP (arrows) with lignified walls in the Xyl zone and with unlignified walls in the Phl. (e) Closely packed Phl rayP cells; radially elongated
ray cells (yellow arrow) are in contact with sclerenchymatous fibers (f) and isodiametric ray cells (white arrow) are in contact with remaining
axPhl elements. (f) Loosely arranged ellipsoidal Phl rayP in the dilRay near the periderm (Per); f – fibers. Sections stained with phloroglucinol-HCl (a–d) and with safranine-astra blue. Scale bars in a, b,c, e, f = 100 µm; scale bar in d = 200 µm.
walls of ray parenchyma cells are lignified in the area of
the secXyl layer but not in the phloem (Figure 6d). Observations of the ray parenchyma in the secPhl demonstrated
that cells inside this tissue are highly vacuolated; specifically, they have a high turgor pressure. The zones of lignified cellular walls of rays in the xylem and completely
unlignified walls in the phloem gradually merge with each
other in the cambial region. This specific dimorphism is
especially remarkable considering the fact that lignin is a
hydrophobic compound. Bearing in mind the high levels
of hydration and anticlinal divisions, axial compression
is likely generated in the phloem ray parenchyma and
released as bulges on transverse surfaces when a phloem
layer is cut off (Figure 2c).
The present results seem to support Böhlmann’s (1971)
hypothesis that phloem ray dilation generates tensile
stresses along lateral branches in T. cordata. This can
be induced through the tangential growth and anticlinal
divisions of the phloem ray parenchyma, namely through
tangential stress in a network of sclerenchymatous fiber
bundles. The dilating ray parenchyma sometimes shows
highly dynamic divisions, and the ray space is filled with
parenchyma cells with thin walls, high vacuolization,
and high axial compression. In the proximal zone of the
cambium and within a wider layer of sclerenchymatous
fibers, the phloem ray cells have a high turgor pressure,
which decreases after disruption of tissue. Therefore,
cutting the phloem sample transversely induced the
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U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata 7
compression of dilating rays and, as a result, specific
bulges were formed, primarily in the phloem ray region
between the sclerenchymatous fibers. This is considered
to be an example of interaction within a complex structure composed of two main elements with different biomechanical properties, but with supplementary activities.
Shortly, TW is an example for biotensegrity, where the
principles of tensional integrity (or floating compression)
are followed based on the interaction of isolated components within an equilibrated network of compression and
tension forces (Ingber 2003a,b; Kasprowicz et al. 2011).
Biomechanical tests
The data are compiled in Table 2. The branches show a
varying flattening (a more or less clear elongation along
the vertical axis in the cross-section), a widening of the
wood and phloem zone above the pith, and, as a result,
increased eccentricity. Flattening ranged from 0 to over
36% and the mean value for all specimens equaled 10%.
The coefficient of variation (CV) for this characteristic
was exceptionally high, amounting to over 80% and it
was higher than the CV for the eccentricity (45%). A clear
eccentric position of the pith in the cross-section, even in
branches with low flattening indicates an asymmetry of
wood rings in the US and LSs of the branches. The mean
width of the wood rings on the LS amounts to 1.1 mm, and
the corresponding value on US is 2.2 mm. A direct proportional linear relationship between pith eccentricity and
the bark width ratio was observed (Figure 7).
The biomechanical measurements of the fresh bark
strips containing phloem with periderm were hampered
because of the heterogeneity, roughness of the strips
obtained from lateral branches with curvature. The bark
MOE show that the mean values collected from the US
are about 9% higher than the corresponding values from
the LS (Table 2). This parameter, however, has a high
variability on both sides of the branches. The differences
between bark densities on both sides of the branches did
not exceed 3% and relatively high variability of MOE data
is obvious.
The density of fresh wood was ca. 640 kg m-3. The MOE of
heart boards of the fresh wood was 2117 MPa (CV = 38%). The
MOE was measured also for the small test pieces of samples
20 × 20 mm with a length of 300 mm and assessed in pairs:
one from the US and the other from the LS. The value of MOE
of the small test pieces is about 15% higher in the specimens
from the US compared to the LS (Table 2). A significant difference was found between the shrinkage along fibers of
Table 2: Characteristics of T. cordata lateral branches and mechanical and physical parameters of their bark (secondary phloem with periderm) and wood measured from the fresh samples collected from upper side (US) and lower side (LS) of lateral branches.
Parameters
Width of annual ring (mm) Flattening, F (%)
Eccentricity, E (%)
Bark thickn. ratio
Bark dens. (kg m-3)
Bark MOE (MPa)
Wood dens. (kg m-3)
Wood MOE (MPa)
Shrinkage of wood:
Longitudinal (%)
Radial (%)
Tangential (%)
Volumetric (%)
US/LS
US
LS
Statistical values
Min
Mean
Max
SE
3.99
1.79
36.6
74.5
3.43
1080
1180
4305
4115
729
3180
3034
0.08
0.05
1.23
2.34
0.08
11.9
15.2
127
126
9
78
63
US
LS
1.33
0.49
0.0
7.4
1.06
780
720
1019
832
508
1232
1051
US
LS
US
LS
US
LS
US
LS
0.18
0.07
4.47
4.08
6.68
7.03
10.87
11.25
0.35
0.19
5.63
5.75
8.74
8.75
14.17
14.17
US
LS
US
LS
2.22
1.07
10.0
35.2
1.86
935
904
2570
2353
641
2011
1712
Mean values of 45 samples from 15 branches; SE, standard error.
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0.79
0.43
7.35
7.32
10.98
10.71
18.32
17.05
0.022
0.012
0.13
0.15
0.18
0.14
0.25
0.22
8 U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata
wood from the US and LS, namely 0.35 and 0.19%, respectively. The values of shrinkage in radial and tangential directions as well as in volume were similar for the wood samples
taken from the two opposite sides of a branch.
The results of biomechanical tests of the strips of
secPhl with periderm, which revealed slightly higher
MOE along the fibers of bark strips from the US seems
to support the results of anatomical observations, which
indicated that secPhl may be an important factor in the
gravitropic response of T. cordata. Nagawa et al. (2012)
reported that phloem fibers formed on the tensional side
of inclined stems of some Japanese hardwoods show
three types of changes in lignin distribution and cell-wall
structures and that the gravitropic reaction is not closely
related to G-fibers in TW. The involvement of the extraxyllary tissues in the gravitropic response has already been
reported by Tomlinson (2001, 2003) for Gnetum, a gymnosperm species that produces gelatinous fibers with a function analogous to those in angiosperm woods.
The results here can be interpreted that the gravitropic
response of T. cordata tree may involve tensional stresses
that develop not only in the layers of the secPhl, but in the
xylem as well. In view of the fact that tensional stresses
also develop during the differentiation of normal wood, it
may be expected that higher tensions should occur in the
eccentric rings on the US of the branches where the rings
are wider. Moreover, the observed differences between the
lumen area of tracheary elements are indicative for the following: there are more xylem fibers (tracheary elements
of smaller lumen area) and less vessels (the elements of
greater lumen area) in the tissues of US compared to those
in the LS. In TW of annual rings of wood stems the relation between xylem fibers and vessels are similar, thus it
seems probable that the secXyl is also involved in the gravitropic response of T. cordata. This hypothesis is also supported by the shrinkage experiments, where the US wood
has a significantly higher shrinkage along fibers than that
from LS. It was also reported recently by Roussel and Clair
(2015) that in Simarouba the G-layer in the TW fibers can
be seen only as a temporary stage of the xylem cell-wall
development. The G-layer is then masked by a late lignification and thin lignified TW fibers cannot be distinguished from normal wood fibers in the mature wood.
Conclusions
Results revealed that the secPhl on the upper side (US)
of T. cordata lateral branches have slightly higher MOE
than that on the LS. Anatomical studies indicate that ray
parenchyma in the wider and longer dilating phloem rays
on the US can cause stronger stretching of phloem fibers
as compared to that on the LS. It is suggested that the
asymmetries in the secPhl may result in development of
axial tensile stresses that are higher on the US of a branch.
In addition to the possible effects of extraxyllary tissues,
the secondary xylem with thin-walled tracheary elements
without gelatinous fibers in TW, has significant impact
on the gravitropic response of T. cordata. The eccentric
shape of a branch cross-section with wider annual rings
is accompanied by higher axial shrinkage and MOE in the
US of a branch is the most important factor responsible for
gravitropic reaction in this tree species.
Acknowledgments: The author wishes to thank Dr. Ignacio
Arganda-Carreras of the Department of Brain and Cognitive Sciences at the Massachusetts Institute of Technology
for his helpful suggestions concerning the selection and
application of computer programs for image analysis. The
generous assistance on the part of Mr. Piotr Banaszczak,
MSc, Head of the Arboretum of the Warsaw University
of Life Sciences in Rogów, and Mr. Kamil Oskroba, MSc,
Młynary Forest District, in collecting the plant material is
also acknowledged. We’re extremely grateful to Professor
Oscar Faix for his constructive criticism and help provided
during development of the original manuscript.
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Figure 7: Plot of pith eccentricity versus bark thickness ratio as
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List of standards
ISO 13061-2:2014 Physical and mechanical properties of wood
– Test methods for small clear wood specimens – Part 2: Determination of density for physical and mechanical tests.
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10 U. Zajączkowska and P. Kozakiewicz: Features of secPhl and xylem in branches of T. cordata
ISO 13061-4:2014 Physical and mechanical properties of
wood – Test methods for small clear wood specimens –
Part 4: Determination of modulus of elasticity in static
bending.
ISO 13061-6:2014 Physical and mechanical properties of wood –
Test methods for small clear wood specimens – Part 6:
Determination of ultimate tensile stress parallel
to grain.
ISO 4858:1982 Wood – Determination of volumetric shrinkage.
(ISO/DIS 13061-14 Physical and mechanical properties of wood
– Test methods for small clear wood specimens – Part 14:
Determination of volumetric shrinkage).
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