WOOD RESEARCH
60 (2): 2015
175-188
THE EFFECT OF DROUGHT ON CELL WALL THICKNESS
AND RADIAL DIMENSION OF TRACHEIDS OF PICEA
ABIES (L.) KARST.
Jakub Hacura, Vladimír Gryc, Hanuš Vavrčík, Jana Hozová
Mendel University in Brno, Faculty of Forestry and Wood Technology
Department of Wood Science
Brno, Czech Republic
Josef Urban
Mendel University in Brno, Faculty of Forestry and Wood Technology, Institute
of Forest Botany, Dendrology and Geobiocoenology
Brno, Czech Republic
(Received June 2014)
ABSTRACT
The presented paper deals with morphological parameters of tracheids of Norway spruce
(Picea abies (L.) Karst.) exposed to long-term drought stress. Microcores were sampled in
Hoxmark, Norway, in 2-week intervals during the growing season of 2010. The microcores
were used to analyse morphological parameters of tracheids – their radial diameter and the cell
wall thickness. These morphological parameters were then correlated with meteorological data.
We used two families of Norway spruce for the experiment. One tree from each family was
located in the control plot, the other one in experimental conditions – drought stress was induced
artificially for the entire growing season. The hypothesis about the negative effect of drought on
morphological parameters of tracheids was not confirmed in this case – no statistically significant
differences were found between the stressed and the control plot trees. The experiment results
indicate that drought affects the resulting tree-ring width but not morphological parameters of
tracheids.
KEYWORDS: Drought stress, tracheid dimension, cambium, Norway spruce.
INTRODUCTION
Wood of Norway spruce consists of two anatomical elements predominantly: Tracheids
and parenchyma cells. The proportion of tracheids in softwood is 87–95 % (Panshin and de
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Zeeuw 1980). Dimensions of tracheids, the thickness of cell walls and their number depend on
their position within a tree ring, the age of the tree, the position within the stem and climatic
conditions of the time of their differentiation. Morphological parameters depend on the speed
of the tracheid differentiation (Horáček 2003). Schweingruber (1990) said that the effects of the
seasonal climate are reflected in the anatomical structure of tree rings, which are at the same time
affected by growth factors. The mean radial size of earlywood and latewood tracheids in Norway
spruce ranges within 40–60 μm and 10–15 μm, respectively; the radial cell wall thickness is stated
to be within 2–3 μm in earlywood tracheids and 2–6 μm in latewood tracheids (Fengel and Stoll
1973, Panshin and de Zeeuw 1980, Horáček 2003, Vavrčík et al., 2006, Gryc and Vavrčík 2006,
Rybníček et al. 2007).
Growth of woody plants consists of two processes, the axial growth and the radial growth
(Wodzicki 1971). These processes are limited to specific periods in the year, based on endogenous
and exogenous factors (Savidge 1996). External factors affect the beginning, progress and end of
the growth of individuals (Wodzicki 2001). The effect of specific climatic factors on the wood
and phloem growth mechanism can be observed in trees growing in their natural environment
or in a control conditions (Antonova and Stasova 1997). Xylem and phloem cell are formed
during periclinal division of cambium followed by the differentiation process (Gričar 2007).
Differentiation leads to cell specialization and involves the phase of postcambial growth, the
phase of secondary cell wall synthesis and lignification and phase of mature cells (Wilson et al.
1966, Panshin and de Zeeuw 1980, Larson 1994, Gričar 2007).
The environment affects a forest ecosystem and thus also the morphological parameters of
tracheids of trees living in this ecosystem. At present, nearly all fields of research are adapting
to the climate change because the future climate change could reinforce damage by drought,
fire, storm, and insect calamities (Michalak 2011). This is also proved by the assertion that
the projected climate change implies not only a change in mean climate parameters, such as
temperature or precipitation, but also that such a change will result in changes in the frequency
and magnitude of extreme weather events (Waggoner 1989, Brabson and Palutikof 2002,
Kjellström 2004). Forests play an essential role in mitigating a climate change and providing
products and ecosystem services that are essential to the prosperity of humankind (State of the
World's Forests 2012). Steppe et al. (2006) stated that changes in water status are necessarily
reflected in the characteristics of xylem and phloem and in the resulting radial growth of the
stem. Drought can be defined in a strict sense, as a situation where a deficiency of precipitation
over some period of time results in a water shortage for some activity, group, or environmental
sector (Trenberth et al. 2007). Several previous investigations have explored the effects of drought
on tree growth or tracheid dimension in natural and controlled environments (Eilmann et al.
2009, Rossi et al. 2009, Swidrak et al. 2011, Eldhuset et al. 2013). Allen et al. (2010) said that
occurrences of intense drought and elevated temperature have been associated with increased
mortality of trees in many regions of the world. Despite the latest knowledge on the cellular,
molecular, and developmental processes underlying wood formation, the recent literature still
strongly recommends more experimental studies to assess how the secondary meristem copes with
extreme drought events (Jentsch et al. 2007, Hartmann 2010).
Norway spruce is widely spread softwood species in Europe. It frequently used to be planted
outside its ecological optimum in the past. For this reason we can expect troubles with its growth
in the future. We are already experiencing troubles – windthrows, bark beetle attacks, soil
degradation and that is why this study focuses on this species. The following research questions
were formulated: 1. Can long-term drought stress affect cambium activity, wood formation and
also morphological parameters of cells? 2. Can temperature and soil water potential affect the
wood formation process?
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MATERIAL AND METHODS
Study site
The study site is located in Hoxmark (Norway, N 59° 40', E 10° 43', 90 m a.s l.). The stand
was planted in former agricultural land in 1990 and its mean height now (in 2010) is 9 m. The
long term values of climatic conditions of Ås (the closest weather station, 2 kilometres from
Hoxmark) were gained by Hansen and Grimenes (2011). The mean annual precipitation there
is 785 mm and the mean annual temperature is 5.3°C (Fig. 1). Actual meteorological conditions
were measured by the meteorological station installed at the open grass area directly at the study
site. Global radiation, air temperature and humidity, wind speed and precipitation were measured
by EMS 11 radiation and EMS 33 temperature and humidity sensors (EMS Brno, Czech
Republic), Met One wind speed sensor and Met One precipitation gauge (MetOne Instruments,
OR, USA), respectively. As this study focuses on the drought effect on morphological parameters
of tracheids, we needed to create artificial conditions to induce the drought stress. In May 2009,
a plastic roof was put up above the stand in the experimental plot. Further, 30 cm deep ditch was
dug to prevent horizontal penetration of water to the soil (Gebauer et al. 2011).
Fig. 1: Monthly mean air temperature and sums of precipitation for individual month during the year
2010 in comparison with long term average.
Two families (referred to as 15 and 18) of Norway spruce were selected for the experiment
(Gebauer et al. 2011, 2012 for more information on the families). Two sample trees of each of
the families were chosen with approximately the same breast height diameters, one in the control
plot (15C, 18C), one in the experimental plot (15D, 18D). The samples were extracted along the
stem perimeter in 14–day intervals during the growing season from May 7, 2010 to September
23, 2010. The sampling was conducted by means of Trephor hand tool (Rossi et al. 2006).
Immediately after sampling, the samples were put in FAA solution and after a week in 50 %
alcohol. Permanent microscopic preparations were made using the microcores and the method of
sample embedding in paraffin (Gryc et al. 2012, Vavrčík et al. 2013). The permanent preparations
were observed using Leica DMLS 2000 light microscope with polarization filter for a more
accurate establishment of the boundaries between the particular phases of radial increment. They
were also recorded using Leica DFC 280 digital camera. The polarization filter allows us to see
the differences between the cells where secondary cell wall formation has started and the cells in
the phase of postcambial growth. We used open source software ImageJ to measure the tree-ring
width, tracheid dimensions and cell wall thickness. We measured three rows in each tree in the
direction from earlywood to latewood. Late-wood tracheids (LWT) were defined according to
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Mork´s formula (1989); the size of the LWT lumen is smaller on the cross section than the cell
wall double (measured in radial direction). The relative position of each tracheid within annual
ring was calculated as RP = P/N where P is absolute position of the tracheid and N is number of
tracheids within the tree ring. The value CFR – cell formation rate (cell/day) was calculated for
each microcore. The total number of cells in all differentiation phases (TOTAL) was divided by
14 in order to find the everyday mean. Another explored relationship was the one between CFR,
temperature and soil water potential. For this purpose, we calculated the average temperature
of the last 14 days. CFR was correlated with: T14min, which is the minimum temperature in the
past 14 days; T14max, i.e. the maximum temperature in the past 14 days; T14 max (avg), which is the
average maximum temperature in the past 14 days; and T14min (avg), i.e. the average minimum
temperature in the past 14 days (Deslauriers and Morin 2005). Linear regressions were performed
for the temperature-related variables. A square root (sqrt) transformation was applied to the cell
formation rate to meet the assumptions of homogeneity of variances (homoscedasticity) when
performing the correlation and regression analyses. Soil water potential (SWP) was measured
in both stands in three depths – 10, 25 and 50 cm. We used calibrated gypsum blocks GB1
(Delmhorst Instrument, NJ, USA) connected to datalogger SP3 (EMS Brno, Czech Republic).
Data were saved in the datalogger memory in ten minute resolution. Week minimum, maximum
and mean values were used for further evaluation. Values obtained were statistically processed in
the Statistica 10.0 application (descriptive statistics, ANOVA).
RESULTS
Cambial activity
At the beginning of measuring, on May 7, 2010, the cambial zone contained 7–10 cells.
The number of cambial cells (CC) then increased until the period from May 21 to June 17, when
the maximum number was observed. Subsequently, there was a slow decrease. At the end of
measuring, on September 23, 2010, the number of CC ranged again between 5 and 8. No new
cells of postcambial growth (PC) were observed in trees 15D, 18C and 18D starting on August
12. This means that the cambial activity in these trees stopped at the beginning of August.
Cambial activity of tree 15C stopped on August 27, 2010. The cambium formed more cells in
clone 15 than in clone 18 (Fig. 2).
Tracheid differentiation
The first PC cells in all trees were observed on May 21, 2010. There was a great difference in
the number of cells in particular phases between tree 15 in control plot (15C) and in experimental
conditions (15D). The first cells in phase of secondary wall thickening (SW) were observed in
all trees on June 3: 8 cells in 15C and 12 cells in 15D and 6 cells in 18C and 18D. The first
cells with a fully lignified cell wall (MT) in 15C, 18C and 18D were found on June 17, while in
15D they were found on July 1. Tree 15C reached the greatest total tree-ring width 2.28 mm.
This value was considerably lower in 15D – 1.78 mm. The total tree ring width was 1.23 mm in
18C and 1.42 in 18D. The results of family 18 were different from family 15 but there were no
recorded differences between the stressed and the control trees. Generally, the numbers of cells
in all differentiation phases of trees 18C and 18D were very similar (Fig. 2).
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Fig. 2: Number of tracheids in each phase of differentiation process and number of cell within cambial
zone during growing season 2010 (7. 5. 2010 – 127. day, 1. 7. 2010 – 182. day).
Effect of temperature on radial increment
The graph (Fig. 3) shows that the temperature affects cell formation positively. Tab. 1 shows
that determination coefficient values were 0.42–0.86 in tree 15C. The determination coefficient
values of the tree stressed by drought 15D were 0.45–0.75. The coefficient was lower in tree
18: 0.04–0.26 for 18C and 0.35–0.64 for 18D. This is also reflected in the values of Pearson
correlation coefficient (Tab. 1). It follows that there is a strong dependence between temperature
and CFR in clone 15 as well as in 18D, while this dependence was not proved for 18C.
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Fig. 3: Dependence of CFR on temperature.
Tab. 1: Linear regression equation coefficient between cell production rate (cell/day) and T 14max, T14min,
T14max (ave), T14min (ave) and SWP 0.5 m.
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Effect of soil water potential on radial increment
The graph (Fig. 4) shows that soil water potential (SWP) affects the trees in the control plot
(15C and 18C) positively. High values of positive dependence (Tab. 1) were found only for the
deepest measuring point in depth 0.5 m. No dependence was found for the soil water potential
measured in shallow soil horizons. No dependence between recorded SWP and CFR was found
at the experimental plot. The tree 18C showed decrease in growth during period from July 27
to August 8. At the control area SWP was found to be -1.1 MPa continued for ten days at a
depth of 25 cm. However, in the depth of 50 cm was value of SWP approximately -0.02 MPa. It
follows that the distribution of root system of the tree 18C is very shallow. It is an obvious need
to measure SWP in more depths, i.e. 10, 25 and 50 cm.
Fig. 4: Dependence of CFR on soil water potential in 0.5 m depth.
Radial diameter of tracheids
The radial diameter of tracheids decreases with the increasing relative position within a tree
ring (Fig. 5). Tracheids spend different times in the phase of radial enlargement. Earlywood
tracheids stay there for a longer time than latewood tracheids. The radial diameter thus decreases
in the direction from earlywood to latewood. The maximum value of tracheid radial diameter
was 44.57 μm in 15C and 46.94 μm in 15D, 43.45 μm in 18C and 41.1 μm in the stressed tree
(18D). The lowest values of tracheid radial diameter were 9.3 μm for 15C, 9.7 μm for 15D,
5.48 μm for 18C and 5.83 μm for 18D. The ANOVA analysis of variance (p < 0.05) did not prove
any statistically significant difference in the radial diameter of tracheids between the stressed and
the control trees.
Fig. 5: Radial diameter of tracheids depending on relative position within a tree ring.
Cell wall thickness in radial direction
Cell wall thickness in the radial direction at the beginning of the growing season is first
small and it increases during the season gradually due to the fact that the tracheids stay in the
secondary cell wall formation phase for a longer time. At the end of the season, the cell wall
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thickness decreases again (Fig. 6). The maximum cell wall thickness was as follows: 5.8 μm in
15C, 5.6 μm in 15D, 5.2 μm in 18C and 4.8 μm in 18D. No statistically significant difference was
found between the stressed and the control trees of both families (p < 0.05).
Fig. 6: Cell wall thickness in radial direction depending on relative position within a tree ring.
Distinction of earlywood and latewood tracheids
The analysis of tracheid numbers shows that more cells were formed in tree family 15 than
in 18. The proportion of early– and latewood tracheids was established based on Mork´s formula
and the results were 6 latewood tracheids in (6.3 %) 15C, 16 latewood tracheids (23.5 %) in 15D,
11 latewood tracheids (23.9 %) in 18C and 23 latewood tracheids (39.6 %) in 18D. On average,
tree 15C formed 95 tracheids, while tree 15D only formed 68 tracheids. The total number of
formed tracheids was different in clone 18: 46 tracheids in the control tree (18C) and 58 tracheids
in the stressed tree (18D).
DISCUSSION
Cambial activity and tracheid differentiation
This paper presents an experiment with long–term drought stress which was artificially
induced during the growth season of 2010 for young Norway spruce in two forest plots in Norway,
with the aim to study the anatomical structure of tracheids of drought exposed and control plots.
Cambial activity started in both research plots on May 7, 2010 (DOY – day of year 127), which
corresponds to the results of Rossi et al. (2009), who said that the treatment affects only slightly
cambial activity in seedlings of Abies balsamea. Gryc et al. (2011) presented the start of cambial
activity of Norway spruce in the first half of April and the same date was presented by Gričar
(2007) and Romagnoli et al. (2011) for Castanea sativa. Gričar and Čufar (2008) found the start
of cambial activity in the Norway spruce in the lowland area at the end of April. In our case, the
later start of cambial activity could be caused by the northern location of experiment in Norway.
During our experiment it was found that the end of cambial activity for tree 15D came on DOY
224 (August 12) and for 15C on DOY 239 (August 27). Gričar (2007) testified to the end of
cambial activity at the end of July, Rossi et al. (2009) presented days DOY 155–246 as the end,
the number of CC (cambial cells) fluctuating between 5 and 9 when the cambial activity stopped.
Also Gričar (2007) said that the number of CC dropped to 6–7 at the end of the growing season.
Gryc et al. (2011) stated that the number of CC is similar at the beginning and the end of the
growing season (for research plot Rájec, Czech Republic). The presented results, not finding
any difference between the stressed and the control trees, show that the stress did not have any
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significant effect on the number of cells in the cambial zone. However, cambial activity and
xylem cell production are largely energy–demanding processes and they mainly need a lot of
sucrose from photosynthesis, so the impact of drought on assimilates can be reflected in a lower
production of wood (Arend and Fromm 2007, Hansen and Beck 1994, Oribe et al. 2003). This
was confirmed only in family 15, where it was found significantly larger width of the annual
rings in tree 15C. Belien et al. (2012) found that the drought applied during summer had no
influence on the total number of cells produced. The same results were acquired by Rossi et
al. (2009), who said that irrigated and non-irrigated plants manifested the same trend of xylem
formation. This contradicts our results, which show that the total number of cells formed during
the growing season is different between the control and the stressed trees in family 15. No
difference was found in family 18. The results show that the explored families reacted to drought
stress differently.
Influence of weather factors on cell production
The results show that the CFR value increases with an increasing temperature. Deslauriers
and Morin (2005), who examined the dependence between Tmin, Week Tmin, Tmax and Week
Tmax, proved that the CFR value is positively affected by air temperature. Tmin had a largest
effect (R 2=0.26) on CFR than Tmax (R 2=0.08) in the period when most tracheids were formed
(Deslauriers and Morin 2005). In our case, values R 2 were considerably higher. T14max (averageand)
had the greatest effect on CFR in trees 15C and 15D (R 2= 0.86 and 0.75, respectively); the values
were markedly lower for family 18. Deslauriers and Morin (2005) found a positive value of the
Pearson correlation coefficient for all the measured temperature features, which corresponds
to our results and can be expected in this range of temperatures. The number of formed cells
decreases with the decreasing supply of water in soil in a depth of 0.5 m in the control plot. No
dependence of CFR and SWP was found for the shallower soil layers, because even if the soil
water was depleted to the wilting point in a depths of 10 and 25 cm (i.e. end of July), SWP never
dropped below -0.03 MPa at the depth of 50 cm thus making water still partly (given the root
distribution) available for the plant. The dependence of CFR on SWP was only observed in the
trees in the control plot due to the fact that the water availability in the stressed plot was quite
stable during the season (i.e. no water was available for plants in depths up to 30 cm while stable
water potential between -0.2 and -0.4 MPa was kept in the depth of 50 cm).
Analysis of tracheid morphological parameters
Morphological parameters of cells affect the resulting properties of wood (Panshin and de
Zeeuw 1980, Gryc and Vavrčík 2006). A significant factor for softwood is also the proportion
of earlywood and latewood. The percentage of latewood is very variable in dependence on the
species and site conditions and varies from 37 % (Belien et al. 2012) to 60 % (Rossi et al. 2009).
The latewood proportion in our case was 23.3 % on average. The drought stressed trees formed
more latewood than the trees in the control plot. The results of measuring show that the radial
diameter of tracheids decreases with an increasing relative position within a tree ring. The radial
diameter decreases in the direction from earlywood to latewood, as also stated by Horáček (2003).
Shepherd (1964) stated that the reduction of the radial diameter is directly caused by the rapid
loss of water potential in the tree. In our case, the tracheid radial diameter ranged from 5.48 μm
to 46.94 μm, which corresponds to Horáček (2003), Rossi et al. (2009) and Wodzicki (2001),
who presented tracheid radial diameter of 13–45 μm. We did not observe a difference in radial
diameter of tracheids between the control and the stressed trees, while Rossi et al. (2009) found
a significant decrease in the radial dimension of tracheid during a 20–day drought stress. This
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difference can be explained by the long–term effects of drought stress throughout the growing
season during our experiment. The maximum cell wall thickness ranged between 4.23 and
4.81 μm and no difference was found between the stressed and the control trees, corresponding
to Rossi et al. (2009), who presented that in both experiments seedlings had a maximum cell wall
thickness of 4.8–6.4 μm; the same results were obtained by Vavrčík et al. (2006). Belien et al.
(2012) presented a cell wall thickness of 2.3–3.9 μm. Our results disprove claims about the impact
of long–term drought stress on cell wall thickness and radial dimension of tracheids.
CONCLUSIONS
In the experiment we investigated the effect of drought and weather factors on morphological
parameters of tracheids and wood formation in two families of Norway spruce during the growing
season of 2010. Generally, drought has a negative effect on tree growth but this may differ in
specific cases. The total number of formed cells was affected in tree family 15, but it was not
affected in tree family 18. No statistically significant difference was found for morphological
parameters between the stressed and the control trees. Out of the weather factors, mainly the
maximum temperature and soil water potential in a depth of 0.5 m affect the cell formation to
the highest extent. A positive dependence was found for temperature and for SWP in the trees
at the control plot. Further research should focus on more tree species and the changes in wood
formation and morphological parameters at various altitudes.
ACKNOWLEDGMENT
This research was funded by Iceland, Liechtenstein and Norway through the EEA Financial
Mechanism together with the Norwegian Financial Mechanism (grant no. A/CZ0046/2/0009),
by project INVID – Indicators for tree vitality (grant no. CZ.1.07/2.3.00/20.0265) and by
Mendel University in Brno (grant IGA 43/2013).
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Jakub Hacura, Vladimír Gryc, Hanuš Vavrčík, Jana Hozová
Mendel University in Brno
Faculty of Forestry and Wood Technology
Department of Wood Science
Zemědělská 3
613 00 Brno
Czech Republic
Corresponding author-mail: [email protected]
Josef Urban
Mendel University in Brno
Faculty of Forestry and Wood Technology
Institute of Forest Botany, Dendrology and Geobiocoenology
Zemědělská 3
613 00 Brno
Czech Republic
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