Trees (2009) 23:85–93
DOI 10.1007/s00468-008-0257-0
ORIGINAL PAPER
Effects of a 20-day-long dry period on cambial and apical
meristem growth in Abies balsamea seedlings
Sergio Rossi Æ Sonia Simard Æ Cyrille B. K. Rathgeber Æ
Annie Deslauriers Æ Carlo De Zan
Received: 13 February 2008 / Revised: 24 July 2008 / Accepted: 24 July 2008 / Published online: 5 August 2008
Ó Springer-Verlag 2008
Abstract There is general agreement that in many
regions additional precipitation with climate change will
not be able to balance the increased evaporation rate
induced by higher air temperatures, causing periods of
intense drought. Although seedlings of Abies balsamea
growing in the boreal forest are known for their resistance
to harsh environmental conditions, the impact of water
stress on their growth still remains largely unexamined.
The aim of this study was to investigate growth responses
of this species during and after a dry period by monitoring
cambial and apical meristem activity at short time scale.
Meristem growth was studied from May to October 2005
on seedlings of A. balsamea submitted to a 20-day-long dry
period in June–July. Lower rates of shoot lengthening were
observed in non-irrigated seedlings only in the first part of
the growing season. Irrigated and non-irrigated trees
showed the same trend of xylem formation and timings of
cell differentiation. Cell production during cambial activity
was estimated at about one xylem cell per day thus
achieving in 100 days 108 tracheids in the tree ring and a
width of 2 mm, with thinner tree rings observed in non-
Communicated by K. Takabe.
S. Rossi (&) S. Simard A. Deslauriers
Département des Sciences Fondamentales, Université du Québec
à Chicoutimi, 555 Boulevard de l’Université, Chicoutimi,
QC G7H 2B1, Canada
e-mail: [email protected]
S. Rossi C. B. K. Rathgeber
INRA, UMR 1092 Laboratoire d’Étude des Ressources
Forêt-Bois (LERFoB), 54000 Nancy, France
A. Deslauriers C. De Zan
Dipartimento TeSAF, Università degli Studi di Padova,
Legnaro, Padova, Italy
irrigated plants. A reduction of up to 50% in lumen area
and cell diameter was observed for the cells produced
during the dry period. Response of A. balsamea seedlings
to a 20-day-long dry period consisted of good resistance of
the cambial meristems during and after water stress, high
sensitivity and rapid recovery of cell sizes during water
depletion and slow but effective recovery of shoot growth
after treatment.
Keywords Boreal forest Cambium Meristem Water stress Wood formation
Introduction
Balsam fir (Abies balsamea (L.) Mill.) is one of the most
important species of the coniferous forest in Canada and an
essential component of much wildlife habitat. Although
considered as shade tolerant, this species grows adequately
and rapidly both in forest gaps and open sites, when either
climatic or edaphic conditions allow an abundant supply of
water for seed germination (Ghent 1958; Bergeron and
Dubuc 1989; Simard et al. 1998). Regeneration of balsam
fir is guaranteed by the favourable climatic conditions
occurring in its natural environment. However, historical
climate analyses and current projections indicate that the
climate is changing at a rate never experienced before. In
the past century, mean annual temperature over southern
regions of Canada has increased by an average of 0.9°C,
with the largest increases during winter and spring (Zhang
et al. 2000). Decreases in the daily temperature range were
caused by greater increases in daily minimum rather than
maximum temperatures, resulting in less cold, and not
hotter, seasons. This temperature increase is affecting the
hydrological cycle by increasing evapotranspiration,
123
86
leading to a modification of the amount and distribution of
precipitation. Consistent with the global trend observed at
high latitudes (Easterling et al. 2000), Canada as a whole is
experiencing increased annual precipitation, mainly in the
form of snowfall (Zhang et al. 2001; Motha and Baier
2005), while extreme conditions, both wet and dry, are
occurring more frequently during summer (Zhang et al.
2000). There is general agreement that the increase in
precipitation will not be able to balance the increase in
evaporation induced by the higher air temperatures, causing declines in soil moisture as well as periods of intense
drought (Easterling et al. 2000; Motha and Baier 2005).
In the boreal forest, tree growth is strongly related to
water supply as a positive effect of precipitation, as
observed in several conifer species of this environment
(Hofgaard et al. 1999; Deslauriers et al. 2003b). Moreover,
at high northern latitudes, recent decreases in radial growth
and carbon uptake were connected to summer drought
stress following air temperature increase (Lloyd and Fastie
2002; Wilmking et al. 2004). Arnott (1973) indicated that
seedbed moisture and temperature are crucial factors for
seed germination and that drought is the main cause of
mortality for regeneration during the first stages of development. However, older, well-established seedlings of
balsam fir are known for their tenacious survival and high
resistance to harsh environmental conditions (Ghent 1958;
Johnson et al. 2003; Morin et al. 2008). Nevertheless, the
level of water stress that the established seedlings can
endure and how this stress will affect seedling growth is
still unknown.
Although the short-term effects of drought on plant
physiology are well documented (Abe and Nakai 1999;
Abe et al. 2003; Arend and Fromm 2007; Giovannelli et al.
2007), the impact of water stress on the dynamics of
seedling growth during the entire growing season remains
still largely unexamined. The water status of a tree is an
important factor determining cambial activity and xylem
development (Denne and Dodd 1981; Larson 1994). A
decrease in the frequency of cambial cell division (Abe and
Nakai 1999) and a reduction in the radial diameter of
tracheids (Zahner 1968; Denne and Dodd 1981; Abe et al.
2003) generally occur in conifers in response to water
deficit. Shepherd (1964) showed that a reduction of the
radial cell diameter was directly caused by a rapid decline
in water potential within a tree. Tracheid growth requires
an increase in cell volume, which depends on the maintenance of high cell turgor pressure, with an irreversible
influence on cell extension and wall polymer deposition
(Proseus and Boyer 2005). Abe et al. (2003) found that cell
diameter reduction occurred during the early stage of water
deficit, and preceded a decrease in cambial cell division.
These results confirm Abe and Nakai (1999)’s hypothesis
that the decrease in water potential directly affects cell
123
Trees (2009) 23:85–93
expansion during the early stage of water deficit, whereas
the rate of cambial cell division declines only during the
later stages of water deficit because of hormone regulation.
However, most of these findings come from short-term
experiments focusing on responses to extreme levels of
stress. How trees respond to a more ‘‘natural’’ moderate
short-term water stress and recover growth during the rest
of the growing season still needs to be investigated.
The aim of this study was to investigate the growth
responses of A. balsamea seedlings by monitoring cambial
and apical meristem activity at short time scale before,
during and after a dry period. A 20-day-long dry period, a
likely occurrence in the natural area of balsam fir, was
chosen according to the possible future scenarios drawn by
current climate models (Zhang et al. 2000). The dry period
took place between June and July, when conifers are
expected to exhibit maximum growth and high sensitivity
to water availability (Hofgaard et al. 1999; Deslauriers
et al. 2003a; Rossi et al. 2006c).
Materials and methods
Experimental design and sampling
The experiment involved two groups of 7-year-old seedlings with uniform height (31.1 ± 3.8 cm) and diameter
(8.4 ± 1.0 mm) and arranged in a completely randomised
design. The seedlings were growing in plastic reversedconic pots (height 15 cm, diameter 11.5 cm at the base and
15 cm at the top) filled with peat moss. They spent March–
May 2005 in an open field close to a greenhouse in Chicoutimi (48°250 N, 71°040 W, 150 m a.s.l., Québec, Canada).
At the end of May, seedlings were transferred into the
greenhouse and each pot was fertilised with 1 g l-1 of
NPK (20-8-20) fertilizer dissolved in 500 ml of water. A
drip irrigation system supplied water to all plants until 17
June. Soil was maintained at 85–90% field capacity in
order to prevent anaerobic soil conditions by supplying an
amount of water equal in weight to the daily losses. On
average, 500 ml of water per seedling was supplied each
day. From 18 June to 8 July, two different treatments were
applied: (1) an irrigation regime (irrigated seedlings) in
which soil water content was maintained at 80% field
capacity and (2) a dry regime (non-irrigated seedlings) in
which no water was supplied. Pre-dawn leaf water potential
[W (MPa)] was measured on needles in the first verticil of
15 seedlings with a pressure chamber (PMS Instrument,
Corvalis, Oregon). After the treatment (9 July), seedlings
were returned to the open field and both treatments were
irrigated until November.
Tree-ring formation was studied from May to October
2005. Stem disks were collected weekly 2 cm above the
Trees (2009) 23:85–93
root collar from 12 randomly selected plants (six irrigated
and six non-irrigated seedlings). The samples were dehydrated with successive immersions in ethanol and Dlimonene, embedded in paraffin and transverse sections of
8–10 lm thickness were cut with a rotary microtome
(Rossi et al. 2006a). For each sampled seedling, stem
diameter and length of apical and lateral shoots belonging
to the first verticil were measured.
Anatomical observations of xylem development
Sections were stained with cresyl violet acetate (0.16% in
water) and examined within 10–25 min with visible and
polarized light at magnifications of 400–5009 to distinguish the developing xylem cells. For each section, the
radial number of (1) cambial, (2) enlarging, (3) cell wall
thickening, and (4) mature cells were counted along three
radial files according to Rossi et al. (2006b). In cross
section, cambial cells were characterized by thin cell walls
and small radial diameters. During cell enlargement, the
tracheids still showed thin primary walls but radial diameters were at least twice those of the cambial cells
(Fig. 1a). Observations under polarized light discriminated
between enlarging and cell wall thickening tracheids
(Fig. 1b). Because of the arrangement of the cellulose
microfibrils, the developing secondary walls glistened
when observed under polarized light, whereas no glistening
was observed in enlargement zones where the cells were
still just composed of primary wall (Fig. 1c, d; Abe et al.
1997). The progress of cell wall lignification was detected
with cresyl violet acetate reacting with the lignin (Antonova and Shebeko 1981). Lignification appeared as a
colour change from violet to blue. A homogeneous blue
colour over the whole cell wall revealed the end of lignification and the reaching of tracheid maturity (Gričar et al.
2005).
As observations were carried out on stem disks collected
from six different seedlings per treatment on each sampling
Fig. 1 Sections of developing stem tissues collected on 24 June from
A. balsamea seedlings. a Cambial zone (CZ) and enlarging xylem
cells (EC) (1009, horizontal bar 20 lm). b Enlarging (EC) and wall
87
day, samples were independent. However, the assumption
of normality in data distribution within each sampling day
was frequently violated. Rank comparisons in the number
of cambial, differentiating and mature cells between irrigated and non-irrigated plants were, therefore, performed
with nonparametric Wilcoxon tests. Linear regression and
analysis of covariance (ANCOVA) were used to assess
pattern of cell production during the growing season (14
sampling days) and to compare regression slopes between
the two treatments. The normality of residual distribution
was verified by the Shapiro–Wilk test.
Anatomical measurements
On the last sampling day, disks were collected from 18
seedlings (nine per treatment) and sections were prepared
as described above. The sections were stained with safranin
(1% in water) and fixed on slides with EukittÒ. A camera
fixed on an optical microscope was used to record
numerical images at magnification of 4009. Cell features
(lumen area, radial cell diameter, and wall thickness) were
measured on three radial files per section using WincellTM
(Deslauriers et al. 2003a). Tracheids were classified as
belonging to earlywood or latewood according to Mork’s
formula, which classified as latewood all cells with lumen
smaller than twice a double cell wall (Denne 1988). The
amount of earlywood and latewood between treatments
was tested using a chi-square test on the number of cells.
Comparisons of cell dimensions between treatments were
performed using the Wilcoxon test.
Results
The experiment monitored meristem and cambial activity
on 264 seedlings, simulating a possible natural situation in
which high water availability from late spring snowmelt
was followed by 20 days without rain. Although peat moss
thickening (WT) cells (409, horizontal bar 50 lm). c, d developing
xylem under visible and polarized light showing phloem (PH) and
wall thickening (WT) cells glistening (209, horizontal bar 100 lm)
123
Stem and shoot growth
Pot weight (kg)
Between the two treatments, height and diameter of the
seedlings were similar both before and after the growing
season. For stem diameter (Fig. 3a), means from non-irrigated seedlings were alternatively above and below means
from irrigated plants and neither a particular growth pattern
nor statistically different groups were observed except on
DOY 183 (Wilcoxon test, v2 = 6.54, P = 0.01).
In the apical shoot, budbreak occurred on DOY 169, at
the beginning of the treatment, on 7 out of 12 seedlings. In
the following week, budbreak was attained in all the
remaining plants. No difference in bud phenology was
observed between the two treatments (chi-square test,
v2 = 0.34, P = 0.55).
Lengthening of the apical shoot proceeded at a lower
weekly rate for non-irrigated seedlings compared to the
irrigated plants until DOY 225 (Fig. 3b). At the end of this
period, between DOY 211 and 218, apical elongation was
even lacking in non-irrigated plants. From DOY 225, statistically different apical shoot lengths between treatments
were observed in only 2 weeks out of 8, and on DOY 246,
non-irrigated plants showed longer apical shoots than irrigated seedlings.
Lateral shoots of the first verticil showed elongation
patterns similar to the apical shoots, with a lower rate of
elongation for the non-irrigated seedlings and statistically
significant differences during the period of highest growth
rates from DOY 183 to 218 (Fig. 3c). As for apical shoots,
2.0
1.5
1.0
∗ ∗
∗ ∗∗ ∗ ∗
Irrigated
Non-irrigated
160
165
170
175
180
∗
185
190
Day of the year
Fig. 2 Pot weight measured on A. balsamea seedlings before and
during the water stress period (grey window). Vertical bars
correspond to the standard deviation between six and eight plants.
Asterisks indicate statistically significant differences in the medians
between the two treatments (Mann–Whitney–Wilcoxon test,
P \ 0.05). Dashed line represents the pot weight with soil at the
field capacity
123
Stem diameter (mm)
helps maintain a moist soil environment, an evident
decrease in pot weight due to water losses was observed
after 5 days from the beginning of the treatment (Wilcoxon
test, P \ 0.05) with lower values achieved on the day of
the year (DOY) 178 and maintained until the end of the
treatment (Fig. 2). On DOY 188, W in the non-irrigated
seedlings was -1.08 ± 0.3 MPa while control plants
showed values of -0.27 ± 0.03 MPa.
16
Apical arrow length (cm)
Trees (2009) 23:85–93
30
Lateral shoot length (cm)
88
14
A
14
12
10
∗
8
25
B
20
15
∗ ∗
∗
10
5
∗
0
12
C
10
∗
8
6
4
∗∗∗
2
0
∗
∗∗
∗
Irrigated
Non-irrigated
120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
Day of the year
Fig. 3 Stem diameter (a mm) and length (cm) of apical (b) and
lateral (c) shoots in the first verticil of the collected A. balsamea
seedlings before, during and after the water stress period (grey
window). Vertical bars correspond to the standard deviation between
six plants. Asterisks indicate statistically significant differences in the
medians between the two treatments (Mann–Whitney–Wilcoxon test,
P \ 0.05)
at the end of the growing season, the differences between
irrigated and non-irrigated seedlings decreased and, from
DOY 239, the Wilcoxon test detected no difference
between treatments, except for DOY 260 (v2 = 6.0,
P = 0.01).
Xylem development
In the cambial zone, very similar annual dynamics and
amounts of cells were observed between the two treatments
(Fig. 4a), with statistical differences observed only on
DOY 204, 239, and 246 (Wilcoxon test, P \ 0.05). In May,
when meristems are supposed to be still inactive, 6–7
closely spaced cells on average were observed in the
cambial zone. From DOY 155 to 246, the number of
cambial cells fluctuated with values ranging between 5 and
9 cells. Once annual activity had ended and the cambium
stopped dividing, the number of cells in the cambial zone
gradually decreased to the minimum value, corresponding
to quiescence conditions of the meristems. Between 4 and
6 cambial, cells were observed in autumn, a lower number
than at the beginning of the season (Fig. 4a).
Irrigated and non-irrigated trees showed the same trend
of xylem formation and timings of cell differentiation:
Trees (2009) 23:85–93
Cambium
9
89
∗
A
8
∗∗
7
6
5
4
Enlarging cells
10
8
∗
B
6
4
2
Wall thickening cells
0
16
C
∗
∗
12
8
∗
∗
4
0
∗
140
Mature cells
120
D
100
80
60
40
20
∗∗
∗ ∗
∗
Irrigated
Non-irrigated
0
120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280
Day of the year
Fig. 4 Radial number of cambial (a), enlarging (b), wall thickening
(c), and mature (d) cells in the developing xylem of A. balsamea
seedlings before, during and after the water stress period (grey
window). Vertical bars correspond to the standard deviation between
six plants. Asterisks indicate statistically significant differences in the
medians between the two treatments (Mann–Whitney–Wilcoxon test,
P \ 0.05)
wall thickening cell numbers presented an evident dip
between DOY 176 and 218 (Fig. 4b, c). This could not be
related to a drought effect because the dynamics of the cell
numbers from both treatments showed very similar
patterns.
In order to assess the pattern of cell production, the
weeks when cambium was active (between DOY 154 and
253) were selected (Fig. 4a, b). The overall xylem cell
production during the season showed a linear pattern
confirmed by the goodness of fit of the regression with R2
of 0.90 and 0.91 for irrigated and non-irrigated plants,
respectively (Fig. 5). Although a slight heteroscedasticity
of the residuals appeared mainly on the first three sampling
days when the amount of growth was still low, the standardized residuals were normally distributed (Shapiro–
Wilk test, W = 0.98, P [ 0.05 for both irrigated and nonirrigated plants) and only 17 out of 159 (10% of the total)
were located outside the boundaries of 1.72 (confidence
limit of 0.05 of the t distribution), indicating a more than
satisfying estimation of the regression. ANCOVA showed
no difference in the slope of the two regressions (F = 0.52,
P = 0.47). Cell production during cambial activity was
estimated at about one xylem cell per day, thus achieving
in 100 days of xylogenesis 108 tracheids in average in the
tree ring and a width of 2.0 mm (Table 1). In respect to
irrigated seedlings, lower final numbers of cells and thinner
tree-ring widths were observed in non-irrigated plants
(Wilcoxon test, v2 of 5.41 and 5.91 for ring width and cell
number, respectively, P \ 0.05).
130
Irrigated
120
y = - 179.5 + 1.15 x
y = - 189.6 + 1.19 x
Non-irrigated
110
100
90
Number of xylem cells
statistical differences were detected only on DOY 169 for
cell enlargement and occasionally for wall thickening
(Fig. 4b, c). The Wilcoxon test indicated 6 days with different amounts of mature cells (Fig. 4d). However, these
different groups were sparse during the season and in most
other cases the two treatments showed very similar or
practically equal (DOY 183, 211, and 239) numbers of
cells. The differentiating cells had a clear pattern of variation during the year in both treatments (Fig. 4), related to
the number of tracheids in the different development phases. At the beginning of cell differentiation (DOY 162), the
number of tracheids undergoing differentiation increased.
During late summer, the number of cells in radial
enlargement gradually reduced, reaching zero on DOY
260, 3 weeks before cells in secondary wall formation. The
curves of mature cells represented the gradual accumulation of mature tracheids in the tree ring and showed a
linear-like trend. The curves representing enlarging and
80
70
60
50
40
30
20
10
0
150
160
170
180
190
200
210
220
230
240
250
Day of the year
Fig. 5 Pattern of xylem cell production in irrigated and non-irrigated
A. balsamea seedlings before, during and after the water stress period
(grey window)
123
90
Trees (2009) 23:85–93
Number of cells
P
2.22 ± 0.37
1.95 ± 0.35
5.41
0.02
112.3 ± 12.8
102.0 ± 14.4
5.91
0.02
2
Results indicate v and relative probability of Mann–Whitney–Wilcoxon test
The observed dynamics of cell production allowed an
assessment of which tracheids were in differentiation during the stress period. By summing the number of enlarging,
wall thickening and mature cells for DOY 169 and 189
(Fig. 4b, c, d), 10 and 37 cells were present at the beginning and end of the dry period, respectively. Similar values
could be extracted directly from Fig. 5 by substituting the
two indicated dates in the regression equations.
Anatomical structure of the tree ring
Measurements provided curves of cell size variation, called
tracheidograms, along radial files of xylem. Twenty-seven
tracheidograms (nine plants 9 three radial files) per treatment were measured on the seedlings collected at the end
of the season. As tracheidograms presented a different
number of tracheids, varying from 70 to 123, the averages
contained a different number of observations, which
gradually reduced from 27 to 0 (Fig. 6).
In both treatments, larger tracheids with thinner cell
walls (2–3 lm thick) were produced at the beginning of the
growing season (Fig. 6). Along the radial file, cell sizes
gradually reduced from 170–220 to 50–80 lm2 of lumen
area and from 18–22 to 13–15 lm in diameter. The last 40
tracheids of the tracheidograms showed wall thickness of
4.8–6.4 lm. When tracheidograms were compared
between irrigated and non-irrigated seedlings, no statistical
difference was observed, except in the middle part of the
earlywood for lumen area and radial cell diameter (Wilcoxon test, P \ 0.05). Statistically significant differences
were also detected for radial cell diameter in the tracheids
located to the boundary between earlywood and latewood
(Wilcoxon test, P \ 0.05). A reduction of up to 50% in
lumen area and cell diameter was observed for the cells
located between rows 18 and 32 in the non-irrigated plants.
This reduction in cell size appeared as a intra-annual
density fluctuation, a layer of tracheids within the tree ring
characterized by diffuse boundaries (Fig. 7). Timings of
xylogenesis demonstrated that these cells were developed
during the dry period (previous paragraph). No reduction
was observed for tracheidograms of irrigated seedlings.
On average, 60% of xylem cells in the tree rings were
classified as latewood (Table 2). No influence of the
123
Lumen area (µm2)
v2
200
150
∗
∗
∗∗
∗ ∗∗ ∗
∗∗
100
50
∗
∗
0
Radial cell diameter (µm)
Ring width (mm)
Non-irrigated
Cell wall thickness (µm)
Irrigated
250
Number of cells
Table 1 Tree-ring width (mm) and number of radial cells in irrigated
and non-irrigated A. balsamea seedlings reported as mean and standard deviation
24
22
20
∗ ∗
∗ ∗
∗∗∗∗∗
18
16
∗
14
∗
∗∗
∗ ∗
∗ ∗∗
12
10
6
5
4
3
Irrigated
Non-irrigated
2
30
20
Irrigated
10
Non-irrigated
0
0
10
20
30
40
50
60
70
80
90
100
110
Radial number of cell along the tree ring
Fig. 6 Cell lumen area (lm2), radial diameter (lm) and wall
thickness (lm) measured on radial files of tracheids along the tree
rings of irrigated (black dots) and non-irrigated (grey dots) A.
balsamea seedlings. The number of measurements for each radial cell
is also reported. Vertical bars represent the standard error. Vertical
lines separate earlywood to latewood. Asterisks indicate statistically
significant differences in the medians between the two treatments
(Mann–Whitney–Wilcoxon test, P \ 0.05)
treatment was observed on the proportion of earlywood and
latewood (chi-square test, v2 = 2.28, P = 0.13). The first
latewood tracheid was located on average at the 46th
position along the cell radial file and was related at first to
increasing wall thickness rather than decreasing cell size
(Fig. 6). This cell was estimated to be produced by cambial
divisions in mid-July, around DOY 195–200 (Fig. 5), when
shoot elongation was completing (Fig. 3).
Discussion
Balsam fir is a typical species of the cold and moist boreal
forest of North America, with a marked sensitivity to water
availability both during seedling establishment (Ghent
1958; Bergeron and Dubuc 1989; Simard et al. 1998) and at
the adult stage (Hofgaard et al. 1999; Deslauriers et al.
2003b). In this study, the effect of a 20-day-long dry period
on seedling growth was analysed in order to investigate
responses of forest regeneration to a plausible change in
Trees (2009) 23:85–93
91
Fig. 7 Intra-annual density
fluctuations (white line) in tree
rings of non-irrigated seedlings
of A. balsamea at
magnifications of 49 (a) and
109 (b)
Table 2 Proportion of earlywood and latewood detected at the end of
the growing season in irrigated and non-irrigated A. balsamea seedlings (chi-square test, v2 = 0.06, P = 0.79)
Irrigated (%)
Non-irrigated (%)
Earlywood
41.08
37.76
Latewood
58.92
62.24
precipitation regime. Although water potential in the stem
xylem rapidly decreased during the drought, because of
reduced water availability in the soil, vigorous growth was
still maintained in the cambial meristems. The results
clearly demonstrated the resistance and plasticity of balsam
fir meristems during and after drought periods, confirming
the tenacity and tolerability of this species to unfavourable
environmental conditions (Ghent 1958; Johnson et al.
2003; Morin et al. 2008).
The dry period involved the first 2–3 weeks of shoot
lengthening, which occurred for a total of 5 weeks in the
seedlings. Different responses to drought were observed
between the meristems of apical and lateral shoots. Under
limited water availability, seedlings exhibited significant
reductions mainly in lengthening of lateral shoots, but this
was compensated for at the end of the growing season.
Consequently, water stress did not hamper seedlings in
maintaining a strong apical dominance to achieve the
required rates of height growth, although to the detriment
of lateral shoot lengthening (O’Connel and Kelty 1994).
Similar results were also found in Finland for Picea abies
seedlings, whose growth in height was not affected by a
drought period shorter than 3 weeks (Helenius et al. 2002).
Although greenhouse and pot conditions represent artificial environments, the dynamics and timing of xylem
formation agreed with the previously observed growth
patterns of adult balsam fir (Deslauriers et al. 2003a; Rossi
et al. 2003). Xylogenesis was observed over about
130 days and the 20-day-long dry period only partially
affected xylem growth, being not more than 15% of the
whole growing season. In our study, water stress occurred
at the beginning of the season, when the first 5–15 cells
produced were differentiating and when cambial activity
was expected to achieve maximum cell production rates
(Rossi et al. 2006c; Ko Heinrichs et al. 2007). However,
linear increases in cell number were observed and small but
statistically significant differences were detected between
treatments at the end of September in the resulting amount
of tracheids in the tree ring. Balsam fir seedlings showed a
good tolerance of cambial meristems to water shortage,
even when occurring at the start of the cell division period.
Water stress is known to affect xylem growth via direct
and indirect effects (Denne and Dodd 1981; Abe and Nakai
1999). At the early stage of water deficit, only cell
expansion is physically inhibited by losses of cell turgor.
As water deficit continues, stress affects the whole plant
physiology, reducing or preventing cell metabolism and
indirectly limiting growth (Larson 1963; Arend and Fromm
2007). Cambial activity and xylem cell development are
considerable sinks of energy and particularly demanding in
sucrose from photosynthesis (Hansen and Beck 1994; Oribe et al. 2003), so the effects of drought on
photoassimilates can manifest in reduced wood production
(Arend and Fromm 2007). The reduction in radial diameter
of earlywood tracheids of the non-irrigated plants revealed
the occurrence of drought stress, which caused declines in
water potential in the xylem resulting in unfavourable
conditions for cell enlargement. The effect led to the formation of a density fluctuation in the tree ring. Although
little information on plant physiology was collected during
the treatment, the results indicated that these conditions did
not affect wall formation and only slightly cambial activity.
Moreover, at the end of the dry period, plants were able to
rapidly recover the pressure potential required for enlarging the remaining produced cells (Abe et al. 2003),
confirming the high plasticity of seedlings of this species
(Ghent 1958).
Reactivation of the vascular cambium is promoted by
indol-3-acetic acid (IAA), a hormone produced in the
younger shoots and exported basipetally into the subjacent
stem to induce the production of xylem and phloem
(Larson 1969; Aloni 2001). According to the IAA basipetal
123
92
movement, periclinal divisions in the cambium are supposed to begin at the base of the buds and spread
downwards toward branches and stem (Larson 1969; Lachaud et al. 1999). As a consequence, cambial activity in
the stem should begin after or at least in the same moment
as budbreak because of the basipetal migration of IAA. On
DOY 169 when budbreak was observed, cambial activity
had begun 2–3 weeks earlier and the first tracheids had
already completed differentiation. There is evidence that
IAA contributes to regulating rate and duration of developmental processes during xylogenesis (Tuominen et al.
1997; Uggla et al. 1998). However, our results indicated
that cambial meristems did not require developing shoots
to reactivate cell division in spring, supporting the
hypothesis that, in late winter, dormant conifer tissues
contain a sufficient amount of IAA for cambial resumption
(Little and Wareing 1981; Sundberg et al. 1991).
Kramer (1964) reported that the transition from earlywood to latewood begins after the first severe water deficit
occurs, but this hypothesis was clearly rejected by our
results as similar amounts of latewood were found in the
two treatments. Latewood initiation could be caused by
decreases in IAA supply (Larson 1964) or, more probably,
by altered IAA concentration gradients in the developing
tissues (Sundberg et al. 2000) following cessation of shoot
growth. In our results, shoot elongation corresponded to
strong reductions in the amount of cells in the differentiation zone, which could not be related to a treatment effect.
Since carbon supply during the vegetative season is not
unlimited, a balance among the growing regions within the
plant is expected (Philipson et al. 1971). As a large supply
of assimilates is required for both shoot growth and secondary wall thickening, the separation in time between
these two metabolic activities reflects the internal competition for carbohydrates. Shoot apices and leaves have a
higher ranking priority than cambium as a sink for carbon
(Minchin and Lacointe 2005). Only after shoots completed
growth could resources be allocated to the expending
process of secondary wall formation of latewood cells.
Conclusions
According to the possible future climate scenarios, models
for North America predict an increasing probability of the
occurrence of short dry periods during wetter summers
(Easterling et al. 2000; Motha and Baier 2005). In cold and
damp climates, these drought periods, even if of short
duration, could affect the regeneration of tree species,
which might not be adapted to tolerate water stress during
growth. The response of balsam fir seedlings to a 20-daylong dry period during early summer consisted of (1) good
resistance of cambial meristems during and after water
123
Trees (2009) 23:85–93
stress, (2) high sensitivity and recovery to changes in water
potential of stem xylem during water deficit as inferred
from the decrease in cell size, and (3) slow but effective
recovery of shoot growth after drought.
The results of this experiment suggest that severe but short
dry periods will not seriously impact either apical or radial
growth of balsam fir regeneration. However, longer
droughts, perhaps associated with increasing summer temperatures, might affect metabolic activity in seedlings,
reducing the supply of photoassimilates and indirectly preventing meristem activity and consequently plant growth.
Acknowledgments This work was funded by Natural Sciences and
Engineering Research Council of Canada, Consortium de Recherche
sur la Forêt Boréale Commerciale and Fondation de l’Université du
Québec à Chicoutimi. The authors thank J. -P. Lebeuf, D. Gagnon and
J. Allaire for technical support.
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