Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 17, 133--140
© 1997 Heron Publishing----Victoria, Canada
Influence of photosynthetic photon flux density on growth and
transpiration in seedlings of Fagus sylvatica
N. T. WELANDER1 and B. OTTOSSON2
1
Swedish University of Agricultural Sciences, Southern Swedish Forest Research Centre, Box 49, S-230 53 Alnarp, Sweden
2
Swedish University of Agricultural Sciences, Department of Horticultural Science, Box 55, S-230 53 Alnarp, Sweden
Received August 28, 1995
Summary Beech seedlings (Fagus sylvatica L.) were grown
in various combinations of three photosynthetic photon flux
densities (PPFD, 0.7, 7.3 or 14.5 mol m −2 day −1) for two years
in a controlled environmental chamber. Dry mass of leaves,
stem and roots, leaf area and number of leaves, and unit leaf
rate were affected by both previous-year and current-year
PPFD. Number of shoots and length of the main shoot were
affected by previous-year PPFD but not by current-year PPFD.
Number of leaves per shoot did not change with PPFD, whereas
leaf dry mass/leaf area ratio was mainly affected by currentyear PPFD. During the first 10 days that newly emerged
seedlings were grown at a PPFD of 0.7 or 14.5 mol m −2 day −1,
transpiration rate per unit leaf area declined. Thereafter, transpiration increased to a constant new rate. Transpiration rate
per seedling was closely related to leaf area but the relationship
changed with time. In two-year-old seedlings grown at various
PPFD combinations of 0.7, 7.3 and 14.5 mol m −2 day −1 during
Years 1 and 2, leaf area and transpiration rate per seedling were
closely correlated at Weeks 7 and 11 after bud burst. Weak
correlations were found between root dry mass and transpiration rate per seedling. During Year 2, transpiration rate per leaf
area was higher at a particular PPFD in seedlings grown at a
previous-year PPFD of 0.7 mol m −2 day −1 than in seedlings
grown at a previous-year PPFD of 14.5 mol m −2 day −1. After
transfer of two-year-old seedlings at the end of the experiment
to a new PPFD (7.3 or 14.5 mol m −2 day −1) for one day,
transpiration rates per leaf area, measured at the new PPFD,
were correlated with leaf area and root dry mass, irrespective
of former PPFD treatment.
Keywords: beech, irradiance, seedling morphology, shading.
Introduction
During natural regeneration, growth of beech seedlings may be
limited by the availability of light because, although seedlings
grow at irradiances as low as 1--2% of the light above the
canopy, maximum dry matter production is only reached under
unshaded conditions (Watt 1923, Suner and Röhrig 1980,
Madsen 1994). The morphology of beech seedlings, including
numbers of leaves, leaf area, branching and length of the main
shoot, is also affected by the light conditions (Burschel and
Huss 1964, Burschel and Schmaltz 1965). Seedlings in the
understory may experience variation in irradiance as a result of
changes in foliage in the upper canopy. In addition, the seedlings may be shaded by fast-growing ground vegetation. The
influence of shelterwood and surrounding vegetation on the
light environment may affect not only growth and morphology
but also transpiration. According to Stickan and Zhang (1992),
transpiration in beech is influenced more by PPFD than by
vapor pressure deficit. A relationship between transpiration
and dry matter production (water use efficiency) can be determined for a seedling; however, it varies with environmental
conditions (Davies and Pereira 1992). Furthermore, in young
Douglas-fir seedlings, transpiration rates are influenced not
only by current environmental conditions but also by previous
growing conditions (Unterscheutz et al. 1974).
The aim of this work was to: (i) investigate the growth,
morphology and transpiration of beech (Fagus sylvatica L.)
seedlings at various PPFDs including a change in PPFD between seasons; and (ii) establish the relationship between
transpiration and growth in young beech seedlings under various light conditions and at various developmental stages of the
seedlings.
Materials and methods
Plant material
Seeds of Fagus sylvatica were collected from trees of unknown
origin at the end of September 1989, in Alnarp, southern
Sweden. All seeds were vernalized at 4 °C for 5 weeks and then
dried at 21 °C until their water content was about 5%. The
dried seeds were stored in plastic bags at 4 °C. Eight weeks
later, the seeds were soaked in tap water for 3 h and placed
under a plastic cover at 5 °C for 5 weeks. The seeds were
redried to about 5% water content and stored at 4 °C. After
8 weeks, the seeds were mixed with moist perlite and germinated at 4 °C. Seeds that germinated within 3 weeks (i.e., when
about 5 mm of the root was visible) were planted in 10-cm-diameter plastic pots containing a 3/1 (v/v) mix of fertilized
compost soil and perlite. The pots were kept at 10 °C. When
the shoot emerged from the soil, the pots were transferred to
the various experimental conditions. The pots were irrigated
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WELANDER AND OTTOSSON
alternately with demineralized water and a commercial nutrient solution (diluted to give N 15.9, P 2.2, K 8.3, Mg 1.7,
S 1.1, Fe 0.031, Mn 0.013, Cu 0.011, Zn 0.0053, B 0.016, Mo
0.00058, Co 0.00035 mM in the final solution) throughout the
experiment. Plants were irrigated once a week in the 0.7 mol
m −2 day −1 PPFD treatment, and plants in 7.3 and 14.5 mol m −2
day −1 were irrigated twice a week. In a preliminary experiment, we ascertained that the effects of the light treatments on
plant growth and development were not limited by the availability of nutrients.
Experiment 1
Newly emerged beech seedlings in pots were placed in a
controlled environment chamber providing a 16-h photoperiod
and a relative air humidity of about 80% throughout the day
(day/night; 1.69/1.25 kPa). Twenty seedlings were used for
each PPFD treatment. The PPFD treatments applied were 0.7,
7.3 or 14.5 mol m −2 day −1 corresponding to about 2, 21 and
43% relative light (total PPFD per day) under field conditions
in southern Sweden. Light was supplied from cool-white fluorescent lamps (Sylvania 215 W). The various irradiances were
attained by regulating the distance between the seedlings and
the lamps, or by using shade cloth. Irradiance (PPFD) was
measured with a quantum meter (LI-189, Li-Cor, Inc., Lincoln,
NE) at the top of the plants. The various irradiances were
measured and adjusted every second week. The variation in
PPFD among plants was about 10% more or less than the given
values. Irradiance was increased in stages during the first two
hours of the day, and was decreased in a similar way at the end
of the day. Peak irradiances during the middle 12 h of the
photoperiod were 15, 150 and 300 µmol m −2 s −1, for the 0.7,
7.3 or 14.5 mol m −2 day −1 PPFD treatments, respectively. The
temperature was 14 ± 0.2 °C during the night and rose continuously to 18 ± 0.2 °C during the first hour of light. A similar
decrease in temperature occurred at the end of the day. After 9
weeks, the light and the temperature conditions were changed
according to Figure 1 to induce dormancy. Twenty-one weeks
later, a second growing period was induced by gradually increasing day length and temperature (Figure 1). At the start of
the second growing period, seedlings from the 0.7 mol m −2
day −1 PPFD treatment were redistributed among the 0.7, 7.3
and 14.5 mol m −2 day −1 PPFD treatments. Similarly, plants
from the 7.3 and 14.5 mol m −2 day −1 PPFD treatments were
also redistributed among all of the PPFD treatments. The
experiment was ended after 62.5 weeks.
At the end of the experiment, dry mass of stem, leaves and
root were determined for each seedling. In addition, the numbers of shoots and leaves, leaf area, seedling height, and length
of the main shoot were recorded. The ratio between increase in
dry mass per seedling and leaf area (unit leaf rate; ULR), which
was calculated as a measure of the productivity of the leaves,
was determined for the second year by subtracting the dry mass
of one-year-old seedlings, determined from a sample taken
14 weeks after planting in the first year. The dry mass of the
sample did not include the dry mass of the leaves because no
leaves were present at the onset of growth in the second year.
Figure 1. (A) Day (--------) and night (• • • •) temperatures, and (B)
daylengths in Experiment 1.
Leaf dry mass/leaf area per seedling was calculated, and leaf
fresh mass per area was also determined in some treatments.
Experiment 2
Newly emerged beech seedlings in pots were grown at 0.7 and
14.5 mol m −2 day −1 (PPFD) in a 16-h photoperiod. Light,
temperature and air humidity conditions were the same as
described for the first 8-week period in Experiment 1. During
the period of leaf expansion, transpiration rates were recorded
over 24-h periods. When the recordings started, the first true
leaves were about 8 and 20 mm long in the 0.7 and 14.5 mol
m −2 day −1 PPFD treatments, respectively. Transpiration rate
was measured by weighing the seedlings and pots to the
nearest 10 mg. During the measurement period, each pot was
placed in a polythene bag that was sealed around the stem of
the seedling. At the start of the measurements, seedlings were
rewatered to approximately the same mass. Between measurements the plastic bag was removed to ensure aeration of the
roots. At the beginning and end of each measurement period,
leaf area per seedling was determined by drawing the outline
of the leaves and determining the area within the outline with
a digital polar planimeter (HAFF No. 330, Gebr. HAFF
GmbH., Pfronten, Germany). Daily transpiration rate, including transpiration in the dark, was expressed per unit leaf area.
In addition, transpiration for the entire 56-day measuring period was calculated for each seedling as the sum of the daily
transpiration rate of the measuring days and the interpolated
values for those days when no measurements were made. Ten
seedlings were used for each PPFD treatment.
Experiment 3
Transpiration rates were measured in 10 seedlings from each
of the nine treatments in Experiment 1 during two periods in
the second year. The first measurements took place when all
seedlings had finished the first flush of growth, i.e., 7 weeks
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
after bud burst. Transpiration rate was measured during three
consecutive 24-h cycles. When the second measurement period was performed a month later (Week 11), no new shoot
formation had occurred. Thus, all transpiration measurements
were performed on shoots that had been initiated in the previous year. During the second period (Week 11), the irradiance
was changed daily (see Figure 7). The procedures for transpiration measurements and leaf area determination were the
same as described for Experiment 2. After the second period
of transpiration measurements, dry mass of leaves, stem and
roots were determined. Correlations between increase in dry
mass and total transpiration per seedling over the second growing season were determined based on integrated values of
transpiration rate per unit leaf area between Weeks 0, 7 and 11
and integrated leaf area between Weeks 0, 2, 7 and 11 weeks.
Diurnal courses of transpiration were recorded in seedlings in
the 0.7, 7.3 and 14.5 mol m −2 day −1 PPFD treatments to assess
the proportion of transpiration during the light and dark periods. Measurements were performed on a balance connected to
a computer that stored the hourly values.
Statistic analysis
Analysis of variance was carried out using the SAS statistical
analysis software (SAS Institute Inc. 1988). A two-tailed t-test
(Cochran and Cox 1957) was used to test the difference between means.
135
Results
Experiment 1
Both previous-year and current-year PPFD influenced leaf,
stem and root dry mass at the end of the second growing season
(P ≤ 0.0001), and there was an interaction between the PPFDs
of the two seasons (P ≤ 0.02) (Figure 2A--C). When related to
previous-year PPFD, dry mass of all plant parts increased with
increasing PPFD. In the current season, dry mass of leaves,
stem and roots were lower in seedlings grown at 0.7 mol m −2
day −1 than in seedlings grown at 7.3 or 14.5 mol m −2 day −1
(P ≤ 0.001). In two-year-old seedlings, ULR increased with
increasing current-year PPFD (P ≤ 0.001) (Figure 2D),
whereas it decreased with increasing previous-year PPFD.
Number of leaves and leaf area per plant increased with
increasing PPFD, but there was no interaction between the
PPFDs of the two seasons (Figures 3A and 3B). Leaf area per
seedling was closely correlated with number of leaves
(r = 0.97), but less closely correlated with mean area per leaf
(r = 0.71) (data not shown). The leaf dry mass/leaf area ratio
increased with increasing current-year PPFD (P ≤ 0.001),
whereas previous-year PPFD had no consistent effect (Figure 3C). Leaf fresh mass per leaf area increased in a similar
way as leaf dry mass with increasing current-year PPFD (data
not shown).
Number of leaves per shoot was not affected by PPFD in
either growing seasons (Figure 3D). Number of shoots per
seedling was affected more by previous-year PPFD
Figure 2. Dry mass of (A) leaves, (B)
stem, (C) roots, and (D) unit leaf rate
(ULR) in two-year-old beech seedlings exposed to various PPFD treatments during the previous and
current year. Previous-year PPFDs
are indicated on the x-axis and the
current-year PPFDs are indicated by
the shading in the columns: (j) 0.7,
(striped square) 7.3, and (h) 14.5
mol m −2 day −1. The vertical bars represent ± SE; n = 20.
136
WELANDER AND OTTOSSON
Figure 3. (A) Number of leaves and
(B) leaf area per seedling, (C) leaf dry
mass/leaf area ratio, (D) number of
leaves per shoot, and (E) number of
shoots on two-year-old beech seedlings exposed to various PPFD treatments during the first and second
year. In (F), total column height indicates the length of the main shoot,
which was oblique or horizontally oriented, and the lower parts indicate
seedling height. Previous-year PPFDs
are indicated on the x-axis and currentyear PPFDs are indicated by the shading in the columns, as in Figure 2.
Vertical bars represent ± SE; n = 20.
(P ≤ 0.0001) than by current-year PPFD (P ≤ 0.05). Fewer
shoots developed in seedlings exposed to a previous-year
PPFD of 0.7 mol m −2 day −1 than in seedlings exposed to
previous-year PPFDs of 7.3 and 14.5 mol m −2 day −1. Currentyear PPFD had no consistent effect on number of shoots per
seedling (Figure 3E). Current-year and previous-year PPFD
treatments had similar effects on seedling height and length of
the main shoot as on number of shoots per seedling (P ≤ 0.01)
(Figure 3F). Seedling height was less than the length of the
main shoot as the main shoot was oblique or horizontally
orientated at all PPFDs.
Experiment 2
During the first 10 days that newly emerged seedlings were
exposed to PPFDs of 0.7 or 14.5 mol m −2 day −1, transpiration
rates per leaf area declined and then remained constant for a
further 10 days (Figure 4). Thereafter, transpiration increased
to a new rate. During the 10-day period of decline in transpiration rate, expansion growth of the first two true leaves ended.
At this time, the total leaf area per seedling comprised the
cotyledons and these first two true leaves. About 20 days later,
new leaves started to expand in seedlings in the 14.5 mol m −2
day −1 PPFD treatment. This expansion period started later than
the increase in transpiration rate. In the 0.7 mol m −2 day −1
PPFD treatment, no leaves were formed after the first true
leaves had expanded.
Close correlations were found between integrated transpiration rate and dry mass per seedling (Figure 5, r = 0.98). In the
14.5 mol m −2 day −1 PPFD treatment, there was a close correlation between leaf area per seedling and transpiration rate
over a 24-h period, both during the first 10 days (r = 0.88) and
between Days 25 and 60 (r = 0.87) (Figure 6) after seedling
emergence. However, there was a change in the relationship
between the two periods as indicated by the difference in the
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
137
Figure 6. Correlation between seedling leaf area and transpiration rate
per seedling during Days 1--10 (closed symbols) and Days 25--56
(open symbols) in seedlings grown at 0.7 (d and s) or 14.5 mol m −2
day −1 (m and n). Each symbol represents an individual seedling.
Figure 4. Effects of PPFD on transpiration rate (open symbols) and
leaf area (closed symbols) during the first 56 days after beech seedling
emergence. Standard errors of the mean are depicted by the vertical
bars, n = 10.
of line) differed between the 0.7 and 14.5 mol m −2 day −1 PPFD
treatments (P ≤ 0.001).
Experiment 3
Figure 5. Correlation between the sum of transpiration over 56 days
and dry mass in beech seedlings grown at (s) 0.7 or (n) 14.5 mol m −2
day −1. Each symbol represents an individual seedling.
slope of the regression line (P ≤ 0.001). A similar correlation
between leaf area and transpiration rate was also observed in
the 0.7 mol m −2 day −1 PPFD treatment (r = 0.85 for Days 1--10
and r = 0.66 for Days 25--60); however, the relationship (slope
Seven weeks after bud burst, transpiration rate per unit leaf
area (measured at the same PPFD as the current-year PPFD
treatment of the seedling) was compared in seedlings from all
nine combinations of previous-year and current-year PPFD
treatments. Transpiration rate increased with current-year
PPFD (P ≤ 0.001) in seedlings from previous-year PPFD
treatments of 0.7 and 7.3 mol m −2 day −1 (Figure 7A--C and
D--F, 7 weeks). In seedlings from the previous-year PPFD
treatment of 14.5 mol m −2 day −1, transpiration rates of seedlings exposed to a current-year PPFD of 7.3 or 14.5 mol m −2
day −1 were higher than transpiration rates of seedlings exposed
to a current-year PPFD of 0.7 mol m −2 day −1 (P ≤ 0.001)
(Figure 7G--I). A close correlation was found between seedling
leaf area and transpiration rate per seedling (not shown in
figure) irrespective of previous-year PPFD (r = 0.90, 0.94 and
0.87 for 0.7, 7.3 and 14.5 mol m −2 day −1).
Eleven weeks after bud burst, rates of transpiration per unit
leaf area were measured, first at the PPFD under which current-year leaf expansion took place, and second after a one-day
transfer to a contrasting PPFD. Before the one-day transfer,
rates of transpiration were similar at Weeks 7 and 11, in all
seedlings exposed to a previous-year PPFD of 0.7 mol m −2
day −1 (Figure 7A--C), whereas in seedlings exposed to a previous-year PPFD of 7.3 mol m −2 day −1, transpiration rates
declined between Weeks 7 and 11 in the current-year 7.3 or
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WELANDER AND OTTOSSON
Figure 7. Influence of various
combinations of PPFD on
transpiration rate per unit leaf
area at Weeks 7 and 11 after
bud burst in two-year-old
beech seedlings. The first column represents the median of
three consecutive days of
measurements at Week 7. The
following three columns represent three consecutive single-day measurements at
Week 11, when the PPFD
was changed daily. The previous-year PPFD was: (A--C)
0.7, (D--F) 7.3, and (G--I)
14.5 mol m −2 day −1. Currentyear PPFD was (A, D and G)
0.7, (B,E and H) 7.3, and (C,
F and I) 14.5 mol m −2 day −1.
The PPFD on the day of
measurement is indicated by
the shading: stippled = 0.7;
crosshatched = 7.3; and open
= 14.5 mol m −2 day −1. Vertical bars represent ± SE;
n = 20.
14.5 mol m −2 day −1 PPFD treatments (P ≤ 0.01) (Figure 7E--F).
A similar decline was seen in the current-year 7.3 mol m −2
day −1 PPFD treatment for seedlings exposed to a previous-year
PPFD of 14.5 mol m −2 day −1 (Figure 7G--H).
In seedlings from all nine combinations of previous-year
and current-year PPFD treatments, the rate of transpiration per
leaf area showed similar changes in relation to current-year
PPFD treatment. However, the overall rate of transpiration
recorded at a PPFD of 0.7 mol m −2 day −1 was higher in
seedlings grown in current-year PPFD of 0.7 mol m −2 day −1
(Figure 7A, D and G, shaded bars, Week 11) than in seedlings
grown in 7.3 and 14.5 mol m −2 day −1 (P ≤ 0.01) (Figure 7B, E,
H and C, F, I, shaded bars, Week 11). In addition, the overall
rate of transpiration was highest in seedlings exposed to a
previous-year PPFD of 0.7 mol m −2 day −1 (Figure 7A--C, Week
11) and lowest in seedlings exposed to a previous-year PPFD
of 14.5 mol m −2 day −1 (Figure 7G--I) (P ≤ 0.01).
Close correlations were seen between leaf area and transpiration rate per seedling on the first day of Week 11 after bud
burst in seedlings exposed to current-year PPFDs of 0.7
(r = 0.90), 7.3 (r = 0.92) and 14.5 mol m −2 day −1 (r = 0.93)
(Figure 8A--C). Close correlations were also found between
root dry mass and transpiration rate for seedlings exposed to
current-year PPFDs of 0.7 (r = 0.80), 7.3 (r = 0.82) and 14.5
mol m −2 day −1 (r = 0.79) (data not shown).
When PPFD was changed daily (one-day PPFD treatment),
the correlations between transpiration rate per seeedling and
leaf area decreased on Days 2 and 3 compared to Day 1.
Measurements at the one-day PPFD of 0.7 mol m −2 day −1
showed a weak correlation between transpiration rate and leaf
area (r = 0.45) (Figure 8D, Week 11), but close correlations
were found between leaf area and transpiration rate per seedling measured at a one-day PPFD of 7.3 (r = 0.76) or 14.5 mol
m −2 day −1 (r = 0.86) (Figures 8E and 8F), irrespective of
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
139
Figure 8. Influence of PPFD on the
correlation between leaf area and
transpiration rate during the first
day of Week 11 after bud burst in
two-year-old seedlings grown at a
PPFD of (A) 0.7, (B) 7.3 or (C)
14.5 mol m −2 day −1, and during the
second and third days of Week 11
after bud burst in two-year-old seedlings grown at a PPFD of (D) 0.7,
(E) 7.3 or (F) 14.5 mol m −2 day −1.
Each symbol represents an individual seedling.
previous-year PPFD or current-year PPFD during the leaf
expansion period. Weak correlations were found for root dry
mass and transpiration (r = 0.36, 0.79 and 0.76) when measured at the one-day PPFDs of 0.7, 7.3 and 14.5 mol m −2 day −1
on Days 2 and 3 of Week 11 (data not shown).
Discussion
Current growth, morphology and transpiration in two-year-old
beech seedlings were all affected by both previous- and current-year light conditions. For example, beech seedlings transferred from a low previous-year PPFD to a higher current-year
PPFD showed a higher ULR than seedlings maintained at the
higher PPFD for both years, indicating that ULR was affected
by both previous-year and current-year PPFD. In oak seedlings
subjected to a similar transfer (Ziegenhagen and Kausch 1995)
a decrease in productivity was found, showing that the response to a year to year change in light environment may vary
among species.
The influence by PPFD on seedling morphology showed
various patterns. Both leaf dry mass/leaf area and leaf fresh
mass/leaf area were only affected by current-year PPFD. In
contrast, Goulet and Bellefleur (1986) found that, in Fagus
grandifolia trees, the light conditions during both bud initiation in the previous year and leaf outgrowth in the current year
affected leaf fresh weight per area.
Leaf area was predetermined by the PPFD prevailing in the
year of leaf initiation and final area was modified by the PPFD
prevailing when the leaves expanded in the following year.
Several morphological parameters appear to determine the leaf
area per seedling. First, the area depends on the number of
leaves as indicated by the close correlation between these
parameters. Second, neither previous-year nor current-year
PPFD influenced the number of leaves per shoot, indicating
that the number of leaves per beech seedling is determined by
the number of shoots. In contrast, Hansen (1959) found that
buds developed in full light contained more leaf initials than
buds developed in the shade. Because the range of PPFDs we
employed was less than in the study of Hansen (1959), it is
possible that the difference between the highest and lowest
PPFDs in our study was not large enough to affect the rate of
leaf initiation significantly. Moreover, in our study, fewer
leaves developed in plants at the lowest current-year PPFD
than in plants at the highest current-year PPFD even when
seedlings had been exposed to the same previous-year PPFD,
indicating that either some leaf initials from the previous year
did not develop, or new leaves were initiated during shoot
elongation in the current year. The number of shoots that
developed in the current season was also determined more by
the PPFD of the previous year when the shoot buds were
initiated than by current-year PPFD.
Transpiration rate per unit leaf area was influenced by PPFD
directly during the transpiration period, and indirectly by the
PPFD when the leaves were expanding, and during the year
when the leaves were initiated. Although transpiration rate per
unit leaf area increased with increasing PPFD, the effect was
less marked as the leaves became older. The decrease with leaf
age was not the result of reduced light caused by increased
self-shading because no new leaves were formed during this
period. Federer (1976) reported that leaf age appears to influence the effect of light on transpiration in various deciduous
tree species.
Transpiration rate per unit leaf area also increased with
increasing PPFD, when PPFD was changed daily, and the
increase was greater in seedlings in which the leaves had
expanded in low PPFD than in medium PPFD. This finding
supports the conclusion of Turner and Heichel (1977) that
stomatal reactivity to radiation depends on the environment of
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WELANDER AND OTTOSSON
the developing leaf rather than on leaf developmental stage. In
Year 2, seedlings exposed to high previous-year PPFDs
showed lower transpiration rates per unit leaf area than seedlings exposed to low previous-year PPFDs, indicating that
some light effects are long lasting. In the year of seedling
emergence, integrated transpiration, based on transpiration
measurements at close intervals, was closely correlated to dry
mass after a 56-day period of growth under conditions of
constant irradiance. However, the correlation was less close
when integrated transpiration was based on only two measuring times in the year and when PPFD was changed between the
two growing seasons, indicating that accurate estimation of
water use efficiency under natural conditions where environmental conditions fluctuate requires repeated measurements of
transpiration at close intervals.
We have demonstrated that previous environmental conditions need to be taken into account when studying the influences
of current environmental parameters on growth, morphology
and transpiration in beech seedlings. These findings have particular relevance for comparative studies of light effects on
current growth and for the study of nursery-grown plant material under field conditions.
Acknowledgment
We thank Dr. Olof Hellgren for valuable comments on the manuscript.
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Watt, A.S. 1923. On the ecology of British beechwoods with special
reference to their regeneration. J. Ecol. 11:1--48.
Ziegenhagen, B. and W. Kausch. 1995. Productivity of young shaded
oaks (Quercus robur L.) as corresponding to shoot morphology and
leaf anatomy. For. Ecol. Manage. 72:97--108.
© 1997 Heron Publishing----Victoria, Canada
Influence of photosynthetic photon flux density on growth and
transpiration in seedlings of Fagus sylvatica
N. T. WELANDER1 and B. OTTOSSON2
1
Swedish University of Agricultural Sciences, Southern Swedish Forest Research Centre, Box 49, S-230 53 Alnarp, Sweden
2
Swedish University of Agricultural Sciences, Department of Horticultural Science, Box 55, S-230 53 Alnarp, Sweden
Received August 28, 1995
Summary Beech seedlings (Fagus sylvatica L.) were grown
in various combinations of three photosynthetic photon flux
densities (PPFD, 0.7, 7.3 or 14.5 mol m −2 day −1) for two years
in a controlled environmental chamber. Dry mass of leaves,
stem and roots, leaf area and number of leaves, and unit leaf
rate were affected by both previous-year and current-year
PPFD. Number of shoots and length of the main shoot were
affected by previous-year PPFD but not by current-year PPFD.
Number of leaves per shoot did not change with PPFD, whereas
leaf dry mass/leaf area ratio was mainly affected by currentyear PPFD. During the first 10 days that newly emerged
seedlings were grown at a PPFD of 0.7 or 14.5 mol m −2 day −1,
transpiration rate per unit leaf area declined. Thereafter, transpiration increased to a constant new rate. Transpiration rate
per seedling was closely related to leaf area but the relationship
changed with time. In two-year-old seedlings grown at various
PPFD combinations of 0.7, 7.3 and 14.5 mol m −2 day −1 during
Years 1 and 2, leaf area and transpiration rate per seedling were
closely correlated at Weeks 7 and 11 after bud burst. Weak
correlations were found between root dry mass and transpiration rate per seedling. During Year 2, transpiration rate per leaf
area was higher at a particular PPFD in seedlings grown at a
previous-year PPFD of 0.7 mol m −2 day −1 than in seedlings
grown at a previous-year PPFD of 14.5 mol m −2 day −1. After
transfer of two-year-old seedlings at the end of the experiment
to a new PPFD (7.3 or 14.5 mol m −2 day −1) for one day,
transpiration rates per leaf area, measured at the new PPFD,
were correlated with leaf area and root dry mass, irrespective
of former PPFD treatment.
Keywords: beech, irradiance, seedling morphology, shading.
Introduction
During natural regeneration, growth of beech seedlings may be
limited by the availability of light because, although seedlings
grow at irradiances as low as 1--2% of the light above the
canopy, maximum dry matter production is only reached under
unshaded conditions (Watt 1923, Suner and Röhrig 1980,
Madsen 1994). The morphology of beech seedlings, including
numbers of leaves, leaf area, branching and length of the main
shoot, is also affected by the light conditions (Burschel and
Huss 1964, Burschel and Schmaltz 1965). Seedlings in the
understory may experience variation in irradiance as a result of
changes in foliage in the upper canopy. In addition, the seedlings may be shaded by fast-growing ground vegetation. The
influence of shelterwood and surrounding vegetation on the
light environment may affect not only growth and morphology
but also transpiration. According to Stickan and Zhang (1992),
transpiration in beech is influenced more by PPFD than by
vapor pressure deficit. A relationship between transpiration
and dry matter production (water use efficiency) can be determined for a seedling; however, it varies with environmental
conditions (Davies and Pereira 1992). Furthermore, in young
Douglas-fir seedlings, transpiration rates are influenced not
only by current environmental conditions but also by previous
growing conditions (Unterscheutz et al. 1974).
The aim of this work was to: (i) investigate the growth,
morphology and transpiration of beech (Fagus sylvatica L.)
seedlings at various PPFDs including a change in PPFD between seasons; and (ii) establish the relationship between
transpiration and growth in young beech seedlings under various light conditions and at various developmental stages of the
seedlings.
Materials and methods
Plant material
Seeds of Fagus sylvatica were collected from trees of unknown
origin at the end of September 1989, in Alnarp, southern
Sweden. All seeds were vernalized at 4 °C for 5 weeks and then
dried at 21 °C until their water content was about 5%. The
dried seeds were stored in plastic bags at 4 °C. Eight weeks
later, the seeds were soaked in tap water for 3 h and placed
under a plastic cover at 5 °C for 5 weeks. The seeds were
redried to about 5% water content and stored at 4 °C. After
8 weeks, the seeds were mixed with moist perlite and germinated at 4 °C. Seeds that germinated within 3 weeks (i.e., when
about 5 mm of the root was visible) were planted in 10-cm-diameter plastic pots containing a 3/1 (v/v) mix of fertilized
compost soil and perlite. The pots were kept at 10 °C. When
the shoot emerged from the soil, the pots were transferred to
the various experimental conditions. The pots were irrigated
134
WELANDER AND OTTOSSON
alternately with demineralized water and a commercial nutrient solution (diluted to give N 15.9, P 2.2, K 8.3, Mg 1.7,
S 1.1, Fe 0.031, Mn 0.013, Cu 0.011, Zn 0.0053, B 0.016, Mo
0.00058, Co 0.00035 mM in the final solution) throughout the
experiment. Plants were irrigated once a week in the 0.7 mol
m −2 day −1 PPFD treatment, and plants in 7.3 and 14.5 mol m −2
day −1 were irrigated twice a week. In a preliminary experiment, we ascertained that the effects of the light treatments on
plant growth and development were not limited by the availability of nutrients.
Experiment 1
Newly emerged beech seedlings in pots were placed in a
controlled environment chamber providing a 16-h photoperiod
and a relative air humidity of about 80% throughout the day
(day/night; 1.69/1.25 kPa). Twenty seedlings were used for
each PPFD treatment. The PPFD treatments applied were 0.7,
7.3 or 14.5 mol m −2 day −1 corresponding to about 2, 21 and
43% relative light (total PPFD per day) under field conditions
in southern Sweden. Light was supplied from cool-white fluorescent lamps (Sylvania 215 W). The various irradiances were
attained by regulating the distance between the seedlings and
the lamps, or by using shade cloth. Irradiance (PPFD) was
measured with a quantum meter (LI-189, Li-Cor, Inc., Lincoln,
NE) at the top of the plants. The various irradiances were
measured and adjusted every second week. The variation in
PPFD among plants was about 10% more or less than the given
values. Irradiance was increased in stages during the first two
hours of the day, and was decreased in a similar way at the end
of the day. Peak irradiances during the middle 12 h of the
photoperiod were 15, 150 and 300 µmol m −2 s −1, for the 0.7,
7.3 or 14.5 mol m −2 day −1 PPFD treatments, respectively. The
temperature was 14 ± 0.2 °C during the night and rose continuously to 18 ± 0.2 °C during the first hour of light. A similar
decrease in temperature occurred at the end of the day. After 9
weeks, the light and the temperature conditions were changed
according to Figure 1 to induce dormancy. Twenty-one weeks
later, a second growing period was induced by gradually increasing day length and temperature (Figure 1). At the start of
the second growing period, seedlings from the 0.7 mol m −2
day −1 PPFD treatment were redistributed among the 0.7, 7.3
and 14.5 mol m −2 day −1 PPFD treatments. Similarly, plants
from the 7.3 and 14.5 mol m −2 day −1 PPFD treatments were
also redistributed among all of the PPFD treatments. The
experiment was ended after 62.5 weeks.
At the end of the experiment, dry mass of stem, leaves and
root were determined for each seedling. In addition, the numbers of shoots and leaves, leaf area, seedling height, and length
of the main shoot were recorded. The ratio between increase in
dry mass per seedling and leaf area (unit leaf rate; ULR), which
was calculated as a measure of the productivity of the leaves,
was determined for the second year by subtracting the dry mass
of one-year-old seedlings, determined from a sample taken
14 weeks after planting in the first year. The dry mass of the
sample did not include the dry mass of the leaves because no
leaves were present at the onset of growth in the second year.
Figure 1. (A) Day (--------) and night (• • • •) temperatures, and (B)
daylengths in Experiment 1.
Leaf dry mass/leaf area per seedling was calculated, and leaf
fresh mass per area was also determined in some treatments.
Experiment 2
Newly emerged beech seedlings in pots were grown at 0.7 and
14.5 mol m −2 day −1 (PPFD) in a 16-h photoperiod. Light,
temperature and air humidity conditions were the same as
described for the first 8-week period in Experiment 1. During
the period of leaf expansion, transpiration rates were recorded
over 24-h periods. When the recordings started, the first true
leaves were about 8 and 20 mm long in the 0.7 and 14.5 mol
m −2 day −1 PPFD treatments, respectively. Transpiration rate
was measured by weighing the seedlings and pots to the
nearest 10 mg. During the measurement period, each pot was
placed in a polythene bag that was sealed around the stem of
the seedling. At the start of the measurements, seedlings were
rewatered to approximately the same mass. Between measurements the plastic bag was removed to ensure aeration of the
roots. At the beginning and end of each measurement period,
leaf area per seedling was determined by drawing the outline
of the leaves and determining the area within the outline with
a digital polar planimeter (HAFF No. 330, Gebr. HAFF
GmbH., Pfronten, Germany). Daily transpiration rate, including transpiration in the dark, was expressed per unit leaf area.
In addition, transpiration for the entire 56-day measuring period was calculated for each seedling as the sum of the daily
transpiration rate of the measuring days and the interpolated
values for those days when no measurements were made. Ten
seedlings were used for each PPFD treatment.
Experiment 3
Transpiration rates were measured in 10 seedlings from each
of the nine treatments in Experiment 1 during two periods in
the second year. The first measurements took place when all
seedlings had finished the first flush of growth, i.e., 7 weeks
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
after bud burst. Transpiration rate was measured during three
consecutive 24-h cycles. When the second measurement period was performed a month later (Week 11), no new shoot
formation had occurred. Thus, all transpiration measurements
were performed on shoots that had been initiated in the previous year. During the second period (Week 11), the irradiance
was changed daily (see Figure 7). The procedures for transpiration measurements and leaf area determination were the
same as described for Experiment 2. After the second period
of transpiration measurements, dry mass of leaves, stem and
roots were determined. Correlations between increase in dry
mass and total transpiration per seedling over the second growing season were determined based on integrated values of
transpiration rate per unit leaf area between Weeks 0, 7 and 11
and integrated leaf area between Weeks 0, 2, 7 and 11 weeks.
Diurnal courses of transpiration were recorded in seedlings in
the 0.7, 7.3 and 14.5 mol m −2 day −1 PPFD treatments to assess
the proportion of transpiration during the light and dark periods. Measurements were performed on a balance connected to
a computer that stored the hourly values.
Statistic analysis
Analysis of variance was carried out using the SAS statistical
analysis software (SAS Institute Inc. 1988). A two-tailed t-test
(Cochran and Cox 1957) was used to test the difference between means.
135
Results
Experiment 1
Both previous-year and current-year PPFD influenced leaf,
stem and root dry mass at the end of the second growing season
(P ≤ 0.0001), and there was an interaction between the PPFDs
of the two seasons (P ≤ 0.02) (Figure 2A--C). When related to
previous-year PPFD, dry mass of all plant parts increased with
increasing PPFD. In the current season, dry mass of leaves,
stem and roots were lower in seedlings grown at 0.7 mol m −2
day −1 than in seedlings grown at 7.3 or 14.5 mol m −2 day −1
(P ≤ 0.001). In two-year-old seedlings, ULR increased with
increasing current-year PPFD (P ≤ 0.001) (Figure 2D),
whereas it decreased with increasing previous-year PPFD.
Number of leaves and leaf area per plant increased with
increasing PPFD, but there was no interaction between the
PPFDs of the two seasons (Figures 3A and 3B). Leaf area per
seedling was closely correlated with number of leaves
(r = 0.97), but less closely correlated with mean area per leaf
(r = 0.71) (data not shown). The leaf dry mass/leaf area ratio
increased with increasing current-year PPFD (P ≤ 0.001),
whereas previous-year PPFD had no consistent effect (Figure 3C). Leaf fresh mass per leaf area increased in a similar
way as leaf dry mass with increasing current-year PPFD (data
not shown).
Number of leaves per shoot was not affected by PPFD in
either growing seasons (Figure 3D). Number of shoots per
seedling was affected more by previous-year PPFD
Figure 2. Dry mass of (A) leaves, (B)
stem, (C) roots, and (D) unit leaf rate
(ULR) in two-year-old beech seedlings exposed to various PPFD treatments during the previous and
current year. Previous-year PPFDs
are indicated on the x-axis and the
current-year PPFDs are indicated by
the shading in the columns: (j) 0.7,
(striped square) 7.3, and (h) 14.5
mol m −2 day −1. The vertical bars represent ± SE; n = 20.
136
WELANDER AND OTTOSSON
Figure 3. (A) Number of leaves and
(B) leaf area per seedling, (C) leaf dry
mass/leaf area ratio, (D) number of
leaves per shoot, and (E) number of
shoots on two-year-old beech seedlings exposed to various PPFD treatments during the first and second
year. In (F), total column height indicates the length of the main shoot,
which was oblique or horizontally oriented, and the lower parts indicate
seedling height. Previous-year PPFDs
are indicated on the x-axis and currentyear PPFDs are indicated by the shading in the columns, as in Figure 2.
Vertical bars represent ± SE; n = 20.
(P ≤ 0.0001) than by current-year PPFD (P ≤ 0.05). Fewer
shoots developed in seedlings exposed to a previous-year
PPFD of 0.7 mol m −2 day −1 than in seedlings exposed to
previous-year PPFDs of 7.3 and 14.5 mol m −2 day −1. Currentyear PPFD had no consistent effect on number of shoots per
seedling (Figure 3E). Current-year and previous-year PPFD
treatments had similar effects on seedling height and length of
the main shoot as on number of shoots per seedling (P ≤ 0.01)
(Figure 3F). Seedling height was less than the length of the
main shoot as the main shoot was oblique or horizontally
orientated at all PPFDs.
Experiment 2
During the first 10 days that newly emerged seedlings were
exposed to PPFDs of 0.7 or 14.5 mol m −2 day −1, transpiration
rates per leaf area declined and then remained constant for a
further 10 days (Figure 4). Thereafter, transpiration increased
to a new rate. During the 10-day period of decline in transpiration rate, expansion growth of the first two true leaves ended.
At this time, the total leaf area per seedling comprised the
cotyledons and these first two true leaves. About 20 days later,
new leaves started to expand in seedlings in the 14.5 mol m −2
day −1 PPFD treatment. This expansion period started later than
the increase in transpiration rate. In the 0.7 mol m −2 day −1
PPFD treatment, no leaves were formed after the first true
leaves had expanded.
Close correlations were found between integrated transpiration rate and dry mass per seedling (Figure 5, r = 0.98). In the
14.5 mol m −2 day −1 PPFD treatment, there was a close correlation between leaf area per seedling and transpiration rate
over a 24-h period, both during the first 10 days (r = 0.88) and
between Days 25 and 60 (r = 0.87) (Figure 6) after seedling
emergence. However, there was a change in the relationship
between the two periods as indicated by the difference in the
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
137
Figure 6. Correlation between seedling leaf area and transpiration rate
per seedling during Days 1--10 (closed symbols) and Days 25--56
(open symbols) in seedlings grown at 0.7 (d and s) or 14.5 mol m −2
day −1 (m and n). Each symbol represents an individual seedling.
Figure 4. Effects of PPFD on transpiration rate (open symbols) and
leaf area (closed symbols) during the first 56 days after beech seedling
emergence. Standard errors of the mean are depicted by the vertical
bars, n = 10.
of line) differed between the 0.7 and 14.5 mol m −2 day −1 PPFD
treatments (P ≤ 0.001).
Experiment 3
Figure 5. Correlation between the sum of transpiration over 56 days
and dry mass in beech seedlings grown at (s) 0.7 or (n) 14.5 mol m −2
day −1. Each symbol represents an individual seedling.
slope of the regression line (P ≤ 0.001). A similar correlation
between leaf area and transpiration rate was also observed in
the 0.7 mol m −2 day −1 PPFD treatment (r = 0.85 for Days 1--10
and r = 0.66 for Days 25--60); however, the relationship (slope
Seven weeks after bud burst, transpiration rate per unit leaf
area (measured at the same PPFD as the current-year PPFD
treatment of the seedling) was compared in seedlings from all
nine combinations of previous-year and current-year PPFD
treatments. Transpiration rate increased with current-year
PPFD (P ≤ 0.001) in seedlings from previous-year PPFD
treatments of 0.7 and 7.3 mol m −2 day −1 (Figure 7A--C and
D--F, 7 weeks). In seedlings from the previous-year PPFD
treatment of 14.5 mol m −2 day −1, transpiration rates of seedlings exposed to a current-year PPFD of 7.3 or 14.5 mol m −2
day −1 were higher than transpiration rates of seedlings exposed
to a current-year PPFD of 0.7 mol m −2 day −1 (P ≤ 0.001)
(Figure 7G--I). A close correlation was found between seedling
leaf area and transpiration rate per seedling (not shown in
figure) irrespective of previous-year PPFD (r = 0.90, 0.94 and
0.87 for 0.7, 7.3 and 14.5 mol m −2 day −1).
Eleven weeks after bud burst, rates of transpiration per unit
leaf area were measured, first at the PPFD under which current-year leaf expansion took place, and second after a one-day
transfer to a contrasting PPFD. Before the one-day transfer,
rates of transpiration were similar at Weeks 7 and 11, in all
seedlings exposed to a previous-year PPFD of 0.7 mol m −2
day −1 (Figure 7A--C), whereas in seedlings exposed to a previous-year PPFD of 7.3 mol m −2 day −1, transpiration rates
declined between Weeks 7 and 11 in the current-year 7.3 or
138
WELANDER AND OTTOSSON
Figure 7. Influence of various
combinations of PPFD on
transpiration rate per unit leaf
area at Weeks 7 and 11 after
bud burst in two-year-old
beech seedlings. The first column represents the median of
three consecutive days of
measurements at Week 7. The
following three columns represent three consecutive single-day measurements at
Week 11, when the PPFD
was changed daily. The previous-year PPFD was: (A--C)
0.7, (D--F) 7.3, and (G--I)
14.5 mol m −2 day −1. Currentyear PPFD was (A, D and G)
0.7, (B,E and H) 7.3, and (C,
F and I) 14.5 mol m −2 day −1.
The PPFD on the day of
measurement is indicated by
the shading: stippled = 0.7;
crosshatched = 7.3; and open
= 14.5 mol m −2 day −1. Vertical bars represent ± SE;
n = 20.
14.5 mol m −2 day −1 PPFD treatments (P ≤ 0.01) (Figure 7E--F).
A similar decline was seen in the current-year 7.3 mol m −2
day −1 PPFD treatment for seedlings exposed to a previous-year
PPFD of 14.5 mol m −2 day −1 (Figure 7G--H).
In seedlings from all nine combinations of previous-year
and current-year PPFD treatments, the rate of transpiration per
leaf area showed similar changes in relation to current-year
PPFD treatment. However, the overall rate of transpiration
recorded at a PPFD of 0.7 mol m −2 day −1 was higher in
seedlings grown in current-year PPFD of 0.7 mol m −2 day −1
(Figure 7A, D and G, shaded bars, Week 11) than in seedlings
grown in 7.3 and 14.5 mol m −2 day −1 (P ≤ 0.01) (Figure 7B, E,
H and C, F, I, shaded bars, Week 11). In addition, the overall
rate of transpiration was highest in seedlings exposed to a
previous-year PPFD of 0.7 mol m −2 day −1 (Figure 7A--C, Week
11) and lowest in seedlings exposed to a previous-year PPFD
of 14.5 mol m −2 day −1 (Figure 7G--I) (P ≤ 0.01).
Close correlations were seen between leaf area and transpiration rate per seedling on the first day of Week 11 after bud
burst in seedlings exposed to current-year PPFDs of 0.7
(r = 0.90), 7.3 (r = 0.92) and 14.5 mol m −2 day −1 (r = 0.93)
(Figure 8A--C). Close correlations were also found between
root dry mass and transpiration rate for seedlings exposed to
current-year PPFDs of 0.7 (r = 0.80), 7.3 (r = 0.82) and 14.5
mol m −2 day −1 (r = 0.79) (data not shown).
When PPFD was changed daily (one-day PPFD treatment),
the correlations between transpiration rate per seeedling and
leaf area decreased on Days 2 and 3 compared to Day 1.
Measurements at the one-day PPFD of 0.7 mol m −2 day −1
showed a weak correlation between transpiration rate and leaf
area (r = 0.45) (Figure 8D, Week 11), but close correlations
were found between leaf area and transpiration rate per seedling measured at a one-day PPFD of 7.3 (r = 0.76) or 14.5 mol
m −2 day −1 (r = 0.86) (Figures 8E and 8F), irrespective of
PHOTON FLUX DENSITY AND GROWTH IN FAGUS
139
Figure 8. Influence of PPFD on the
correlation between leaf area and
transpiration rate during the first
day of Week 11 after bud burst in
two-year-old seedlings grown at a
PPFD of (A) 0.7, (B) 7.3 or (C)
14.5 mol m −2 day −1, and during the
second and third days of Week 11
after bud burst in two-year-old seedlings grown at a PPFD of (D) 0.7,
(E) 7.3 or (F) 14.5 mol m −2 day −1.
Each symbol represents an individual seedling.
previous-year PPFD or current-year PPFD during the leaf
expansion period. Weak correlations were found for root dry
mass and transpiration (r = 0.36, 0.79 and 0.76) when measured at the one-day PPFDs of 0.7, 7.3 and 14.5 mol m −2 day −1
on Days 2 and 3 of Week 11 (data not shown).
Discussion
Current growth, morphology and transpiration in two-year-old
beech seedlings were all affected by both previous- and current-year light conditions. For example, beech seedlings transferred from a low previous-year PPFD to a higher current-year
PPFD showed a higher ULR than seedlings maintained at the
higher PPFD for both years, indicating that ULR was affected
by both previous-year and current-year PPFD. In oak seedlings
subjected to a similar transfer (Ziegenhagen and Kausch 1995)
a decrease in productivity was found, showing that the response to a year to year change in light environment may vary
among species.
The influence by PPFD on seedling morphology showed
various patterns. Both leaf dry mass/leaf area and leaf fresh
mass/leaf area were only affected by current-year PPFD. In
contrast, Goulet and Bellefleur (1986) found that, in Fagus
grandifolia trees, the light conditions during both bud initiation in the previous year and leaf outgrowth in the current year
affected leaf fresh weight per area.
Leaf area was predetermined by the PPFD prevailing in the
year of leaf initiation and final area was modified by the PPFD
prevailing when the leaves expanded in the following year.
Several morphological parameters appear to determine the leaf
area per seedling. First, the area depends on the number of
leaves as indicated by the close correlation between these
parameters. Second, neither previous-year nor current-year
PPFD influenced the number of leaves per shoot, indicating
that the number of leaves per beech seedling is determined by
the number of shoots. In contrast, Hansen (1959) found that
buds developed in full light contained more leaf initials than
buds developed in the shade. Because the range of PPFDs we
employed was less than in the study of Hansen (1959), it is
possible that the difference between the highest and lowest
PPFDs in our study was not large enough to affect the rate of
leaf initiation significantly. Moreover, in our study, fewer
leaves developed in plants at the lowest current-year PPFD
than in plants at the highest current-year PPFD even when
seedlings had been exposed to the same previous-year PPFD,
indicating that either some leaf initials from the previous year
did not develop, or new leaves were initiated during shoot
elongation in the current year. The number of shoots that
developed in the current season was also determined more by
the PPFD of the previous year when the shoot buds were
initiated than by current-year PPFD.
Transpiration rate per unit leaf area was influenced by PPFD
directly during the transpiration period, and indirectly by the
PPFD when the leaves were expanding, and during the year
when the leaves were initiated. Although transpiration rate per
unit leaf area increased with increasing PPFD, the effect was
less marked as the leaves became older. The decrease with leaf
age was not the result of reduced light caused by increased
self-shading because no new leaves were formed during this
period. Federer (1976) reported that leaf age appears to influence the effect of light on transpiration in various deciduous
tree species.
Transpiration rate per unit leaf area also increased with
increasing PPFD, when PPFD was changed daily, and the
increase was greater in seedlings in which the leaves had
expanded in low PPFD than in medium PPFD. This finding
supports the conclusion of Turner and Heichel (1977) that
stomatal reactivity to radiation depends on the environment of
140
WELANDER AND OTTOSSON
the developing leaf rather than on leaf developmental stage. In
Year 2, seedlings exposed to high previous-year PPFDs
showed lower transpiration rates per unit leaf area than seedlings exposed to low previous-year PPFDs, indicating that
some light effects are long lasting. In the year of seedling
emergence, integrated transpiration, based on transpiration
measurements at close intervals, was closely correlated to dry
mass after a 56-day period of growth under conditions of
constant irradiance. However, the correlation was less close
when integrated transpiration was based on only two measuring times in the year and when PPFD was changed between the
two growing seasons, indicating that accurate estimation of
water use efficiency under natural conditions where environmental conditions fluctuate requires repeated measurements of
transpiration at close intervals.
We have demonstrated that previous environmental conditions need to be taken into account when studying the influences
of current environmental parameters on growth, morphology
and transpiration in beech seedlings. These findings have particular relevance for comparative studies of light effects on
current growth and for the study of nursery-grown plant material under field conditions.
Acknowledgment
We thank Dr. Olof Hellgren for valuable comments on the manuscript.
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