Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 17, 125--132
© 1997 Heron Publishing----Victoria, Canada
Effects of light quality on growth and N accumulation in birch
seedlings
P. J. APHALO1,2 and T. LEHTO1,2
1
2
Finnish Forest Research Institute, Suonenjoki Research Station, 77600 Suonenjoki, Finland
Current address: University of Joensuu, Faculty of Forestry, P.O. Box 111, 80101 Joensuu, Finland
Received July 10, 1995
Summary We studied the effects of light quality and nutrient
supply on growth and nitrogen accumulation in silver birch
(Betula pendula Roth) seedlings to test three hypotheses: (1)
growth of birch seedlings is sensitive to changes in light
quality; (2) the response of birch seedling growth to light
quality depends on nutrient supply; and (3) assimilation and
allocation of nitrogen by birch seedlings are affected by light
quality. The two light regimes simulated the spectral quality of
sunlight and shadelight, but did not differ in photosynthetic
photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of mineral
nutrients.
Accumulation and allocation of dry weight and nitrogen
were strongly affected by nutrient supply regime, but light
quality had little effect. During the first 15 days of the experiment, the largest effect of light quality was on height growth,
which was greater in seedlings in simulated shadelight than in
seedlings in simulated sunlight. Light quality had little effect
on dry weight and nitrogen allocation to the stem during this
period. However, at the end of the experiment (Day 29), there
was an increase in N concentration per unit dry weight in
leaves and stems of seedlings in the simulated shadelight plus
high nutrient supply treatment.
Keywords: allocation, assimilation, nitrogen, nutrient supply,
photomorphogenesis, shadelight, sunlight.
Introduction
Because plants both modify their light environment and respond to it, a detailed understanding of plant responses to light
is needed to elucidate the mechanisms underlying the morphological and physiological processes in canopies and plant communities (e.g., Ballaré 1994, Aphalo and Ballaré 1995).
Shade-avoiding species such as silver birch (Betula pendula
Roth) (Atkinson 1992) tend to be more responsive to changes
in light quality than shade-tolerant species (Smith 1994).
In many species, the perception of low R/FR ratios by
phytochrome triggers several responses, including increased
stem elongation, reduced leaf/stem dry weight ratio, increased
shoot/root dry weight ratio, and reduced photosynthetic capacity, which are presumably adaptive for plants growing in dense
populations (e.g., Smith 1994, Barreiro et al. 1992). Blue light
also triggers several photomorphogenic responses, including
partial inhibition of hypocotyl elongation. Moreover, phytochrome-mediated responses and those dependent on blue light
often interact (Mohr 1994).
Most research on plant photomorphogenesis has centered on
the effects of light quality, e.g., R/FR, on carbon assimilation
and partititoning. This has been a useful approach for studying
agricultural crops and their weeds, which usually grow in rich
fertilized soils. However, tree seedlings from boreal forests
grow in nutrient-poor soils where nutrient supply is often a
growth-limiting factor. Under these conditions, a study of plant
photomorphogenesis that focuses on the effects of light quality
on the acquisition and partitioning of mineral nutrients is likely
to be rewarding. Depression of plant growth and a concurrent
increase in allocation of dry matter to roots (Charles-Edwards
et al. 1986, Levin et al. 1989), and inhibition of leaf area
expansion (McDonald et al. 1992, Taylor et al. 1993) are well
known responses to low nutrient supply. However, there are
few reports on the interactions of light quality and mineral
nutrition. Root growth was depressed by end-of-day FR in
Glycine max (L.) (Kasperbauer et al. 1984). The activity of
enzymes involved in N assimilation is enhanced by red light
(R) (nitrate and nitrite reductase in several species (Johnson
1990, Mohr et al. 1992) and glutamine synthetase in Pinus
sylvestris (L.) (Elmlinger and Mohr 1992)). These effects
could alter the mineral nutrient economy of a plant in response
to shading by its neighbors (see Aphalo and Ballaré 1995).
The objective of this study was to test three hypotheses: (1)
growth of birch seedlings is sensitive to changes in light
quality; (2) the response of birch seedling growth to light
quality depends on nutrient supply; and (3) assimilation and
allocation of nitrogen by birch seedlings are affected by light
quality. We conducted a 2 × 2 factorial experiment, with light
quality and mineral nutrient supply as the experimental factors. The two light regimes simulated the spectral quality of
sunlight and shadelight, but did not differ in photosynthetic
photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of
mineral nutrients. Data from sequential harvests were used for
growth analysis.
126
APHALO AND LEHTO
Materials and methods
Plants
Betula pendula Roth (B. verrucosa Ehr.) seeds were germinated on sand in a greenhouse. The seeds were from a seed
orchard established with material from different Finnish sites
between 60°40′ N and 63°30′ N. When the seedlings had one
or two true leaves visible, they were transplanted to Ray-Leach
cells (165 cm3, SC-10 super cell, Stuewe and Sons, Corvallis,
OR) containing a mix of sand and unfertilized peat (1/2, v/v).
Fifteen days before the first harvest, all seedlings were watered
once with a balanced nutrient solution containing 50 mg N
dm −3. Nine days before the first harvest, the seedlings were
placed in a growth chamber that provided a day/night air
temperature of 20/15 °C, and a day/night water vapor saturation deficit of 5.8/3.5 mmol mol −1 (relative humidity 75/80%).
Treatments
The light quality treatments began on the day the seedlings
were transferred to the growth chamber. The light source in the
chamber consisted of 10 metal halide discharge lamps (HRI-T
250 W, Radium, Germany), supplemented with six incandescent lamps (25 W). The photoperiod was 20 h, with the lamps
switched on in three incremental steps (4 metal halide + 2
incandescent lamps, 7 + 4, and 10 + 6) during the first 2 h of
the photoperiod, and switched off in reverse order during the
last 2 h. With all lamps on, the photosynthetic photon flux
density (I) at the top of the plants was 225 µmol m −2 s −1
(measured with an LI-190SB quantum sensor, Li-Cor, Inc.,
Lincoln, NE). Taking into account the steps, the daily photon
flux was 14.7 mol m −2 day −1, which is approximately 35% of
the mean natural PAR in central Finland in May--July (Finnish
Meteorological Institute 1993).
Light quality was modified with filters located approximately 5 cm above the top of the seedlings (height adjusted
weekly). The frame holding the filters fitted closely against the
walls of the growth chamber, and although no vertical filters or
barriers were used, border plants were used to prevent interference by stray light. Two kinds of filters were used: neutral
(clear polyethylene film plus silver black scrim, Strand filter
No. 270, Strand Lighting Ltd., Isleworth, UK) and green (moss
green, Strand filter No. 422). The I under these filters differed
by less than 1%, but the red (655--665 nm) to far-red (725-735 nm) photon flux ratio (R/FR) was 2.1 for the neutral filter
(simulated sunlight, SU) and 0.5 for the green filter (simulated
shadelight, SH). The photon flux density of blue light (450 nm)
under the green filter was 20% of that under the neutral filter.
Spectra are shown in Figure 1.
Nutrient treatments were started on the day of the first
harvest (Day 0). Nutrients were supplied at two rates, but the
proportions of the different elements were kept constant. A
complete nutrient solution balanced for silver birch (Ingestad
1970) was provided at a rate of 0.86 mg N day −1 plant −1 for the
high nutrient supply treatment (HN), and at a rate of 0.17 mg
N day −1 plant −1 for the low nutrient supply treatment (LN).
The solutions were prepared by dilution of equal amounts of
two stock solutions. Solution A contained (per dm3): K2SO4
Figure 1. Spectral photon flux density of the simulated sunlight (broken line) and shadelight (solid line), normalized to equal photosynthetic photon flux density (I = 225 µmol m −2 s −1).
24.48 g, K2HPO4 16.81 g, KH2PO4 15.44 g, KNO3 24.62 g,
NH4NO3 110.0 g. Solution B contained (per dm3):
Ca(NO3)2.4H20 20.68 g, Mg(NO3)2.6H2O 44.88 g, micronutrients stock solution 25 cm3, HNO3 (70%) 1.8 g. The micronutrient stock solution contained (per dm3): Fe(NO3)3.9H2O
101.28 g, Mn(NO3)2.4H2O 36.56 g, Zn(NO3)2.4H2O 4.798 g,
CuCl2.2H2O 1.610 g, Na2MoO4.2H2O 0.353 g, H3BO3 22.88 g,
HNO3 (70%) 19.8 g.
Measurements and chemical analysis
Leaf area was measured with a leaf area meter (LI-3000,
Li-Cor, Inc., Lincoln, NE) and stem height was measured to
the nearest mm. Each seedling harvested was divided into leaf
blades, stem plus petioles, and roots, and the plant parts were
dried at 65 °C for 48 h before weighing. Each plant part was
ground separately, but the four replicates were combined. For
the larger plants, the samples were first coarsely ground in a
mill, subsampled, and then reground in a mortar with a pestle.
Nitrogen concentration was determined with an automatic
analyzer (Model CHN-9000, Leco Co., St. Joseph, MI).
Chlorophyll concentration was estimated in vivo from transmittance spectra measured in the second youngest fully expanded leaf with a spectroradiometer equipped with an
integrating sphere (Li-Cor LI-1800). The in vivo transmittance
at 660 nm was calibrated against chlorophyll concentration per
unit leaf area, measured in N,N-dimethylformamide extracts
from leaf discs of birch seedlings using the equations of Inskeep and Bloom (1985). The R2 of the polynomial regression
obtained was 0.98 (n = 32). The emission spectra of the light
sources used was measured with the same spectroradiometer.
Experimental design, growth analysis calculations and
statistical methods
The chamber was divided into four regions representing four
blocks (true replicates); each region contained four groups of
seedlings that were assigned at random to the treatments. Rows
of border plants were used to reduce interference between
treatments. The plants harvested at each date were selected at
random, and the border seedlings were moved to keep the
density of the experimental plants uniform in space. When the
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
127
number of remaining plants was small enough to allow a new
spatially uniform arrangement, the seedlings were arranged at
a lower density to reduce shading (the maximum leaf area
index was 2.7 during the experiment).
The functional growth analysis method was used. Functions
were fitted to the harvest means of the measured variables with
time as the independent variable. The fitted functions were
exponential polynomials. Relative growth rate (Rw) and relative N accumulation rate (RN ) were calculated as the derivatives of the regressions for dry weight (W) and nitrogen content
(N) (see Table 2):
^y′ = b + 2b t,
1
2
(1)
and confidence limits as:
^y′ ± t × sd(^y′),
α/2
(2)
where:
sd(y^′) = √
Var(b1) + 4t2Var(b 2) + 4tCov(b1,b2) .
(3)
Multivariate or univariate ANOVA was used to test the
significance of differences between treatments. Height, leaf
area, dry weight, and nitrogen content values and allometric
ratios were ln-transformed before analyses or calculations, and
are presented in the figures on a log scale.
Results
Growth
Plant dry weight (W ) increased throughout the experiment, but
the relative rate of increase was smaller during the later stages
than during the first 15 days of the experiment. At the final
harvest on Day 29, W was between 12 and 18 times as much
as on Day 0, depending on the treatments (Figure 2a). Shadelight (SH) had no effect on final W (P = 0.59), but the low
nutrient supply rate (LN) decreased final W by 33% (P <
0.001), and there was a small but significant interaction (P =
0.029).
The relative rate of leaf area expansion decreased throughout the experimental period in all treatments (Figure 2b). Final
leaf area (L) was 10 and 18 times as much as initial L for the
LN- and HN-treated seedlings, respectively. Compared with
final L of plants in the HN treatment, LN resulted in a 45%
decrease in final L (P < 0.001), whereas shadelight had no
effect on L (P > 0.20).
In the SH treatment, stem height (H) was initially increased
by about 40%, but later in the study, the stem elongation rate
was greater in SU-treated seedlings than in SH-treated seedlings (Figure 3a). By the end of the experiment, H was similar
in both light quality treatments (P > 0.20 for main effect and
P = 0.086 for the interaction). In contrast, H was decreased by
LN so that LN-treated plants were shorter than HN-treated
plants at the final harvest (P < 0.001). A plot of H versus W
(Figure 3b) shows that the effect of LN is mostly explained by
Figure 2. Dry weight and morphological attributes of silver birch
seedlings. (a) Whole-plant dry weight (W ) and (b) whole-plant leaf
area (L) versus time. Low nutrient supply, LN (s,d); high nutrient
supply, HN (h,j); simulated sunlight, SU (s,h); simulated shadelight, SH (d,j).
its effect on total plant size, whereas the effect of light quality
is not.
Nitrogen content of the seedlings increased more slowly
than W. On Day 2, the N content was 2.4 mg per seedling.
Because the final N content was 4.2 and 9.5 times as much as
the initial content in LN- and HN-treated seedlings, respectively, the final N content of LN-treated seedlings was only
44% of that of HN-treated seedlings (P < 0.001). Nitrogen
content was only increased 3.4% by the SH treatment
(P = 0.056). When plotted against W, differences in N content
in response to LN remained large (data not shown).
Because N content increased at a slower rate than W accumulation, N concentration decreased during the course of the
experiment. On Day 2, N concentrations in leaves per unit dry
weight and per unit leaf area were 31 mg g −1 and 0.105 mg
cm −2, respectively. Leaf N concentration per unit leaf area also
decreased throughout the experiment, and this decrease was
larger in LN-treated seedlings than in HN-treated seedlings
(~50% in LN and ~25% in HN). At the time of the final harvest
(Day 29), N concentrations in leaves and stems of LN-treated
seedlings were only half those in HN-treated seedlings,
whereas N concentration in the roots was less affected by
nutrient supply (Table 1). There was no effect of light quality
on N concentration in roots. However, in leaves and stems, the
effect of light quality on N concentration depended on the
nutrient supply. In the HN-treated seedlings, the SH treatment
increased N concentration (P = 0.013 leaf, P= 0.052 stem),
whereas in the LN-treated seedlings, SH treatment slightly
decreased N concentration (P = 0.054 leaf, P = 0.100 stem).
128
APHALO AND LEHTO
Figure 3. Stem growth of silver
birch seedlings. (a) Stem height
versus time from the start of the
nutrient supply treatments, (b)
stem height versus total plant dry
weight (W ), (c) specific stem
length (SSL) versus time and (d)
SSL versus total plant dry weight
(W ). Low nutrient supply, LN
(s,d); high nutrient supply, HN
(h,j); simulated sunlight, SU
(s,h); simulated shadelight, SH
(d,j).
Table 1. Nitrogen concentration in leaves, stems, and roots, and specific leaf area (SLA) of birch seedlings exposed to simulated sunlight (SU) or
shadelight (SH), and high (HN) and low (LN) nutrient supply regimes. Values are the means of two harvests carried out on Day 29.
Treatment
Nitrogen concentration
−1
SLA
−1
Leaf (mg g )
Stem (mg g )
Root (mg g )
Leaf (mg cm )
cm2 g −1
SU + HN
SH + HN
SU + LN
SH + LN
22.0
25.0
14.5
12.6
13.0
14.2
8.1
7.1
15.5
17.5
12.0
12.3
0.076
0.079
0.054
0.053
289
317
270
237
Source of variation
Probability from ANOVA
Nutrition
Light quality
Interaction
< 0.001
0.338
0.008
< 0.001
0.687
0.026
0.003
0.162
0.306
0.001
0.754
0.654
0.014
0.843
0.063
Nitrogen concentration per unit leaf area was not affected by
light quality (Table 1).
At the end of the experiment, chlorophyll concentration
(expressed per unit leaf area) was reduced by LN (P < 0.001)
but was not affected by the SH treatment (main effect P = 0.56,
interaction P = 0.65). Estimated chlorophyll concentrations
(means and standard error, in mg m −2) were: SU + HN: 196,
SU + LN: 129, SH + HN: 209, SH + LN: 130, SE = 11.6.
Allocation
Because dry weight allocation is usually correlated with plant
size (e.g., Evans and Hughes 1961), we plotted weight and N
fractions against W (cf. Evans 1972, Peace and Grubb 1982).
Otherwise, the effects of the treatments on allocation could
−1
−2
become confounded with their effects on plant size and ontogenic drift (Bourdôt et al. 1984). For example, leaf weight ratio
(wleaf ) at the final harvest was not significantly affected by any
of the treatments (P = 0.191 for nutrient supply, P > 0.177 for
light quality and P > 0.20 for the interaction). However, when
wleaf was plotted against W, a small but consistent decrease in
wleaf in response to LN was observed (Figure 4a, P < 0.001). A
similar response pattern was observed for leaf N content ratio
(Nleaf ) (Figure 5a, P = 0.023). The SH treatment had no significant effect on wleaf or Nleaf when the whole data set was
considered (P > 0.20). However, at Day 29, N leaf was slightly
higher in SU-treated seedlings than in SH-treated seedlings
(P = 0.110) and slightly higher in HN-treated seedlings compared with LN-treated seedlings (P = 0.029). Neither wleaf nor
Nleaf changed appreciably with plant size.
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
129
In contrast, stem weight ratio (w stem ) increased with plant
size, and the magnitude of the increase was mainly dependent
on nutrient supply (Figure 4b, P < 0.001), but also slightly
dependent on light quality (P = 0.028). In HN-treated seedlings, the proportion of total W allocated to stem increased
from 8% at the initial harvest to 25% at the final harvest,
whereas in LN-treated seedlings wstem increased from 8 to
17%. Only a small part of this difference is explained by the
difference in total W. Stem N content ratio (N stem ) followed a
similar pattern (P = 0.006 for nutrient supply and P > 0.20 for
light quality and interaction); however, N allocation to the stem
in LN-treated seedlings decreased toward the end of the experiment (Figure 5b). The SH treatment had no effect on wstem or
N stem when the whole data set was considered (P > 0.20).
As the size of the plants increased, root weight ratio (wroot )
decreased, but it stabilized toward the end of the experiment.
This decrease was smaller in LN-treated seedlings than in
HN-treated seedlings (Figure 4c, P < 0.001). A similar pattern
was observed for root N content ratio (N root ) (P = 0.005),
except that Nroot increased toward the end of the experiment in
LN-treated seedlings (Figure 5c). The SH treatment had no
effect on wroot or Nroot when the whole data set was considered
(P > 0.20).
Leaf area ratio (L/W ) first increased and later decreased as
plant size increased (Figure 6). The small difference in response to the LN treatment observed at the final harvest,
increased when leaf area ratio was plotted against W. Leaf area
ratio followed the same temporal pattern in LN- and HNtreated seedlings, however, its value was lower in LN-treated
seedlings. By Day 29, specific leaf area (SLA) was decreased
by LN (Table 1).
Specific stem length (SSL) decreased during the experiment, and when plotted against time, an effect of LN was
apparent toward the end of the experiment (Figure 3c). On Day
29, SSL was increased by 50% by the LN treatment
(P < 0.001). However, when SSL was plotted against W instead of time, the apparent effect of nutrition on SSL disappeared (Figure 3d). The SH treatment increased SSL during
the first part of the experiment when W was less than 120 mg
per seedling; however, the effect disappeared as the seedlings
grew (Figure 3d).
Figure 4. Dry weight allocation of silver birch seedlings. (a) Leaf
weight ratio (w leaf ), (b) stem weight ratio (w stem ), and (c) root weight
ratio (w root ) versus total plant dry weight (W ). Low nutrient supply,
LN (s,d); high nutrient supply HN (h,j); simulated sunlight, SU
(s,h); simulated shadelight, SH (d,j).
Figure 5. Nitrogen allocation of silver birch seedlings. (a) Leaf N ratio
(N leaf ), (b) stem N ratio (N stem ), and (c) root N ratio (N root ) versus total
plant dry weight (W ). Low nutrient supply, LN (s,d); high nutrient
supply, HN (h,j); simulated sunlight, SU (s,h); simulated shadelight, SH (d,j).
130
APHALO AND LEHTO
Figure 6. Leaf area ratio versus total plant dry weight of silver birch
seedlings. Low nutrient supply, LN (s,d); high nutrient supply,
HN (h,j); simulated sunlight, SU (s,h); simulated shadelight,
SH (d,j).
Functional growth analysis
We used functional growth analysis methods to calculate time
courses of relative growth rate (Rw) and relative N accumulation rate (RN). Because there was no effect of SH on W or N
content, the data from the two light quality treatments were
pooled before fitting the growth curves (Table 2). Relative
growth rate (the slope of the curves in Figure 2a) decreased
during the experiment, and the LN treatment further decreased
it. Although Rw and RN had similar values at Day 0, RN decreased more quickly than Rw during the experiment and was
also affected by the LN treatment. In the second half of the
experiment, net assimilation rate per unit leaf area was lower
in LN- than in HN-treated plants (data not shown). However,
net assimilation rate per unit leaf N was similar in LN- and
HN-treated seedlings throughout the experiment, although it
varied with time (4.0 g day −1 g −1 N at Day 0, and 7.1 g day −1
g −1 N at Day 29).
Discussion
Nutrient supply
Silver birch is a pioneer species that prefers fertile forest sites
and abandoned crop fields. Under conditions of optimal light
and nutrient supply, seedlings can attain Rw values of up to 0.27
day −1 (Ingestad and Ågren 1992, Table 1).
Initial Rw was 0.12 day −1, which is lower than the value of
aproximately 0.2 day −1 reported by Ingestad and McDonald
(1989) under conditions of similar I but optimum nutrition.
The N concentration of our plants was suboptimal from the
beginning of the experiment (cf. seedlings under optimal nutrition in Ingestad and Ågren 1992), and decreased even further
during the second half of the study period. Although we took
care to minimize shading, Rw decreased toward the end of the
experiment; this was probably because we used a constant
nutrient addition rate.
The response of the seedlings to altered nutrient supply was
greater than the response to light quality. Similar increases in
growth and decreases in allocation to roots with increased
nutrient supply have been documented in silver birch both in
the field (Viro 1974) and in controlled experiments (Ågren
1985, Ågren and Ingestad 1987), as well as in other broadleaved tree species (e.g., Lehto and Grace 1994).
The nutrient supply treatments and plant size had little effect
on leaf weight ratio. Most of the changes in allocation affected
only roots and stems, a result qualitatively similar to that
obtained in Eucalyptus by Sheriff (1992). Both in birch and in
Eucalyptus, N leaf was more sensitive than w leaf to nutrient
supply. However, in birch, N stem was decreased by LN, whereas
in Eucalyptus, it was not affected by N supply (Sheriff 1992).
Light quality
We attempted to simulate only the spectral quality of sun- and
shadelight. The results would have been different had we
simulated both the spectral quality and photon flux density of
these two kinds of light. Furthermore, although the spectrum
of sunlight is fairly constant, that of shadelight varies both with
the leaf area index of the canopy and with characteristics of the
foliage forming the canopy. Values of R/FR photon flux ratio
in the range of 0.1--0.75 are common in vegetation canopies
(Smith 1994); the value of 0.5 in the present experiment
corresponds to a relatively sparse canopy. The R/FR photon
flux ratio in simulated sunlight was 2.1, which is higher than
the ratio of 1.05--1.25 in real sunlight; however, this difference
has only a small effect on the photoequilibrium of phytochrome.
In the early stages of the experiment, stem elongation was
enhanced in SH-treated seedlings. However, there was no large
effect of shadelight quality on assimilation and allocation of N
or W. The explanation for the lack of effect of light quality on
the growth rate of birch seedlings is that neither wleaf , leaf area
Table 2. Parameters of growth equation. Time series data were fitted to ^y = b0 + b1t + b2t2, where y is the ln of the variable under consideration and
t is time. Dry weight (W ) and N content are in mg per plant and time is in days. Means of four replicates from each treatment and harvest were
used, with the two light treatments bulked (n = 8 for N content, n = 22 for W).
Variable
Treatment
b0
SE
b1
SE
b2
SE
R2
Dry weight (W)
LN
HN
4.234
4.180
0.0475
0.0540
0.1090
0.1186
0.00747
0.00848
−0.000933
−0.000638
0.000238
0.000270
0.989
0.990
N Content
LN
HN
0.928
1.035
0.0274
0.0716
0.1047
0.1168
0.00473
0.01236
−0.001946
−0.001531
0.000141
0.000368
0.998
0.994
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
ratio nor net assimilation rate were affected. Rice and Bazzaz
(1989) also found that light treatments have no effect on leaf
area ratio when the results are expressed on a dry weight basis.
The lack of an effect of light quality on net assimilation rate
per unit leaf area can be explained by its lack of effect on both
N and chlorophyll concentrations per unit leaf area.
When plants were compared at equal size, the main effect of
light quality on birch seedlings was a change in SSL, which
was smaller in SU-treated seedlings than in SH-treated seedlings during the early stages of the experiment. However, this
effect disappeared when the plants reached a certain size,
possibly indicating the operation of a developmental switch. A
similar decrease in responsiveness to natural canopy shade as
seedlings grow has been reported for cucumber (Ballaré et al.
1991).
In herbaceous plants, an increase in stem extension is usually accompanied by an increase in W of the stem, and these
increases occur at the expense of both leaves and roots
(McLaren and Smith 1978, Morgan and Smith 1979), resulting
in a reduction in Rw. In our experiment, increased stem elongation was not accompanied by a decrease in Rw. Similar
responses have been observed in other woody plants: in Fuchsia magellanica Lam., increased stem elongation in response
to end-of-day FR occurs through changes in SSL that do not
affect Rw (Aphalo et al. 1991). In seedlings of the tropical tree
Terminalia ivorensis A. Chev, a low R/FR increased stem
elongation without decreasing Rw or increasing w stem , except at
very low I (Kwesiga and Grace 1986).
If W accumulation is not depressed by low R/FR, the elongation response has no apparent cost in terms of carbon use.
However, there are risks involved in a high elongation rate even
if this can be achieved without carbon cost. For example, a
seedling with a long thin stem is more exposed to mechanical
damage by wind and rain, and to browsing by animals.
The increase in extension rate in response to shade depends
on the habitat of the species (Kwesiga and Grace 1986, Warrington et al. 1989). For example, Kwesiga and Grace (1986)
observed that, in Terminalia ivorensis, the main effect of increased R/FR was a substantial increase in leaf area ratio,
whereas the leaf area ratio of Khaya senegalensis (Desr.) A.
Juss. did not react to changes in R/FR. They attributed this
difference to the different ecologies of the species. Terminalia
is a pioneer species, whereas Khaya can survive extended
periods shaded in the forest, and will start growing rapidly
when a gap appears and I and R/FR increase. However, although birch is a pioneer species, we observed no effect of light
quality on leaf area ratio.
We have discussed the response to light quality observed
only in terms of R/FR; however, the light treatments also
differed in their blue light content. Because phytochrome mediated-responses are altered by blue light (Casal and Smith
1989, Mohr 1994), the effect of the simulated shadelight treatment is probably the result of a complex interaction involving
two or more photoreceptors. In an outdoor experiment, Dale
and Causton (1992) observed that shading by vegetation affects the assimilation of mineral nutrients and their allocation
to different organs. In their experiment, in which the natural
shade differed in both light quality and flux density from
131
sunlight, which was used as the control, the effects of shading
on nutrient allocation differed from those on carbon allocation.
In our experiment, in which light treatments differed only in
light quality, allocation of dry matter was not affected and N
allocation was only slightly different in seedlings from the two
light treatments. These small differences caused significant
changes in N concentration. Moreover, the magnitude and
possibly also the direction of the changes in leaf N concentration depended on the nutrient supply rate.
We conclude that: (1) stem elongation was affected by light
quality at the early stages of the experiment, but only until
plants reach a certain size; (2) there was no interaction between
the effect of light quality on stem elongation and seedling
nutritional status; (3) assimilation and allocation of N and W
were only slightly affected by light quality; and (4) there was
an effect of light quality on N concentration at the end of the
experiment, evident in the shadelight-induced increase in N
concentration per unit dry weight in leaves and stems of the
seedlings receiving the higher nutrient supply.
Thus, although our results cannot be directly extrapolated to
other species in which light quality has a more profound and
long-lasting effect on dry weight allocation, we have demonstrated that light quality can alter the mineral nutrition of silver
birch seedlings, and that this effect depends on the rate of
mineral nutrient supply.
Caveat
A time series of growth data allows the use of two bases for
comparison: size and age. Use of one rather than the other
sometimes influences the interpretation of the results, but the
appropriateness of one or the other is difficult to establish. It
has been argued that plant size is preferable (e.g., Rice and
Bazzaz 1989), because plant dry weight may reflect physiological age better than time (Bourdôt et al. 1984, and references therein). For example, dry weight allocation to stems is
probably related to mechanical constraints that depend on
plant size rather than on chronological age. It should be also
noted that, in our data set, some effects and interactions present
when data were plotted against time disappeared when the
same data were plotted against W, but the opposite was not
true. This is unlikely to happen by chance, and provides circumstantial evidence for preferring W as a basis for comparison.
Whichever basis was used, size or age, LN-treated seedlings
always had a lower N concentration than HN-treated seedlings,
demonstrating that, in both cases, the objective of testing for
the effect of light quality in seedlings with different nutritional
status was achieved.
Acknowledgments
This study was based on ideas discussed between the authors and
Carlos L. Ballaré, and also with Jorge J. Casal and Rodolfo A.
Sánchez. We thank Juha Lappi for statistical advice, and Raija Kuismin, Mervii Ahonpää and Pekka Voipio for skillful technical assistance.
132
APHALO AND LEHTO
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© 1997 Heron Publishing----Victoria, Canada
Effects of light quality on growth and N accumulation in birch
seedlings
P. J. APHALO1,2 and T. LEHTO1,2
1
2
Finnish Forest Research Institute, Suonenjoki Research Station, 77600 Suonenjoki, Finland
Current address: University of Joensuu, Faculty of Forestry, P.O. Box 111, 80101 Joensuu, Finland
Received July 10, 1995
Summary We studied the effects of light quality and nutrient
supply on growth and nitrogen accumulation in silver birch
(Betula pendula Roth) seedlings to test three hypotheses: (1)
growth of birch seedlings is sensitive to changes in light
quality; (2) the response of birch seedling growth to light
quality depends on nutrient supply; and (3) assimilation and
allocation of nitrogen by birch seedlings are affected by light
quality. The two light regimes simulated the spectral quality of
sunlight and shadelight, but did not differ in photosynthetic
photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of mineral
nutrients.
Accumulation and allocation of dry weight and nitrogen
were strongly affected by nutrient supply regime, but light
quality had little effect. During the first 15 days of the experiment, the largest effect of light quality was on height growth,
which was greater in seedlings in simulated shadelight than in
seedlings in simulated sunlight. Light quality had little effect
on dry weight and nitrogen allocation to the stem during this
period. However, at the end of the experiment (Day 29), there
was an increase in N concentration per unit dry weight in
leaves and stems of seedlings in the simulated shadelight plus
high nutrient supply treatment.
Keywords: allocation, assimilation, nitrogen, nutrient supply,
photomorphogenesis, shadelight, sunlight.
Introduction
Because plants both modify their light environment and respond to it, a detailed understanding of plant responses to light
is needed to elucidate the mechanisms underlying the morphological and physiological processes in canopies and plant communities (e.g., Ballaré 1994, Aphalo and Ballaré 1995).
Shade-avoiding species such as silver birch (Betula pendula
Roth) (Atkinson 1992) tend to be more responsive to changes
in light quality than shade-tolerant species (Smith 1994).
In many species, the perception of low R/FR ratios by
phytochrome triggers several responses, including increased
stem elongation, reduced leaf/stem dry weight ratio, increased
shoot/root dry weight ratio, and reduced photosynthetic capacity, which are presumably adaptive for plants growing in dense
populations (e.g., Smith 1994, Barreiro et al. 1992). Blue light
also triggers several photomorphogenic responses, including
partial inhibition of hypocotyl elongation. Moreover, phytochrome-mediated responses and those dependent on blue light
often interact (Mohr 1994).
Most research on plant photomorphogenesis has centered on
the effects of light quality, e.g., R/FR, on carbon assimilation
and partititoning. This has been a useful approach for studying
agricultural crops and their weeds, which usually grow in rich
fertilized soils. However, tree seedlings from boreal forests
grow in nutrient-poor soils where nutrient supply is often a
growth-limiting factor. Under these conditions, a study of plant
photomorphogenesis that focuses on the effects of light quality
on the acquisition and partitioning of mineral nutrients is likely
to be rewarding. Depression of plant growth and a concurrent
increase in allocation of dry matter to roots (Charles-Edwards
et al. 1986, Levin et al. 1989), and inhibition of leaf area
expansion (McDonald et al. 1992, Taylor et al. 1993) are well
known responses to low nutrient supply. However, there are
few reports on the interactions of light quality and mineral
nutrition. Root growth was depressed by end-of-day FR in
Glycine max (L.) (Kasperbauer et al. 1984). The activity of
enzymes involved in N assimilation is enhanced by red light
(R) (nitrate and nitrite reductase in several species (Johnson
1990, Mohr et al. 1992) and glutamine synthetase in Pinus
sylvestris (L.) (Elmlinger and Mohr 1992)). These effects
could alter the mineral nutrient economy of a plant in response
to shading by its neighbors (see Aphalo and Ballaré 1995).
The objective of this study was to test three hypotheses: (1)
growth of birch seedlings is sensitive to changes in light
quality; (2) the response of birch seedling growth to light
quality depends on nutrient supply; and (3) assimilation and
allocation of nitrogen by birch seedlings are affected by light
quality. We conducted a 2 × 2 factorial experiment, with light
quality and mineral nutrient supply as the experimental factors. The two light regimes simulated the spectral quality of
sunlight and shadelight, but did not differ in photosynthetic
photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of
mineral nutrients. Data from sequential harvests were used for
growth analysis.
126
APHALO AND LEHTO
Materials and methods
Plants
Betula pendula Roth (B. verrucosa Ehr.) seeds were germinated on sand in a greenhouse. The seeds were from a seed
orchard established with material from different Finnish sites
between 60°40′ N and 63°30′ N. When the seedlings had one
or two true leaves visible, they were transplanted to Ray-Leach
cells (165 cm3, SC-10 super cell, Stuewe and Sons, Corvallis,
OR) containing a mix of sand and unfertilized peat (1/2, v/v).
Fifteen days before the first harvest, all seedlings were watered
once with a balanced nutrient solution containing 50 mg N
dm −3. Nine days before the first harvest, the seedlings were
placed in a growth chamber that provided a day/night air
temperature of 20/15 °C, and a day/night water vapor saturation deficit of 5.8/3.5 mmol mol −1 (relative humidity 75/80%).
Treatments
The light quality treatments began on the day the seedlings
were transferred to the growth chamber. The light source in the
chamber consisted of 10 metal halide discharge lamps (HRI-T
250 W, Radium, Germany), supplemented with six incandescent lamps (25 W). The photoperiod was 20 h, with the lamps
switched on in three incremental steps (4 metal halide + 2
incandescent lamps, 7 + 4, and 10 + 6) during the first 2 h of
the photoperiod, and switched off in reverse order during the
last 2 h. With all lamps on, the photosynthetic photon flux
density (I) at the top of the plants was 225 µmol m −2 s −1
(measured with an LI-190SB quantum sensor, Li-Cor, Inc.,
Lincoln, NE). Taking into account the steps, the daily photon
flux was 14.7 mol m −2 day −1, which is approximately 35% of
the mean natural PAR in central Finland in May--July (Finnish
Meteorological Institute 1993).
Light quality was modified with filters located approximately 5 cm above the top of the seedlings (height adjusted
weekly). The frame holding the filters fitted closely against the
walls of the growth chamber, and although no vertical filters or
barriers were used, border plants were used to prevent interference by stray light. Two kinds of filters were used: neutral
(clear polyethylene film plus silver black scrim, Strand filter
No. 270, Strand Lighting Ltd., Isleworth, UK) and green (moss
green, Strand filter No. 422). The I under these filters differed
by less than 1%, but the red (655--665 nm) to far-red (725-735 nm) photon flux ratio (R/FR) was 2.1 for the neutral filter
(simulated sunlight, SU) and 0.5 for the green filter (simulated
shadelight, SH). The photon flux density of blue light (450 nm)
under the green filter was 20% of that under the neutral filter.
Spectra are shown in Figure 1.
Nutrient treatments were started on the day of the first
harvest (Day 0). Nutrients were supplied at two rates, but the
proportions of the different elements were kept constant. A
complete nutrient solution balanced for silver birch (Ingestad
1970) was provided at a rate of 0.86 mg N day −1 plant −1 for the
high nutrient supply treatment (HN), and at a rate of 0.17 mg
N day −1 plant −1 for the low nutrient supply treatment (LN).
The solutions were prepared by dilution of equal amounts of
two stock solutions. Solution A contained (per dm3): K2SO4
Figure 1. Spectral photon flux density of the simulated sunlight (broken line) and shadelight (solid line), normalized to equal photosynthetic photon flux density (I = 225 µmol m −2 s −1).
24.48 g, K2HPO4 16.81 g, KH2PO4 15.44 g, KNO3 24.62 g,
NH4NO3 110.0 g. Solution B contained (per dm3):
Ca(NO3)2.4H20 20.68 g, Mg(NO3)2.6H2O 44.88 g, micronutrients stock solution 25 cm3, HNO3 (70%) 1.8 g. The micronutrient stock solution contained (per dm3): Fe(NO3)3.9H2O
101.28 g, Mn(NO3)2.4H2O 36.56 g, Zn(NO3)2.4H2O 4.798 g,
CuCl2.2H2O 1.610 g, Na2MoO4.2H2O 0.353 g, H3BO3 22.88 g,
HNO3 (70%) 19.8 g.
Measurements and chemical analysis
Leaf area was measured with a leaf area meter (LI-3000,
Li-Cor, Inc., Lincoln, NE) and stem height was measured to
the nearest mm. Each seedling harvested was divided into leaf
blades, stem plus petioles, and roots, and the plant parts were
dried at 65 °C for 48 h before weighing. Each plant part was
ground separately, but the four replicates were combined. For
the larger plants, the samples were first coarsely ground in a
mill, subsampled, and then reground in a mortar with a pestle.
Nitrogen concentration was determined with an automatic
analyzer (Model CHN-9000, Leco Co., St. Joseph, MI).
Chlorophyll concentration was estimated in vivo from transmittance spectra measured in the second youngest fully expanded leaf with a spectroradiometer equipped with an
integrating sphere (Li-Cor LI-1800). The in vivo transmittance
at 660 nm was calibrated against chlorophyll concentration per
unit leaf area, measured in N,N-dimethylformamide extracts
from leaf discs of birch seedlings using the equations of Inskeep and Bloom (1985). The R2 of the polynomial regression
obtained was 0.98 (n = 32). The emission spectra of the light
sources used was measured with the same spectroradiometer.
Experimental design, growth analysis calculations and
statistical methods
The chamber was divided into four regions representing four
blocks (true replicates); each region contained four groups of
seedlings that were assigned at random to the treatments. Rows
of border plants were used to reduce interference between
treatments. The plants harvested at each date were selected at
random, and the border seedlings were moved to keep the
density of the experimental plants uniform in space. When the
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
127
number of remaining plants was small enough to allow a new
spatially uniform arrangement, the seedlings were arranged at
a lower density to reduce shading (the maximum leaf area
index was 2.7 during the experiment).
The functional growth analysis method was used. Functions
were fitted to the harvest means of the measured variables with
time as the independent variable. The fitted functions were
exponential polynomials. Relative growth rate (Rw) and relative N accumulation rate (RN ) were calculated as the derivatives of the regressions for dry weight (W) and nitrogen content
(N) (see Table 2):
^y′ = b + 2b t,
1
2
(1)
and confidence limits as:
^y′ ± t × sd(^y′),
α/2
(2)
where:
sd(y^′) = √
Var(b1) + 4t2Var(b 2) + 4tCov(b1,b2) .
(3)
Multivariate or univariate ANOVA was used to test the
significance of differences between treatments. Height, leaf
area, dry weight, and nitrogen content values and allometric
ratios were ln-transformed before analyses or calculations, and
are presented in the figures on a log scale.
Results
Growth
Plant dry weight (W ) increased throughout the experiment, but
the relative rate of increase was smaller during the later stages
than during the first 15 days of the experiment. At the final
harvest on Day 29, W was between 12 and 18 times as much
as on Day 0, depending on the treatments (Figure 2a). Shadelight (SH) had no effect on final W (P = 0.59), but the low
nutrient supply rate (LN) decreased final W by 33% (P <
0.001), and there was a small but significant interaction (P =
0.029).
The relative rate of leaf area expansion decreased throughout the experimental period in all treatments (Figure 2b). Final
leaf area (L) was 10 and 18 times as much as initial L for the
LN- and HN-treated seedlings, respectively. Compared with
final L of plants in the HN treatment, LN resulted in a 45%
decrease in final L (P < 0.001), whereas shadelight had no
effect on L (P > 0.20).
In the SH treatment, stem height (H) was initially increased
by about 40%, but later in the study, the stem elongation rate
was greater in SU-treated seedlings than in SH-treated seedlings (Figure 3a). By the end of the experiment, H was similar
in both light quality treatments (P > 0.20 for main effect and
P = 0.086 for the interaction). In contrast, H was decreased by
LN so that LN-treated plants were shorter than HN-treated
plants at the final harvest (P < 0.001). A plot of H versus W
(Figure 3b) shows that the effect of LN is mostly explained by
Figure 2. Dry weight and morphological attributes of silver birch
seedlings. (a) Whole-plant dry weight (W ) and (b) whole-plant leaf
area (L) versus time. Low nutrient supply, LN (s,d); high nutrient
supply, HN (h,j); simulated sunlight, SU (s,h); simulated shadelight, SH (d,j).
its effect on total plant size, whereas the effect of light quality
is not.
Nitrogen content of the seedlings increased more slowly
than W. On Day 2, the N content was 2.4 mg per seedling.
Because the final N content was 4.2 and 9.5 times as much as
the initial content in LN- and HN-treated seedlings, respectively, the final N content of LN-treated seedlings was only
44% of that of HN-treated seedlings (P < 0.001). Nitrogen
content was only increased 3.4% by the SH treatment
(P = 0.056). When plotted against W, differences in N content
in response to LN remained large (data not shown).
Because N content increased at a slower rate than W accumulation, N concentration decreased during the course of the
experiment. On Day 2, N concentrations in leaves per unit dry
weight and per unit leaf area were 31 mg g −1 and 0.105 mg
cm −2, respectively. Leaf N concentration per unit leaf area also
decreased throughout the experiment, and this decrease was
larger in LN-treated seedlings than in HN-treated seedlings
(~50% in LN and ~25% in HN). At the time of the final harvest
(Day 29), N concentrations in leaves and stems of LN-treated
seedlings were only half those in HN-treated seedlings,
whereas N concentration in the roots was less affected by
nutrient supply (Table 1). There was no effect of light quality
on N concentration in roots. However, in leaves and stems, the
effect of light quality on N concentration depended on the
nutrient supply. In the HN-treated seedlings, the SH treatment
increased N concentration (P = 0.013 leaf, P= 0.052 stem),
whereas in the LN-treated seedlings, SH treatment slightly
decreased N concentration (P = 0.054 leaf, P = 0.100 stem).
128
APHALO AND LEHTO
Figure 3. Stem growth of silver
birch seedlings. (a) Stem height
versus time from the start of the
nutrient supply treatments, (b)
stem height versus total plant dry
weight (W ), (c) specific stem
length (SSL) versus time and (d)
SSL versus total plant dry weight
(W ). Low nutrient supply, LN
(s,d); high nutrient supply, HN
(h,j); simulated sunlight, SU
(s,h); simulated shadelight, SH
(d,j).
Table 1. Nitrogen concentration in leaves, stems, and roots, and specific leaf area (SLA) of birch seedlings exposed to simulated sunlight (SU) or
shadelight (SH), and high (HN) and low (LN) nutrient supply regimes. Values are the means of two harvests carried out on Day 29.
Treatment
Nitrogen concentration
−1
SLA
−1
Leaf (mg g )
Stem (mg g )
Root (mg g )
Leaf (mg cm )
cm2 g −1
SU + HN
SH + HN
SU + LN
SH + LN
22.0
25.0
14.5
12.6
13.0
14.2
8.1
7.1
15.5
17.5
12.0
12.3
0.076
0.079
0.054
0.053
289
317
270
237
Source of variation
Probability from ANOVA
Nutrition
Light quality
Interaction
< 0.001
0.338
0.008
< 0.001
0.687
0.026
0.003
0.162
0.306
0.001
0.754
0.654
0.014
0.843
0.063
Nitrogen concentration per unit leaf area was not affected by
light quality (Table 1).
At the end of the experiment, chlorophyll concentration
(expressed per unit leaf area) was reduced by LN (P < 0.001)
but was not affected by the SH treatment (main effect P = 0.56,
interaction P = 0.65). Estimated chlorophyll concentrations
(means and standard error, in mg m −2) were: SU + HN: 196,
SU + LN: 129, SH + HN: 209, SH + LN: 130, SE = 11.6.
Allocation
Because dry weight allocation is usually correlated with plant
size (e.g., Evans and Hughes 1961), we plotted weight and N
fractions against W (cf. Evans 1972, Peace and Grubb 1982).
Otherwise, the effects of the treatments on allocation could
−1
−2
become confounded with their effects on plant size and ontogenic drift (Bourdôt et al. 1984). For example, leaf weight ratio
(wleaf ) at the final harvest was not significantly affected by any
of the treatments (P = 0.191 for nutrient supply, P > 0.177 for
light quality and P > 0.20 for the interaction). However, when
wleaf was plotted against W, a small but consistent decrease in
wleaf in response to LN was observed (Figure 4a, P < 0.001). A
similar response pattern was observed for leaf N content ratio
(Nleaf ) (Figure 5a, P = 0.023). The SH treatment had no significant effect on wleaf or Nleaf when the whole data set was
considered (P > 0.20). However, at Day 29, N leaf was slightly
higher in SU-treated seedlings than in SH-treated seedlings
(P = 0.110) and slightly higher in HN-treated seedlings compared with LN-treated seedlings (P = 0.029). Neither wleaf nor
Nleaf changed appreciably with plant size.
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
129
In contrast, stem weight ratio (w stem ) increased with plant
size, and the magnitude of the increase was mainly dependent
on nutrient supply (Figure 4b, P < 0.001), but also slightly
dependent on light quality (P = 0.028). In HN-treated seedlings, the proportion of total W allocated to stem increased
from 8% at the initial harvest to 25% at the final harvest,
whereas in LN-treated seedlings wstem increased from 8 to
17%. Only a small part of this difference is explained by the
difference in total W. Stem N content ratio (N stem ) followed a
similar pattern (P = 0.006 for nutrient supply and P > 0.20 for
light quality and interaction); however, N allocation to the stem
in LN-treated seedlings decreased toward the end of the experiment (Figure 5b). The SH treatment had no effect on wstem or
N stem when the whole data set was considered (P > 0.20).
As the size of the plants increased, root weight ratio (wroot )
decreased, but it stabilized toward the end of the experiment.
This decrease was smaller in LN-treated seedlings than in
HN-treated seedlings (Figure 4c, P < 0.001). A similar pattern
was observed for root N content ratio (N root ) (P = 0.005),
except that Nroot increased toward the end of the experiment in
LN-treated seedlings (Figure 5c). The SH treatment had no
effect on wroot or Nroot when the whole data set was considered
(P > 0.20).
Leaf area ratio (L/W ) first increased and later decreased as
plant size increased (Figure 6). The small difference in response to the LN treatment observed at the final harvest,
increased when leaf area ratio was plotted against W. Leaf area
ratio followed the same temporal pattern in LN- and HNtreated seedlings, however, its value was lower in LN-treated
seedlings. By Day 29, specific leaf area (SLA) was decreased
by LN (Table 1).
Specific stem length (SSL) decreased during the experiment, and when plotted against time, an effect of LN was
apparent toward the end of the experiment (Figure 3c). On Day
29, SSL was increased by 50% by the LN treatment
(P < 0.001). However, when SSL was plotted against W instead of time, the apparent effect of nutrition on SSL disappeared (Figure 3d). The SH treatment increased SSL during
the first part of the experiment when W was less than 120 mg
per seedling; however, the effect disappeared as the seedlings
grew (Figure 3d).
Figure 4. Dry weight allocation of silver birch seedlings. (a) Leaf
weight ratio (w leaf ), (b) stem weight ratio (w stem ), and (c) root weight
ratio (w root ) versus total plant dry weight (W ). Low nutrient supply,
LN (s,d); high nutrient supply HN (h,j); simulated sunlight, SU
(s,h); simulated shadelight, SH (d,j).
Figure 5. Nitrogen allocation of silver birch seedlings. (a) Leaf N ratio
(N leaf ), (b) stem N ratio (N stem ), and (c) root N ratio (N root ) versus total
plant dry weight (W ). Low nutrient supply, LN (s,d); high nutrient
supply, HN (h,j); simulated sunlight, SU (s,h); simulated shadelight, SH (d,j).
130
APHALO AND LEHTO
Figure 6. Leaf area ratio versus total plant dry weight of silver birch
seedlings. Low nutrient supply, LN (s,d); high nutrient supply,
HN (h,j); simulated sunlight, SU (s,h); simulated shadelight,
SH (d,j).
Functional growth analysis
We used functional growth analysis methods to calculate time
courses of relative growth rate (Rw) and relative N accumulation rate (RN). Because there was no effect of SH on W or N
content, the data from the two light quality treatments were
pooled before fitting the growth curves (Table 2). Relative
growth rate (the slope of the curves in Figure 2a) decreased
during the experiment, and the LN treatment further decreased
it. Although Rw and RN had similar values at Day 0, RN decreased more quickly than Rw during the experiment and was
also affected by the LN treatment. In the second half of the
experiment, net assimilation rate per unit leaf area was lower
in LN- than in HN-treated plants (data not shown). However,
net assimilation rate per unit leaf N was similar in LN- and
HN-treated seedlings throughout the experiment, although it
varied with time (4.0 g day −1 g −1 N at Day 0, and 7.1 g day −1
g −1 N at Day 29).
Discussion
Nutrient supply
Silver birch is a pioneer species that prefers fertile forest sites
and abandoned crop fields. Under conditions of optimal light
and nutrient supply, seedlings can attain Rw values of up to 0.27
day −1 (Ingestad and Ågren 1992, Table 1).
Initial Rw was 0.12 day −1, which is lower than the value of
aproximately 0.2 day −1 reported by Ingestad and McDonald
(1989) under conditions of similar I but optimum nutrition.
The N concentration of our plants was suboptimal from the
beginning of the experiment (cf. seedlings under optimal nutrition in Ingestad and Ågren 1992), and decreased even further
during the second half of the study period. Although we took
care to minimize shading, Rw decreased toward the end of the
experiment; this was probably because we used a constant
nutrient addition rate.
The response of the seedlings to altered nutrient supply was
greater than the response to light quality. Similar increases in
growth and decreases in allocation to roots with increased
nutrient supply have been documented in silver birch both in
the field (Viro 1974) and in controlled experiments (Ågren
1985, Ågren and Ingestad 1987), as well as in other broadleaved tree species (e.g., Lehto and Grace 1994).
The nutrient supply treatments and plant size had little effect
on leaf weight ratio. Most of the changes in allocation affected
only roots and stems, a result qualitatively similar to that
obtained in Eucalyptus by Sheriff (1992). Both in birch and in
Eucalyptus, N leaf was more sensitive than w leaf to nutrient
supply. However, in birch, N stem was decreased by LN, whereas
in Eucalyptus, it was not affected by N supply (Sheriff 1992).
Light quality
We attempted to simulate only the spectral quality of sun- and
shadelight. The results would have been different had we
simulated both the spectral quality and photon flux density of
these two kinds of light. Furthermore, although the spectrum
of sunlight is fairly constant, that of shadelight varies both with
the leaf area index of the canopy and with characteristics of the
foliage forming the canopy. Values of R/FR photon flux ratio
in the range of 0.1--0.75 are common in vegetation canopies
(Smith 1994); the value of 0.5 in the present experiment
corresponds to a relatively sparse canopy. The R/FR photon
flux ratio in simulated sunlight was 2.1, which is higher than
the ratio of 1.05--1.25 in real sunlight; however, this difference
has only a small effect on the photoequilibrium of phytochrome.
In the early stages of the experiment, stem elongation was
enhanced in SH-treated seedlings. However, there was no large
effect of shadelight quality on assimilation and allocation of N
or W. The explanation for the lack of effect of light quality on
the growth rate of birch seedlings is that neither wleaf , leaf area
Table 2. Parameters of growth equation. Time series data were fitted to ^y = b0 + b1t + b2t2, where y is the ln of the variable under consideration and
t is time. Dry weight (W ) and N content are in mg per plant and time is in days. Means of four replicates from each treatment and harvest were
used, with the two light treatments bulked (n = 8 for N content, n = 22 for W).
Variable
Treatment
b0
SE
b1
SE
b2
SE
R2
Dry weight (W)
LN
HN
4.234
4.180
0.0475
0.0540
0.1090
0.1186
0.00747
0.00848
−0.000933
−0.000638
0.000238
0.000270
0.989
0.990
N Content
LN
HN
0.928
1.035
0.0274
0.0716
0.1047
0.1168
0.00473
0.01236
−0.001946
−0.001531
0.000141
0.000368
0.998
0.994
EFFECTS OF LIGHT QUALITY ON BIRCH SEEDLINGS
ratio nor net assimilation rate were affected. Rice and Bazzaz
(1989) also found that light treatments have no effect on leaf
area ratio when the results are expressed on a dry weight basis.
The lack of an effect of light quality on net assimilation rate
per unit leaf area can be explained by its lack of effect on both
N and chlorophyll concentrations per unit leaf area.
When plants were compared at equal size, the main effect of
light quality on birch seedlings was a change in SSL, which
was smaller in SU-treated seedlings than in SH-treated seedlings during the early stages of the experiment. However, this
effect disappeared when the plants reached a certain size,
possibly indicating the operation of a developmental switch. A
similar decrease in responsiveness to natural canopy shade as
seedlings grow has been reported for cucumber (Ballaré et al.
1991).
In herbaceous plants, an increase in stem extension is usually accompanied by an increase in W of the stem, and these
increases occur at the expense of both leaves and roots
(McLaren and Smith 1978, Morgan and Smith 1979), resulting
in a reduction in Rw. In our experiment, increased stem elongation was not accompanied by a decrease in Rw. Similar
responses have been observed in other woody plants: in Fuchsia magellanica Lam., increased stem elongation in response
to end-of-day FR occurs through changes in SSL that do not
affect Rw (Aphalo et al. 1991). In seedlings of the tropical tree
Terminalia ivorensis A. Chev, a low R/FR increased stem
elongation without decreasing Rw or increasing w stem , except at
very low I (Kwesiga and Grace 1986).
If W accumulation is not depressed by low R/FR, the elongation response has no apparent cost in terms of carbon use.
However, there are risks involved in a high elongation rate even
if this can be achieved without carbon cost. For example, a
seedling with a long thin stem is more exposed to mechanical
damage by wind and rain, and to browsing by animals.
The increase in extension rate in response to shade depends
on the habitat of the species (Kwesiga and Grace 1986, Warrington et al. 1989). For example, Kwesiga and Grace (1986)
observed that, in Terminalia ivorensis, the main effect of increased R/FR was a substantial increase in leaf area ratio,
whereas the leaf area ratio of Khaya senegalensis (Desr.) A.
Juss. did not react to changes in R/FR. They attributed this
difference to the different ecologies of the species. Terminalia
is a pioneer species, whereas Khaya can survive extended
periods shaded in the forest, and will start growing rapidly
when a gap appears and I and R/FR increase. However, although birch is a pioneer species, we observed no effect of light
quality on leaf area ratio.
We have discussed the response to light quality observed
only in terms of R/FR; however, the light treatments also
differed in their blue light content. Because phytochrome mediated-responses are altered by blue light (Casal and Smith
1989, Mohr 1994), the effect of the simulated shadelight treatment is probably the result of a complex interaction involving
two or more photoreceptors. In an outdoor experiment, Dale
and Causton (1992) observed that shading by vegetation affects the assimilation of mineral nutrients and their allocation
to different organs. In their experiment, in which the natural
shade differed in both light quality and flux density from
131
sunlight, which was used as the control, the effects of shading
on nutrient allocation differed from those on carbon allocation.
In our experiment, in which light treatments differed only in
light quality, allocation of dry matter was not affected and N
allocation was only slightly different in seedlings from the two
light treatments. These small differences caused significant
changes in N concentration. Moreover, the magnitude and
possibly also the direction of the changes in leaf N concentration depended on the nutrient supply rate.
We conclude that: (1) stem elongation was affected by light
quality at the early stages of the experiment, but only until
plants reach a certain size; (2) there was no interaction between
the effect of light quality on stem elongation and seedling
nutritional status; (3) assimilation and allocation of N and W
were only slightly affected by light quality; and (4) there was
an effect of light quality on N concentration at the end of the
experiment, evident in the shadelight-induced increase in N
concentration per unit dry weight in leaves and stems of the
seedlings receiving the higher nutrient supply.
Thus, although our results cannot be directly extrapolated to
other species in which light quality has a more profound and
long-lasting effect on dry weight allocation, we have demonstrated that light quality can alter the mineral nutrition of silver
birch seedlings, and that this effect depends on the rate of
mineral nutrient supply.
Caveat
A time series of growth data allows the use of two bases for
comparison: size and age. Use of one rather than the other
sometimes influences the interpretation of the results, but the
appropriateness of one or the other is difficult to establish. It
has been argued that plant size is preferable (e.g., Rice and
Bazzaz 1989), because plant dry weight may reflect physiological age better than time (Bourdôt et al. 1984, and references therein). For example, dry weight allocation to stems is
probably related to mechanical constraints that depend on
plant size rather than on chronological age. It should be also
noted that, in our data set, some effects and interactions present
when data were plotted against time disappeared when the
same data were plotted against W, but the opposite was not
true. This is unlikely to happen by chance, and provides circumstantial evidence for preferring W as a basis for comparison.
Whichever basis was used, size or age, LN-treated seedlings
always had a lower N concentration than HN-treated seedlings,
demonstrating that, in both cases, the objective of testing for
the effect of light quality in seedlings with different nutritional
status was achieved.
Acknowledgments
This study was based on ideas discussed between the authors and
Carlos L. Ballaré, and also with Jorge J. Casal and Rodolfo A.
Sánchez. We thank Juha Lappi for statistical advice, and Raija Kuismin, Mervii Ahonpää and Pekka Voipio for skillful technical assistance.
132
APHALO AND LEHTO
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