1689
Photosynthesis and growth of Douglas-fir seedlings when grown
in different environments 1
FRANK
C.
SORENSEN
Forestry Sciences Laboratory, Pacific Northwest Forest and Range Experiment Station, Forest Service,
U.S. Department of Agriculture, Corvallis, Oregon 97331
AND
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w.
K.
FERRELL
Forest Research LaboratOIJ', Oregon State University, Corvallis, Oregon 97331
Received December 22, 1972
SoRENSEN, F. C., and W. K. FERRELL. 1973. Photosynthesis and growth of Douglas-fir seedlings when
grown in different environments. Can. J. Bot. 51: 1689-1698.
Douglas-fir seedlings from western Montana and western Oregon seed sources were grown in a cool
(18°C day- 4°C night) or warm (36°C day- 21 oc night) growth chamber. Photosynthetic and respiratory
rates were measured at 20°, 27.5°, and 35°C at cotyledon stage (15 to 18 days old), 55 days old, and 100
days old. Needle, stem, and root dry weights were also determined. Photosynthetic rates declined with
age, were higher for Oregon than for Montana seedlings, particularly at lower measurement temperatures,
were higher for high-temperature-grown than for low-,temperature-~rown plants, and were lowest at the
highest measurement temperature. Each of these factors was modified by the others in its effect on the
photosynthetic rates. Respiration rates declined with age, were slightly higher for Montana than for
Oregon seedlings, were much higher for the cold-temperature-grown than for the warm-temperature­
grown seedlings, and were highest at the highest measurement temperature. When plants from the two
growth environments were interchanged, the photosynthetic rates (measured at 27.5°C only) of the
seedlings changed to those of the new environment within 10 days. Comparison of seedlings grown at
high and low temperatures showed the latter to have lower photosynthetic rates but higher growth rates,
probably as a result of the higher proportion ofleaf tissue produced by the low-temperature-grown plants.
SoRENSEN, F. C., et W. K. FERRELL. 1973. Photosynthesis and growth of Douglas-fir seedlings when
grown in different environments. Can. J. Bot. 51: 1689-1698.
Des plantules de sapin Douglas obtenues de graines provenant de !'ouest du Montana et de !'ouest de
!'Oregon ont ete cultivees en chambre de croissance froide (18°C Ie jour, 4°C Ia nuit) ou chaude (36°C Ie
jour, 21 oc Ia nuit). Les taux de res(liration et de photosynthese ont ete mesures a 20", 27.5° et 35°C au
stade cotyledonaire (15 a 18 jours), a !'age de 55 jours eta !'age de 100 jours. Les poids sees des aiguilles,
des tiges et des racines ont egalement ete determines. Les taux de photosynthese ont diminue avec !'age;
ils etaient plus eleves pour les plantules de !'Oregon que celles du Montana, en particulier Iorsque mesures
aux temperatures Ies plus basses; ils etaient plus eleves pour les plantules cultivees aux temperatures
elevees que pour celles cultivees aux basses temperatures; et ils etaient les plus bas lorsque mesures aux
temperatures les plus elevees. Chacun de ces facteurs etait modifie par les autres dans ses effets sur les
taux photosynthetiques. Les taux de respiration ont diminue avec !'age; ils etaient legerement plus eleves
chez les plantules du Montana que chez celles de !'Oregon; ils etaient beaucoup plus eleves chez Ies
plantules cultivees a des temperatures chaudes; et ils etaient les plus eleves Iorsque mesures aux tempera­
tures Ies plus elevees. Lorsque des plantes des deux milieux de croissance ont ete interchangees, Ies taux
photosynthetiques (mesures a 27.5°C seulement) des plantu1es ont adopte ceux du nouveau milieu en
moins de 10 jours. La comparaison de plantules cultivees a hautes eta basses temperatures ont montre
que ces dernieres ont des taux photosynthetiques plus bas mais des taux de croissance plus eleves, pro­
bablement a cause de Ia plus forte proportion du tissu foliaire produit par les plantes cultivees aux
basses temperatures.
[Traduit par Ie journal]
Introduction Sorensen 1967; Sweet 1965). Such growth differ­
ences
could result from inherent differences in
Douglas fir is a species with a large geographic
seasonal
duration of active growth (Irgens­
range in western North America. Seedlings of
different geographic origins differ in growth even Moller 1958), from differences in C02 exchange
under common environmental conditions (Ching rates, or from differences in the distribution of
and Bever 1960; Hermann and Lavender 1968 ; assimilate. The purpose of this paper is to
examine some of these factors as they relate to
1 Work prepared under auspices of National Science
seedlings of two sources growing under two
Foundation Contract G-9576. This paper is based on a different temperatures.
dissertation submitted to Oregon State University in
The results were considered from two points
partial fulfillment of the requirements for the degree of
of view. First, the effect of seed source, tempera­
Doctor of Philosophy.
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1690
CAN. J. BOT. VOL. 51, 1973
ture at which seedlings were grown, temperature
at which C02 exchange was measured, and age
was analyzed to determine if the sources showed
differentiation in temperature patterns of photo­
synthesis and respiration. Second, photosyn­
thetic rates were related to total plant growth and
to growth of needles, stems, and roots. This was
done to see how photosynthetic rates were
related to growth rates.
Campbell and Rediske (1966), testing !-year­
old Douglas-fir seedlings which were the product
of controlled crosses within a natural stand of
about 200 ac, found significant differences among
families and a moderate heritability for photo­
synthetic efficiency. Such within-stand variation
could lead to formation of varieties or ecotypes
with different photosynthetic abilities. However,
studies on ecotypic variability, even on seedlings
from sources as diverse as northern Rocky
Mountains and Pacific Northwest coast, have
indicated that ecotypic variability in photosyn­
thetic responses is not present or not equally
strong under all measurement conditions
(Krueger and Ferrell 1965; Zavitkovski 1964;
Zavitkovski and Ferrell 1968, 1970). To have a
sample of these interactions in the present
investigation, age and measurement temperature
variables were included.
Temperature was selected for manipulation
because it was easy to control with our facilities,
it had been shown to affect Douglas-fir seedling
growth (Hellmers and Sundahl 1959), and it was
known to exert a strong effect on photosyn­
thetic and respiratory responses of many plants
both at the time of measurement and when used
as a pretreatment (Semikhatova 1960; Simonis
1960).
Materials and Methods
Tests were run on seedlings from western Oregon and
western Montana. Oregon seeds were collected at 700-ft
elevation on McDonald Forest of Oregon State Univer­
sity, 10 mi north of Corvallis. About 10 individual mother
trees were represented in a bulked collection. Rocky
Mountain seeds were collected from three trees at 4300-ft
elevation in the foothills of the Garnet Mountains, 30 mi
east of Missoula, Montana.
Seedlings were grown in two controlled-environment
chambers which differed primarily in the temperature
regimes, a 36°C day- 21 oc night (called warm environ­
ment) and an 18°C day- 4°C night (cool environment).
Day temperature was held for 10 h, night temperature for
8 h, and there were 3-h temperature transition periods in
the morning and evening.
About 1000 ft-c were supplied by a combination of
three types of lights: cool white fluorescent, deluxe warm
white fluorescent, and incandescent. Photoperiod was
16 h. During the light period, relative humidity was about
70% in the warm conditions and 90% in the cool con­
ditions.
Plants were watered daily until seed coats were shed.
Thereafter, seedlings in both environments were watered
heavily once a week and given nutrient solution as
required tile rest of the week. Because of greater moisture
loss, plants in tile warm environment received three
applications of nutrient solution per week compared with
two applications per week to plants in the cool en­
vironment.
Carbon dioxide exchange rates were determined at
three ages: cotyledon stage (cotyledons horizontal, first
needles just beginning to emerge), 55 days after planting,
and 100 days after planting. The cotyledon stage occurred
15 to 18 days after planting.
Photosynthetic rates at all ages were measured at
5000 ft-c (saturating light intensity) at 20°, 27.SO, and
35°C. In addition, measurements were made at 12.5° and
42.5°C on 55-day-old plants. Respiratory rates were
determined in darkness at the same temperature.
Rates of gas exchange for cotyledon stage and 55-day­
old plants were measured on pairs of seedlings free from
mutual shading. Photosynthetic rate determinations at
the 100-day age were usually made on individual seed­
lings because it was difficult to avoid mutual shading.
Gas exchange determinations were made in a closed
system at C0 2 concentrations between 330 and 370 ppm.
Temperature in the plant chamber or cuvette was con­
trolled by an overhead, flowing-water filter of about 4 em
depth and a circulating water heat sink in the walls.
Volume of the system used in photosyntiletic measure­
ments was 2.95 liters; volume of the system used in
respiratory measurements was 1.63 liters. Air movement
in the cuvette was sufficient to minimize boundary layer
resistance, as demonstrated by lack of change of C02
uptake with a 50% decrease in air flow rate in the plant
chamber. A more complete description of the apparatus
is given in Krueger and Ferrell (1965).
Rates were measured after a 20-min equilibration
period in the cuvette at the proper temperature. Two
consecutive photosynthetic determinations with a maxi­
mum duration of 30 min were made on each seedling to
check the adequacy of the equilibration period. The
average rate was used in subsequent analyses. The only
exception was for determinations at 42.5°C. Here, a
prolonged decrease in rate began about 45 min after first
exposure to the high temperature, so only one measure­
ment was made. Respiratory determinations took con­
siderably longer, so the two halves of a single run were
compared to check if the equilibration time had been
long enough.
These rate determinations were made throughout the
second half of the 16-h photoperiod. During the day,
there was no indication of change in rate for plants from
either the warm or cool chamber.
After measurement, the needles were stripped from the
seedlings and weighed after drying for 24 h at 100°C.
Photosynthetic and respiratory rates were expressed on
the basis of milligrams C0 2 exchanged per gram needle
dry weight per hour.
Some of the seedlings were also used for further dry
weight determinations. At 18, 55, and 100 days from
1691
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SORENSEN AND FERRELL: DOUGLAS-FIR SEEDLINGS
planting, about 40 plants of each source from each
environment were removed and separated into needles,
stems, and roots. This material was dried for 24 h at
l00°C and weighed.
Two additional tests involved the transfer of plants
from one growth environment to the other. Only Oregon
seedlings were transferred; no more Montana seeds or
seedlings were available. For the photosynthetic and
respiratory measurements, the plants were grown to age
45 days in one temperature chamber, then transferred to
the other. C0 2 exchange rates were determined at 27.5°C,
10 days after the transfer. For the growth measurements,
the seedlings were grown 28 days in one environment,
then transferred to the other. Dry weights of needles,
stems, and roots were determined at age 55 days.
Analysis
Gas exchange rates were analyzed according to
completely random analyses of variance with
fixed treatment effects. Treatments were based
on variation of source of seed, age at which gas
exchange rate was measured, temperature at
which seedlings were grown (called "growth
temperature"), and temperature at which gas
exchange rate was measured (called "measure­
ment temperature"). Ten seedlings or 10 pairs
of seedlings were used to determine photosyn­
thetic rates at each treatment level; 5 seedlings
or pairs of seedlings were used for respiratory
rates.
In both analyses, variances were found to be
heterogeneous when evaluated with Bartlett's
test. Logarithmic and square-root transforma­
tions were tried but did not successfully remove
the heterogeneity. In the analysis of photosyn­
thetic rates, the transformations had little effect
on heterogeneity or on test statistics. Conse­
quently, untransformed values were used in the
analysis.
Respiratory rate, within fairly wide limits,
varied exponentially with temperature. This
would lead to interactions between measurement
temperature and other treatments if untrans­
formed rates were used. In this analysis of
respiratory rates, the log transformation elimi­
nated or greatly reduced interactions involving
temperature, although it did not eliminate the
heterogeneity of variance. Under these circum­
stances, the log scale was considered best for
presentation of the data (Kempthorne 1957;
Mather and Jinks 1971 , pp. 56-64) and was
used here.
Results
Photosynthetic Rates
Seed source, growth temperature, age, and
measurement temperature all had highly signifi­
cant effects on variation in photosynthetic rates
(Table I, Fig. I). On the average, the Corvallis
seedlings absorbed C02 at a rate of 14.4 mgjg
needle dry weight per hour; Missoula plants
photosynthesized at a rate of 13.5 mg C02/g
needle dry weight per hour. The effect of the
TABLE 1
Analysis of variance of photosynthetic rates of Douglas-fir seedlings from
western Oregon and western Montana
Source of variation
DF
Mean squares
Total
Seed source (S)
Age (A)
SXA
Growth temperature (G)
SXG
AXG
SXAXG
Measurement temperature (M)
S XM
AXM
GXM
SXAXM
SXGXM
AXGXM
SXAXGXM
Error
359
1
2
2
1
1
2
2
2
2
4
2
4
2
4
4
324
6.62t
785.04t
7 .14t
211.26t
0.36
34 .32t
l.13t
92.52t
4.30t
33.11 t
13 .88t
2.3lt
0.02
1.62t
0.39
Components
of variance*
0.035
0.113
0.000
0.027
0.066
0.099
0.000
*Components given only for the sources of variation which include effect of seed source.
tCalculated F exceeds 5% tabular F.
~Calculated F exceeds 1% tabular F.
1692
CAN. J. BOT. VOL. 51, 1973
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other treatments was greater. Plants from the
cool environment assimilated at an average rate
of 11.3 mg C02 fg needle dry weight per hour;
plants from the warm environment photosynthe­
sized at an average rate of 16.6 mg COzfg needle
dry weight per hour. Average rates at the three
temperatures of measurement (20°, 27.5°, and
35°C) were, respectively, 16.0, 15.3, and 10.5
40
------------------,
COTYLEDON STAGE
35
c:
:::> 30
0
I
....
~
25
w
s:
>­
0:
0
20 w
--'
0
w
w
z
l
15 2
'J
~
-----COOL --WARM
o
OREGON
M:;;=___.~ _
~10 ~-~
0
<(
0
u
_ ______l_
15
I
12.5
20
27.5
35
42.5
MEASUREMENT TEMPERATURE ("C)
FIG. 1. Photosynthetic rates at three ages and three or
five measurement temperatures of seedlings from Oregon
(open circles) or Montana (solid circles) after growth in
warm (solid line) or cool (broken line) environment.
mg COz/g needle dry weight per hour. Average
rates at the three ages (cotyledon stage, 55 days
of age, and 100 days of age) were, respectively,
24.1, 9.5, and 9.2mgCOz/g needle dry weight
per hour.
Several of the interactions were also signifi­
cant. Seed source X age had the highest com­
ponent of variance of all the effects involving
source (Table 1). Montana and Oregon seedlings
had, according to this interaction, different
rate-age curves. Oregon seedlings had a higher
average photosynthetic rate than Montana
plants at the cotyledon stage. By 55 days,
Montana seedlings had a significantly higher
rate. Oregon seedlings again had the higher rate
at 100 days, primarily because of the poor per­
formance of Montana seedlings from the warm
environment (Fig. 1).
A source X measurement-temperature inter­
action resulted from photosynthetic rates of
Oregon plants exceeding the rates of Montana
plants more at low measurement temperatures
than at high temperatures (Fig. I). Krueger and
Ferrell ( 1965) found the same relationship be­
tween coastal British Columbia and Montana
seedlings. This interaction was not significantly
altered by age or growth temperature.
In general, plants from the warm environment
had higher measured photosynthetic rates than
plants from the cooler environment. Results
from other organisms both agree (Woledge and
Jewiss 1969) and disagree (Langridge 1963;
Sorokin 1958) with these results. This lack of
consistency could be associated partially with
the temperatures used or with length of time the
plants remained in their respective environments.
Growth temperature interacted significantly
with both age and measurement temperature.
The growth temperature X age interaction was
revealed through a reduced effect of environ­
ment on growth with increasing plant age or
with the change from cotyledons alone to a mix­
ture of cotyledons and needles. Growth tempera­
ture X measurement-temperature interaction
occurred because plants grown at low tempera­
ture tended to have lower optimum temperatures
for photosynthesis (Fig. 1). In fact, the measure­
ment temperature interacted significantly with
both age and growth temperature, and an esti­
mate of an optimum temperature or temperature
range for photosynthesis would not mean much
unless these factors were taken into account.
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SORENSEN AND FERRELL: DOUGLAS-FIR SEEDLINGS
Net photosynthetic rates of 55-day-old plants
were determined at 12.5° and 42.5°C, in addition
to the three temperatures used previously (Fig.
1). Assimilation rates of plants from the high­
temperature environment remained higher than
those from the low-temperature environment.
These results were similar to those reported by
Mooney and West (1964), who gave examples of
species whose photosynthetic rates at tempera­
tures from 10° to 30°C were greater after acclima­
tion at a "desert station" than after acclimation
at a "subalpine station."
As in most tests of this type (Lyutova 1958;
Pisek and Winkler 1958; Rook 1969; Semikha­
tova 1960; Tranquillini 1957; Woledge and
Jewiss 1969), the effect of environment was of
short duration. The duration of effect was tested
for the Douglas-fir seedlings by growing the
plants of the Corvallis source for 45 days in
either warm or cool conditions, then moving
them to the other environment and determining
photosynthetic rates 10 days later. Rates were
compared with nontransferred seedlings (Table
2). Seedlings transferred from one chamber to
another behaved, with respect to photosynthesis,
almost as if they had spent their entire lives in
the latter environment. Mooney and Harrison
(1970) showed that such changes, involving com­
plex modifications of stomatal, mesophyll, and
biochemical behaviors, can occur within a 24-h
period.
1693
Missoula: 2.53 mg COz evolved/g needle
dry wt. h
Corvallis: 2.46 mg C02 evolved/g needle dry
wt. h
Effects due to age were highly significant,
with the rate dropping rapidly with age.
Cotyledon stage: 4.39 mg COz evolvedfg
needle dry wt. h
55 days old: 2.33 mg COz evolved/g needle
dry wt. h
100 days old: 1.51 mg COz evolved /g needle
dry wt. h
Environment in which the plants were raised
also had a strong effect on their subsequent
respiration. Plants from the cool environment
respired almost twice as rapidly as plants from
the warmer environment.
l8°C day-4°C night, 3.43 mg COz evolved/g
needle dry wt. h
36°C day- 21 oc night, 1.81 mg COz evolv­
ed/g needle dry wt. h
Difference between measurement temperatures
was 7.5°C. Average respiratory rates at the three
temperatures for all plants were as follows:
20°C: 1.60 mg C02 evolved fg needle dry wt. h
27.SOC: 2.57 mg COz evolved/g needle dry
wt. h
35°C: 3.78 mg C0 2 evolved/g needle dry wt. h
Dark Respiration
When averages were taken over all treatment
combinations, there was no difference between
seed sources. Average rates 2 for sources were
as follows.
The only significantly different response in­
volving seed source was with environment in
which the plants were grown. Missoula plants
respired more rapidly than Corvallis plants after
growth at cool temperatures; and Corvallis
plants, more rapidly than Missoula plants after
growth in warm temperatures.
TABLE 2 The only other significant interaction was
Photosynthetic rates at 27.5°C of transferred and between age and growth temperature. Plants in
non transferred 55-day-old seedlings of the Corvallis source; rates are in mg COz absorbed /g needle dry wt. the cool chamber respired more rapidly than
per hour plants in the warm chamber at all ages, but the
difference was greater at the cotyledon stage
Treatment
Rate
than at older ages.
Warm chamber: 55 days
Cool chamber: 45 days; warm chamber: 10 days
Wann chamber: 45 days; cool chamber: 10 days
Cool chamber: 55 days
12.8/J*
14.3a
7.4c
S.lc
*Any two means with different l~ttcrs are significantly clilfcrcnt at
the 0.05 level by DtE1C~m·s new multiple r.:wgc test.
2 These averages are retransformed from the logarithms
of the original rates.
Dry Weight
In general, high growth temperature decreased
rate of dry matter accumulation, the difference
increasing with advancing age up to 100 days.
On the basis of whole plant dry weight, seedlings
of the two sources were similar when grown in
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1694
CAN. J. BOT. VOL. 51, 1973
the l8°-4°C environment but quite different
when grown in the 36°-21 oc regime (Fig. 2).
All plants in the cool environment had about
the same weight distribution of needle, stem, and
root tissues up to age 55 days (Fig. 3). By 100
days, the Missoula plants had lower needle
weight and greater stem and root weight than
the Corvallis plants, apparently as a result of
Missoula seedlings in cool environment setting
buds between 55 and 100 days. Bud set did not
affect photosynthetic rates (Fig. 1) but did stop
terminal elongation, which in turn reduced
needle dry weight increment. Excess of photo­
synthate apparently went to roots and stems and
increased their growth rate. Missoula plants in
200
- - - Co.ol en.vironme.. nt plants Warm environment plants [
160
o
Corvallts seedl1ngs •
Mtssoula seedlmgs ill
:;;:
<>:
a:
"'j
120
~
f-
I
"'"'5:
80
>­
a:
0
40
DL-----~----~-----~~----~
10
30
50
70
90
110
SEEDLINGS AGE (DAYS)
FIG. 2. Average whole plant dry weights of Corvallis
and Missoula seedlings raised under two temperature
environments at various ages.
55 DAYS
Missoula
Corvallis
~
18/4 36/2118/4 36/21
the cool environment added little photosynthetic
tissue, which reduced their total increment rela­
tive to nondormant Corvallis seedlings.
Discussion
Photosynthesis
The main features of the photosynthetic
results related to seed source may be summarized
as follows. First, Oregon plants had higher
photosynthetic rates than Montana plants at the
cotyledon stage (Fig. I). This relationship
changed with age and led to a significant source
X age interaction (Table 1). Second, Oregon
seedlings had higher rates at low measurement
temperatures (Fig. 1), and this interaction was
also significant. Third, photosynthetic rates of
the two sources responded differently to the
prolonged exposure to high growth tempera­
tures, as shown by the significant source X
growth temperature X age interaction (Table 1).
In general, growth environment had a much
greater effect on net photosynthesis than did
seed source. Rates of warm chamber plants were
higher than those of cold chamber plants at all
measurement temperatures, with greatest differ­
ences at higher measurement temperatures.
Summed over all treatments, average net photo­
synthetic rates of warm chamber plants exceeded
those of cool chamber plants by 47%.
On the average, Oregon seedlings had rates
about 5% higher than the Montana plants.
Larger differences between sources emerged
when other factors were considered. For ex­
ample, the biggest differences were in the
cotyledon stage, when rates for Oregon seedlings
were 12% higher than rates for Montana seed­
100 DAYS
Corvallis
Missoula
18/4 36/2118/4 36/21
100
LU
u
a:
:5:
80
60
40
>­
a:
0
...J
20
<>:
f­
0
f-
FIG. 3. Relative dry weights of needles, stems, and roots of the two sources, at two ages and after growth
in the two environments.
SORENSEN AND FERRELL: DOUGLAS-FIR SEEDLINGS
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lings. The differences between the sources essen­
tially disappeared with increasing age. The non­
persistent early differences could have been due
to maternal effects. Short-lived maternal effects
could be of significance in establishment of
seedlings in their respective environments but
would not seem to be of significance to longer­
term growth.
Relationship Between Photosynthesis and Growth
Increment in the warmer environment was
considerably less than in the cooler conditions.
Yet for the entire 100 days that the plants
developed, all material from the warm chamber,
when measured under the same conditions as
plants from the cooler chamber, assimilated
more rapidly than the latter plants. For example,
at 55 days of age, seedlings from the warm
environment were 13% lighter in total dry weight
than seedlings from the cool environment. The
difference was even greater at 100 days. But in
their comparative photosynthetic rates, the
warm-chamber plants were about 50% higher.
Others have reported similar results for other
species (Neuwirth 1967; Hanson 1971 ). In
attempting to explain this seeming paradox, we
considered five contributing factors.
1. The plants were grown at about 1000 ft-c
light intensity, but photosynthesis was measured
at a saturating light intensity of 5000 ft-c. If
initial slopes of the light intensity curves were
greatly different for plants from the two environ­
ments, the photosynthetic rate at saturating light
intensity might be a poor indicator of what was
occurring in the growth chamber light intensity.
2. Photosynthetic rates for warm-chamber
plants were always higher than rates for cool­
chamber plants when compared at the same
temperatures. But the proper rate comparison
for growth should be at the particular tempera­
tures being used in the growth chambers.
3. Respiration at night for plants from the
two environments might have been different.
4. Root respiration may have been different
for the two environments.
5. Distribution ofphotosynthate to needle and
nonneedle tissue was different and would have
contributed to the difference in plant size.
We will show the effect of these five points in
turn, using 55-day-old plants as a specific
example.
1695
Effect of Light Intensity
One early study (reported in Lyubimenko
1963) indicated that light intensity required for
light saturation became lower as the growth
temperature became higher. To test this for the
warm and cool environments used here, two
groups of Corvallis seedlings were grown to 55
days, one in each temperature regime, then light
intensity curves were determined from photo­
synthetic rates measured at 500, 1000, 2000, and
5000 ft-c. Warm-chamber plants were tested at
their day-growth temperature of 36°C, cool­
chamber plants at their day-growth temperature
of l8°C. For warm-chamber plants the photo­
synthetic rate at 1000 ft-c was 80% of the rate at
5000 ft-c; for cool-chamber plants, the figure
was 65%. As a result, the photosynthetic rate of
warm-chamber plants at 1000 ft-c turned out to
be almost twice the rate of cool-chamber plants.
The absolute difference in milligrams C02
assimilated /g needle dry weight per hour be­
tween plants from the two chambers was almost
the same at 1000 ft-c as at 5000 ft-c.
Effect of Measurement Temperature
During the 16-h photoperiod in each chamber,
the seedlings were 10 hat the maximum tempera­
ture and 3 h morning and 3 h evening in a transi­
tion between maximum and minimum tempera­
tures. Thus, warm-chamber plants spent 10 h
at 36°C and 6 h in changes between 36°C and
21 °C. Cool-chamber seedlings spent 10 h at
l8°C and 3 h twice daily in transition between
l8°C and 4°C. Average transition temperature
was 28.5°C for the warm environment and 11 oc
for the cool environment.
When photosynthetic rates were calculated
for 55-day-old plants at these temperatures, the
following hourly and daily accumulations were
obtained: (I) for warm-chamber seedlings, 10 mg
COz absorbed jg needle dry weight per hour for
the 10 hat 36°C and 13.6 mgjg per hour for the
transition periods; (2) for cool-chamber plants,
8.7 mgjg per hour at l8°C and 7.3 mgjg per
hour during the transition periods. On the basis
of grams needle dry weight per day, warm­
chamber plants absorbed 182 mg C02 and cool­
chamber plants, 131 mg. These rates were at
5000 ft-c. When they were adjusted to 1000-ft-c
light intensity, comparative daily photosynthate
accumulation amounted to 145 and 85 mg C02
absorbed per gram needle dry weight per day for
warm- and cool-environment seedlings, respec­
1696
CAN. 1. BOT. VOL. 51, 1973
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tively. In this instance, as with data below for
respiration, these unit values must be multiplied
by actual needle dry weights to obtain compar­
able seedling values. Therefore, if a seedling base
is considered, the difference between the two
groups is considerably reduced.
Effect of Seedling-top Respiration
Net photosynthetic rates have been used, so
only the respiration rates for the 8 h of darkness
had to be subtracted from the light C02 absorp­
tion totals. Respiration rates were estimated
from rates used for Fig. 15 in Sorensen (1964)
and were 1.1 mg C02 evolved Jg needle dry
weight per hour for warm-chamber plants (at
21 °C) and 0.3 mg Jg per hour for cool-chamber
plants (at 4°C). For the 8 h of darkness, this
amounted to 9 mg and 2 mg, respectively, on the
basis of grams needle dry weight per day. This
accounts, in a small way, for the discrepancy
between growth rates and photosynthetic rates;
however, the difference on a seedling-weight
base is not this large because the cool-environ­
ment plants were a little heavier.
Effect of Root Respiration
It seemed possible that root respiration for
warm-chamber plants was higher than for cool­
chamber plants. However, no separate measure­
ments of root respiration were made. lt was
assumed that, on a tissue-weight basis, root
respiratory rates were equivalent to seedling-top
respiratory rates.
Using warm-chamber temperatures of 10 h
at 36°C, 6 hat 28.5°C, and 8 hat 21 oc and cool­
chamber temperatures of 18°, 11°, and 4°C,
respectively, the estimated daily evolution of
C02 by the roots was 20 mg C02 Jg needle dry
weight per day for the former and 9 mg Jg per day
for the latter. This again accounts for part of the
discrepancy between photosynthetic rates and
growth rates.
Effect of Photosynthate Distribution
In spite of the "roughness" of some of the
estimates, it seemed certain from these figures
that on a needle-dry-weight basis the warm­
chamber plants were indeed assimilating more
rapidly than the cool-chamber plants, although
the latter had a more rapid dry weight increment.
One of the most striking differences between
plants grown in the two temperature environ­
ments was the relative amount of needle or
photosynthetic tissue. At the cotyledon stage,
when the seedlings had just shed the nutritive
tissue of the female gametophyte, the cool­
chamber plants had a slightly smaller proportion
of needle tissue. At both 55 and 100 days of age,
the cool-chamber plants had, relative to total
plant weight, 20% more needle tissue. Presum­
ably this adjustment is only another example of
the adjustment of plant growth to the environ­
ment (Blackman and Wilson 1954). However,
it is apparent that a 20% larger photosynthetic
apparatus plus a continual channeling of more
photosynthate into more photosynthetic tissue
could compensate for a lower photosynthetic
rate.
Borsdorf (1967), Brix (1967), Sweet and
Wareing (1968), Ledig and Perry (1969), and
Ledig (1969) had similar results; they found that
it was necessary to take leaf-weight /plant-weight
ratios or some equivalent into consideration to
make growth predictions from photosynthetic
rates.
Because of the apparent influence of photo­
synthate distribution on growth rate, we were
interested to determine if a certain distribution,
once started, was stable if the environment
changed. To test this, seedlings were planted and
grown in either a warm or cool environment for
27 days, then transferred to the other environ­
ment and harvested 28 days later. Plants which
spent the last half of the time in the cool en­
vironment had a needle proportion of 0.58;
those grown last in the warm environment had a
needle proportion of 0.52. Needle proportions
for plants which had been raised all 55 days in
either cool or warm environments were 0.61 and
0.50, respectively.
Thus, various plant tissues of the seedlings
seemed to respond to different environments.
In one environment, this was a relatively small
top or needle portion; in another environment,
a relatively small root. But in both cases, the
photosynthetic rate was apparently somewhat
dependent upon some desirable or fixed relation­
ship of needle to nonneedle tissue.
Table 2 shows that photosynthetic rates
adjusted quite rapidly when the plants were
moved from one growth environment to another.
If we look more closely at the assimilation rates
of transferred and nontransferred seedlings, we
can see that the average assimilation rate of the
plants transferred into the warm environment
was higher than that of the regular warm­
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SORENSEN AND FERRELL: DOUGLAS-FIR SEEDLINGS
chamber plants (14.3 mg C02 absorbed fg needle
dry weight per hour and 12.8, respectively), and
that the average assimilation rate of plants trans­
ferred into the cool environment was lower than
that of the regular cool-chamber plants (7.4 and
8.1, respectively). This indicated that the trans­
ferred plants not only were adjusting the dis­
tribution of current photosynthate to suit new
environmental conditions but also were growing
toward some fixed balance between the different
tissues. Plants which spent half their lives in
both environments had needle:nonneedle weight
ratios quite similar to expected ratios for the
second environment. This could hardly have
occurred if there had not been at least a tempo­
rary overadjustment by the transferred seedlings
in distribution of photosynthate.
In this test it was not possible, because of
insufficient Montana seed for some of the final
comparisons, to relate racial differences in
photosynthetic rate to differences in photosyn­
thate distribution. But Zavitkovski ( 1964), using
Douglas-fir seedlings from northeastern Wash­
ington and western Oregon, measured photo­
synthetic rates at 2 and 3 months after exposing
plants to various moisture stresses. If only
measurements made under favorable moisture
conditions were considered, photosynthetic rates
for the two sources were equivalent at 2 months,
and rates at 3 months for the seedlings from
northeastern Washington were higher than the
rates for the plants from western Oregon.
Nevertheless, western Oregon seedlings were 7%
heavier than northeastern Washington seedlings
at 2 months and 25% heavier at 3 months. Again,
this reverse relationship between photosynthetic
rate and growth seemed to be reasonable only
when distribution of plant weights was taken into
account. Northeastern Washington plants at
2 months had an average top-root ratio of 2.16,
western Oregon plants, a ratio of 2.58. At 3
months, the top-root ratios were 1.31 and 1.93,
respectively. Thus, the higher photosynthetic
rate of the northeastern Washington seedlings
was not a reflection of faster top growth but
probably of faster root growth relative to top
growth.
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