Reprinted from ECOLOGY, Vol. 48, No. 2.
Early Spring 1967.
Purchased by the Forest Service, U.S. Department of Agriculture,
for official use.
PATTERNS OF P HOTOSY NTHESIS U NDER NATURAL E NVIRO N ME NTAL COND ITIONS JOHN D. HODGES1 Ulli'Vcrsity of rVashington, Seattle, T,Vas/lillgton
(Accepted for publication August 6, 1966)
Abstract. Net photosynthesis in six different conifers was studied under various natural
environmental conditions. Changes in the pattern of photosynthesis on clear days, especially
the midday decrease, are apparently primarily controlled by changes in leaf water potential.
In noble fir and Scots pine, water potential probably acts mainly through its influence on
stomatal movement. In grand fir, Douglas-fir, hemlock, and Sitka spruce, however, some other
mechanism, probably mesophyll resistance to CO2 diffusion, seems to play a more important
role. Both mechanisms probably operate concurrently in all species. Daily variations in leaf
water potential seem to occur primarily in response to changes in atmospheric moisture, or,
more precisely, vapor pressure gradient from leaf to atmosphere. Variation in carbohydrate
content, through its influence on solute concentration, may also influence leaf water potential.
INTRODUCTION
The pattern of assimilation in forest trees dif­
fers markedly on days with different local weather
conditions and, most often, the highest rates of
assimilation occur on cloudy or overcast days
(Polster 1950, Pisek and Tranquillini 1954,
Gentle 1963, Helms 1963) . On overcast days,
net photosynthesis normally reaches a peak about
midday and then gradually decreases throughout
the remainder of the day. On clear days, there is
normally a morning peak followed by a marked
depression in photosynthesis and a secoGd, lower
peak (Rabinowitch 1945, Polster 1950, Stocker
1960) . The objective of this research was to
study intensively the pattern of net photosynthe­
sis in conifer seedlings of the Pacific Northwest
and, if possible, to explain these patterns by mea­
surement of various plant and environmental fac­
tors. The research was part of a larger investiga­
tion which was also designed to study and compare
photosynthetic rates and efficiency of seedlings of
west coast conifers under a wide range of natural
environmental conditions and to use the informa­
l Present address: Southern Forest Experiment Sta­
tion, Alexandria, Louisiana.
tion in explaining ecological differences between
species. Results of the measurements of photo­
synthetic rates and efficiency will be presented in
a subsequent paper.
MATERIALS AND METHODS
Plant material and environment
Seedlings (2-0 stock) of six species-Douglas­
fir (Pseudotsuga menziesii ( Mirb.) Franco) ,
grand fir (Abies grandis (Lindley) ) , western
hemlock (Tsuga heterophylla. (Rafinesque) Sar­
gent), Sitka spruce (Picea sitchensis (Bongard)) ,
noble fir (Abies procera), and Scots pine (Pinus
silvestris (L.) ) -were out-planted in plots along
north-south transects extending from deep within
a 35- to 40-year-old Douglas-fir stand into an
adjacent open area cleared of all vegetation in­
cluding herbaceous plants. The four most inter­
esting environmental situations were those des­
ignated as open (full exposure) , outside stand
border, inside stand border, and deep inside stand
(deep shade). The location designated as "out­
side stand border" was immediately adjacent to
the outer margin of the Douglas-fir stand, while
the one designated as "inside stand border" was
Early Spring 1967
PATTERNS OF PHOTOSYNTHESIS
approximately 20 ft inside the stand. Thus the
envirollment at both locations was influenced by
the Douglas-fir stand, but the influence of the
stand was mllch greater at the location just inside
the stand border. The location deep inside the
stand was approximately 125 ft fr0111 the margin
on the stand.
Light intensity was often greater than 11,000
ft-c in the open on clear summer days. Intensity
at the stand border (outside) was normally in the
range of 1,000-2,000 ft-c except in late afternoon
when the area received direct sunlight for a short
period. Inside the stand border the plants re­
ceived little direct sunlight and intensity was nor­
mally less than 1,000 ft-c. Light intensity deep
inside the stand was normally less than 100 ft-c
and seldom over 200 ft-c except during occasional
"sun flecks." Environmental factors other than
light were also modified by the Douglas-fir stand.
The effect of the stand on air temperature, as com­
pared to the open area, was to reduce the maxi­
mum temperatures and to slightly increase the
minimum temperatures. At locations influenced
by the stand, relative humidity, especially on clear
days, was higher than in the open. A more de­
tailed description of the environment at the dif­
ferent locations will be presented in a subsequent
paper dealing with ecological differences between
the six species.
Seedlings were also planted at locations inter­
mediate in position between the open area and
the stand border (outside), and between the 10­
cation inside the stand border and deep shade.
These seedlings were not studied intensively, how­
'
ever, since their environment and rates and pat­
terns of net photosynthesis were almost identical
either to seedlings in the open area or in deep
shade.
Experimental plantings were made in January
1962 and January 1963. Two transects of plots
were established in 1962 and one transect in 1963.
The 1962 plots contained six seedlings of each
species, while those established in 1963 contained
20 seedlings of each species. Thus 32 seedlings
of each species were planted at each experimental
location. The study area was located on Charles
Lathrop Pack Demonstration Forest which is
owned by the College of Forestry of the Univer­
sity of Washington and located near LaGrande,
Washington.
Measu,remen! of plant factors and processes
An infrared gas analyzer was used for the mea­
surement of net photosynthesis during the day­
light hours and of respiration at night. Gas
samples were collected from a small plastic (poly­
propylene) cuvette which surrounded the foliage
235
sample. The total needle mass sampled was kept
at about 0.5 g, ovendry weight. For all samples,
a single branch from the uppermost whorl of
branches which developed during the second grow­
ing season in the field was used. Thus, seedlings
llsed fr0111 June 1963 to about June 1964 were
planted in January 1962 and those used from June
1964 until the end of the study were planted in
January 1963. In all cases, the needles were fully
developed and, depending on the species, 2-4
inches of the branch tip were placed in the cuvette.
An automatic switching mechanism at the CO2
analyzer permitted cyclic sampling at six separate
points. Since at least one point (line) was always
used to monitor ambient CO2 concentration, up
to five seedlings could be sampled at any time.
The sampling procedure was varied so that at
times all sample lines were placed in one location
in order to compare photosynthesis in different
species under the same environment. At other
times the sample lines were placed along the en­
vironmental transect, and rates and patterns of
assimilation were compared for the same species
over a range of environments. Net photosynthe­
sis was normally determined for a period of 3
days on each sample seedling after which the
foliage sample was removed and needle dry weight
determined. Seedlings were normally used only
once except at the location deep inside the stand
where survival was poor. The investigation ex­
tended over a 2-year period beginning in June
1963. Thus measurements were made in all sea­
sons of the year. A total of 390 seedlings was
used in the investigation.
Plant factors which were measured in the
spring, summer, and fall of 1964, included leaf
water potentiaJ,2 sugar content of the foliage, leaf
temperature, and relative stomafal aperture. Leaf
water potential and sugar concentrations were
determined from foliage samples of the same.seed­
ling taken at periodic intervals throughout the
day. An electric psychrometer, similar to the one
described by Spanner (1951), was used for water
potential measurements. Sugars, reducing and
non-reducing, were determined colorimetrically by
a. modification of the method given by Bernfield
(1955). Leaf temperature was measured con­
tinttously by a thermocouple attached to the under
surface of the needle. On two occasions, field
measurements of relative stomatal aperture were
made throughout the day and into the night. In
addition, stomatal measurements were made in the
• Leaf water potential is the difference
or chemical potential between pure water
contained in the leaf cells. The potential
is zero. Therefore, the potential of water
less than zero (a negative number).
in free energy
and the water
of pure water
in leaf cells is
236
JOHN D. HODGES
laboratory under controlled conditions of light,
temperature, and humidity. All stol11atal measure­
ments were made by an infiltration technique
described by Fry ( 1965). Measurements listed
above as well as the measurement of environmen­
tal factors were made concomitantly with mea­
surements of CO2 assimilation.
0
"
....
Ecology, Vol. 48, No.2
50
12
40
10
8
.,
Q.
E
.,
I-
2
6
4
MeaSlIre111ent of environmental factors
Continuous measurements of light intensity, air
temperature, and relative humidity were made for
the duration of the study. Light intensity was
measured by means of selenium photocells and/or
radiometers. Temperature and humidity were
measured with wet-dry bulb thermocouples and
with hygrothermographs. All light intensity and
thermocouple temperature measurements were re­
corded on a 24-point millivolt recorder. Soil
moisture was determined by gravimetric sampling.
Under fully exposed conditions, the typical pat­
tern of photosynthesis on overcast or cloudy days
was one in which net photosynthetic rates in­
creased to a maximum about noon, then either
Light
2
i
t
i!
20
15
0
..
..,
1 0
..
U
..
;:
15
...
§
.c
5
'"
.c
:::;
6.0
Temp.
'3:<
1 0
r: 3 :tJ
x!"
'"
5
u
0
Grand Fir
5.0
4.0
3.0
2.0
1.0
...
"..
E
c
0
;::
.!!
]
::
«
0.0
-1·0 -2'()
-3.0
12
M
.2
4
6
8
10
12
N
2
..
6
.8
10
12
M
Time
Typical patterns of net CO2 assimilation under
full exposure on overcast days in the summer months.
Curves shown are for individual grand fir and noble fir
seedlings, but the pattern is representative of all six spe­
cies under similar conditions. V.P.D. vapor pressure
deficit.
FIG. 1.
=
...
co
;;.
E
c
0
'iii
..
<
Patterns of net photosynthesis
20
2
'E
RESULTS AND DISCUSSION
6
e...
Light
-I.
12
M
2
4
6
8
10
12
N
Time
2
4
6
8
10
12
M
FIG. 2.
CO2 assimilation for single seedlings under full
exposure on a selected clear day in the summer months
( 1964). Curves shown were chosen as typical for above
conditions. V.P.D. = vapor pressure deficit.
decreased or remained constant' for an hour or
two and finally decreased (Fig. 1). On bright
sunny days in the growing season, assimilation
normally increased rapidly, reaching a peak at
about 9-12 AM, then decreased until late after­
noon when it again increased and reached a second
but much lower peak (Fig. 2). Patterns were
basically the same at all seasons of the year, but
in the colder months the period of photosynthesis
was shorter, rates of net photosynthesis were
lower, and the depression on sunny days was less
than in the warmer months.
The patterns described above for exposed con­
ditions were typical for all six species. vVith
Scots pine, however, the midday decrease on clear
days was often not so marked as for the other
species (Fig. 2), and rates were much more vari­
able. Also with Scots pine, the average daily rate
of photosynthesis increased after several days of
sunny weather (Table 1), but with the other
species a progression of clear days resulted in a
progressive decrease in total photosynthate pro­
duction. Furthermore, there was a noticeable
interaction between weather conditions and species
in terms of assimilation rates. Species showing
the highest rate on clear days (Scots pine and
noble fir) did not necessarily show the highest
PATTERNS OF PHOTOSYNTHESIS
Early Spring 1967
TARLE 1.
Effect of successive days of clear weather on
average rate of CO.) assimilation (same seedlings used
throt1ghout-sumll1e , 1964)
Average
No. days
with sun
t rmp<'ratu ro
('C)
1........ ..
2..........
3..........
23
26
28
26
4 ..........
.\.vrrap;e
V.P.D.
(10m H )
12.80
15.88
20.13
15.63
Av£'rnp;c dailY rate of assimilation
(lOg! gUl/hr)
--
-------.
Douglas-fir
1.44
0.76
0.59
0.52
Noble
1.89
1.14
1.04
1.07
fir
---
Scot. pine
---
2.26 2.43 2.60
2.76 rate on overcast days (compare noble fir and
grand fir) Fig. 1 and 2). In general, net photo­
synthesis in Scots pine, and to a lesser extent
noble fir, appeared less adversely affected by
sunny conditions than in the other species.
At the locations near the stand border (outside
and inside the stand) the basic patterns of net
photosynthesis were the same as for the open area.
However, there were differences in rate of photo­
synthesis, timing of the peak rate of photosynthe­
sis, and in the magnitude of the depression on
clear days. For all species except Scots pine and
noble fir it was quite common for peak rates of
photosynthesis and total photosynthate production
to be higher outside the border than in the open
area even on overcast days. Also, the depression
on clear days was usually not so great as in the
open. Inside the border peak rates of photosyn­
thesis occurred later in the day and the depression
on clear days, although still very apparent, was
much less than in the open. Daily photosynthate
production by the seedlings was lower inside the
stand border than in the open on overcast days
but, except for Scots pine and noble fir seedlings,
was higher on bright sunny days. Short-term
fluctuations in rates of photosynthesis were much
more common at the stand border, especially in­
side the border, than in the open due to greater
short-term variation in light intensity. As with
the open area, daily weather conditions seemed to
control the basic pattern of net photosynthesis so
that no great seasonal differences were apparent.
In the colder months, however, rates of net photo­
synthesis were lower, the photosynthesizing period
was shorter, and midday depression was less on
clear days.
Under deep shade the pattern of photosynthesis
was controlled by variations in light intensity.
Peaks of photosynthesis as well as rate were de­
termined by the occasional sun flecks which pene­
trated the dense canopy.
The discussion which follows is concerned with
the general pattern of photosynthesis which was
common to all locations, except deep shade. As
will be seen, however, comparisons of rates and
pattern of photosynthesis under the different en­
237
vironmental situations were invaluable in explain­
ing the general patterns. Unless otherwise noted,
data which are cited are for the fully exposed
environment.
Expla.nat. ions of patterns
Much has been written concerning daily patterns
of photosynthesis and especially the midday de­
crease in photosynthesis. The literature suggests
at least nine possibilities that might account for
the depression, as follows: (a) a daily decrease in
the CO2 content of the air; (b) bleaching of
chlorophyll (Bohning 1949); (c) increased chlo­
roplast temperature (Kozlowski 1957); (d) pho­
tooxidative deactivation of enzymes (Rabinowitch
1945); (e) increased respiration (Kozlowski
1957); (f) accumulation of photosynthetic end
products (Thomas and Hill 1949); (g) stomatal
closure ( Maskell 1928, StMelt 1935, Nutman
1937); (h) increased mesophyll resistance to CO2
diffusion (Gaastra 1959, Shimshi 1963, Bierhuizen
and Slatyer 1964); and (i) water de ficit in the
leaves (Pisek and Tranquillini 1954).
In the present investigation, the first five of
these possibilities could not account for the differ­
ences in pattern of photosynthesis on overcast and
clear days. As for possibility (a), the CO2 con­
tent of the atmosphere was remarkably constant
and there was often a marked decrease in net
photosynthesis even though there was no change
in CO2 concentration. The data for photosynthe­
sis under various environmental conditions (along
the environmental transect extending from within
the stand to the open field) indicate that sugges­
tions (b), (c), (d), and (e) can also be eliminated
as major causes of the midday decrease. The
decrease was C0111mon at the location just inside
the stand border on clear days and sometimes oc­
curred deep inside the stand. This would strongly
suggest that bleaching of chlorophyll and photo­
oxidative deactivation of enzymes were not factors
in the depression because light at both locations
was from a diffuse source, and intensity was less
than 1,000 ft-c near the border and normally less
than 200 ft-c deep inside the stand.
\Vhen the data are considered in relationship to
daily variation in temperature, there is little like­
lihood that high chloroplast temperature and in­
creased rates of respiration (resulting from in­
creased temperature) could account for the ob­
served decrease in assimilation. On August 17,
1963, for example, at the location just inside the
stand, there was a 5970 reduction in net assimila­
tion of Sitka spruce from 9 :30 AM (maximum
rate) until 3 :30 PM (minimum rate), after which
the rate again increased. During this period the
air temperature increased only 3.0°C (15.0° to
238
JOHN D. HODGES
18.00). Leaf tel11perature was never 111000e than
0.50 higher than air temperature. Uncler more
exposed conditions, increased temperature did re­
sult in some decrease in net photosynthesis due to
an increase in respiration. On several occasions,
however, calculations were made of the reduction
in net photosynthesis which could be expected
fr0111 the increase in temperature. Even under
the most extreme conditions (hot summer days)
respiration could not account for all the reduction,
and on cooler days increased respiration c0111monly
accounted for less than 15% of the midday reduc­
tion in assimilation.
The possibility that reductions in the rate of
photosynthesis result from a clogging of the photo­
synthetic mechanism due to an accumulation of
photosynthetic end products cannot be substan­
tiated by the findings of this study. Sugar con­
centration in the foliage did increase during the
period when rate of photosynthesis was decreas­
ing; but normally, sugar content continued to
increase even after net photosynthesis began to
increase in late afternoon (Fig. 3). Also, the rate
of sugar accumulation was as high and sometimes
higher on overcast mornings than on clear morn­
ings. It is not implied, however, that sugar
accumulation has no influence on patterns of pho­
tosynthesis. As will be shown later, sugar con­
8
c
0
+=
1:1 .. ....
c
4D
U
C
0
(,)
7
Ecology, Vol.48, No.2
centrations may affect photosynthesis through an
influence on cell water relations.
Leaf water status and photosynthesis
Of the possible explanations for the diurnal
decrease in photosynthesis, (f), (g), and (h) are
all related in some way to the water status of the
plant. They may either directly affect the status
of the water in the plant or be affected by it. The
last named possibility, water deficit, is a "catch
all" in that the possible influence of the water
status is recognized but no explanation is given as
to how the water deficit acts to bring about a
reduction in photosynthesis.
Since these possibilities are related to the status
of the water in the plant, it was decided to first
determine whether or not photosynthetic rates are
related to water regime in the foliage. Vvater
status was determined by periodic measurement
of leaf water potential with an electric psy­
chrometer.
In agreement with the work of Boyer (1964) ,
photosynthesis was found to be well correlated
with changes in leaf water potential. In Douglas­
fir, grand fir, hemlock, and Sitka spruce correla­
tion coefficients were 0.96, 0.95, 0.90, and 0.95
respectively and were statistically significant in
all cases. On sunny days, photosynthesis tended
to decrease as leaf water potential decreased and
to increase when it increased (Fig. 4). The after­
noon increase in water potential apparently began
about an hour before the increase in net photo­
6
e
- 5 ..-.--,--.-,--.-,--,--,-,--.-.--,
.,[ -10
5
'I:
.,
-15
l-20
-25
...
..
4.0
c
o
;:
.!!?
·s
..
.;; .s::.
D. Fir 4.0
3.0
3.0
e2.0
2.0
",.
E 1.0
01
(\I'
c>1.0
g
o
(,)1:
'E
6
FIG. 3.
Photosynthesis and reducing-sugar content of
the foliage in single seedlings of Douglas-fir and grand
fir on a selected representative day in July 1964. Pattern
is basically the same for all species except that in Scots
pine the change in sugar concentration appeared to be
much less than for the other species.
0.0
·:i-I.O
c(
12
M
Time
FIG. 4. Leaf water potential and CO2 assimilation in
a Douglas-fir seedling on a clear summer day (1964).
Relationship is basically the same for grand fir, hemlock,
and Sitka spruce.
synthesis, but this could 1I0t he definitely estab­
lished because of the limitatio1ls of the sampling
procedure.
The relationship of leaf water potential to photo­
synthesis was not so obviolts in noble fir and
Scots pine as in the other species. Morning photo­
synthesis in noble fir decreased as water potential
decreased (Fig. 5), after which there appeared to
be an almost inverse relationship between the two.
\'Vith Scots pine there was normally a decrease
e
.,
!!! - 1 5
CI
;:
c:
!
0
II.
..
•
of0
...
.t: 'E
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--
-20 - 25
-30
3.0
2.0
1.0
c
o
0.0
]
: - 1.0
c
12
M
2
4
239
PATTERNS OF PHOTOSYNTHESIS
Early Spring 1967
6
8
10
12
N
Time
2
M
FIG. 5. Leaf water potential and CO2 assimilation in
single noble fir and Scots pine seedlings in early sum­
mer, 1964.
in photosynthesis during midday, and there ap­
peared to be a tendency for a slight diurnal de­
crease in water potential. A preliminary exami­
nation, however, indicated no direct relationship
between the two.
From these preliminary observations it might
be concluded that plant water status, as evidenced
by leaf water potential, is not an important factor
in controlling the daily pattern of photosynthesis
in all species tested. However, this does not seem
probable. More likely, water potential is impor­
tant in the control of photosynthesis in all species
tested, and the apparent difference between spe­
cies is due to a difference in the mechanism by
which water potential acts to control photosyn­
thesis.
Stomalal and t11esophyll resistance
to CO2 diffusion
Leaf water potential could conceivably influence
patterns of photosynthesis through its influence
on stomatal closure and/or mesophyll resistance
to CO2 diffusion. Although the exact mechanism
of stomatal movement has not been definitely
established, there is ample evidence that opening
and closing ultimately depend on increasing and
decreasing the supply of water to the guard cells
(Zelitch 1963). In the liquid phase of CO2 diffu­
sion, control of photosynthesis could be exerted
through resistance to gaseous movements into or
within the mesophyll cells. Changes in the water
regime of the mesophyll cells could have a marked
influence on CO2 solubility and diffusion. Rawl­
ings (1963) concluded that mesophyll resistance
was not constant, but was a function of water
potential and could vary with transpiration.
To determine if either or both of these possible
resistances are involved in the midday reduction
of photosynthesis, measurements of relative stoma­
tal aperture were made in the field and in the
laboratory in a controlled environment chamber.
Measurements were made in the summer and fall
of 1964 on foliage of the current year. In the
laboratory relative stomatal aperture was deter­
mined under conditions of decreasing relative
humidity and increasing temperature. A marked
difference in stomatal movemeilt was found be­
tween species. In one experiment the stomata of
grand fir were less responsive than those of noble
fir (Table 2). Stomata of grand fir and Douglas­
fir tended to close gradually while those of noble
TABLE 2.
Relative stomatal aperture in four species as determined by infiltration and expressed in pounds of pres­
sure at various temperatures and levels of humidity (A single seedling of each species was used throughout the
test. )
Time"
9:45...............................
11:00...............................
13:00...............................
14:30...............................
15:30...............................
16:30 .....
. ...
........................
Relative
humidity
Infiltration pressure (psi)h
(%)
Temperature
(OF)
Grand fir
92
60
40 20
15
15
70 75 80
85 98
98
5.5
6.0
6.5
9.0
17.0
38.0
-----
-Humidity and temperature were changed to next level immediately after the time shown. indicates that the stomata are effectively closed. bA reading of 30-40 psi
Noble fir
5.0
8.5
25.0
40.0
40.0
40.0+
Douglas-fir
5.5
7.5
13.0
27.0
40.0+
40.0+
Scots pine
10.0
30.0
26.0
19.3
30.0 31.4 240
JOHN D. HODGES
fir closed very rapidly once closure had begun.
Bannister (1964) has also noted that all species
do not react in the same manner to internal water
deficits in the leaves, stomatal closure occurring
much sooner in some than in others.
A second laboratory experiment on stomatal
movement confirmed the findings of the first. In
addition, grand fir showed a marked reduction in
photosynthesis before there was any appreciable
change in stomatal aperture. In noble fir, how­
e\'er, the pattern of photosynthesis appeared to be
more strongly related to stomatal movements
(Fig. 6).
.45°t R.H.20%
40 R.H.96"/t R.H.75% r·H
Nobl Fi r
:::
(1)3 0
0.:
20
e
c
o CII
... '"
...... 10
o
:;.,'20
OJ C
.. .,
jlO
-30
7.0
CD
..
..
C
" ...
OJ '"
:;
:;
...
-.
::;:
..,
C>
...
:;; g'
ol!!
6.0
.:: 5.0
.....
E
go
E
4.0
:;::g 3.0
:§.. 2.0
..
«
_D"""
x.-I(
1.0
8
9
0
II
Grand Fir
12
N
2
3
4
5
Time
FIG. 7.
---- ------ --
40
r:
it: =
E
·: 't
.,
Grand Fir
;: :
!:a.
Ecology, Vol. 48, No.2
ft. c.
I.
.
.
__------
Ti
2500
ft.
c.
::::: :::
--
--------
me
FIG. 6.
CO2 assimilation and relative stomatal aperture
as determined by infiltration in noble fir and grand fir.
A reading of 30-40 psi indicates that the stomata are
effectively closed. Results are for a controlled laboratory
test conducted in September 1964. Current-year needles
from a single seedling of each species were used through­
out.
Laboratory measurements of stomatal aper­
ture in Scots pine indicated that, under condi­
tions of decreasing relative humidity, the stomata
may close and later open again. This possibly
occurs many times in the course of a day. Other
researchers (Ehrler, Nakayama, and van Bave!
1965) have recently reported opening and closing
cycles of less than 30 min. Hemlock and Sitka
spruce seedlings were not used in the laboratory
experiments. Based on field observations, how­
ever, it appears that stomatal movements in these
species more closely resemble those of grand fir
and Douglas-fir than those of noble fir or Scots
pine.
Field tests of stomatal movement confirm the
laboratory observations. In Douglas-fir and grand
fir there was a reduction in photosynthesis with
Photosynthesis, leaf water potential, and sto­
matal aperture in Douglas-fir and grand fir under field
conditions. Curves shown are for single seedlings on a
selected representative day in late summer, 1964.
very little change in stomatal aperture (Fig. 7).
Concomitant measurements of photosynthesis and
stomatal aperture were not made for the other
species, but the stomata of noble fir and Scots pine
were much more responsive to closure than were
those of the other species. At night the stomata
of Scots pine and noble fir showed complete
closure, but stomatal closure was not observed in
grand fir. Douglas-fir was intermediate between
the two extremes. Hemlock and Sitka spruce
stomata showed some closure but not so much as
Douglas-fir.
These findings are consistent with the observed
patterns of change in both water potential and
photosynthesis. In species where stomatal closure
is of primary importance, it would be expected
that there would first be a decline in water poten­
tial until the stomata closed. After closure, water
loss by the plant would then be reduced so that
water potential would again increase. Later, the
stomata might open, followed by a second decline
in water potential due to increased transpiration.
Fig. 5 shows that this is consistent with the pat­
tern of change of water potential in noble fir. In
Scots pine there is evidence that the same pattern
is followed, but that the stomata open and close
much more rapidly than in noble fir. This would
account for the more rapid short-term fluctuation
in photosynthesis in Scots pine as compared to
the other species. It might also account for the
net diurnal decrease in water potential being less
Early Spring 1967
241
PATTERNS OF PHOTOSYNTHESIS
than in other species, and for the rate of photo­
synthesis being less affected by a progression of
warm, clear days. During periods of stomatal
closure transpiration would be reduced and partial
recharge might occur. To thoroughly investigate
the relationship between water potential and pho­
tosynthesis in this species woulcl require short
term (5-10 min) sampling of both water potential
and stomatal aperture throughout the photosyn­
thesizing period.
In this investigation, the reduction in net photo­
synthesis not accounted for by stomatal closure
has been attributed to mesophyll resistance. No
quantitative measurements were made, but of the
many possibilities considered, this phenomenon is
the only one that will fully account for the ob­
served reductions in all cases. There is ample
evidence in the literature of the existence of such
a phenomenon, and in several cases it has been
reported to be of more importance than stomatal
resistance in reducing photosynthesis (Gaastra
1959, Rawlings 1963, Bierhuizen and Slatyer 1964,
Fry 1965).
The two phenomena, stomatal closure and meso­
phyll resistance, do not operate to the mutual ex­
clusion of each other. Stomatal closure occurs in
all species, and under conditions such as low soil
moisture and low humidity, it is of primary im­
portance in regulating photosynthesis. There was
also an indication that in noble fir a certain amount
of reduction in photosynthesis occurred before
stomatal closure became important.
Environmental factors and leaf water potential
Daily variations in leaf water potential were
most closely related to variations in the vapor
pressure deficit (VPD) of the atmosphere. \iVith
a decrease in moisture content of the air, the vapor
pressure gradient between air and foliage increases,
resulting in more rapid water loss from the plant.
This in turn results in water stress conditions
within the plant and possible reduction in photo­
synthesis because of hydroactive stomatal closure
and/or mesophyll resistance. This agrees with
the later work of Philpot (1965) who reported
that diurnal fluctuation in moisture content of
ponderosa pine and whiteleaf manzanita leaves
was correlated with changes in relative humidity.
Soil moisture also influences leaf water poten­
tial. As soil moisture is lowered, leaf water po­
tential decreases daily and the effect of low
atmospheric moisture is intensified.
Sugar accumulation and patterns of photosynthesis
Another factor which has some influence on
daily variation in photosynthesis is sugar accumu­
lation in the leaves (Fig. 8). Sugar accumulation
-- Doullioa Fir
It---IIGra nd FI r
12
T olal
Toiol
10
Red uclnll
.,
CIt
8
c
'"
u
...
<IJ
a.
6
2
R e d u clnll
N on-Reducinll
4
Non-Rllduclnll
2
4
8
6
Time
FIG. 8. Variation in total, redncing, and non-reducing
sugar content of foliage during daylight hours. Results
are for a typical test in late spring 1964. Samples for
analysis were collected from single seedlings of each
species. Sugar content expressed as percentage of dry
weight.
may affect photosynthesis by directly influencing
stomatal movements as suggested by Yemm and
Willis (1954), or it may affect osmotic pressure of
the cells and membrane permeability as suggested
by Shreve (1931) and Boon-Long (1941). Sugar
accumulation, however, seems to be of less impor­
tance than vapor pressure deficit. Even on over­
cast days there is an increase in sugar, with rela­
tively little decrease in photosynthesis or leaf water
potential as compared to sunny days. The daily
increase in sugar is in the reducing form, which
results in part from an accumulation of the end
product of photosynthesis (glucose). It appears,
however, that there is also a conversion from the
non-reducing to the reducing form since reducing
sugars increased at a faster rate than total sugars
(Fig. 8).
ACKNOWLEDGMENTS
The author expresses appreciation to the U. S. Forest
Service and to the National Science Foundation for
providing partial support for this study; and to Dr. David
R. M. Scott for advice and assistance in the study and
for reviewing the manuscript. The work was accom­
plished while the author was a candidate for the Ph.D.
degree at the University of Washington.
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