COMJ\ruNITIES OF COTTON PLANTS
PHOTOSYNTHESIS IN ARTIFICIAL COMMUNITIES
IN RELATION TO LEAF AREA
I. EXPERIMENTS WITH PROGRESSIVE DEFOLIATION OF MATURE PLANTS
By L. J. LUDWIG,*t T. SAEKI,*t and L. T. EVANS*
[Manuscript received May 26, 1965]
[Manu8cript
1965J
Summary
The relation between the leaf area index (L.A.I.) of artificial communities
of cotton plants and their rates of dark respiration and of net photosynthesis at
three light intensities was examined. The L.A.I. was varied by the removal of
successive layers of leaves, working from the base of the canopy upwards. Experiments were carried out at 20, 30, and 40°0 to vary the relative magnitudes of
respiration and photosynthesis.
comAt all temperatures and light intensities, net photosynthesis by the com·
munities increased with increase in L.A.I. to values of 3-4. At 20°0, there was no
change in net photosynthesis with further increase in L.A.I. to 7·6. At 30°0 there
was a slight decrease in net photosynthesis as L.A.I. increased from 4 to 6·5,
6· 5, and
at 40°0 the decrease in net photosynthesis at the highest L.A.I. values was more
marked. At 40°0 the maximum L.A.I. that could be maintained was, however,
only 5·4. The maximum net photosynthetic rates by the communities, under a
002/dm22 of ground surfacejhr
surface/hr
light intensity of 3300 f.c., were 32, 25, and 18 mg 002jdm
at 20, 30, and 40°0 respectively.
The lack of a marked decline in net photosynthetic rates of the cotton communities at high L.A.I. values is shown to be due to the low rates of respiration in
the lower leaves of the canopy, the compensation point falling progressively with
depth in the canopy.
I. INTRODUCTION
The development of mathematical models relating photosynthesis in plant
communities to environmental conditions could be of value in two directions. On the
one hand, they could be used for the estimation of the potential yields of various
crop plants in a range of environments (cf. Nichiporovich 1956). On the other, they
could lead to a greater understanding of the photosynthetic structure of plant communities and of the ways in which this interacts with the various plant and climatic
parameters. To be useful in either context, such models must be complex enough
to simulate real plant communities to a reasonable degree. Many assumptions must
be made in setting up even simple models, and these assumptions must then be
subject to further experiment and analysis, if the models are to become progressively
more complex and realistic.
The early model of Davidson and Philip (1958) assumed that neither the light
extinction coefficient nor the mean rate of respiration per unit leaf area varied with
leaf area index (L.A.I.). To account for an equal rate of respiration in the upper and
* Division
t
t
of Plant Industry, OSIRO, Oanberra.
Present address: Department of Biology, Queen's University, Kingston, Ontario.
Present address: Department of Biology, University of Tokyo.
AUBt. J. Biol.
Bioi. Sci., 1965, 18, 1103-18
Aust.
1104
L. J. LUDWIG, T. SAEKI, AND L. T. EVANS
lower leaves they assumed that the lower leaves were "parasitic in the sense that
they must depend for their persistence on the translocation of material from the
productive regions". Nichiporovich (1956) had also assumed that lower, shaded
leaves are parasitic upon the plant. These assumptions lead to a clearly defined
optimum L.A.I., and a sharp fall in net photosynthesis per unit of ground area as
the L.A.I. of a stand increases beyond the optimum.
In the field a fall in growth rate at L.A.I. values beyond the optimum has been
found by Watson (1958), Davidson and Donald (1958), Konda and Sato (1963),
and Black (1963). Brougham (1956), on the other hand, found no fall in growth rate
as the L.A.I. of a pasture increased from 5 to 9. Similarly, with artificial communities
of wheat, rice, sunflower, and Perilla, Wang and Wei (1964) found no decrease in
net photosynthesis with increase in L.A.I. to high values. Nichiporovich and Malofeyev (1965) also found no decrease in net photosynthetic rate in artificial communities
of cabbage plants as their L.A.I. increased to 5·7. These latter results are difficult to
reconcile with the model developed by Davidson and Philip.
Verhagen, Wilson, and Britten (1963) developed a model in which the extinction
coefficient changed to lower values in the course of growth, to account for plant
communities in which there is no decrease in production at high L.A.I. values
Such changes in extinction coefficient may occur, as when a grass community develops
flowering stems, but there is as yet little evidence for them.
There is more evidence that the mean respiration rate may fall in the course
of growth. The models developed by Davidson and Philip (1958), Saeki (1960), and
Verhagen, Wilson, and Britten (1963) all assumed that the mean rate of respiration
does not change with increase in L.A.I. Because of this, and because he did not
consider that lower leaves could be parasitic, Saeki (1960) concluded that aboveoptimal L.A.I. values will only rarely and transiently be present.
In their comprehensive analysis of the growth of Helianthu8 tuber08u8 communities, Hogetsu et al. (1960) found that the respiration rates of mature and old
leaves fell to extremely low values, and that the respiration rate of young leaves on
old plants was also much lower than that of young leaves on young plants. In some
experiments on the effect of temperature on the respiration rate of leaves of several
varieties of cotton, we found a similar phenomenon to occur. Moreover, the extent
of the fall in mean respiration rate with age was such that one would not expect a
marked fall in net photosynthesis in plant communities beyond the optimum L.A.I.
L.A. I.
We therefore decided to examine the dependence of rates of photosynthesis
and respiration in artificial communities of cotton plants on their L.A.I. Our procedure was to use a controlled-environment cabinet as an assimilation chamber for
a segment of a crop. With this coupled to an infrared gas analyser, the dependence
of photosynthesis and respiration on L.A.I. was examined in two ways. The first,
used in the experiments reported in this paper, was by progressively defoliating a
dense community of mature plants from the base upwards, measuring photosynthesis
and respiration after removal of each layer of leaves. The second, used in the experiments to be described in a later paper, was to measure photosynthesis and respiration
at various stages during the growth of a community of cotton plants within the
cabinet. The results of the two methods were in close agreement with one another.
PHOTOSYNTHESIS IN COTTON PLANT COMMUNITIES. I
II.
1105
MATERIALS AND METHODS
(a) General
Most of the experiments were carried out with cotton (Gossypium hirsutum L.
cv. Deltapine 15). The cotton cultivars Acala, Pima, and Pope were also used in some
experiments, as were sunflowers.
The plants were grown initially in one of the glasshouses of the Canberra
phytotron (Morse and Evans 1962), at a day temperature of 30°C and a night
temperature of 25°C. Plants were grown singly in 6-in. diameter pots containing
perlite, under a layer of vermiculite, and were given Hoagland's nutrient solution
and water daily. Lots of 64 uniform well-grown plants were transferred from the
glasshouse to an artificially lit cabinet 1-2 weeks prior to making the measurements
of photosynthesis and respiration. This allowed the plants to adapt to the different
spatial arrangements and conditions oflight and temperature for the experimental run.
During the adaptation period the plants were exposed to light of about 3500 f.c.
intensity for 12 hr each day. For most experiments the temperature during the
light period was 30°C, although one experiment was carried out at 20°C, and one
at 40°C. The temperature of the 12-hr dark period was 25°C in all experiments.
(b) Assimilation Ohambers
A diagram of the general layout of the equipment is given in Figure 1. A type
LB cabinet, as described by Morse and Evans (1962) and Pescod, Read, and Cunliffe
(1963), was used as an assimilation chamber for the crop. It was artificially lit by
an arched sealed canopy of 28 125-W high-output, internal-reflector fluorescent
lamps, supplemented by four 75-W incandescent lamps. The arched canopy resulted
in a very uniform light intensity of up to 4500 f.c. across the cabinet with slightly
lower intensities at the front and back of the growing area. Pairs of fluorescent tubes
could be switched off to vary the light intensity over a wide range. The inside walls
of the cabinet were covered with highly reflective aluminium foil to reduce edge
effects. The plant growing area was 1· 38 by 1·16 by 1· 4 m high. The air flow, as
indicated in Figure 1, was downwards through the plant canopy at a rate between
0·25 and O· 5 m/sec, depending on the density of the foliage. The fresh air inlet to
the cabinet was coupled to a compressed air supply via a rotameter measuring flows
up to 2000 l/min.
For work with single leaves, a Perspex assimilation chamber 20 by 16 by 2 cm
high was used. This was made in two halves which could be bolted together to
enclose a single attached leaf. A network of nylon threads held the leaf in the central
plane of the chamber, and kept it in contact with the thermocouples.
(c ) Measurements
The absolute C02 level in the air supply to the assimilation chamber was
continuously monitored by a three,box Grubb-Parsons infrared gas analyser, sensitive
to 2 p.p.m. CO 2. A second infrared gas analyser, a Grubl;>-Parsons model LM.L,
sensitive to O· 5 p.p.m. CO 2, was then used to monitor the difference in CO 2 concentration between the air entering and that leaving the assimilation chamber. The
llOG
1106
L. J. LUDWIG, T. SAEK1,
SAEKI, AND L. T. EVANS
difference in CO 2 concentration between the two streams was indicated directly on
the dial of the instrument, and was also recorded on a Varian G-I0 recorder. Before
entering the gas analysers, the sample air streams were dried, first by condensation
in coiled
eoiled copper tubes immersed in ice, and then by passing over columns of calcium
chloride. The concentration of CO 2 in the input air, drawn from a basement room,
was always close to O·03%(vjv), while that in the cabinet was never more than 15%
below the input level.
LIGHT CANOPY
/
\
//
///
FRESH AIR
INPUT
Fig. l.-Diagram indicating the general layout for measuring photosynthesis, respiration, and
transpiration in artificial communities of cotton plants. A, three-box
three· box infrared gas analyser and
recorder; B, 1M1
IMI gas analyser and recorder; 0, humidity indicator; D, temperature recorder; E,
cabinet air outlet.
Since the volume of fresh air entering the cabinets was controlled and known
from the rotameter readings, the amount of CO 2 consumed in photosynthesis, or
evolved by respiration, under equilibrium conditions could be calculated.
The temperatures and relative humidities of the incoming and outgoing air
streams were measured with an electric hygrometer (Hygrodynamics Inc., Silver
Spring, Maryland). Air temperature in the cabinet was also measured with a Degussa
Hanau W85K platinum resistance element, and recorded on a Honeywell Universal
Model 15 Electronik Recorder. Leaf temperatures were measured with 30-gauge
PHOTOSYNTHESIS IN COTTON PLANT COMMUNITIES. I
1107
copper-constantan thermocouples held against the lower surface of the leaves,
their output also being recorded by a Honeywell recorder. Air movement rates
were determined with an Alnor thermoanemometer.
Light intensities at various positions above and in the crop canopy were
2 ) selenium
measured with an EEL photometer, and also by the use of a small (1·4 cm
cm2)
cell combined with an opal Perspex and a Kodak (CC40M) filter, mounted on the
end of a rod. This could readily be inserted at various levels into the canopy without
disturbing leaf arrangement. Readings at each level were taken at 7 or 14 positions
along the centre line across the cabinet from back to front, yielding mean values for
the characterization of the light profile within each plant community.
Leaf areas were determined in two ways. The first was by taking samples of
known area, and determining the area/dry weight relation for the leaves on the
primary shoot, and for those on the axillary shoots, in each leaf layer. The area of
each leaf layer could then be estimated from the total dry weight for each layer.
The second method was by comparing each leaf with a set of standards of known
area, and covering the range of leaf shape met in the experimental plants (cf. Williams
'Williams
1954).
(d) Experimental Method
Each experimental run required 2 or 3 days for completion. When the rate of
photosynthesis by the full leaf canopy, with an L.A.I. of 6-8, had stabilized at the
highest light intensity (over 3000 f.c.), the intensity was reduced to about 2000 f.c.
until the CO 22 concentration reached the new equilibrium value. This usually required
about 10 min, but longer times were sometimes required for transpiration rates to
reach their new equilibrium. After 20-30 min, the light intensity was reduced to
about 1000 f.c., and the equilibrium levels of photosynthesis and transpiration
again recorded. The lights were then switched off entirely and the equilibrium rate
of respiration in darkness obtained. The transpiration rate in darkness decreased
slowly and continuously, and equilibrium rates could not be obtained. All lights were
then switched on again, and the cabinet was opened to allow the lowest layer of
leaves to be cut off and the average light intensity at the level of these leaves
to be determined.
About 1 hr was required for the conditions within the cabinet to reach their
new equilibrium, during which time the area of the leaves removed was estimated
and their dry weight obtained. The measurements were then repeated on the reduced
community, working from the highest to the lowest light intensity and to darkness
in each case. The reason for following this sequence was that, as Kuiper (1961) had
found, stomatal closing in response to lowered light intensity was more rapid than
the opening in brighter light, and less time was therefore needed to reach equilibrium
levels of photosynthesis and transpiration. After these had been determined, the
lowermost layer of leaves was again cut off, their area and dry weight determined,
their light environment measured, and the sequence of measurements repeated.
The final sequence of measurements was made on completely defoliated plants to
assess the level of photosynthesis and respiration of the non-foliar tissue.
llOS
L. J. LUDWIG, T. SAEKI, AND L. T. EVANS
(e) Some Comments on the Experimental Methods
(i) Possible Errors in the Use of the Progressive Defoliation Technique
It was assumed that the light environment of the upper leaves would not be
noticeably affected by removal of lower layers of leaves, and this was confirmed by
actual measurement.
Removal of lower leaves might conceivably affect the rate of photosynthesis in
upper leaves quite apart from changes in their light environment. This possibility was
examined directly in four experiments by enclosing single upper leaves in the small
assimilation chamber, and measuring photosynthesis under given conditions before
and after removal of the lower leaves. In no case did their removal have any effect
on the photosynthetic rate of the upper leaves.
These results, and the close agreement between photosynthetic profiles obtained
by progressive defoliation and those obtained at various stages of growth, suggest
that the technique of progressive defoliation does not introduce serious errors. Its
use is, however, limited to plants such as cotton and sunflower, in which the leaves
are horizontally disposed in fairly regular layers.
(ii) Possible Errors Due to the Time Period Required for Each Experiment
The photosynthetic profiles obtained in one experiment were corrected for
leaf area increase during the 3-day period of the experiment. This was found to
make little difference to the results, and the corrections were not applied in the
results reported below.
A more serious problem was posed by diurnal trends in photosynthesis, transpiration, leaf temperature, and stomatal opening during each 12-hr period of high
intensity illumination. No experimental measurements were made during the first
2 hr of each light period, to allow stomatal opening and photosynthesis to reach
equilibrium values. Variation in net photosynthetic rate during the remainder of
the light period was largely due to two opposed effects. One was a fall in respiration
rate as the day progressed, most markedly at the higher temperatures (see Fig. 6),
giving higher values for net photosynthesis in the afternoons [Fig. 6(c)]. The other
was a fall in net photosynthesis each day after the lights had been on for about 9 hr.
This was found to be due to pronounced stomatal closing, leading to a reduction
in photosynthesis and transpiration, and to a rise in leaf temperature. Skidmore
and Stone (1964) found a marked decline in the transpiration rate of leaves of Acala
cotton plants growing in solution culture under constant conditions, after about 6 hr
in light, and their results suggest that this was due to stomatal closing following an
increased root impedance to the uptake of water. The magnitude of this effect can
be seen in Figure 6(a).
(iii) N
eCe8sity for a Period of Adaptation
Necessity
This was suggested by the results of our earlier work on the rate of respiration
in the lower leaves of cotton plants of several varieties. In this, we found that
although the lower leaves had a low respiration rate at all temperatures when held
under low light intensity conditions, their respiration rate was high and very sensitive
PHOTOSYNTHESIS IN COTTON PLANT COMMUNITIES. I
1109
to temperature following a period under high intensity light (cf. Fig. 3). Thus, taking
plants from the wide-spacing and high-intensity light conditions of the glasshouse
to the densely packed cabinet communities would probably give
give very different photosynthetic profiles if these were measured immediately rather than after several days
of adaptation. The results of such experiments are presented in a later paper, and
emphasize the importance of an adaptation period if results relevant to natural
plant communities are to be obtained.
(iv) Profile Conditions within the Artificial Plant Community
The uniformly luminous ceiling and the decrease in light intensity with distance
from the source in growth cabinets are not characteristic of natural environments.
The effect of a uniformly luminous ceiling on the photosynthetic profile was examined
14
12
z
o
;::
"0:: ~
o:~
m---
10
w "W
0:
0:
~"u."
"
B
(] W
~~
ffiW---~
6
~o
z u
,. f!J
m:l:
4
o~
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DARK RESP1RATI~
RESP1RATI,':O;;:.N....._
2
o
10
If.7
20
TEMPERATURE (Oe)
(OC)
,p
~
30
. .•.
40
Fig. 2.-Net photosynthesis and dark respiration rates for a fully grown second
leaf of Deltapine cotton, as influenced by leaf temperature. Photosynthesis
measured for a light intensity at the leaf surface of 1700 f.c. and C02
concentration of 0'026%
o· 026% (v/v).
for us by Professor W. G. Duncan, using his computer model (unpublished data) for
estimating photosynthesis in corn communities. The profiles he obtained were similar
to those with sky light only, and much less attenuated than those where a point
source contributed most of the luminous flux.
In an empty cabinet the net light flux was found to decrease by 20% from
30 cm below to 150 cm below the glass panel under the light canopy, due to absorption
of light by the walls. However, when the light extinction pattern was measured
within a dense cotton community 85 cm tall, it was calculated that absorption of
light by the walls below the top of the plants was only 2· 5%. Luminance measured
at the base of the empty cabinet gradually decreased from the zenith to lower angles
giving a light environment between the brightness distribution on fine days and
that on overcast days in a natural environment.
L. J. LUDWIG, T. SAEKI, AND L. T. EVANS
1110
11lO
The fairly uniform rate of vertical air movement downwards through the plant
community at 25-50 cm/sec is also not characteristic of natural environments. Such
a rate of air movement in the upper foliage is not unusual, but the rate of air movement among the lower leaves is higher in the artificial communities than would
normally occur in field communities during daylight hours (Jarman 1959; Lemon
1960; Penman and Long 1960). This could lead to a somewhat higher rate of photosynthesis by the lower leaves than occurs in natural communities, but since photosynthesis by these leaves is severely light-limited, the increase is unlikely to be
important.
3'0
2'5
ir
WI
..: ..:
I- - - 2'0
0: W
0:
Z ..:
Qu..
t- ..:
~ ~ "5
iLN
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0:,
N
"'0
n:: U
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1'0
o~
0·5
o
10
20
30
40
TEMPERATURE (Oe)
(DC)
Fig. 3.-Equilibrium
3.~Equilibrium dark respiration rate of various leaves of a Pope
cotton plant, as influenced by temperature. 0 Tenth leaf, half
expanded at measurement; X sixth leaf; 6 fourth leaf after a day
exposed to high intensity light; ....
... the same fourth leaf after a day
in darkness.
u.
III.
RESULTS
(a) Photosynthesis and Respiration in Single Leaves
The effect of temperature on the rates of respiration and net photosynthesis
of a mature leaf of Deltapine cotton is shown in Figure 2. The plotted values are
steady state rates for a leaf of a young plant grown at 30/25°C,
30/25°0, the photosynthesis
measurements being made with a light intensity of 1700 f.c. at the leaf surface.
Whereas the dark respiration rate increased progressively with increase in temperature up to 40°C,
40°0, net photosynthesis was highest at 25°C
25°0 and the fall in photosynthetic rate at higher temperatures was much more than could be accounted for
by increased dark respiration.
The effects of age and light environment of a leaf on its respiration rate are
shown in Figure 3, for various leaves on a plant of Pope cotton. The uppermost
curve is for the tenth leaf, which was half expanded at the time of measurement.
PHOTOSYNTHESIS IN COTTON
OOTTON PLANT OOMMUNITIES.
COMMUNITIES. I
I
lli
1111
Older leaves had a much lower respiration rate at'
at·· all temperatures. The lowest
curve is for the fourth leaf, following a day in which it was not exposed to light,
its respiration rate then being low and relatively insensitive to temperature. After
being exposed to light on the following day, itR
its respiration rate was much higher,
particularly at the higher temperatures.
Similar dependence of the dark respiration rates of
ofleaves
leaves on their light environment during the preceding day was found in other cotton varieties, and even in young
leaves. For example, the respiration rate of the second leaf of a Pima plant was
LEAF AREA INDEX
o0
0·25
I
0'50
0-50
I
0'75
0·75
I
1'00
I
LEAF
LAYER
16
15
LEAF
AREA
REMOVED INDEX
14
::1
13
12
12
p
~
VI
1·00
1'00
V
V
1·35
~ O»JI
II
0: 11
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::;
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0
0
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9
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1·05
7
III
1·10
II
1·02
6
5
4
3
1·10
2
I
o0
40
80
120
160
LEAF AREA (OM2)
o
LEAVES ON PRIMARY SHOOT
f::::?:$:,3
E~:f~a LEAVES
~ LEAVES
ON AXILLARY SHOOTS
BEGINNING TO SENESCE
Fig. 4.-Productive structure for the Deltapine cotton com30°C, and the leaf layers
munity used in the fifth experiment at 30°0,
removed at successive defoliations.
C02(dm2(hr
stable at 2·24 mg C0
2 jdm 2 jhr in darkness at 25°C following a day when it was
exposed to light of 2000 f.c. intensity, and at 1·05
1· 05 mg C0
CO 22 (dm
jdm 22(hr
jhr following a day
of 220 f.c. intensity.
In a search for differences between varieties, a large number of curves showing
the relation between temperature and dark respiration rate has been obtained,
for the cultivars Deltapine, Pope, Acala, and Pima. No clear differences between
varieties have been found, and certainly none to compare with the great differences
within each variety due to varying age and light environment of the leaves. Young
leaves of all varieties showed steady state values similar to those given in the uppermost curve of Figure 3, with an approximately nine-fold increase in respiration
L. J. LUDWIG, T. SAEKI, AND L. T. EVANS
1112
rate between 5 and 35°0.
35°C. Goodman and Wedding (1956) present a response curve
for Acala leaves in which there is an approximately 25-fold increase in respiration
rate over the same range of temperature. We have obtained response curves similar
to theirs from respiration rates measured 3,S
as soon as leaf temperatures were raised.
But the high rates found soon after transfer to high temperatures are due to overshoot
(cf. Burton 1939), and the steady state values obtained after about half an hour have
always
always'yielded
yielded results like those given in Figure 3.
1000
u.s
>-
n.
~
o
z
<:
«
u
3:
~
o...Jg
..•
100
Fig. 5.-Light
5 ,-Light profile within the Deltapine
cotton communities used in the fifth experiment
at 30°0
30°C (0) and in that at 20°0
20°C (e).
(e) .
W
al
m
>-~
....
iii
Ui
z
w
W
I....
Z
~
....
l-
I
l!l
t!l
J
o
10
•
,.
I
I
I
I
W
I
1~0----~----~2-----3~--~4~--~5----~6~--~7~--~8
o
2 3 4 5 6 7 8
LEAF AREA INDEX
INDEX OF CANOPY
(b) Photosynthesis and Respiration in Artificial Communities
Oommunities
Seven cotton communities, and one of sunflower plants, have been analysed
for the relation between photosynthesis and L.A.I.
The first five experiments were with Deltapine cotton, at a day temperature
of 30°0
30°C and a night temperature of 25°0.
25°C. In the early experiments the rate of air
renewal in the cabinet was relatively low, and the maximum leaf areas were not
30j25°C
as high as in the last two runs. However, the results of all five experiments at 30/25°0
agreed closely.
Figure 4 indicates the productive structure of one of the communities at the
time of the experiment, and the leaf layers which were removed at each defoliation.
Figure 5 indicates the light profile in the community, as a function of cumulative
L.A.I.
L.A. I. measured from the top of the canopy. The light profile conforms to Beer's
COMMUNITIES. I
PHOTOSYNTHESIS IN COTTON
OOTTON PLANT OOMMUNITIES.
1113
law, the extinction coefficient being 1·06. Clearly, there was no change in extinction
coefficient with depth in the community, such as Verhagen, Wilson, and Britten
(1963) proposed.
Figure 6 (b) indicates the dependence of dark respiration and net photosynthesis
on L.A.I. at 30°C. Respiration rate increased progressively with increase in L.A.I.,
but by no means in proportion to it. The respiration rate at zero L.A.I. was high,
due to respiration by stems and roots. As noted previously, afternoon respiration
rates were lower than those obtained in the morning, which confuses the dependence
of respiration rate on L.A.I. Taking the morning determinations for L.A.I. values of
40
-
(I)
(J)'"
_
I0:
_I
(J)er
~oW
30
z
IZ
~5
.... ::J
(1)(9
3~
~~
1-:;;
00
I-
!
~
•
o0
(b) 30°C
30130
20
'"
0.0'
"0'
.... u
I-U
~
o
0•
o
~~~
" 10
6,0
-
~
10
•
0
3
i=
c;(
~
0
~N
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~~0
~
4
5
6
77
8
-!
2
3
I
4
LEAF AREA INDEX
~
z"
00
u
6"
20
(c) 40°C
30
20
20
20
~
I
....
~/-'
zO
,.0:
>-'"
40
40
4T
(a) 20°C
----
10
I
5
I
6
I
7
01J!rr7~'L
I ~•
20L
20
2J
•
•
I
6
•
Fig. 6.-Relation between dark respiration or net photosynthesis at three light intensities and
leaf area index of the Deltapine cotton communities: (a) at 20°C;
20°0; (b) at 30°C
30°0 (fifth experiment);
(c) at 40°C.
(a)
40°0. Solid symbols, morning values; open symbols, afternoon values .
• , 0 Net photosynthesis under 3300 f.c. . , D Net photosynthesis under 1200 f.c.
,6.,
6 Net photosynthesis under 2100 f.c. 'f',
"', 6.
T, v
V Dark respiration.
/dm 2 of
4·5 and 6·5, the respiration rate at the higher L.A.I. was only 1·3 mg CO 2/dm
ground surface/hr higher. The mean respiration rate for these lower leaves was thus
only 0·65 mg C02/dm2 of leaf area/hr, a value close to that at 30°C for the lowest
curve in Figure 3. On the other hand, the mean respiration rate for the two uppermost
layers of leaves was 3·1 mg C02/dm2
CO 2/dm2 leaf area/hr, a value similar to that for the
youngest leaf given in Figure 3.
The net photosynthesis curves show an increase in photosynthesis with increase
in L.A.I. to values of about 3, at the higher light intensities, and only a slight fall at
still higher L.A.I. values. At the highest light intensity, photosynthesis by the
stems equalled their dark respiration rate, and even at 1200 f.c. the stems were
largely self-supporting.
L. J. LUDWIG, T. SAEKI, AND L. T. EVANS
1114
Photosynthesis increased with light intensity at all L.A.I. values. The rates
of respiration and of photosynthesis at the various light intensities, for the top layer
of leaves (L.A.I. = 1), derived from the results given in Figure 6, are presented in
Figure 8. Clearly, photosynthesis by single leaves of cotton increases sharply with
increase in light intensity to at least 3300 f.c.,
fc., although Bohning and Burnside (1956)
found only a slight increase at intensities beyond 2000 f.c.
fc. The rates of photosynthesis for the top layer of leaves in the plant community at 1700 f.c.
fc. are similar
to those given in Figure 2, and to those given by Bohning and Burnside. They are,
however, much lower than those found by EI-Sharkawy and Hesketh (1964) for
Deltapine cotton leaves under a much higher light intensity.
Figure 7 shows the relation between evapotranspiration and L.A.I. in one of
the cotton communities at 30°C. As with photosynthesis, evapotranspiration rose
3·0
Fig. 7.~Relation
7.-Relation between evapotrans·
evapotranspiration at three light intensities and
leaf area index of a Deltapine cotton
community at 30°C. Solid symbols,
morning values; open symbols, after·
afternoon values.
., 0 3300 Lc.
... , D 2100 f.c.
., 0 1200 Lc.
.,0
A
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oz ~
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-<
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Cl
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o
2 3 4 5 6 7
LEAF AREA INDEX
with increase in L.A.I. to 2, to reach values of about 3 g water/dm 2/hr under the
highest light intensity. At this intensity, the incoming radiation was equivalent to
about 0·185 cal/cm 2 /min, whereas the evapotranspiration rate was equivalent to
about O·
0·33 cal/cm 2 /min. Thus, the energy for much of the evapotranspiration was
being provided by the advection of warm air, and for this reason the range of light
intensities used had less effect on evapotranspiration than on photosynthesis by
the plant community.
Figure 6 (a) presents the results for the experiment in which the day temperature
was maintained at 20°C instead of at 30°C. The light profile for the plant community
used in this experiment is given in Figure 5, and agreed closely with that for the
experiment at 30°C. At this lower day temperature, the dark respiration rate of both
leaves and stems was considerably lower than at 30°C and net photosynthesis was
correspondingly higher at all L.A.I. values. The increase in respiration rate with
increase in L.A.I. was even less marked than at 30°C, and only at the lowest light
intensity was even a slight decline in net photosynthesis evident at the highest
L.A.I. values. The dark respiration rate for the uppermost leaves was about 1·5 mg
CO 2 /dm 2 1eaf area/hr, compared with a rate of 1·2 mg CO 2 /dm 2 leaf area/hr at 20°C
for the youngest leaf in Figure 3. That ofthe
of the lowest leaves was about 0·2 mg CO 2 /dm 2
PHOTOSYNTHESIS IN COTTON PLANT COMMUNITIES. I
1115
ofleaf area/hr, compared with a rate of 0·3 mg 002/dm2/hr at 20°0 from the lowest
curve in Figure 3.
3 .. The rate of net photosynthesis at an intensity of 1700 f.c. by the
uppermost leaf layer in the plant community at 20°0 (derived by interpolation from
cf.
Fig. 8) also agreed closely with that for a single leaf given in Figure 2 (U'8,
(11· 8, of.
12·4 mg 002/dm2/hr, respectively).
The results of the experiment in which the day temperature was maintained
at 40°0 are given in Figure 6(c). The highest L.A.I. that could be maintained at
40°0 (5,4)
(5·4) was rather lower than at 20°0 (7·6) and 30°0 (6'
(6· 5), due to greater leaf
loss at the bottom of the canopy. At 40°0, the dark respiration rate of the stems
18
16
~
I?
I:r:
14
;;-;;;-:;:
::E
.e..
.e.
12
ON
0
()
U
C)
t!l
.6
~
10
Fig. S.-Rate of dark respiration, and
of net photosynthesis at three light
intensities in the uppermost layer of
leaves in Deltapine cotton communities
at temperatures of 20, 30, and 40°C.
I/)
Ul
iii
w
I:r:
8
It-
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Ul
0
e6
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o
0
I:r:
n.
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ItW
Z
4
2
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HJ~-2
e;~
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0
toOO
1000
2000
3000
LIGHT INTENSITY (F.e.)
0.
'" 0~
'" 8
'"'" '"
"
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0
0
4
and roots was higher than that in the community at a day temperature of 30°0. As
at 30°0, the respiration rates from afternoon measurements were lower than those
measured in the mornings. Net photosynthesis was greatly reduced at 40°0, the rate
for the top layer ofleaves
of leaves being only 6·4 mg 002/dm21eaf area/hr at 1700 f.c. (Fig. 8),
which is close to the rate for single leaves given in Figure 2.
Since stem photosynthesis was less than half the rate of dark respiration by
stems and roots, there was negative assimilation by the community at the lowest
light intensity, at the lowest and at the highest L.A.I. values. At the higher light
intensities, net photosynthesis declined at the highest L.A.I. values, although not
sharply. However, the decline was greater than would be expected from the increase
in dark respiration rate at the highest L.A.I. value.
The results with the community of sunflower plants at 30/25°0 will not be
presented, as they were very similar to those with cotton plants at the same
temperature.
1116
L. J.
LUD~G,
T. SAEKI,AND L. T. EVANS
IV.
DISCUSSION
The relation found here between net photosynthesis and L.A.I.
L.A.1. in cotton
communities is like that suggested by Brougham's (1956) results, and by those of
Wang and Wei (1964) and Nichiporovich and Malofeyev (1965). Net photosynthesis
increased with increase in L.A.I.
L.A.1. values up to 2-3, and remained high with further
increase in L.A. I. values up to about 7, particularly at the higher light intensities
and lower temperatures. Comparable data for field communities of cotton plants
are sparse. Ashley, Ross, and Bennett (1965) found the yield of seed cotton to
increase progressively with increase in L.A.I.
L.A.1. to more than 5. Bar'etas (1964), however, found that removal of distal leaves on the axillary branches of a cotton crop
could increase fibre yields, and Mednis (1955) found an optimum L.A.I.
L.A.1. of about 2
for fibre yields of cotton, but adverse conditions at the end of the growing season
may have made this optimum value lower than that for vegetative growth, as Nichiporovich (1956) has noted. The values for net assimilation rate obtained by Mednis
were only 15-30% of the highest net photosynthetic rate measured in our cotton
communities, 32 mg CO 2 /dm 2 ground surface/hr at 3300 f.c., which is in turn about
half of the maximum photosynthetic rate recorded for cotton crops in the field (Baker
1965).
With the artificial cotton community at 20°C, under a light intensity of 3300 f.c.,
there was no change in the net photosynthetic rate with increase in the L.A.I.
L.A.1. from
3 to 7·6 [Fig.
[Fig.6(a)].
6(a)]. This implies that each of the lower leaf layers was close to its
compensation point, although the mean light intensity below the canopy was 130, 45,
15, and 5 f.c. under canopies of L.A.I.
L.A.1. 3, 4, 5, and 6, respectively.
Such results suggest that, at least for our cotton and sunflower communities,
the absence of a marked fall in net photosynthesis by communities with aboveoptimal L.A.I.
L.A.1. values is due to the very low respiration rates of the lower leaves.
This was suggested by the results from direct measurement of respiration rate in
single leaves, such as those given in Figure 3. Calculation of the respiration rates
of the youngest and oldest leaves in the plant communities, from the results in
Figure 6, were in close agreement with those from the highest and lowest curves
in Figure 3.
At the higher temperatures, where the respiration rate of all leaves is higher
relative to the rate of photosynthesis, there was some decline in the rate of net
photosynthesis of the communities at high L.A.I.
L.A.1. values, indicating that the lower
leaves were below their compensation point. The decline was slight at 30°C, but
was more marked at 40°C.
The plants used in the experiment at 40°C were initially similar to those used
in the experiments at 20°C and 30°C, the L.A.I.
L.A.1. being about 7 at the beginning of the
adaptation period. But within 10 days at a day temperature of 40°C, the L.A.I.
L.A.1.
dropped from 7 to 5·4 through loss of
ofthe
the lower leaves. These leaves were presumably
well below compensation point, and senesced rapidly.
Davidson and Philip (1958) assumed that such leaves could be parasitic on the
plant, deriving the assimilates they required by translocation from the upper leaves.
PHOTOSYNTHESIS IN COTTON PLANT COMMUNITIES. I
1117
The experiment at 40°0 suggests, however, that older leaves below their compensation
point soon die. This conclusion is supported by the results of translocation studies
with many plants in which lower, shaded, old leaves have been found not to import
assimilates from upper, younger leaves (Belikov 1958; Jones, Martin, and Porter
1959; Thaine, Ovenden, and Turner, 1959; Shiroya et al. 1961; Thrower 1962;
Hale and Weaver 1962; Evans and Wardlaw 1964; Hartt, Kortschak, and Burr
1964). Thus, if the lower, shaded leaves are to survive for any length of time, their
respiration rate must be greatly reduced. A point of some interest is the rapidity with
which their respiration rate rises and falls depending on their light environment
during the preceding day (Fig. 3).
With plants of a growth habit different from that of cotton and sunflower,
there may be a quite different relation between net photosynthesis and L.A.I. of
the community. In grass and cereal communities, for example, the much lower
light extinction coefficient will lead to greater light penetration to the base of the
leaf canopy, and a higher rate of photosynthesis by the lower leaves. On the other
hand, there is usually more young tissue at the lower levels of grass communities than
in our cotton communities. These may act as sinks for assimilates translocated
from the upper leaves, and may have much higher rates of respiration than the
lower leaves in the cotton community. Their higher respiration rates may balance
their higher photosynthetic rates, so that there is little fall in net photosynthesis at
high L.A.I. values, as found by Wang and Wei (1964) with wheat and rice.
v.
V.
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