Carbon Dioxide Compensation Points of Leaves and Stems and Their Relation to Net Photosynthesis - Plant Physiology
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Plant Physiol. (1971) 48, 607-612
Carbon Dioxide Compensation Points of Leaves and Stems and
Their Relation to Net Photosynthesis
Received for publication March 12, 1971
B. BRAVDO
Department of Pomology and Viticulture, The Faculty of Agriculture, Rehovot, The Hebrew University of
Jerusalem, Israel
ABSTRACT way have low CO2 compensation values (around zero) while
The interactions between C02 and H20 vapor exchange of species having photorespiration have higher values (4, 17, 19, 20).
the leaf and respirant organs like stems were studied in tobacco
Some of the discrepancies reported could be explained by dif-
plants. The results were analyzed according to a suggested
ferences in plant material used: whole potted plants (18), de-
tached shoots (1, 4, 20), detached leaves (8, 15).
model. Good agreement between open and closed system meas- Since plants utilizing the Calvin CO2 fixation pathway vary
urements supported the validity of the model. considerably in their r and it has recently been used as a measure
The measured over-all resistance to C02 of a leaf and a stem for photosynthetic efficiency, it is important to investigate further
enclosed in a measuring cuvette was the same as the measured its nature and its relation to photosynthetic efficiency.
resistance of the leaf when measured alone provided the This paper evaluates photosynthetic and r measurements of
resistance of the stem to C02 is relatively high. The combined leaves of different size and efficiency measured alone or together
C02 compensation concentration of a leaf and stem having high with other plant parts, i.e., stems.
resistance to C02 was higher than the C02 compensation point
of the leaf alone, by the magnitude of rate of C02 evolution
from the stem multiplied by the overall resistance of the leaf. THEORY
C02 evolution into C02-free air was found to be higher in According to the model of a photosynthesizing leaf that I
light than in dark in leaves, while the reverse was true for have proposed (2), the leaf is taken as a unit within which all
stems. It was concluded that normally the CO2 compensation sinks and sources of CO2 are represented by one sink and one
point of a leaf is unaffected by stomata and boundary layer source while one CO2 stream flows in a single pathway between
resistance while the combined CO2 compensation point of a the sinks and sources. The model is presented in Figure 1 (leaf
leaf and a stem differs in its nature since it represents a steady and mesophyll) and its algebraic solution in equation 1 (Ip was
state of photosynthesis in which stem contribution, I, is equal eliminated).
to net photosynthesis, I.. Interpretation of the experimental
data shows tht respiration in the light is unaffected by external E- IRp
C02 concentration (at the range of 0-300 ,ul liter) and by R. + Rp (1)
intensity of photosynthesis.
Is is the CO2 stream entering and leaving the leaf (net photosyn-
thesis), I, the CO2 stream entering the photosynthetic site, I the
internal CO. source, S stomata, p photosynthetic site, Es external
CO2 concentration. Rp and Rs are resistances between the junc-
tion, J (Fig. 1), and p and s, respectively.
A linear relationship exists between net photosynthesis, Is,
The phenomenon of CO.2 compensation point was investigated and external CO2 concentration, Es, provided that I, Rp, and
and interpreted differently in the literature (9, 13). It was termed RS are constant. Under constant light and temperature conditions,
as CO2 compensation point (6, 23) or r (15). Gabrielsen (8) the physical and biochemical resistances combined in Rp are
regarded it as a threshold below which no assimilation occurs supposed to be constant and so is physical resistance in the liquid
while others interpreted it as a balance between photosynthesis phase of Rs (7). The gas phase resistance, which is part of Rs,
and respiration (5, 6) or as a "minimum concentration that must depends mainly upon the stomatal resistance. However, stomata
be maintained in the intercellular spaces to give a rate of assimila- are known to be affected by CO2 concentrations but plant species
tion sufficiently high to balance the respiration rate" (10, 11). probably differ in this respect. Stomata of turnip were little af-
Heath (10) rejected Blackman's suggestion (1) that this is a fected by between 0 and 400 41l/liter CO2 (7) while stomata of
function of nongreen tissue and claimed the existence of a other plants were affected even at 100 Al/liter CO2 (17). It is
"buffering system." Orchard suggested a "third process" in addi- generally accepted (7, 13) that up to about 300 ll/'liter CO2 the
tion to ordinary dark respiration and photosynthesis (22). The net photosynthesis is linearly related to the external CO2 con-
effect of temperature (5, 6, 12, 17, 18, 27, 28), and water strain on centration in most plant species. Rs, Rp, and I are constant, being
r and its relation to stomatal opening was also investigated (2, 14, the parameters of a straight line. The slope of the lines in Figure 5
15, 19). equal 1,/(R, + Rp) and their two intercepts at Is = 0 and E, = 0
The magnitude of r reported in the literature varied consider- are IRp and IRp/(R, + Rp), respectively. IRp is the CO2 compen-
ably, even for the same species. Some workers proposed a uni- sation concentration, r, and is dependent upon the internal CO2
versal value for all plants (8, 12, 18, 23), but lately it is commonly source, I, and the internal resistance, Rp, only while the second
accepted that plants which fix CO2 via the C4 dicarboxylic path- intercept equals the compensation concentration, IRp, divided
607
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Qs
= IRP A e- ft,;'(1?,+RP) C I I (4)
Ca
where A is a constant. Under constant light, temperature, air
circulation, and increasing CO2 concentrations from zero to the
CO2 compensation concentration 1, Rp, and RS were found to be
constant (2).
On introduction of the stem or other respiring organs having a
high over-all resistance, R. + Rp, into the cuvette, equation 3
becomes,
Q'.(Rs-+ Rp) - C-+
Cs
IRp +,I(RJ + Rp) = 0 (5)
(the stem resistance is neglected since it is connected in parallel to
the leaf resistance), differentiating it for time yields,
FIG. 1. A model for photosynthesizing leaf. p: Photosynthetic Cs
= E, = IRp + I,(R, -4 Rp) - A e[t/(RS+Rp)Cs (6)
site; s: stomata; I: C02 source; p: C02 sink; Ia: C02 source; II: C02
entering and leaving the leaf; Ip: C02 entering the photosynthetic Under conditions where Q',/ Cs is zero at zero time
site; J: hypothetical junction; R.: resistance to C02 between stomata
(including boundary layer resistance) and the junction J; R,: re- A = IRp + Ia(Rs-+ Rp) (7)
sistance to CO2 between the junction J and the photosynthetic
site p. Likewise the open system F of the leaf will be achieved when Q., =
0, but CO2 will continued to accumulate in the system because of
by the over-all resistance, R, + Rp (2). Equation 1 is applicable the contribution of the respiring organ Ia until Q, = - a, when
to open system measurements; namely, a green leaf is enclosed in a steady state will be achieved at a concentration of
a transparent, well illuminated cuvette, and net photosynthesis,
Cs IRp + I(Rs + Rp)
I, ,is calculated from the differences in CO2 concentration at the = (8)
inlet and outlet and the rate of air flowing through the cuvette. If
the air is thoroughly mixed, the concentration at the outlet is in The exponents e-[tI(Rs+Rp)Cs] remains the same for equations 4
fact that which exists in the cuvette. and 6.
In cases where plant parts, other than the leaves, are introduced
into the measured cuvette they usually have a high over-all re- MATERIALS AND METHODS
sistance, R, + R,, to CO2 and therefore have a very low or even
negative ratio of photosynthesis to respiration. If R, + Rp is Leaves and shoots from which all leaves except one were re-
very high, then the slope, 1 (JR + R,), is very small and conse- moved were enclosed in a double-walled Plexiglas cuvette. Con-
quently these organs evolve CO, at an almost constant rate at stant temperature was maintained in the cuvette by circulating
external CO2 concentrations between 0 and 300 4l liter. Models water from a temperature-controlled bath through the double
for leaf, and leaf having a constant external CO2 source are repre- wall. Illumination was provided by two iodine quartz lamps
sented in Figure 1 (mesophyll, leaf, and chamber). Introducing cooled by streaming water. The cuvette was used with either the
this external CO2 source, Ia , into equation I gives equation 2. open or closed system described in Figure 2. CO2 concentration
F. - IR p in the air stream of the open system was controlled by mixing
P = ls+Ia = R+ Ia (2) C02-free air with a stream of 10' CO., from a cylinder by means
of a pressure reducer, a needle valve, and a capillary pipe. The
air flow through the measuring cuvette was measured by two
Since Ia is constant, a linear relation exists here between net flowmeters before and after the cuvette. Circulation inside the
photosynthesis, p, and external CO., concentration, Es The slope cuvette was achieved by a circulation pump connected to the
is the same as in equation 1, 1 (R. + R,), but the intercepts are
IRp + Ia(R., + Rp) at I + I, = p = 0 and (IRp R, + Rp) + Ia FM
at Es = 0. The CO2 compensation concentration in that case dif-
fers from F as defined by Heath et al. (15) by the magnitude of
Ia(Rs + Rp). This value is dependent upon stomatal resistance,
which is a part of R, . Although no change in CO2 concentration
occurs between the inlet and the outlet of the cuvette at the
compensation point the leaf itself apparently consumes CO, at a
rate of Ia -
CLOSED SYSTEM ANALYSIS. For the purpose of a closed system
analysis where CO2 concentration is constantly being changed,
equation 1 should be treated as a differential equation,
Q', (R, + Rp) - -
Cs
+ IRp = 0 (3)
FIG. 2. A general sketch of the measuring system. A: Measuring
where Q stands for C02 charge and C, for the volume of the en- cuvette; FM: flowmeter; B: constant temperature water bath; E:
tire closed system. As was already shown (2) if I, Rp, and Rs electrical humidity elements; D: drier (MgCl04); IRGA: infrared
are constant, then the differentiation for equation 3 for time gas analyser; R: recorder; As: Ascarite tower; H: humidifier: p:
yields, pump; w: wlre.
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V8CO, COMPENSATION POINT AND PHOTOSYNTHESIS 609
cuvette in a separate closed system. A bypass permitted measure- synthesis of the leaf and external CO2 concentrations in the meas-
ment of the H20 and CO2 concentration in the air entering the uring cuvette should increase proportionally.
cuvette. The air was passed through a chamber containing LiCl CO2 compensation point of leaf and stem together differs from
elements (Hygrodynamics) placed in a constant temperature the F of the leaf since it is dependent upon both internal and ex-
bath, and part of the air was dried and flowed through an IRGA
Beckman model 320A for CO2 measurements. The humidity was
controlled by mixing moist with dry air using the plant as a 601 b b
humidifier while only part of the air was dried and analyzed for a
I
a
CO2 .0
The same system also served for closed circuit measurements -----fr2- - C _
except that the CO2 cylinder was kept closed and there was no 30 0.6 C
mixing Of C02-free air with that flowing from the humidifier and 0
the CO2 analyzer. The ascarite was used for reducing the CO2 E
content of the air, and part of the air was bubbling through a ox 4
washing bottle containing water and immersed in a constant bath c0
(in)I
temperature, in order to control humidity. The volume of the 0 Tim 20 40
entire closed system was 2600 cm', the volume of the cuvette 1,000 Time (min)
cm3, light intensity was 0.8 cal min-' temperature in the
CM-2,
measuring cuvette was kept constant at 27 C, humidity of the FIG. 3. CO2 accumulation during CO2 compensation point meas-
air varied between 20 and 50%C relative humidity, rate of air flow urement. a: Leaf; b: leaf and stem; c: transpiration.
was 5 liters min-', and leaf temperature was measured by attached
thermocouples. I I i
Tobacco plants (Nicotiana tabacum var. Samson) were grown
in pots at a constant temperature of 25 C and 12-hr day and night
cycles. Fluorescent lamps, Sylvania F40, supplied light of about I.6
2500 ft-c. A few days before the start of the experiment all leaves
except one were removed, and gas exchange analysis was carried
out as follows. A leaf was measured first for CO2 compensation
point and for dark CO2 evolution into C02-free air in a closed
system, and then for net photosynthesis in 300 Al/liter CO2 in an . 15
01
-
open system. The stem was then introduced, and the measure- 1.3
ments were repeated for leaf and stem together. Finally the leaf 0
was cut and the stem was measured alone.
RESULTS
Figures 3, 4, and 5 show the results obtained with plant 10 0 5 10 15
(TableI). The CO2 compensation increased owing to the presence Time (min)
of the stem in the measuring cuvette from 46 to 58 MI/liter (Fig.
Log of the difference between CO2 compensation
3). Curves a and b of Figure 3 were analyzed as suggested pre- andFIG. 4.concentration point
viously (2). Plotting log of the difference between CO2 compensa- C02 in the measuring cuvette. a: Leaf; b: leaf and
tion concentration and the external CO2 concentration against stem.
time showed linearity and a similar slope in a and b. This indi-
cates no change in the over-all resistance to C02, R8 + Rp, due
to the presence of the stem in the measuring cuvette. Net photo-
synthesis, I, , was calculated using equation 3. A difference of
0.6 mg CO2 dM-2 hr-1 was calculated between the leaf and the
leaf plus stem measurement for all concentrations in the range of
0 to 0.03% CO2. Open system measurements confirmed this E~~~~~~~~~~~~~~~
value (Fig. 5). The absolute values of net photosynthesis of plant
10 (Tables 1, II) varied between -2.41 and 10.6 mg CO2 dM-2
hr-'. Thus a 0.6-mg difference amounts to from 100 to 6% of net
photosynthesis for the CO2 concentrations used. Transpiration
i- X
was constant during the entire measuring period (about 25 min),
and so were leaf and air temperatures. Calculated values of net
photosynthesis, 18 , for external CO2 concentration of 0.03 % using 0 10 00/0
over-all resistance values, R, + Rp, obtained by a closed system 0 1
agreed well with the values obtained in an open system (Fig. 6).
Clearly the model enables calculation of photosynthesis for a
wide range of CO2 concentrations under which the over-all re-
sistance, R8 + Rp, and the internal CO2 source, I, remain con-
stant. Similar values of R8 + Rp were obtained in all 14 plants C02 ml/liter
when leaves were measured with or without stems (Table I).
The value of Ia(Rs + Rp) (stem respiration multiplied by on net photosyn-
FIG. 5. Effect of external C02 concentration leaf
over-all resistance) gave a good estimate of the increase in the thesis and stem; c:
measured by an open system. a: Leaf; b:
CO2 compensation due to the presence of the stem. Thus if we leaf, calculated for intercellular space
provide a leaf with increasing external CO2 sources, net photo- CO,2concentration.
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Table I. CO2 Compenisation Pointts anzd Resistantce of Leaves
Measuired with anid without Stems
R8 < Rp Leaf and Differ-
Leaf Leaf + Ia(R,+ Stem ence 202 1
Plant Stem Rp) (Calcu- between U~~~~~~~~~
Leaf Leaf and
stem lated) 2 and 6
(IlL
I 2 3 4 5 6
j,u/liter sec cm-3 cm' sec-1 Mi/liter
X 10-6
(mg * d 2*r =0.973
1 40.5 54 0.282 0.250 10.5 52.0 +2
2 46.0 57 0.172 0.170 10.3 56.3 -0.7 0~~~~~~~
-1.6 0 0
3 39.0 46 0.099 0.099 5.4 44.4 l0o
4 36.0 59 0.171 0.178 2.1 57.0 -2.0
5 46.0 56 0.103 0.103 8.3 54.3 -1.7
6 48.0 57 0.336 0.336 7.2 55.2 -1.8
7 58.0 69 0.325 0.318 11.7 69.7 +0.7 10 20
8 46.0 55 0.300 0.300 7.5 53.5 -1.5
9 46.0 52 0.220 0.220 7.3 53.3 +1.3 Net photosynthesis calculated
10 46.0 58 0.230 0.230 12.0 58.0 0 :E_ (mg dm2-. hr -'CO02)
11 40.0 70 0.210 0.210 29.0 69.0 -1.0 FIG. 6. Correlation between net photosynthesis of a leaf (calcu-
12 45.0 74 0.340 0.340 30.0 75.0 +1.0 lated from a closed system for 300 1dl/liter C02) and open system
13 39.0 75 0.300 0.305 35.7 74.7 -0.3 measurements.
14 35.0 93 0.380 0.380 55.5 90.5 -2.5
Average 43.6 62.5 0.246 0.245 16.6 61.6
0
V -0
Table II. Net Photosynithesis of Leaves Measured with Cr
c° *s~~~ T_
anid without Stems az 0 ° 0
Leaf and
Plant Leaf in C02- Leaf and Stem Leaf in 300 Stem in
free Air in CO2-free Air iA/liter C02 300 MI/liter Leaf Area
COs 5 0 0
CD ~~~~~0
mg dm2 hr-' cm2
1 -1.74 -2.33 11.2 10.6 63 r0r0.696
2 -1.89 -2.31 10.5 10.1 100 CM
3 -3.02 -3.56 20.0 19.7 93 clE ~00
00
0
4 -1.76 -2.90 12.9 11.8 85 a1)
z
5 -2.16 -2.63 11.9 11.4 147
6 -1.24 -1.47 6.5 6.27 82
7 -2.58 -3.07 10.7 10.3 50
8 -1.27 -1.53 7.08 6.84 85 C02 concentration ml/liter
9 -2.79 -3.16 15.5 15.1 53
10 -1.93 -2.41 10.6 10.0 74 FIG. 7. The correlation between CO2 concentrations (formed in
11 -3.55 -6.20 23.1 20.0 38 the measuring cuvette due to the interaction between leaf and
12 -2.19 -3.65 12.4 11.0 43 stem) and net photosynthesis calculated for this concentration.
13 -2.64 -5.1 17.6 15.2 35
14 -2.42 -6.45 18.3 for 0 and 0.03% CO2 concentrations for all 14 tobacco plants
14.35 27
shown in Table II. The difference between the net photosynthesis
of the leaf measured with and without the stem was almost con-
ternal CO2 sources of respiration. It is achieved at a concentration stant through the entire range of 0.03%7, CO2 for all plants and
where net photosynthesis of the leaf, Is, equals the stem contri- amounted to 1.1 mg CO2 dM-2 hr-'.
bution, Ia,. Equation 2 can now be written as follows The dark respiration measurements of stems varied from 0.12
to 0.58 mg CO2 g-' hr-' according to the nature of the stem used.
E8 1.(R. + Rp) + IRp
= (9) Results in Table III show that stems evolved more CO2 into
C02-free air in the dark than in the light, while the opposite was
where E, is the external CO2 concentration (,ul/liter), RS + Rp is true for leaves. CO2 evolution into C02-free air calculated on a
the over-all resistance to CO2 (sec cm-3), and IRp is the CO2 fresh weight basis was on an average almost 25 % higher in leaves
compensation point of the leaf (,l'liter). than in stems for the 14 plants measured.
Net photosynthesis, Is, of leaves only was calculated for the
concentrations prevailing in the measuring cuvette at the CO2 DISCUSSION
compensation point when each leaf and stem were measured to-
gether. A positive correlation was found to exist between these The use of the term r rather than CO2 compensation point was
CO2 concentrations and the intensity of net photosynthesis of the suggested in order to avoid any implications as to the precise
leaf for all 14 plants measured (Fig. 7). The effect of CO2 con- relations between respiration and assimilation (15). The results
tributed by the stem on the net photosynthesis, I was calculated , in this paper show that the physiological meaning of this value
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depends greatly upon the plant material measured. CO, com- Table III. CO2 Evoluttioni into CO2-free Air of Leaves
pensation points of leaves are supposed to have mainly an internal Measured wit/h and without Stems
CO2 source of respiration, while when stems and leaves are meas-
ured together an extra CO2 source external to the leaf is involved. Plant Leaf in Dark Leaf in Light Leaf in Dark Darkin
Stem Lightin
Stem
In both cases "true" photosynthesis (i.e., the amount of CO2
reaching the chloroplasts) equals respiration while net photo- mg. dm2 hr-1 mg. g' hr-'
synthesis differs. Therefore the term r cannot be used for both 1 0.88 1.74 0.264 0.148 0.113
cases. In the case of a leaf the CO2 compensation point is unaf- 2 0.50 1.89 0.183 0.177 0.150
fected by stomatal resistance while in the case of a leaf and stem 3 1.40 3.02 0.462 0.207 0.147
measured together, R., which includes stomatal resistance, is 4 1.40 1.76 0.576 0.348 0.330
also involved (equations 2, 9). Whether or not a leaf has also 5 1.05 2.16 0.429 0.406 0.235
external or intracellular CO2 sources is an open question. How- 6 0.52 1.24 0.164 0.104 0.037
ever, any fitness between experimental and calculated values 7 2.10 2.53 0.663 0.234 0.080
based upon the model suggests that it is negligible. This was also 8 0.92 1.27 0.390 0.260 0.064
supported by Heath (11). It must be borne in mind that the model 9 1.00 2.79 0.266 0.255 0.119
suggested here is an idealized form combining all CO2 streams 10 0.65 1.91 0.240 0.207 0.154
entering and leaving the plant organelles as single streams. The 11 0.43 3.50 0.117 0.236 0.233
junction J (Fig. 2) is a hypothetical point inside the mesophyll 12 0.59 2.19 0.167 0.235 0.157
cells of the leaf. Its location is determined by the average of all 13 0.66 2.64 0.178 0.195 0.172
internal streams. If all cell organelles which evolve and fix CO2 14 0.56 2.42 0.125 0.183 0.192
are randomly distributed in a uniform cytoplasm, then J is Average 0.90 2.21 0.301 0.228 0.155
located at the center of the cell. The leaf is taken as a uniform
unit, although differences in photosynthesis along the leaf may
occur (25, 26). Whether or not such differences exist is not clear result in changes in IRp only while Rs + Rp would remain un-
(25, 26); however, our test is the experimental result which fits changed. Both IRp and RS + Rp are easy to measure, and it is
the model (Figs. 3, 4, 5). important to use them properly for comparing efficiency of dif-
If the over-all resistance, R, + Rp, of the stem is very large in ferent plants or changes within a plant, due to external and
comparison to the over-all resistance of the leaf, its effect on the internal factors.
combined over-all resistance should be negligible since these The ratio IRp:R, + Rp represents the CO2 evolution into C02-
resistances are parallel. However, its effect on the net photosyn- free air (equation 1). Net photosynthesis depends not only upon
thesis measured on leaf and stem together depends upon its CO2 this ratio but also upon the absolute values of these two parame-
compensation point, IRp. Stems which in addition to a high ters when the plant is subjected to CO2 concentrations higher
over-all resistance have IRp values similar to, or smaller than, than zero.
those of the leaf, will have a very low net photosynthesis, 1,, Our findings agree with those of Jackson and Volk (16), who
and therefore will hardly affect the measured photosynthesis of a questioned all extrapolation methods used for estimation of
leaf. The stems we used had a very high CO2 compensation point photorespiration since the intercept of zero external CO2 con-
as compared to the leaves and therefore had a pronounced effect centration equals IRp: R, + Rp and not l as claimed (equation 1).
on the combined photosynthesis. When an entire plant is enclosed Extrapolating the values of internal CO2 concentration instead of
in a measuring cuvette, green stems are probably shaded and as a external concentrations (24) does not help much since net photo-
consequence their CO2 compensation point increases, causing synthesis at the intercept is expressed by equation 11
reduction in net photosynthesis and increase in the CO2 compen-
sation point of the entire plants.
The agreement between the values obtained from open and is = IRsA-JIRP (11)
R., + RP
closed system measurements and between predicted and measured
values support the validity of the proposed model. where IsRs stands for internal CO2 concentrations (21) and R, is
The straight lines obtained by plotting net photosynthesis, I,, the liquid phase part of Rs The intercept at zero CO2 concentra-
versus external CO2 concentration, Es (Fig. 5), indicate that R, tion is accordingly
and Rp are constant. However, this does not prove that photo-
respiration, I, is constant. If I varies with photosynthesis and ex- lRp (12)
ternal CO2 compensation, namely, I = IK or I = Ki + I,K, R8, + Rp
then 1, and Es will also be linear. r in this case will be either zero
or KIRP, respectively. However, this possibility is completely re- A calculated line of internal C02 source versus net photosynthesis,
jected by a closed system analysis. Any attempt to introduce I, for plant 10 is shown in Figure 5, line c. This line intercepts
IR, = f(Q',) into equation 3 results in a solution of the type the abscissa at the CO2 compensation point IRp while the slope is
Q,:C, = Ae-'. This type of curve differs completely from that changed from I: R, + Rp (the over-all resistance) to I:R51 + Rp
found experimentally. I have shown before (2) that the CO2 (the "mesophyll resistance"). Thus IRp:Rsl + Rp may represent
compensation point can be used for estimation of the effect of photorespiration, I, only if R,j is very small and neglected.
respiration on net photosynthesis by equation 10 When IRp is constant within a plant, the relationship between
the over-all resistance, Rs + Rp (varying due to changes in R,),
=1- -P (10) and net photosynthesis for each concentration of CO2 is repre-
ISO E. sented by a hyperbole. When Rs + Rp is constant and only IRp
varies (with changes in 1), the relationship between them is linear.
Recently many used F as a measure for photosynthetic efficiency
(18, 19, 4). LITERATURE CITED
Our findings show that the over-all resistance, R. + Rp , should 1. BLACKMAN, F. F. 1895. Experimental researches on vegetable assimilation
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