Plant Physiol. (1980) 66, 1032-1036
0032-0889/80/66/ 1032/05/$00.50/0
Effect of CO2, 02, and Light on Photosynthesis and
Photorespiration in Wheat
Received for publication April 10, 1980 and in revised form June 30, 1980
ALAIN GERBAUD AND MARCEL ANDRE
Departement de Biologie, Service de Radioagronomie, CEN Cadarache, BP 1, 13115 Saint-Paul-Lez-Durance,
France
ABSTRACT
Unidirectional 02 fluxes were measured with 1802 in a whole plant of
wheat cultivated in a controOled environment. At 2 or 21% 02, 02 uptake
was maximum at 60 microliters per liter CO2. At lower CO2 concentrations,
it was strongly inhibited, as was photosynthetic 02 evolution. At 2% 02,
there remained a substantial 02 uptake, even at high CO2 level; the 02
evolution was inhibited at CO2 concentrations under 330 microliters per
liter. The 02 uptake increased linearly with light intensity, starting from
the level of dark respiration. No saturation was observed at high light
inten-sities. No significant change in the gas-exchange patterns occurred
during a long period of the plant life. An adaptation to low light intensities
was observed after 3 hours illumination. These results are interpreted in
relation to the functioning of the photosynthetic apparatus and point to a
regulation by the electron acceptors and a specific action of CO2. The
behavior of the 02 uptake and the study of the CO2 compensation point
seem to indicate the persistence of mitochondrial respiration during photosynthesis.
Whereas the metabolism of photorespiration has been the subject of numerous studies, relatively few data are available concerning the associated gas fluxes in organs or whole plants, although such gas-exchange measurements were at the origin of the
discovery of, first, the Warburg effect, then the loss of CO2 (16)
and the uptake of 02 in light (8, 22), which are the main expressions of photorespiration. Interest in the gas exchanges ebbed after
these discoveries, as the complexity of the underlying mechanisms
was uncovered through biochemistry, challenging the value of gas
flux measurements and showing the difficulty of their interpretation.
Nonisotopic methods do not allbw a measurement of photorespiration during photosynthesis; moreover, they are not consistent.
For example, the Warburg effect depends on CO2 concentration
but not on light intensity (6, 34), whereas the evolution of CO2 in
C02-free air and the CO2 postillumination burst (14, 37) increase
with light intensity but are independent of CO2 (7). All results
agree on the stimulating action of 02 on photorespiration.
Isotopic methods also have their flaws. As in the preceding case,
several phenomena may be involved. Methods with 14C are used
to measure true photosynthesis (CO2 uptake) (25) or CO2 evolution
(41), but contradictions have arisen (13). Unfortunately, the results
are underestimated because of internal recycling phenomena (10,
32) which can only be estimated. However, tests with 14C have
been used to estimate the photorespiratory loss of CO2 at 10 to
20% of P,' independently of CO2 concentration (17, 25).
The use of 1 02 gives access to the complementary aspect of
photorespiration, the uptake of 02, as well as to the gross 02
evolution. The information obtained is not at all symmetrical with
the preceding one. In particular, the flux rates are much higher,
which may be due in part to the difference in reactions involving
02 or CO2 and in part to the underestimation of CO2 evolution,
which is avoided by the use of 1802 (3, 19, 33).
In spite of this advantage in precision, 1802 was seldom used
after the first studies on algae (8, 40) or higher plants (27, 38, 39).
The main defect of these studies was the lack of CO2 regulation.
The problem was reassessed recently on a new technical basis
(whole plants grown in automatic culture, computer monitoring,
and analysis), revealing the existence of photorespiration at high
light intensity in C4 plants (3) and, in C3 plants, a surprisingly
high level of 02 uptake, the competition between 02 and CO2 for
reducing power, and the independence of the production of reducing power from CO2 level (5, 19, 30). An international team
has confirmed these results and further explored the influences of
light and 02 concentration (11). The study presented here is
complementary to theirs and presents new data on the regulation
of reducing power production, the continuation ofdark respiration
in light, the influence of plant age, and the adaptation to low light
intensity.
MATERIALS AND METHODS
The experiments were conducted on whole wheat plants (Tritiaestivum L var Champlein) grown from the 8th day after
sowing in a "C23A mini-chamber" (volume, 6 to 18 liters). PAR
during growth was 175 w/m2 (610 ,IE/m2 s-1), day/night temperatures were 20 ± lC/15 ± IC, and the CO2 level was kept at 330
tdl F'. The root compartment was separated from the aerial part.
Techniques were described in detail by Andre et al. (1, 2).
The plants were grown for 40 to 70 days. As the plants had not
undergone winter frosts, vegetative growth still continued after 70
days without flowering. Apparent photosynthesis was around 100
ml/h at the end of the experiments, still increasing by 2 to 4%/
day; it was limited mainly by the reciprocal shading and the
disturbance of the ventilation by the leaves.
The roots were placed in a beaker containing 2.3 liters nutrient
solution at pH 6.5. This solution was changed every day and
analyzed for nutrient uptake. The volume of solution was enough
to provide a roughly constant concentration of elements, except
NH4', which was exhausted after a few hours of photosynthesis.
The experiments were done in the same growth chamber.
cum
Abbreviations: P, apparent photosynthesis or net CO2 assimilation; P',
or net 02 evolution; E, 02 evolution; U, 02 uptake; PS, net
photosynthesis in standard conditions; R, dark respiration; For clarity the
gas fluxes are schematized in Figure 1.
1032
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'
apparent
Copyright © 1980 American Society of Plant Biologists. All rights reserved.
Plant Physiol. Vol. 66, 1980
02 EXCHANGES IN WHEAT
0
1033
PS~
100
ale(
0
0
100
200
300
400
500
9I1-1.' CO2
b 2%
62
E
FIG. 1. Scheme of the gas exchanges of a plant, with unidirectional and
net fluxes.
C,,p
0.
Different light intensities were obtained by the use of the proper
number of lamps or by placing grids under the lamps for lower
intensities. 02 concentration was lowered by sweeping the chamber
with N2 just before the measuring period. After that, the concentration increased again; at the end of the period, it ranged from 3
to 7% 02- Periods of measurement in nonstandard conditions were
usually limited to 6 to 7 h, separated by 1.5-day "rest" periods to
avoid any adaptation or stress of the plant. In doing so, we
ascertained that the growth curves of the plant were not perturbed
by the experiments, which made it possible to interpolate the
apparent photosynthesis in standard conditions (175 w/m2, 330
Au 1 C02, 21% 02) to the precise time of the measurement; most
of the results here are expressed in per cent of this standard
photosynthesis PS.
Photorespiration was measured through the decrease of the
concentration of the 18018O isotope of 02, compared with that of
an inert reference gas ( 19). The initial concentration of the isotope
in the chamber was about 1%. Gaseous concentrations were
measured with a gas spectrometer Riber QMM 17 and CO2 levels
were measured with an IR gas analyzer (ADC). All raw data were
processed in real time by a Telemecanique T 1600 minicomputer
(2).
RESULTS
INFLUENCE OF A LOW
02
PRESSURE
Figure 2 shows the 02 and net CO2 gas exchanges of a wheat
plant as a function of the concentration of CO2 at either the 21%
(a) or 2% (b) 02 level.
Warburg Effect. The lowering of the 02 concentration increases
photosynthesis (Warburg effect) and decreases U. In absolute
value, the increase in photosynthesis is nearly independent of CO2
(about 40% PS), whereas the decrease in U is greater at low CO2.
Residual Respiration. At 330 Lp1 1-1 C02, reducing the 02 pressure 10 times reduces U by less than 4 times. Even at higher C02,
U does not fall under 20%o of PS, just the level of R. The
identification of this reaction with dark respiration will be discussed below.
Competition between 02 and CO2. The increase of U and
decrease of P when CO2 decreases from 300 to 80 ,ul I-, due to the
competition between 02 and CO2 for reducing power which was
already noticed at 20% 02 (11, 19), also occurs at 2% 02. In that
case, however, the curves are not symmetrical but are decreased
by the reduction of the available quantity of reducing power.
Inhibition of Electron Transport by Lack of Acceptors. We
0
100
P
0
100
200
300
400
900
11.1 '1 CO2
FIG. 2. Variations of the photosynthesis and oxygen uptake rates of a
wheat plant as a function of the concentration of CO2 at 21% 02 (a) or 2%
02 (b). Light intensity was 175 w m-2 (610 AE m-2 s-'); temperature was
20 C; plant age was 45 to 70 days. Standard photosynthesis (PS) is defined
as the net photosynthesis at 175 w m-2 light intensity, 21% 02, and 330 ,u
-1 C02.
observed a decrease in the production of 02 at 20% 02 under 120
,A I-' CO2 or at 2% 02 under 330 ,ul I` CO2. In both cases, this
occurs when the availability of acceptors is about 40% lower than
normal, if we take the affinities of the plant for 02 and CO2 into
account. Since reducing power, in the form of NADPH, is an end
product of electron transport, this inhibition could be a simple
end-product regulation of the reaction by mass action law or by
the effect of an allosteric enzyme.
Inhibition of Electron Transport by Lack of CO2. The preceding
considerations cannot explain why, at still lower CO2 concentrations, U becomes inhibited also, in spite of the continuing availability of 02 and diminishing competition from C02; this is clearly
a new phenomenon. At both 02 concentrations, E is inhibited
when CO2 is less than 60 Ll 1-1. This cannot be due to the
exhaustion of the pool of ribulose bisphosphate, the substrate of
the 02-consuming carboxylase reaction, because the exhaustion
would occur much sooner at 20o than at 2% 02, whereas, remarkably, the inhibition occurs at the same level of CO2. Therefore, it
must be due to a specific effect of CO2 on the photosynthetic
apparatus.
INFLUENCE OF LIGHT INTENSITY
Figure 3 shows the variations in gas exchanges of a young (40
days old) wheat plant as a function of light intensity. The values
of R are those measured on a night following a whole day at the
corresponding light intensity. When a given light intensity was
maintained for only half a day, the effect on R was similar, but of
lower amplitude. The balance of photosynthesis was positive for
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GERBAUD AND ANDRE
1034
Plant Physiol. Vol. 66, 1980
C,)
0.
4R
100
C.)
10
LuA
R
0
-
100
200
300
Light intensity (W.M-2)
400
FIG. 3. Effect of light intensity on the gas exchanges of a wheat plant
at 330 ill -'; 1 w m-2 = 3.48 ,uE m-2 s-'.
40
light intensities greater than 12.5 w m-2 (43.5 ,tE m-2 s-') and
evolved afterwards following a classical curve. The ratio of P to U
was highest at 175 w m-2. This means that the reducing power
was used most efficiently at the standard light intensity, which was
about half-saturating.
At higher intensities, P approached saturation but U continued
growing linearly. Canvin et al. (I 1) have shown that U could even
exceed P at high light intensities in the C3 plant Hirschfeldia
incana Lowe. This indicates that the reaction with 02 has a low
affinity but a high maximum rate (higher than that of photosynthesis), which allows an efficient elimination of excess reducing
power. Nevertheless, we can see that E does not increase linearly
with light intensity; this could be due to a beginning of saturation
of the photosystems or of the 02 uptake reaction.
50
60
PLANT AuE (DAYS)
70
FIG. 4. Evolution with plant age of the gas exchanges of wheat. As the
plot is semilogarithmic, the linear part of the curves (left) corresponds to
the exponential growth period; the flat part of the curves (right) corresponds to a linear growth period.
7.5
C)0.
0
5
AGE OF PLANT
When the plant grows, mutual shading of leaves increases and
the lighting of central leaves becomes weaker and less homogeneous, which changes the shapes of gas-exchange curves and
makes comparisons more difficult.
The decrease of P and increase of U relative to E that is
observed when the plant ages (Fig. 4) does not exceed 10%Yo. It may
be due in part to the effect of diminished light intensity (Fig. 3).
2.5
0
5
ADAPTATION
The wheat plant adapts itself to low light intensities (less than
60 w m 2 or 210 ,utE m-2 s-'). When a new, lower intensity
illumination is given from the beginning of a day, photosynthesis,
initially low, begins to increase after 3 h, the light intensity
remaining constant (Fig. 5). The variation of photosynthesis [e.g.
from 0.5 ml/h initially to 9 ml/h, when the light intensity is 20 w
m-2 (70 ,uE m-2 s-1)] is quite small when compared to the standard
photosynthesis of the plant (116 ml/h), but it is worth noticing
that the same increase of photosynthesis would be obtained if
there were no adaptation at 30 w m-2 (105 ,uE m-2 s-'), that is, a
50% increase in light intensity. No reverse effect was observed at
the return to normal lighting. It is possible that the reverse
adaptation is fast or that the adaptation is efficient only at low
illumination.
The speed of this adaptation process shows the risk of dealing
with a modified or rapidly evolving plant when measurements are
made in apparently steady-state conditions, in particular in whole
plant experiments. All our measurements at low light intensity
were taken during the first 3 h so that the values correspond to
plants in their initial standard state. Other measurements correspond to half-day or whole-day means, but there was no variation
of the gas exchanges during the period of measurement.
10
14
HOURS
FIG. 5. Hourly evolution of photosynthesis of a wheat plant during its
adaptation to a low light intensity [20 w m-2 (70 ,uE m-2 s-'), about onetenth of normal intensity (175 w m-2)]. The average level of U during the
same period was 36 ml/h. Net photosynthesis in normal light intensity
was 116 ml/h. Plant age was 63 days.
ZERO LIGHTING
When zero lighting was realized during the day, U and the CO2
evolution rates were near that of the respiration of the preceding
night and a little higher than the extrapolation of R to 0 w m
(Fig. 3). No rhythm appeared.
DARK RESPIRATION IN LIGHT
The fact that the variation of U with light follows the equation
U = R + k(light intensity) is a clue in favor of the continuation
of mitochondrial respiration in the light. Supplementary information is given by the study of the CO2 compensation point r as
a function of the 02 concentration. It is assumed that the level of
dark respiration is independent of 02 concentration between 1
and 21% and that the level of photorespiration tends towards zero
with 02 concentration. In this hypothesis, the extrapolation to
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1035
02 EXCHANGES IN WHEAT
Plant Physiol. Vol. 66, 1980
photorespiratory gas exchanges, all respiration tests are made
under conditions that inhibit photorespiration, sometimes even in
the absence of photosynthesis (21). It is known that photorespiration influences the ADP/ATP ratio and that some of its reactions
occur in the mitochondria (36). Photorespiration must have an
influence on respiration, the nature and direction of which are still
unknown. However, such a regulation could hardly act as an on/
off switch but, rather, could modulate the level of dark respiration
so as to allow a finite rate of activity even under the less favorable
conditions and apparently, a rate near that of night respiration
under the experimental conditions reported here.
Regulation of Photosynthetic Apparatus. The observed effects
of 02 and CO2 (Fig. 2) may be related to what is known from the
biochemistry of the photosynthetic apparatus.
DISCUSSION
The functioning of the electron transport chain necessitates that
Continuation of Dark Respiration in Light. Several points, of NADP+ be regenerated from NADPH. This is normally done
our study suggest the presence of dark respiration during photo- through CO2 and 02 uptake. Inhibition appears when the availasynthesis. This is an important point in the interpretation of all 02 bility of acceptors is insufficient; this happens at 21% 02 when
CO2 concentration is under 100 d F1-1 or, which is equivalent for
uptake measurements.
The convergence of U towards the level of R at zero light the acceptor efficiency, at 2% 02 when CO2 concentration is under
intensity (Fig. 3) is best ex?lained by a dark respiration. Similar 330 [l I-.
A specific action of C02, the lack of which inhibits U or E,
measurements done with 802 with algae (22, 40) have given
contradictory results. More recently, Mulchi et al. (27) came to regardless of the 02 concentration, has been pointed out. Three
the same conclusion as was reached here, whereas Canvin et al. mechanisms could explain it. (a) The inhibition of the electron
(11) found a convergence of U towards zero. First, it is possible transport in the absence of CO2 was discovered by Warburg, who
that the measurements of Canvin et al. may be slightly underes- believed that CO2 was the source of the 02 evolved during
timated, as shown by the convergence below zero of several curves; photosynthesis. It has been found since then that CO2 catalyzes
second, as shown below, experimental conditions may be impor- the transport of electrons (35), but the exact mechanism is still a
matter of discussion (20). (b) The transport of electrons needs the
tant in this respect.
The convergence of U towards the level of R either at very high regeneration of ADP from ATP, which is assumed by the assimiCO2 concentrations (19) or at high CO2 at 2% 02 (Fig. 2b), lation of CO2 in the Calvin cycle, but not by photorespiration.
conditions which are known to suppress photorespiration, shows This hypothesis seems less probable because the inhibition of E
would then be photosynthesis-dependent rather than C02-dethe presence of dark respiration.
The convergence of the CO2 compensation point towards 10 ,tl pendent. (c) The inactivation of the enzyme ribulose bisphosphate
1-1 CO2 is consistent with the preceding data. The use of a whole carboxylase-oxygenase at low CO2 level (4) also may play a role,
plant could explain the disagreement with the result of Forrester. impeding the regeneration of ADP, and, except for the small part
In vitro studies have proven the respiratory. activity of the that may be due to the Mehler reaction, of NADP. It is not yet
mitochondria in the light (26), this activity being regulated at the possible to judge of the relative importance of these three mechlevel of substrates, in particular ADP, or of enzymes (12, 31). This anisms.
Potential Role of Photorespiration. The exact role of plant
regulation of R could be an alternate explanation of the variations
in labeling of the emitted CO2 after a period of photosynthesis in photorespiration is still not known, although several functions
'4Co2. Fock et al. (17) attribute these variations to the participation have been proposed, most notably the protection of the photosynthetic apparatus in the case of various stresses (9, 15, 28).
of carbohydrate reserves in photorespiration.
The high maximum rate of U and the regulation of electron
Because it is not known how to distinguish respiratory from
transport could help to protect the plant whenever CO2 assimilation cannot cope with the supply of reducing power, e.g. too strong
light, closed stomata (water stress), or cold weather causing a
slowing down of photosynthesis. It was observed that strong light
40
can actually enhance the damaging effect of cold (23, 24). This
view is also confirmed by experiments that show a durable inhibition of photosynthesis in white mustard (15) or bean (29) after
30
treatment at low concentrations of 02 and C02, but we did not
observe it in wheat, which suggests that the regulation of the
electron transport that has been shown here is quite efficient in
~20
wheat, but not in white mustard or bean. In a wheat-type plant,
efficient regulation could be a necessary component of plant
resistance to low temperatures.
10
zero CO2 of the values of r measured between I and 21%'02
depends only on the level of dark respiration in this range (18).
Forrester et al. (18) found that, in soybean leaves, r extrapolated
to zero with 02 and concluded that dark respiration was inhibited
in the light. Our result is different: r extrapolated to 10 ,ul 1-' C02
(Fig. 6). If we suppose that the level of the compensation point is
approximately proportional to the rate of CO2 evolution, we
roughly estimate that there is 5 times more CO2 evolved at 20%
02 than at 2%. This does not make it possible to determine the
precise value of the CO2 evolution rate at r CO2 and 2% 02, but
it indicates the existence of a nonnegligible dark respiration in the
light in the conditions of the experiment.
Acknowledgments-The authors are grateful to Mr. A. Daguenet and Mrs. J.
Massimino for their contributions to the experiments.
0
5
15
10
%
20
02
FIG. 6. Influence of 02 concentration on the CO2 compensation point
of a wheat plant. The arrows show the order in which the points were
determined. Rest periods of about 20 min were allowed between the
determination of each point. Light intensity was 175 w m-2; plant age was
48 days.
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