Relationship betweenPhotosynthesis and Respiration

[Pages:8]Plant Physiol. (1983) 71, 574-581

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Relationship between Photosynthesis and Respiration

THE EFFECT OF CARBOHYDRATE STATUS ON THE RATE OF CO2 PRODUCTION BY RESPIRATION IN DARKENED AND ILLUMINATED WHEAT LEAVES

Received for publication June 21, 1982 and in revised form October 25, 1982

JOAQUIN AZCON-BIETO AND C. BARRY OSMOND Department of Environmental Biology, Research School of Biological Sciences, The Australian National University, P.O. Box 475, Canberra City, A. C. T. 2601, Australia

ABSTRACT

The rate of dark CO2 efflux from mature wheat (Triium aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO2 efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO2 efflux includes a large component which can be abolished by transferring Muminated leaves to 3% 0 and 330 microbar CO2 before darkening. After 30 minutes of darkness, a relatively steady rate of CO2 efflux was obtained. The temperature dependence of steady-state dark CO2 efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO2 effiux followin photosynthesis is correlated with accumulated net CO2 assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO2 co_msation point in 21% 02, and an increase in the light compensation point. The interactions between CO2 efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO2 efflux by respiration Is comparable in darkened and illuminated wheat leaves.

The rate of CO2 efflux by respiration from single leaves and whole plants in the dark is linearly related to the rate of previous photosynthesis when the latter is varied by changing the light level or CO2 concentration (18, 21). McCree (21) fitted an empirical equation in which the rate of dark CO2 efflux is proportional to photosynthesis and dry weight of living material on the plant. This served as a basis for development of the theoretical concepts of growth and maintenance respiration by Penning de Vries (24). Both of these components of respiration are thought to involve, principally, carbohydrate oxidation through glycolysis, the pentose phosphate pathway and the tricarboxylic acid cycle. Growth respiration appears to be less sensitive to temperature than maintenance respiration (22). Explanations of the complex interactions between photosynthesis, temperature, and dark respiration are uncertain, although it is probable that the interaction is mediated by carbohydrate level (6, 8).

The extent to which tricarboxylic acid cycle respiration continues in the light in green leaves is uncertain. Graham (12) concluded that biochemical evidence suggests the tricarboxylic acid cycle continues to operate in illuminated leaves at about the same rate as it does in darkness. Physiological evidence is contradictory; some experiments are best explained in terms of significant CO2 efflux in illuminated leaves from sources other than photorespiration (3, 12) whereas others suggest that these other sources are negligible (7, 19, 23). The lack of methods for direct measurement ofthe rate ofrespiration during photosynthesis greatly complicates

the resolution of this question. A new experimental approach is attempted in this paper.

The experiments described here investigate the relationship between photosynthesis in mature wheat leaves, its products (particularly carbohydrates), temperature, and CO2 efflux by respira-

tion in the light (Rd) in the dark (Rn) (see Ref. 3 for terminology). We conclude that the increase in dark CO2 efflux (Rn) after a

period of photosynthesis is correlated with the amount of carbohydrate synthesized, and that the temperature dependence of dark CO2 efflux varies with leaf carbohydrate concentration. We also conclude that the rate of respiration in the light (Rd) is comparable to Rn and that it makes a significant contribution to total CO2 efflux in illuminated wheat leaves.

MATERIALS AND METHODS

Plant Material. Triticum aestivum cv Gabo, Viciafaba, Eucalyp-

tus grandis, and Lolium perenne plants were grown from seed in a cabinet in 12-cm pots of soil. They were watered twice a day, and were fertilized every other day with nitrate-type Hewitt solution. Total nitrate concentration was 12 mm. Quantum flux (400-700

nm) was 500 to 600 pEm2.s-'. The day/night temperature

regime was 25/20?C with a daylength of 13 h. RH was between 60 and 80%. Mature leaves of 30-d-old plants were selected at the end of the night period.

Gas Exchange Techniques. CO2 and water exchanges were measured in leaves using an open system gas analysis apparatus, utilizing an IR CO2 analyzer (model 865, Beckman Instruments) which was operated in both differential and absolute modes, and a dew point hygrometer (model 880; Cambridge Systems, Waltham, MA).

One or two attached intact leaves were inserted in a wellventilated aluminum leaf chamber (boundary layer conductance

to diffusion of water vapor was 2.2 mol_m-2_s-'). Illumination

was provided by a 2.5 kw water-cooled, high-pressure, xenon-arc lamp (model XBF 2500, Osram), the UV and IR components being removed with a Schott KG 2B filter. Quantum flux (400700 nm) incident on the leaf was 1000 ,uE_m 2_s- , and it was measured with a quantum sensor (model LI-190 SR, Lambda Instruments). Leaf temperature, which was controlled by circulating water through a jacket, was measured with two copper-constantan thermocouples (0.1 mm diameter) in contact with the lower surface.

Air with the desired partial pressure of CO2 was obtained by injection of 5% CO2 in air into C02-free air through a stainless steel capillary tubing. A self-venting pressure regulator (model MAR-1P; Clippard Minimatic, cincinnati OH) and a pressure gauge were used to control the injection rate. C02-free air with

different 02 concentrations was obtained by mixing compressed

ambient air with N2 from a cylinder, and then by passing the

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RESPIRATION AND PHOTOSYNTHESIS

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resulting gas through two columns of soda lime (Carbosorb, self-

indicating; BDH Chemicals Ltd, Poole, England). The 02 concentration was measured with an 02 electrode (model 5331, YSI). The gas was then humidified in a gas washing bottle with a scintered disc. The dew point of the gas was maintained by passing the gas through a glass condenser, the temperature of the latter being controlled by circulating the water from a temperature-controlled

water bath. Air flow through the leaf chamber was monitored with a mass flowmeter (model AFSC-IOK; Hastings, Hampton, VA). Flowmeters with needle valves were used to distribute gas flow throughout the system. Copper tubing was used in the circuit.

The outputs of all sensors were registered on a digital voltmeter, and the outputs from the CO2 analyzer and dew point hygrometer were continuously recorded. The outputs from the sensors allowed

calculation of the rate of net CO2 assimilation, stomatal conduct-

ance, intercellular CO2 partial pressure, and dark CO2 efflux in air. All these parameters were calculated according to the equations given in (30). Leaf area was measured with an electronic planimeter (model LI-3050A, Lambda Instruments).

Measurement of the CO2 and Light Compensation Points. The CO2 compensation point, r, was either measured by using a closed system and allowing the leaf to equilibrate with its CO2 atmosphere, or by interpolation of a curve of 'net CO2 assimilation versus intercellular CO2 partial pressure' to zero assimilation. The gas exchange apparatus was modified by the inclusion of a closed system. A metal bellows pump (model MB-21E; Metal Bellows Corp., Sharon, MA) circulated the air through the system. Plug valves (model B-4P4T; Nupro Co., Cleveland, OH) were used to

manually switch from open to closed system or vice versa. Both methods were compared in the same leaf of wheat at two 02

concentrations (21% and 3%) yielding identical results (not shown). When r was measured in closed system, its value was taken

after 60 min in order to obtain steady-state values; then, the rate of dark CO2 efflux was measured 30 min after the light was switched off.

The light compensation point was measured by interpolation of a curve of 'net CO2 assimilation versus quantum flux' to zero assimilation. Light intensity was changed by interposing copper screens.

Effect of a Period of Photosynthesis on the Rate of CO2 Efflux in the Dark. A pair of mature wheat leaves from the same plant was enclosed in the photosynthetic chamber and the rate of dark CO2 efflux in ambient air was monitored for 2 h at the end of the night and after a period of photosynthesis of 6.25 h at ambient CO2 and 02 levels. Leaf temperatures were 13.5, 20, 24, 27, and 30?C in darkness. Leaf temperatures during the light period were 2 to 40C higher than in the dark period. This experiment was repeated three times at every leaf temperature. A different plant

was used every time.

In a similar experiment, the rate of dark CO2 efflux was monitored for 1 h at the end of the night and after a period of photosynthesis of 6.25 h in which the 02 concentration in the air in the last 20 min was 3%. In this experiment, temperature was kept constant in the light and in the dark. In the experiments

performed at 30?C, the 02 concentration in the dark period was 21% or 3%. Three replicates were done at every 02 concentration, but no difference was found in the time course of dark CO2 efflux after the light period (not shown). This experiment was also performed at 200C in leaves selected from six plants, but the 02 concentration in the dark period was 21%.

The results of these experiments are shown in Figures 1 and 2. The curves representing the time course of dark CO2 efflux are averages of three or six individual curves. The statistical variation of the data was very small and it is not shown. The standard errors were less than 5% of the absolute values and ranged between 0.01

and 0.08 ,umol CO2* m2* s', the lower values being more common specially at lower temperatures.

2_ \

30 C

In

-5

E

x

G) 0

-

o-

27C

24C

II

20 C

13.5 C

Uri

IIII

30

60

90

120

Time in the dark (minutes)

FIG. 1. Time course of dark CO2 efflux of mature wheat leaves after a period of photosynthesis of 6.25 h at ambient CO2 and 02 levels ( ). (-- -), rate of dark CO2 efflux at the end of the preceding night period.

Relationship between Dark CO2 Efflux and Leaf Carbohydrate Status. Mature wheat leaves were allowed to photosynthesize for variable periods of time up to 7 h, at ambient and high (800 ,ubar)

external CO2 partial pressures; then, the rate of dark CO2 efflux was measured 30 min after the termination of the photosynthetic

period. Leaves were immediately killed in liquid N2 and stored frozen for carbohydrate determination (see below). Leaf temperature was 21?C in darkness and 23.50 in the light.

Study of the Temperature Dependence of Dark CO2 Efflux. The rate of dark CO2 efflux of mature wheat leaves selected at the end

of the night was measured at different temperatures up to 40?C. The first measurement was made at 11 C. Other leaves were

allowed to photosynthesize for 6.25 h at 22?C, at ambient CO2 and 02 levels. Then, dark CO2 effux was measured at 20?C, 30 min after the light was switched off; leaf temperature was steeply raised to 42?C in some experiments or decreased to about 8?C in other experiments.

Carbohydrate Determination. Leaves killed in liquid N2 were freeze-dried. Carbohydrates were extracted in boiling water for 15 min and analyzed using an enzymic method. Free glucose plus fructose were measured from the leaf extract using a glucosespecific assay (Calbiochem-Behring Glucose s.v.r. No. 870104), after converting fructose to glucose with P-glucoisomerase (Sigma P-538 1). Glucose was converted to glucose 6-P by hexokinase, and then oxidized by glucose 6-P dehydrogenase, reducing a molar equivalent of NADP. The change in A at 340 nm is proportional

576

AZCON-BIETO AND OSMOND

Plant Physiol. Vol. 71, 1983

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-

%02

30C

and increased within 30 min to the level of leaves kept in 21% 02

2

-

2 21 duri 30Cng throughtohuet

the photosynthetic

measurement period

period. in the

If the 02 concentration dark was lowered to 3%,

the result was the same.

E

3

-- >- _

These experiments suggest that the initially high rates of dark

0 1 -- - - - -

x

CO2 efflux are in part due to O2-dependent processes in the light

-

period. Presumably, large pools of photorespiratory intermediates

continue to be decarboxylated in the dark, for about 30 min at

-6

0

20?C and 30?C.

The rate of dark CO2 efflux 30 min after the termination of the

4)

/??2

2

21

20 C photosynthetic period increased in proportion with the total net

CO2 assimilation which had occurred during this period (Fig. 3).

Dark CO2 efflux was also positively correlated with specific leaf

------

weight which greatly increased during the light period due to the

1 /3

accumulation of products derived from photosynthesis, mostly carbohydrates (not shown). Dark CO2 efflux was correlated with

several leaf carbohydrate fractions (Fig. 4). The relationship be-

_

I __

tween dark CO2 efflux and fructosans was not investigated here.

0

10 20 30 40 50 60

The temperature dependence of dark CO2 efflux at the end of

the night period was compared with that of leaves after 6.25 h of

Time in the dark (minutes)

FIG. 2. Time course of dark CO2 efflux of mature wheat leaves after a period of photosynthesis of 6.25 h in which the 02 concentration in the air

in the last:20 min was 3% (-). The time course of dark CO2 efflux after a period off 6.25 h of photosynthesis at ambient 02 concentration (see Fig. 1) is given for comparison --- -). The 'end of night' level of dark CO2

efflux is at[so shown (- ).

photosynthesis in air. At the end of the night, dark CO2 efflux

showed an exponential relationship with temperature, with a

single apparent activation energy, Ea, of 12.9 kcal . mol' in the

range from I I to 40?C (Fig. 5). However, CO2 efflux after a long period of photosynthesis presented a very different pattern in

response to temperature. Its rate was higher at all temperatures,

but Ea declined in the range from 200C to 400C while it increased in the range from 10?C to 20?C.

to the gluicose concentration in the range from 0 to 10 ng-mf'

wfaanarncosdempwetaorhcsfecor4unarraeemrmparepeslmd.e.uedrdeSatducwwtriotsihentVwsmaarphsieeardnhn ry6dw3rh4o. elssynpztee6 rcdtorpobhp3 yiohtotitanoo4 cctmmuheetabtetdaentrbrbg.i.enngThtaeahbleeaaqbsslosoeratayf

extract at 370C for 2 h in a water bath with invertase (Sigma I-

5875) in C. N acetate buffer (pH 4.6). Inasmuch as invera also

has been reported to hydrolyze some small fructosans (29), the sugars ressulting from the action of this enzyme are described as the inverttase fraction. This fraction was obtained by subtracting the amouint offree glucose plus fructose from total glucose assayed Starch co:incentration was obtained by incubating the leaf extract

at 370C fFor 48 h in a water bath with 0.5% 'Clarase 900' (Miles Laboratonries) in 0.1 N acetate buffer (pH 4.6). Clarase 900 is a mixture o)f several digestive enzymes which hydrolyze starch and sucrose to) hexoses. Starch was obtained by subtracting the glucose in glucose plus fructose and invertase fractions from total glucose assayed in the Clarase digest.

RESULTS

daRelCaOti2onasnhdipLbiegthtweCenomDarnktCiOo2n EPfofiinxts,.CCaoribnochiyddernattewiStthattuhse,

increase in the rate of respiration following a period of photosyn-

r thesis, in 21% 02 also increased in the same leaf (Table I; Fig.

6). Interestin.gl-y, the values for r showed the

treatments i the rate of

cawrhbiochhydprhaotteorfeosrpmmaattiioonn

would would

largest increases after have been lleast bbuutt

have been maximal

(800 bar a another set

CO2f 2% O2). of expenments

These correlations

in whichlaeafCwas

were confirmed in initially illuminated

for 4 h in air containing 750 obar CO2, 21% 02 (ow photorespir-

ation), then in air containing low CO2 prereses (slightly above

F) for a second period of 4 h (high photorespiration). Table II

shows agin that F was higher after h peri iodcwhh ichthe rates

of photorespiration were lower and the rates of carbohydrate

formation were higher.

Measurements of r from many experiments in which dark CO2

c4A 1 .S

A

E

Properties and Temperature Dependence of Dark CO2 Efflux. Dark CO2 efflux measured after a period of photosynthesis was much higher than at the end of the preceding night period (Fig. 1). This effect occurred at all temperatures studied. However, the increase in total dark CO2 efflux due to the effect ofphotosynthetic activity was relatively higher at lower temperatures (eg. 20?C). At higher temperatures (e.g. 300C), the rate of dark CO2 effluX returned to the level at the end of the night within 2 h. At lower temperatures, it took longer (e.g. 5 h at 200C). That is, the effect of the photosynthetic activity on dark CO2 efflux was more accentuated and lasted longer at lower temperatures.

It was commonly found that the rate of dark CO2 efflux was higher in the first 30 min after illumination and did not attain a steady slow rate of change until after about 60 min of darkness. When the 02 concentration of the atmosphere was lowered from 21% to 3% during the last 20 min of the light period and the rate of dark CO2 efflux measured in 21% 02, a different pattern was obtained (Fig. 2). The rate of dark CO2 efflux was initially low

E

1.Ok

M.

1.0

0/

x

0*

a 0.5F- .0 CM 0

o

In

ons *

0

-

0

r

100

---

300

500

700

Integrated net CO2 assimilation (mmol rmf2)

FIG. 3. Relationship between dark CO2 efflux and integrated net CO2 assimilation in mature wheat leaves. (0), Leaves selected at the end of the night; (0), leaves photosynthesizing at ambient CO2 levels (A), leaves

photosynthesizing at 800,ubar CO2.

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E

0

E

C

a-

wRESPIRATION AND PHOTOSYNTHESIS

Carbohydrate concentration ( g glucose equiv m-2)

50

100

150

200

250

577

Carbohydrate concentration (m mol Cmr2)

FIG. 4. Relationship between dark CO2 efflux and several carbohydrate fractions in mature wheat leaves.

Leaf temperature (?C)

50

40

30

20

10

0

A~~~~~~~

I

N.,

0

4 7'

E 2

E

1

x

-1

I

310

U0)

0.3 0 --v. l. 0 a)

320

330

340

350

360

Inverse of absolute temperature x105

FIG. 5. Arrhenius plots for dark CO2 efflux of mature wheat leaves selected at the end of the git period (0) or at the end of a period of photosynthesis of 6.25 h at ambient CO2 and 02 pressures (A). Apparent activation energies (E.) are expressed in kcal- mol'. They can be converted to Qlo values by using the formula 'log Qio = 2190.EJ/T T2,' where T,-T2 = 10 K.

effiux (R.) was varied by varying CO2 and 02 partial pressures during the period of photosynthesis are shown in Figure 7. Extrapolation of this relationship to zero R. presumably yields the photorespiratory component of r in these mature wheat leaves.

The correlation between an increase in r and R. following a period of photosynthesis was also observed at temperatures other than 21PC (e.g. 15?C and 30?C) (data not shown) and in other

species (Table III). The increase in r following a period of photosynthesis was

reflected in a decrease in net rate of photosynthesis over a range of CO2 partial pressures and was not due to a change in the slope of the curve of net CO2 assimilation versus intercellular CO2

partial pressure (Fig. 6). The displacement of this curve was 1.0

? 0.2,umol C02-m 2-s , which is an average value obtained in

four experiments including that shown in Figure 6. This value compared well with the increase in the rate of dark CO2 efflux observed after a period of photosynthesis. The rate of CO2 efflux into C02-free air in the light was also higher following a period of

photosynthesis (Fig. 6). The light compensation point also increased in the same leaf

after a period of active photosynthesis. Figure 8 shows the correlation between the light compensation point and dark CO2 efflux which was varied by the period of prior photosynthesis under different conditions of temperature and CO2 partial pressure.

578

AZC5N-BIETO AND OSMOND

Plant Physiol. Vol. 71, 1983

Table I. r and Dark CO2 Efflux, R,, Measured at the End ofthe Night and after a Period of Photosynthesis of 5 Hours in the Same Leaf of Wheat.

r was measured in closed system (see "Material and Methods"). r and R. were measured in 21% 02. Leaf temperature was 21?C in the light and in the dark. Net CO2 assimilation rates were about 24 (A) and 30 (B) nmo1

C02m .2.s'. The values shown are means ? SE of three to four independent experiments.

CO2 and O2 Levels during the Photosynthetic At the End of the Night

Period

F

After 5 h Light

F

A. 370 ubar C02, 21% 02 B. 800 ,ubar CO2, 21% 02 or 800 ,ubar CO2, 2%

02

pbar Ab~a~rubar JImMo-l2.Css O-2I. pm br JMo-l2 *CsO-I2

35 ? 1.5 0.65 ? 0.12 36.5 ? 2.5 0.62 ? 0.03

38 ? 0.5 0.94 ? 0.04 42 ? 1.5 1.18 ? 0.16

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CA

8s

E

6L

E

0

E

Before

-

0

After

30

40 50 60 70 80 90

Intercellular CO2 partial pressure (pbar)

z) z

CO2 Evolution in CO2 - free air

FIG. 6. Curve of net CO2 assimilation versus intercellular CO2 partial pressure measured at the end of the night and after a period of photosynthesis of 3 h at 800 g2bar CO2 in the same leaf of wheat. Leaf temperature was 21 'C. Measurements proceeded from high to low CO2 partial pressures, as indicated by the arrows.

Table II. r and R. ofa Mature Wheat LeafMeasured after Two enough to sustain glycine decarboxylation in the dark for about

Consecutive Periods ofPhotosynthesis of 4 Hours at High and Low C02 30 min at the rates observed in our experiments. This observation

Partial Pressures

contrasts with the idea that the photorespiratory postillumination

02 concentration was 21%. The rates ofnet CO2 assimilation during the burst in leaves is restricted to the first 2 to 5 min in darkness, as

firt (A) and second (B) periods were about 23 and 2 jAmol C02m2s1, commonly found in many species including wheat (1 1). However,

respectively. Two independent experiments were performed. The rest of this discrepancy may be explained by the short length of the

conditions were the same as in Table I.

preceding light period utilized in previous studies (often only a

C02Partial Pressure during the r

Photosynthetic Period ,Abar

R

pAmol CO2 m-2.S1

few min) compared to the present experiments. The remaining CO2 efflux, was closely correlated with several carbohydrate frac-

tions. This CO2 presumably arose from tricarboxylic acid cycle and pentose phosphate pathway oxidation of carbohydrate de-

A. 750 ubar CO2

43 ? 1

1.48 ? 0.02

rived substrates. A similar correlation between dark CO2 efflux

B. 50-75 ,barCO2

37 ? 2

0.80 ? 0.05

and carbohydrates has also been found in leaves of Cucumis sativa

(8).

DISCUSSION

The linear relationship between the rate of photosynthesis and the subsequent rate of dark CO2 effluX (see also Ref. 18) can be

Prperties and Temerature D epedence of Dark CO2 Eflux. explained in terms of quantitative changes in carbohydrates, com-

Dark C02 efflux (R.) of mature wheat leaves increased consider- mon metabolites to both processes.

ably after a long period of photosynthesis, as did that of tomato The enhancement of leaf respiration by carbohydrates cannot

leaves (18). At least two groups of substrates contributed to the be primarily related to growth requirements since mature leaves

CO2 efflux. Because 15% to 20% of the CO2 evolved in the first 30 were used. Alternatively, excess respiration may be used for

min ofd ss was abolished ifleaves were kept in low 02 during synthesis of compounds (e.g. amino acids) in the leaf which can

the latter part of the photosynthetic period, we conclude that this be utilized for growth in other parts of the plant and for providing

CO2 are from photorespiratory substrates. In this sense, the energy for transport of assimilates (15). However, when the rate

levels of glycine measured in wheat leaves during the ligt period, of sugar export from the leaf is reduced by e.g. lowering sink

1.5 to 2 mmol-m2 (M. Berger, personal communication), are high demand, carbohydrates accumulate in this organ and the rate of

RESPIRATION AND PHOTOSYNTHESIS

579

Table III. r and R. of Mature Leaves of Several Plant Species Measured at the End of the Night and after a Period of Photosynthesis of5 Hours

A is the rate of net CO2 assimilation. The rest of the conditions were the same as in Table I.

Species

CO2 Pressures during

the Photosynthetic

Period

At the End of the Night After 5 h Light

A

r

RFr

R,

Abar

aiMOl C02m2

pLMOI C02m2

Atibar

pAbhar JIMOl Cs00,2 M2

Eucalyptus gran-

340

dis

800

Viciafaba

800

Lolium perenne

800

13

40.5

0.82

45

1.07

17

39

1.20

45

1.50

21

38

0.77

41

1.05

22

34

0.65

38

0.87

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cf%r-

-0

* */

0

20

CN-

IE

UJ

y=13.4x r=0.99

0

L-

L-._

0.0

0

0

0

c 15p

0

40H

0

_ @00

._

0.6

0 *.10 *

an

C Q0 0c

**

in-

0

000

01-11 0 0

y=30+9.4 x r = 0.83

.C5_

0.

10k-

ac

0

a-

0

I0.)

30-

0

E

U01

5

-I

U1

_.V

0

0.5

1.0

1.5

0?

0.5

1.0

1.5

Dark CO2efflux, Rn (pmol m-2s-1)

Dark CO2 efflux, Rn (pmol m2 S1)

FIG. 7. Relationship between the CO2 compensation point and dark CO2 efflux of mature wheat leaves at 21 'C.

respiration increases (2, 14). This suggests that carbohydrate may be wastefully oxidized in some conditions in the absence of any apparent major requirement.

The rate of respiration extrapolated to a positive value at zero carbohydrate (Fig. 4), which may represent maintenance respiration (24). Respiration at the end of the night, when carbohydrate content of wheat leaves was very low, may be principally associated with maintenance processes. As other authors have found (6, 22), it increased exponentially with temperature. However, when carbohydrates accumulated in the leaf as a result of the photosynthetic activity, the rate of dark CO2 efflux increased, and the shape of its temperature dependence changed dramatically, showing different activation energies (Ea) above and below 20?C. It is unlikely that this break in the Arrhenius plot for dark CO2 efflux at high leaf sugar level could be attributed to membrane phase transitions (26) because mitochondrial respiration is presumably involved in both instances. The mechanism underlying this response may involve the effect of substrate concentration on the temperature dependence of enzymatic reactions. The apparent Ea of enzymatic reaction decreases at low substrates availability since the Km of enzymes for their substrates generally increases with temperature (10). Therefore, a fixed substrate concentration could be saturating or limiting depending on temperature, and Ea should

FIG. 8. Relationship between the light compensation point and dark CO2 efflux in mature wheat leaves. (0), Leaf temperature, 20?C; external CO2 partial pressure, 330 ,ubar. (0), 30?C, 330 ,ubar CO2. (A), 20?C, 640

Obar CO2. (A), 30?C, 640,ubar CO2.

be consequently affected. Considered as a multienzyme system, respiration could be saturated by substrates at low temperature after a period of photosynthesis, and its Ea should be very high. However, E. would decline at higher temperatures as soon as substrates are present at concentrations close or below the Km of key enzymes.

These explanations of the interaction between carbohydrate levels and temperature on respiratory CO2 efflux assume that there is a direct regulation of respiration by substrate availability. The data suggest that glycolysis and mitochondrial reactions in leaves are not necessarily limited by the energy charge in a very narrow range, at least when substrate levels are low (cf. Beevers, 4). A similar conclusion was reached by Sagho and Pradet (27) who have shown that 02 uptake of maize root tips varied widely in response to sugars while the energy charge remained constant. However, these results do not exclude the possibility that respiration is regulated in a complex way. As will be reported later, the wheat leaves studied here show a large cyanide resistant component of respiration when carbohydrate levels are high (J. Azc6nBieto, H. Lambers, and D. A. Day, unpublished). The Arrhenius plot of the cyanide-resistant respiration of wheat coleoptile mitochondria also shows a discontinuity at 17.5?C (20).

Relationship between Dark CO2 Effux and the CO2 and Light

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AZCON-BIETO AND OSMOND

Plant Physiol. Vol. 71, 1983

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Compensation Points. r of mature leaves of wheat and other C3 species varied during the photoperiod, its value being low at the end of the night period, but increasing during the day period. Similar changes of r after periods of light or darkness have been reported in leaves of wheat (23) and Rumex acetosa (16). In contrast to these results, r did not vary after prolonged exposure to darkness, leading to starvation in leaves of Nicotiana tabacum (13).

To investigate the nature of the changes in r in wheat leaves,

we varied the CO2 and 02 partial pressures in the atmosphere during the photosynthetic period to produce different rates of photorespiration and photosynthetic carbohydrate formation. We concluded that variations in r during the photoperiod were principally related to processes other than photorespiration, and presumably were associated with respiration, because r and R. increased maximally after periods in which the gas composition of the air favored high rates of photosynthetic carbohydrate formation and minimal rates of photorespiration (low 02 and high CO2 pressures). Conversely, r and Rn were lower following a period in which the rate of photorespiration was maximal and the rate of carbohydrate synthesis was reduced (low CO2 and ambient 02 pressures). This conclusion is further supported by the strong

correlation found between r and R. (Fig. 7). From this relationship, r has a positive value when R. is zero, which presumably represents the photorespiratory component. Assuming an average

value of 1 ,umol C02 m2.s-1 (at 21VC) for Rn, we estimate that

Rd, the CO2 efflux due to respiration, contributes about 25% of the CO2 efflux measured at r. This value for wheat leaves is similar to that found in L. perenne (3). However, the contribution of Rd to r is variable, and it is correlated with the carbohydrate level. This conclusion is consistent with the fact that externally added sugars increase the CO2 compensation point and the rate of respiration of leaves (28, 29, and unpublished results).

The light compensation point of mature wheat leaves also increased during the day, being correlated with the rate of respiration. This relationship extrapolated to the origin suggesting that respiration is a major component of the light compensation point.

The rate of respiration in the light (Rd) can be estimated from the displacement on the curve of net CO2 assimilation versus intercellular CO2 partial pressure by varying the rate ofrespiration in the dark (Rn) through changes in the leaf carbohydrate concentration. It can be concluded that the rates of Rd and Rn are comparable in wheat leaves.

Peisker and Apel (23) also analyzed the responses of r, its 02 dependence, and respiration in the dark in wheat leaves after a dark period and after an extended light period (18 h) at high CO2 concentrations. They found that respiration increased following the extended period ofphotosynthesis, that the 02 dependency of the CO2 compensation point increased, but that the latter increased by only 30%o of the value expected on the basis of their model. Our data confirm their observations, but the different analysis of our data does not support Peisker and Apel's (23) conclusion that respiration in the light is inhibited by 70%0. Whether the expectations of their model, or technical discrepancies, are relevant to our

different conclusions remains to be resolved.

Graham (12) reviewed the literature and concluded that glycolysis and tricarboxylic acid cycle can operate in illuminated green cells although some modifications probably occur in relation to the dark pattern. This is suggested by the increase in the malate/ aspartate ratio and the different labeling patterns after administration of radioactive carbon compounds (e.g. CO2, tricarboxylic acid cycle intermediates, amino acids, sugars) into citrate and other tricarboxylic acid cycle intermediates and related compounds, such as glutamate, glutamine, etc. (5, 12). The evidence is consistent with the suggestion that glycolysis and tricarboxylic acid cycle are modified in the light to allow a continuous anaplerotic carbon flow for supplying a-oxoacids which the chloroplast

is unable to make (17). These compounds can be used for a variety of synthetic reactions including amino acid and lipid formation. Important features of this anaplerotic flow are the probable operation of P-enolpyruvate carboxylase in the cytosol and malic enzyme and the mitochondrion to replenish carbon loss from the tricarboxylic acid cycle (1, 9). It is not known if the tricarboxylic acid cycle operates beyond succinate oxidation, and the operation of the mitochondrial electron chain in the light is a more uncertain aspect of the problem. If, however, respiration in leaves in the light is cyanide insensitive, control of electron transport via energy charge is likely to be less effective. The CO2 arising from the above mentioned reactions (e.g. 1 mol CO2 released/mol of glutamine formed) could well be responsible for most of the rate of respiration in illuminated leaves observed in our experiments. Non-green cells in the leaf also contribute to Rd, but it is not known whether photosynthesis exerts the same influence on their respiratory metabolism as in green cells.

The effect of the photosynthetic activity on the rate of respiration in the light may be mediated by the supply of P-enolpyruvate from recently synthesized triose-P or from sugars. The latter alternative seems more unlikely inasmuch as the exogenous glucose is not metabolized through glycolysis in illuminated leaves including wheat (5, 12). High CO2 concentration enhances the carbon traffic through the tricarboxylic acid cycle and related compounds, presumably by increasing the supply of substrates for P-enolpyruvate carboxylase (25). This may help to explain why some authors have failed to find significant CO2 efflux by respiration in illuminated leaves into C02-free air conditions (19). This also suggests that respiration in daytime (Rd) may be underestimated at the CO2 compensation concentration.

Acknowledgments-We are very grateful to Dr. S. C. Wong for his help in designing equipment and to Drs. G. D. Farquhar, D. A. Day, and H. Lambers for useful comments. We also thank Mr. M. Berger for making available unpublished

data.

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