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Variations in the Effect of Temperature on Oxygen Dependence of CO2 Gas Exchange in Wheat Leaves
Biochem. Physiol. Pflanzen 174, 391-397 (1979)
Variations in the Effect of Temperature on Oxygen Dependence of C0 2 Gas Exchange in Wheat Leaves MARTIN PEISKER1 ), INGRID TICHA2) and PETER APEL1 ) 1)
Zentralinstitut fiir Genetik und Kulturpflanzenforschung der AdW der DDR, Gatersleben, DDR; 2 ) Institute of Experimental Botany, CSA V, Praha, CSSR
Key Term Index: 00 2 gas exchange, 00 2 compensation concentration, oxygen dependence, leaf temperature, intracellular resistance; Triticum aestivum.
Summary The influence of leaf temperature on the slope of the linear oxygen dependence of the intracellular Tesistance to 00 2 transfer ({J) and on the slope of the linear oxygen depedence of the 00 2 compensation concentration (y) was determined in wheat leaves. The variation in the temperature dependence of fJ found in individual leaves was interpreted as an interaction of the effect of 0 2 on carboxylation of ribulose 1,5-biphosphate and other temperature dependent processes. The strong correlation between y and leaf temperature suggests the absence of an influence of such processes on the relationship between photosynthesis and photorespiration.
Introduction
It is widely recognized that in C3 plants the oxygenation of ribulose 1,5-biphosphate (RubP) is the main reaction providing glycolate - the substrate for photorespiration (LoRIMER et al. 1978). In addition this reaction has been suggested as an explanation of the oxygen inhibition of RubP carboxylation. This concept is supported by similarity in the effect of 0 2 on C0 2 gas exchange at different temperatures and temperature dependencies of kinetic constants determined in vitro for RubP carboxylase-oxygenase (LAING et al. 1974; PEISKER and APEL 1977). However, there are some observations which do not fit this direct correspondence between gas exchange and enzymological data. For instance, in different selections of sunflower different degrees of stimulation of net C0 2 uptake by low 0 2 concentrations were observed at 350 ll-1 C0 2 whereas the C02 compensation concentrations (F) were relatively constant (LLOYD and CAN'VIN 1977). This means that the slopes of the C0 2 response curves at 2% and 21% 0 2 from leaf to leaf varied greatly in relation to one another. Similarly, in flag leaves of wheat, variation in the 0 2 dependence of the intracellular resistance to C0 2 transfer was observed. rand its 0 2 dependence were also in this case practically unchanged (PEISKER and APEL 1976). In the present paper a variation in the influence of temperature on the 0 2 dependence of intracellular resistance in individual leaves is described. Material and Methods Single plants of spring wheat cv. "Carola" were grown in plastic pots with sandy soil in a growth chambeT (SAXCIL, U.K.). The conditions during growth were 16 h of light at an air temperature of 21 oo and a photon flux density (400-700 nm) of 310 f.l Einstein m- 2 s-1 • A temperature of 16 oc was, 26
Biochem. Physiol. Pflanzen, Bd. 174
1\I.
392
PEISKER,
I. TrcnA and
P. APEL
maintained during the 8 h of darkness. The plants were allowed to develop the main culm only. All developing tillers were regularly cut off. Measurements were made on 20 fully developed 5th leaves before leaf area expansion of the 6th leaf was complete. Net C0 2 uptake, transpiration, and leaf temperature of attached leaves were measured at C0 2 concentrations between 0 and 4 · 10-4 kg· m- 3 as described by PEISKER and APEL (1975). In contrast to former experiments, a leaf chamber was used which was equiped with a device for regulating leaf temperature by means of an electric heater in a manner similar to that described by WEST and GAFF (1976). The measurements were conducted at three of four different temperatures ranging between 21 and 34 oc and at a saturating photon flux density (400-700 nm) of 950 p, Einstein m - 2 s-1 • From the linear part of the relationship between net C0 2 uptake and intercellular C0 2 concentration, the intracellular resistance to C0 2 transfer (r;) and the C0 2 compensation concentration (F) were determined. Three 0 2 concentrations were chosen between 0.27 and 0.65 kg m-3 to determine the 0 2 dependencies, fJ andy, of r; and r, respectively. These parameters are defined by the equations r;
=
r
= 'YJ
x
+ fJ [0 2] and + r co2)
and were calculated by linear regression for each leaf temperature
(PEISKER
and
APEL
1977).
Results
The influence of 0 2 concentration and temperature on the C0 2 dependence of net photosynthesis is shown for three leaves selected from twenty experiments. The pattern seen in these responses varies greatly from leaf to leaf (Fig. 1). The slopes of the C0 2 dependencies at atmospheric 0 2 concentration of 0.27 kg m- 3 were similar at 23 and 33 °C. However, at the higher 0 2 concentration of 0.62 kg m- 3 different inhibitory effects were observed at the two temperatures. The slope of the C0 2 dependency at the lower temperature was at times less than (Fig. 1A) and greater than (Fig. 1C) that measured at the higher temperature. In some cases the slopes were nearly equal for both temperatures (Fig. 1 B). In other words, the effect of leaf temperature on the 0 2 dependence of intracellular resistance varied from leaf to leaf (Fig. 2).
51 A
c
B
1(1) 4 N
'E
...""'"'
3
~ 2
0
2
30
1
2
30
1
Jntercellular C0 2 concentration f 10-4 kg rn- 3 J
2
3
Fig. 1. Relationship between net C0 2 uptake and intercellular C0 2 concentration. The measurements were conducted at temperatures of about 23 oc (full lines) and 33 oc (broken lines) and at 0~ concentrations of about 0.27 kg m- 3 (full circles) and 0.62 kg m- 3 (open circles). The results from three individual leaves 3re shown (A, B, C).
393
Effeet of Temperature on 0 2 Dependence of C0 2 Exchange
In 12 experiments a pronounced decrease of f3 with increasing leaf temperature as in Fig. 2A was found. f3 did not change markedly with temperature (Fig. 2B) in 7 experiments. In one experiment a distinct increase of f3 at the higher leaf temperature was observed (Fig. 2 C). If the results of all experiments were considered jointly, an overall
c
800A
. ------0,3
~-
0,4
--
~~
0,5
-~
0,3 0;4 o,s 0,6 ~ concentration (kg rri3)
0,6
0,3
0,4
0,5
0.6
Fig. 2. Relat·ionship between intracellular resistance to CU 2 transfer and 0 2 concentration. The 0 2 dependence of ri at temperatures of about 23 oc (full lines) and 33 oc (broken lines) in three individual leaves is shown (A, B, C).
15
•
•
~OJ _;£
"',: (j,
10-
•• •
N
!2
• •
• •
CQ.
5-
• r exp=0.55
•
~---------.-----------,----------,
20
25
30 35 Temperature ( °C l
Fig. 3. Influence of leafiemperawre on the 0 2 dependence of the intracellular resistance to 002 transfer ( {J). The curve represents the exponential regression line for values of {J from 20 leaves according to {J = {J0 exp (26*
~) (p
< 0.001, kp
=
-3348 K).
394
M.
PEISKER,
I. TrcHA and
P. APEL
decrease of {J with increasing leaf temperature was seen (Fig. 3). However, the considerable variation in {J, which was due in part to variation in its temperature dependence, resulted in a weak correlation. Using the exponential expression {J
=
flo exp -
(~)
a correlation coefficient of 0.55 and a temperature constant of kfl = -3348 K at p < 0.001 were obtained. In individual leaves, the values of kfl varied between -9598 and +4843 K. The 0 2 dependence of the C0 2 compensation concentration increased with increasing leaf temperature (Fig. 4). The exponential equation
-
( T-ky)
y -Yo exp -
was fit to the data using the values of y from all the experiments. This resulted in a value for ky of +3358 K and a correlation coefficient of 0.90. The difference in the correlation coefficients for {J andy indicates that at a given leaf temperature the relative deviations from the means were much higher in {J than in y. For instance, at temperatures
• 4
~
'0> ~
0>
3
~ ~
';2
""
...·' . ,,._. /~ ·=-..
•
r exp =0.90
2
20
25
30
35
Temperature(°C)
Fig. 4. Influence of leaf temperature on the 0 2 dependence of the CO~ compensation concentration (y). The curve represents the exponential regression line for values of y from 20 leaves according to y = y0 exp ( -
i) (p < 0.001, kfl
=
+3358 K).
Effeet of Temperature on 0 2 Dependence of C0 2 Exchange
395
between 32 and 34 oc fJ = (558 ± 183) s m2 kg-1 andy = (3.67 ± 0.26) · 10-4 kg kg- 1 were obtained. The relative values of the standard deviations were 33 and 7%, respectively. Discussion
In accordance with data on wheat flag leaves (PEISKER and APEL 1976) a large variation in fJ and a considerably smaller variation in y were obtained (Figs. 3 and 4). These results are comparable with differing degrees of 0 2 inhibition of net C0 2 uptake and concomitant constant values of r observed in various sunflower selections (LLOYD and CANVIN 1977). The decline in fJ and the increase in y with increasing leaf temperatures found for the experimental material as a whole (Figs. 3 and 4), agree qualitatively with former results on wheat flag leaves (PEISKER and APEL 1977). However, in individual leaves deviations in the temperature dependence of fJ from the mean response were observed. Deviations included a more marked decrease with temperature (Fig. 2A), a lack of an influence of temperature (Fig. 2 B), and in one case even a reversal in comparison to the mean response (Fig. 2 C). Previous workers have attempted to explain the effects of 0 2 on carboxylation efficiency or on intracellular resistance to C0 2 transfer and also the temperature dependence of these effects by the kinetic properties of RubP carboxylase-oxygenase (LAING et al. 1974; PEISKER and APEL 1977). However, the variations in{J and its temperature dependence described here suggest that other processes which interfere with carboxylation and oxygenation of RubP must also be involved. On the other hand, the smaller variation in y and the better correlation of y with leaf temperature indicate that carboxylation and oxygenation of RubP must be similarly influenced by such processes. This suggests to us that the relationship between C0 2 fixation and photorespiration is not essentially modified by other factors than properties of RubP carboxylase-oxygenase. Temperature sensitivities of fJ andy as expressed by the constants kfJ and ky, respectively, were calculated without considering the temperature dependence of the solubility ratio of 0 2 and C0 2 (Ku and EDWARDS 1977). The increase of this ratio with rising temperature, when described with the Arrhenius equation, results in a temperature constant of 920 K. This value must be subtracted from the calculated values of kfJ and kY to correct for changing solubility ratio. These corrections do not give rise to qualitative changes but result only in quantitative modifications of the calculated temperature responses. Variations in intracellular resistance to C0 2 transfer without changes of C0 2 compensation concentration have been reported by LAISK (1977) to occur in one leaf after different treatments. After a period of net C02 uptake ri was smaller than after a period of C0 2 evolution into C0 2-frec gas. LAISK has interpreted such variations as the result of different capacities of the Calvin cycle for regenerating the C0 2 acceptor due to different sizes or the pools of participating intermediates. It seems possible to explain the variations in fJ observed here as well by differing sizes of the pools of cycle intermediates. However, nothing is known about the causes of such variations in pool sizes
396
M. PEISKER, I. TICHA and P. APEL
and the question remains to be answered as to the causes of measured different temperature responses in {J. In addition to the competitive action of 0 2 on RubP carboxylation some other influences of 0 2 on reactions and processes participating in C0 2 fixation have been suggested. KRAUSE et al. (1978) have pointed out that under limiting light 0 2 may also inhibit C0 2 fixation by photorespiratory ATP consumption. This was concluded from studies of the 0 2 dependence of light scattering and of chlorophyll fluorescence in intact leaves of spinach. An influence of 0 2 on chlorophyll fluorescence has also been observed by BJORKMAN (1966) in leaves of Plantago lanceolata. It is possible that such effects could also modify the 0 2 inhibition of C02 fixation under conditions of light saturation. An inhibition of the activity of ribulose 5-phosphate kinase by 0 2 in intact chloroplasts of spinach has been described by LATZKO et al. (1970). At light and C0 2 saturation VrrL et al. (1977) have found a stimulation of net C0 2 uptake by 0 2 in leave~ of Populus trernula which has been ascribed to an influence on some component of the electron transport chain. However, it is not known whether these or other effects of 0 2 are relevant to the variations in {J and in its temperature dependence as observed in our experiments. We are indebted to Dr. J.D. TENHUNEN ;md :\Irs. L. C. TENHUNEN for valuable help with the preparation of the manuscript.
References BJt1RK~AN,
0.: The effect of oxygen concentration on photosynthesis in higher plants. PhysioL Plant. 19, 618-633 (1966). KRAUSE, G. H., LORIMER, G. H., HEBER, U., and KIRK, M. R.: Photorespiratory energy dissipation in leaves and chloroplasts. In: Photosynthesis 77 (Edit. HALL, D. 0., CooMBS, J., and GooDWIN, T. W.). Pp. 299-310. The Biochemical Society, London 1978. Ku, S.-B., and EDWARDS, G. E.: Oxygen inhibition of photosynthesis. I. Temperature dependence and rehttion to 0 2 {C0 2 solubility ratio. Plant Physiol. o9, 986-990 (1977). LAING, W. A., OGREN, W. L., and HAGEMAN, R. H.: Regulation of soybean net photosynthetic C0 2 fixation by the interaction of C0 2 , 0 2 , and ribulose 1,5-diphosphate carboxylase. Plant Physiol. ii4, 678-685 (1974). LAISK, A. H.: Kinetics of photosynthesis and photorespiration in C3 plants (Kinetika fotosinteza i. fotodychaniya C3 -rastenij). Nauka, }foscow 1977. LATZKO, E., GARNIER, R. v., and GmBs, M.: Effect of photosynthesis, photosynthetic inhibitors and oxygen on the activity of ribulose 5-phosphate kinase. Biochem. Biophys. Res. Commun. 39, 1140-1144 (1970). LLOYD, N.D. H., and C.\NVIN, D. T.: Photosynthesis and photorespiration in sunflower selections. Can. J. Bot. rio, 3006-3012 (1977). LORIMER, G. H., Woo, K. c., BERRY, J. A., and OsMOND, c. B.: The c2 photorespiratory carbon oxidation cycle in leaves of higher plants: Pathway and consequences. In: Photosynthesis 77 (Edit. HALL, D. 0., CooMBS, J., and GooDWIN, T. W.). Pp. 311-322. The Biochemical Society, London 1978. PEISKER, M., and APEL, P.: Influence of oxygen on photosynthesis and photorespiration in leaves of Triticum aestivum L. 1. Relationship between oxygen concentration, C0 2 compensation point. and intracellular resistance to C0 2 uptake. Photosynthetica 9, 16-23 (1975).
Effect of Temperature on 0 2 Dependence of 00 2 Exchange
397
Influence of oxygen on photosynthesis and photorespiration in leaves of Triticum aestivum L. 2. Response of 00 2 gas exchange to oxygen at various leaf ages and its variability. Photosynthetica 10, 140-146 (1976). - -- Influence of oxygen on photosynthesis and photorespiration in leaves of Triticum aestivum L. 3. Response of 00 2 gas exchange to oxygen at various temperatures. Photosy.athetica 11, 29-37 (1977). Vm., J., LAISK, A., 0JA, V., and PXRNIK, T.: Enhancement of photosynthesis caused by oxygen under saturating irradiance and high 00 2 concentrations. Photosynthetica 11, 251-259 (1977). WEST, D. W., and GAFF, D. F.: A controlled-environment leaf chamber to allow measurement of gas exchange by leaves undergoing rapid fluctuations in temperature. J. Exp. Bot. 27, 205-213 (1976). Received January 29, 1979.
Authors' addresses: Dr. MARTIJ\ PEISKER, Zentralinstitut Iiir Genetik und Kulturpflanzenforschung, DDR- 4325 Gatersleben; Dr. INGRID TICHA, Institute of Experimental Botany, CSA V, Flerningovo narn. 2, cs- 16000 Praha 6, USSR; Dr. PETER APEL, Zentralinstitut fiir Genetik und Kulturpflanzenforschung, DDR- 4325 Gatersleben.
Variations in the Effect of Temperature on Oxygen Dependence of C0 2 Gas Exchange in Wheat Leaves MARTIN PEISKER1 ), INGRID TICHA2) and PETER APEL1 ) 1)
Zentralinstitut fiir Genetik und Kulturpflanzenforschung der AdW der DDR, Gatersleben, DDR; 2 ) Institute of Experimental Botany, CSA V, Praha, CSSR
Key Term Index: 00 2 gas exchange, 00 2 compensation concentration, oxygen dependence, leaf temperature, intracellular resistance; Triticum aestivum.
Summary The influence of leaf temperature on the slope of the linear oxygen dependence of the intracellular Tesistance to 00 2 transfer ({J) and on the slope of the linear oxygen depedence of the 00 2 compensation concentration (y) was determined in wheat leaves. The variation in the temperature dependence of fJ found in individual leaves was interpreted as an interaction of the effect of 0 2 on carboxylation of ribulose 1,5-biphosphate and other temperature dependent processes. The strong correlation between y and leaf temperature suggests the absence of an influence of such processes on the relationship between photosynthesis and photorespiration.
Introduction
It is widely recognized that in C3 plants the oxygenation of ribulose 1,5-biphosphate (RubP) is the main reaction providing glycolate - the substrate for photorespiration (LoRIMER et al. 1978). In addition this reaction has been suggested as an explanation of the oxygen inhibition of RubP carboxylation. This concept is supported by similarity in the effect of 0 2 on C0 2 gas exchange at different temperatures and temperature dependencies of kinetic constants determined in vitro for RubP carboxylase-oxygenase (LAING et al. 1974; PEISKER and APEL 1977). However, there are some observations which do not fit this direct correspondence between gas exchange and enzymological data. For instance, in different selections of sunflower different degrees of stimulation of net C0 2 uptake by low 0 2 concentrations were observed at 350 ll-1 C0 2 whereas the C02 compensation concentrations (F) were relatively constant (LLOYD and CAN'VIN 1977). This means that the slopes of the C0 2 response curves at 2% and 21% 0 2 from leaf to leaf varied greatly in relation to one another. Similarly, in flag leaves of wheat, variation in the 0 2 dependence of the intracellular resistance to C0 2 transfer was observed. rand its 0 2 dependence were also in this case practically unchanged (PEISKER and APEL 1976). In the present paper a variation in the influence of temperature on the 0 2 dependence of intracellular resistance in individual leaves is described. Material and Methods Single plants of spring wheat cv. "Carola" were grown in plastic pots with sandy soil in a growth chambeT (SAXCIL, U.K.). The conditions during growth were 16 h of light at an air temperature of 21 oo and a photon flux density (400-700 nm) of 310 f.l Einstein m- 2 s-1 • A temperature of 16 oc was, 26
Biochem. Physiol. Pflanzen, Bd. 174
1\I.
392
PEISKER,
I. TrcnA and
P. APEL
maintained during the 8 h of darkness. The plants were allowed to develop the main culm only. All developing tillers were regularly cut off. Measurements were made on 20 fully developed 5th leaves before leaf area expansion of the 6th leaf was complete. Net C0 2 uptake, transpiration, and leaf temperature of attached leaves were measured at C0 2 concentrations between 0 and 4 · 10-4 kg· m- 3 as described by PEISKER and APEL (1975). In contrast to former experiments, a leaf chamber was used which was equiped with a device for regulating leaf temperature by means of an electric heater in a manner similar to that described by WEST and GAFF (1976). The measurements were conducted at three of four different temperatures ranging between 21 and 34 oc and at a saturating photon flux density (400-700 nm) of 950 p, Einstein m - 2 s-1 • From the linear part of the relationship between net C0 2 uptake and intercellular C0 2 concentration, the intracellular resistance to C0 2 transfer (r;) and the C0 2 compensation concentration (F) were determined. Three 0 2 concentrations were chosen between 0.27 and 0.65 kg m-3 to determine the 0 2 dependencies, fJ andy, of r; and r, respectively. These parameters are defined by the equations r;
=
r
= 'YJ
x
+ fJ [0 2] and + r co2)
and were calculated by linear regression for each leaf temperature
(PEISKER
and
APEL
1977).
Results
The influence of 0 2 concentration and temperature on the C0 2 dependence of net photosynthesis is shown for three leaves selected from twenty experiments. The pattern seen in these responses varies greatly from leaf to leaf (Fig. 1). The slopes of the C0 2 dependencies at atmospheric 0 2 concentration of 0.27 kg m- 3 were similar at 23 and 33 °C. However, at the higher 0 2 concentration of 0.62 kg m- 3 different inhibitory effects were observed at the two temperatures. The slope of the C0 2 dependency at the lower temperature was at times less than (Fig. 1A) and greater than (Fig. 1C) that measured at the higher temperature. In some cases the slopes were nearly equal for both temperatures (Fig. 1 B). In other words, the effect of leaf temperature on the 0 2 dependence of intracellular resistance varied from leaf to leaf (Fig. 2).
51 A
c
B
1(1) 4 N
'E
...""'"'
3
~ 2
0
2
30
1
2
30
1
Jntercellular C0 2 concentration f 10-4 kg rn- 3 J
2
3
Fig. 1. Relationship between net C0 2 uptake and intercellular C0 2 concentration. The measurements were conducted at temperatures of about 23 oc (full lines) and 33 oc (broken lines) and at 0~ concentrations of about 0.27 kg m- 3 (full circles) and 0.62 kg m- 3 (open circles). The results from three individual leaves 3re shown (A, B, C).
393
Effeet of Temperature on 0 2 Dependence of C0 2 Exchange
In 12 experiments a pronounced decrease of f3 with increasing leaf temperature as in Fig. 2A was found. f3 did not change markedly with temperature (Fig. 2B) in 7 experiments. In one experiment a distinct increase of f3 at the higher leaf temperature was observed (Fig. 2 C). If the results of all experiments were considered jointly, an overall
c
800A
. ------0,3
~-
0,4
--
~~
0,5
-~
0,3 0;4 o,s 0,6 ~ concentration (kg rri3)
0,6
0,3
0,4
0,5
0.6
Fig. 2. Relat·ionship between intracellular resistance to CU 2 transfer and 0 2 concentration. The 0 2 dependence of ri at temperatures of about 23 oc (full lines) and 33 oc (broken lines) in three individual leaves is shown (A, B, C).
15
•
•
~OJ _;£
"',: (j,
10-
•• •
N
!2
• •
• •
CQ.
5-
• r exp=0.55
•
~---------.-----------,----------,
20
25
30 35 Temperature ( °C l
Fig. 3. Influence of leafiemperawre on the 0 2 dependence of the intracellular resistance to 002 transfer ( {J). The curve represents the exponential regression line for values of {J from 20 leaves according to {J = {J0 exp (26*
~) (p
< 0.001, kp
=
-3348 K).
394
M.
PEISKER,
I. TrcHA and
P. APEL
decrease of {J with increasing leaf temperature was seen (Fig. 3). However, the considerable variation in {J, which was due in part to variation in its temperature dependence, resulted in a weak correlation. Using the exponential expression {J
=
flo exp -
(~)
a correlation coefficient of 0.55 and a temperature constant of kfl = -3348 K at p < 0.001 were obtained. In individual leaves, the values of kfl varied between -9598 and +4843 K. The 0 2 dependence of the C0 2 compensation concentration increased with increasing leaf temperature (Fig. 4). The exponential equation
-
( T-ky)
y -Yo exp -
was fit to the data using the values of y from all the experiments. This resulted in a value for ky of +3358 K and a correlation coefficient of 0.90. The difference in the correlation coefficients for {J andy indicates that at a given leaf temperature the relative deviations from the means were much higher in {J than in y. For instance, at temperatures
• 4
~
'0> ~
0>
3
~ ~
';2
""
...·' . ,,._. /~ ·=-..
•
r exp =0.90
2
20
25
30
35
Temperature(°C)
Fig. 4. Influence of leaf temperature on the 0 2 dependence of the CO~ compensation concentration (y). The curve represents the exponential regression line for values of y from 20 leaves according to y = y0 exp ( -
i) (p < 0.001, kfl
=
+3358 K).
Effeet of Temperature on 0 2 Dependence of C0 2 Exchange
395
between 32 and 34 oc fJ = (558 ± 183) s m2 kg-1 andy = (3.67 ± 0.26) · 10-4 kg kg- 1 were obtained. The relative values of the standard deviations were 33 and 7%, respectively. Discussion
In accordance with data on wheat flag leaves (PEISKER and APEL 1976) a large variation in fJ and a considerably smaller variation in y were obtained (Figs. 3 and 4). These results are comparable with differing degrees of 0 2 inhibition of net C0 2 uptake and concomitant constant values of r observed in various sunflower selections (LLOYD and CANVIN 1977). The decline in fJ and the increase in y with increasing leaf temperatures found for the experimental material as a whole (Figs. 3 and 4), agree qualitatively with former results on wheat flag leaves (PEISKER and APEL 1977). However, in individual leaves deviations in the temperature dependence of fJ from the mean response were observed. Deviations included a more marked decrease with temperature (Fig. 2A), a lack of an influence of temperature (Fig. 2 B), and in one case even a reversal in comparison to the mean response (Fig. 2 C). Previous workers have attempted to explain the effects of 0 2 on carboxylation efficiency or on intracellular resistance to C0 2 transfer and also the temperature dependence of these effects by the kinetic properties of RubP carboxylase-oxygenase (LAING et al. 1974; PEISKER and APEL 1977). However, the variations in{J and its temperature dependence described here suggest that other processes which interfere with carboxylation and oxygenation of RubP must also be involved. On the other hand, the smaller variation in y and the better correlation of y with leaf temperature indicate that carboxylation and oxygenation of RubP must be similarly influenced by such processes. This suggests to us that the relationship between C0 2 fixation and photorespiration is not essentially modified by other factors than properties of RubP carboxylase-oxygenase. Temperature sensitivities of fJ andy as expressed by the constants kfJ and ky, respectively, were calculated without considering the temperature dependence of the solubility ratio of 0 2 and C0 2 (Ku and EDWARDS 1977). The increase of this ratio with rising temperature, when described with the Arrhenius equation, results in a temperature constant of 920 K. This value must be subtracted from the calculated values of kfJ and kY to correct for changing solubility ratio. These corrections do not give rise to qualitative changes but result only in quantitative modifications of the calculated temperature responses. Variations in intracellular resistance to C0 2 transfer without changes of C0 2 compensation concentration have been reported by LAISK (1977) to occur in one leaf after different treatments. After a period of net C02 uptake ri was smaller than after a period of C0 2 evolution into C0 2-frec gas. LAISK has interpreted such variations as the result of different capacities of the Calvin cycle for regenerating the C0 2 acceptor due to different sizes or the pools of participating intermediates. It seems possible to explain the variations in fJ observed here as well by differing sizes of the pools of cycle intermediates. However, nothing is known about the causes of such variations in pool sizes
396
M. PEISKER, I. TICHA and P. APEL
and the question remains to be answered as to the causes of measured different temperature responses in {J. In addition to the competitive action of 0 2 on RubP carboxylation some other influences of 0 2 on reactions and processes participating in C0 2 fixation have been suggested. KRAUSE et al. (1978) have pointed out that under limiting light 0 2 may also inhibit C0 2 fixation by photorespiratory ATP consumption. This was concluded from studies of the 0 2 dependence of light scattering and of chlorophyll fluorescence in intact leaves of spinach. An influence of 0 2 on chlorophyll fluorescence has also been observed by BJORKMAN (1966) in leaves of Plantago lanceolata. It is possible that such effects could also modify the 0 2 inhibition of C02 fixation under conditions of light saturation. An inhibition of the activity of ribulose 5-phosphate kinase by 0 2 in intact chloroplasts of spinach has been described by LATZKO et al. (1970). At light and C0 2 saturation VrrL et al. (1977) have found a stimulation of net C0 2 uptake by 0 2 in leave~ of Populus trernula which has been ascribed to an influence on some component of the electron transport chain. However, it is not known whether these or other effects of 0 2 are relevant to the variations in {J and in its temperature dependence as observed in our experiments. We are indebted to Dr. J.D. TENHUNEN ;md :\Irs. L. C. TENHUNEN for valuable help with the preparation of the manuscript.
References BJt1RK~AN,
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Authors' addresses: Dr. MARTIJ\ PEISKER, Zentralinstitut Iiir Genetik und Kulturpflanzenforschung, DDR- 4325 Gatersleben; Dr. INGRID TICHA, Institute of Experimental Botany, CSA V, Flerningovo narn. 2, cs- 16000 Praha 6, USSR; Dr. PETER APEL, Zentralinstitut fiir Genetik und Kulturpflanzenforschung, DDR- 4325 Gatersleben.