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Theoretical and Experimental Observations on O2 Sensitivity of C3 Photosynthesis
Abstract
Photosynthesis in C3 plants is usually inhibited by air levels of O2. Because C3 plants have no accessory metabolic pathways for concentrating CO2, the characteristics of photosynthesis such as oxygen sensitivity, measured with intact leaves can be readily interpreted at the biochemical level. This paper will discuss the biochemical basis for O2 sensitivity of C3 photosynthesis, the predictions of O2 sensitivity which follow from known mechanisms, and the metabolic information which O2 sensitivity measurements can give.
... ylation, and (c) consuming RuBP which would otherwise be available for carboxylation (18). Across the physiological range of temperatures, 02 removal should stimulate photosynthesis from 5 to 60% (14,18). ...
... ylation, and (c) consuming RuBP which would otherwise be available for carboxylation (18). Across the physiological range of temperatures, 02 removal should stimulate photosynthesis from 5 to 60% (14,18). Numerous studies, however, have described cases where photosynthesis is either unaffected or is inhibited by a 90% reduction of 02 (5,9,13,16,17,25). ...
... In order to determine the importance of feedback limitations in the field, we have used a field gas exchange system to measure the occurrence of 02 and CO2 insensitive photosynthesis over a range of temperatures in a variety of field grown plants. THEORY The sensitivity of photosynthesis to 02 may be predicted for Rubisco or RuBP regeneration limited photosynthesis using the equations ofFarquhar and von Caemmerer (6) and Sharkey (18). Oxygen sensitivity will depend upon whether photosynthesis is Rubisco or RuBP regeneration limited (18). ...
The sensitivity of photosynthesis to O(2) and CO(2) was measured in leaves from field grown plants of six species (Phaseolus vulgaris, Capsicum annuum, Lycopersicon esculentum, Scrophularia desertorum, Cardaria draba, and Populus fremontii) from 5 degrees C to 35 degrees C using gas-exchange techniques. In all species but Phaseolus, photosynthesis was insensitive to O(2) in normal air below a species dependent temperature. CO(2) insensitivity occurred under the same conditions that resulted in O(2) insensitivity. A complete loss of O(2) sensitivity occurred up to 22 degrees C in Lycopersicon but only up to 6 degrees C in Scrophularia. In Lycopersicon and Populus, O(2) and CO(2) insensitivity occurred under conditions regularly encountered during the cooler portions of the day. Because O(2) insensitivity is an indicator of feedback limited photosynthesis, these results indicate that feedback limitations can play a role in determining the diurnal carbon gain in the field. At higher partial pressures of CO(2) the temperature at which O(2) insensitivity occurred was higher, indicating that feedback limitations in the field will become more important as the CO(2) concentration in the atmosphere increases.
... The occnrrence of photorcspiration depends on the partial prcssures of 0, and GO, in a straight foruard way (Laing et al.: 1974;Sharkey, 1988). T h e sensitivity of photosvntllesis tu 0, is less srraight form-ard because of the many ways that photorespiration affects the rate of photosynthesis (Farcluhar et nl.. 1980;Sharkey, 1986). However, the interpretation that photorcspiration is involved in feedback effects (lliardlau, and Eckhardt, 1987) is wrong, O:! sensitivity is just an assay for-the occurrence of feedback effects (for example sec AzcBn-Bieto, 1983). ...
... The patchy stomatal closure causes the effective p(C0,) inside the leaf to be lower than that estimated from gas exchange (Downton et al., 1988;Terashima et al., 1988). Since 0, sensitivity depends on the p(C0,) inside the leaf (Sharkey, 1986), hypersensitivity to 0, is an excellent clue that patchy stomatal closure is occurring. I made similar observations with water-stressed leaves. ...
... While most studies of rubisco activation and RuBP levels are interpreted in terms of either light or CO2 limitations, Walker and Herold (30) pointed out that TPU could also limit the rate of photosynthesis. Recent studies (22)(23)(24)(25)(26) have related the phenomenon of 02 insensitive photosynthesis in C3 plants to a TPU limitation of photosynthesis. This can be observed consistently under conditions of high light intensity and high partial pressure of CO2 (500 ,ubar internal). ...
... Under these conditions the activation state of carboxylase was also 20% less in leaves freeze-killed in 30 mbar 02 than in leaves killed in 180 mbar 02 When Ci was controlled at 190 Abar in 30 mbar 02, photosynthesis was slightly lower than when C, was 500 jubar (Table I) and the activation state of rubisco was nearly the same as in 180 The effect of light on 02 sensitivity of activation is shown in Table II. At 200 smol quanta m-2 s-', CO2 assimilation exhibited the 02 sensitivity expected under these conditions (22). The activation of rubisco was the same at both partial pressures of 02. ...
The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (rubisco) activity in Phaseolus vulgaris was studied under moderate CO(2) and high light, conditions in which photosynthesis in C(3) plants can be insensitive to changes in O(2) partial pressure. Steady state RuBP concentrations were higher, the calculated rate of RuBP use was lower and the activation state of rubisco was lower in low O(2) relative to values observed in normal O(2). It is suggested that the reduced activity of rubisco observed here is related to feedback effects which occur when the rate of net CO(2) assimilation approaches the maximum capacity for starch and sucrose synthesis (triose phosphate utilization). The activation state of rubisco was independent of O(2) partial pressure when light or CO(2) was limiting for photosynthesis. Reduced activity of rubisco was also observed at limiting light. However, in this species light dependent changes in the concentration of an inhibitor of rubisco controlled the apparent V(max) of rubisco in low light while changes in the CO(2)-Mg(2+) dependent activation of rubisco controlled the apparent V(max) in high light.
... It does this as a competitive inhibitor of Rubisco, as well as diverting energy from carboxylation to oxygenation (T. Sharkey, 1986). ...
With climate change, night temperatures are expected to increase faster than day temperatures. In several studies, high night temperatures have been reported to decrease the yield potential of rice. With rice being the primary staple for more than half of the world’s population, projected yield decreases imply a major threat to food security. Nevertheless, physiological responses of rice plants to varying day and night temperatures are not fully understood and both positive and negative effects of high night temperatures have been described with regard to CO2 assimilation and growth. Whereas respiratory losses have been shown to increase as a result of higher night temperatures, leaf conductance and net assimilation rates during the day were reported to be higher. It was hypothesized in this study that higher daytime net assimilation rates were potentially the result of a compensation mechanism in response to depleted carbohydrate pools within the leaf, associated with adjustments in regards to mesophyll conductance or reduction in photorespiration, or even if these adjustments are themselves direct response to higher night temperatures.
In the present study, four-week-old IR64 rice plants were exposed to different day and night temperatures for 12 days in a growth chamber experiment. The temperature treatments did not lead to any significant changes in morphology, except for the ratios indicating carbon allocation, specific leaf area and root to shoot ratio. Shifts in carbon allocation was also demonstrated by the increase in sucrose utilization or export as night temperature increased. However, there was no link found between the assimilation rate and the status of carbohydrates in the leaves. The assimilation rate and its component processes significantly responded to increases in day temperature, whereas mesophyll conductance showed no significant response either to day or night temperature. Photorespiration also responded solely to increases in day temperature. Further research is needed in rice in regards to the diurnal dynamics of sucrose transport and utilization, as well the interaction of the limited starch reserves to the assimilation rate.
... Since about 15% more electrons are needed for the oxygenation reaction than for car boxylation (the theoretical ratio is 5.25:4.5; the ex perimental 5.19:4.62 (Sharkey, 1986)), ; GE, the electron-transport rate used for Rubisco reactions, should hardly be affected. The m easured;GE, how ever, decreases in parallel with A and p c{. ...
A method was developed for carrying out gas-exchange and chlorophyll-fluorescence measurements simultaneously during fumigation of spruce twigs with peroxidic photooxidants. It is thus now possible to investigate how a pollutant affects distinct sectors of the photosynthetic apparatus of the plant: whereas fluorescence reveals any changes in the primary light reaction, CO2 gas-exchange measurements supply information about the biochemical reactions of the Calvin cycle. Results of short-time fumigation with 750 ppb ozone are presented here. Gas-exchange and fluorescence data are affected strongly in early summer, but not in autumn. The assimilation rate decreases significantly: primarily as a result of Rubisco activity and possibly because of direct inhibition of the electron-transport chain as well. Closure of the stomata leads to further reduction in the assimilation rate. Though no damage becomes visible on the needles, the perturbance of the photosynthetic apparatus caused by ozone fumigation is not reversible within 24 h. © 1994 Verlag der Zeitschrift für Naturforschung. All rights reserved.
... feedback-inhibited conditions, the reactions of triosephosphate transport and consumption cannot keep up with the rate of photosynthetic triose phosphate production, and it is well established that such conditions ultimately lead to a reduced rate of ATP and NADPH synthesis due to low stromal P i (Sharkey, 1985a;Sharkey and Vanderveer, 1989;Sharkey and Vassey, 1989;Lee et al., 2014). Under such conditions, the sensitivity of photosynthesis to changes in CO 2 and O 2 concentrations decreases, and the photosynthesis might eventually even respond negatively to increases in CO 2 concentration and decreases in O 2 concentration (Sharkey, 1985a(Sharkey, , 1985b(Sharkey, , 1986. Indeed, positive O 2 sensitivity of photosynthesis ( Fig. 4B) or negative CO 2 sensitivity of photosynthesis ( Fig. 5) was observed in our experiments, confirming the feedback-limited conditions. ...
Plant isoprene emissions respond to light and temperature similarly to photosynthesis, but CO2-dependencies of isoprene emission and photosynthesis are profoundly different with photosynthesis increasing and isoprene emissions decreasing with increasing CO2 concentration due to reasons not yet understood. We studied isoprene emission, net assimilation rate and chlorophyll fluorescence under different CO2 and O2 concentrations in the strong isoprene emitter hybrid aspen (Populus tremula x P. tremuloides), and used rapid changes in ambient CO2 or O2 concentrations or light level to induce oscillations. As isoprene-emitting species support a very high steady-state chloroplastic pool sizes of the primary isoprene substrate, dimethylallyl diphosphate (DMADP), that can mask the effects of oscillatory dynamics on isoprene emission, the size of DMADP pool was experimentally reduced by either partial inhibition of isoprenoid synthesis pathway by fosmidomycin-feeding or by changes in ambient gas concentrations leading to DMADP pool depletion in intact leaves. In feedback-limited conditions, oscillations in photosynthesis and isoprene emission were repeatedly induced by rapid environmental modifications in both partly fosmidomycin-inhibited leaves and in intact leaves. The oscillations in net assimilation rate and isoprene emission in feedback-inhibited leaves were in the same phase, and relative changes in the pools of photosynthetic metabolites and DMADP estimated by in vivo kinetic methods were directly proportional through all oscillations induced by different environmental perturbations. We conclude that the oscillations in isoprene emission provide direct experimental evidence demonstrating that the response of isoprene emission to changes in ambient gas concentrations is controlled by the chloroplastic reductant supply.
... The stimulation of steady-state photo synthesis and alteration of the condition under which oscillations in the rate of photosynthesis occur, which can be elicited by feeding Pi, demonstrates that, at high CO2, the cytosolic reactions that liberate Pi (this will include all of the reactions of the cytosol to a greater or lesser degree because of the conserva tion of cytosolic Pi and presumably any processes involved in the movement of Pi from the vacuole to the cytosol) partially limit the rate of photosynthesis. Similar conclusions have been drawn by workers examining the sensitivity of the rate of photosynthesis to O2 either at high CO2 levels and slightly depressed temperatures (221,222) or at ambient CO2 and a low temperature (153). ...
This chapter discusses the carbon dioxide assimilation in plants. All oxygenic (oxygen-evolving) organisms from the simplest prokaryotic cyanobacteria to the most complicated land plants have a common pathway for the reduction of CO2 to sugar phosphates. This pathway is known as the reductive pentose phosphate (RPP), Calvin-Benson or C3 cycle. Although the RPP cycle is the fundamental carboxylating mechanism, a number of plants have evolved adaptations in which CO2 is first fixed by a supplementary pathway and then released in the cells in which the RPP cycle operates. One of these supplementary pathways, the C4 pathway, involves special leaf anatomy and a division of biochemical labor between cell types. Plants endowed with this pathway, through greater efficiency, are able to flourish under conditions of high light intensity and elevated temperatures. A second supplementary pathway was first found in species of the Crassulaceae and is called Crassulacean acid metabolism (CAM). These plants are often found in dry areas and fix CO2 at night into C4 acids. During the day, the leaves can close their stomata to conserve water, while CO2 released from the C4 acids is converted to sugar phosphates by the RPP cycle using absorbed light energy. CO2 fixation is also found in many bacteria, both photosynthetic and non-photosynthetic. The purple sulfur and purple nonsulfur bacteria employ the RPP cycle as do plants. The photosynthetic green bacteria, however, use a group of ferredoxin-linked carboxylases in a pathway known as the reductive carboxylic acid cycle.
Atmospheric carbon dioxide concentration has increased globally from about 280 ppm before the Industrial Revolution (Pearman 1988) to about 353 ppm in 1990. That increase, and the continuing increase at a rate of about 1.5 ppm per annum, owing mainly to fossil fuel burning, is likely to cause change in climate, in primary productivity of terrestrial vegetation (managed and unmanaged), and in the degree of net sequestration of atmospheric CO2 into organic form. The quantitative role of the latter in attenuating the increase in atmospheric CO2 concentration itself is an important but uncertain element of the global carbon-cycle models that are required to predict future increases of atmospheric CO2 concentration.
Photorespiration is the light-dependent evolution of CO2, which accompanies photosynthesis in C3plants. The four best known methods of measuring the rate of photorespiration have theoretical or technical problems, which make the results unreliable. However, the rate of photorespiration can be calculated from the rate of net CO2assimilation and the partial pressures of CO2and O2. Estimates of rates of photorespiration in the past and future can be made. The rate of photorespiration as a proportion of the rate of photosynthesis will fall to one half the current rate when the CO2level in the atmosphere doubles.
The Paleozoic stands out in geochemical reconstructions as a time of unusually high organic carbon burial and disturbed O2 and CO2 regimes. The anomaly is apparently of terrestrial origin. Evidence and explanations for reconstructed trends are sought in the terrestrial record. The correspondence between reconstructed O2 and fusinite abundance in coal, a possible proxy for charcoal formation and fire activity, is ambiguous due to problems of data quality and interpretation. Patterns in Silurian, Devonian, and Carboniferous plant evolution are consistent with stress imposed by progressive CO2 starvation followed by leveling out, as predicted in reconstructions. I hypothesize that the Paleozoic disturbance was caused by bottlenecks in the cycling of lignin and other refractory compounds synthesized by land plants, and that these bottlenecks were eventually reduced by the evolution of more effective decomposer organisms and/or reduction of the refractory fraction in land plant biomass.
The success of extracorporeal membrane oxygenation (ECMO) for the treatment of acute respiratory failure has led to consideration of the development of a more portable, and perhaps even implantable, artificial lung. The authors suggest a bioregenerative life support system that includes a photo-synthetic organism that can remove CO2 and produce O2 in the presence of an energy source. To build a model of such a photosynthetic artificial lung, the photosynthetic capability of a high temperature strain of the algae Chlorella pyrenoidosa was maximized at a cell density of 25 million cells/ml to serve as the O2 producer and CO2 remover. The "patient" in this model was comprised of 1 L of medium or 350 ml of blood, interfaced with the photosynthetic system across a gas transfer membrane. The experiments demonstrated the ability of the plant cells to supply O2 and remove CO2 from the "patient" with a maximum rate of 0.55 mmoles/L/hr under the most favorable measured operating conditions. The projected rate of 1.0 mmoles/L/hr required for physiologic applications is not totally ab absurd idea, with a slightly modified set-up. Modifications may be in the form of regulating the photosynthetic pathway or genetically engineering a hybrid strain with enhanced O2 producing and suppressed photoinhibition capacity.
The effects of two levels of salinity on photosynthetic properties of olive (Olea europea L.) leaves were observed either in low or in high H(2)O vapor pressure deficit (vpd). Under moderate salt stress, stomata were found to be less open and responsive both to light and vpd, but the predominant limitation of photosynthesis was due to the mesophyll capacity of CO(2) fixation. We elaborate a procedure to correlate mesophyll capacity and liquid phase diffusive conductance. The estimated liquid phase diffusive conductance was reduced by salt and especially by high vpd; morphological and physiological changes could be responsible for this reduction. As a result, the chloroplast CO(2) partial pressure was found to decrease both under salt and vpd stress, thus resulting in a ribulose-1,5-bisphosphate carboxylase limitation of assimilation. However, under combined salt and vpd stress, O(2) sensitivity of assimilation increased, as would be expected under conditions of limiting ribulose 1,5-bisphosphate regeneration. Fluorescence induction measurements indicated that, under these conditions, energy supply may become limiting. When Cl(-) concentration exceeded 80 millimolar in tissue water, zero growth and 50% leaf drop was observed. Fluorescence induction showed irreversible damage at Cl(-) levels higher than 200 millimolar and basal leaves reached this concentration earlier than the apical ones.
Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.
The partial pressure of CO(2) inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO(2) and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO(2) in the closed system rapidly reached a steady pressure which it is believed was identical to the CO(2) pressure inside the leaf, because there was no flux of CO(2) across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO(2) and vapor at the other surface.
Although open systems have been used for the study of transients in leaf CO(2) exchange such as the postillumination burst, these systems frequently do not permit reliable estimates of transient rates due to their nonsteady state nature. A nonsteady state mathematical approach is described which predicts changes in CO(2) concentration in the leaf chamber and infrared gas analyzer measuring cell as a function of leaf CO(2) exchange rate in Nicotiana tabacum vars John Williams Broadleaf and Havana Seed. With the aid of a computer, a numerical formula simulates the mixing and dilution which occurs as CO(2) passes through the finite volume of the measuring cell of the analyzer. The method is presented with special relevance to photorespiration as manifested by the postillumination burst of CO(2). The latter is suggested to decline with the first order kinetics following darkening of a C(3) leaf. This approach provides a basis for reliable estimation of the initial and, hence, maximal rate of CO(2) evolution during the postillumination burst under a variety of environmental conditions.
Leaves of C(3) plants which exhibit a normal O(2) inhibition of CO(2) fixation at less than saturating light intensity were found to exhibit O(2)-insensitive photosynthesis at high light. This behavior was observed in Phaseolus vulgaris L., Xanthium strumarium L., and Scrophularia desertorum (Shaw.) Munz. O(2)-insensitive photosynthesis has been reported in nine other C(3) species and usually occurred when the intercellular CO(2) pressure was about double the normal pressure. A lack of O(2) inhibition of photosynthesis was always accompanied by a failure of increased CO(2) pressure to stimulate photosynthesis to the expected degree. O(2)-insensitive photosynthesis also occurred after plants had been water stressed. Under such conditions, however, photosynthesis became O(2) and CO(2) insensitive at physiological CO(2) pressures. Postillumination CO(2) exchange kinetics showed that O(2) and CO(2) insensitivity was not the result of elimination of photorespiration.It is proposed that O(2) and CO(2) insensitivity occurs when the concentration of phosphate in the chloroplast stroma cannot be both high enough to allow photophosphorylation and low enough to allow starch and sucrose synthesis at the rates required by the rest of the photosynthetic component processes. Under these conditions, the energy diverted to photorespiration does not adversely affect the potential for CO(2) assimilation.
This paper describes studies we have undertaken to extend our knowledge of rate limitations to photosynthesis at the level of organization of the leaf. The ultimate aim of this research is to determine those processes which prevent the leaf from having a higher rate of CO2 assimilation. Recently, progress has been made in studies of the stomatal (Sharkey et al. 1982) and mesophyll (J.A. Berry, T.D. Sharkey and J.R. Evans, unpublished) resistances to CO2 diffusion. This paper will describe work on the biochemical limitations to photosynthesis.
This chapter describes the C2 chemo- and photorespiratory carbon oxidation cycle. All lithotrophic organisms—chemolithotrophic or photolithotrophic—share a common mechanism for the reduction of CO2 to carbohydrate. The oxidation of inorganic substrates is coupled, through NADPH and ATP, to the reduction of CO2 to carbohydrate. The most common mechanism for the reduction of CO2 is the C3 photo- or chemosynthetic carbon reduction cycle (C3 cycle). Hydrogen atoms derived from water are incorporated during all three phases, carboxylation→ reduction→ regeneration, of the C3 cycle. The operation of the C2 cycle results in the consumption of ATP and reducing equivalents. Dark respiration involves both substrate level and oxidative phosphorylation so that 35–40 % of the energy available from the oxidation of glucose is conserved in the form of ATP. There is no net conservation of energy associated with the C2 cycle. On the contrary, an input of energy is required to drive the C2 cycle.
A series of experiments is presented investigating short term and long term changes of the nature of the response of rate of CO2 assimilation to intercellular p(CO2). The relationships between CO2 assimilation rate and biochemical components of leaf photosynthesis, such as ribulose-bisphosphate (RuP2) carboxylase-oxygenase activity and electron transport capacity are examined and related to current theory of CO2 assimilation in leaves of C3 species. It was found that the response of the rate of CO2 assimilation to irradiance, partial pressure of O2, p(O2), and temperature was different at low and high intercellular p(CO2), suggesting that CO2 assimilation rate is governed by different processes at low and high intercellular p(CO2). In longer term changes in CO2 assimilation rate, induced by different growth conditions, the initial slope of the response of CO2 assimilation rate to intercellular p(CO2) could be correlated to in vitro measurements of RuP2 carboxylase activity. Also, CO2 assimilation rate at high p(CO2) could be correlated to in vitro measurements of electron transport rate. These results are consistent with the hypothesis that CO2 assimilation rate is limited by the RuP2 saturated rate of the RuP2 carboxylase-oxygenase at low intercellular p(CO2) and by the rate allowed by RuP2 regeneration capacity at high intercellular p(CO2).
The response of CO2-assimilation rate to the intercellular partial pressure of CO2 (p(CO2)) is used to analyse the effects of various growth treatments on the photosynthetic characteristics of P. vulgaris. Partial defoliation caused an increase in CO2-assimilation rate at all intercellular p(CO2). A change in the light regime for growth from high to low light levels caused a decrease of CO2-assimilation rate at all intercellular p(CO2). Growth in a CO2-enriched atmosphere resulted in lowered assimilation assimilation rates compared with controls at comparable intercellular p(CO2). Short-term water stress initially caused only a decline in the CO2-assimilation rate at high intercellular p(CO2), but not at low intercellular p(CO2). Except under severe water stress, changes in the initial slope of the response of CO2-assimilation rate to intercellular p(CO2) were in parallel to those of the in-vitro activity of ribulose-1,5-bisphosphate (RuBP) carboxylase. From the results, we infer that partial defoliation, changes in the light regime for growth, and growth in a CO2-enriched atmosphere cause parallel changes in RuBP-carboxylase (EC 4.1.1.39) activity and the "capacity for RuBP regeneration", whereas short-term water stress initially causes only a decline in the RuBP-regeneration capacity.
The relationship between the gas-exchange characteristics of attached leaves of Phaseolus vulgaris L. and the pool sizes of several carbon-reduction-cycle intermediates was examined. After determining the rate of CO2 assimilation at known intercellular CO2 pressure, O2 pressure and light, the leaf was rapidly killed (<0.1 s) and the levels of ribulose-1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), fructose-1,6-bisphosphate, fructose-6-phosphate, glucose-6-phosphate, glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate were measured. In 210 mbar O2, photosynthesis appeared RuBP-saturated at low CO2 pressure and RuBP-limited at high CO2 pressure. In 21 mbar (2%) O2, the level of RuBP always appeared saturating. Very high levels of PGA and other phosphate-containing compounds were found with some conditions, especially under low oxygen.
The substrate specificity factor, V cKo/VoKc, of spinach (Spinacia oleracea L.) ribulose 1,5-bisphosphate carboxylase/oxygenase was determined at ribulosebisphosphate concentrations between 0.63 and 200 μM, at pH values between 7.4 and 8.9, and at temperatures in the range of 5° C to 40° C. The CO2/O2 specificity was the same at all ribulosebisphosphate concentrations and largely independent of pH. With increasing temperature, the specificity decreased from values of about 160 at 5° C to about 50 at 40° C. The primary effects of temperature were on K c [Km(CO2)] and V c [Vmax (CO2)], which increased by factors of about 10 and 20, respectively, over the temperature range examined. In contrast, K o [Ki (O2)] was unchanged and V o [Vmax (O2)] increased by a factor of 5 over these temperatures. The CO2 compensation concentrations (Γ) were calculated from specificity values obtained at temperatures between 5° C and 40° C, and were compared with literature values of Γ. Quantitative agreement was found for the calculated and measured Γ values. The observations reported here indicate that the temperature response of ribulose 1,5-bisphosphate carboxylase/oxygenase kinetic parameters accounts for two-thirds of the temperature dependence of the photorespiration/photosynthesis ratio in C3 plants, with the remaining one-third the consequence of differential temperature effects on the solubilities of CO2 and O2.
Kinetic properties of soybean net photosynthetic CO(2) fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO(2) concentration, and O(2) concentration. With leaves, O(2) inhibition of net photosynthetic CO(2) fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO(2) fixation at higher temperatures was caused by a reduced affinity of the leaf for CO(2) and an increased affinity of the leaf for O(2). With purified ribulose 1,5-diphosphate carboxylase, O(2) inhibition of CO(2) incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O(2) sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO(2) and a slightly increased affinity of the enzyme for O(2). The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO(2) and O(2) provides further evidence that the carboxylase regulates the O(2) response of photosynthetic CO(2) fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO(2) and O(2) fixation in vivo is proposed.Oxygen competitively inhibited carboxylase activity with respect to CO(2), and CO(2) competitively inhibited oxygenase activity with respect to O(2). Within the limits of experimental error, the Michaelis constant (CO(2)) in the carboxylase reaction was identical with the inhibition constant (CO(2)) in the oxygenase reaction, and the Michaelis constant (O(2)) in the oxygenase reaction was identical with the inhibition constant (O(2)) in the carboxylase reaction. The Michaelis constant, (ribulose 1,5-diphosphate) was the same in both the carboxylase and oxygenase reactions. This equality of kinetic constants is consistent with the notion that the same enzyme catalyzes both reactions.
The instantaneous rate of photosynthetic CO2 assimilation in C3 plants has generally been studied in model systems such as isolated chloroplasts and algae. From these studies and from theoretical analyses of gas exchange behavior it is now possible to study the biochemistry of photosynthesis in intact leaves using a combination of methods, most of which are nondestructive. The limitations to the rate of photosynthesis can be divided among three general classes: (1) the supply or utilization of CO2, (2) the supply or utilization of light, and (3) the supply or utilization of phosphate. The first limitation is most readily studied by determining how the CO2 assimilation rate varies with the partial pressure of CO2 inside the leaf. The second limitation can be studied by determining the quantum requirement of photosynthesis. The third limitation is most easily detected as a loss of O2 sensitivity of photosynthesis. Measurement of fluorescence from intact leaves can give additional information about the various limitations. These methods are all non-destructive and so can be observed repeatedly as the environment of a leaf is changed. In addition, leaves can be quick-frozen and metabolite concentrations then measured to give more information about the limitations to intact leaf photosynthesis rates. In this review the physics and biochemistry of photosynthesis in intact C3 leaves, and the interface between physiology and photosynthesis—triose phosphate utilization—are discussed.
An oxygen-dependent production of phosphoglycolate is catalyzed by purified soybean ribulose diphosphate carboxylase and by crude extracts of soybean and corn leaves. It is suggested that the phosphoglycolate produced in this reaction is the source of glycolate metabolized in photorespiration.
The magnitude of the percentage inhibition of photosynthesis by atmospheric levels of O(2) in the C(3) species Solanum tuberosum L., Medicago sativa L., Phaseolus vulgaris L., Glycine max L., and Triticum aestivum L. increases in a similar manner with an increase in the apparent solubility ratio of O(2)/CO(2) in the leaf over a range of solubility ratios from 25 to 45. The solubility ratio is based on calculated levels of O(2) and CO(2) in the intercellular spaces of leaves as derived from whole leaf measurements of photosynthesis and transpiration. The solubility ratio of O(2)/CO(2) can be increased by increased leaf temperature under constant atmospheric levels of O(2) and CO(2) (since O(2) is relatively more soluble than CO(2) with increasing temperature); by increasing the relative levels of O(2)/CO(2) in the atmosphere at a given leaf temperature, or by increased stomatal resistance. If the solubility ratio of O(2)/CO(2) is kept constant, as leaf temperature is increased, by varying the levels of O(2) or CO(2) in the atmosphere, then the percentage inhibition of photosynthesis by O(2) is similar. The decreased solubility of CO(2) relative to O(2) (decreased CO(2)/O(2) ratio) may be partly responsible for the increased percentage inhibition of photosynthesis by O(2) under atmospheric conditions with increasing temperature.
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