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... When data obtained at high CO 2, high light and low temperature proved inconsis-tent with the model, it was modified to include limitations arising from inadequate utilization of triose phosphate (Sharkey 1985a). The formulation presented by Sharkey establishes a Pilimited ceiling for assimilation but does not lead to reversed sensitivity of assimilation to either CO 2 or 02, as has been frequently observed (Jolliffe and Tregunna 1973, Viii et al. 1977, McVetty and Canvin 1981, von Caemmerer and Farquhar 1981, Woo and Wong 1983, Sharkey 1985a, Brown et al. 1986, Leegood and Furbank 1986, Sage and Sharkey 1987, Sharkey and Vassey 1989. We propose a simple modification which explains the reversed sensitivity of net photosynthesis to both CO 2 and 0 2 under conditions where Pi limits assimilation. ...
... This formulation for carboxylation under Pilimited conditions places a ceiling on net photosynthesis such that when Pi limits carboxylation, net assimilation becomes totally insensitive to both CO 2 and 0 2. Indeed, the presence of 0 2 insensitivity has been used as an indication that assimilation is TPU-limited (Sharkey 1985b). On the other hand, data in which A vs. C i curves measured at ambient 0 2 (210 mbar) and low 0 2 (10 or 20mbar) cross over, such that low 0 2 becomes inhibitory, have been reported repeatedly (Jolliffe and Tregunna 1973, Viii et al. 1977, McVetty and Canvin 1981, Sharkey 1985a, Brown et al. 1986, Leegood and Furbank 1986, Sharkey and Vassey 1989. This phenomenon is generally associated with high irradiance and superambient CO 2, although at low temperatures, the crossover of A vs. C i curves measured at 20 and 210mbar 0 2 may occur at ambient CO 2 or below Furbank 1986, Sage andSharkey 1987). ...
... The effects of 0 2 on starch production warrant further study, but we believe that the reduction in phosphoglucoisomerase activity caused by PGA is unlikely to be a general effect. Viii et al. (1977), for instance, report that twice as much C is incorporated into starch in Phaseolus at 0.5% 0 2 than at 21% Harris et al. (1983), Sharkey (1985a,b) and Walker et al. (1986) have suggested that the observed stimulation of assimilation by 0 2 under TPU-limited conditions might involve the contribution of Pi released in the PCO cycle. Usuda and Edwards (1982) showed that the Pi optimum for photosynthesis in wheat chloroplasts was increased by lowering the 0 2 concentration from 21 to 6%, and proposed that conditions which favored glycollate synthesis (i.e., oxygenation of RuBP and de-phosphorylation of phosphoglycollate) might alleviate the potential for Pi limitations to occur. ...
Current models of C3 photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO2 and O2, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO2 and O2, i.e., increasing O2 stimulates assimilation and increasing CO2 inhibits assimilation. We propose a model which explains reversed sensitivity to CO2 and O2 by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO2, O2, and light is demonstrated.
... Photosynthesis was in steady state at the light intensity I of 74 mW cm −2 (strongly saturating) and CO 2 concentration C a of 4080 ng cm −3 (2200 ppm). Oscillations were generated after CO 2 supply or illumination were stopped for 1 min (Laisk 1977, p. 101) (Laisk et al. 1992) photosynthesis in the presence of oxygen-which happens when CO 2 concentration is high enough to out-compete RuBP oxygenation (Viil et al. 1972(Viil et al. , 1977Laisk 1977). A sudden removal of atmospheric oxygen induces an oscillatory transient, damping finally on a 10-20% lower CO 2 uptake rate than it was in the presence of oxygen (Fig. 32). ...
... Important herewith is that P700 reduction is oscillating in counterphase with the CO 2 uptake rate, suggesting that removal of oxygen worsened the ATP/NADPH ratio, due to which the steady state equilibrates at a slightly lower and more acceptor-limited rate. This enhancement of photosynthesis by oxygen was ascribed to its role as an alternative electron acceptor (Viil et al. 1977). ...
Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-Km in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome b6f, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt b6f to P700+ and the proton-coupled linear flow from PSII to Cyt b6f to P700+. Analyzing responses of O2 evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the QA-reduced PSII units after their QB site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e−. Feedback regulation of this ratio leads to oscillations under saturating light and CO2, when the rate is Pi-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e− ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O2 reduction), are only a fraction of a per cent. The Pi limitation causes slip in the ATP synthase, slightly increasing the basic 12H+/3ATP requirement.
... We estimated R d and Γ* with the "Laisk method" (Viil et al., 1977) for the three species, i.e., from the intersection of three A-C i curves recorded at PPFDs of 100, 50 and 25 ...
... We used the apparent CO 2 photorespiratory compensation point (C i *) as a proxy for Γ* (von Caemmerer et al. 1994). We estimated R d and C i * with the Laisk method (Viil et al. 1977) for the three species from the intersection of three A-C i curves (C e of 125, 100 and 50 µmol mol -1 ) measured at three irradiance levels (100, 50 and 25 µmol m -2 s -1 ). C i * did not differ between species (t-test >0.05), so we used the mean (C i * =38.7 ± 0.51 µmol mol -1 , n=11) as a common value for the three species. ...
This work was focused on the relationship between isotopic discrimination of 13C during photosynthesis (delta13C) and leaf gas exchange. The model of Farquhar and colleagues (Farquhar et al. 1982) predicts delta13C by accounting for diffusion, carboxylation and decarboxylation processes during the photosynthesis. This relationship is widely used and delta13C is frequently considered as a proxy water use efficiency (WUE, the amount of water required to fix a amount of carbon), an interesting parameter in the context of climate change, crop production and sylviculture. The delta13C model is also used to assess mesophyll conductance to CO2 (gm), that strongly limits photosynthesis via the availability of carbon in the chloroplast. Along this work we analyzed the delta13C model and identified the most important parameters, and highlighted that using the "simple form" of the model (which ignores gm and the decarboxylations) could lead to misestimating WUE. We also focused on the possible rapid variations of gm, a subject still under debate. We confirmed that gm was sensitive to rapid variations of CO2 and irradiance in all species tested in this study. We also showed that apparent rapid variations of gm could not be induced by variations of other parameters in the model, with the exception of parameter b (discrimination during carboxylation). We propose that future studies should focus on (i) the possible environmental and genetic variability of parameter b, and (ii) the physiological processes able to change gm at short time scales (aquaporins and carbonic anhydrase).
... The shortage of Pi may be exacerbated by low 02 since under conditions of 02-insensitivity of AP, lowering 02 concentration has been found to raise the levels of RuBP and 3-P-glyceraldehyde (TD Sharkey, personal communication). Therefore, under conditions of TPU limitation, 02 may have no effect or may actually stimulate AP because less Pi is tied up in phosphorylated intermediates and phosphoglycolate produced by reaction of 02 and RuBP is also a source of Pi. 02-insensitivity or stimulation of AP has been observed only at low temperatures (8,12), at CO2 levels above atmospheric (12,17,18,20,21), or at concentrations near atmospheric in plants under stress ( 18). In this paper we report the 02 response of AP in a plant of the species, F. linearis, in which AP is stimulated by 210 ml L0 02 at atmospheric CO2 concentrations, and under nonstress growth conditions. ...
... However, no reports of 02 stimulation of AP have been made for temperatures as high as 30(C with ci as low as 200 ul L-'. In addition, inhibition of AP by exposure to low 02 has in several cases been transitory, with recovery requiring only a few minutes (5,15,20). Response of AP of 84-9 to C02 and 02 concentrations most closely resembles the results of Cornic and Louason (8) for measurements conducted with C3 species at 5(C. They observed stimulation of AP by 210 ml L`02 at C02 concentrations of 200 to 400 Ml L`or higher. ...
A plant was found in the C(3)-C(4) intermediate species, Flaveria linearis, in which apparent photosynthesis is stimulated by atmospheric O(2) concentrations. A survey of 44 selfed progeny of the plant showed that the O(2) stimulation of apparent photosynthesis was passed on to the progeny. When leaves equilibrated at 210 milliliters per liter O(2) were transferred to 20 milliliters per liter O(2) apparent photosynthesis was initially stimulated, but gradually declined so that at 30 to 40 minutes the rate was only about 80 to 85% of that at 210 milliliters per liter O(2). Switching from 20 to 210 milliliters per liter caused the opposite transition in apparent photosynthesis. All other plants of F. linearis reached steady rates within 5 minutes after switching O(2) that were 20 to 24% lower in 210 than in 20 milliliters per liter O(2). At low intercellular CO(2) concentrations and low irradiances, O(2) inhibition of apparent photosynthesis of the aberrant plant was similar to that in normal plants, but at an irradiance of 2 millimoles quanta per square meter per second and near 300 microliters per liter CO(2) apparent photosynthesis was consistently higher at 210 than at 20 milliliters per liter O(2). In morphology and leaf anatomy, the aberrant plant is like the normal plants in F. linearis. The stimulation of apparent photosynthesis at air levels of O(2) in the aberrant plant is similar to other literature reports on observations with C(3) plants at high CO(2) concentrations, high irradiance and/or low temperatures, and may be related to limitation of photosynthesis by triose phosphate utilization.
... The role of the Mehler reaction: enhancement of photosynthesis by oxygen Oxygen inhibits C 3 photosynthesis by inducing photorespiration at atmospheric CO 2 concentration, but at saturating CO 2 concentrations, when photorespiration is suppressed, oxygen enhances photosynthesis (Viil et al. 1972). This effect was suggested to reflect the Mehlertype O 2 reduction, increasing the ATP/NADPH ratio (Viil et al. 1977). O 2 /CO 2 , O 2 vs. CO 2 exchange ratio during the light response and CO 2 response curves; dP700e À /dt, the rate of electron accumulation in P700; dPSIe À /dt, the rate of electron accumulation in PSI donor side carriers after the O 2 concentration was decreased from 210 to 20 mmol mol À1 at 1,500 mmol CO 2 mol À1 ; dPSIe À /J C , rate of electron accumulation at PSI relative to the linear electron transport rate; J F , J I and J C , electron transport rates through PSII, PSI and for BPGA reduction, respectively; the presented differences were measured at light saturation; J max , maximum electron transport rate at CO 2 and light saturation; MDH, fully activated reaction capacity of NADP-MDH; n.m., not measured. ...
... We evaluated the rate of the Mehler-type O 2 reduction from the inhibition of CO 2 -and light-saturated photosynthesis after the removal of most oxygen, discovered several decades ago (Viil et al. 1972, Viil et al. 1977. In the present work we recorded the 810 nm transmittance signal in parallel with the CO 2 uptake, showing that the inhibition of photosynthesis under the low O 2 level was accompanied by the additional reduction of electron carriers around PSI. ...
Measurements of 810 nm transmittance changes in leaves, simultaneously with Chl fluorescence, CO(2) uptake and O(2) evolution, were carried out on potato (Solanum tuberosum L.) leaves with altered expression of plastidic NADP-dependent malate dehydrogenase. Electron transport rates were calculated: J(C) from the CO(2) uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, J(O) from the O(2) evolution rate, J(F) from Chl fluorescence parameters and J(I) from the post-illumination re-reduction speed of PSI donors. In the absence of external O(2), J(O) equaled (1.005 +/- 0.003) J(C), independent of the transgenic treatment, light intensity and CO(2) concentration. This showed that nitrite and oxaloacetate reduction rates were very slow. The Mehler-type O(2) reduction was evaluated from the rate of electron accumulation at PSI after the O(2) concentration was decreased from 210 to 20 mmol mol(-1), and resulted in <1% of the linear flow. J(F) and J(I) did not differ from J(C) while photosynthesis was light-limited, but considerably exceeded J(C) at saturating light. Then, typically, J(F) = 1.2 J(C) and J(I) = 1.3 J(C), and J(F) -J(C) and J(I) -J(C) depended little on CO(2) and O(2) concentrations. The results showed that the alternative and cyclic electron flow necessary to compensate variations in the ATP/NADPH ratio were only a few percent of the linear flow. The data do not support the requirement of 14H(+)/3ATP by the chloroplast ATP synthase. We suggest that the fast PSI cyclic electron flow J(I) - J(C), as well as the fast J(F) - J(C) are energy-dissipating cycles around PSI and PSII at light saturation.
... These regulatory mechanisms are the only aspects of TPU limitation typically observed in steady-state gas exchange. While TPU limitation results in and can be assessed through gas exchange as O 2and CO 2insensitive photosynthesis (Sharkey, 1985) or reverse sensitivity to O 2 (Viil et al., 1977) or CO 2 (Jolliffe & Tregunna, 1973), it is easier to assess by the decline in electron transport rate associated with q E when CO 2 is increased or O 2 is decreased. The appearance of transient effects on photosynthesis associated with TPU limitation (Ogawa, 1982;Walker et al., 1983) lead us to believe that, in the steady state, the rate of photosynthesis is not set by TPU, but instead, the rate is set by regulatory mechanisms that match the rates of carbon input to and carbon output from the organic phosphate pool. ...
Triose‐phosphate utilization (TPU) limits the maximum rate at which plants can photosynthesize. However, TPU is almost never found to be limiting photosynthesis under ambient conditions for plants. This, along with previous results showing adaptability of TPU at low temperature, suggest that TPU capacity is regulated to be just above the photosynthetic rate achievable under the prevailing conditions. A set of experiments were performed to study the adaptability of TPU capacity when plants are acclimated to elevated CO2 concentrations. Plants held at 1500 ppm CO2 were initially TPU limited. After 30 hours they no longer exhibited TPU limitations but they did not elevate their TPU capacity. Instead, the maximum rates of carboxylation and electron transport declined. A timecourse of regulatory responses was established. A step increase of CO2 first caused PSI to be oxidized but after 40 s both PSI and PSII had excess electrons as a result of acceptor‐side limitations. Electron flow to PSI slowed and the proton motive force increased. Eventually, non‐photochemical quenching reduced electron flow sufficiently to balance the TPU limitation. Over several minutes rubisco deactivated contributing to regulation of metabolism to overcome the TPU limitation. This article is protected by copyright. All rights reserved.
... As photorespiration releases CO 2 , it is counterintuitive that altering the gas composition to favor carboxylation would result in decreased carbon assimilation. Yet data dating back decades show that once at high CO 2 , increasing CO 2 can cause a decrease in net assimilation (Jolliffe and Tregunna, 1973;Canvin, 1978;von Caemmerer and Farquhar, 1981), and increasing O 2 can cause an increase in net assimilation (Viil et al., 1977). ...
Chlorophyll fluorescence measurements of electron transport rate are an important companion of gas exchange analysis of photosynthesis. Detailed models allow prediction of gas exchange behavior based on fluorescence measurements, critical for converting low‐throughput photosynthetic measurements to greater scales (Damm et al., 2010). Recently, a problem with using fluorescence‐estimated electron transport rates in red versus blue light was discussed (Evans et al., 2017). The explanation was that high absorptance of blue light leads to saturation of photosystems near the light‐exposed surface, but the red measuring beam may sample deeper in the leaf (Vogelmann, 1993; Vogelmann & Han, 2000).
... As photorespiration releases CO 2 , it is counterintuitive that altering the gas composition to favor carboxylation would result in decreased carbon assimilation. Yet data dating back decades show that once at high CO 2 , increasing CO 2 can cause a decrease in net assimilation (Jolliffe and Tregunna, 1973;Canvin, 1978;von Caemmerer and Farquhar, 1981), and increasing O 2 can cause an increase in net assimilation (Viil et al., 1977). ...
During photosynthesis, plants fix CO2 from the atmosphere onto ribulose-bisphosphate, producing 3-phosphoglycerate, which is reduced to triose phosphates (TPs). The TPs are then converted into the end products of photosynthesis. When a plant is photosynthesizing very quickly, it may not be possible to commit photosynthate to end products as fast as it is produced, causing a decrease in available phosphate and limiting the rate of photosynthesis to the rate of triose phosphate utilization (TPU). The occurrence of an observable TPU limitation is highly variable based on species and especially growth conditions, with TPU capacity seemingly regulated to be in slight excess of typical photosynthetic rates the plant might experience. The physiological effects of TPU limitation are discussed with an emphasis on interactions between the Calvin-Benson cycle and the light reactions. Methods for detecting TPU-limited data from gas exchange data are detailed and the impact on modeling of some physiological effects are shown. Special consideration is given to common misconceptions about TPU.
... We used the apparent CO 2 photorespiratory compensation point (C i *) as a proxy for G* (von Caemmerer et al. 1994). We estimated R d and C i * following the Laisk method (Viil et al. 1977) for the three species from the intersection of three curves plotting the net CO 2 assimilation rate (A) against C i (at C e of 125, 100 and 50 mmol mol -1 ) measured at three irradiance levels (100, 50 and 25 mmol m -2 s -1 ). C i * did not differ between species (t-test >0.05), so we used the mean (C i * = 38.7 AE 0.51 mmol mol -1 , n = 11) as a common value for the three species. ...
Mesophyll conductance to CO2 (g
m) limits the diffusion of CO2 to the sites of carboxylation, and may respond rapidly (within minutes) to abiotic factors. Using three Eucalyptus species, we tested the rapid response of g
m to irradiance under 21% and 1% O2. We used simultaneous measurements of leaf gas exchange and discrimination against 13CO2 with a tuneable diode laser absorption spectrometer. Measurements under 1% O2 were used to limit uncertainties due to 13C–12C fractionation occurring during photorespiration. Switching irradiance from 600 to 200 µmol m–2 s–1 led to a ≈60% decrease of g
m within minutes in all species under both 21% O2 and 1% O2. The g
m response to irradiance is unlikely to be a computation artefact since using different values for the parameters of the discrimination model changed the absolute values of g
m but did not affect the relative response to irradiance. Simulations showed that possible rapid changes of any parameter were unable to explain the observed variations of g
m with irradiance, except for13C–12C fractionation during carboxylation (b), which, in turn, is dependent on the fraction of leaf C assimilated by phospho-enol pyruvate carboxylase (PEPc) (β). g
m apparently increased by ≈30% when O2 was switched from 21% to 1% O2. Again, possible changes of β with O2 could explain this apparent g
m response to O2. Nevertheless, large irradiance or O2-induced changes in β would be required to fully explain the observed changes in g
m, reinforcing the hypothesis that g
m is responsive to irradiance and possibly also to O2.
... This lack of stimulation by 2% 02 is not due to the absence of photorespiration at such temperatures (Arrabaca et al. 1981 ) nor is photorespiration absent in many other circumstances in which 2% 02 fails to stimulate photosynthesis (Harris et al. 1983; Sharkey 1985 ). The only feasible explanations for this anomalous effect of 2% Oz at low temperatures are that (i) the potential increase in the rate of photosynthesis due to the presence of 2% 02 is offset by a decrease in the activity of ribulose-l,5-bisphosphate (RuBP) carboxylase, either by a change in its activation state (Schnyder et al. 1984 ) or else by a change in one of its effectors at low temperatures (Arrabaca et al. 1981), or (ii) the efficiency of electron transport is altered by the presence of low oxygen (Viil et al. 1977; Canvin 1978 ), or (iii) the system is phosphate-limited (Leegood 1985; Sharkey 1985). For example, measurements of the influence of the phosphatesequestering agent, mannose, on spinach leaf discs indicate that phosphate deficiency results in a lack of response to 2% 02 (Harris et al. 1983). ...
The effect of phosphate feeding on the influence of low (2%) oxygen on photosynthetic carbon assimilation has been investigated in leaf discs of spinach (Spinacia oleracea L.) at 12°C. The following observations were made. First, after the transition from 20% O2 to 2% O2, the rate of CO2 uptake was inhibited at CO2 concentrations between about 250 and about 800 μl CO2·l(-1). Second, phosphate feeding stimulated the rate of CO2 uptake in 20% O2 at higher concentrations of CO2 (500-900 μl·l(-1)). Third, phosphate feeding stimulated the rate of CO2 uptake in 2% O2 at all but the highest (900 μl·l(-1)) and lowest 74 (μl·l(-1)) concentrations of CO2 employed. Phosphate thereby restored the stimulation of photosynthesis by 2% O2 and it did so over a wide range of lower temperatures. Fourth, oscillatory behaviour, however generated, was dampened by phosphate feeding, even at very low concentrations of CO2. Contents of leaf metabolites were measured during the transition to 2% O2 in control and phosphate-fed leaf discs. During this period the ratio glycerate-3-phosphate/triose phosphate rose steeply, but fell again only in the phosphate-treated leaf discs. These data, taken together with measured ATP/ADP ratios, showed that assimilatory power, the ratio [ATP]·[NAD(P)H]/[ADP]·[Pi]·[NAD(P)], decreased when leaves were exposed to 2% O2, but that this decrease was minimised by previous feeding of phosphate. The mechanism of phosphate limitation is discussed in the light of the results.
... 'Supported by the It has been observed that decreasing the 02 concentration from 210 to 21 mbar does not always lead to an increase of photosynthesis, especially when the light intensity is high and the temperature is below 20°C (3,12,14,24). It has been proposed that this 02 insensitivity of photosynthesis is due to a limitation of photosynthate utilization, in particular triose-P utilization for sucrose and starch synthesis (17,18). ...
The occurrence of O2-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves. In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions. Upon switching to low O2 in O2-insensitive conditions the ATP/ADP ratio fell dramatically within one minute. The P-glycerate pool increased over the same time. Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air. The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O2 of 21 millibars than at 210 millibars. These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O2-insensitive conditions.
... The rate of photosynthesis in several species of terrestrial C3 plants decreased when the O2 concentration was reduced from 21 to 2% at high light intensities (Viil and Parnik 1974, Viil et al. 1977, Canvin 1978, McVetty and Canvin 1981. This inhibition was only a transient effect (McVetty and Canvin 1981), in contrast to the inhibition by anaerobiosis in Chroomonas sp. (Fig. 3A). ...
The effect of oxygen on photosynthetic 14CO2 fixation in the air-grown freshwater flagellate Chroomonas sp. (Cryptophyta) was studied. Considerable inhibition by anaerobiosis was observed only under light-saturated conditions with no effect from the CO2 concentration. This inhibition was reversed by 2% O2. With, more than 2% O2, the rate of 14CO2 fixation was inhibited; 100% O2 caused about 20% inhibition which could be reversed by 2% O2. The degree of inhibition was only slightly higher at low concentrations (less than 0.43 mM NaHCO3) than at high CO2 concentrations, indicating that photorespiration is not a main cause of this inhibition. Possible causes of the inhibitions by anaerobiosis and by oxygen are discussed.
... Model parameters R d and C* were estimated with the 'Laisk method' (Viil et al., 1977) for the three species (i.e. from the intersection of three A–C i curves recorded at PPFDs of 100, 50, and 25 lmol photon m À2 s À1 and C e of 125, 100, and 50 lmol mol À1 ). The 'Laisk method' provides C i * or the 'apparent' CO 2 compensation point in the absence of day respiration (Von Caemmerer et al., 1994), and was used as a proxy of C*. ...
Mesophyll conductance (gm) is now recognized as an important limiting process for photosynthesis, as it results in a significant decrease of CO2 diffusion from substomatal cavities where water evaporation occurs, to chloroplast stroma. Over the past decade, an increasing
number of studies proposed that gm can vary in the short term (e.g. minutes), but these variations are still controversial, especially those potentially induced
by changing CO2 and irradiance. In this study, gm data estimated with online 13C discrimination recorded with a tunable diode laser absorption spectrometer (TDL-AS) during leaf gas exchange measurements,
and based on the single point method, are presented. The data were obtained with three Eucalyptus species. A 50% decrease in gm was observed when the CO2 mole fraction was increased from 300 μmol mol−1 to 900 μmol mol−1, and a 60% increase when irradiance was increased from 200 μmol mol−1 to 1100 μmol mol−1 photosynthetic photon flux density (PPFD). The relative contribution of respiration and photorespiration to overall 13C discrimination was also estimated. Not taking this contribution into account may lead to a 50% underestimation of gm but had little effect on the CO2- and irradiance-induced changes. In conclusion, (i) the observed responses of gm to CO2 and irradiance were not artefactual; (ii) the respiratory term is important to assess absolute values of gm but has no impact on the responses to CO2 and PPFD; and (iii) increasing irradiance and reducing the CO2 mole fraction results in rapid increases in gm in Eucalyptus seedlings.
... CO 2 assimilation is often stimulated by subatmospheric levels of O 2 in C 3 plants because of the reduced oxygenase activity of the bifunctional enzyme Rubisco (Sage and Sharkey, 1987). However, in some cases, such as at low temperatures (Leegood and Furbank, 1986;Sage and Sharkey, 1987;Sun et al., 1997) or at high CO 2 levels (Viil et al., 1977;Sharkey, 1985), C 3 plants exhibit O 2 -insensitive photosynthesis or even reversed O 2 sensitivity. Reversed O 2 sensitivity is also seen in a Flaveria mutant that contains reduced levels of the cytosolic Fru bisphosphatase (Sharkey et al., 1995) and in a tobacco starchless mutant with phosphoglucomutase deficiency (Hanson, 1990;Eichelmann and Laisk, 1994). ...
Wild-type Arabidopsis plants, the starch-deficient mutant TL46, and the near-starchless mutant TL25 were evaluated by noninvasive in situ methods for their capacity for net CO2 assimilation, true rates of photosynthetic O2 evolution (determined from chlorophyll fluorescence measurements of photosystem II), partitioning of photosynthate into sucrose and starch, and plant growth. Compared with wild-type plants, the starch mutants showed reduced photosynthetic capacity, with the largest reduction occurring in mutant TL25 subjected to high light and increased CO2 partial pressure. The extent of stimulation of CO2 assimilation by increasing CO2 or by reducing O2 partial pressure was significantly less for the starch mutants than for wild-type plants. Under high light and moderate to high levels of CO2, the rates of CO2 assimilation and O2 evolution and the percentage inhibition of photosynthesis by low O2 were higher for the wild type than for the mutants. The relative rates of 14CO2 incorporation into starch under high light and high CO2 followed the patterns of photosynthetic capacity, with TL46 showing 31% to 40% of the starch-labeling rates of the wild type and TL25 showing less than 14% incorporation. Overall, there were significant correlations between the rates of starch synthesis and CO2 assimilation and between the rates of starch synthesis and cumulative leaf area. These results indicate that leaf starch plays an important role as a transient reserve, the synthesis of which can ameliorate any potential reduction in photosynthesis caused by feedback regulation.
... If ATP deficiency exists, it must be compensated for by O 2dependent alternative e À transport processes, mentioned above. The importance of O 2 is supported by the clear positive effect of this gas on photosynthesis in the absence of photorespiration at saturating CO 2 concentrations [42,43]. Our failure to detect cyclic e À transport in many leaves is at variance with the suggested principle role of cyclic e À transport as a process that corrects imbalances in ATP/NADPH stoichiometry accompanying linear e À transport alone [44]. ...
The light-dependent control of photosynthetic electron transport from plastoquinol (PQH(2)) through the cytochrome b(6)f complex (Cyt b(6)f) to plastocyanin (PC) and P700 (the donor pigment of Photosystem I, PSI) was investigated in laboratory-grown Helianthus annuus L., Nicotiana tabaccum L., and naturally-grown Solidago virgaurea L., Betula pendula Roth, and Tilia cordata P. Mill. leaves. Steady-state illumination was interrupted (light-dark transient) or a high-intensity 10 ms light pulse was applied to reduce PQ and oxidise PC and P700 (pulse-dark transient) and the following re-reduction of P700(+) and PC(+) was recorded as leaf transmission measured differentially at 810-950 nm. The signal was deconvoluted into PC(+) and P700(+) components by oxidative (far-red) titration (V. Oja et al., Photosynth. Res. 78 (2003) 1-15) and the PSI density was determined by reductive titration using single-turnover flashes (V. Oja et al., Biochim. Biophys. Acta 1658 (2004) 225-234). These innovations allowed the definition of the full light response curves of electron transport rate through Cyt b(6)f to the PSI donors. A significant down-regulation of Cyt b(6)f maximum turnover rate was discovered at low light intensities, which relaxed at medium light intensities, and strengthened again at saturating irradiances. We explain the low-light regulation of Cyt b(6)f in terms of inactivation of carbon reduction cycle enzymes which increases flux resistance. Cyclic electron transport around PSI was measured as the difference between PSI electron transport (determined from the light-dark transient) and PSII electron transport determined from chlorophyll fluorescence. Cyclic e(-) transport was not detected at limiting light intensities. At saturating light the cyclic electron transport was present in some, but not all, leaves. We explain variations in the magnitude of cyclic electron flow around PSI as resulting from the variable rate of non-photosynthetic ATP-consuming processes in the chloroplast, not as a principle process that corrects imbalances in ATP/NADPH stoichiometry during photosynthesis.
... There is no doubt that, in vitro, CO2 competitively inhibits oxygenase activity of RuBPCase (2) but is this also the explanation for the apparent lack of effect of 02 on in vivo photosynthesis at high CO2 levels (13)? Recent reports (5,14,19) show that at high CO2 concentrations, where the capacity of the leaf for CO2 fixation is fully taxed, there is an inhibition of photosynthesis when the 02 concentration is decreased from 21 to 2%. These data suggest that at high CO2 concentrations the capacity of the leaf to fix CO2 in 2% 02 is less, or, after an adaptation period, only equal to the capacity of the leaf to fix CO2 in 21%02. ...
The isotopic CO(2) technique for measuring photorespiration was shown to be a valid technique for measuring the unidirectional inward and outward fluxes of CO(2) from a sunflower (Helianthus annuus L.) leaf in the light. The rate of photorespiration was decreased little as the CO(2) concentration was increased from 20 to 1,150 microliters per liter. This finding contradicts the widely held assumption that photorespiration is suppressed at high CO(2) concentrations. Some discussion regarding this apparent conflict is presented.
Mesophyll resistance for CO 2 diffusion ( r m ) is one of the main limitations for photosynthesis and plant growth. Breeding new varieties with lower r m requires knowledge of its distinct components.
We tested new method for estimating the relative drawdowns of CO 2 concentration ( c ) across hypostomatous leaves of Fagus sylvatica . This technique yields values of the ratio of the internal CO 2 concentrations at the adaxial and abaxial leaf side, c d / c b , the drawdown in the inter cellular air space (IAS), and intra cellular drawdown between IAS and chloroplast stroma, c c / c bd . The method is based on carbon isotope composition of leaf dry matter and epicuticular wax isolated from upper and lower leaf sides. We investigated leaves from tree‐canopy profile to analyse the effects of light and leaf anatomy on the drawdowns and partitioning of r m into its inter‐ ( r IAS ) and intracellular ( r liq ) components. Validity of the new method was tested by independent measurements of r m using conventional isotopic and gas exchange techniques.
73% of investigated leaves had adaxial epicuticular wax enriched in ¹³ C compared to abaxial wax (by 0.50‰ on average), yielding 0.98 and 0.70 for average of c d / c b and c c / c bd , respectively. The r IAS to r liq proportion were 5.5:94.5% in sun‐exposed and 14.8:85.2% in shaded leaves. c c dropped to less than half of the atmospheric value in the sunlit and to about two‐thirds of it in shaded leaves.
This method shows that r IAS is minor but not negligible part of r m and reflects leaf anatomy traits, i.e. leaf mass per area and thickness.
The effect of oxygen on photosynthesis depends on plant species and on some internal and environmental conditions. Oxygen inhibits the photosynthesis of C3 plants at atmospheric CO2 concentration (Warburg effect). The main cause of this phenomenon is the competitive inhibition of RuBP carboxylation and stimulation of RuBP oxygenation, the latter resulting in formation of glycolate and photorespiratory CO2. Oxygen inhibition of photosynthesis may also be due to the decrease of RuBP regeneration rate and diminution of the pool size of this substrate (1–16).
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 short-term, in-vivo response to elevated CO2 of ribulose-1,5-bisphosphate carboxylase (RuBPCase, EC 4.1.1.39) activity, and the pool sizes of ribulose 1,5-bisphosphate, 3-phosphoglyceric acid, triose phosphates, fructose 1,6-bisphosphate, glucose 6-phosphate and fructose 6-phosphate in bean were studied. Increasing CO2 from an ambient partial pressure of 360-1600 μbar induced a substantial deactivation of RuBPCase at both saturating and subsaturating photon flux densities. Activation of RuBPCase declined for 30 min following the CO2 increase. However, the rate of photosynthesis re-equilibrated within 6 min of the switch to high CO2, indicating that RuBPCase activity did not limit photosynthesis at high CO2. Following a return to low CO2, RuBPCase activation increased to control levels within 10 min. The photosynthetic rate fell immediately after the return to low CO2, and then increased in parallel with the increase in RuBPCase activation to the initial rate observed prior to the CO2 increase. This indicated that RuBPCase activity limited photosynthesis while RuBPCase activation increased. Metabolite pools were temporarily affected during the first 10 min after either a CO2 increase or decrease. However, they returned to their original level as the change in the activation state of RuBPCase neared completion. This result indicates that one role for changes in the activation state of RuBPCase is to regulate the pool sizes of photosynthetic intermediates.
Various CO2 fluxes were measured in Phaseolus vulgaris and Nicotiana tabacum leaves under 0.03 % CO2, 1.5 or 21 % O2, and limiting or saturating light(5 or 35 mW cm-2, respectively). A combined radio- and gasometric method was applied. From the components of the CO2 exchange, the rate of the turnover of ribulose-1,5-diphosphate(RuDP) and of 3-phosphoglycerate(PGA) was calculated. In the saturating light, the consumption of RuDP in oxygenation was compensated by its enhanced synthesis in 21 % O2. Therefore the inhibition of carboxylation accounted for the Warburg effect only in the limiting light while in the saturating light the effect was due only to photorespiration and the intrafoliar re-assimilation. At the light saturation, the PGA turnover proceeded also faster in 21 % O2. A quantitative model of the reductive pentose phosphate(RPP) cycle and the associated glycolate pathway has been proposed for the two oxygen concentrations. From the model it follows that in the saturating light the RPP cycle works in 21 % O2 twice as fast as in 1.5 % O2. Energetic requirements per mole CO2 bound at carboxylation and per ng atom carbon incorporated into end products are in 21 % O2 nearly two fold higher than in 1.5 % O2. In the air, the re-entry of carbon from the glycolate pathway into the RPP cycle is obligatory for the supporting of the steady state photosynthesis.
Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO2 enrichment up to 6000 μl CO2 l−1. This high CO2 optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO2 was less or absent following four days growth in 2% O2. The Leaf Area Ratio decreased in response to CO2 enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO2 enrichment up to 1500 μl CO2 l−1 during measurement, showing only small increases with further CO2 enrichment up to 5000 μl CO2 l−1 at a photon flux density of 210 μmol m−2 s−1 and small decreases at 2000 μmol m−1 s−1. The actual rate of photosynthesis of those plants cultivated at high CO2 levels, however, was less than the air grown plants. The response of photosynthesis to O2 indicated that the enhancement of growth and photosynthesis by CO2 enrichment was a result of decreased photorespiration. Plants cultivated in low O2 produced abnormal morphological features and after a short time showed a reduction in growth.
S ummary
Measurements of photosynthetic CO 2 and O 2 exchange, and associated chlorophyll fluorescence were made on leaves (from plants grown in complete nutrient medium) while the internal inorganic phosphate concentration was increased or decreased by feeding solutions through the vascular tissue. The data indicate that orthophosphate supply to the chloroplast can, in some circumstances, become the process that limits photosynthesis in vivo.
Photosynthesis is the incorporation of carbon, nitrogen, sulphur and other substances into plant tissue using light energy from the sun. Most of this energy is used for the reduction of carbon dioxide and, consequently, there is a large body of biochemical and biophysical information about photo synthetic carbon assimilation. In an ecophysiological context, we believe that most of today’s biochemical knowledge can be summarized in a few simple equations. These equations represent the rate of ribulose bisphosphate (RuP2)-saturated carboxylation, the ratio of photorespiration to carboxylation, and the rates of electron transport/photophosphorylation and of “dark” respiration in the light. There are many other processes that could potentially limit CO2 assimilation, but probably do so rarely in practice. Fundamentally this may be due to the expense, in terms of invested nitrogen, of the carboxylase and of thylakoid functioning. To reach our final simple equations we must first discuss the biochemical and biophysical structures — as they are understood at present — that finally reduce the vast number of potentially rate-limiting processes to the four or five listed above. A diagrammatic representation of these processes is given in Fig. 16.1.
It has been shown that atmospheric O2 can either depress or stimulate the rate of apparent photosynthesis of white mustard depending on the environmental conditions: CO2 concentration, light intensity and temperature. Stimulation by O2 was observed only under high photon fluence rate and at high CO2 concentrations. The critical CO2 concentration below which O2 was inhibiting and above which it was stimulating was dependent on the temperature of the assay: for plants grown at 12°C the critical CO2 concentration was 13.35 mmol at 5° C and 21.92 mmol at 10° C. Stimulation by O2 depended also on the growth temperature: for measurements at 26.31 mmol m−3 CO2, O2 was stimulating at temperatures less than 12°C for plants grown at 12°C and less than 19°C for plants grown at 27°C. The efficiency of the O2-dependent stimulation of net photosynthesis was maximum at 9.21 mol m−3 O2 at 26.31 mmol m−3 CO2.
Oxygen-stimulation of net photosynthesis was detected in Nicotiana tabacum L. var Samsun, Lycopersicum esculentum L. and Chenopodium album L. At 5°C and under high photon fluence rate, O2 increased the carboxylation capacity of the photosynthetic apparatus of mustard and decreased its affinity for CO2. The O2 inhibition of the net CO2 uptake observed at low CO2 concentrations was the result of a decrease in the affinity for carbon dioxide. The nature of the mechanism which causes the stimulation of photosynthesis is discussed.
Photosynthetic electron transport rate and partitioning of electrons between CO2 assimilation and O2 reduction were estimated in vivo at different temperatures using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence emission on intact leaves of both hairy-willow herb (Epilobium hirsutum L.) and French bean (Phaseolus vulgaris L.). In E. hirsutum leaves at low temperatures (below 15 °C), leaf net CO2 assimilation, A, was stimulated by normal O2 compared with air containing 2% O2, while at high temperatures the inhibition of A by normal O2 via stimulation of the oxygenase function of RubisCo masked the stimulatory effect of O2 on A. As a consequence of stimulation of A, the non-cyclic electron flow rate always remained higher in normal air than in air with 2% O2. In P. vulgaris, switching to non-photorespiratory conditions by increasing CO2 concentration stimulated A but did not change electron transport rates, indicating that only the partitioning of electrons between CO2 and O2 was changed. Two methods used to estimate the total photosynthetic electron transport rate gave similar results. The validity of the technique used was also tested by estimation of the CO2/O2 specificity factor of RubisCo, S. In both species, the estimated values of S agreed with that of the literature, obtained using spinach enzyme in vitro within a wide range of temperature (18 to 32 °C). However, in E. hirsutum S was considerably higher at low temperatures (below 18 °C). Overall, our results suggest that in both species, at temperatures above 18 °C, carboxylation and photorespiration are the main processes consuming photosynthetic electrons, the processes of O2 reduction other than photorespiration being negligible.
Bean (Phaseolus vulgaris L. cv. Golden Saxa) plants were grown under low artificial light or under natural daylight. The rate of net photosynthesis (PN) was measured at: CO2 partial pressure, p(CO2), of 0.03, 0.09 or 0.15 kPa; O2 partial pressure, p(O2), of 2, 21 or 31 kPa and at light intensities of 350 or 1000 μmol m−2 s−1 (photosynthetically active radiation). In plants which had been grown under natural light, stimulation of PN at 21 kPa p(O2) was found only at elevated p(CO2) and high light. It is proposed that this phenomenon is dependent on a high capacity of the photosynthetic apparatus to regenerate ribulose 1.5-bisphosphate.
Photosynthetic carbon metabolism is affected by a range of environmental factors. In this chapter we focus on the effect of temperature on photosynthesis in relation to other environmental factors. Plants grow over a wide range of temperatures and, apart from encountering large seasonal variations in temperature (including freezing), the aerial parts of a plant may face temperature variations of tens of degrees centigrade in a single day and smaller temperature changes in a matter of minutes. These are often coupled with changes in photon flux density and require a variety of regulatory responses. In addition, the mechanisms of photosynthesis associated with different photosynthetic types, C3, C4 and CAM, enable plants to perform better at specific temperatures, although they do not enable plants to tolerate temperature extremes which cause irreversible damage.
A computer model comprised of light reactions in PS II and PS I, electron—proton transport reactions in mesophyll and bundle
sheath (BS) chloroplasts, all enzymatic reactions, and most of the known regulatory functions of NADP-malic enzyme type C4 photosynthesis, has been developed as a system of differential budget equations for intermediate compounds. Rate-equations
were designed on principles of multisubstrate-multiproduct enzyme kinetics. The model provided good simulations for rates
of photosynthesis and pool sizes of intermediates under varying light, CO2 and O2. A principle novelty of the model for NADP-ME type species is the hypothesis that electrons transported into the BS chloro-plasts
via the malate shuttle enter the electron transport chain with the help of NAD(P)H-plastoquinone oxyreductase (NDH, or an
enzyme of similar function). In the model, the electrons from reduced plastoquinone pass through the Q-cycle and photosystem
I (PS I) only once, without cycling around PS I, as commonly assumed. With this the ratio of 2 ATP/NADPH, satisfying the energy
requirements process, is fixed, provided that 2 H+/e− are transported by the Q-cycle and 2 H+/e− by NDH, and 4 H+are utilized per ATP generated. The hypothesis is based on modeling results showing that there must be fine control of the
ATP/NADPH ratio in BS chloroplasts for optimum function of C4 photosynthesis. The CO2 concentrating function of NADP-ME type C4 photosynthesis, which occurs as the rate of the C4 cycle exceeds the rate of CO2 assimilation in BS cells (overcycling), can be explained on the basis of two processes. First, alternative consumption of
some ATP in BS chloroplasts to support other processes (e.g. starch and protein synthesis) reduces the ATP/NADPH ratio available
in BS. As a result, some CO2 imported into BS remains unassimilated and accumulates, resulting in overcycling back to the mesophyll. Second, the residual
photorespiratory activity alternatively consumes some ribulose 1,5-bisphosphate for oxygenation; as with the alternative consumption
of ATP, some CO2 imported into BS remains unassimilated and accumulates, causing overcycling. The CO2 evolved from photorespiration in BS also contributes to the CO2 pump in C4 plants.
The kinetic properties of photosynthesis (both transient and steady-state) were monitored using three non-invasive techniques to evaluate limitations on triose-phosphate (triose-P) conversion to carbohydrate in rice. These included analyzing the O2 sensitivity of CO2 fixation and the assimilatory charge (AC) using gas exchange (estimate of the ribulose 1,5- bisphosphate pool) and measuring Photosystem II activity by chlorophyll fluorescence analysis under varying light, temperature and CO2 partial pressures. Photosynthesis was inhibited transiently upon switching from 20 to 2 kPa O2 (reversed O2 sensitivity), the degree of which was correlated with a terminal, steady-state suppression of low O2 enhancement of photosynthesis. Under current ambient levels of CO2 and moderate to high light, the transient pattern was more obvious at 18 C than at 26 C while at 34 C no tra nsient response was observed. The transient inhibition at 18 C ranged from 15% to 31% depending on the pre-measurement temperature. This pattern, symptomatic of feedback, was observed with increasing light and CO2 partial pressures with the degree of feedback decreasing from moderate (18 C) up to high temperature (34 C). Under feedback conditions, the rate of assimilation is shifted from being photorespiration limited to being triose-P utilization limited. Transitory changes in CO2 assimilation rates (A) under low O2 indicative of feedback coincided with a transitory drop in assimilatory charge (AC) and inhibition of electron transport. In contrast to previous studies with many C3 species, our studies indicate that rice shows susceptibility to feedback inhibition under moderate temperatures and current atmospheric levels of CO2.
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.
A computer model of C3 photosynthesis comprising light reactions, electron-proton transport, enzymatic reactions and regulatory functions is presented
as a system of differential budget equations for intermediate compounds. Carbon and nitrite reduction systems are linked assuming
that nitrite reduction is the dominant proton-coupled alternative electron transport pathway compensating for ATP consumption
by starch synthesis and other non-photosynthetic processes. The principal theoretical hypothesis is that the carbon skeletons
for the freshly synthesized amino acids are partitioned from the pool of phosphoglyceric acid (PGA) before its reduction in
photosynthesis. Consequently, the rate of nitrite reduction is controlled by ferredoxin reduction and PGA levels. The latter
simultaneously controls the rate of starch synthesis — the major alternative ATP consumer linking nitrite reduction with starch
synthesis. The model reproduces light and CO2 response curves of photosynthesis, chlorophyll fluorescence and 810 nm transmittance signals during steady state, as well
as during induction and oscillations. The model explains the integral Nitrogen/Carbon (N/C) ratios of plant tissues and predicts
that the availability of nitrogen may limit the photosynthetic rate in natural communities.
In spinach (Spinacia oleracea) and barley (Hordeum vulgare) leaves, chlorophyll a fluorescence and O(2) evolution have been measured simultaneously following re-illumination after a dark interval or when steady state photosynthesis has been perturbed by changes in the gas phase. In high CO(2) concentrations, both O(2) and fluorescence can display marked dampening oscillations that are antiparallel but slightly out of phase (a rise or fall in fluorescence anticipating a corresponding fall or rise in O(2) by about 10 to 15 seconds). Infrared gas analysis measurements showed that CO(2) uptake behaved like O(2) evolution both in the period of oscillation (about 1 minute) and in its relation to fluorescence. In the steady state, oscillations were initiated by increases in CO(2) or by increases or decreases in O(2). Oscillations in O(2) or CO(2) did not occur without associated oscillations in fluorescence and the latter were a sensitive indicator of the former. The relationship between such oscillations in photosynthetic carbon assimilation and chlorophyl a fluorescence is discussed in the context of the effect of ATP or NADPH consumption on known quenching mechanisms.
Carbon dioxide and water vapor exchange of tomato (Lycopersicon esculentum Mill. cv Rheinlands Ruhm) leaves were measured before and after 24 h of soil flooding to characterize both stomatal and nonstomatal responses to the stress. Leaf epidermal conductance to water vapor decreased by 47% after flooding, accompanied by an increase in the sensitivity of stomata to changes in CO(2) concentration. Assimilation rates under ambient conditions fell by 27%, and the inhibition could not be overcome by elevated CO(2) partial pressures. Stomatal conductance limited the assimilation rate to approximately the same degree both before and after flooding. The reduction in photosynthetic capacity was not due to a decrease in apparent quantum yield or to an increase in photorespiration. The results were analyzed according to a recent model of photosynthesis, and possible mechanisms underlying the flooding effect are discussed.
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.
The occurrence of O(2)-insensitive photosynthesis at high quantum flux and moderate temperature in Spinacia oleracea was characterized by analytical gas exchange measurements on intact leaves. In addition photosynthetic metabolite pools were measured in leaves which had been rapidly frozen under defined gas conditions. Upon switching to low O(2) in O(2)-insensitive conditions the ATP/ADP ratio fell dramatically within one minute. The P-glycerate pool increased over the same time. Ribulose bisphosphate initially declined, then increased and exceeded the pool size measured in air. The pools of hexose monophosphates and UDPglucose were higher at a partial pressure of O(2) of 21 millibars than at 210 millibars. These results are consistent with the hypothesis that the rate of sucrose synthesis limited the overall rate of assimilation under O(2)-insensitive conditions.
Photosynthesis of C(3) plants is occasionally inhibited upon switching from normal to low partial pressure of O(2). Leaves of Solanum tuberosum exhibited this effect reproducibly under saturating light and 700 microbars of CO(2). We determined the partitioning of recent photosynthate between starch and sucrose and measured the concentration of hexose monophosphates in the stroma and cytosol after nonaqueous fractionation. The reduction in the rate of photosynthesis upon switching to low partial pressure of O(2) was caused by reduced starch synthesis. The concentration of hexose monophosphates in the stroma fell and the glucose 6-phosphate to fructose 6-phosphate to fructose 6-phosphate ratio fell from 2.7 to 1.3, indicating an inhibition of phosphoglucoisomerase as described by K-J Dietz ([1985] Biochim Biophys Acta 839: 240-248). The concentration of hexose monophosphates in the cytosol increased, ruling out a sucrose synthesis limitation by reduced transport from the chloroplast as the explanation for low O(2) inhibition of photosynthesis.
Oxygen may enhance CO2-saturated photosynthesis in intact leaves, which display the Warburg effect when illuminated at the current atmospheric level of CO2 and O2, of about 350 μl l−1 and 21%, respectively. The magnitude of the stimulation depends on irradiance. The K
M(O2) of the stimulation is 128 μM (10.6% O2). Maximum enhancement in wheat leaves is 6.1 and 5.3 μmol m−2 s−1 under 27.9 and 18.7 mW cm−2, respectively, corresponding to a 25–30% increase in the ribulose 1,5-bisphosphate (RuBP) turnover rate if compared with O2-free ambient gas phase. The stimulation appears in 5–10 s after a sharp increase in O2. In response to a decrease in O2, the new stabilized rate is reached in 5–7 min. The stimulation does not involve any increase in the activity of Rubisco. The effect correlates with increased concentration of RuBP. Oxygen enhances CO2-saturated photosynthesis by acting as a terminal electron acceptor in the photosynthetic electron transport. The magnitude of the effect may be adopted as an index of the pseudocyclic photophosphorylation in vivo.
Sunflower (Helianthus annuus L.) and tobacco (Nicotiana tabacum L.) were grown in the laboratory and leaves were taken from field-grown birch trees (Betula pendula Roth). Chlorophyll fluorescence, CO2 uptake and O2 evolution were measured and electron transport rates were calculated, JC from the CO2 uptake rate considering ribulose-1,5-bisphosphate (RuBP) carboxylation and oxygenation, JO from the O2 evolution rate, and JF from Chl fluorescence parameters. Mesophyll diffusion resistance, rmd, used for the calculation of JC, was determined such that the in vivo Rubisco kinetic curve with respect to the carboxylation site CO2 concentration became a rectangular hyperbola with Km(CO2) of 10 μM at 22.5°C. In sunflower, in the absence of external O2, JO = 1.07JC when absorbed photon flux density (PAD) was varied, showing that the O2-independent components of the alternative electron flow to acceptors other than CO2 made up 7% of JC. Under saturating light, JF, however, was 20–30% faster than JC, and JF − JC depended little on CO2 and O2 concentrations. The inter-relationship between JF − JC and non-photochemical quenching (NPQ) was variable, dependent on the CO2 concentration. We conclude that the relatively fast electron flow JF − JC appearing at light saturation of photosynthesis contains a minor component coupled with proton translocation, serving for
nitrite, oxaloacetate and oxygen reduction, and a major component that is mostly cyclic electron transport around PSII. The
rate of the PSII cycle is sufficient to release the excess excitation pressure on PSII significantly. Although the O2-dependent Mehler-type alternative electron flow appeared to be under the detection threshold, its importance is discussed
considering the documented enhancement of photosynthesis by oxygen.
A computer model comprising light reactions, electron-proton transport, enzymatic reactions, and regulatory functions of C3 photosynthesis has been developed as a system of differential budget equations for intermediate compounds. The emphasis is on electron transport through PSII and PSI and on the modeling of Chl fluorescence and 810 nm absorptance signals. Non-photochemical quenching of PSII excitation is controlled by lumenal pH. Alternative electron transport is modeled as the Mehler type O2 reduction plus the malate-oxaloacetate shuttle based on the chloroplast malate dehydrogenase. Carbon reduction enzymes are redox-controlled by the ferredoxin-thioredoxin system, sucrose synthesis is controlled by the fructose 2,6-bisphosphate inhibition of cytosolic FBPase, and starch synthesis is controlled by ADP-glucose pyrophosphorylase. Photorespiratory glycolate pathway is included in an integrated way, sufficient to reproduce steady-state rates of photorespiration. Rate-equations are designed on principles of multisubstrate-multiproduct enzyme kinetics. The parameters of the model were adopted from literature or were estimated from fitting the photosynthetic rate and pool sizes to experimental data. The model provided good simulations for steady-state photosynthesis, Chl fluorescence, and 810 nm transmittance signals under varying light, CO2 and O2 concentrations, as well as for the transients of post-illumination CO2 uptake, Chl fluorescence induction and the 810 nm signal. The modeling shows that the present understanding of photosynthesis incorporated in the model is basically correct, but still insufficient to reproduce the dark-light induction of photosynthesis, the time kinetics of non-photochemical quenching, 'photosynthetic control' of plastoquinone oxidation, cyclic electron flow around PSI, oscillations in photosynthesis. The model may find application for predicting the results of gene transformations, the analysis of kinetic experimental data, the training of students.
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