Evaluation of the light/dark C assay of photorespiration: tobacco leaf disk studies with glycidate and glyoxylate

Preincubation of illuminated tobacco (Nicotiana tabacum L.) leaf disks in glycidate (2,3-epoxypropionate) or glyoxylate inhibited photorespiration by about 40% as determined by the ratio of ¹⁴CO₂ evolved into CO₂-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark ¹⁴C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO₂ exchange parameters, including the CO₂ compensation concentration in 21% O₂, the inhibitory effect of 21% O₂ on net photosynthesis in 360 microliters per liter of CO₂ and the rate of net photosynthetic ¹⁴CO₂ uptake in air.

The effects of both glycidate and glyoxylate on the ¹⁴C assay are inconsistent with other measures of photorespiratory CO₂ exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of ¹⁴CO₂ efflux as an assay for relative rates of photorespiration (Zelitch 1968, Plant Physiol 43: 1829-1837). The results of this study specifically indicate that neither glycidate nor glyoxylate reduces photorespiration or stimulates net photosynthesis by tobacco leaf disks under physiological conditions of pO₂ and pCO₂, contrary to previous reports.

     

Variation in photorespiration. The effect of genetic differences in photorespiration on net photosynthesis in tobacco

The hypothesis that net photosynthesis is diminished in many plant species because of a high rate of CO₂ evolution in the light has been tested further. High rates of CO₂ output in CO₂-free air in comparison with dark respiration were found in Chlamydomonas reinhardi, wheat leaves, tomato leaves, and to a lesser extent in Chlorella pyrenoidosa by means of the ¹⁴C-photorespiration assay. In tobacco leaves high photorespiration was characteristic of a standard variety, Havana Seed, and a possibly still higher rate was found in a yellow heterozygous mutant, JWB Mutant. However, the dark homozygous sibling of the latter, JWB Wild, had a low photorespiration for the tobacco species. The relative rates of photorespiration were in the same sequence when measured by the ¹⁴CO₂ released in normal air from leaf disks supplied with glycolate-1-¹⁴C in the light.

As would be predicted by the hypothesis, the maximal net rate of photosynthesis at 300 ppm of CO₂ in the air in JWB Wild leaves was greater (24%) than in Havana Seed, while JWB Mutant had less CO₂ uptake than the standard variety (21%). At 550 ppm of CO₂ the differences in net photosynthesis were not as great between the 2 siblings as at 200 ppm. The relative leaf expansion rates of seedlings of the 3 tobacco varieties in a greenhouse had the same relationship as their rates of CO₂ assimilation.

Thus within the tobacco species, as in a comparison between tobacco and maize, low photorespiratory CO₂ evolution was correlated with higher photosynthetic efficiency. Therefore it seems that increased CO₂ uptake should be achieved by genetic interference with the process of photorespiration.

     

Metabolism of <Superscript>14</Superscript>C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of ¹⁴C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO₂ concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO₂), true photosynthesis (at 0.03% CO₂), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO₂ evolution resulting from metabolization of exogenous ¹⁴C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both ¹⁴C-glycine absorption and ¹⁴CO₂ evolution were lower in the light as compared with darkness, which is explained by CO₂ reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO₂ evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO₂ evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO₂ evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.     

The effect on net photosynthesis of pedigree selection for low and high rates of photorespiration in tobacco

A normal appearing plant with a low rate of photorespiration (ratio of ¹⁴CO₂ released light/dark = 1.6) was found in an unselected tobacco (Nicotiana tabacum) cultivar. The plant was self-pollinated, and further selections were made on several successive generations. Excised leaves from the progeny of the selections were examined for photorespiration and net CO₂ assimilation in normal air during photosynthesis. Similar measurements were made of plants derived from selfed parents with high rates of photorespiration (ratio of ¹⁴CO₂ released light/dark = 3.0 or greater). Efficient photosynthetic plants (greater than 22.0 mg of CO₂ dm⁻² hr⁻¹) with low rates of photorespiration produced a larger proportion of efficient progeny (about 25%) than did selfing inefficient plants (about 6%), but this proportion did not increase in successive generations.     

Photosynthesis, Photorespiration and Respiration of Detached Spruce Twigs as Influenced by Oxygen Concentration and Light Intensity

The effects of oxygen concentration and light intensity on the rates of apparent photosynthesis, true photosynthesis, photorespiration and dark respiration of detached spruce twigs were determined by means of an infra-red carbon dioxide analyzer (IRCA). A closed circuit system IRCA was filled with either 1 per cent of oxygen in nitrogen, air (21 % O₂) or pure oxygen (100 % O₂). Two light intensities 30 × 10³ erg · cm ^?2· s^?1 and 120 × 10³ erg · cm^?2· s^?1 were applied. It has been found that the inhibitory effect of high concentration of oxygen on the apparent photosynthesis was mainly a result of a stimulation of the rate of CO₂ production in light (photorespiration). In the atmosphere of 100 % O₂, photorespiration accounts for 66–80 per cent of total CO₂ uptake (true photosynthesis). Owing to a strong acceleration of photorespiration by high oxygen concentrations, the rate of true photosynthesis calculated as the sum of apparent photosynthesis and photorespiration was by several times less inhibited by oxygen than the rate of apparent photosynthesis. The rates of dark respiration were essentially unaffected by the oxygen concentrations used in the experiments. An increase in the intensity of light from 30 × 10³ erg · cm^?3· s^?1 to 120 · 10³ erg · cm^?2· s^?1 enhanced the rate of photorespiration in the atmospheres of 21 and 100 % oxygen but not in 1 % O₂. The rate of apparent photosynthesis, however, was little affected by light intensity in an atmosphere of 1 % oxygen.     

Peroxisomal malate dehydrogenase is not essential for photorespiration in Arabidopsis but its absence causes an increase in the stoichiometry of photorespiratory CO2 release

Peroxisomes are important for recycling carbon and nitrogen that would otherwise be lost during photorespiration. The reduction of hydroxypyruvate to glycerate catalyzed by hydroxypyruvate reductase (HPR) in the peroxisomes is thought to be facilitated by the production of NADH by peroxisomal malate dehydrogenase (PMDH). PMDH, which is encoded by two genes in Arabidopsis (Arabidopsis thaliana), reduces NAD⁺ to NADH via the oxidation of malate supplied from the cytoplasm to oxaloacetate. A double mutant lacking the expression of both PMDH genes was viable in air and had rates of photosynthesis only slightly lower than in the wild type. This is in contrast to other photorespiratory mutants, which have severely reduced rates of photosynthesis and require high CO₂ to grow. The pmdh mutant had a higher O₂-dependent CO₂ compensation point than the wild type, implying that either Rubisco specificity had changed or that the rate of CO₂ released per Rubisco oxygenation was increased in the pmdh plants. Rates of gross O₂ evolution and uptake were similar in the pmdh and wild-type plants, indicating that chloroplast linear electron transport and photorespiratory O₂ uptake were similar between genotypes. The CO₂ postillumination burst and the rate of CO₂ released during photorespiration were both greater in the pmdh mutant compared with the wild type, suggesting that the ratio of photorespiratory CO₂ release to Rubisco oxygenation was altered in the pmdh mutant. Without PMDH in the peroxisome, the CO₂ released per Rubisco oxygenation reaction can be increased by over 50%. In summary, PMDH is essential for maintaining optimal rates of photorespiration in air; however, in its absence, significant rates of photorespiration are still possible, indicating that there are additional mechanisms for supplying reductant to the peroxisomal HPR reaction or that the HPR reaction is altogether circumvented.     

Photorespiration and Glycolate Metabolism: A Re-examination and Correlation of Some Previous Studies

Some previous studies of photorespiration and glycolate oxidation were re-examined and correlated by infra-red CO₂ analysis. Data about rate of photosynthesis and oxygen sensitivity indicated that complete inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1 dimethyl urea (DCMU) allowed dark respiration to continue in the light. Photorespiration was also inhibited. The oxygen sensitivity of glycolate-stimulated CO₂ production was found to be compatible with the proposal that glycolate is a substrate of photorespiration. Both `in vivo' and `in vitro' studies of the alga Nitella flexilis have revealed a pathway of glycolate oxidation similar to that of higher plants. DCMU inhibition of photosynthesis by Nitella gave results similar to those for the monocotyledons tested. Under very low light intensity, carbon dioxide compensation in corn was measurable but was not sensitive to high oxygen concentration. It appears that the lack of photorespiration in this plant is not the end result of efficient internal recycling of CO₂ to photosynthesis.     

Erratum

Glycolate synthesis was inhibited 40–50% in illuminated tobacco leaf disks, which have rapid rates of photorespiration, when floated on 20 mm potassium glycidate (2,3-epoxypropionate), an epoxide similar in structure to glycolate. The inhibitor also decreased the release of photorespiratory CO₂ about 40%, and the specificity of glycidate was demonstrated by the 40–50% increase in rate of photosynthetic CO₂ uptake observed in its presence. The importance of glycolate synthesis and metabolism in the production of photorespiratory CO₂ and the role of glycolate in diminishing net photosynthesis in species with rapid rates of photorespiration was thus further confirmed. L-(or 2S)-Glycidate was slightly more active than DL-glycidate, but glycidate was more effective as a specific inhibitor in leaf tissue than several other epoxide analogs of glycolate examined. The products of photosynthetic ¹⁴O₂ fixation after 3 or 4 min of uptake were proportionately altered in the presence of glycidate, and the specific radioactivity of the [¹⁴C]glycolate produced was closer to that of the ¹⁴CO₂ supplied. Glycidate inhibited glycolate synthesis in tobacco leaf disks irreversibly, since the degree of inhibition was the same for at least 2 hr after the inhibitor solution was removed. Glycidate also blocked glycolate synthesis in maize leaf disks, tissue with low rates of photorespiration, but large increases in net photosynthesis were not observed in maize with glycidate, because glycolate synthesis is normally only about 10% as rapid in maize as in tobacco. The demonstration of increases in net photosynthesis of 40–50% when glycolate synthesis (and photorespiration) is blocked with glycidate indicates in an independent manner that the biochemical or genetic control of photorespiration should permit large increases in plant productivity in plant species possessing rapid rates of photorespiration.     

Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms

The submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal exhibited different photosynthetic pulse-chase labeling patterns. In Hydrilla, over 50% of the ¹⁴C was initially in malate and aspartate, but the fate of the malate depended upon the photorespiratory state of the plant. In low photorespiration Hydrilla, malate label decreased rapidly during an unlabeled chase, whereas labeling of sucrose and starch increased. In contrast, for high photorespiration Hydrilla, malate labeling continued to increase during a 2-hour chase. Thus, malate formation occurs in both photorespiratory states, but reduced photorespiration results when this malate is utilized in the light. Unlike Hydrilla, in low photorespiration Myriophyllum, ¹⁴C incorporation was via the Calvin cycle, and less than 10% was in C₄ acids.

Ethoxyzolamide, a carbonic anhydrase inhibitor and a repressor of the low photorespiratory state, increased the label in glycolate, glycine, and serine of Myriophyllum. Isonicotinic acid hydrazide increased glycine labeling of low photorespiration Myriophyllum from 14 to 25%, and from 12 to 48% with high photorespiration plants. Similar trends were observed with Hydrilla. Increasing O₂ increased the per cent [¹⁴C]glycine and the O₂ inhibition of photosynthesis in Myriophyllum. In low photorespiration Myriophyllum, glycine labeling and O₂ inhibition of photosynthesis were independent of the CO₂ level, but in high photorespiration plants the O₂ inhibition was competitively decreased by CO₂. Thus, in low but not high photorespiration plants, glycine labeling and O₂ inhibition appeared to be uncoupled from the external [O₂]/[CO₂] ratio.

These data indicate that the low photorespiratory states of Hydrilla and Myriophyllum are mediated by different mechanisms, the former being C₄-like, while the latter resembles that of low CO₂-grown algae. Both may require carbonic anhydrase to enhance the use of inorganic carbon for reducing photorespiration.

     

A Re-evaluation of Soybean Leaf Photorespiration

Recalculations of soybean photorespiration indicate that mean rates are closer to 16.1 than 5.6 milligrams of CO₂ per square decimeter per hour as previously reported. Photorespiration of soybean thus amounts to at least a 30% carbon turnover of light-saturated photosynthesis. Photorespiration showed no significant relationship to net photosynthesis. Negative correlations were found between CO₂ efflux and stomatal resistance as well as between corrected photorespiration and residual intracellular resistance of the leaf to CO₂ uptake.     

Intraspecific measurements of photorespiration

The relative magnitudes of (a) CO₂ compensation concentration, (b) zero CO₂ intercept of the CO₂ response curve, (c) O₂ suppression of net photosynthesis, (d) differential ¹²CO₂ and ¹⁴CO₂ uptake, and (e) ¹⁴CO₂ efflux into CO₂-free air were determined in the dry bean (Phaseolus vulgaris L.) varieties Michelite-62 (M-62) and Red Kidney (RK). In comparing the two varieties for each of the above processes, there were three categories of response, M-62 > RK, M-62 = RK, and M-62 < RK. Since these processes did not give the same relative difference for the two varieties being studied, it was concluded that these phenomena cannot validly be used to estimate the magnitude of photorespiration, although they do identify its presence. The results suggest that photorespiration is but one component of O₂ inhibition of net photosynthesis and that photorespiration itself has two or more component metabolic pathways.     

Effect of Glyoxylate on the Sensitivity of Net Photosynthesis to Oxygen (the Warburg Effect) in Tobacco

The addition of glyoxylate to tobacco (Nicotiana tabacum) leaf discs inhibited glycolate synthesis and photorespiration and increased net photosynthetic ¹⁴CO₂ fixation. This inhibition of photorespiration was investigated further by studying the effect of glyoxylate on the stimulation of photosynthesis that occurs when the atmospheric O₂ level was decreased from 21 to 3% (the Warburg effect). The Warburg effect is usually ascribed to the increased glycolate synthesis and metabolism that occurs at higher O₂ concentrations. Photosynthesis in control discs increased from 59.1 to 94.7 micromoles of CO₂ per gram fresh weight per hour (a 60% increase) when the O₂ level was lowered from 21 to 3%, while the rate for discs floated on 15 millimolar glyoxylate increased only from 82.0 to 99.7 micromoles of CO₂ per gram fresh weight per hour (a 22% increase). The decrease in the O₂ sensitivity of photosynthesis in the presence of glyoxylate was explained by changes in the rate of glycolate synthesis under the same conditions.

The rate of metabolism of the added glyoxylate by tobacco leaf discs was about 1.35 micromoles per gram fresh weight per hour and was not dependent on the O₂ concentration in the atmosphere. This rate of metabolism is about 10% the amount of stimulation in the rate of CO₂ fixation caused by the glyoxylate treatment on a molar carbon basis. Glyoxylate (10 millimolar) had no effect on the carboxylase/oxygenase activity of isolated ribulose diphosphate carboxylase. Although the biochemical mechanism by which glyoxylate inhibits glycolate synthesis and photorespiration and thereby decreases the Warburg effect is still uncertain, these results show that cellular metabolites can regulate the extent of the Warburg effect.

     

Chemical inhibition of the glycolate pathway in soybean leaf cells

Isolated soybean (Glycine max [L.] Merr.) leaf cells were treated with three inhibitors of the glycolate pathway in order to evaluate the potential of such inhibitors for increasing photosynthetic efficiency. Preincubation of cells under acid conditions in α-hydroxypyridinemethanesulfonic acid increased ¹⁴CO₂ incorporation into glycolate, but severely inhibited photosynthesis. Isonicotinic acid hydrazide (INH) increased the incorporation of ¹⁴CO₂ into glycine and reduced label in serine, glycerate, and starch. Butyl 2-hydroxy-3-butynoate (BHB) completely and irreversibly inhibited glycolate oxidase and increased the accumulation of ¹⁴C into glycolate. Concomitant with glycolate accumulation was the reduction of label in serine, glycerate, and starch, and the elimination of label in glycine. The inhibitors INH and BHB did not eliminate serine synthesis, suggesting that some serine is synthesized by an alternate pathway. The per cent incorporation of ¹⁴CO₂ into glycolate by BHB-treated cells or glycine by INH-treated cells was determined by the O₂/CO₂ ratio present during assay. Photosynthesis rate was not affected by INH or BHB in the absence of O₂, but these compounds increased the O₂ inhibition of photosynthesis. This finding suggests that the function of the photorespiratory pathway is to recycle glycolate carbon back into the Calvin cycle, so if glycolate metabolism is inhibited, Calvin cycle intermediates become depleted and photosynthesis is decreased. Thus, chemicals which inhibit glycolate metabolism do not reduce photorespiration and increase photosynthetic efficiency, but rather exacerbate the problem of photorespiration.     

Comparison of the Oxygen Exchange between Photosynthetic Cell Suspensions and Detached Leaves of Euphorbia characias L

Using a mass-spectrometric ¹⁶O₂/¹⁸O₂-isotope technique, we compared the nature and the relative importance of oxygen exchange in photomixotrophic (PM) and photoautotrophic (PA) suspensions of Euphorbia characias L. with those in intact leaves of the same species. Young and mature leaves, dividing and nondividing cell suspensions were characterized in short-term experiments. On chlorophyll basis, the gross photosynthetic activities at CO₂ saturating concentration of PA and PM suspensions varied little from those of leaves. On dry weight basis, gross photosynthesis of PA suspensions was equal to that of leaves because of their similar chlorophyll content. This was not the case in PM suspensions where gross photosynthesis was lower and largely varied during the growth cycle. The CO₂ compensation point of PA cells (155-265 parts per million) was much higher than that of leaves (50-80 ppm). Oxygen uptakes were analyzed in terms of mitochondrial respiration, photorespiration and light stimulation of oxygen uptake (LSOU), often identified to Mehlertype reactions. In PA and PM suspensions, mitochondrial respiration rates were higher than in leaves by a factor of 1.5 to 4.5. In PM suspensions, photorespiration and LSOU were observed only in nondividing cells. Photorespiration and LSOU rates were comparable in PA suspensions and leaves. Our results demonstrate that photorespiration of PA suspensions has not been affected by the 2% CO₂ concentration imposed during 2 years of culture.     

Photosynthesis, photorespiration and productivity of wheat and soybean genotypes

The results of the numerous measurements obtained during the last 40 years on gas exchange rate, photosynthetic carbon metabolism by exposition in ¹⁴CO₂ and activities of primary carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RuBPC/O), in various wheat and soybean genotypes grown over a wide area in the field and contrasting in photosynthetic traits and productivity are presented in this article. It was established that high productive wheat genotypes (7–9 t ha^?1) with the optimal architectonics possess higher rate of CO₂ assimilation during the leaf ontogenesis. Along with the high rate of photosynthesis, high values of photorespiration are characteristic for the high productive genotypes. Genotypes with moderate (4–5 t ha^?1) and low (3 t ha^?1) grain yield are characterized by relatively low rates of both CO₂ assimilation and photorespiration. A value of photorespiration constitutes 28–35% of photosynthetic rate in contrasting genotypes. The activities of RuBPC and RuBPO were changing in a similar way in the course of the flag leaf and ear elements development. High productive genotypes are also characterized by a higher rate of biosynthesis and total value of glycine–serine and a higher photosynthetic rate. Therefore, contrary to conception arisen during many years on the wastefulness of photorespiration, taking into account the versatile investigations on different aspects of photorespiration, it was proved that photorespiration is one of the evolutionarily developed vital metabolic processes in plants and the attempts to reduce this process with the purpose of increasing the crop productivity are inconsistent.     

Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature

An open system associated with an infrared gas analyzer was employed to study transients in CO₂ exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO₂ exchange rate. A computer was used to evaluate parameters of the leaf CO₂ release rate to provide an estimate of the initial rate of postillumination CO₂ evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O₂, the decline in rate of CO₂ evolution upon darkening followed first order kinetics. Initial rates of CO₂ evolution following darkening were relatively independent of the prior ambient CO₂ concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO₂ concentration at 21% O₂. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO₂ fixation rate at 32°C and high irradiance. The rapid initial CO₂ evolution observed upon darkening at 21% O₂ was absent in 3% O₂. Rates of photorespiration under normal atmospheric concentrations of CO₂ and O₂ as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C₃ leaf tissue.     

The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells

The effect of pH, O₂ concentration, and temperature on the CO₂ compensation point (Г[CO₂]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO₂ depending upon experimental conditions. The Г(CO₂) increases with increasing temperature. The rate of increase is dependent upon the O₂ concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O₂ concentrations. The differential effect of temperature on Г(CO₂) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O₂-sensitive component of Г(CO₂) remains constant over this range. The Г(CO₂) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O₂ concentration. It is suggested that the pH-dependence of Г(CO₂) is related to the ability of the cell to take up CO₂ from the aqueous environment. The correlation between high HCO₃⁻ concentrations and low Г(CO₂) at alkaline pH indicates that extracellular HCO₃⁻ facilitates the uptake of CO₂, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O₂− and temperature-dependence of Г(CO₂) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO₂ to a level sufficient to reduce or suppress photorespiration.     

Modelling O2 and O2 unidirectional fluxes in plants. III: Fitting of experimental data by a simple model

Photosynthetic assimilation of CO₂ in plants results in the balance between the photochemical energy developed by light in chloroplasts, and the consumption of that energy by the oxygenation processes, mainly the photorespiration in C₃ plants. The analysis of classical biological models shows the difficulties to bring to fore the oxygenation rate due to the photorespiration pathway. As for other parameters, the most important key point is the estimation of the electron transport rate (ETR or J), i.e. the flux of biochemical energy, which is shared between the reductive and oxidative cycles of carbon. The only reliable method to quantify the linear electron flux responsible for the production of reductive energy is to directly measure the O₂ evolution by ¹⁸O₂ labelling and mass spectrometry. The hypothesis that the respective rates of reductive and oxidative cycles of carbon are only determined by the kinetic parameters of Rubisco, the respective concentrations of CO₂ and O₂ at the Rubisco site and the available electron transport rate, ultimately leads to propose new expressions of biochemical model equations. The modelling of ¹⁸O₂ and ¹⁶O₂ unidirectional fluxes in plants shows that a simple model can fit the photosynthetic and photorespiration exchanges for a wide range of environmental conditions. Its originality is to express the carboxylation and the oxygenation as a function of external gas concentrations, by the definition of a plant specificity factor Sp that mimics the internal reactions of Rubisco in plants. The difference between the specificity factors of plant (Sp) and of Rubisco (Sr) is directly related to the conductance values to CO₂ transfer between the atmosphere and the Rubisco site. This clearly illustrates that the values and the variation of conductance are much more important, in higher C₃ plants, than the small variations of the Rubisco specificity factor. The simple model systematically expresses the reciprocal variations of carboxylation and oxygenation exchanges illustrated by a “mirror effect”. It explains the protective sink effect of photorespiration, e.g. during water stress. The importance of the CO₂ compensation point, in classical models, is reduced at the benefit of the crossing points Cx and Ox, concentration values where carboxylation and oxygenation are equal or where the gross O₂ uptake is half of the gross O₂ evolution. This concept is useful to illustrate the feedback effects of photorespiration in the atmosphere regulation. The constancy of Sp and of Cx for a great variation of P under several irradiance levels shows that the regulation of the conductance maintains constant the internal CO₂ and the ratio of photorespiration to photosynthesis (PR/P). The maintenance of the ratio PR/P, in conditions of which PR could be reduced and the carboxylation increased, reinforces the hypothesis of a positive role of photorespiration and its involvement in the plant-atmosphere co-evolution.     

Increased rate of net photosynthetic carbon dioxide uptake caused by the inhibition of glycolate oxidase

There is considerable variation among species in their rate of photorespiration, and photorespiration increases greatly at higher temperatures. The addition of an inhibitor of glycolate oxidase, α-hydroxy-2-pyridinemethanesulfonic acid, to tobacco leaf disks at 35° stimulated photosynthetic ¹⁴CO₂ uptake at least 3-fold, but ¹⁴CO₂ uptake was not changed by the inhibitor at 25°. The inhibitor did not increase photosynthesis in maize leaf disks at either temperature.