Photorespiratory phenomena in maize: oxygen uptake, isotope discrimination, and carbon dioxide efflux

13CO2 labeling kinetics in maize reveal impaired efficiency of C4 photosynthesis under low irradiance

C4 photosynthesis allows faster photosynthetic rates and higher water and nitrogen use efficiency than C3 photosynthesis, but at the cost of lower quantum yield due to the energy requirement of its biochemical carbon concentration mechanism. It has also been suspected that its operation may be impaired in low irradiance. To investigate fluxes under moderate and low irradiance, maize (Zea mays) was grown at 550 µmol photons m-2 s-l and 13CO2 pulse-labeling was performed at growth irradiance or several hours after transfer to 160 µmol photons m-2 s-1. Analysis by liquid chromatography/tandem mass spectrometry or gas chromatography/mass spectrometry provided information about pool size and labeling kinetics for 32 metabolites and allowed estimation of flux at many steps in C4 photosynthesis. The results highlighted several sources of inefficiency in low light. These included excess flux at phosphoenolpyruvate carboxylase, restriction of decarboxylation by NADP-malic enzyme, and a shift to increased CO2 incorporation into aspartate, less effective use of metabolite pools to drive intercellular shuttles, and higher relative and absolute rates of photorespiration. The latter provides evidence for a lower bundle sheath CO2 concentration in low irradiance, implying that operation of the CO2 concentration mechanism is impaired in this condition. The analyses also revealed rapid exchange of carbon between the Calvin-Benson cycle and the CO2-concentration shuttle, which allows rapid adjustment of the balance between CO2 concentration and assimilation, and accumulation of large amounts of photorespiratory intermediates in low light that provides a major carbon reservoir to build up C4 metabolite pools when irradiance increases.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Integrated operation of the photorespiratory cycle and cytosolic metabolism in the modulation of primary nitrogen assimilation and export of organic N-transport compounds from leaves: a hypothesis

Photorespiration is generally considered to be an essentially dissipative process, although it performs some protective and essential functions. A theoretical appraisal indicates that the loss of freshly assimilated CO2 due to photorespiration in well-watered plants may not be as high as generally believed. Even under moderately adverse conditions, these losses may not exceed 10%. The photorespiratory metabolism of the source leaves of well-watered and well-nourished crop plants ought to be different from that of other leaves because the fluxes of the export of both carbohydrates and organic N-transport compounds in source leaves is quite high. With a heuristic approach that involved the dovetailing of certain metabolic steps with the photorespiratory cycle (PR-cycle), a novel network is proposed to operate in the source-leaves of well-watered and well-nourished plants. This network allows for the diversion of metabolites from their cyclic-routes in sizeable quantities. With the removal of considerable quantities of glycine and serine from the cyclic route, the number of RuBP oxygenation events would be several times those of the formation of hydroxypyruvate. Thus, to an extreme extent, photorespiratory metabolism would become open-ended and involve much less futile recycling of glycine and serine. Conversion of glyoxylate to glycine has been proposed to be a crucial step in the determination of the relative rates of the futile (cyclic) and anabolic (open-ended) routes. Thus, in the source leaves of well-watered and well-nourished plants, the importance of the cyclic route is limited to the salvaging of photorespiratory intermediates for the regeneration of RuBP. The proposed network is resilient enough to coordinate the rates of the assimilation of carbon and nitrogen in accordance with the moisture and N-fertility statuses of the soil.

Zea mays iRS1563: a comprehensive genome-scale metabolic reconstruction of maize metabolism

The scope and breadth of genome-scale metabolic reconstructions have continued to expand over the last decade. Herein, we introduce a genome-scale model for a plant with direct applications to food and bioenergy production (i.e., maize). Maize annotation is still underway, which introduces significant challenges in the association of metabolic functions to genes. The developed model is designed to meet rigorous standards on gene-protein-reaction (GPR) associations, elementally and charged balanced reactions and a biomass reaction abstracting the relative contribution of all biomass constituents. The metabolic network contains 1,563 genes and 1,825 metabolites involved in 1,985 reactions from primary and secondary maize metabolism. For approximately 42% of the reactions direct literature evidence for the participation of the reaction in maize was found. As many as 445 reactions and 369 metabolites are unique to the maize model compared to the AraGEM model for A. thaliana. 674 metabolites and 893 reactions are present in Zea mays iRS1563 that are not accounted for in maize C4GEM. All reactions are elementally and charged balanced and localized into six different compartments (i.e., cytoplasm, mitochondrion, plastid, peroxisome, vacuole and extracellular). GPR associations are also established based on the functional annotation information and homology prediction accounting for monofunctional, multifunctional and multimeric proteins, isozymes and protein complexes. We describe results from performing flux balance analysis under different physiological conditions, (i.e., photosynthesis, photorespiration and respiration) of a C4 plant and also explore model predictions against experimental observations for two naturally occurring mutants (i.e., bm1 and bm3). The developed model corresponds to the largest and more complete to-date effort at cataloguing metabolism for a plant species.

High glycolate oxidase activity is required for survival of maize in normal air

A mutant in the maize (Zea mays) Glycolate Oxidase1 (GO1) gene was characterized to investigate the role of photorespiration in C4 photosynthesis. An Activator-induced allele of GO1 conditioned a seedling lethal phenotype when homozygous and had 5% to 10% of wild-type GO activity. Growth of seedlings in high CO2 (1%-5%) was sufficient to rescue the mutant phenotype. Upon transfer to normal air, the go1 mutant became necrotic within 7 d and plants died within 15 d. Providing [1-14C]glycolate to leaf tissue of go1 mutants in darkness confirmed that the substrate is inefficiently converted to 14CO2, but both wild-type and GO-deficient mutant seedlings metabolized [1-14C]glycine similarly to produce [14C]serine and 14CO2 in a 1:1 ratio, suggesting that the photorespiratory pathway is otherwise normal in the mutant. The net CO2 assimilation rate in wild-type leaves was only slightly inhibited in 50% O2 in high light but decreased rapidly and linearly with time in leaves with low GO. When go1 mutants were shifted from high CO2 to air in light, they accumulated glycolate linearly for 6 h to levels 7-fold higher than wild type and 11-fold higher after 25 h. These studies show that C4 photosynthesis in maize is dependent on photorespiration throughout seedling development and support the view that the carbon oxidation pathway evolved to prevent accumulation of toxic glycolate.

Photorespiratory rates in wheat and maize as determined by o-labeling

A method was devised to quantify short-term photorespiratory rates in terrestrial plants using (18)O-intermediates of the glycolate pathway, specifically glycolate, glycine, and serine. The pathway intermediates were isolated and analyzed on a GC/MS to determine molecular percent (18)O-enrichment. Rates of glycolate synthesis were determined from (18)O-labeling kinetics of the intermediates, derived rate equations, and nonlinear regression techniques. Glycolate synthesis in wheat (Triticum aestivum L.), a C(3) plant, and maize (Zea mays L.), a C(4) plant, was stimulated by high O(2) concentrations and inhibited by high CO(2) concentrations. The synthesis rates were 7.3, 2.1, and 0.7 micromoles per square decimeter per minute under a 21% O(2) and 0.035% CO(2) atmosphere for leaf tissue of wheat, maize seedlings, and 3-month-old maize, respectively. Photorespiratory CO(2) evolution rates were estimated to be 27, 6, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue support the hypothesis that C(4) plants photorespire, albeit at a reduced rate in comparison to C(3) plants, and that the CO(2)/O(2) ratio in the bundle sheath of maize is higher in mature tissue than in seedling tissue. The pool size of the three photorespiratory intermediates remained constant and were unaffected by changes in either CO(2) or O(2) concentrations throughout the 10-minute labeling period. This suggests that photorespiratory metabolism is regulated by other mechanism besides phosphoglycolate synthesis by ribulose-1,5-bisphosphate carboxylase/oxygenase, at least under short-term conditions. Other mechanisms could be alternate modes of synthesis of the intermediates, regulation of some of the enzymes of the photorespiratory pathway, or regulation of carbon flow between organelles involved in photorespiration. The glycolate pool became nearly 100% (18)O-labeled under an atmosphere of 40% O(2). This pool failed to become 100% (18)O-enriched under lower O(2) concentrations.

Hill Reaction, Hydrogen Peroxide Scavenging, and Ascorbate Peroxidase Activity of Mesophyll and Bundle Sheath Chloroplasts of NADP-Malic Enzyme Type C(4) Species

Intact mesophyll and bundle sheath chloroplasts wee isolated from the NADP-malic enzyme type C(4) plants maize, sorghum (monocots), and Flaveria trinervia (dicot) using enzymic digestion and mechanical isolation techniques. Bundle sheath chloroplasts of this C(4) subgroup tend to be agranal and were previously reported to be deficient in photosystem II activity. However, following injection of intact bundle sheath chloroplasts into hypotonic medium, thylakoids had high Hill reaction activity, similar to that of mesophyll chloroplasts with the Hill oxidants dichlorophenolindophenol, p-benzoquinone, and ferricyanide (approximately 200 to 300 micromoles O(2) evolved per mg chlorophyll per hour). In comparison to that of mesophyll chloroplasts, the Hill reaction activity of bundle sheath chloroplasts of maize and sorghum was labile and lost activity during assay. Bundle sheath chloroplasts of maize also exhibited some capacity for 3-phosphoglycerate dependent O(2) evolution (29 to 58 micromoles O(2) evolved per milligram chlorophyll per hour). Both the mesophyll and bundle sheath chloroplasts were equally effective in light dependent scavenging of hydrogen peroxide. The results suggest that both chloroplast types have noncyclic electron transport and the enzymology to reduce hydrogen peroxide to water. The activities of ascorbate peroxidase from these chloroplast types was consistent with their capacity to scavenge hydrogen peroxide.

Light-Dependent Oxygen Uptake, Glycolate, and Ammonia Release in l-Methionine Sulfoximine-Treated Chlamydomonas

Glycolate and ammonia excretion plus oxygen exchanges were measured in the light in l-methionine-dl-sulfoximine treated air-grown Chlamydomonas reinhardii. At saturating CO(2) (between 600 and 700 microliters per liter CO(2)) neither glycolate nor ammonia were excreted, whereas at the CO(2) compensation concentration (<10 microliters per liter CO(2)) treated algae excreted both glycolate and ammonia at rates of 37 and 59 nanomoles per minute per milligram chlorophyll, respectively. From the excretion values we calculate the amount of O(2) consumed through the glycolate pathway. The calculated value was not significantly different from the component of O(2) uptake sensitive to CO(2) obtained from the difference between O(2) uptake of the CO(2) compensation point and at saturating CO(2). This component was about 40% of stationary O(2) uptake measured at the CO(2) compensation point. From these data we conclude that glyoxylate decarboxylation in air-grown Chlamydomonas represents a minor pathway of metabolism even in conditions where amino donors are deficient and that processes other than glycolate pathway are responsible for the O(2) uptake insensitive to CO(2).

Incorporation of Oxygen into Glycolate, Glycine, and Serine during Photorespiration in Maize Leaves

Glycolate, glycine, and serine extracted from excised Zea mays L. leaves which had been allowed to photosynthesize in the presence of (18)O(2) were analyzed by gas chromatography-mass spectrometry. In each case, only one of the oxygen atoms of the carboxyl group had become labeled. The maximum enrichment observed in glycine and serine was attained after 5 minutes and 15 minutes of exposure to (18)O(2) at the CO(2) compensation point; the labeling was very high, reaching 70 to 73% of that in the applied O(2). Thus, it appears that all or nearly all of the glycine and serine are synthesized in maize leaves via fixation of O(2). In the presence of CO(2) (380 or 800 microliters per liter), (18)O-labeling was markedly slower.Glycolate enrichment was variable and much lower than that in glycine and serine. It is possible that there are additional pathways of glycolate synthesis which do not result in the incorporation of (18)O from molecular oxygen. An estimation of the metabolic flow of O(2) through the photorespiratory cycle was made. It appeared that less than 75% of the O(2) taken up by maize leaves is involved in this pathway. Therefore, other processes of O(2) metabolism must occur in the light.

Photoreduction of oxygen in mesophyll chloroplasts of c(4) plants: a model system for studying an in vivo mehler reaction

Mesophyll chloroplasts of three C(4) sub types, Panicum miliaceum (NAD-malic enzyme), Panicum maximum (PCK), and Zea mays (NADP-malic enzyme), were prepared from protoplast extracts and used to study the photoreduction of O(2). The processes of O(2) uptake and evolution in these preparations, which lack ribulose 1,5-bisphosphate carboxylase/oxygenase, were studied simultaneously using stable isotopes of O(2) and mass spectrometry. The responses of O(2) uptake to O(2) tension and addition of various substrates (3-phosphoglycerate, pyruvate, and oxaloacetate) were studied in detail. The addition of photosynthetic substrates differing in ATP to NADPH demands indicated that photoreduction of O(2) in these chloroplast preparations is linked to ATP production and strongly regulated by NADP(+) levels. The results clearly indicate that photoreduction of O(2) could be of physiological relevance in balancing the ATP to NADPH requirements of C(4) mesophyll chloroplasts.

Effect of CO(2), O(2), and Light on Photosynthesis and Photorespiration in Wheat

Unidirectional O(2) fluxes were measured with (18)O(2) in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O(2), O(2) uptake was maximum at 60 microliters per liter CO(2). At lower CO(2) concentrations, it was strongly inhibited, as was photosynthetic O(2) evolution. At 2% O(2), there remained a substantial O(2) uptake, even at high CO(2) level; the O(2) evolution was inhibited at CO(2) concentrations under 330 microliters per liter. The O(2) uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO(2). The behavior of the O(2) uptake and the study of the CO(2) compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.

Oxygen effect on photosynthetic and glycolate pathways in young maize leaves

To study the effect of O(2) on the photosynthetic and glycolate pathways, maize leaves were exposed to (14)CO(2) during steady-state photosynthesis in 21 or 1% O(2). At the two O(2) concentrations after a (14)CO(2) pulse (4 seconds) followed by a (12)CO(2) chase, there was a slight difference in CO(2) uptake and in the total amount of (14)C fixed, but there were marked changes in (14)C distribution especially in phosphoglycerate, ribulose bisphosphate, glycine, and serine. The kinetics of (14)C incorporation into glycine and serine indicated that the glycolate pathway is inhibited at low O(2) concentrations. In 1% O(2), labeling of glycine was reduced by 90% and that of serine was reduced by 70%, relative to the control in 21% O(2). A similar effect has been observed in C(3) plants, except that, in maize leaves, only 5 to 6% of the total (14)C fixed under 21% O(2) was found in glycolate pathway intermediates after 60 seconds chase. This figure is 20% in C(3) plants. Isonicotinyl hydrazide did not completely block the conversion of glycine to serine in 21% O(2), and the first carbon atom of serine was preferentially labeled during the first seconds of the chase. These results supported the hypothesis that the labeled serine not only derives from glycine but also could be formed from phosphoglycerate, labeled in the first carbon atom during the first seconds of photosynthesis.Another noticeable O(2) effect concerned differential labeling of phosphoglycerate and ribulose bisphosphate. Phosphoglycerate is more labeled than ribulose bisphosphate in air; the reverse is observed in 1% O(2). Changes in ribulose bisphosphate and phosphoglycerate pools exhibit similar trends. To understand the effect of O(2) on the distribution of (14)C in these two intermediates, it was postulated that, in air, there remains an oxygenase function which produces additional phosphoglycerate at the expense of ribulose bisphosphate.

Oxygen exchange in leaves in the light

Photosynthetic O(2) production and photorespiratory O(2) uptake were measured using isotopic techniques, in the C(3) species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO(2) and normal O(2), O(2) production increased linearly with light intensity. At low O(2) or low CO(2), O(2) production was suppressed, indicating that increased concentrations of both O(2) and CO(2) can stimulate O(2) production. At the CO(2) compensation point, O(2) uptake equaled O(2) production over a wide range of O(2) concentrations. O(2) uptake increased with light intensity and O(2) concentration. At low light intensities, O(2) uptake was suppressed by increased CO(2) concentrations so that O(2) uptake at 1,000 microliters per liter CO(2) was 28 to 35% of the uptake at the CO(2) compensation point. At high light intensities, O(2) uptake was stimulated by low concentrations of CO(2) and suppressed by higher concentrations of CO(2). O(2) uptake at high light intensity and 1000 microliters per liter CO(2) was 75% or more of the rate of O(2) uptake at the compensation point. The response of O(2) uptake to light intensity extrapolated to zero in darkness, suggesting that O(2) uptake via dark respiration may be suppressed in the light. The response of O(2) uptake to O(2) concentration saturated at about 30% O(2) in high light and at a lower O(2) concentration in low light. O(2) uptake was also observed with the C(4) plant Amaranthus edulis; the rate of uptake at the CO(2) compensation point was 20% of that observed at the same light intensity with the C(3) species, and this rate was not influenced by the CO(2) concentration. The results are discussed and interpreted in terms of the ribulose-1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of O(2).

Photosynthesis and photorespiration in whole plants of wheat

Wheat was cultivated in a small phytotronic chamber. (18)O(2) was used to measure the O(2) uptake by the plant, which was recorded simultaneously with the O(2) evolution, net CO(2) uptake, and transpiration. At normal atmospheric CO(2) concentration, photorespiration, measured as O(2) uptake, was as important as the net photosynthesis. The level of true O(2) evolution was independent of CO(2) concentration and stayed nearly equal to the sum of net CO(2) photosynthesis and O(2) uptake. We conclude that at a given light intensity, O(2) and CO(2) compete for the reducing power produced at constant rate by the light reactions of photosynthesis.

Metabolic regulation of carbon flux during C4 photosynthesis : II. In situ evidence for reffixation of photorespiratory CO2 by C 4 phosphoenolpyruvate carboxylase

The potential for glycolate and glycine metabolism and the mechanism of refixation of photorespiratory CO2 in leaves of C4 plants were studied by parallel inhibitor experiments with thin leaf slices, different leaf cell types and isolated mitochondria of C3 and C4 Panicum species. CO2 evolution by leaf slices of P. bisulcatum, a C3 species, fed glycolate or glycine was light-independent and O2-sensitive. The C4 P. maximum and P. miliaceum leaf slices fed glycolate or glycine evolved CO2 in the dark but not in the light. In C4 species, dark CO2 evolution was abolished by the addition of phosphoenolpyruvate (PEP)(4). The addition of maleate, a PEP carboxylase inhibitor, resulted in photorespiratory CO2 efflux by C4 leaf slices in the light also. However, PEP and maleate had no effect on either glycolate-dependent O2 uptake by the C4 leaf slices or on glycolate and glycine metabolism in C3 leaf slices. The rate of photorespiratory CO2 evolution in the C3 Panicum species was 3 times higher than that observed with the C4 species. The ratio of glycolate-dependent CO2 evolution to O2 uptake in both groups was 1:2. Isolated C4 mesophyll protoplasts or their mitochondria did not metabolize glycolate or glycine. However, both C3 mesophyll protoplasts and C4 bundle sheath strands readily metabolized glycolate and glycine in a light-independent, O2-sensitive manner, and the addition of PEP or maleate had no effect. C4 bundle sheath- and C3-mitochondria were capable of oxidizing glycine. This oxidation was linked to the mitochondrial electron transport chain, was coupled to three phosphorylation sites and was sensitive to electron transport inhibitors. C4 bundle sheath- and C3-mitochondrial glycine decarboxylation was stimulated by oxaloacetate and NAD had no effect. In marked contrast, mitochondria isolated from C4 mesophyll cells were incapable of oxidizing or decarboxylating added glycine. The results suggest that in leaves of C4 plants bundle sheath cells are the primary site of O2-sensitive photorespiratory CO2 evolution and the PEP carboxylase present in the mesophyll cells has the Potential for efficiently refixing CO2 before it escapes out of the leaf. The relative role of the PEP carboxylase mediated CO2 pump and reassimilation of photorespiratory CO2 are discussed in relation to the apparent lack of photorespiration in leaves of C4 species.

Photoreduction of O(2) Primes and Replaces CO(2) Assimilation

A mass spectrometer with a membrane inlet system was used to monitor directly gaseous components in a suspension of algae. Using labeled oxygen, we observed that during the first 20 seconds of illumination after a dark period, when no net O(2) evolution or CO(2) uptake was observed, O(2) evolution was normal but completely compensated by O(2) uptake. Similarly, when CO(2) uptake was totally or partially inhibited, O(2) evolution proceeded at a high (near maximal) rate. Under all conditions, O(2) uptake balanced that fraction of the O(2) evolution which could not be accounted for by CO(2) uptake.From these observations we concluded that O(2) and CO(2) are in direct competition for photosynthetically generated reducing power, with O(2) being the main electron acceptor during the induction process and under other conditions in which CO(2) reduction cannot keep pace with O(2) evolution. The high rate of the O(2) uptake reaction observed in the presence of iodoacetamide, KCN, or carbonyl cyanide p-trifluoromethyoxyphenylhydrazone, suggests that a special high capacity oxidase distinct from ribulose diphosphate oxygenase exists in whole cells. The rapid reduction of molecular O(2) after a period of darkness probably serves as a priming reaction for the photosynthetic apparatus. The high steady state rate of the O(2) cycle in the absence of CO(2) fixation suggests that the regulation of photosynthesis does not involve significant changes in the rate of photochemical electron transport.

Conversion of photosynthetic products in the light in CO2-free O 2 and N 2 in leaves of Zea mays L. and Phaseolus vulgaris L

After 10 min illumination of segments of bean (Phaseolus vulgaris L.) or maize (Zea mays L.) leaves in air with (14)CO2, the atmosphere was changed to CO2-free O2 or N2 and conversion of photosynthetic products in the light was investigated. The experiments have shown that after the (14)CO2 assimilation period the bean leaves contain the pool of weakly fixed (14)C (WF-(14)C) which is converted into stable products during the subsequent period of illumination in CO2-free N2. In O2 atmosphere the WF-(14)C pool is initially the main source of CO2 evolved. The marked decrease in radioactivity of sucrose and starch during illumination of bean leaves in O2 atmosphere indicates that these compounds were also the source of CO2 evolved in the light. The total amount of previously fixed (14)C remained almost on the same level during illumination of maize leaves in N2 as well as in O2. However, oxygen changed the distribution of (14)C in photosynthetic products, which is suggested to be the consequence of the photorespiration process in maize.

Reduction of oxygen by the electron transport chain of chloroplasts during assimilation of carbon dioxide

In photosynthetically competent chloroplasts from spinach the quantum requirements for oxygen evolution during CO2 reduction were higher, by a factor often close to 1.5, than for oxygen evolution during reduction of phosphoglycerate. Mass spectrometer experiments performed under rate-limiting light indicated that an oxygen-reducing photoreaction was responsible for the consumption of extra quanta during carbon dioxide assimilation. Uptake of 18O2 during reduction of CO2 was considerably higher than could be accounted for by oxygen consumption during glycolate formation and by the Mehler reaction of broken chloroplasts which were present in the preparations of intact chloroplasts. The oxygen reducing reaction occurring during CO2 assimilation resulted in the formation of H2O2. This was indicated by a large stimulation of CO2 reduction by catalase, but not of phosphoglycerate reduction. Catalase could be replaced as a stimulant of photosynthesis by dithiothreitol or ascorbate, compounds known to react with superoxide radicals. There was no effect of dithiothreitol and ascorbate on phosphoglycerate reduction. A main effect of superoxide radicals and/or H2O2 was shown to be at the level of phosphoglycerate formation. Evidence for electron transport of oxygen was also obtained from 14CO2 experiments. The oxidation of dihydroxyacetonephosphate during a dark period or after addition of carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone in the light was studied. The results indicated a link between the chloroplast pyridine nucleotide system and oxygen. Oxygen reduction during photosynthesis under conditions where light is rate limiting is seen as important in supplying the ATP which is needed for CO2 reduction but is not provided during electron transport to NADP. A mechanism is discussed which would permit proper distribution of electrons between CO2 and oxygen during photosynthesis.

Carbon-13 Nuclear Magnetic Resonance Analysis of Metabolism in Soybean Labeled by CO(2)

Fourier transform (13)C nuclear magnetic resonance spectra have been obtained of intact, fresh soybean ovules (Glycine max L. cv. Dare) harvested from pods subtended by a trifoliolate exposed to (13)CO(2) 1 to 3 days earlier. The high resolution spectra are interpreted in terms of the labeled sugars and lipids in the ovule. Comparison of the spectra taken over the 3-day period permits qualitative estimates of sugar metabolism and rates of lipid synthesis. The spectra also contain information about the distribution of labels within the lipid chains. This information leads to a method of estimating the extent to which glucose degradation in the synthesizing soybean ovule is involved in the reactions of the phosphogluconate pathway.

The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves: III. A Comparison between Some C(3) and C(4) Plants

The chlorophyll-based specific activity of cytochrome oxidase and three exclusively mitochondrial enzymes of the tricarboxylic acid cycle showed little variation between leaves of C(3) and C(4) plants or between mesophyll and bundle sheath cells of Atriplex spongiosa and Sorghum bicolor. However, a large, light-dependent transfer of label from intermediates of the tricarboxylic acid cycle to photosynthetic products was a feature of leaves of C(4) plants. This light-dependent transfer of label was barely detectable in leaves of C(3) plants and in leaves of F(1) and F(3) hybrids of Atriplex rosea (C(4)) and Atriplex patula spp hastata (C(3)). The light-dependent transfer of label to photosynthetic products in leaves of C(4) plants was inhibited by the tricarboxylic acid cycle inhibitors malonate and fluoroacetate. The requirement for continued tricarboxylic acid cycle activity was also indicated in experiments with specifically labeled succinate-(14)C. These experiments, together with the distribution of (14)C in glucose prepared from sucrose-(14)C formed during the metabolism of succinate-2,3-(14)C, confirmed that the photosynthetic metabolism of malate and aspartate derived from the tricarboxylic acid cycle, and not the refixation of respiratory CO(2), was the main path of carbon from the cycle to photosynthesis.

Changes in specific radioactivities of corn-leaf metabolites during photosynthesis in (14)CO 2 and (12)CO 2 at normal and low oxygen

The (12)CO2- and (14)CO2-exchange of illuminated corn leaf discs were measured at normal (21%) and low (1%) oxygen. After periods of exposure to (14)CO2 or to (14)CO2 followed by (12)CO2, the discs were killed and the specific activities of some metabolites were determined. At both O2 concentrations the specific activity of 3-PGA increased and decreased rapidly during the first 5 min of (14)CO2-feeding or (12)CO2-flushing but did not equilibrate with that of the CO2 in the assimilation chamber even after 15 min. The specific activity of aspartic acid also showed bimodal kinetics during both feeding and flushing. The specific activities of 3-phosphoglyceric acid (3-PGA), aspartic acid and alanine were higher at 1% O2 than at 21% O2, but glycine and serine were lower in specific activity at 1% O2. The results are in agreement with the proposed initial fixation of CO2 into C4-dicarboxylic acids and subsequent transfer of this carbon to 3-PGA. Indirect evidence supports the idea that at 21% O2, CO2 was produced by the corn leaf discs in the light and was refixed into C4-dicarboxylic acids. At 1% O2, the photorespiratory process could also have been active although the flux of carbon through the glycolate pathway was probably smaller than at 21% O2.