Photorespiratory Carbon Oxidation Cycle Research Articles

Enhanced Rubisco activation associated with maintenance of electron transport alleviates inhibition of photosynthesis under low nitrogen conditions in winter wheat seedlings.

Studying the response of photosynthesis to low nitrogen (N) and the underlying physiological mechanism can provide a theoretical basis for breeding N-efficient cultivars and optimizing N management. We conducted hydroponic experiments using two wheat (Triticum aestivum) cultivars, Zaoyangmai (low N sensitive) and Yangmai158 (low N tolerant), with either 0.25 mM N as a low N (LN) treatment or 5 mM N as a control. Under LN, a decrease in net photosynthetic rate (Pn) was attributed to reduction in the maximum Rubisco carboxylation rate, which then accelerated a reduction in the maximum ribulose-1,5-bisphosphate regeneration rate, and the reduction in Pn was 5-35% less in Yangmai158 than in Zaoyangmai. Yangmai158 maintained a 10-25% higher Rubisco concentration, especially in the upper leaves, and up-regulated Rubisco activase activity compared with Zaoyangmai to increase the Rubisco activation to sustain Rubisco carboxylation under LN conditions. In addition, Yangmai158 increased electron flux to the photorespiratory carbon oxidation cycle and alternative electron flux to maintain a faster electron transport rate and avoid photodamage. In conclusion, the LN-tolerant cultivar showed enhanced Rubisco activation and sustained electron transport to maintain a greater photosynthetic capacity under LN conditions.

Response of carbon assimilation and chlorophyll fluorescence to soybean leaf phosphorus across CO2: Alternative electron sink, nutrient efficiency and critical concentration

To evaluate the response of CO2 assimilation rate (PN) and various chlorophyll fluorescence (CF) parameters to phosphorus (P) nutrition, soybean plants were grown in controlled environment with sufficient (0.50mM) and deficient (0.10 and 0.01mM) phosphate (P) supply under ambient and elevated CO2 (aCO2, 400 and eCO2, 800μmolmol−1, respectively). Measurements were made at ambient (21%) and low (2%) O2 concentrations. Results showed strong correlation of leaf P concentration with PN and CF parameters. The P deficiency showed parallel decreases in PN, and CF parameters including quantum efficiency (Fv′/Fm′), quantum yield of photosystem II (ΦPSII), electron transport rate (JF), and photochemical quenching (qP). The Fv′/Fm′ decreased as a result of greater decline in maximal (Fm′) than minimal (Fo′) fluorescence. The eCO2 stimulated PN especially under higher leaf P concentrations. Low O2 also stimulated PN but only at aCO2. The photosynthetic carbon reduction (PCR, signified by PN) and photorespiratory carbon oxidation cycles (PCO, signified photorespiration as indicated by ratio of JF to gross PN and % increase in PN at 2% O2) was the major electron sinks. However, the presence of alternative electron sink was also evident as determined by the difference between the electron transport calculated from chlorophyll fluorescence and gas exchange measurements. Alternative electron sink declined at lower leaf P concentration suggesting its minor role in photochemical energy consumption, thus dissipation of the excess excitation pressure of PSII reaction center under P deficiency. The JF/PG and % increase in PN at 2 versus 21% O2 remained consistent across leaf P concentration suggesting PCO cycle as an important mechanism to dissipate excess excitation energy in P deficient leaves. The severe decline of Fv′/Fm′, ΦPSII, JF and qP under P deficiency also suggested the occurrences of excess radiant energy dissipation by non-photochemical quenching mechanisms. Critical leaf P concentration (CLPC) needed to achieve 90% of the maximum value was greater for PN than CF parameters. Moreover, CLPC was always higher at eCO2 suggesting increased sensitivity of soybean to P deficiency under eCO2. An increased phosphorus utilization efficiency of PN and CF parameters was also achieved but with the expense of net CO2 assimilation in P-deficient leaves.

Alternative Electron Flows (Water–Water Cycle and Cyclic Electron Flow Around PSI) in Photosynthesis: Molecular Mechanisms and Physiological Functions

An electron flow in addition to the major electron sinks in C(3) plants [both photosynthetic carbon reduction (PCR) and photorespiratory carbon oxidation (PCO) cycles] is termed an alternative electron flow (AEF) and functions in the chloroplasts of leaves. The water-water cycle (WWC; Mehler-ascorbate peroxidase pathway) and cyclic electron flow around PSI (CEF-PSI) have been studied as the main AEFs in chloroplasts and are proposed to play a physiologically important role in both the regulation of photosynthesis and the alleviation of photoinhibition. In the present review, I discuss the molecular mechanisms of both AEFs and their functions in vivo. To determine their physiological function, accurate measurement of the electron flux of AEFs in vivo are required. Methods to assay electron flux in CEF-PSI have been developed recently and their problematic points are discussed. The common physiological function of both the WWC and CEF-PSI is the supply of ATP to drive net CO(2) assimilation. The requirement for ATP depends on the activities of both PCR and PCO cycles, and changes in both WWC and CEF-PSI were compared with the data obtained in intact leaves. Furthermore, the fact that CEF-PSI cannot function independently has been demonstrated. I propose a model for the regulation of CEF-PSI by WWC, in which WWC is indispensable as an electron sink for the expression of CEF-PSI activity.

Modeling scalar and heat sources, sinks, and fluxes within a forest canopy during and after rainfall events

A one‐dimensional, multilayer model that infers the fluxes of heat, water vapor, and CO2 in a biosphere‐atmosphere system, as well as their sources and sinks, was described and evaluated in a secondary broad‐leaved forest. Coupling of a third‐order closure model allows the model to infer profiles of scalar fluxes, sinks, and sources with the computed velocity statistics. Furthermore, the model combines a leaf water process model and makes it possible to capture the impacts of rainfall events on the fluxes, sinks, and sources. The model test was conducted for the two periods (rain and nonrain events) to investigate whether the proposed model can reproduce the profiles of scalar fluxes, sinks, and sources in different weather conditions in the experimental forest. Modeled scalar and heat fluxes for the nonrainfall period showed generally good agreement with measurements. Model calculation showed that CO2 sink of the canopy during rainfall decreased, since the deep canopy layers work as a source due to least radiation transferred to the deep layers. After rainfall, evaporation from wetted leaves caused low vapor pressure deficit (VPD) between the leaf and air, creating favorable conditions for stomatal conductance and intercellular CO2 concentration, which consequently increases carboxylation rates in the photorespiratory carbon oxidation cycle. These results suggest that photosynthesis rates during and after rainfall events are primarily influenced by radiation as well as a variety of environmental factors, particularly VPD. The model results also indicated that the maximum source/sink strengths of heat and H2O were distributed within the top 30% of the canopy and that the CO2 distribution had the feature of the vegetation shape. These differences imply that the roughness heights of individual components are different, causing different effective exchange heights.

Crystal structure of activated ribulose-1,5-bisphosphate carboxylase/oxygenase from green alga Chlamydomonas reinhardtii complexed with 2-carboxyarabinitol-1,5-bisphosphate

Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the initial steps of photosynthetic carbon reduction and photorespiratory carbon oxidation cycles by combining CO 2 and O 2, respectively, with ribulose-1,5-bisphosphate. Many photosynthetic organisms have form I rubiscos comprised of eight large (L) and eight small (S) subunits. The crystal structure of the complex of activated rubisco from the green alga Chlamydomonas reinhardtii and the reaction intermediate analogue 2-carboxyarabinitol-1,5-bisphosphate (2-CABP) has been solved at 1.84 Å resolution ( R cryst of 15.2 % and R free of 18.1 %). The subunit arrangement of Chlamydomonas rubisco is the same as those of the previously solved form I rubiscos. Especially, the present structure is very similar to the activated spinach structure complexed with 2-CABP in the L-subunit folding and active-site conformation, but differs in S-subunit folding. The central insertion of the Chlamydomonas S-subunit forms the longer βA-βB loop that protrudes deeper into the solvent channel of rubisco than higher plant, cyanobacterial, and red algal (red-like) βA-βB loops. The C-terminal extension of the Chlamydomonas S-subunit does not protrude into the solvent channel, unlike that of the red algal S-subunit, but lies on the protein surface anchored by interactions with the N-terminal region of the S-subunit. Further, the present high-resolution structure has revealed novel post-translational modifications. Residue 1 of the S-subunit is N α-methylmethionine, residues 104 and 151 of the L-subunit are 4-hydroxyproline, and residues 256 and 369 of the L-subunit are S γ-methylcysteine. Furthermore, the unusual electron density of residue 471 of the L-subunit, which has been deduced to be threonine from the genomic DNA sequence, suggests that the residue is isoleucine produced by RNA editing or O γ-methylthreonine.

Growth in Elevated CO2 Can Both Increase and Decrease Photochemistry and Photoinhibition of Photosynthesis in a Predictable Manner. Dactylis glomerata Grown in Two Levels of Nitrogen Nutrition

Biochemically based models of C(3) photosynthesis can be used to predict that when photosynthesis is limited by the amount of Rubisco, increasing atmospheric CO(2) partial pressure (pCO(2)) will increase light-saturated linear electron flow through photosystem II (J(t)). This is because the stimulation of electron flow to the photosynthetic carbon reduction cycle (J(c)) will be greater than the competitive suppression of electron flow to the photorespiratory carbon oxidation cycle (J(o)). Where elevated pCO(2) increases J(t), then the ratio of absorbed energy dissipated photochemically to that dissipated non-photochemically will rise. These predictions were tested on Dactylis glomerata grown in fully controlled environments, at either ambient (35 Pa) or elevated (65 Pa) pCO(2), and at two levels of nitrogen nutrition. As was predicted, for D. glomerata grown in high nitrogen, J(t) was significantly higher in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). This was due to a significant increase in J(c) exceeding any suppression of J(o). This increase in photochemistry at elevated pCO(2) protected against photoinhibition at high light. For plants grown at low nitrogen, J(t) was significantly lower in plants grown and measured at elevated pCO(2) than for plants grown and measured at ambient pCO(2). Elevated pCO(2) again suppressed J(o); however growth in elevated pCO(2) resulted in an acclimatory decrease in leaf Rubisco content that removed any stimulation of J(c). Consistent with decreased photochemistry, for leaves grown at low nitrogen, the recovery from a 3-h photoinhibitory treatment was slower at elevated pCO(2).

A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2 -grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3− and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3– of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3− channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3−, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.

A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2-grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO3− and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO3– of the CO2 produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO3− channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO3−, thus limiting the potential for CO2 leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.

Determination of the rate of photoreduction of O2 in the water-water cycle in watermelon leaves and enhancement of the rate by limitation of photosynthesis.

A study was performed to determine how the electron fluxes for the photosynthetic carbon reduction (PCR) and the photorespiratory carbon oxidation (PCO) cycles affect the photoreduction of O2 at PSI, which is the limiting step in the water-water cycle. Simultaneous measurements were made of CO2-gas exchange, transpiration and quantum yield of PSII [phi(PSII)] using leaves of watermelon (Citrullus lanatus). The total electron flux in PSII[Je(PSII)], as estimated from phi(PSII), was always larger than the total electron flux required for the PCR and PCO cycles at various partial pressures of CO2 and O2 and 1,100 micromol photons m(-2)s(-1). This observation suggested the existence of an alternative electron flux (Ja). Ja was divided into O2-dependent [Ja(O2-depend)] and O2-independent [Ja(O2-independ)] components. The magnitude of half Ja(O2-depend), 7.5 to 9.5 micromol e- m(-2)s(-1), and its apparent Km for O2, about 8.0 kPa, could be accounted for by the photoreduction of O2 at PSI either mediated by ferredoxin or catalyzed by monodehydroascorbate reductase. The results indicated that Ja(O2-depend) was driven by the water-water cycle. A decrease in the intercellular partial pressure of CO2 from 23 to 5.0 Pa at 21 kPa O2 enhanced Ja(O2-depend) by a factor of 1.3. Saturation of the activities of both the PCR and PCO cycles by increasing the photon flux density induced Ja. These results indicate the electron flux in PSII that exceeds the flux required for the PCR and PCO cycles induces the photoreduction of O2 in the water-water cycle.

Photosynthetic electron sinks in transgenic tobacco with reduced amounts of Rubisco: little evidence for significant Mehler reaction.

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of the small subunit of Rubisco were used to investigate the role of O2 as an electron acceptor during photosynthesis. The reduction in Rubisco has reduced the capacity for CO2-fixation in these plants without a similar reduction in electron transport capacity. Concurrent measurements of chlorophyll fluorescence and CO2 assimilation at different CO2 and O2 partial pressures showed close linear relationships between chloroplast electron transport rates calculated from chlorophyll fluorescence and those calculated from CO2-fixation. These relationships were similar for wild-type and transgenic plants, indicating that the reduced capacity for CO2 fixation in the transgenic plants did not result in extra electron transport not associated with the photosynthetic carbon reduction (PCR) or photorespiratory carbon oxidation (PCO) cycle. This was further investigated with mass spectrometric measurements of 16O2 and 18O2 exchange made concurrently with measurements of chlorophyll fluorescence. In all tobacco lines the rates of 18O2 uptake in the dark were similar to the 18O2 uptake rates at very high CO2 partial pressures in the light. Rates of oxygenase activity calculated from 18O2 uptake at the compensation point were linearly related to the Rubisco content of leaves. The ratios of oxygenase to carboxylase rates were calculated from measurements of 16O2 evolution and 18O2 uptake at the compensation point. These ratios were lower in the transgenic plants, consistent with their higher CO2 compensation points. It is concluded that although there may be some electron transport to O2 to balance conflicting demands of NADPH to ATP requirements, this flux must decrease in proportion with the reduced demand for ATP and NADPH consumption in the transgenic lines. The altered balance between electron transport and Rubisco capacity, however, does not result in rampant electron transport to O2 or other electron transport acceptors in the absence of PCR and PCO cycle activity.

The past, present and future of nitrogenous compounds in the atmosphere, and their interactions with plants

Ammonia was necessary for the origin of life. However, NH3 would not have been a significant component of the neutral or mildly reducing (N2, CO2, H2O) atmosphere which characterized the Hadean earth (>3·8 Gyr ago), especially in view of the u.v. lability of NH3, the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH3 source. Lightning, or bolides (meteorites and comets), generated NOx., and Fe2+ in the sea could have reduced NO2− resulting from the NOx. to NH4+; this disequilibrium NH4+ level could have supported the production of the nitrogenous organic building blocks of life. NOx. and NHy thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH4+, and biological N2 fixation could have evolved in its present Fe‐demanding, O2‐sensitive forms in the Archaean O2‐free, Fe2+‐rich environment. Organic matter could have driven biological denitrification reactions based on NO2− and NO3− generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NOx. and producing N2O. Any atmospheric NH3 leaking from oceanic biology would have been subject to u.v. breakdown and rain‐out. Although O2‐evolving photosynthesis probably began in the Archean some 3·5 Gyr ago, any O2 accumulation was local until 2·0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe2+ and S2−. However, localized O2 accumulation before 2·0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O2‐consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NOx. production. Oxygen accumulation globally from 2·0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification‐like aerobic production of NO. by nitric oxide synthetase, while lightning and bolides could now generate NO. from N2 and O2.Evidence for terrestrial ecosystems is found from 1·0 Gyr onwards; NOx. and NH3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO2 levels fell and O2 levels rose, NH3 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity and N cycling on land, with greater quantitative possibilities for NOx. and NH3 escape to the atmosphere. The evolution of lignin (and related phenylpropanoids) at least 400 Myr ago, with associated NH3 recycling in vascular land plant, does not seem (on present evidence) to increase NH3 loss to the atmosphere significantly. Biomass burning occurred at least 350 Myr ago with lightning as the likely ignition source; such burning yielded NO., NH3, N2O and N2 from organic N. As in earlier times, the existence of terrestrial embryophyte vegetation has been punctuated by major bolide impacts, with the Cretaceous‐Tertiary boundary impact generating perhaps 104 as much NO. as a year's thunderstorms do today, although the toxicity of NOx.per se to vegetation might not have been the major effect of the NOx. on biota. Terrestrial plants suffered fewer extinctions at this time than did many other major taxa. Despite such very significant generators of atmospheric combined N as major bolide impacts, the mean levels of NHy, NOx. and N2O (and O3) in the atmosphere during the 450 Myr existence of embryophyte vegetation was lower than the current, globally averaged, anthropogenically influenced values. Current globally averaged levels of atmospheric combined N are thus not without precedent in the history of life, globally for NOx., and locally for NHy, so vegetation has had evolutionary experience of high atmospheric combined N. However, this should not make us complacent about the impact of current wide‐spread and continuing anthropogenic inputs of combined N, in view of the rate and extent of the inputs, and their combination with other aspects of local anthropogenic influence (e.g. SO2 from burning of high‐S coal), stratospheric O3 depletion, and global environmental change (increasing CO2, temperature and sea‐level).

T-protein of glycine decarboxylase from Solanum tuberosum.

The Gly decarboxylase multienzyme complex (Gly cleavage system, EC 2.1.2.10) catalyzes the conversion of Gly into COz, NH3, and 5,10-methylene-tetrahydrofolate, reducing NAD+ to NADH. The enzyme has been isolated from animals, plants, and bacteria-in animals and plants it occurs exclusively in the mitochondrial matrix, and in photosynthetic tissue it can be found in very large amounts (Bourguignon et al., 1988). In plants it is involved in the photorespiratory carbon oxidation cycle because it is the site of release of photorespiratory C 0 2 and NH3 (Keys et al., 1978). The enzyme complex is composed of four subunits: 100kD P-protein (pyridoxal phosphate binding), 14-kD H-protein (lipoamide containing carrier), 45-kD T-protein (tetrahydrofolate-dependent methylenetransferase), and 59-kD L-protein (lipoamide dehydrogenase), with a stoichiometry of 1 L-dimer:2 P-dimers:27 H:9 T (Oliver et al., 1990). The T-protein catalyzes the release of ammonia from the methyleneamine intermediate bound to H-protein after the decarboxylation of Gly and transfers the remaining one-carbon unit to tetrahydrofolate (Fujiwara et al., 1984). cDNA sequences for a11 of the subunits of the Gly cleavage system from bacteria, animals, and human have been reported including the T-protein from chicken (Okamura-Ikeda et al., 1992), bovine (Okamura-Ikeda et al., 1991), and human (Hayasaka et al., 1993). Most of the cloning of plant Gly decarboxylase multienzyme complex components has been performed with pea, and the sequences of P, H, and L subunits have been published (Macherel et al., 1990; Tumer et al., 1992a, 199213). This is the first report of successfully cloning a cDNA encoding the complete T-precursor protein in a plant species. In comparison with the T-precursor protein from bovine, the nucleotide and derived amino acid sequences are 57.1 and 48.9% homologous, respectively. The mature T-protein from potato is 51.5% homologous with both human and bovine sequences and 48.7% with the T-protein from chicken. From Southern analysis of Solanum tuberosum genomic DNA, it is highly probable that T-protein is encoded by a single gene.

The role of phosphoenolpyruvate carboxykinase in a marine macroalga with C4-like photosynthetic characteristics.

Udotea flabellum is a marine, macroscopic green alga with C4-like photosynthetic characteristics, including little O2 inhibition of photosynthesis, a low CO2 compensation point, and minimal photorespiration; but it lacks anatomical features analogous to the Kranz compartmentation of C4 plants, and phosphoenolpyruvate carboxylase [PEPC; orthophosphate:oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31] activity is negligible. Phosphoenolpyruvate carboxykinase (PEPCK) activity (carboxylating) in Udotea extracts was equivalent to that of ribulose-bisphosphate carboxylase [Rubisco; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. When PEPCK activity was inhibited in vivo with 3-mercaptopicolinic acid (MPA), thallus photosynthesis decreased by 70% and became sensitive to O2. Codium decorticatum, a related species that lacks C4-like photosynthetic features and PEPCK activity, showed no increase in O2 inhibition upon exposure to MPA. Rubisco and PEPC activities in Udotea were not inhibited by MPA. Labeling of the early photosynthetic products malate and aspartate was reduced 66% by MPA, while intermediates of the photorespiratory carbon oxidation cycle showed a 3-fold increase. Udotea evolved O2 in the light in the absence of inorganic carbon, suggesting it had an endogenous carbon source for photosynthesis. Exogenous malate stimulated this process, while MPA inhibited it. PEPCK was not involved in Crassulacean acid metabolism or dark CO2 fixation. These MPA studies establish a direct link between PEPCK activity and the low O2 inhibition of photosynthesis and low photorespiration in Udotea. The data are consistent with carboxylation by a cytosolic PEPCK providing a C4 acid, such as malate, to the chloroplast for decarboxylation to elevate the CO2 concentration at the Rubisco fixation site. Udotea is to date the most primitive plant with a C4-like form of photosynthesis.

Photoassimilation of Inorganic Carbon in Chlamydomonas segnis during Cell Development at Low and High Carbon Dioxide Tension

Summary Rates of photoassimilation of inorganic carbon at non-limiting (4.8 mM) and limiting (0.14 mM) concentrations of NaHCO 3 were compared in air- and 5% CO 2 -adapted cells of Chlamydomonas segnis during development in continuously illuminated synchronous cultures. At the limiting concentration of inorganic carbon and depending on the algal developmental stage, 5% CO 2 -adapted cells exhibited only 1–6% of the rates shown at the non-limiting concentration, compared to 30–90% in air-adapted cells. As indicated by these low and high rates, adaptation of 5% CO 2 zoospores to air occurred faster (4 h) than that of air zoospores to 5% CO 2 (24 h). The longer period required by the latter (i.e. the completion of one cell cycle) could be explained in terms of a requirement for structural changes. Analysis of the patterns of 14 C distribution among the ethanol-water soluble products of short-term photosynthesis (1 min) at the limiting concentration of inorganic carbon revealed significant differences between air- and 5% CO 2 -adapted cells. At early stages of development, the flow of carbon to the C2 photorespiratory carbon oxidation cycle appeared to be suppressed in the former, but not in the latter cells which were characterized by comparatively high rates of carbon flow into C4 compounds. At later stages of development (early to mid S-phase), the major difference was seen in the greater ability of 5% CO 2 -adapted cells to fix inorganic carbon via phosphoenolpyruvate (PEP) carboxylation. It was argued that PEP derived from the degradation of carbohydrate reserves in these cells, rather than PEP recently produced from the C3 photosynthetic carbon reduction cycle, would be responsible for the enhancement of β-carboxylation reaction(s) at S-phase.

The mechanism of oxalate biosynthesis in higher plants: investigations with the stable isotopes 18 O and 13 C

Substantial incorporation of 18 O 2 into photorespiratory carbon oxidation cycle intermediates in illuminated Spinacia oleracea leaves confirms that oxygenase activity of the enzyme ribulose biphosphate carboxylase–oxygenase is a major source of glycollate in illuminated leaves. No 18 O 2 incorporation into oxalate was detected in these experiments, although 13 C incorporation from 13 CO 2 shows that oxalate synthesis is occurring under the experimental conditions. This result tends to minimize the role of a direct oxidation of glyoxylate derived (via phosphoglycollate and glycollate) from ribulose biphosphate oxygenase activity in oxalate synthesis in Spinacia . Measurements of δ 13 C show (in confirmation of earlier reports) that oxalate from Spinacia is less depleted in 13 C than is bulk organic C in the plant; it is possible the phosphoenolpyruvate carboxylase is involved in the production of the oxalate precursor. Of the plants tested, Mercurialis and Pelargonium shared with Spinacia the high δ 13 C value, while Chenopodium (closely related to Spinacia ), Oxalis (more distantly related to Pelargonium ) and two members of the Polygonaceae had oxalate δ 13 C values close to the whole-leaf δ 13 C value, which suggests derivation of both oxalate C atoms from carboxylase activity of the enzyme ribulose biphosphate carboxylase–oxygenase.

A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species

Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.

Photosynthetic and Photorespiratory Carbon Metabolism in Mesophyll Protoplasts and Chloroplasts Isolated from Isogenic Diploid and Tetraploid Cultivars of Ryegrass (Lolium perenne L.)

Photosynthetic (14)CO(2) fixation, [(14)C]glycolate formation, and the decarboxylation of [1-(14)C]glycolate and [1-(14)C]glycine by leaf mesophyll protoplasts isolated from isogenic diploid and tetraploid cultivars of ryegrass (Lolium perenne L.) were examined. The per cent O(2) inhibition of photosynthesis in protoplasts from the tetraploid cultivar was less than that of the diploid line at both 21 and 49% O(2). Kinetic studies revealed that the K(m) (CO(2)) for photosynthesis by the diploid protoplasts was about twice that of the tetraploid line. In contrast, the K(i) (O(2)) for protoplast photosynthesis was similar in both cultivars, as was the potential for oxidizing glycolate and glycine to CO(2) via the photorespiratory carbon oxidation cycle. Although the maximal rates of glycolate accumulation by the isolated protoplasts in the presence of 21% O(2) and a glycolate oxidase inhibitor were similar in the two cultivars, the percentage of total fixed (14)C entering the [(14)C]glycolate pool and the ratio of the rate of [(14)C]glycolate formation to (14)CO(2) fixation at 21% O(2) and low pCO(2) were about two times greater in protoplasts and intact chloroplasts isolated from the diploid line compared to the tetraploid. These results fully support the recent observation that a doubling of ploidy in various ryegrass cultivars reduced the K(m) (CO(2)) of purified ribulose bisphosphate carboxylase-oxygenase by about one-half without affecting the K(i) (O(2)) (Garrett 1978 Nature 274: 913-915).

Inhibition of the Capacity and Efficiency of Photosynthesis in Bean Leaflets Illuminated in a CO2-Free Atmosphere at Low Oxygen: A Possible Role for Photorespiration

Mature, intact, attached leaflets of Phaseolus vulgaris illuminated at 2000 μE m-2 s-1 in CO2-free N2 containing 1% O2 for a 3-h treatment period showed an 80% reduction in the light-saturated CO2 assimilation rate. Young bean leaflets showed photoinhibition but exhibited a partial recovery of CO2 assimilation rate within 2 h of treatment whereas mature leaflets did not. Photoinhibition by illumination under conditions which prevent photorespiration was markedly dependent upon the exposure time. Photoinhibition was not observed following illumination in the presence of CO2-free N2 containing above 7% O2 or in N2 containing 1% O2 and an intercellular CO2 partial pressure equivalent to the CO2 compensation point in 21% O2 (62 μbar, 6.2 Pa). Photoinhibition involved concomitant reduction in the light-saturated and the light-limited rate of CO2 assimilation; apparent quantum yield was reduced following treatment. These experiments show that internal generation of CO2 by the photorespiratory carbon oxidation cycle is sufficient to prevent the photoinhibition observed when photorespiration is prevented. Possible mechanisms and implications of these data are discussed.