Photosynthetic/photorespiratory characteristics of C3−C4 intermediate species

The extent of photorespiration, the inhibition of apparent photosynthesis (APS) by 21% O₂, and the leaf anatomical and ultrastructural features of the naturally occurring C₃–C₄ intermediate species in the diverse Panicum, Moricandia, and Flaveria genera are between those features of representative C₃ and C₄ plants. The greatest differences between the photosynthetic/photorespiratory CO₂ exchange characteristics of the C₃–C₄ intermediates and C₃ plants occur for the parameters which are measured at low pCO₂ (i.e., the CO₂ compensation concentration and rates of CO₂ evolution into CO₂-free air in the light). The rates of APS by the intermediate species at atmospheric pCO₂ are similar to those of C₃ plants.The mechanisms which are responsible for reducing photorespiration in the C₃–C₄ intermediate species are poorly understood, but two proposals have been advanced. One emphasizes the importance of limited C₄ photosynthesis which reduces O₂ fixation by ribulose 1,5-bisphosphate carboxylase/oxygenase, and, thus, reduces photorespiration by a CO₂-concentrating mechanism, while the other emphasizes the importance of the internal recycling of photorespiratory CO₂ evolved from the chloroplast/mitochondrion-containing bundle-sheath cells. There is no evidence from recent studies that limited C₄ photosynthesis is responsible for reducing photorespiration in the intermediate Panicum and Moricandia species. However, preliminary results suggest that some, but not all, of the intermediate Flaveria species may possess a limited C₄ cycle. The importance of a chlorophyllous bundle-sheath layer in the leaves of intermediate Panicum and Moricandia species in a mechanism based on the recycling of photorespiratory CO₂ is uncertain.Therefore, although they have yet to be clearly delineated, different strategies appear to exist in the C₃–C₄ intermediate group to reduce photorespiration. Of major importance is the finding that some mechanism(s) other than Crassulacean acid metabolism or C₄ photosynthesis has (have) evolved in at least the majority of these terrestrial intermediate species to reduce the seemingly wasteful metabolic process of photorespiration.Abbreviations APS apparent (net) photosynthesis - CAM Crassulacean acid metabolism - CE carboxylation efficiency - T CO₂ compensation concentration - IRGA infrared gas analysis - P_i orthophosphate - PEP phosphoenolpyruvate - RuBP ribulose 1,5-bisphosphate Published as Paper No. 7383, Journal Series, Nebraska Agricultural Experiment Station.     

Short-term carbon-isotope discrimination in C3−C4 intermediate species

Short-term discrimination in assimilation of stable isotopes of carbon was measured for leaves of the C₃ speciesPhaseolus vulgaris L. cv. Hawkesbury Wonder andFlaveria pringlei Gandoger, the C₄ speciesAmaranthus edulis Speg., and the C₃–C₄ intermediate speciesPanicum milioides Nees ex. Trin,Flaveria floridana Johnson, andFlaveria anomala B.L. Robinson. Discriminations in the C₃ and C₄ species were similar to those expected from theoretical considerations. When ambient CO₂ pressure was 330 bar the mean discriminations in the C₃ species andPanicum milioides were similar, whereas the mean discriminations inF. floridana andF. anomala were less than discrimination in C₃ species andPanicum milioides. When ambient CO₂ pressure was 100 bar the mean discriminations inPanicum milioides andF. anomala were greater, and that inF. floridana was less, than that inPhaseolus vulgaris. We conclude that the pattern of discrimination inPanicum milioides is consistent with the presence of a glycine shuttle; inF. floridana andF. anomala, discrimination is consistent with the presence of a C₄ pathway coupled with the operation of a glycine shuttle.Abbreviations and symbols PEP phosphoenolpyruvate - Rubisco ribulose, 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) - p _a ambient CO₂ pressure - p _i intercellular CO₂ pressure - carbon-isotope discrimination - carbonisotope composition relative to PeeDee Belemnite     

Regulation of photosynthetic carbon assimilation in leaves of C3–C4 intermediate species ofMoricandia andFlaveria

The relationship between the gas-exchange characteristics, the contents of photosynthetic intermediates and the quantum yield of photosystem II was examined at different intercellular partial pressures of CO₂ (p _i) in attached leaves of Moricandia arvensis L. (D.C.) and Flaveria floridana J.R. Johnson (both C₃–C₄ intermediate plants) and, for comparison, in F. pringlei Gandoger (a C₃ plant) and in F. bidentis (a C₄ plant). Both C₃–C₄ intermediate species had pools of phosphoenolpyruvate, pyruvate, alanine and aspartate intermediate to those of the C₃ and C₄ species examined. Moricandia arvensis had large pools of glycine at low p _i, consistent with the operation of a glycine shuttle from mesophyll to bundle-sheath cells. It also had a high pool of triose-phosphate at ambient partial pressures of CO₂, indicating that a glycerate-3-phosphate/triose-phosphate shuttle could operate in this species. This was not the case in F. floridana. A decline in the ribulose-1,5-bisphosphate and triose-phosphate pool in M. arvensis, and a rise in the pools of glycerate-3-phosphate and pyruvate in F. floridana, at low p _i, show different patterns of metabolic regulation in M. arvensis and F. floridana at low p _i in comparison to C₃ and C₄ plants.Abbreviations Frul,6bisP fructose-1,6-bisphosphate - PEP phosphoenolpyruvate PGA-glycerate-3-phosphate - p _i intercelular CO₂ pressure - PPFD photosynthetic photon flux density; - RuBP ribulose-1,5-bisphosphate - triose-P triose phosphates This work was done while R.C.L. was a Visiting Fellow at the Australian National University, and was sponsored by the Royal Society. We are grateful to Kathy Britt for assistance with the analysis of amino acids.     

Interspecific variation in assimilation of 14CO2 into C4 acids by leaves of C3, C4 and C3−C4 intermediate Flaveria species near the CO2 compensation concentration

The assimilation of ¹⁴CO₂ into the C₄ acids malate and aspartate by leaves of C₃, C₄ and C₃–C₄ intermediate Flaveria species was investigated near the CO₂ compensation concentration ^* in order to determine the potential role of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) in reducing photorespiration in the intermediates. Relative to air concentrations of CO₂, the proportion of CO₂ fixed by PEP carboxylase at ^* increased in all six C₃–C₄ intermediate species examined. However, F. floridana J.R. Johnston and F. ramosissima Klatt were shown to be markedly less responsive to reduced external CO₂, with only about a 1.6-fold enhancement of CO₂ assimilation by PEP carboxylase, as compared to a 3.0- to 3.7-fold increase for the other C₃–C₄ species examined, namely, F. linearis Lag., F. anomala B.L. Robinson, F. chloraefolia A. Gray and F. pubescens Rydb. The C₃ species F. pringlei Gandoger and F. cronquistii A.M. Powell exhibited a 1.5- and 2.9-fold increase in labeled malate and aspartate, respectively, at ^*. Assimilation of CO₂ by PEP carboxylase in the C₄ species F. trinervia (Spreng.) C. Mohr, F. australasica Hook., and the C₄-like species F. brownii A.M. Powell was relatively insensitive to subatmospheric levels of CO₂. The interspecific variation among the intermediate Flaverias may signify that F. floridana and F. ramosissima possess a more C₄-like compartmentation of PEP carboxylase and ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) between the mesophyll and bundle-sheath cells. Chasing recently labeled malate and aspartate with ¹²CO₂ for 5 min at ^* resulted in an apparent turnover of 25% and 30% of the radiocarbon in these C₄ acids for F. ramosissima and F. floridana, respectively. No substantial turnover was detected for F. linearis, F. anomala, F. chloraefolia or F. pubescens. With the exception of F. floridana and F. ramosissima, it is unlikely that enhanced CO₂ fixation by PEP carboxylase at the CO₂ compensation concentration is a major mechanism for reducing photorespiration in the intermediate Flaveria species. Moreover, these findings support previous related ¹⁴CO₂-labeling studies at air-levels of CO₂ which indicated that F. floridana and F. ramosissima were more C₄-like intermediate species. This is further substantiated by the demonstration that F. floridana PEP carboxylase, like the enzyme in C₄ plants, undergoes a substantial activation (2.2-fold) upon illuminating dark-adapted green leaves. In contrast, light activation was not observed for the enzyme in F. linearis or F. chloraefolia.Abbreviations and symbols PEP phosphoenolpyruvate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - CO₂ compensation concentration - ^* a subatmospheric level of CO₂ approximating Published as Paper No. 8832, Journal Series, Nebraska Agricultural Research Division     

Immunofluorescent localization of phosphoenolpyruvate carboxylase and ribulose 1,5-bisphosphate carboxylase/oxygenase proteins in leaves of C3, C4 and C3−C4 intermediate Flaveria species

The concentrations of 17 nucleotides and three nucleosides have been determined in a batch suspension culture of Datura innoxia using a new procedure for extraction, purification and high-performance liquid chromatography separation of these compounds. The nucleotide pools change appreciably in the different phases of growth. These changes indicate the preparation for and initiation of cell proliferation, and reflect metabolic events during cell division, cell elongation and starvation. The main components of the nucleotide pool are uracil nucleotides, with uridine 5-diphosphate sugars as the predominant fraction, and the adenine nucleotides. Although their concentrations vary by a factor of more than 6 the ratio of the uracil to adenine nucleotides is kept fairly constant during growth. The energy charge is maintained at a rather high value. The correlation of these events with nutrient uptake and macromolecular synthesis by the batch culture is presented in the following paper.Abbreviations Glc glucose - GlcNAc 2-acetamido-2-deoxy-d-glucose - HPLC high performance liquid chromatography - UDP uridine 5-diphosphate     

Molecular properties of phosphoenolpyruvate carboxylase from C3, C3−C4 intermediate,and C4 Flaveria species

Phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) from Flaveria trinervia Mohr (C₄), F. floridana Johnston (C₃–C₄), and F. cronquistii Powell (C₃) leaves were compared by electrotransfer blotting/enzyme-linked immunoassay (Western-blot analysis), mobility of the native enzyme in polyacrylamide gels and in isoelectric focusing (IEF) gels, peptide mapping, and in-vitro translation of RNA isolated from each plant. The PEPCases from the C₃ and C₃–C₄ plants were very similar to each other in terms of electrophoretic mobilities on gels and isoenzyme patterns on IEF gels, and identical in peptide mapping. Quantitative differences were noted, however, in that the C₃–C₄ intermediate plant contained more PEPCase overall and that the relative activity of individual isoenzymes shifted between the C₃ and C₃–C₄ intermediate PEPCases. The PEPCase from the C₄ plant had a different isoenzyme pattern, a different peptide map, and was far more abundant than the other two enzymes. Western blot analysis demonstrated the cross-reactivity of PEPCases from all three Flaveria species with antibody raised against maize PEPCase. The results provide evidence, at the molecular level, that supports the view of C₃–C₄ intermediate species as C₃-like plants with some C₄-like photosynthetic characteristics, but there are differences from the C₃ plant in the quantity and properties of the PEPCase from the C₃–C₄ intermediate plant.Abbreviations IEF isoelectric focusing - kDa kilodalton - PEPCase phosphoenolpyruvate carboxylase - Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase     

In-situ immunofluorescent localization of phosphoenolpyruvate and ribulose 1,5-bisphosphate carboxylases in leaves of C3, C4, and C3−C4 intermediatePanicum species

The in-situ inter- and intracellular localization patterns of phosphoenolpyruvate (PEP) and ribulose 1,5-bisphosphate (RuBP) carboxylases in green leaves of severalPanicum species were investigated using an indirect immunofluorescence technique. Four species were examined and compared:P. miliaceum (C₄),P. bisulcatum (C₃), andP. decipiens andP. milioides (C₃–C₄ intermediates which have Kranz-like leaf anatomy and reduced photorespiration). In the C₄ Panicum, PEP carboxylase was located in the cytosol of the mesophyll cells and RuBP carboxylase was restricted to the bundle-sheath chloroplasts. In contrast, in the C₃ Panicum species, PEP carboxylase was found throughout the leaf chlorenchyma, in both the cytosol and chloroplasts, and RuBP carboxylase was located in the chloroplasts. For the C₃–C₄ intermediate plants, the patterns depended on the species examined. ForP. decipiens, the in-situ localization of both carboxylases was similar to that described forP. bisulcatum and other C₃ plants. However, inP. milioides, PEP carboxylase was found exclusively in the cytosol of the mesophyll cells, as inP. miliaceum and other C₄ species, whereas RuBP carboxylase was distributed in both the mesophyll and bundle-sheath chloroplasts.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose 1,5-bisphosphate     

Production of somatic hybrids between Brassica oleracea and the C3–C4 intermediate species Moricandia nitens

Protoplast fusion between Brassica oleracea and Moricandia nitens, a C₃–C₄ intermediate wild species, was carried out. Four hundred and twenty five plants were regenerated from 1995 calli. More than 90% of the regenerated plants were verified as true intergeneric hybrids on the basis of morphological observation and molecular-marker analysis. The hybrids were morphologically intermediate between both fusion parents. Variations in flower color and petal number were also observed. The chromosome number and pollen fertility varied across the individual hybrids. Although after self-pollination pollen germinated on the stigma and pollen tubes were visible in the style, the pods did not develop properly without in vitro culture. Measurements of the CO₂ compensation point revealed that six out of eight hybrid plants expressed a gas-exchange character that was intermediate between the C₃–C₄ M. nitens and C₃ B. oleracea parents. Received: 20 January 1999 / Accepted: 16 June 1999     

Ecotypic differences in the C3 and C4 photosynthetic activity in Mollugo verticillata,a C3−C4 intermediate

Four populations of Mollugo verticillata L. were compared on the basis of their photosynthetic products, photosynthetic rates, enhancement under low oxygen concentration, and CO₂ compensation points. In addition, pulse-chase labeling experiments were conducted using one of the four populations. Depending on the plant population, C₄ acids ranged from 40% to 11% of the primary products under short-term exposure to ¹⁴CO₂. These compounds were also metabolized during pulse-chase experiments. All four populations had significantly different photosynthetic rates and those rates were correlated with the amounts of labelled C₄ acids produced and C₄-acid turnover. Three populations of M. verticillata had similar compensation points (40 l/l) and degrees of photosynthetic enhancement under low [O₂] (20%), the fourth population was much lower in both characteristics (CO₂ compensation, 25 l/l; low-O₂ enhancement, 12%). The results verify the intermediate nature of photosynthesis in this species, and illustrate populational differences in its photosynthetic and photorespiratory carbon metabolism.Abbreviations PGA 3-phosphoglyceric acid - Kan Kansas - Mass Massachusetts - Mex Mexico     

Plant regeneration from hypocotyls,cotyledons and stem segments of the C3–C4 intermediate species Moricandia arvensis

Techniques have been developed for the regeneration of Moricandia arvensis from complex explants. Hypocotyl segments and cotyledonary explants regenerated shoots, but the most efficient plant regeneration was from stem sections taken from in vitro grown shoots. Regeneration from these three explant types was tested on a range of concentrations of benzylaminopurine and either naphthylene acetic acid or indole acetic acid. Regeneration from all three explants was much higher on indole acetic acid than on naphthylene acetic acid and the ratio of auxin to cytokinin was also significant in determining the response of explants. Optimum regeneration was on 1mg/l IAA with 1mg/l BAP. Plants could be transferred to soil and grown to flowering in the glasshouse.Abbreviations GDC glycine decarboxylase - BAP benzyl aminopurine - NAA naphthalene acetic acid - IAA indole acetic acid     

Thyroliberin is rapidly transferred to the nucleus of GH3 pituitary cells at both 4°C and 37°C

The time-course of thyroliberin transfer to the nucleus of GH3/B6 rat pituitary prolactin cells was studied by both autoradiography and cell fractionation of intact cells exposed to [³H]thyroliberin at 4°C or 37°C. It was previously shown that thyroliberin is not degraded in these conditions. It is found by autoradiography that [³H]-thyroliberin is transferred to the nucleus of GH3/B6 cells within 5 min at least at both 37° C and 4°C. Consistent results are obtained by fractionation of cells exposed to [³H]thyroliberin at 37°C. However after binding at 4°C 50% of the cell radioactivity is extractible by glutaraldehyde and after fractionation the isolated nuclei retain only 1–1.5% of the cell radioactivity. This suggests the existence of both tightly bound and loosely bound internalized thyroliberin molecules.     

The relative contributions of reduced photorespiration,and improved water-and nitrogen-use efficiencies,to the advantages of C3−C4 intermediate photosynthesis in Flaveria

Summary Analyses of carbon-assimilation patterns in response to intercellular CO₂ concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C₃, a C₄, and three C₃–C₄ species in the genus Flaveria in order to determine some of the advantages and disadvantages of C₃–C₄ intermediate photosynthesis. Operational intercellular CO₂ partial pressures (pi), determined when the atmospheric CO₂ partial pressure (pa) was approximately 330 bar, in the C₃–C₄ species were generally equal to, or greater than, those observed in the C₃ species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C₃–C₄ species. The only case in which higher WUEs were observed in the C₃–C₄ species, compared to the C₃ species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 mol CO₂ m^-2s^-1. At higher photosynthesis rates, the C₃–C₄ species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C₃ species. Comparing slopes for the linear regions of the relationship between leaf nitrogen content and net photosynthesis rate (taken as an index of photosynthetic nitrogen-use efficiency, NUE), the C₄ species exhibited the highest NUE, followed by the C₃–C₄ species, F. ramosissima, with the other two C₃–C₄ species and the C₃ species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C₃–C₄ species F. pubescens and F. floridana suggest that in some C₃–C₄ Flaveria species C₄-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO₂-concentrating mechanism of fully-expressed C₄ plants.     

Transition from C3 to proto-Kranz to C3–C4 intermediate type in the genus Chenopodium (Chenopodiaceae)

The Chenopodiaceae is one of the families including C₄ species among eudicots. In this family, the genus Chenopodium is considered to include only C₃ species. However, we report here a transition from C₃ photosynthesis to proto-Kranz to C₃–C₄ intermediate type in Chenopodium. We investigated leaf anatomical and photosynthetic traits of 15 species, of which 8 species showed non-Kranz anatomy and a CO₂ compensation point (Γ) typical of C₃ plants. However, 5 species showed proto-Kranz anatomy and a C₃-like Γ, whereas C. strictum showed leaf anatomy and a Γ typical of C₃–C₄ intermediates. Chenopodium album accessions examined included both proto-Kranz and C₃–C₄ intermediate types, depending on locality. Glycine decarboxylase, a key photorespiratory enzyme that is involved in the decarboxylation of glycine, was located predominantly in the mesophyll (M) cells of C₃ species, in both M and bundle-sheath (BS) cells in proto-Kranz species, and exclusively in BS cells in C₃–C₄ intermediate species. The M/BS tissue area ratio, number of chloroplasts and mitochondria per BS cell, distribution of these organelles to the centripetal region of BS cells, the degree of inner positioning (vacuolar side of chloroplasts) of mitochondria in M cells, and the size of BS mitochondria also changed with the change in glycine decarboxylase localization. All Chenopodium species examined were C₃-like regarding activities and amounts of C₃ and C₄ photosynthetic enzymes and δ¹³C values, suggesting that these species perform photosynthesis without contribution of the C₄ cycle. This study demonstrates that Chenopodium is not a C₃ genus and is valuable for studying evolution of C₃–C₄ intermediates.

     

Evidence for a light-dependent system for reassimilation of photorespiratory CO2, which does not include a C4 cycle,in the C3−C4 intermediate species Moricandia arvensis

Three methods of estimating photorespiratory rate in leaves of the C₃–C₄ intermediate species Moricandia arvensis and the related C₃ species Moricandia moricandioides were compared. The results indicated that the photorespiratory rate in M. arvensis is less than in M. moricandioides, and that this is caused partly by reduced carbon flux through the photorespiratory pathway, and partly by the presence of a mechanism for enhanced photorespiratory CO₂ reassimilation in the intermediate species. Measurements of the CO₂ compensation point () in the two species supported this conclusion. A functional C₄ pathway is unlikely to be involved in the reduction of photorespiratory rate in M. arvensis since pulse-chase experiments showed that carbon did not move from C₄ acids to the reductive pentose-phosphate pathway in attached leaves under steady-state conditions at .Abbreviations and symbols APR apparent photosynthetic rate - C_i, C_e intercellular, external CO₂ concentration - CO₂ compensation point - PAR photosynthetically active radiation - PFD photon flux density     

A model of photosynthetic CO2 assimilation and carbon-isotope discrimination in leaves of certain C3−C4 intermediates

A model of leaf, photosynthesis has been developed for C₃–C₄ intermediate species found in the generaPanicum, Moricandia, Parthenium andMollugo where no functional C₄ pathway has been identified. Model assumptions are a functional C₃ cycle in both mesophyll and bundle-sheath cells and that glycine formed in the mesophyll, as a consequence of the oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39), diffuses to the bundle sheath, where most of the photorespiratory CO₂ is released. The model describes the observed gas-exchange characteristics of these C₃–C₄ intermediates, such as low CO₂-compensation points () at an O₂ pressure of 200 mbar, a curvilinear response of to changing O₂ pressures, and typical responses of CO₂-assimilation rate to intercellular CO₂ pressure. The model predicts that bundle-sheath CO₂ concentration is highest at low mesophyll CO₂ pressures and decreases as mesophyll CO₂ pressure increases. A partitioning of 5–15% of the total leaf Rubisco into the bundle-sheath cells and a bundlesheath conductance similar to that proposed for C₄ species best mimics the gas-exchange results. The model predicts C₃-like carbon-isotope discrimination for photosynthesis at atmospheric levels of CO₂, but at low CO₂ pressures it predicts a higher discrimination than is typically found during C₃ photosynthesis at lower CO₂ pressures.Abbreviations and symbols PEP phosphoenolpyruvate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) - RuBP ribulose-1,5-bisphosphate - p(CO₂) partial pressure of CO₂ - p(O₂) partial pressure of O₂. See also p. 471     

Effects of low atmospheric CO2 and elevated temperature during growth on the gas exchange responses of C3, C3–C4 intermediate, and C4 species from three evolutionary lineages of C4 photosynthesis

This study evaluates acclimation of photosynthesis and stomatal conductance in three evolutionary lineages of C(3), C(3)-C(4) intermediate, and C(4) species grown in the low CO(2) and hot conditions proposed to favo r the evolution of C(4) photosynthesis. Closely related C(3), C(3)-C(4), and C(4) species in the genera Flaveria, Heliotropium, and Alternanthera were grown near 380 and 180 μmol CO(2) mol(-1) air and day/night temperatures of 37/29°C. Growth CO(2) had no effect on photosynthetic capacity or nitrogen allocation to Rubisco and electron transport in any of the species. There was also no effect of growth CO(2) on photosynthetic and stomatal responses to intercellular CO(2) concentration. These results demonstrate little ability to acclimate to low CO(2) growth conditions in closely related C(3) and C(3)-C(4) species, indicating that, during past episodes of low CO(2), individual C(3) plants had little ability to adjust their photosynthetic physiology to compensate for carbon starvation. This deficiency could have favored selection for more efficient modes of carbon assimilation, such as C(3)-C(4) intermediacy. The C(3)-C(4) species had approximately 50% greater rates of net CO(2) assimilation than the C(3) species when measured at the growth conditions of 180 μmol mol(-1) and 37°C, demonstrating the superiority of the C(3)-C(4) pathway in low atmospheric CO(2) and hot climates of recent geological time.     

The diurnal course of leaf gas exchange of the C4 species Amaranthus retroflexus under field conditions in a ‘cool’ climate: Comparison with the C3 species Glycine max and Chenopodium album

Summary The gas exchange of leaves of Amaranthus retroflexus (C₄) measured under fluctuating environmental conditions in an experimental garden in Würzburg was compared with that of Glycine max and Chenopodium album (C₃). Consistent with previous observations, Amaranthus had higher leaf diffusion resistance than the C₃ species and low internal air space carbon dioxide concentration. Due to high photosynthetic capacity, Amaranthus fixed as much carbon during the light as the C₃ species, even at low temperatures and low light intensities. Low rates of dark respiration of leaves potentially enhances the ability of Amaranthus to grow rapidly after establishment in a disturbed habitat. The data suggest that some populations of Amaranthus retroflexus are adapted to cool climate conditions but are also able to exploit high temperature situations.     

Purification and properties of glycolate oxidase from plants with different photosynthetic pathways: Distinctness of C4 enzyme from that of a C3 species and a C3–C4 intermediate

Glycolate oxidase (GO; EC 1.1.3.1) was purified from the leaves of three plant species:Amaranthus hypochondriacus L.(NAD-ME type C₄ dicot),Pisum sativum L. (C₃ species) andParthenium hysterophorus L. (C₃–C₄. intermediate). A flavin moiety was present in the enzyme from all the three species. The enzyme from the C₄ plant had a low specific activity, exhibited lower K_M for glycolate, and required a lower pH for maximal activity, compared to the C₃ enzyme. The enzyme from the C₄ species oxidized glyoxylate at <10% of the rate with glycolate, while the GO from the C₃ plant oxidized glyoxylate at a rate of about 35 to 40% of that with glycolate. The sensitivity of GO from C₄ plant to -hydroxypyridinemethane sulfonate, 2-hydroxy-3-butynoate and other inhibitors was less than that of the enzyme from C₃ source. The properties of GO fromParthenium hysterophorus, were similar to those of the enzyme fromPisum sativum. The characteristics of glycolate oxidase from leaves of a C₄ plant,Amaranthus hypochondriacus are different from those of the C₃ species or the C₃–C₄ intermediate.     

Photorespiration in C3–C4 intermediate species of Alternanthera and Parthenium: Reduced ammonia production and increased capacity of CO2 refixation in the light

The pattern of photorespiratory ammonia (PR–NH₃) formation and its modulation by exogenous bicarbonate or glycine were investigated in C₃–C₄ intermediates of Alternanthera (A. ficoides and A. tenella) and Parthenium hysterophorus in comparison to those of C₃ or C₄ species. The average rates of PR–NH₃ accumulation in leaves of the intermediates were slightly less than (about 25% reduced) those in C₃ species, and were further low in C₄ plants (40% of that in C₃). The levels of PR–NH₃ in leaf discs decreased markedly when exogenous bicarbonate was present in the incubation medium. The inhibitory effect of bicarbonate on PR–NH₃ accumulation was pronounced in C₃ plants, very low in C₄ species and was moderate in the C₃–C₄ intermediates. Glycine, an intermediate of photorespiratory metabolism, raised the levels of PR–NH₃ in leaves of not only C₄ but also C₃–C₄ intermediates, bringing the rates close to those of C₃ species. The rate of mitochondrial glycine decarboxylation in darkness in C₃–C₄ intermediates was partially reduced (about 80% of that in C₃ species), corresponding to the activity-levels of glycine decarboxylase and serine hydroxymethyltransferase in leaves. The intermediates had a remarkable capacity of reassimilating photorespiratory CO₂ in vivo, as indicated by the apparent refixation of about 85% of the CO₂ released from exogenous glycine in the light. We suggest that the reduced photorespiration in the C₃–C₄ intermediate species of Alternanthera and Parthenium is due to both a limitation in the extent of glycine production/decarboxylation and an efficient refixation/recycling of internal CO₂.Abbreviations GDC glycine decarboxylase - GS glutamine synthetase - GOGAT glutamate-oxoglutarate aminotransferase - -HPMS -hydroxy-2-pyridinemethanesulfonic acid - INH isonicotinyl hydrazide - MSO L-methionine sulfoximine - PR–NH₃ photorespiratory-ammonia - SHMT serine hydroxymethyltransferase