Stoichiometry of carbon dioxide release and oxygen uptake during glycine oxidation in mitochondria isolated from spinach (Spinacia oleracea) leaves.

Mitochondria isolated from spinach (Spinacia oleracea) leaves oxidized glycine with a stoichiometry of CO2 evolution to O2 uptake of 2 : 1. In the absence of added substrate, the mitochondria exhibited an extremely low endogenous rate of O2 uptake.     

Calibration of simultaneous measurements of photosynthetic carbon dioxide uptake and oxygen evolution in leaves

The stoichiometric ratio of O2 evolution to CO2 uptake during photosynthesis reveals information about reductive metabolism, including the reduction of alternative electron acceptors, such as nitrite and oxaloacetate. Recently we reported that in simultaneous measurements of CO2 uptake and O2 evolution in a sunflower leaf, O2 evolution changed by 7% more than CO2 uptake when light intensity was varied. Since the O2/CO2 exchange ratio is approximately 1, small differences are important. Thus, these gas exchange measurements need precise calibration. In this work, we describe a new calibration procedure for such simultaneous measurements, based on the changes of O2 concentration caused by the addition of pure CO2 or O2 into a flow of dry air (20.95% O2) through one and the same capillary. The relative decrease in O2 concentration during the addition of CO2 and the relative increase in O2 concentration during the addition of O2 allowed us to calibrate the CO2 and O2 scales of the measurement system with an error (relative standard deviation, RSD) of <1%. Measurements on a sunflower leaf resulted in an O2/CO2 ratio between 1.0 and 1.03 under different CO2 concentrations and light intensities, in the presence of an ambient O2 concentration of 20-50 micromol mol(-1). This shows that the percentage use of reductive power from photochemistry in synthesis of inorganic or organic matter other than CO2 assimilation in the C3 cycle is very low in mature leaves and, correspondingly, the reduction of alternative acceptors is a weak source of coupled ATP synthesis.     

The uptake of carbon dioxide by plant roots

Summary The uptake of C¹⁴O₂ by the roots of intact tomato plants from solution containing Na₂C¹⁴O₃ was studied at different light intensities as well as in darkness.Where plants had previously been starved for CO₂ for 12 hours, a higher rate of C¹⁴ uptake was observed than with plants which had been transferred directly from the soil to the radioactive solution.In general, the C¹⁴ content of the roots was slightly higher than that of the shoots. At light intensities under the compensation point and in darkness the C¹⁴ content of the shoots relative to the roots decreased. This was accompanied by release of C¹⁴O₂ during respiration, indicating that the absorbed C¹⁴ was readily translocated upwards and released as C¹⁴O₂ under these conditions. At light intensities above the compensation point no C¹⁴O₂ was released.     

On the dynamics of internal leaf carbon dioxide uptake

A model is proposed to investigate the hypothesis that the observed time course of whole leaf photosynthetic responses to changes in incident light energy are caused by diffusional limitations. The model leaf consists of a continuously distributed mesophyll with diffusion of CO₂ in the leaf interior governed by a reaction-diffusion equation. Biochemical activation is assumed to occur on a fast time scale. Both the cases of a homogeneous and a nonhomogeneous leaf interior are investigated. Using parameter estimates available in the literature, the model predicts CO₂ uptake equilibration times which are much smaller than those observed. The results strongly suggest that diffusional limitations do not significantly affect photosynthetic dynamics in variable light environments.     

The role of carbon dioxide and oxygen in determining chlorophyll fluorescence quenching during leaf development

Chlorophyll fluorescence emission at 680 nm (F680) and the rate of CO₂ fixation were measured simultaneously in sections along the length of wheat and maize leaves. These leaves possess a basal meristem and show a gradation in development towards the leaf tip. The redox state of the primary electron acceptor, Q, of photosystem II was estimated using a non-invasive method. Distal mature leaf sections displayed typical F680 induction curves which were generally anti-parallel with CO₂ fixation and during which Q became gradually oxidised. In leaf-base sections net assimilation of CO₂ was not detectable, F680 quenched slowly and monotonously without displaying any of the oscillations typical of mature tissue and Q remained relatively reduced. Sections cut from mid-regions of the leaf showed intermediate characteristics. There were no major differences between the wheat and maize leaf in the parameters measured. The results support the hypothesis that generation of the transthylakoid proton gradient and associated ATP production is not a major limitation to photosynthesis during leaf development in either C₃ or C₄ plants. Removal of CO₂ from the mature leaf sections caused little change in steady-state F680 and produced about 50% reduction of Q. When O₂ was then removed, F680 rose sharply and Q became almost totally reduced. In immature tissue unable to assimilate CO₂, removal of O₂ alone caused a similar large rise in F680 and reduction of Q whilst removal of CO₂ had negligible effects on F680 and the redox state of Q. It is concluded that in leaf tissue unable to assimilate CO₂, either because CO₂ is absent or the tissue is immature, O₂ acts as an electron acceptor and maintains Q in a partially oxidised state. The important implication that O₂ may have a role in the prevention of photoinhibition of the photochemical apparatus in the developing leaf is discussed.Abbreviations F680 chlorophyll fluorescence emission at 680 nm - PSI photosystem I - PSII photosystem II - Q PSII primary electron acceptor - pH transthylakoid proton gradient     

Apparatus for monitoring the oxygen uptake and carbon dioxide production of fermentations

The requirements of the continuous analysis of effluent gas streams from aerated flash and tank fermentors are described, as are instrumental devices for measuring the oxygen and carbon dioxide content of fermentor gases. The use of a specially designed sequential gas sample for monitoring four fermentations simultaneously and a system for precise control of low air flow and pressure is explained. Equations for calculating carbon dioxide production or oxygen consumption rates and respiratory quotients are given. A discussion of the operating characteristics of a device for automatic translation of aeration data between fermentors is presented.     

The uptake and release of calcium by heart mitochondria

The kinetics of uptake of Ca(2+) by rat heart mitochondria were studied by a spectrophotometric method with Arsenazo III indicator. The exponential rate coefficients measured with or without added phosphate increase with the amount of Ca(2+) added up to about 24mum. Evidence is given that the effect is attributable to a combination of formation of chelates at low concentrations to act as Ca(2+) buffers, with co-transport of substrate to provide more respiratory fuel. The inhibitory effect of Mg(2+) depends on the Ca(2+) concentration, so with a constant [Mg(2+)] the low concentrations of Ca(2+) are most inhibited, and the rate coefficients are still more Ca(2+)-dependent. Ca(2+) uptake is slowed by local anaesthetics such as butacaine and dibucaine, and also by propranolol and palmitoyl-CoA. After an uptake, the release of Ca(2+) was investigated. The spontaneous release involves an initially slow and small appearance of free Ca(2+) and is followed by an auto-accelerated phase. The release is accompanied by a gradual decrease in internal ATP; it is initiated by palmitoyl-CoA (reversed by carnitine), by lysophosphatidylcholine, by Na(+) salts (reversed by oligomycin) and by K(+) salts added to a K(+)-free medium containing valinomycin. The process is probably a response to an increased energy load imposed on the mitochondria by the various conditions, which include the spontaneous action of phospholipase activated by traces of Ca(2+). The problem of how much mitochondrial activity is participating in normal heart Ca(2+) turnover is discussed, and experiments showing only 7-14% exchange of the mitochondrial Ca(2+) occurring in vivo in 10 or 20min are reported.     

Effect of oxygen concentration on leaf photosynthesis and resistances to carbon dioxide diffusion

Summary Net photosynthesis of tropical legume leaves increased by 44% and that of tropical grass leaves was unaffected when oxygen concentration was reduced from 21 to 0.2%. Stomatal resistance to carbon dioxide diffusion was unaltered in both cases but mesophyll resistance of legume leaves decreased with oxygen concentration. It is proposed that the decrease in mesophyll resistance is accompanied by decreases in excitation and carboxylation resistances.     

Simultaneous oxidation of glycine and malate by pea leaf mitochondria

Mitochondria isolated from pea leaves (Pisum sativum L.) readily oxidized malate and glycine as substrates. The addition of glycine to mitochondria oxidizing malate in state 3 diminished the rate of malate oxidation. When glycine was added to mitochondria oxidizing malate in state 4, however, the rate of malate oxidation was either unaffected or stimulated. The reason both glycine and malate can be metabolized in state 4 appears to be that malate only used part of the electron transport capacity available in these mitochondria in this state. The remaining electron transport capacity was used by glycine, thus allowing both substrates to be oxidized simultaneously. This can be explained by differential use of two NADH dehydrogenases by glycine and malate and an increase in alternate oxidase activity upon glycine addition. These results help explain why photorespiratory glycine oxidation and its associated demand for NAD do not inhibit citric acid cycle function in leaves.     

Effects of glycine hydroxamate, carbon dioxide, and oxygen on photorespiratory carbon and nitrogen metabolism in spinach mesophyll cells

The effects of added glycine hydroxamate on the photosynthetic incorporation of ¹⁴CO₂ into metabolites by isolated mesophyll cells of spinach (Spinacia oleracea L.) was investigated under conditions favorable to photorespiratory (PR) metabolism (0.04% CO₂ and 20% O₂) and under conditions leading to nonphotorespiratory (NPR) metabolism (0.2% CO₂ and 2.7% O₂). Glycine hydroxamate (GH) is a competitive inhibitor of the photorespiratory conversion of glycine to serine, CO₂ and NH₄⁺. During PR fixation, addition of the inhibitor increased glycine and decreased glutamine labeling. In contrast, labeling of glycine decreased under NPR conditions. This suggests that when the rate of glycolate synthesis is slow, the primary route of glycine synthesis is through serine rather than from glycolate. GH addition increased serine labeling under PR conditions but not under NPR conditions. This increase in serine labeling at a time when glycine to serine conversion is partially blocked by the inhibitor may be due to serine accumulation via the “reverse” flow of photorespiration from 3-P-glycerate to hydroxypyruvate when glycine levels are high. GH increased glyoxylate and decreased glycolate labeling. These observations are discussed with respect to possible glyoxylate feedback inhibition of photorespiration.     

Hydrogen peroxide production and the release of carbon dioxide during glycollate oxidation in leaf peroxisomes

Summary The rate at which H₂O₂ becomes available during glycollate oxidation for further oxidation reactions, especially that of glyoxylate to formate and CO₂, in peroxisomes from spinach-beet (Beta vulgaris L., var. vulgaris) leaves has been determined by measuring O₂ uptake in the presence and absence of added catalase. The rates observed under air and pure O₂ were sufficient to account for the ¹⁴CO₂ released from [l-¹⁴C]glycollate under these conditions; the two reactions showed similar characteristics. In the course of the reaction, a fall in catalase activity was observed concomitant with an increase in ¹⁴CO₂ release. There is no evidence that catalase was disproportionately lost from the peroxisomes during isolation, and it is argued that the CO₂ release observed contributes to the photorespiratory CO₂ loss in intact leaves.Abbreviations DCPIP 2,6-dichlorophenolindophenol - FMN Flavin mononucleotide     

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

Concurrent O₂ evolution, O₂ uptake, and CO₂ uptake by illuminated maize (Zea mays) leaves were measured using ¹³CO₂ and ¹⁸O₂. Considerable O₂ uptake occurred during active photosynthesis. At CO₂ compensation, O₂ uptake increased. Associated with this increase was a decrease in O₂ release such that a stoichiometric exchange of O₂ occurred. The rate of O₂ exchange at CO₂ compensation was directly related to O₂ concentration in the atmosphere at least up to 8% (v/v).     

The interaction between oxygen and carbon dioxide movements in fishes

Many teleost fishes have haemoglobins which possess a Root effect, a large Haldane effect and a low buffer capacity. This combination of characteristics influences the interaction between movements of oxygen and carbon dioxide in the red cell, in the respiratory epithelium, and in the tissues. The presence of the Root effect may limit oxygen uptake at the gills due to an accumulation of Bohr protons released upon oxygenation. However, the Root effect is probably important in maintaining or elevating blood PO₂ during muscle capillary transit, enhancing oxygen delivery to the tissues.Bohr protons are reversibly bound to haemoglobin. The release of Bohr protons during oxygenation facilitates bicarbonate dehydration at the gills, while Bohr proton binding facilitates CO₂ hydration at the tissues. In some teleost fishes, most of the Bohr protons are released and bound to haemoglobin, between 50 and 100% of haemoglobin-oxygen saturation (27). This trait is probably significant in maximizing oxygen uptake at the gills and in conserving body CO₂ stores during exposure to hypoxia and exercise, when the lower reaches of the haemoglobin-oxygen equilibrium curve are used.