Carbon Dioxide Fixation in the Carbon Economy of Developing Seeds of Lupinus albus (L.)

The effects of CO₂ concentration and illumination on net gas exchange and the pathway of ¹⁴CO₂ fixation in detached seeds from developing fruits of Lupinus albus (L.) have been studied.

Increasing the CO₂ concentration in the surrounding atmosphere (from 0.03 to 3.0% [v/v] in air) decreased CO₂ efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500 to 600 micro-Einsteins per square meter per second) in full sunlight or held in darkness. Above 1% CO₂ detached seeds made a net gain of CO₂ in the light (up to 0.4 milligrams of CO₂ fixed per gram fresh weight per hour) but ¹⁴CO₂ injected into the gas space of intact fruits (containing around 1.5% CO₂ naturally) was fixed mainly by the pod and little by the seeds.

Throughout development seeds contained ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39), especially in the embryo (up to 99 micromoles of CO₂ fixed per gram fresh weight per hour) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) in both testa (up to 280 micromoles of CO₂ fixed per gram fresh weight per hour) and embryo (up to 355 micromoles of CO₂ fixed per gram fresh weight per hour).

In kinetic experiments the most significant early formed product of ¹⁴CO₂ fixation in both light and dark was malate but in the light phosphoglyceric acid and sugar phosphates were also rapidly labeled. ¹⁴CO₂ fixation in the light was linked to the synthesis of sugars and amino acids but in the dark labeled sugars were not formed.

     

Enhanced Dark CO(2) Fixation by Preilluminated Chlorella pyrenoidosa and Anacystis nidulans

The products of short time photosynthesis and of enhanced dark ¹⁴CO₂ fixation (illumination in helium prior to addition of ¹⁴CO₂ in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the ¹⁴C assimilated during enhanced dark fixation while hexose and pentose phosphates accounted for the largest fraction of isotope assimilated during photosynthesis. During the enhanced dark fixation period, glycerate 3-phosphate is carboxyl labeled and glucose 6-phosphate is predominantly labeled in carbon atom 4 with lesser amounts in the upper half of the C₆ chain and traces in carbon atoms 5 and 6. Tracer spread throughout all the carbon atoms of photosynthetically synthesized glycerate 3-phosphate and glucose 6-phosphate. During the enhanced dark fixation period, there was a slow formation of sugar phosphates which subsequently continued at 5 times the initial rate long after the cessation of ¹⁴CO₂ uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO₂ fixation, the suggestion is made that most of the reductant mediating these effects did not have its origin in the preillumination phase.

It is concluded that a complete photosynthetic carbon reduction cycle operates to a limited extent, if at all, in the dark period subsequent to preillumination.

     

Regulation of chloroplast photosynthetic activity by exogenous magnesium

Magnesium was most inhibitory to photosynthetic reactions by intact chloroplasts when the magnesium was added in the dark before illumination. Two millimolar MgCl₂, added in the dark, inhibited CO₂-dependent O₂ evolution by Hordeum vulgare L. and Spinacia oleracea L. (C₃ plants) chloroplasts 70 to 100% and inhibited (pyruvate + oxaloacetate)-dependent O₂ evolution by Digitaria sanguinalis L. (C₄ plant) mesophyll chloroplasts from 80 to 100%. When Mg²⁺ was added in the light, O₂ evolution was reduced only slightly. O₂ evolution in the presence of phosphoglycerate was less sensitive to Mg²⁺ inhibition than was CO₂-dependent O₂ evolution.

Magnesium prevented the light activation of several photosynthetic enzymes. Two millimolar Mg²⁺ blocked the light activation of NADP-malate dehydrogenase in D. sanguinalis mesophyll chloroplasts, and the light activation of phosphoribulokinase, NADP-linked glyceraldehyde-3-phosphate dehydrogenase, and fructose 1,6-diphosphatase in barley chloroplasts. The results suggest that Mg²⁺ inhibits chloroplast photosynthesis by preventing the light activation of certain enzymes.

     

Relation between Mesophyll Surface Area, Photosynthetic Rate, and Illumination Level during Development for Leaves of Plectranthus parviflorus Henckel

The influence of illumination level during leaf development on the mesophyll cell surface area per unit leaf area (A^mes/A), CO₂ resistances, and the photosynthetic rate was determined for leaves of Plectranthus parviflorus Henckel. The relative importance of A^mes/A versus CO₂ resistances in accounting for observed changes in photosynthesis was quantitatively evaluated using equations based on analogies to electrical circuits.     

Photosynthetic Induction in a C(4) Dicot, Flaveria trinervia: I. Initial Products of CO(2) Assimilation and Levels of Whole Leaf C(4) Metabolites

Labeling patterns from ¹⁴CO₂ pulses to leaves and whole leaf metabolite contents were examined during photosynthetic induction in Flaveria trinervia, a C₄ dicot of the NADP-malic enzyme subgroup. During the first one to two minutes of illumination, malate was the primary initial product of ¹⁴CO₂ assimiltion (about 77% of total ¹⁴C incorporated). After about 5 minutes of illumination, the proportion of initial label to aspartate increased from 16 to 66%, and then gradually declined during the following 7 to 10 minutes of illumination. Nutrition experiments showed that the increase in ¹⁴CO₂ partitioning to aspartate was delayed about 2.5 minutes in plants grown with limiting N, and was highly dampened in plants previously treated 10 to 12 days with ammonia as the sole N source. Measurements of C₄ leaf metabolites revealed several transients in metabolite pools during the first few minutes of illumination, and subsequently, more gradual adjustments in pool sizes. These include a large initial decrease in malate (about 1.6 micromoles per milligram chlorophyll) and a small initial decrease in pyruvate. There was a transient increase in alanine levels after 1 minute of illumination, which was followed by a gradual, prolonged decrease during the remainder of the induction period. Total leaf aspartate decreased initially, but temporarily doubled in amount between 5 and 10 minutes of illumination (after its surge as a primary product). These results are discussed in terms of a plausible sequence of metabolic events which lead to the formation of the intercellular metabolite gradients required in C₄ photosynthesis.     

Enhancement of carbon dioxide fixation by alteration of illumination duringChlorella vulgaris-Buitenzorg's growth

Alteration of illumination with optimum carbon dioxide fixation-based curve in this research successfully enhanced the CO₂-fixation (q_co2) capability ofChlorella vulgaris Buitenzorg cultivated in a bubble column photo bioreactor. The level of CO₂ fixation was up to 1.91 times that observed from cultivation with intensification of illumination on an optimum growth-based curve. During 144 h of cultivation, alteration of light intensity on an optimum CO₂-fixation-based curve produced a q_CO2 of 6.68 h^?1. Increases in light intensity based on a curve of optimum CO₂-fixation produced a final cell concentration of about 5.78 g/L. Both cultivation methods were carried out under ambient pressure at a temperature of 29°C with a superficial gas velocity of 2.4 m/h (U_G). Cells were grown on Beneck medium in a 1.0 L Bubble Column Photo bioreactor illuminated by aPhillips Halogen Lamp (20 W/12 V/50 Hz). The inlet gas had a carbon dioxide content of 10%.     

Active Transport of CO(2) by the Cyanobacterium Synechococcus UTEX 625 : Measurement by Mass Spectrometry

Mass spectrometry has been used to confirm the presence of an active transport system for CO₂ in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO₃ in the absence of Na⁺ (to prevent HCO₃⁻ transport). Upon illumination the cells rapidly removed almost all the free CO₂ from the medium. Addition of carbonic anhydrase revealed that the CO₂ depletion resulted from a selective uptake of CO₂, rather than a total uptake of all inorganic carbon species. CO₂ transport stopped rapidly (<3 seconds) when the light was turned off. Iodoacetamide (3.3 millimolar) completely inhibited CO₂ fixation but had little effect on CO₂ transport. In iodoacetamide poisoned cells, transport of CO₂ occurred against a concentration gradient of about 18,000 to 1. Transport of CO₂ was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO₂ transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na⁺ (<100 microequivalents per liter), but not of K⁺, stimulated CO₂ transport as much as 2.4-fold. Unlike Na⁺-dependent HCO₃⁻ transport, the transport of CO₂ was not inhibited by high concentrations (30 milliequivalents per liter) of Li⁺. During illumination, the CO₂ concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO₂ transport was continuously occurring at a rapid rate, since the continuing dehydration of HCO₃⁻ to CO₂ would rapidly raise the CO₂ concentration to its equilibrium value if transport ceased. Measurement of the rate of dissolved inorganic carbon accumulation under these conditions indicated that at least part of the continuing CO₂ transport was balanced by HCO₃⁻ efflux.     

CO(2) Fixation in Opuntia Roots

Nonautotrophic CO₂ metabolism in Opuntia echinocarpa roots was studied with techniques of manometry and radiometry. The roots were grown in a one-quarter strength nutrient solution for several days; the distal 2 cm was used for physiological studies. The roots assimilated significant quantities of ¹⁴CO₂ and appeared to show a crassulacean-type acid metabolism with respect to quality and quantity. Most of the ¹⁴C activity was associated with the distal portion of the elongating root indicating correlation with metabolic activity. The ¹⁴CO₂ assimilation was comparable to a crassulacean leaf succulent, but 3 times greater than that found for stem tissue of the same Opuntia species.

The rates of O₂ and CO₂ exchange and estimated CO₂ fixation were 180, 123, and 57 μl/g per hour. A respiratory quotient of 0.66 was found.

The products of ¹⁴CO₂ fixation were similar in most respects to reported experiments with leaf succulents. Equilibration of the predominant malic acid with isocitric, succinic, and fumaric acids was not evident. The latter observation was interpreted as metabolic isolation of the fixation products rather than poor citric acid cycle activity.

A rapid turnover of the fixed ¹⁴CO₂ was measured by following decarboxlyation kinetics and by product analysis after a postincubation period. The first order rate constant for the steady state release was 4.4 × 10⁻³ min⁻¹ with a half-time of 157.5 minutes. Amino acids decayed at a more rapid rate than organic acids.

     

Simple oscillations in photosynthesis of higher plants

Illumination of leaves of Vicia faba L. provoked oscillations in the rates of CO₂ uptake and O₂ evolution. The oscillations were marked under anaerobic conditions, but were absent at 20% O₂. The minimum CO₂ concentration required for the appearance of oscillations was 600 μl · l^?1. The higher the CO₂ concentration, the stronger the oscillations. The effect of CO₂ concentration was saturated at 1000 μl CO₂ · l^?1. The period of the oscillations was 5–6 min at a light intensity of 80 nE · cm^?2 · s^?1 and became longer on lowering of the intensity. No oscillations appeared at intensities below 12 nE · cm^?2 · s^?1. Oscillations could also be generated by increasing the CO₂ concentration in the atmosphere during strong illumination under anaerobic conditions. The chlorophyl a fluorescence yield showed oscillations, similar in shape and frequency to those of photosynthesis, after such an environmental change. Oscillations were also observed in photosynthesis of other C₃ plants, Lycopersicon esulentum Mill and Glycine max Merrill, under the same conditions as those required for V. faba, but were absent for the C₄ plants, Zea mays and Amaranthus retroflexus L.     

Malate synthesis by dark carbon dioxide fixation in leaves

The rates of dark CO₂ fixation and the label distribution in malate following dark ¹⁴CO₂ fixation in a C-4 plant (maize), a C-3 plant (sunflower), and two Crassulacean acid metabolism plants (Bryophyllum calycinum and Kalanchoë diagremontianum leaves and plantlets) are compared. Within the first 30 minutes of dark ¹⁴CO₂ fixation, leaves of maize, B. calycinum, and sunflower, and K. diagremontianum plantlets fix CO₂ at rates of 1.4, 3.4, 0.23, and 1.0 μmoles of CO₂/mg of chlorophyll· hour, respectively. Net CO₂ fixation stops within 3 hours in maize and sunflower, but Crassulaceans continue fixing CO₂ for the duration of the 23-hour experiment.

A bacterial procedure using Lactobacillus plantarum ATCC No. 8014 and one using malic enzyme to remove the β-carboxyl (C₄) from malate are compared. It is reported that highly purified malic enzyme and the bacterial method provide equivalent results. Less purified malic enzyme may overestimate the label in C₄ as much as 15 to 20%.

The contribution of carbon atom 1 of malate is between 18 and 21% of the total carboxyl label after 1 minute of dark CO₂ fixation. Isotopic labeling in the two carboxyls approached unity with time. The rate of increase is greatest in sunflower leaves and Kalanchoë plantlets. In addition, Kalanchoë leaves fix ¹⁴CO₂ more rapidly than Kalanchoë plantlets and the equilibration of the malate carboxyls occurs more slowly. The rates of fixation and the randomization are tissue-specific. The rate of fixation does not correlate with the rate of randomization of isotope in the malate carboxyls.

     

Some observations on the carbon dioxide burst in chlorella and chlamydomonas

In the course of mass spectrometer studies with the algae Chlorella and Chlamydomonas, data were obtained which indicate that the CO₂ burst and gulp are sensitive to oxygen. Furthermore, the CO₂ burst was found to be strongly suppressed when wave lengths shorter than 460 nanometers were blocked at intensities adequate to saturate photosynthesis. Under appropriate conditions at 30°, the CO₂ burst was interrupted by a brief CO₂ gulp and the post illumination gulp by a brief burst of CO₂. The post illumination gulp of CO₂ could be induced during illumination by interposition of a filter blocking wave lengths shorter than 460 nanometers. These data are discussed in relation to earlier reports of the phenomenon and briefly as they affect the detection of photorespiration.     

Conversion of photosynthetic products in the light in CO2-free O2 and N2 in leaves of Zea mays L. and Phaseolus vulgaris L.

Summary After 10 min illumination of segments of bean (Phaseolus vulgaris L.) or maize (Zea mays L.) leaves in air with ¹⁴CO₂, 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 ¹⁴CO₂ assimilation period the bean leaves contain the pool of weakly fixed ¹⁴C (WF-¹⁴C) which is converted into stable products during the subsequent period of illumination in CO₂-free N₂. In O₂ atmosphere the WF-¹⁴C 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 ¹⁴C remained almost on the same level during illumination of maize leaves in N₂ as well as in O₂. However, oxygen changed the distribution of ¹⁴C in photosynthetic products, which is suggested to be the consequence of the photorespiration process in maize.Abbreviation WF-¹⁴C weakly fixed ¹⁴C     

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

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

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

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

     

Photooxidative Damage in Photosynthetic Activities of Chromatium vinosum

The capacity of photosynthetic CO₂ fixation in the anaerobic purple-sulfur bacterium, Chromatium vinosum is markedly impaired by strong illumination (9 × 10⁴ lux) in the presence of 100% O₂. In the absence of HCO₃⁻, decline in activity occurred gradually, with about 40% of the initial activity remaining after a 1-hour incubation. The addition of 50 millimolar HCO₃⁻ to the incubation medium resulted in a measurable delay (about 30 minutes) of the inactivation process. Ribulose-1,5-bisphosphate carboxylase activity and light-dependent O₂ uptake (electron flow) or crude extracts prepared after pretreatment of the bacterial cells with O₂ and light were not affected but the photophosphorylation capacity of either bacterial cells or chromatophores was drastically reduced. The inhibition of photophos-phorylation in the chromatophore preparations was significantly reduced by the addition of either an O₂⁻ scavenger, Tiron, or an ¹O₂ scavenger, α-tocopherol. These results suggest that the active O₂ species, O₂⁻ or ¹O₂, might take part in the observed inactivation.

The pretreatment of the bacteria with O₂ and light inhibited CO₂ assimilation through the Calvin-Benson cycle, while relatively stimulating the formation of aspartate and glutamate. It also inhibited the conversion of glycolate to glycine, resulting in a sustained extracellular excretion of glycolate. The inactivation of photosynthetic CO₂ fixation by intact cells was enhanced by low temperature, KCN, or methylviologen addition during the pretreatment with O₂ and light. The mechanism(s) of O₂-dependent photoinactivation of photosynthetic activities in Chromatium are discussed in relation to the possible role of photorespiration as a means of producing CO₂ in the photosynthetic system.

     

Influence of adenosine phosphates and magnesium on photosynthesis in chloroplasts from peas, sedum, and spinach

¹⁴CO₂ photoassimilation in the presence of MgATP, MgADP, and MgAMP was investigated using intact chloroplasts from Sedum praealtum, a Crassulacean acid metabolism plant, and two C₃ plants: spinach and peas. Inasmuch as free ATP, ADP, AMP, and uncomplexed Mg²⁺ were present in the assays, their influence upon CO₂ assimilation was also examined. Free Mg²⁺ was inhibitory with all chloroplasts, as were ADP and AMP in chloroplasts from Sedum and peas. With Sedum chloroplasts in the presence of ADP, the time course of assimilation was linear. However, with pea chloroplasts, ADP inhibition became progressively more severe, resulting in a curved time course. ATP stimulated assimilation only in pea chloroplasts. MgATP and MgADP stimulated assimilation in all chloroplasts. ADP inhibition of CO₂ assimilation was maximal at optimum orthophosphate concentrations in Sedum chloroplasts, while MgATP stimulation was maximal at optimum or below optimum concentrations of orthophosphate. MgATP stimulation in peas and Sedum and ADP inhibition in Sedum were not sensitive to the addition of glycerate 3-phosphate (PGA).

PGA-supported O₂ evolution by pea chloroplasts was not inhibited immediately by ADP; the rate of O₂ evolution slowed as time passed, corresponding to the effect of ADP on CO₂ assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhibition of PGA-dependent O₂ evolution became more severe with time. This did not mirror CO₂ assimilation, which was inhibited immediately by AMP. In Sedum chloroplasts, PGA-dependent O₂ evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedum, the magnitude of MgADP and MgATP stimulation of PGA-dependent O₂ evolution was not much larger than that given by ATP, and it was much smaller than MgATP stimulation of CO₂ assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisphosphate carboxylase was inhibited by ADP and stimulated by MgADP in Sedum chloroplasts.

The appearance of label in the medium was measured when [U-¹⁴C] ADP-loaded Sedum chloroplasts were challenged with ATP, ADP, or AMP and their Mg²⁺ complexes. The rate of back exchange was stimulated by the presence of Mg²⁺. This suggests that ATP, ADP, and AMP penetrate the chloroplast slower than their Mg²⁺ complexes. A portion of the CO₂ assimilation and O₂ evolution data could be explained by differential penetration rates, and other proposals were made to explain the remainder of the observations.

     

Diurnal and Seasonal Patterns of Photosynthesis and Respiration by Stems of Populus tremuloides Michx

The photosynthetic and respiratory rates of 5- to 7-year-old aspen stems (Populus tremuloides Michx.) were monitored in the field for 1 year to determine the seasonal patterns. The stem was not capable of net photosynthesis, but the respiratory CO₂ loss from the stem was reduced by 0 to 100% depending on the time of year and the level of illumination as a result of bark photosynthesis. The monthly dark respiratory rate ranged from 0.24 mg CO₂/dm²· hr in January to a maximum 7.4 mg CO₂/dm²· hr in June. Individual measurements ranged from 0.02 mg CO₂/dm²· hr in February to 12.3 mg CO₂/dm²· hr in June. Gross photosynthesis followed a pattern similar to the dark respiratory rate. The mean monthly rate was highest in June (1.65 mg CO₂/dm²· hr) and lowest in December (0.02 mg CO₂/dm²· hr). Individual measurements ranged from 0.0 mg CO₂/dm²· hr in winter to 5.5 mg CO₂/dm²· hr in July.     

Partitioning of CO2 Fixation in the Colonial Cyanobacterium Microcystis aeruginosa: Mechanism Promoting Formation of Surface Scums

Constraints on inorganic carbon (C_i) availability stimulated buoyancy in natural, photosynthetically active populations of the colonial blue-green alga (cyanobacterium) Microcystis aeruginosa. In nonmixed eutrophic river water and cultures, O₂ evolution determinations indicated C_i limitation of photosynthesis, which was overcome either by CO₂ additions to the aqueous phase or by exposure of buoyant colonies to atmospheric CO₂. Microautoradiographs of M. aeruginosa colonies revealed partitioning of ¹⁴CO₂ fixation and photosynthate accumulation between peripheral and internal cells, particularly in large colonies. When illuminated colonies were suspended in the aqueous phase, peripheral cells accounted for at least 90% of the ¹⁴CO₂ assimilation, whereas internal cells remained unlabeled. However, when ¹⁴CO₂ was allowed to diffuse into colonies 15 min before illumination, a more uniform distribution of labeling was observed. Resultant differences in labeling patterns were most likely due to peripheral cells more exclusively utilizing CO₂ when ambient C_i concentrations were low. Among colonies located at the air-water interface, internal cells showed an increased share of photosynthate production when atmospheric ¹⁴CO₂ was supplied. This indicated that C_i transport was restricted in large colonies below the water surface, forcing internal cells to maintain a high degree of buoyancy, thus promoting the formation of surface scums. At the surface, C_i restrictions were alleviated. Accordingly, scum formation appears to have an ecological function, allowing cyanobacteria access to atmospheric CO₂ when the C_i concentration is growth limiting in the water column.     

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

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

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

     

Prior illumination and the respiration of maize leaves in the dark

The course of respiration of attached maize (Zea mays L.) leaves was measured by infrared gas analysis of CO₂ efflux in the dark following illumination in atmospheres of 300 microliters of CO₂ per liter of air, CO₂-free air, and CO₂-free N₂ containing 400 microliters of O₂ per liter. CO₂ efflux from control leaves started 3 to 4 minutes after darkening, increased to a maximum after about 20 minutes, and returned to a steady minimum after 2 to 3 hours. Respiration was quantitatively related to prior illumination, independent of net CO₂ fixation in the light, and depressed by N₂. Light, but not air, was required to produce a substrate for respiration in the subsequent dark period; air was required for oxidation of the substrate to CO₂. The stimulation of respiration by prior illumination in maize leaves differs in its slower onset and greater duration from the postillumination burst of photorespiration.     

Light-Induced Alkalinization of the Suspending Medium of Guard Cell Protoplasts from Vicia faba L

The light-dependent pH changes in the suspending medium of guard cell protoplasts (GCP) from Vicia faba were studied. Upon illumination, the medium was initially slightly alkalinized and then acidified. The extent of alkalinization was lower in CO₂-free air than in normal air. This initial alkalinization was inhibited by DCMU. Acidification in CO₂-free air became observable in shorter duration of light exposure than that in normal air. The rate of acidification was higher in CO₂-free air than in normal air. The CO₂ level of the medium decreased in the light, and increased in the dark. ¹⁴CO₂ uptake was enhanced 2- to 3-fold by light, but not in the presence of DCMU. These results indicate that photosynthetic CO₂ fixation does take place in GCP and that the initial alkalinization is due to this photosynthetic CO₂ uptake. Diethylstilbestrol, a nonmitochondrial membrane-bound ATPase inhibitor, inhibited the acidification, suggesting that the acidification resulted from H⁺ extrusion by GCP. The acidification in light was also prevented by KCN, and partly by DCMU. Possible mechanisms of alkalinization and acidification are discussed in relation to guard cell metabolism.