Relation of CO(2) Compensation Concentration to Apparent Photosynthesis in Maize

Significant differences in CO₂ compensation concentration measured in the field among varieties of the species Zea mays L. are reported for the first time. CO₂ compensation concentrations were significantly (P≤ 0.01) and negatively correlated with apparent photosynthesis at 300 μl CO₂/liter air. The Michaelis constant (as defined) for a leaf was significantly (P≤ 0.01) and positively correlated with apparent photosynthesis among varieties. While the first correlation is similar to behavior of CO₂ compensation among species of different photosynthetic efficiency, the latter correlation is the converse of the behavior of Km among species.     

The Regulation of Photosynthesis in Leaves of Field-Grown Spring Wheat (Triticum aestivum L., cv Albis) at Different Levels of Ozone in Ambient Air

Wheat (Triticum aestivum L. cv Albis) was grown in open-top chambers in the field and fumigated daily with charcoal-filtered air (0.015 microliters per liter O₃), nonfiltered air (0.03 microliters per liter O₃), and air enriched with either 0.07 or 0.10 microliters per liter ozone (seasonal 8 hour/day [9 am-5 pm] mean ozone concentration from June 1 until July 10, 1987). Photosynthetic ¹⁴CO₂ uptake was measured in situ. Net photosynthesis, dark respiration, and CO₂ compensation concentration at 2 and 21% O₂ were measured in the laboratory. Leaf segments were freeze-clamped in situ for the determination of the steady state levels of ribulose 1,5-bisphosphate, 3-phosphoglycerate, triose-phosphate, ATP, ADP, AMP, and activity of ribulose, 1,5-bisphosphate carboxylase/oxygenase. Photosynthesis of flag leaves was highest in filtered air and decreased in response to increasing mean ozone concentration. CO₂ compensation concentration and the ratio of dark respiration to net photosynthesis increased with ozone concentration. The decrease in photosynthesis was associated with a decrease in chlorophyll, soluble protein, ribulose bisphosphate carboxylase/oxygenase activity, ribulose bisphosphate, and adenylates. No decrease was found for triose-phosphate and 3-phosphoglycerate. The ratio of ATP to ADP and of triosephosphate to 3-phosphoglycerate were increased suggesting that photosynthesis was limited by pentose phosphate reductive cycle activity. No limitation occurred due to decreased access of CO₂ to photosynthetic cells since the decrease in stomatal conductance with increasing ozone concentration did not account for the decrease in photosynthesis. Ozonestressed leaves showed an increased degree of activation of ribulose bisphosphate carboxylase/oxygenase and a decreased ratio of ribulose bisphosphate to initial activity of ribulose bisphosphate carboxylase/oxygenase. Nevertheless, it is suggested that photosynthesis in ozone stressed leaves is limited by ribulose bisphosphate carboxylation possibly due to an effect of ozone on the catalysis by ribulose bisphosphate carboxylase/oxygenase.     

Effect of the Long-Term Elevation of CO(2) Concentration in the Field on the Quantum Yield of Photosynthesis of the C(3) Sedge, Scirpus olneyi

CO₂ concentration was elevated throughout 3 years around stands of the C₃ sedge Scirpus olneyi on a tidal marsh of the Chesapeake Bay. The hypothesis that tissues developed in an elevated CO₂ atmosphere will show an acclimatory decrease in photosynthetic capacity under light-limiting conditions was examined. The absorbed light quantum yield of CO₂ uptake (ø_abs and the efficiency of photosystem II photochemistry were determined for plants which had developed in open top chambers with CO₂ concentrations in air of 680 micromoles per mole, and of 351 micromoles per mole as controls. An Ulbricht sphere cuvette incorporated into an open gas exchange system was used to determine ø_abs and a portable chlorophyll fluorimeter was used to estimate the photochemical efficiency of photosystem II. When measured in an atmosphere with 10 millimoles per mole O₂ to suppress photorespiration, shoots showed a ø_abs of 0.093 ± 0.003, with no statistically significant difference between shoots grown in elevated or control CO₂ concentrations. Efficiency of photosystem II photochemistry was also unchanged by development in an elevated CO₂ atmosphere. Shoots grown and measured in 680 micromoles per mole of CO₂ in air showed a ø_abs of 0.078 ± 0.004 compared with 0.065 ± 0.003 for leaves grown and measured in 351 micromoles per mole CO₂ in air; a highly significant increase. In accordance with the change in ø_abs, the light compensation point of photosynthesis decreased from 51 ± 3 to 31 ± 3 micro-moles per square meter per second for stems grown and measured in 351 and 680 micromoles per mole of CO₂ in air, respectively. The results suggest that even after 3 years of growth in elevated CO₂, there is no evidence of acclimation in capacity for photosynthesis under light-limited conditions which would counteract the stimulation of photosynthetic CO₂ uptake otherwise expected through decreased photorespiration.     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

Photosynthesis, Transpiration, Leaf Stomatal and Mesophyll Resistance Measurements by the Use of a Ventilated Diffusion Porometer

A ventilated diffusion porometer was modified and adapted for simultaneous measurements of leaf resistance and photosynthesis (using ¹⁴C). The system enables measurements to be made under field and laboratory conditions with different concentrations of CO₂ and vapor pressure gradients between the evaporating surfaces inside the leaf and the external atmosphere. The leaf is subjected to the porometer's atmosphere only for short periods (up to 30 seconds) and it is assumed that stomata are not affected. Establishing the linear regression of the effect of CO₂ concentration on net photosynthesis makes it possible to extrapolate for CO₂ compensation point, to calculate the overall resistance to CO₂ and the mesophyll resistance to CO₂.     

Land plants equilibrate O2 and CO2 concentrations in the atmosphere

The role of land plants in establishing our present day atmosphere is analysed. Before the evolution of land plants, photosynthesis by marine and fresh water organisms was not intensive enough to deplete CO₂ from the atmosphere, the concentration of which was more than the order of magnitude higher than present. With the appearance of land plants, the exudation of organic acids by roots, following respiratory and photorespiratory metabolism, led to phosphate weathering from rocks thus increasing aquatic productivity. Weathering also replaced silicates by carbonates, thus decreasing the atmospheric CO₂ concentration. As a result of both intensive photosynthesis and weathering, CO₂ was depleted from the atmosphere down to low values approaching the compensation point of land plants. During the same time period, the atmospheric O₂ concentration increased to maximum levels about 300 million years ago (Permo-Carboniferous boundary), establishing an O₂/CO₂ ratio above 1000. At this point, land plant productivity and weathering strongly decreased, exerting negative feedback on aquatic productivity. Increased CO₂ concentrations were triggered by asteroid impacts and volcanic activity and in the Mesozoic era could be related to the gymnosperm flora with lower metabolic and weathering rates. A high O₂/CO₂ ratio is metabolically linked to the formation of citrate and oxalate, the main factors causing weathering, and to the production of reactive oxygen species, which triggered mutations and stimulated the evolution of land plants. The development of angiosperms resulted in a decrease in CO₂ concentration during the Cenozoic era, which finally led to the glacial-interglacial oscillations in the Pleistocene epoch. Photorespiration, the rate of which is directly related to the O₂/CO₂ ratio, due to the dual function of Rubisco, may be an important mechanism in maintaining the limits of O₂ and CO₂ concentrations by restricting land plant productivity and weathering.     

Changes in the Water Balance and Photosynthesis of Onion, Bean and Cotton Plants under Saline Conditions

The osmotic concentration (osmotic potential) of onion leaf sap did not adjust to chloride salinity, and consequently water potential, turgor, stomatal aperture and transpiration were reduced. Although osmotic concentration of bean and cotton leaf sap did adjust to a saline root medium and turgor was no less in the salinized plants than in the controls, stomata of the salinized plants remained only partly open and transpiration was reduced. Net photosynthesis of onion plants was reduced by salinity (this effect being much enhanced in a hot dry atmosphere) but it could be rapidly raised to the level of the controls by inducing elevated leaf turgor. Stomatal closure was initially responsible for most of the ～30 % reduction in photosynthesis of salinized beans. This was due to interference with CO₂ diffusion and could be overcome by raising the CO₂ concentration in the air. At a later stage of growth, salinity affected the light reaction of bean photosynthesis, and elevation of the air CO₂ had little effect. Closure of stomata of salinized cotton plants had only a relatively small effect on net photosynthesis. Light intensity and CO₂ concentration experiments showed that salinity was reducing the photosynthesis of cotton leaves mainly by affecting the light reaction of photosynthesis. It is concluded that chloride salinity does affect the water balance and rate of photosynthesis of plants and that the nature and degree of the effects will depend upon climatic conditions and may be very different between plant species and in the same species at different periods of growth.     

Photosynthetic characteristics of a photoautotrophic cell suspension culture of soybean

A soybean suspension culture (SB-P) which can grow photoautotrophically in 5% CO₂ will not grow in ambient CO₂ levels. This elevated CO₂ requirement seems to be due to the additive effects of a number of factors. The in vivo activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) is much lower in the SB-P cells, compared to soybean plants. This may be due to the low light intensity used to culture the cells, which has been shown to decrease both the amount and activity in whole plants, resulting in a low rate of net photosynthesis. The RuBPcase activation level is also lowered in air CO₂ levels. The presence of the liquid medium raises the cells CO₂ compensation concentration (the CO₂ concentration reached when the rates of CO₂ fixed by photosynthesis and the CO₂ respired by the cells are equal). These factors, coupled with the high respiratory loss of CO₂ all contribute to reduced net photosynthesis in air, resulting in a photosynthetic capacity that is inadequate for cell survival. Active cell division, low photosynthetic capacity, elevated respiration, and a low ratio of RuBPcase(initial)/phosphoenolpyruvate carboxylase are traits that SB-P cells share with young leaf cells, indicating SB-P cell physiology may be comparable to that of young expanding leaves rather than to that of mature leaves.     

Oxygen inhibition of photosynthesis and stimulation of photorespiration in soybean leaf cells

The occurrence of photorespiration in soybean (Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O₂-dependent CO₂ compensation concentration, a nonlinear time course for photosynthetic ¹⁴CO₂ uptake at low CO₂ and high O₂ concentrations, and an O₂ stimulation of glycine and serine synthesis which was reversed by high CO₂ concentration. The compensation concentration was a linear function of O₂ concentration and increased as temperature increased. At atmospheric CO₂ concentration, 21% O₂ inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO₂ and increased with increasing temperature. The Km (CO₂) of photosynthesis was also temperature-dependent, increasing from 12 μm CO₂ at 15 C to 38 μm at 35 C. In contrast, the Ki (O₂) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 mm bicarbonate and 100% O₂, where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O₂ inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO₂ and O₂ concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO₂ and O₂.     

Effect of Temperature, CO(2) Concentration, and Light Intensity on Oxygen Inhibition of Photosynthesis in Wheat Leaves

The effect of 21% O₂ and 3% O₂ on the CO₂ exchange of detached wheat leaves was measured in a closed system with an infrared carbon dioxide analyzer. Temperature was varied between 2° and 43°, CO₂ concentration between 0.000% and 0.050% and light intensity between 40 ft-c and 1000 ft-c. In most conditions, the apparent rate of photosynthesis was inhibited in 21% O₂ compared to 3% O₂. The degree of inhibition increased with increasing temperature and decreasing CO₂ concentration. Light intensity did not alter the effect of O₂ except at light intensities or CO₂ concentrations near the compensation point. At high CO₂ concentrations and low temperature, O₂ inhibition of apparent photosynthesis was absent. At 3% O₂, wheat resembled tropical grasses in possessing a high rate of photosynthesis, a temperature optimum for photosynthesis above 30°, and a CO₂ compensation point of less than 0.0005% CO₂. The effect of O₂ on apparent photosynthesis could be ascribed to a combination of stimulation of CO₂ production during photosynthesis, and inhibition of photosynthesis itself.     

Carbonic Anhydrase-Deficient Mutant of Chlamydomonas reinhardii Requires Elevated Carbon Dioxide Concentration for Photoautotrophic Growth

A mendelian mutant of the unicellular green alga Chlamydomonas reinhardii has been isolated which is deficient in carbonic anhydrase (EC 4.2.1.1) activity. This mutant strain, designated ca-1-12-1C (gene locus ca-1), was selected on the basis of a high CO₂ requirement for photoautotrophic growth. Photosynthesis by the mutant at atmospheric CO₂ concentration was very much reduced compared to wild type and, unlike wild type, was strongly inhibited by O₂. In contrast to a CO₂ compensation concentration of near zero in wild type at all O₂ concentrations examined, the mutant exhibited a high, O₂-stimulated CO₂ compensation concentration. Evidence of photorespiratory activity in the mutant but not in wild type was obtained from the analysis of photosynthetic products in the presence of ¹⁴CO₂. At air levels of CO₂ and O₂, the mutant synthesized large amounts of glycolate, while little glycolate was synthesized by wild type under identical conditions. Both mutant and wild type strains formed only small amounts of glycolate at saturating CO₂ concentration. At ambient CO₂, wild type accumulated inorganic carbon to a concentration several-fold higher than that in the suspension medium. The mutant cells accumulated inorganic carbon internally to a concentration 6-fold greater than found in wild type, yet photosynthesis was CO₂ limited. The mutant phenotype was mimicked by wild type cells treated with ethoxyzolamide, an inhibitor of carbonic anhydrase activity. These observations indicate a requirement for carbonic anhydrase-catalyzed dehydration of bicarbonate in maintaining high internal CO₂ concentrations and high photosynthesis rates. Thus, in wild type cells, carbonic anhydrase rapidly converts the bicarbonate taken up to CO₂, creating a high internal CO₂ concentration which stimulates photosynthesis and suppresses photorespiration. In mutant cells, bicarbonate is taken up rapidly but, because of a carbonic anhydrase deficiency, is not dehydrated at a rate sufficiently rapid to maintain a high internal CO₂ concentration.     

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).     

A dynamic model for photosynthesis by an aquatic plant, Egeria densa

Abstract This paper describes a dynamic model for photosynthesis by an aquatic plant, Egeria densa. The model takes into account an HCO^?₃ pump, high diffusion resistances and PEP carboxylase, and develops a set of differential equations to form the time-dependent solutions for photosynthesis. The predicted changes in pH, [CO₂]_aq and total inorganic carbon are compared with experimental data and the model is found to describe the data. The model is then used to examine the effect of O₂ on photosynthesis under these conditions, and shows that the increase in internal CO₂ concentration due to the recycling of photorespiratory CO₂ directly stimulates gross CO₂ fixation and can more than compensate for the O₂ inhibition of gross photosynthesis. The importance of the HCO^?₃ pump in O₂ inhibition is also examined. The CO₂ compensation point (where inorganic carbon influx and efflux are equal) is examined and the importance of the HCO^?₃ pump and PEP carboxylase in reducing the compensation concentration is discussed. The model was developed in order to study the photosynthesis of an aquatic weed, which will be reported in a later paper.     

Photosynthesis, Respiration and Post-illumination Fixation of CO2 by Corn Leaves as Influenced by Light and Oxygen Concentration

The effect of oxygen concentration on the rate of CO₂-uptake in continuous and intermittent light was studied as well as the CO₂-fixation during a short dark period after light was turned off. In addition the dark respiration and the CO₂-compensation point of attached and detached corn leaves were determined. Leaves of 4 to 22-day old plants were used as experimental material. A closed circuit system of an infrared carbon dioxide analyzer was employed to measure the rate of CO₂-exchange. It was found that in an atmosphere consisting of 100 % oxygen, there was about 50 per cent inhibition of the rate of CO₂-uptake in continuous and intermittent light compared to that in an atmosphere consisting of 21% oxygen. The same was true of the rate of CO₂-fixation in darkness during a short period after the light was turned off. Since the response to oxygen concentration of the CO₂-uptake in light and of the CO₂-fixation in darkness after the light was turned off were similar, it is concluded that the fixation of CO₂ in the short dark period represents an over- shoot of photosynthesis. The rate of dark respiration was little affected by the oxygen concentration in the ranges used in the experiments. The carbon dioxide compensation point which has been observed in leaves of 4 to 14-day old plants was not influenced by either oxygen concentration or light intensity. Since the changes in the rate of CO₂-uptake due to changes in the concentration of oxygen and light intensity had no effect on the CO₂-compensation point, it is concluded that a reabsorption of respiratory CO₂ by photosynthesis could not account for the low value of this point. These results are interpreted as a further corroboration of the statement that the leaves of corn lack the process of photorespiration and that dark respiration is inhibited in light. It was observed that the rate of the CO₂-uptake gradually increased in plants which were from 4 to 22-days old. The inhibitory effect of high concentration of oxygen on the rate of CO₂-uptake was relatively higher in old plants than in young ones.     

Photosynthesis: action spectra for leaves in normal and low oxygen

The action spectrum of apparent photosynthesis for attached radish (Raphanus sativus L. var. Early Scarlet Globe) and corn (Zea mays L. var. Pride V.) leaves was measured at 300 μl/l CO₂ and both 21% and 2% O₂. The spectra were measured at light intensities where apparent photosynthesis was proportional to intensity. For radish, a high compensation point plant, oxygen had an inhibiting effect on photosynthesis at all wavelengths from 402 to 694 mμ. If a constant rate of photosynthesis at 21% O₂ for the different wavelengths was chosen, then the percent increase in net CO₂ fixation at 2% O₂ was constant. For corn, a low compensation point plant, no inhibitory effect of oxygen concentration from 2% to 21% O₂ was found over the visible spectrum. The CO₂ compensation point for light intensities greater than the light compensation point was found to be constant and independent of wavelength for both radish and corn leaves. For radish, the lowering of the oxygen concentration from 21% to 2% at these intensities was found to reduce the CO₂ compensation point by the same amount for the wavelengths studied.     

Effects of desiccation and CO2 concentrations on emersed photosynthesis in Porphyra haitanensis (Bangiales,Rhodophyta), a species farmed in China

The effects on photosynthesis of CO₂ and desiccation in Porphyra haitanensis were investigated to establish the effects of increased atmospheric CO₂ on this alga during emersion at low tides. With enhanced desiccation, net photosynthesis, dark respiration, photosynthetic efficiency, apparent carboxylating efficiency and light saturation point decreased, while the light compensation point and CO₂ compensation point increased. Emersed net photosynthesis was not saturated by the present atmospheric CO₂ level (about 350?ml?m^?3), and doubling the CO₂ concentration (700?ml?m^?3) increased photosynthesis by between 31% and 89% at moderate levels of desiccation. The relative enhancement of emersed net photosynthesis at 700?ml?m^?3 CO₂ was greater at higher temperatures and higher levels of desiccation. The photosynthetic production of Porphyra haitanensis may benefit from increasing atmospheric CO₂ concentration during emersion.     

A method for the study of CO2 exchanges in in vitro cultured Vitis rupestris plantlets

A CO₂ assay circuit adapted to in vitro culture was designed to investigate CO₂ exchanges in test tube-grown Vitis rupestris plantlets. The CO₂ concentration of the air in culture tubes was measured by injection of samples in the open circuit. It was observed under the culture conditions used that the CO₂ content stabilized during the light phase at 3 times the CO₂ compensation point.Measurements of dark respiration under closed circuit conditions at every two-hour interval during the night did not reveal any limiting by lack of the substrate under mixotrophic culture conditions. A mathematical model of the influence of ambient CO₂ concentration on net CO₂ uptake rates under closed circuit conditions was devised and used to compare net photosynthesis at different lighting levels. Measurement of CO₂ evolution into CO₂-free air under open circuit conditions revealed a post-illumination burst characteristic of photorespiration which increased with the temperature.     

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID‐TOLERANT,FREE‐LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)1

The processes of CO₂ acquisition were characterized for the acid‐tolerant, free‐living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO₃^? uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO₂ formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO₂ uptake and O₂ evolution by membrane‐inlet mass spectrometry demonstrated that air‐grown cells reduced the CO₂ concentration in the medium to an equilibrium concentration of 15 μM, but AZA‐treated cells caused a drop in extracellular CO₂ concentration to a compensation concentration of 27 μM at pH 8.0. CO₂‐pulsing experiments with cells in the light indicated that the cells do not actively take up CO₂. An internal pool of unfixed inorganic carbon was not detected at the CO₂ compensation concentration, probably because of the lack of active CO₂ uptake, but was detectable at times before compensation point was reached. These results indicate that this free‐living Coccomyxa possesses a CO₂‐concentrating mechanism (CCM) due to an active bicarbonate‐uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.     

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.

     

Dependence of the Post-Illumination Burst of CO2 on Temperature, Light, CO2, and O2 Concentration in Wheat (Triticum aestivum)

The effect of environmental factors on the post-illumination burst of CO₂ (PIB) and O₂ inhibition of apparent photosynthesis (APS) in wheat (Triticum aestivum L.) was studied in an open gas exchange system utilizing the mathematics of non-steady-state systems. Two components of inhibition by O₂ are suggested: one is caused by photorespiration as measured from the maximum rate of the PIB, and the second is direct inhibition as taken as APS_2%O2— (APS_x%O2+ PIB_x%O2) where X is the oxygen concentration. A primary PIB which occurred from 16–28 s after the darkening of the foliage was attributed to photorespiration. No primary PIB was observed at 2% O₂. At a CO₂ concentration of 100 μ/1 in the atmosphere (about 2.5 μM based on leaf intercellular concentration) and at 30°C and 145 nE/cm² nE/cm²·s, APS decreased curve-linearly with increasing O₂ and reached an O₂ compensation point of 560 μM (48% by volume), above which there was a net loss of CO₂ in the light. The PIB increased with increasing O₂ and became saturated at about 500 μM O₂ but decreased above 900 μM O₂. Direct inhibition of photosynthesis by O₂ increased with increasing O₂ concentration. Decreasing CO₂ concentration had an effect on the magnitude of the PIB similar to that of increasing O₂. At 30°C and 21% O₂, the PIB increased with decreasing CO₂ down to the CO₂ compensation point (I) of 1.4 μM (47 μM/l). Below Γ, both PIB and CO₂ evolution into the air in the light (at 21% O₂) increased and then decreased at CO₂ below 0.8 μM. The ratio of the PIB to APS_{2% o} O₂ increased linearly with increasing O₂/CO₂ ratio where O₂ was held constant at 21% and CO₂ was varied from 1.4 to 8.5 μM, while direct inhibition of photosynthesis expressed as a proportion of APS_2%O2 remained constant over this range. At low CO₂ concentration photorespiration as estimated by the PIB is the major part of O₂ photosynthesis, while at atmospheric CO₂ levels, direct inhibition is the major component. The PIB and APS at 2% and 21% O₂ increased hyperbolically with increasing irradiance and all became light-saturated at about 65 nE/cm₂ s. The percentage total O₂ inhibition of photosynthesis remained constant with increasing irradiance as did the relative contribution of direct O₂ inhibition or photorespiration (PIB) to total O₂ inhibition. The PIB and APS at 21% O₂ had similar temperature optima of 30°C when experimental conditions were adjusted to provide a constant internal O₂/CO₂ solubility ratio at varying temperatures. However, with a constant external CO₂ concentration, the temperature optimum for the PIB shifted upward to 35°C while that for APS at 21% O₂ remained at 30°C, which may be due to an increased O₂/CO₂ concentration in the leaf with increasing temperature.