A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger,J. 1985. A CO₂ concentrating system in leaves of higher C₃-plantspredicted by a model based on RuBP carboxylase/oxygenase kineticsand ¹⁴CO₂/¹²CO₂ exchange.–J. exp. Bot. 36: 1542–1550. A model is presented which compares the ratio of the two activitiesof the enzyme nbulose bisphosphate carboxylase/oxygenase asdetermined in vitro with the ratio of photosynthesis to photorespirationin leaves as determined from differential ¹⁴CO₂/¹²CO₂ uptakeor from CO₂ compensation concentration. Discrepancies betweenmeasurements made in vitro and in vivo are attributed to theeffect of a CO₂ concentrating system in the leaf cells. Interferencefrom dark respiration is discussed. A CO₂ concentrating systemis postulated which is efficient mainly at low temperature andlow CO₂ concentration. Key words: —Photosynthesis, photorespiration, ribulose bisphosphate carboxylase/oxygenase     

Stimulation of Ribulose Bisphosphate Carboxylase Activity by Inorganic Orthophosphate without an Increase in Bound Activating CO2: Co-operativity between the Subunits of the Enzyme

Parry, M. A. J., Schmidt, C. N. G., Cornelius, M. J., Keys,A. J., Millard, B. N. and Gutteridge, S. 1985. Stimulation ofribulose bisphosphate carboxylase activity by inorganic orthophosphatewithout an increase in bound activating CO²: co-operativitybetween the subunits of the enzyme.—J. exp. Bot. 36: 1396–1404 Ribulose bisphosphate carboxylase/oxygenase (E.C. 4.1.1.39 [EC] )from wheat (Triticum aestivum L.), already activated by reactionwith CO² and Mg²⁺, was increased in activity on addition ofinorganic orthophosphate. This further activation took placewithout a significant increase in the amount of bound activatingCO² and the effect was relatively greater with smaller amountsof bound CO². With less than 2·0 mol of CO² bound permol holoenzyme, phosphate increased activity about five-foldwhilst with 7·0 mol of bound activating CO² per mol holoenzyme,phosphate increased activity by a factor of only 1 ·8.This decrease in the effect of orthophosphate with increasein bound activating CO² suggests negative co-operativity betweenactivated sites. The stimulation of activity by inorganic orthophosphatemust be a process distinct from activation by CO²; it was observedwith both the slow and the rapidly activating forms of ribulosebisphosphate carboxylase/oxygenase from wheat. Key words: Ribulose bisphosphate carboxylase, activation, inorganic orthophosphate CO², co-operativity     

Ribulose Bisphosphate Carboxylase Activity and Photosynthesis during Leaf Development in the Tomato

Besford, R. T., Withers, A. C. and Ludwig, L. J. 1985. Ribulosebisphosphate carboxylase activity and photosynthesis duringleaf development in the tomato.—J. exp Bot. 36: 1530–1541. The carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenaseand of phosphoenolpyruvate carboxylase, and the light saturatedrate of net photosynthesis were measured in the developing 5thleaf of tomato plants. Values for light saturated net photosynthesiswere also calculated from the measured carboxylase activitiesand estimates of internal CO₂ and oxygen concentrations. Thecalculated rate using the activity of ribulose bisphosphatecarboxylase alone for net CO₂ assimilation in 300 mm³ dm^–3CO₂ was greater than the measured rate at 80% and full expansionbut less than the measured rate in younger leaves. When theactivities of both the carboxylases were taken into accountbetter agreement was evident for young leaves but the rate wasfurther overestimated for older leaves The calculated rate forphotosynthesis in 1200 mm³ dm^–3 CO², assuming saturationof ribulose bisphosphate carboxylase with RuBP, was an overestimatefor young leaves but was close to the observed values for leavesnear full expansion. The results are discussed in terms of measuredconductances for CO₂ and the availability of RuBP in the leaf Key words: Tomato, leaf development, photosynthesis, RuBP carboxylase, oxygenase     

CO2-Fixation, Glycolate Formation, and Enzyme Activities of Photorespiration and Photosynthesis during Greening and Senescence of Sunflower Cotyledons

Time-courses of ¹⁴CO₂-fixation and of enzyme activities involvedin photorespiration and photosynthesis were determined duringthe life span of cotyledons from sunflower seedlings (Helianthusannuus L.). Glycolate formation in vivo was estimated from theresults of combined labelling and inhibitor experiments. NADPH-glyceraldehyde-3-phosphatedehydrogenase, NADPH-glyoxylate reductase and chlorophyll werewell correlated with the time-course of ¹⁴CO₂-fixation (photosynthesis).There was, however, a considerable discrepancy between the developmentalsequence of photosynthesis and that of both ribulose-l,5-bisphosphatecarboxylase and glycolate oxidase. Furthermore, time-coursesof glycolate oxidase activity in vitro and of glycolate formationin vivo differed significantly. Therefore, the use of glycolateoxidase as a marker for the activity of photorespiration ingreening sunflower cotyledons may be questionable. Results from¹⁴CO₂-labelling experiments with cotyledons treated with theglycolate oxidase inhibitor 2-hydroxy butynoic acid suggestthat glycolate formation relative to CO₂-fixation is reducedin senescent cotyledons. Key words: Development, glycolate oxidase, photorespiration, ribulose-l,5-bisphosphate carboxylase, oxygenase     

Inhibition of Ribulose-P2 Carboxylase/Oxygenase by Fluoride

Fluoride is a potent inhibitor of both reactions of ribulose-P₂carboxylase/oxygenase. Inhibition is almost totally competitivewith respect to CO₂, but uncompetitive with respect to ribulose-P₂Inhibition of photosynthesis after exposure to HF may resultfrom the inhibition of ribulose-P₂ carboxylase by F– accumulatedin the leaves. Key words: Fluoride inhibition, Ribulose P₂ carboxylase, Activation     

Effect of Oxygen and Carbon Dioxide on Photorespiratory Flux Determined from Glycine Accumulation in a Mutant of Arabidopsis thaliana

Exposure to atmospheric conditions which promote photorespirationstrongly inhibits photosynthesis in a mutant of Arabidopsislacking mitochondrial serine transhydroxymethylase activity,and glycine accumulates as a stable end-product of photorespiratorycarbon and nitrogen flow. By providing exogenous serine andammonia to leaves of the mutant, wild-type photosynthesis ratescan be temporarily maintained in the absence of photorespiratoryCO₂ evolution. In these circumstances, the rate of glycine accumulationprovides a direct measure of photorespiratory flux which isnot complicated by the efflux and refixation of photorespiredCO₂, the dilution of radioactive label by endogenous metabolicpools, or non-specific effects of metabolic inhibitors. At thestandard atmospheric concentration of CO₂, the rate of glycineaccumulation in the mutant was proportional to the oxygen concentration,amounting to 53% of the rate of gross CO₂-fixation at 21% O₂.At normal levels of O₂, glycine accumulation was maximal atabout 475 µl CO₂1^–1 and was reduced at higher orlower CO₂ concentrations, being almost abolished at 3000µ1CO₂1^–1. These observations are discussed in the contextof a model of photorespiration based on the properties of ribulose1, 5-bisphosphate carboxylase/oxygenase, and in relation tothe results of previous attempts to measure photorespiration.Preliminary evidence from ¹⁴CO₂-labelling experiments whichsuggests a non-photorespiratory pathway of serine synthesisis also presented. Key words: Arabidopsis mutant, Photorespiration, Serine transhydroxymethylase     

Responses of Photosynthesis and Dark Respiration to Temperature

This analysis suggests that a model of the temperature dependenceof carbon exchange by a plant can be developed based upon absolutereaction rate theory. Component temperature-dependent physiologicalprocesses necessary to describe net photosynthesis over thebiological temperature range include the light reaction, darkreaction (carboxylase CO₂ uptake, oxygenase photorespiration)and mitochondrial dark respiration. An essential assumptionof the model is the reversibility of thermal inhibition. Supportingevidence for this assumption is provided within the biologicalrange. Thermodynamic constants were found to be strongly correlatedwith the thermal environment to which they were adapted. Therewas little difference in non-photorespiration thermodynamicconstants between C₂ andC₄species within thermal habitat types.The model shows the observed shift in temperature optima withlight intensity as a natural consequence of enzyme kineticsand absolute reaction rate theory. photosynthesis, photorespiration, dark respiration, temperature response, carbon exchange, mathematical model     

Effect of CO2 Concentration During Growth on a CO2 Concentrating Mechanism in White Clover as Predicted from Differential 14CO2/12CO2Uptake

Lehnherr, B. M?chler, F. and N?sberger, J. 1985. Effect of CO₂concentration during growth on a CO₂ concentrating mechanismin white clover as predicted from differential ¹⁴CO₂/¹²CO² uptake.-J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and100 Pa p(CO₂). The CO₂ response of net photosynthesis and differentialuptake of ¹⁴CO₂ and ¹²CO₂ by leaves were measured at varioustemperatures and at various O₂ and CO₂ partial pressures andcompared with predictions from ribulose bisphosphate carboxylase/oxygenasekinetics. Discrepancies between the observed gas exchange characteristicsfor the leaves and those predicted from the enzyme kineticswere interpreted as being due to a CO₂ concentrating mechanism.Plants grown at 20 Pa p(CO₂) showed a higher affinity for CO₂than plants grown at 100 Pa p(CO₂) when measured at 10 ?C. Nodifference in affinity was found at 30 ?C. The postulated CO₂concentrating effect was greater in plants grown at low CO₂than in plants grown at high CO₂ concentration and occurredonly at low temperature and low CO₂ partial pressure. It issuggested that plants grown at the lower CO₂ partial pressurehave a higher affinity for CO₂ due to a more efficient CO₂ concentratingsystem than plants grown at the higher CO₂ partial pressure. Key words: Photosynthesis, CO₂, concentration, RuBP carboxylase/oxygenase     

Some Properties of Ribulose Bisphosphate Carboxylase Extracted from Tomato Leaves

Ribulose bisphosphate carboxylase (EC 4.1.1.39 [EC] ) activity wasvery low in tomato leaf extracts unless prepared in the presenceof Mg²⁺, and polyclar AT. With young leaves, but not with fully-expanded leaves, the RuBP carboxylase activityextracted was increased by prolonged illumination of the leaves(2 h). The main effect of the light treatment was to increasethe specific activity of the enzyme but there was also a smallincrease in RuBP carboxylase protein. Tomato leaf RuBP carboxylasein extracts had specific activities in the range 0.2–0–6µmol CO₂ min^–1 mg^–-1 total protein extracted,or 0.5–1.2 µmol CO₂ min^–1 mg^–1 RuBPcarboxylase, and an apparent K_m (CO₂) at 20 ?C of 9.3 ? 1.2µM (using a of 6.407). Key words: Tomato leaf, RuBP carboxylase, Properties     

Phosphoenolpyruvate Carboxylase and CarbonEconomy of Apple Seedlings

Seeds of apple cv. Golden Delicious were germinated and cultivatedin the greenhouse until the third leaf emerged. Respirationofgerminating seeds or photosynthesis of the first leaves wasmeasured by infra-red gas analysis and porometry, respectively.To study the role of phosphoenolpyruvate carboxylase (PEPC),the dominant carboxylase in the carbon economy, its CO₂ refixationpotentialwas related to the amount of CO₂ lost in respiration. With arange of 0.2 (dry seeds) to 18 (cotyledons) µmol CO₂ h^–1g^–1 PEPC activity resembled or exceeded the amount ofC0₂ lost in respiration before the third leaf developed. Itis concludedthat PEPC largely contributes to economize the carbonmetabolism of apple seedlings before they become photosyntheticallycompetent. Key words: Apple (Malus pumila Mill.) seedling, carbon economy, phosphoenolpyruvate carboxylase, photosynthesis, respiration     

Temperature dependence of the enzymic carboxylation and oxygenation of ribulose 1,5-bisphosphate in relation to effects of temperature on photosynthesis

Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5 to 35°C either at constant O₂ and CO₂ concentrations or where the O₂ and CO₂ simulated the concentrations in water equilibrated at each temperature with the same gaseous phase. At constant CO₂ (14 micromolar) and O₂ (0.34 millimolar), the oxygenase to carboxylase ratio remained constant at 0.21 between 5 and 25°C but increased to 0.26 at 35°C. At O₂ and CO₂ concentrations near those expected in water equilibrated with air (21% [v/v] O₂) containing 300 μl/l CO₂ at the various temperatures, the ratio of oxygenase to carboxylase activity increased 2.2-fold between 15 and 35°C. At CO₂ and O₂ concentrations expected in water in equilibrium with subatmospheric concentrations of CO₂ in air (21% [v/v] O₂, 210 μl/l CO₂), the oxygenase to carboxylase ratio increased from 0.25 at 10°C to 0.56 at 35°C. Between 20 and 30°C, the apparent Q₁₀ value for the oxygenase reaction was 1.78 and that for the carboxylase was 1.26. Hence, the different responses of photosynthesis and photorespiration to temperature are due more to changes in the relative solubilities of CO₂ and O₂ (the solubility ratio) than to changes in kinetic parameters of the reactions catalyzed by ribulose bisphosphate carboxylase.     

Effect of the age of tobacco leaves on photosynthesis and photorespiration

Relationships among the activities of enzymes related to photosynthesisand photorespiration, and ¹⁴CO₂ photosynthetic products wereinvestigated with individual tobacco leaves attached to thestalk from the bottom to the top. P-glycolate phosphatase ofthe chloroplasts and glycolate oxidase of the peroxisomes hadtheir maximum activities in the 25th leaf from the dicotyledons.Maximum photorespiration was similarly distributed. The highestratio of serine-¹⁴C to glycine-¹⁴C in the photosynthesates andmaximum glycolate formation were also observed in the 25th leaf.Glutamateglyoxylate aminotransferase, serine hydroxymethyltransferaseand glycine decarboxylase were more active in the upper leaves.RuDP carboxylase had nearly constant activity in all leaves,except for the youngest in which activity decreased. MaximumCO₂ photosynthesis and enzyme activity for the C₄ dicarboxylicacid cycle occurred in the upper, youngest leaf. Distributionof photosynthetic CO² fixation among the leaves did not coincidewith RuDP carboxylase activity. The photosynthetic capacityappeared to be better related to the distribution pattern forenzymes of the C₄ dicarboxylic acid pathway, i.e. PEP carboxylase,pyruvate Pi dikinase and 3-PGA phosphatase in the upper leaves.The results suggest that the C₄ dicarboxylic acid pathway participates,to some extent, in photosynthesis in young leaves of tobacco,a dicotyledonous plant. ¹This work was reported at the Annual Meeting (1970) of theJapanese Plant Physiologists in Kobe. ²The Central Research Institute, Japan Monopoly Corporation1-28-3, Nishishinagawa, Shinagawaku, Tokyo, 141 Japan. (Received November 2, 1972; )     

Photosynthesis of Stands of Tomato, Cucumber and Sweet Pepper Measured in Greenhouses under Various CO2-concentrations

The rate of net photosynthesis (P) of whole plant stands oftomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativusL.) and sweet pepper (Capsicum annuum L.) was measured in sixlong-term experiments in large greenhouses under normal operatingconditions and CO₂-concentrations between 200 and 1200 µmolmol^-1. The objective was to quantify the responses to lightand carbon dioxide and to obtain data sets for testing simulationmodels. The method of measuring canopy photosynthesis involvedan accurate estimation of the greenhouse CO₂ balance, usingnitrous oxide (N₂O) as tracer gas to determine, on-line, theexchange rate between greenhouse and outside air. The estimatedrelative error in the observed P was about ± 10%, exceptthat higher relative errors could occur under particular conditions. A regression equation relating P to the photosynthetically activeradiation, the CO₂ concentration and the leaf area index explained83-91% of the variance. The main canopy photosynthesis characteristicscalculated with the fitted regression equations were: canopyP_max 5-9 g m^-2 h^-1 CO₂ uptake; ratio P_max/LAI 1·5-3 gm^-2 h^-1; light compensation point 32-86 µmol s^-1 m^-2;light use efficiency (quantum yield) at low light 0·06-0·10µmol µmol^-1 and CO₂ compensation point 18-54 µmolmol^-1. The results were related to the prevailing conditions.Copyright1994, 1999 Academic Press Canopy photosynthesis, Capsicum annuum L., carbon dioxide, CO₂, CO₂ balance, CO₂ use efficiency, cucumber, _{Cucumis sativus} L., glasshouse, greenhouse, light use efficiency, Lycopersicon esculentum Mill., sweet pepper, tomato, tracer gas     

Glyoxylate Inhibition of Ribulosebisphosphate Carboxylase/Oxygenase Activation State in vivo

Photosynthetic CO₂ exchange in photorespiration mutants of Arabidopsisthaliana showed a time-dependent inhibition at 350 µl/literCO₂ in 50% O₂ but not in 2% O₂. In a glycolate-P phos-phatasedeficient mutant, inhibition of photosynthesis was due to adepletion of ribulosebisphosphate. In the remaining mutants,which have defects in photorespiratory enzymes which metabolizeamino acids, reduced photosynthesis was accompanied by a declinein the activation level of ribulosebisphosphate carboxylase/oxygenase(Chastain and Ogren 1985), a decline in ribulosebisphosphateconcentration, and an accumulation of glyoxylate. Addition ofglyoxylate at submillimolar concentrations to intact spinach(Spinacea oleracea L.) chloroplasts inhibited light activationof ribulosebisphosphate carboxylase/oxygenase (rubisco) andCO₂ fixation. Similar concentrations of glyoxylate had no effecton A. thaliana rubisco activity in vitro. These results suggestthat glyoxylate accumulation indirectly inhibited rubisco activationstate in vivo. The inhibition of photosynthesis in mutants whichaccumulate glyoxylate may be attributed to a decline in ribulosebisphosphateconcentration, a reduction in rubisco activation state, or acombination of both phenomena. ³Present address: CSIRO, Division of Plant Industry, GPO Box1600, Canberra, ACT 2601, Australia. (Received May 12, 1989; Accepted July 8, 1989)     

Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

Distribution of enzymes related to C3 and C4 pathway of photosynthesis between mesophyll and bundle sheath cells of Panicum hians and Panicum milioides

Enzymes of the C₄, C₃ pathway and photorespiration have beenanalyzed for P. hians and P. milioides, which have chlorenchymatousbundle sheath cells in the leaves. On whole leaf extracts thelevels of PEP carboxylase are relatively low compared to C₄species, RuDP carboxylase is typical of C₃ species, and enzymesof photorespiratory metabolism appear somewhat intermediatebetween C₃ and C₄. Substantial levels of PEP carboxylase, RuDPcarboxylase, and photorespiratory enzymes were found in bothmesophyll and bundle sheath cells. Low levels of C₄-acid decarboxylatingenzymes may limit the capacity for C₄ photosynthesis in P. hiansand P. milioides. The results on enzyme activity and distributionbetween mesophyll and bundle sheath cells are consistent withCO₂ fixation via C₃ pathway in these two species. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and bythe University of Wisconsin Research Committee with funds fromthe Wisconsin Alumni Research Foundation; and by the NationalScience Foundation Grant BMS 74-09611. (Received September 16, 1975; )     

Photosynthetic and Photorespiratory Enzymes in Widely Divergent Plant Species with Special Reference to the Moss Ceratodon purpureus: Properties of Ribulose Bisphosphate Carboxylase/ Oxygenase, Phosphoenolpyruvate Carboxylase and Glycolate Oxidase

Rintamäki, E. and Aro, E.-M. 1985. Photosynthetic and photorespiratoryenzymes in widely divergent plant species with special referenceto the moss Ceratodon purpureus: Properties of ribulose bisphosphatecarboxylase/oxygenase, phosphoenolpyruvate carboxylase and glycolateoxidase.—J. exp. Bot. 36: 1677–1684. Km(CO₂) values and maximal velocities of ribulose bisphosphatecarboxylase/oxygenase (E.C. 4.1.1.39 [EC] ) were determined for sixplant species growing in the wild, consisting of a moss, a fernand four angiosperms. The maximum velocities of the RuBP carboxylasesvaried from 0.13 to 0.;62 µmol CO₂ fixed min^–1 mg^–1soluble protein and the Km(CO₂) values from 15 to 22 mmol m^–3CO₂. The highest Km(CO₂) values found were for the moss, Ceratodonpurpureus, and the grass, Deschampsia flexuosa. These plantsalso had the highest ratios of the activities of RuBP carboxylaseto RuBP oxygenase. Glycolate oxidase (E.C. 1.1.3.1 [EC] ) activitieswere slightly lower in D.flexuosa, but not in C. purpureus,than for typical C₃ species. Phosphoenolpyruvate carboxylase(E.C. 4.1.1.31 [EC] ) was not involved in the photosynthetic carboxylationby these two plants. However, another grass, Phragmites australis,was intermediate in PEP carboxylase activity between C₃ andC₄ plants The properties of RuBP carboxylase/oxygenase are discussedin relation to the activities of PEP carboxylase and glycolateoxidase and to the internal CO₂ concentration. Key words: RuBP carboxylase, oxygenase, Km(CO₂), moss     

Affixing the O to Rubisco: discovering the source of photorespiratory glycolate and its regulation

The source of glycolate in photorespiration and its control, a particularly active and controversial research topic in the 1970s, was resolved in large part by several discoveries and observations described here. George Bowes discovered that the key carboxylation enzyme Rubisco (ribulosebisphosphate carboxylase/oxygenase) is competitively inhibited by O₂ and that O₂ substitutes for CO₂ in the initial `dark' reaction of photosynthesis to yield glycolate-P, the substrate for photorespiration. William Laing derived an equation from basic enzyme kinetics that describes the CO₂, O₂, and temperature dependence of photosynthesis, photorespiration, and the CO₂ compensation point in C₃ plants. Jerome Servaites established that photosynthesis cannot be increased by inhibiting the photorespiratory pathway prior to the release of photorespiratory CO₂, and Douglas Jordan discovered substantial natural variation in the Rubisco oxygenase/carboxylase ratio. A mutant Arabidopsis plant with defective glycolate-P phosphatase, isolated by Chris Somerville, definitively established the role of O₂ and Rubisco in providing photorespiratory glycolate. Selection techniques to isolate photorespiration-deficient plants were devised by Jack Widholm and by Somerville, but no plants with reduced photorespiration were found. Somerville's approach, directed mutagenesis of Arabidopsis plants, was subsequently successful in the isolation of numerous other classes of mutants and revolutionized the science of plant biology. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthesis and photorespiration in soybean [Glycine max (L.) Merr.] chronically exposed to elevated carbon dioxide and ozone

The effects of elevated carbon dioxide (CO₂ and ozone (O₃) onsoybean (Glycine max (L.) Merr.] photosynthesis and photorespiration-relatedparameters were determined periodically during the growing seasonby measurements of gas exchange, photorespiratory enzyme activitiesand amino acid levels. Plants were treated in open-top fieldchambers from emergence to harvest maturity with seasonal meanconcentrations of either 364 or 726 µmol mol^–1 CO₂in combination with either 19 or 73 nmol mol^–1 O₃ (12h daily averages). On average at growth CO₂ concentrations,net photosynthesis (A) increased 56% and photorespiration decreased36% in terminal mainstem leaves with CO₂ enrichment. Net photosynthesisand photorespiration were suppressed 30% and 41%, respectively,by elevated O₃ during late reproductive growth in the ambientCO₂ treatment, but not in the elevated CO₂ treatment. The ratioof photorespiration to A at growth CO₂ was decreased 61% byelevated CO₂ There was no statistically significant effect ofelevated O₃ on the ratio of photorespiration to A. Activitiesof glycolate oxidase, hydroxypyruvate reductase and catalasewere decreased 10–25% by elevated CO₂ and by 46–66%by elevated O₃ at late reproductive growth. The treatments hadno significant effect on total amino acid or glycine levels,although serine concentration was lower in the elevated CO₂and O₃ treatments at several sampling dates. The inhibitoryeffects of elevated O₃ on photorespiration-related parameterswere generally commensurate with the O₃-induced decline in A.The results suggest that elevated CO₂ could promote productivityboth through increased photoassimilation and suppressed photorespiration. Key words: Photorespiration, CO₂-enrichment, ozone, climate change, air pollution     

Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.