Plant Physiol. (1968) 43, 923-929

Photorespiration and Glycolate Metabolism: A Re-examinationand Correlation of Some Previous Studies'

W. J. S. Downton and E. B. TregunnaDepartment of Botany, The University of British Columbia, Vancouver 8, British Columbia

Received January 22, 1968.

A bstract. Some previous studies of photorespiration and glycolate oxidation were re-examined and correlated by infra-red CO, analysis. Data about rate of photosynthesis andoxygen sensitivity indicated that complete inhibition of photosynthesis with 3. (3,4-dichloro-phenyl) -1,1 dimethyl urea (DCMU) allowed dark respiration to continue in the light.Photorespiration was also inhibited. The oxygen senisitivity of glycolate-stimulated CO2production was found to be compatible with the proposal that glycolate is a substrate ofphotorespiration. B-oth 'in vivo' and 'in vitro' studies of the alga Nitella flexilis haverevealed a pathway of glycolate oxidation similar to that of higher plants. DCMU inhibitionof photosynthesis by Nitella gave results similar to those for the monocotyledons tested.Under very low light intensity, carbon dioxide compensation in corn was measurable but wasnot sensitive to high oxygen concentration. It appears that the lack of photorespiration in thisplant is not the end result of efficient internal recycling of CO, to photosynthesis.

Photorespiration has been described as a CO,producing process that operates during photosyn-thesis in some leaves and algae. The newer evidencesupporting this concept is based on the response ofphotorespiration to 0. concentration. It has beenshown that dark respiration in leaves is saturatedby about 2 % 02 whereas photorespiration has amuch higher 0., requirement (7, 25). 'Much recentliterature also implicates glycolic acid as a substrateof photorespiration. The correlation which we havestudied is the O., requirement for glycolate-stimu-lated CO, production. These results led to anotherstudv of whether corn leaves produce CO., by photo-respiration during photosynthesis.

A recent paper by El-Sharkawy et al. (6) hasprovided data on CO. production in the light anddark when C0, source-sink relationships werechanged by inhibiting photosynthesis with DCM\IU.Under these conditions the rates in light and darkwere identical and these data were used as evidencefor a common source of CO, evolved both duringphotosynthesis and in the dark. We re-examinedthis work and extended it with information aboutthe effect of 02 tension on rates of CO., production.As a check against the possibility that the gasexchange results from the DCMU-treated mono-cotyledons were an artifact of stomatal behavior,the gas exchange of a DCMU-treated alga, Nitellaflexilis, was studied. This alga has been shown tohave a photorespiratory mechanism similar to thatof some land plants (3). Results obtained by in-hibiting photosynthesis by non-chemical means have

1 Supported by the National Researc-h Council ofCanada.

also been useful in determining whether photores-piratory C'O., is being produced during photosyn-thesis by leaves with very low compensation values(14, 15). The term, "dark respiration", is used inthis paper to denote that process of CO., productionwhich normally occurs in the dark, and whose rateis not influenced by changes in 0. concentrationbetween 2 % and 21 %.

Materials

Seedlings of wheat (Triticumi vulgare L.), oats(Arventa sativa L.), and corn (Zea mays L.) weregrown at a 16 hour photoperiod with a 240 dav/19'night temperature in a controlled environmentchamber. Illumination was supplied by banks ofCool White Fluorescent tubes supplemented withincandescent bulbs. The intensitv as measured witha Weston 756 Illuminometer was 1200 ± 100 ft-c.All seeds were planted in flats of vermiculite andwatered daily. Corn seeds were pretreated with75 % Captan powder to control a seed and rootfungus. Seedling age for experimental trials rangedfrom 2 to 3 weeks.

Nitella flexilis L. (C. Agardh), a macroscopicgreen alga, was grown in an aquarium in tapwaterat room light and temperature.

Methods

DC,I U Feedings. Detached shoots of corn andwheat were submerged in distilled water or a satu-rated aqueous solution of DCMU [3- (3,4-dichloro-phenyl)-1,1 dimethyl urea] and placed in a vacuumdesiccator. The system was evacuated for 1 hour

923

924 PLANT

with a water asl)irator. Then the cut ends of theshoots wvere placed in small vials containing thesame solutioin that they wAere infiltrated with, andset under a fluorescent light having an intensity of700 ft-c. Shoots were tested 3 hours later by whichtime photosynthesis had been inhibited completelyin the DCMU-infiltrated shoots.

For tlle test, a vial containing shoots was placedin a glass cylinder wNhich served as a leaf chamber.The CO., concelntration was measured in a closedsysteml consistinig of a diaphragmii pump, the leafchanmber alnd a Beckman Model IR 215 infra-redCO, analyzer. The volume of the systenm wlas215 ml; the flow rate, 1.1 liters per miniute. Theleaf chamber wNas submerged in a water bath, whichstabilized the tenmperature at 22.5 + 1.50. Theintensity from a General Electric "Cool Beam" lampwas 2000 ft-c. Changes in the CO. concentrationwere mleasured under the following conditions.Rates of photosynthesis were determined between325 and 275 ppm CO_. The rate of respiration inthe dark was theni mleasured. Following this, thechamiber was re-illumiinated and flushed with N., tolower the O., concentration to between 1 to 3 % asmonitored w-ith a Clark Polarographic oxygen elec-trode. CO. was introduced to raise the CO., con-centration to the former level and another set ofrates in light and dark wxas determlined. The systemwas again flushed with air and rates taken todetermine the reversibility of the 02 effect.

The apparatus used to study -Vitclla was almostidentical to that described above. The chamber wasagain a glass columinl but with a side-arml near thesintered base w-hiclh allowed rapid drainage of solu-tions. The sintered disc broke up the air-streamwNrhiclh bubbled through the liquid in wlhichi the algawas suspended. The flow rate wvas 0.475 liters perrminute. The light intensity was 1250 + 250 ft-c.Temperature ranged from 21.5 to 23.00.

Rates of photosynthesis and resl)iration weredetermined under 21 % and 1 to 3 % 02 concentra-tions wvith the alga in 11 mM phosplhate buffer(KH..PO, and NaH.,PO, 1:1) adjusted to pH 4.9to 5.0. Followinig these determinations, the bufferwas removed via the side-arm and a solution of11 mI phosphate buffer saturated with DCIMU wNasintroduced. Photosynthesis was inhibited within 3minutes and at this time changes in the CO. con-centration w,ere mleasured in light and dark at the2 O., concenitrationis.

Glycolate Fccdinigs. Detached shoots of corn,wheat, aind oats were placed in vials containingsolutions of 0.05 Mi glycolic acid or 0.05 M aceticacid adjusted to pli 5.0 with solid sodiuml bicar-bonate, or distilled water adjusted to the same pHwith 0.1 NT hydrochloric acid. A minimum feedingperiod of 3 to 4 hoturs under a fluorescent light of700 ft-c intensity ensued before samples wrere tested.Individual vials wN-ere then placed in a darkened leafchamber. Rates of CO., evolution were calculatedaccording to the tinme required for the slhoots to

PHYSIOLOGY

raise the CO., concentration from 62 to 100 ppm inthe closed system. Two different 02 tensions wereused, N., being used to lower the concentration fromatmosphleric to 1 to 3 %. The order of applicationof the 2 tensions was at random and rates representaverages of 2 rates at eachi tension.

The system used to feed glvcolate to AVitella wasthe samle as that described for the DCMU feedingsexcept that all rate determinations wvere carried outin darkness. Rates were deternmined under the 2 O2tensioils with the alga in 11 nlm phosphate buffer.Following this, the solution was replaced by 25 marglycolic acid in 11 nim phosphate buffer at pH 4.9to 5.1. To insure that handling had not damagedthe alga, its abilit) to assimilate CO., by photosyn-thesis was determined before rates of respirationwere determined under the 2 different O. tensions.

Ens ,yne Assay for Nitella. Crude preparationsof glycolic acid oxidase were made by grinding 15grams freslh weiglht of Nitella in 15 nml of 0.05 AIK.HPO4 in a W;aring Blendor for 1 mlinute. Thlebrei was ground further in a mortar, filtered throughcheeseclotlh, centrifuged at 200 X g for 5 miinutesand the supernatant aerated at room temperaturefor 10 minutes to lower endogenous oxidation. Asample of the crude preparation w,as placed in a1.55 nml reaction vessel and stirred by a magnet.Rates of O., consumption were measured with theClark Polarographic oxygen electrode at 200. Theactivity of glycolic acid oxidase was determlined bythe chalnge in rate of O., consumlption upon additionof 1(50 ,ul of 0.05 M glycolic acid.

Oxy+1gen Sensitivity of tf/e Comipensation l alieUnder Low Light Intensity. Two-week-old w\Nheatseedlings were detached and placed in the leafchamber at a light intensity of 60 + 5 ft-c asmeasured with a Gossen Tri-lux lightmeter. Thelight was attenuated witlh sheets of WAhatmiiain _No. 1chromatography paper. The system was scrubbedfree of CO2. with Ascarite, closed aind the CO.2comlpeinsation of the wrheat shoots in 21 % O, wasdetermined. The system was thein flushed with N.,to lower the O., concentration to 1 to 3 % and CO.compensation measured again. The temperatureduring these experiments wvas 21.50. Twenty-day-old corn seedlings were subjected to the abovetreatment but the light intensity required to producea mleassurable conlpensation was 18 - 2 ft-c at 220.

Resultsand Discussion

The Effect of DCAIU on CO., Produtction. Fromtable I it is evident that DCMU treatment inhibitedphotosynthesis in wheat and corn shoots allowingCO., production to be measured in the light. Thesimilarity of the rates in both light and dark indicatethe complete suppression of photosynthesis. Theresults are in agreement with those of El-Sharkawy-et al. (6) for corn, sugarbeet and Awnaralthuscduflis under conditions where photosynthesis appears

DOWNTON AND TREGUNNA-PHOTORESPIRATION AND GLYCOLATE

Tab,le I. Effect of DCMU aild Oxygen Concentration on the Gas Exchlanige of Detached Wheat anid Cor0i Shoots

Infiltration Rate at 21 % 0 Rate at 2 % 0,Plant solution Light Dark Light Dark

Ag CO./mbin X g fr zwtWheat WVater -24.61 +3.2 -37.5 +3.2

-26.2 + 3.5-28.1 +3.4 36.5 +3.3

29.2 +3.5

DC\IU + 3.6 +3.4 + 3.6 +3.2+ 3.2 +3.2

4- 2.2 +2.7 + 2.2 ...

+ 2.2 +2.9

Corni WN ater -31.2 +3.3 -31.2 +3.3-34.4 4-3.2

-42.0 +3.4 2.0 +3.3-42.0 +3.5

DCMU + 2.9 +2.7 + 3.2 +2.7"9 + 3.4 +3.4 + 3.6 +3.2

+ 3.2 4-3.01 CO. produced (±) or absorbed (-).

to be completely inhibited. They have suggested sistent with its apparent lack of photorespirationthat the CO., released in the light in the presence of (4, 5, 7).DCMU is that which is normally completely re- The similarity of the rates of CO2 production incycled to photosynthesis in plants which do not light and dark by DCMU-inhibited plants to thoseproduce CO. into C02-free air, and implied a com- of the dark controls indicates that the lack of 02mon source of CO, production both during normal response by shoots fed DCMU was not a result ofphotosvnthesis and in darkness. The lack of sensi- stomatal closure brought about by the chemical. Astivitv of rates of 00, production to 02..tensions a check, however, Nitella flexilis was subjected tobetween 2 and 21 % by wheat shoots in DC'MU DCMU treatment. Brown and Tregunna (3) have(table I) indicates that the usual pathway of CO, shown that Nitella in acidic medium had a highproduction which occurs during photosynthesis compensation value which was lowered by reducing(photorespiration) has been replaced by another the 09 concentration. Such a reduction did notprocess. The observation of CO, production in the affect the dark respiratory rates. They concludedlight by DCMU-treated corn shoots which normally that respiration was inhibited during photosynthesisdo not produce CO2 during photosynthesis suggests and replaced by another pathway of CO evolutionthe switching on of a CO.,-evolving pathway when (photorespiration) having a high 0, requirement.photosynthesis is completely inhibited. The control Rates of photosynthesis with the alga in bufferexperiments support these conclusions. Control (table II) show an enhancement under low 02wheat seedlings when placed under low 02 tension similar to that reported for temperate plants withexhibit a great enhancement in photosynthetic rate. photorespiration (4). In this experiment, all photo-This effect can be explained as a reduction in CO2 svnthetic activity was inhibited within 3 minutesrelease during photosynthesis bv suppressed photo- after addition of DCMU compared to the 3 hoursrespiratory activity under low 0., The lack of required for such inhibition in the monocotyledons.such an enhancement for the corn controls is con- The inhibition of photosynthesis of Nitella results

Table II. Effect of DCMU anzd Oxygent Concentrationz on7 the Gais Exchanige of Nitella flexilis

Rate at 21 % 0 Rate at 2 % 02Sample Solution Light Dark Light Dark

Ag CO0/mnbin X g fr7 ZatBuf fer -4.08' +2.16 -6.26 +1.95DCMU +2.14 +2.10 +2.10 +2.10

2 Buffer -4.41 +2.94 -6.01 +2.94DCMU +2.60 +2.48 +2.48 +2.44

CO2 produced (+) or absorbed (-).

925

PLANT PHYSIOLOGY

in CC). production in liglht wlhlich is e(qual to therate in the dark and not 0., sensitive between 2 and21 % oxygen (table II). This result is identical tothat from the DCMU-inllibited monocotvledons andindicates that photorespiration has been replaced bythe pathway whiclh is saturated at a lower 0. con-

centration.The DCMU data of Poskuta et al. (19) were

based on rates of CO, production in light and dark-when the apparent rate of photosvnthesis was zero.

That is, the CO, compensation concentration was

approximately 300 ppm. Therefore some photosvn-thetic activity was occurring. Under these condi-tions, CO, evolution in light was found to be greatlyinhibited compared to the controls. There was a

negligible effect of DCMU on dark respiratorvrates. Similarlv, thev have shown that during inhi-bition of apparent photosynthesis by 2,4-dinitro-phenol, increasing the 0, concentration from atmos-

pheric to 100 % greatly increased CO., productionin the light, but had a negligible effect on darkrespiration. Hence under conditions where some

photosynthesis is occurring the process of CO.2evolution in light is different from dark respiration.Tregunna et al. (24) have shown that albino corn,

whiclh lacks a functional photosynthetic apparatusand therefore photosynthesis, produces CO., at thesamle rate in light and dark indicating a common

source of CO., production when photosynthetic ac-

tivitv is absent. Normal corn, on the other hand.does not produce CO, in the light as indicated by

its low compensation point and lack of CO., produlc-tion into CO,-free air (6, 7).

7/he Oxrvgen tSenisitivity of Glycolatc Oxidationin Relation to Pliotorespirationt. The effect of O.0concentration on glvcolic acid oxidation in dlarknessis recorded in tables III and IV. Shoots fed glv-colate exhibited a great increase in CO., evolutiontunder 21 % O.. compared to the rates for water andacetate feedings. This stimulation in wlheat andcorn shoots was suppressed under 2 % O., to thelev.el of the shoots fed water or acetate. Oats dif-fered to soimie degree in its response in that anincrease in O., concelntration had a stimulatory effecton CO., production for leaves fed acetate. Themagnitude of this stimulation, however, was mluclless than that for the glvcolate feedings. WA"henglycolate was fed to ANitella (table IV). a similarstimulation of CO., evolution under 21 % O., was

observed. The O., effect was completely reversiblefor all shoots whiclh were fed glvcolate. Fturthersupl)ort for the existence of glv-colic acid oxidaseactivity in Nitella came froml crude enzyme prepara-tions. Upon addition of glv-colic acid to the prep-aration, the encdogenlotus rates of oxygen consumptionfor 2 saml)les increased from 6.8 and 9.3 Aul O., perhour per graml fresh wveight of tissue extracted tovalues of 22.3 anid 21.7 respectively.

Trhe mleasturemiient of CO., evolution in the (larkallowed glvcolic acid oxidation (photorespiration ?)to be studied in the absence of photosynthesis. Thediffering 02 requirements for glycolic acid oxidationi

Table IIT. 'lfc Effc(-t of Substrate anld Oxygen Concenttrationl onI Rates of Co., Evolution by DetachedMonlocotvxledoni Shoots in Darkness

Plant Sub4strate

\ lilat W\ ater

Acetate 0.05 'M

Glhcoanbte 0.05 -m

WRaterAcetaite 0.05 At

GIvcolate 0.05 AT

Cor-in WaterA\ceit-lte O.QS -M

Glycolate 0.05 -m

Rate at Rate atReplicates 2 % O., (A) 21 % 0, (B)

yg CO.,/nuni,l X (l f,i ret4.06 4.234.92 5.305.14 9.02

66

10

466

6

6

8

5.504.987.46

4.283.964.11

5.445.8611.34

4.554.476.61

Oxygen

stimulatioi(B/A)Ratio1.051.08

I .75

0.991.18

1.061.131.61

Sutbstrnate rate/vater

rate in 21 % O.,

Raitio

1.25;2.13

1.082.08

0.981.45

Table IV. 1Effect of Glycolatc (and Oxygen Concenitration on CO, Producf'tion b) Nitella flexilis in Darkncss

Rate at 2 % 02 Ra,te at 21 % 02, Oxygen stimiulationSample Solution (A) (B) (B/A)

,Ug CO ,/mninl >1 / fr Ze' Ratio1 Buffer 2.67 3.07 1.15

Glycolatc 3.02 5.33 1.77

2 Buffer 2.86 3.15 1.10Glvcolatc 3.70 5.46 1.48

926

0O-Its

927DOWNTON AND TREGUNNA-PHOTORESPIRATION AND GLYCOLATE

and dark respiration allowed separation of theprocesses. Under low 0, both glycolate oxidationand the CO2 compensation value become negligible.The 0. effect is also readily reversible for both.These results, then, are compatible with the high O.,requirement for photorespiration and the implicationthat glycolate is the substrate of this process.

Glycolic acid oxidase activity in wheat, corn andoats has been demonstrated by manv workers (18,21,22). Hess and Tolbert (12,23) failed to detectglvcolic acid oxidase activity in some unicelluilaralgae and have generalized to state that "glycolatemetabolism represents a major difference betweenalgae and higher plants". Our studies have providedboth 'ini vivo' and 'in vitro' evidence for an activeglycolate oxidation mechanism in Nitella.

Glv colic acid, as well as being ubiquitous inleaves, is one of the earliest products of photosyn-thesis particularly at lower CO., concentrations andhigher O. concentrations (1, 2,11, 20, 30). In addi-tion, Ludwig and Krotkov (13) have provided evi-dence for some early photosynthetic product beingoxidized to CO., in light. Measurenments of "4CO.,uptake by sunflower demonstrated that the initial rate

of uptake decreased soon after the introduction of"CO.,, indicating that 14CO0, was being evolved.Similarly, Goldsworthy (8) has shown that tobaccosegments when subjected to a streanm of CO2-free airfollowing a period of 14C0., incorporation, producedCO., in the light with a higher specific activity thanin the dark. The rate of CO., production in the lightwas greatly stinmulated by high O., concentrationwhereas the dark rate was not. The use of purportedinhibitors of glvcolate metabolism reduced the spe-

cific activity of CO., released in the light to a level

comparable to that released in the dark.The investigations of Zelitch and Mloss have been

a major support for the tenet that the source of

CO., evolved in the light is glvcolate ( 16, 28, 29,30, 31). Much of this work has involved the use

of competitive inhibitors of glvcolic acid oxidase.

The feeding of labeled glycolate to tobacco resulted

in 14CO., production. The source xvas specificallvfrom the carboxvl carbon of glvcolate. iCorn was

an exception and produced little 1"CO., compared to

tobacco, altlhough it assimilated as much glvcolate.Tobacco leaf discs at 350 in the presence of the

inhibitor took up at least 3 times as much 14CO2 as

the controls in water. There was no effect on corn.

It was presumed that the inhibitor blocked photo-

respiration in tobacco and increased the concentra-

tion gradient between the atmosphere and the chloro-

plast resulting in a higher rate of photosynthesis.

Moss (16), in recent studies, has shown that rates

of CO., evolution in light into CO,-free air exceeded

dark rates, despite the likelv occurrence of re-utiliza-

tion of CO2 bv photosynthesis. The inhibitor re-

duced this evolution of CO2 in the light to a rate

lower than that in darkness, probablv bv blocking

photorespiration. The amount of glvcolate that

accumulated in the presence of the inhibitor was

sufficient to account for the difference in CO,evolution between controls and leaves fed inhibitor.The inhibitor had no effect on respiration rates inthe dark.

Contrary to what would be expected from thework of Tregunna (26), corn was capable of oxi-dizing glvcolate in the dark in the absence ofexogenously added FMIN. The results of Moss (17)show a 30 % stimulation in CO., production bv cornleaves fed 0.1 M glycolate compared to the watercontrols. The absence of data for corn leaves fedacetate or glucose does not allow comparison of theeffects of different substrates on stimulation of CO,evolution. Our data of the ratio: (rate of CO)evolution by leaves fed glycolate) :(rate by the leavesin water) show that interspecific differences do occur.

Under 21 % 0, the rates of CO., production doubledfor the wheat and oat leaves which were fed glycolatecompared to the water controls. For corn shoots,however, the rate increased only by 45 %. Thislower stimulation in corn may reflect a species differ-ence in the transport of glycolate to the site of oxida-tion or in the ability to metabolize it. The ability ofcorn to produce CO., from glycolate in the dark butnot in the light (31) would suggest that either theplant is extremelv efficient in the internal recyclingof photorespiratory CO. to photosynthesis and doesnot leak CO., to the outside, or that the pathwav ofglycolate metabolism is not the same in the light as

in the dark. The mechanism proposed by Tregunna(26) appears incorrect in view of the results offeeding glcolate.

Oxylgenl Sensitivity of the CO., ComtpensationValte at Low Lighlt Initensitv. Complete inhibitionof photosynthesis with DCMIU resulted in the re-

placement of photorespiration by another process.Hence the CO., source-sink relations must be changedmore subtly to determine whether corn simply re-

cycles the photorespiratory CO.,. The manipulationof a photosynthetic parameter such as light intensityprovides one approach. If corn does possess thephotorespiratory pathw,av common to wheat and oats(5), then under conditions where CO., is not ratelimiting, such as very low light intensity, some ofthis photorespiratory CO., should leak out. Thisshould be detectable by measuring the effect of 0_concentration on the compensation value. Table Vshows the results obtained using wheat and corn

shoots. \Vrheat as a control under 65 ± 5 ft-c hadan average compensation of 170 ppm in 21 % O0.The lack of measurable rates of gas exchange at

CO., concentrations above the compensation concen-

tration indicated that CO., was not limiting at thisconcentration. Under low O., the compensationvalue was 27 ppm. Therefore, in 21 % O., and lowlight intensity, the compensation value was a com-

posite of dark respiration, photorespiration, andphotosynthesis. Photorespiration was the majorsource of CO., in 21 % O.,.

The light intensity reqtuired to produce a measur-

able compensation value in corn was very low

Table V. Effect of Oxygen Concentration on the CO2 Compensation Concentration of Wheait and Corn Shootswhen Light is a Limiting Factor

C02 compesationPlant Sampte 21 % 09 2 % 02 Light intensity

ppM1 pp II ft-cWheat 1 157 25 65 ± 5

2 183 29 65 ± 5

Corni 1 43 40 18 ± 22 37 38 18 ± 2

(18 ± 2 ft-c) and narrow in range. The compen-sation point was not 02. sensitive between 2 and 21 %02, and therefore represented equilibrium betweenphotosynthesis and dark respiration. Under condi-tions sufficient to produce a measurable compensa-tion value in corn, there was no loss of CO. pro-duced by photorespiration. This indicates that cornlacks photorespiration and that the negligible com-pensation value at higher light intensities is not aresult of efficient internal recycling of photorespira-torv CO2 to photosynthesis by decreased diffusionresistances. Meidner (14, 15) has demonstrated thatcorn leaves placed under water strain had an elevatedcompensation value which was essentiallv unaffectedby 02 concentrations between 21 and 100 %. Suchplants also lacked a post-illumination burst. Theseobservations are compatible with the absence of anoperational photorespiratory mechanism in corn.

Although corn does have an active glycolic acidoxidase pathway, it appears unlikely that the pathwayor reaction products are the same in the light duringphotosynthesis as in the dark. The lack of a com-pensation point, lack of CO, production into C'02-free air and negligible production of 14C09 fromlabeled glycolate in light support this (6,31). Thestudies of Hatch et al. (9, 10O have shown thatwheat and oats form intermediates of the Calvincycle as the earliest products of photosynthesis.These plants also have photorespiration (5). Cornand many tropical grasses, which lack photorespira-tion, have a different carboxylation sequence inwhich C-4 compounds are labeled first (5, 10). Thisappears to be true also for low compensation mem-bers of the Amaranthaceae and Chenopodiaceae (27,unpublished data). Further studies are necessary toclarify the relationship of glycolate to these paths ofcarbon in photosynthesis.

The opposite nature of the gas exchange proc-esses of photorespiration to those of the concomittantphotosynthesis constitutes a difficulty in the studyof photorespiration by the infra-red method. Itsapparent dependence upon photosynthesis does notallow for its independent study wlhen photosvnthesisis completely blocked. Hence the sensitivitv ofphotorespiration to 02 concentration provides al.owerful means of manipulation in the further eluci-dation of its mechanism and its importance in termsof the carbon balance and dry matter production inplants.

Acknowledgment

Dr. Janet Stein kindly provided the Nitella flexilis.

Literature Cited

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2. BENSON. A. A. AND M. CALVIN. 1950. The pathof carbon in photosynthesis, VII. Respirationand photosynthesis. J. Exptl. Botany 1: 63-68.

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4. DOWNES, R. XV. AND J. D. HESKETH. 1968. En-hanced photosynthesi-s at low oxygen concentra-tions: Differential response of temperate andtropical grasses. Planta 78: 79-84.

5. DOWNTON, \W. J. S. AND E. B. TREGUNNA. 1968.Carbon dioxide compensation - its relation tophotosynthetic carboxyl,ation reactions, systematicsof the Grantineae, and leaf anatomy. Can. J.Botany 46: 207-15.

6. EL-SHARKAWY, M. A., R. S. LooMIs, AND W. A.WILLIAMS. 1967. Apparent reassiimilation ofrespiratory carbon dioxide by different plant spe-cies. Physiol. Plantarum 20: 171-86.

7. FORRESTER, M. L., G. KROTKOV, AND C. D. NELSON.1966. Effect of oxygen on photosynthesis, photo-respiration and respiration in detached leaves.I Soybean. II Corn and other monocotyledons.Plant Physiol. 41: 422-31.

8. GOLDSWORTHY, A. 1966. Experiments on the or-igin of CO., released by tobacco leaf segments inthe light. Phytochemistry 5: 1013-19.

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928 PLANT PHYSIOLOGY

DOWNTON AND TREGUNNA-PHOTORESPIRATION AND GLYCOLATE

13. LUDWIG, L. J. AND G. KROTKOV. 1967. The kineticsof labeling of the substrates for CO2 evolution bysunflower leaves in the light. Plant Physiol. 42:S-47.

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