Summinuarv. The effect of O., onl the CO., exchange of (letaclhe(I soybean leaxves was miieasured with a Clark oxygen electrode and infrare(d carlbon (lioxide alnalvsers in both open and closed systems.The rate of apparent photosynthesis was inhibited 1y O. while the steady rate of respiration after a few nminutes in the dark was not affected. Part of the inhibition of apparent photosynthesis was shown to be a result of increased photorespiration. This stimulation of photorespirationi by O. was manifested by an increase in the CO, compensation poiInt.The differential effects of 0., Oli dark respiration (n1o effect) anid( photorespiration (stimulatioln) indicated that these were 2 different processes.Moreover the extrapolation of the CO., compensationi poilnt to zero at zero O. indicated that dark respiration was suippressed in the light at least at zero 0. concelntrationi.The rate of apparent photosynthesis has been showlv to be inhibited 1)v 02 in a -ivide range of plant species (13). Until recently, however. 110 satisfactory explanationi of this phenomiienioni has been proposed. The possibility that part of this inhibitioll miiight be due to a stimiiulation of respiration was not considere(d because A) the respiratory-process wlhiclh operates in the dark was thought to continiue during photosynthesis (3, 13), and B) O. has no effect on the clark respirationi of green leaves (7. 9, 13).Recent evidence stiggests that (lark respiration is inhibited in the light in green leaves andaIlgae and is replaced by a different respiratory process photorespirationi (4, 6, 12).The questionl now to he answered is whether O. has anl effect on photorespiration i.e. onl the evolution of CO., ill light. Il the experimiienits reported below these effects of O3 were iilvestigated miore fully, Using a (liffereilt planit species, a wider range of O3 anid CO., conicenltratioris aild steadv staite conditions. Materials and MethodsSoybean plailts, Glxcinic nma.mlMerr. v-ar. Coimiet, were growvn in pots of v-ermiiicuilite in a growtlh cllanllber. Thle ligllt intensitv was 1,500 ft-c, and(I the dav lengtll 16 hoturs. The teiimperature was 22.50 dturing tile da ( and 190

.Su;Immary. 'T'lhc -ffect of 0. oni the CO. exchange of (letaclle(l leaves ot corli (Zca ways) wlleat (Triticitmn -,u/gare), oats (Ar'eiia sativa)., barley (Hor(iciiii vidgarc), tinmothy (Ph/cor)n pratcnsc ) anld cat-tail (T'I'phla anigstifoli(a) was m1leasuired witlh a Clarl-k oxvgeni electro(le an(l inifrared carbon dioxide analysers in both open aiid closed systenis.Corn leaves did niot pro(luce CO., in the light at anly 0., conicelntration, as was show-ni by the zero CO., compensation point anid the absenice of a CO., burst in the first minute of darkness. The rate of photosynithesis was inhibited by 02 anlI the inihibition was not completely reversible. On the other haiid the stead) rate of respiration after a few\ minutes in the dark wNas niot affected bv 0._ These results wer-e interpreted as inidicating the albsence of allv measurable resl)iration during photosynthesis. Twelve (liffereint varieties of cornl studied all responded to O., in the same way.Tihe other 5 imonocotyledonis studied did produce CO., in the light. AIoreover. the CO2, compensation l)oint inicreased linearly with 02 in(licatinig a stimulation of photorespirationl.The imiplications of the lack of photorespiration in studies of primary productivity are discussed.I n previous communications fromii this laboratory it was show-ni that lpart of the inhibition of apparent photosynthesis by O., in tobacco and(I soybean leaves was duie to a stimulation of photorespiration which (liffere(l fromii dark respiration (1 3. 6). Hoever, ini addition to stimlulating plhotorespiration, 0., also had a seconid effect, which w-as attributed to a (lirect inihibitioni of p)hotosynlthesis.This seconid effect cani be sttilie(1 (firectlv iM a plant wlhich does nlot produce CO.. in thle light (i.e.-hicil lias n1o Ilhotorespiration '). Corln is suich a plailt sinice, in air, younig corin leaves lack both a llleasural)le CO., co.llpelnsationi point all( a Illeasurale CO., blirst i.e. tile iniitial Iligil rate of CO., prodtictioni during the first minute of (larkiless followinga light period (2,6). 'T'his mllealls tilat eitlier tilere is 110 CO., produced by leaxves of this species ill the light, or the CO., prodiuce(l is re-utilized before it can escape ilitO the atillosphere. If this latter explaliatioil is correct, then ilicreasing the 02 coIlcenltratioll from 1 to 100 % would ilncrease tile rate of CO., production in the light anid decrease the rate of photosynthesis, as showil ill the prece(ling paper for soybeall (1). This should result in ani increased CO.2 compelnsatioll poinit aidl ail ilicrease ill the illagniitilde of 1 The fiilalicial sul)port for this wsork camIle froli the Nationlal Researclh Council of Caniada.