The carbon dioxide compensation concentration of members of the Gramineae and a few other plants was determined with an infrared CO2 analyzer. These results were then considered in relation to the new photosynthetic carboxylation pathway proposed by Hatch et al., rates of photosynthesis, grass systematics, leaf anatomy, and distribution of starch in the leaf. Plants possessing the new carboxylation pathway had low compensation values whereas those having the Calvin carboxylation reaction had high values. Low compensation plants also had a well-developed parenchyma bundle sheath containing a high concentration of chloroplasts which accumulated large amounts of starch. Little or no starch was present in the mesophyll cells. Cyperus was exceptional in that it also formed appreciable starch in the mesophyll. Those low compensation members of the Gramineae tested belonged either to the chloridoid–eragrostoid or the panicoid lines of evolution. A literature survey indicated that low compensation grasses have photosynthetic rates that are about double those of plants with photorespiration correlated with a temperature optimum for photosynthesis of about 35 °C. Those plants with photorespiration have optima within the range 10–25 °C. Some simple assay procedures proposed on the basis of the above correlations allow rapid determination of the physiological and biochemical status of plants with respect to photosynthesis.

The rate of carbon dioxide exchange in both light and darkness by detached tobacco leaves placed at various oxygen concentrations was measured by an Infra‐Red CO2 Analyzer and a Clark oxygen electrode. It was observed that during illumination oxygen had two different effects. One was to stimulate carbon dioxide evolution and the other to inhibit carbon dioxide absorption. Concentration of carbon dioxide at compensation point was found to be a linear function of oxygen concentration and this has been explained as due mainly to an increased evolution of carbon dioxide. Such an evolution during illumination has been called photorespiration. Increased concentrations of oxygen also had a stimulating effect on the magnitude of the initial post‐illumination burst of carbon dioxide in darkness, but no effect on the subsequent steady rates. These data have been explained as due to the suspension of regular respiration in darkness and its replacement by a different process, tentatively called photorespiration. A second effect of oxygen was to reduce the efficiency (called “carboxylation efficiency”) with which a leaf was able to remove carbon dioxide from the atmosphere.

Abstract. The effect of 21 % O., and 3 CO O., on the CO., exchange of detached wheat leaves was measured in a closed system with an infrared carbon dioxide analyzer. Temperature was varied between 20 and 430, CO., concentration between 0.000 to and 0.050 to and light intensity between 40 ft-c and 1000 ft-c. In most conditions, the apparent rate of photosynthesis was inhibited in 21 % 0, compared to 3 % O.,. In the plreseilt investigationi. thle effect of ( )oi on pllotosyntllesis in wheat was examined at (liffeleilt temil)eratures, (O.) concentratiolis andl( light intensities to determiine the miagiliti(le anld signlificanlce of the inhlibitionl of phlotosyntlhesis. Materials and MethodsSeeds of Triticuin sativuii L. \vere planted in vermiculite, wvatered daily. and grown in a growth roonm at 2(0)0 ft-c light, 16 hour da-y anid 21 to 26/18 to 220 day/night temperature. In most experimlents, ahout twenty-five 10 to 14 dav-old slhoots were detached, immnlediately iiiimersedl in water. and recut 5 mnm l)elow the node. The ctut shoots were then transferred to a plexiglas chamber. When intact plants were used, the seeds were planted in a row so that shoots of the salmle age could be sealed into a plexiglas chamber by a rubber gasket coated with silicone grease. An 860 nml closed system was used for all of the experiments, and a constant flow rate of 2 liters per minute Nwcas maintainied by a gas pump. The CO., concentration in the gas stream was miieasured from 0 to 0.060 % CO., by a Beckman

A closed gas exchange system was used to survey the apparent rates of photosynthesis by young wheat shoots in a wide range of O2 concentrations (O2), CO2 concentrations (CO2), temperatures (T), and radiant flux densities. The results are expressed graphically and by equations. The carbon dioxide compensation point (ΓH) conformed to the equation ΓH = O2e(0.0428T − 12.1). The relationship between the apparent rates of photosynthesis in the presence (PH) and absence (PL) of inhibitory concentrations of oxygen was described by the equation PH = PLk loge(CO2/ΓH) where k was independent of the prevailing environmental conditions and [Formula: see text].

An infrared CO2 analyzer was used to measure production and assimilation of CO2 by 11 marine or freshwater algae in acidic media. Considerable differences among the algae were found in the concentration of CO2 required to saturate photosynthesis and in the compensation concentration of CO2. High compensation values were always lowered by lowering the O2 concentration, even to concentrations which did not reduce respiration in the dark. It was concluded that respiration was inhibited during photosynthesis. In four of the algae, respiration was replaced by a different process of CO2 production during photosynthesis.