Carbon dioxide compensation—its relation to photosynthetic carboxylation reactions, systematics of the Gramineae, and leaf anatomy

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.