Comparison of the Kinetics of Photo-synthetic Carbon Dioxide Fixation in Maize, Sugar Cane and Tobacco, and its Relation to Photorespiration

MOST plants exhibit the phenomenon of photorespiration : that is to say they give out a certain amount of carbon dioxide in the light. This effect is usually masked by the occurrence of photosynthesis which takes up carbon dioxide at a greater rate than photorespiration releases it. Photorespiration can, however, usually be demonstrated by allowing leaves to photosynthesize in an enclosed space when they are unable to reduce the carbon dioxide concentration of the atmosphere below about 50–100 p.p.m. This is because at this point carbon dioxide starvation reduces the rate of photosynthesis to a level at which it equals photorespiration and therefore no further net uptake of carbon dioxide is observed. More recently, a small group of plants, including maize and sugar cane, has been investigated, which can reduce the carbon dioxide concentration of a closed system virtually to zero1. This could be the result either of their having a poor photorespiratory mechanism or of their possessing an unusually efficient photosynthetic mechanism. There is evidence that the mechanism of photosynthetic carboxylation at least is different in maize and sugar cane from that in most other plants, for the initial product of photo-synthetic carbon dioxide fixation in maize and sugar cane is oxalo-acetate, rather than the phosphoglycerate which is the normal product of most plants2. Thus if different carbon dioxide fixing enzymes were operating in maize and sugar cane, this could well account for an increased affinity for carbon dioxide in these species.