Carbondioxide compensation concentration and its relationship to photorespiration and net carbon exchange—A review

Photosynthesising plant or their parts when placed in a closed gas exchange system and exposed to light intensities above compensation point, cause an equilibrial steady state between photosynthesis and respiration and this steady CO₂ concentration (CO_aCC) depends upon temperature, O₂ concentration, light intensity, physiological stage of growth development and many other factors. Plants have been classified in two major groups: one exhibiting CO₂CC of 40 ppm or more and the other 0 to 10 ppm. Certain plants (or leaves in the same plant) exhibiting intermediate CO₂CC have also been reported as exception to above classification. CO₂ compensation concentration is also used as an indication or measure of photorespiration, screening for efficient C₃ types and as a criterion to distinguish C₃ and C₄ plants. However, these attempts have met with little success so far.

Although the CO₂CC can be used as a measure for photo-respiration it is not sensitive enough to assess small changes in photorespiration. It can be regarded as reverse approximation of net photosynthesis with some exception. Work carried out in this laboratory shows that it varies with growth and development. In case of wheat (Triticum aestivum Vill var. Sonalika) CO₂CC was significantly low after anthesis and also during grain development. Some of the leaves of sorghum exhibited intermediate and higher CO₂ compensation concentration characteristic of C₃ plants.

A scheme about the modification of Calvin Cycle at various points of CO₂ compensation concentration is presented. It is suggested that various aspects of CO₂CC need further investigation.