The kinetics of photosynthesis

The influence of the various controlling factors on the rate of photosynthesis is one of the most interesting problems met with in the study of the living plant. If the photosynthetic cycle be considered as a problem in physical chemistry, the three important controlling factors are temperature, intensity of irradiation, and concentration of carbon dioxide in the surrounding medium. During recent years accurate measurements have been made by Warburg and by Emerson of the influence of each of these on the rate of photosynthesis, but no explanation of that influence has been put forward. Of the three controlling factors, the most important is the temperature, since an explanation of the whole process must depend on a complete understanding of the relation between the velocity of photosynthesis and the temperature. It is obvious from the fact that the velocity increases with the temperature that there must be associated with the primary photosynthetic reaction a dark reaction which has a temperature coefficient. This was first recognized by F. F. Blackman (1905), who also proved that when the intensity of illumination is very small the rate of photosynthesis becomes independent of the temperature, a fact which was later confirmed by Warburg (1919). It is evident from this that the Blackman reaction cannot be the precursor of the primary photosynthetic reaction, and there seems little doubt that it is the restoration to its initial state of the chlorophyll which has undergone some change as the result of the primary reaction (Emerson and Arnold, 1932,a ).