THE DEPRESSANT EFFECT OF CARBON DIOXIDE UPON PHOTOSYNTHESIS

Abstract

S OME of the earliest studies in photosynthesis were concerned with the relation of the assimilatory activity of green plants to the carbon dioxide concentrations of their environments. Although these were made at a time when the importance of the interaction of factors determining the rate was not understood, it was established that with increasing concentration of carbon dioxide the rate of assimilation rose to a maximum and then declined. The focus of interest was the establishment of 'optima'. The optimal concentration varied for different species. While Godlewski (I873), Kreusler (I885) and others showed that the concentration above which assimilatory activity declined was about 7-IO %, other estimates were as high as 20 0. It has generally been assumed that a concentration of carbon dioxide of from 4 to 5 %0 is distinctly lower than that causing a decline in activity. Willstatter & Stoll (I9I8) performed the great majority of their large range of experiments in air streams containing concentrations of 4-6 %0, and many modern workers have followed their methods. Apart from the known effects of very high concentrations of carbon dioxide upon stomatal aperture, the possible causes of the decline of assimilation rate when the 'optimal' concentration of carbon dioxide is exceeded have been incompletely understood and are usually referred to in vague terms. Thus Blackman (I905) says: 'Ultimately, if the supply of carbon dioxide in the air current be increased up to 30, 50, 70 %0, the carbon dioxide will have a general depressing effect on the whole vitality, and before suspension of all function a diminution of assimilation undoubtedly occurs. This is, however, quite a separate process.' Both Stiles (I925,