Experimental researches on vegetable assimilation and respiration. XXI.—Induction phases in photosynthesis and their bearing on the mechanism of the process

In many reactions proceeding in living cells the concentration of the reactants changes with time, and hence our knowledge of the nature of the mechanism cannot proceed very far without means of determining, directly or indirectly, the changes of concentration. In photosynthesis the reactants carbon dioxide and water can often be readily maintained at a constant level, and consequently investigations have been centred on the relation between the rate of photosynthesis, when it has reached a stationary value, and the concentration of the reactants and various inhibitors and accelerators. Transitions from one stationary state to another, consequent upon a change in intensity of one of the determining factors, have received little attention, Investigation of such transitions may throw light on the mechanism of the process itself, and possibly on the interaction of this and other processes in the cell. The transition to be considered in this paper is that taking place in the rate of photosynthesis when chlorophyll-containing cells pass from darkness to illumination. Perhaps, in view of the interactions between photosynthesis and respiration which we are going to suggest, as well as for other reasons, we ought to say net oxygen production or carbon dioxide consumption rather than photosynthesis. In 1918 Osterhout and Haas (1918) gave the data for one experiment on Ulva in sea water exposed to bright sunlight which, according to the authors, showed "that Ulva which has been kept in the dark begins photosynthesis as soon as it is exposed to light, and that the rate steadily increases until a constant speed is attained." Actually their experiments tell us nothing about the rate in the earliest stages.; only, that in the first 35⋅7 minutes there was as great a change in the concentration of hydrogen ions of the sea water as was eventually achieved in 20⋅4 minutes. The authors developed two theories to explain their set of data.