Evidence for Existence of Light and Dark Reactions

The process of photosynthesis was known in its bare outline already at the beginning of this century. But the phenomenon was still much like a mysterious black box and scientists had little idea of the events that proceeded inside the box. Unfortunately, the box could also not be opened then since photosynthesis ceased immediately if one broke up the cell or tissue where photosynthesis was going on. For a long time there was no clue, till at the turn of the century, an English plant physiologist, F.F. Blackman, working in Cambridge, started his experiments.

Figure: Diagram to show evolution of concepts concerning the mechanism of photosynthesis.

He was the first to give the idea that photosynthesis consists of at least two kinds of reactions -the light and dark reaction(s). He estimated Q_l0 of the photosynthetic reaction and found it about 2.5, provided that photosynthesis was studied under optimal conditions, especially of adequate light and supply of carbon dioxide. That photosynthesis involved some photochemical reaction(s) was of course obvious because of  the necessity  of  light and chlorophyll pigments.