Evidences for the Existence of Light and Dark Reactions
(a) Intermittent light experiments
Warburg (1919) found that if light was supplied to a plant in short intense flashes separated by dark periods, the photosynthetic yield per second of illumination was higher than if light of the same intensity was given with uninterrupted period of illumination. The Calvin cycle (dark phase) regenerates ADP and NADP required for light reaction. However in continuous light, the dark reactions are slow and fail to regenerate sufficient ADP and NADP for use in light phase. Thus dark phase acts as a rate limiting step for the process of photosynthesis.
In the following reaction sequence:
In the reaction A ——> B proceeds at a faster rate than B ——> C so that there is a tendency for some B to accumulate. In the intermittent light the reaction B ——> C proceeds in the dark utilizing the compound B.
(b) Q10 Experiments
The ratio of the rates of reactions at toC and (t + 10)oC is denoted by Q10.
Q10 = Reaction rate at (t + 10)oC/Reaction rate at toC
F.F. Blackmann was the first to give the idea that photosynthesis consists of at least two kinds of reactions—the light and dark reactions. He estimated Q10 of the photosynthetic reaction and found it around 2.5, provided that photosynthesis was studies under optimal conditions, specially 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. However, the derivation of Q10 value gave a unique insight into the complexity of the reactions. The Q10 photochemical reaction is 1, whereas for chemical reactions it is generally 2-3. By a simple study of the effect of temperature and analysis of results indicated that photosynthesis consists also of chemical reaction(s) often called ‘dark’ reactions since they do not need light.
The present study of photosynthesis demonstrated the existence of two phases, a light phase and a dark phase. The reaction of light phase requires light and hence also called photochemical reaction whereas reactions of dark phase require no light (dark) and are purely chemical reactions.