Two light wavelength effects

Many experiments have now been performed with chloroplasts and other photosynthetic material in which electrons are injected or withdrawn from the photoelectron transport pathway by the addition of electron donors or acceptors. These studies, together with the enhancement and chromatic transient experiments, have led to the formulation of a currently popular scheme for photoelectron transport, shown in Fig. 3. 

According to this scheme, there are two pigment systems. These are labeled Pigment System 1 and Pigment System 2 for the 

With this scheme before us, let us consider some of the biochemical evidence in its support. One can choose only a few of the many interesting experiments which have been published. More complete descriptions of these experiments may be found in collections of papers presented at symposia such as that sponsored by the National Academy of Sciences (1963) and the Centre National de la Recherche Scientiflque (1963). 

Ash et al. (1961) first showed that tetrazolium blue and methyl red could be photoreduced in a Hill reaction by spinach chloro-plasts. Since no electron donor was added, it was presumed that this reaction was accompanied by the evolution of oxygen. Reduction of both dyes was strongly inhibited by the addition of dichlorophenylmethylurea (DCMU), which is thought to block 

Losada et al. (1961) reported the separation of the two light reactions in noncyclic photophosphorylation and NADP+ reduction in green plants. They blocked System 2, which oxidizes water, by adding the inhibitor DCMU and then adding a dye as an electron donor. Under these conditions they were able to demonstrate the photochemical reduction of NADP+ and the simultaneous noncyclic photophosphorylation. 

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