Action spectra of the reducing and oxidizing systems in spinach chloroplasts

The reducing moiety of the photosynthetic cycle, resulting in TPN⁺ reduction, and the oxidizing moiety (cytochromes), resulting in the main part of ATP production, were excited by separate light reactions and showed different action spectra. The two light reactions were also characterized by specific changes in the absorption spectra of the pigments. The reducing system was more sensitive to blue-green light, oxidized carotenoids (negative band at 500 mμ) and reduced an initial reductant. The oxidizing system was specifically sensitive to red light, oxidized the cytochrome system and reduced the pigments via chlorophyll and unknown factors showing negative bands at 640 mμ. The action spectra showed a multilateral linkage between pigments and enzyme systems and considerable activity also in green at higher light intensities. It is suggested that the enzyme systems together form a balancing steady state, which operates also in the dark and is sensitive to variations in the concentrations of O₂ and various substrates. The site of O₂-production (photolysis of H₂O) is probably at the junction of th two redox systems, viz. at the autoxidable cytochrome, b₃ and flavoprotein. Its action spectrum showed characteristics of both the reducing and oxidizing systems. The action spectra are influenced by light scattering.