Evidence from Chloroplast Fragments for Three Photosynthetic Light Reactions

Previous reports from this laboratory described a new concept of three light reactions in plant photosynthesis comprising two short-wavelength (λ < 700 nm) photoreactions belonging to Photosystem II and one long-wavelength (λ > 700 nm) photoreaction belonging to Photosystem I. Among the electron carriers assigned to Photosystem II were cytochrome b₅₅₉ and plastocyanin and to Photosystem I, cytochrome f. According to a widely held view, the light-induced reduction of NADP by water requires the collaboration of Photosystems I and II and involves specifically cytochrome f and P700 (a portion of chlorophyll a peculiar to Photosystem I). By contrast, the new concept ascribes the light-induced reduction of NADP by water solely to the two photoreactions of Photosystem II, without the participation of Photosystem I and its components, cytochrome f and P700. Further evidence in support of the new concept has now been obtained from chloroplast fragments. Two kinds of chloroplast fragments have been prepared: (a) one with Photosystem II activity, capable-in the presence of plastocyanin-of photoreducing NADP with water but lacking P700 and functional cytochrome f and (b) another having only Photosystem I activity, lacking plastocyanin, and enriched in P700.