The Emerson Enhancement Effect and Evidence for Two Photosystems

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

To explain the Emerson enhancement effect, Govindjee and Rahinowitch ° obtained evidence from the action spectra of the enhancement effect the presence of two distinct forms of Chl-a in vivo presumably present in two different pigment systems. Soon, new experimental evidence in support of the two-photo-system concept was reported independently by Govindjee etal and by Kautsky eta/. from evidence obtained from fluorescence and by Kok and Hoch Duysens and Witt based on measurements of absorbance changes. We present here two results based on absorbance changes associated with changes in the redox state of electron carriers in the chloroplast. 

In the meantime, Witt and colleagues observed similar effects on the redox state of cytochrome / when red and far-red light were alternately used for illumination. They also observed similar two-light effects with respect to a species that was later identified as plastoquinone, as well as another species, P700 discovered earlier by Kok. Fig. 17 (A) shows the spectroscopic results of the two-light effect on P700 in broken spinach chloroplasts. Illumination of chloroplasts with 720-nm light of 0.5- ec duration 

Wavelength of excitation light (at equal number of incident photons) 

Rumberg et al. measured an absorption change due to plastoquinone that is known to be located on the reducing side of the PS-II reaction center. An absorption decrease at 254 nm attributable to the reduction of plastoquinone could be induced by a 638-nm flash. This photoreduced plastoquinone could then be completely reoxidized by far-red light at 720 nm. It was also noted that the electron-transfer reactions in the re-reduction of P700 and re-oxidation of reduced PQ were kinetically coupled. 

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