SIMULTANEOUS MEASUREMENT OF ACTION SPECTRA FOR PHOTOREACTIONS I AND II OF PHOTOSYNTHESIS

— We have devised a method of obtaining simultaneous action spectra for photoreactions I and II by analysis of direct and indirect effects involved in enhancement. The method requires previous determination of the neutral wavelength which gives maximum quantum yield by virtue of equal fractions of open reaction centers (p and q) for each photoreaction. A sufficient intensity of the neutral wavelength is used as a constant background. Upon addition of a weak modulated measuring light of intensity Im and wavelength λm two amperometric signals are obtained for rate of oxygen evolution. A modulated signal (AC¯) isolates the direct effect of Im and gives action of photoreaction II as AC/Im. An increment in total rate (ΔDC) also includes an indirect effect of Im in perturbing reaction center conditions (p and q). From analysis of interaction of the two photoreactions, action for photoreaction I can be estimated as (2 ΔDC-AC)/Im. The method is applicable to whole cells, properly scales the two action spectra to each other, and removes contribution of the State 1-State 2 phenomena. Action spectra were obtained for Chlorella.