A second chlorophyll reaction in the electron chain of photosynthesis--registration by the repetitive excitation technique.

Light absorbed in the far red limit of the absorption spectrum of green plants shows a poor yield of photosynthesis but is enhanced to much greater efficiency when it is supplemented by shorter wave length light1. These two colour effects were a first hint to suppose 20 years ago that the primary reaction of photosynthesis may perhaps be sensitized by two pigments, a long wavelength and a shorter wavelength chlorophyll. — Since then, very different hypotheses have been postulated to explain how a cooperation of two pigments may occur 2 5 . The problem remained an open question until those substances which are in action during light excitation could directly be measured by absorption changes. Three techniques have been developed: a static method with continuous light excitation6, a high sensitive single flash light method 7, and a repetitive flash light technique 8. Using the two colour effects in connection with measurements of the absorption changes, it has been possible to demonstrate directly the existence of two pigment systems coupled in series9 1 1 . Especially by the flash light methods, seven different types of absorption changes and the products and intermediates which cause these changes have been identified. The detailed kinetic analysis of the reactions of these substances between 10~6 and 1 0 1 sec leads to a reaction scheme depicted in fig. 1 12. Electrons are transferred from water to NADP® through at least nine intermediates. The flow is generated by the excitation of a chlorophyll-ai and by the excitation of an unknown second pigment Pn . The promoting dilorophyll-ai-reaction can be directly followed by absorption changes13 '14. The difference spectrum of these changes in the region of the typical chlorophyll absorption bands in the red is depicted in fig. 3. The life time is 2 • 10~2 sec (22 °C). Based on the measurements of action spectra, it was shown that chlorophyll-aj can be excited with wave lengths ^<730nm, the pigment II, however, only with wave length A < 7 0 0 n m (see fig. 1) . Chlorophyll-a[-430-703 is, therefore, the proposed long wave length chlorophyll-a. Pigment II must correspond to a shorter wave length chlorophyll-a with an absorption band just beyond 700 nm, probably with a maximum at ~ 690 nm. A detailed description of the characteristics of the chlorophyll-ai-reaction is given elsewhere 1 3 '1 4 . The reaction of the second pigment P n , which is especially active in the cleavage of water has, however, not been discovered. The reasons are as follows.