L.N.M, Duyaens 



Evidence for the occurrence of the redox couples shown in 

 Fig. 1 will be discussed below. We restrict ourselves here to 

 a few general statements. According to the scheme* upon onset 

 of illumination a reduction will occur of all compounds at the 

 right hand side at the top of the scheme, and an oxidation of 

 H2O and ZH. Upon illumination with light of a wavelength, which 

 ia mainly absorbed by system 1 (and which we call light 1. such 

 as 430 and 680 mn in Porphyridium , or 700 mjx in Chlorella ) , 

 redox components in between the two systems, such as ^ and PH, 

 tend to accumulate in the oxidized state; in light 2 (such as 

 light of 560 mM in Porphyridium and of wavelength shorter than 

 680 mji in Chlorella ) these redox couples may be expected to ac- 

 cumulate in the reduced state. Upon darkening, the initial re- 

 dox state in the dark is usually reestablished due to reactions 

 with redox substances shown in the scheme, or with other cell 

 constituents. The action spectrum for the initial r^te of re- 

 duction of X and all compounds written at the right hand side 

 of Xwill be proportional to the action spectrum of system 1; the 

 same will be true for the initial rate of oxidation of the sub- 

 stances between system 1 and 2, with the exception of ^, if the 

 electron or hydrogen transport between Q and the substances is 

 interrupted by an inhibitor such as DCMU, In the presence of 

 this inhibitor, the action spectrum for the initial rate of re- 

 duction of <^ will be proportional to the action spectrum of 

 system 2. The overall electron or hydrogen transport during 

 steady state photosynthesis will be limited by that system which 

 has the smallest activity at the wavelength of the actinic 

 light. Thus, the action spectrum of photosynthesis will follow 

 the lower of two curves shown in the inserts of Pig. 1. If, on 

 the other hand, the action spectrum of photosynthesis is mea- 

 sured in the presence of a strong constant background of light » 

 mainly absorbed "by system 2, this action spectrum will be pro- 

 portional to the action spectrum of eystoaO. ^ , and vice versa . 

 This was in fact the way French and Myers ineasured action spectra 

 of Chlorella , and we have interpreted these spectra as action 

 spectra of system 1 and 2. The spectra for Porphyridium , also 

 shown in Fig. 1, were measured in various ways (see below). 



The scheme of Fig. 1 makes it possible/to understand the 

 '• Emerson effect ", Emerson's discovery ^ ' stimulated research 

 that established and identified the two algal photosynthetic 

 systems. This effect may be formulated and "explained" as fol- 

 lows (cf. ), If light beams 1 and 2 (e.g. light of 700 and 

 650 m^. respectively in Chlorella , or 68O and 56O mjx in Porphy- 

 ridium ) are applied simultaneously, then the rate of photosyn- 



