BESSFJ. KOK AM) (.EORGE HOCH 407 



the jMcvious section) . The lull curves indicate the background con- 

 centration ot "P 700" as dei^endent upon the wavelength of the 

 actinic beam. The spectrinn measured with strong detecting light 

 shows an increase of "P 700" absorption sensitized by phycocyanin 

 (the accessory pigment) , and indicates that a decrease of "P 700" con- 

 centration is induced by chlorophyll a and carotenoid. The over- 

 hipping absorjjtion spectrum of the two pigments probably accounts 

 for the peculiar shape of the activation spectrum around 650 m^a. 



Noteworthy in Fig. 6 is that a 30-fold decrease of the intensity of 

 the detecting beam results in complete disappearance of the positive 

 phycocyanin-sensiti/ed effect and a strong enhancement of the nega- 

 tive (background) change induced by chlorophyll a. (In contrast, 

 the fast reversible bleaching is less influenced by the intensity //,) . 



There is an obvious explanation for the fact that the positive 

 effect, in order to occur at all, requires a considerable intensity of 

 the detecting beam. This latter beam has to be of a wavelength 

 around 700 m^ (absorbed by chlorophyll a and by 'T 700" directly) , 

 and therefore will itself bleach "P 700" and thus lower its steady- 

 state concentration. Only if this occurs to a sufficiently large extent, 

 will the 630 ni/x beam find enough bleached material to convert 

 and cause a detectable reformation of "P 700." 



Conversely, the establishment of a low steady-state "P 700" con- 

 centration by a strong detecting beam will decrease the negative effect 

 of the actinic light, since both compete for the same substrate. Fig. 6 

 shows that the positive change seems to be largely sensitized by 

 phycocyanin in Anacystis. In green cells chlorophyll b is the main 

 sensitizer, but there is a definite involvement of part of the chlorophyll 

 a (18). 



Is Photosynthesis Driven by Two Light Reactions? 



The observations discussed in the above sections strongly indicate 

 the occurrence of two different light-induced reactions: the first sensi- 

 tized by chlorophyll a and a direct photochemical bleaching of 'T 

 700"; the second, sensitized by accessory pigment, acting indirectly 

 via the mediation of dark steps, and restoring "P 700." 



So far we have no way to approach the quantum yield of the second 

 step, but, as discussed in the beginning, photosynthetic action spectra 

 invariably indicate a high conversion efficiency of sensitization by 

 accessory pigment. 



We also should recall that fluorescence data, if significant, indicate 

 that accessory pigments transfer their energy efficiently to chlorophyll 



