13 



L.N.M* Dujsens 



ronoxua- 



fluorescence in Porphyridixim cruentum shows Hnich more pron( 

 ced activity of phycoerythrin than of chlorophyll a^ ' ,(see 

 also^ ). Prom this it follows not only that excitation energy 

 is transferred from phycoerythrin to the fluorescing chloro- 

 phyll a, but also that the fluorescence is mainly caused by the 

 chlorophyll a of system 2. From the observation that the ratio 

 of the value at the phycoerythrin maximum and that at the chlo- 

 rophyll a maximum was larger for the action spectrum of chloro- 

 phyll a fluorescence than for the absorption spectrum, it had 

 been already earlier concluded that two forms of chlorophyll a 

 occurred. One of these is a fluorescent form, receives its ener- 

 gy to a large extent from phycoerythrin and/the\other is non- 

 or weakly fluorescent form of chlorophyll a^^'-'', Thes^gforms 

 were recently called chlorophyll a^ and chlorophyll a^^ • 



Upon illumination of a suspension of Porphyridium cruentum, 

 which previously had been in the dark, with green light (light 

 2) the fluorescence yield rapidly increased, by a factor of 

 about two, and then slowly declined to a steady state value, 

 which was only slightly higher than the value in the dark. The 

 excitation spectrum for this fluorescence increase revealed 

 that it was caused by chlorophyll &2' ^^* ^* ^^^ maximum of the 

 increase in fluorescence yield, or shortly thereafter a strong 

 blue beam was admitted in addition to the green actinic beam, 

 the fluorescence yield rapidly dropped to a value not much 

 higher than the steady state value. This phenomenon was obser- 

 ved for all species of algae of different groups investigated: 

 for various red and blue-breen\algae, the green alga Chlorella 

 and for spinach chloroplasts^ « Light 2 was found to strongly 

 increase the chlorophyll a^ fluorescence and light 1 to lower 

 this enhanced fluorescence. If these changes are attributed to 

 chemical changes in a compound Ci present in the neighbourhood 

 of chlorophyll &2, then it can be concluded from scheme 1 that 

 ^ is located between systems 2 and 1. Apparently the oxidized 

 form "^ quenches the fluorescence, and the reduced form GiH does 

 not quench the chlorophyll a^ fluorescence. DGMU does not inhi- 

 bit the increase in fluorescence, but it abolishes the decrease 

 in fluorescence (or the reoxidation of ^iH)by light 1. Apparently 

 ^ ±3 different from C 420, P 700, or quinone, since the reduc- 

 tion of these compounds and not the oxidation is inhibited by 

 DCMU. Furthermore the first two compounds are in the dark in 

 general present in the reduced form, while Q, is present in the 

 oxidized form. In spinach chloroplasts the chlorophyll fluores- 

 cence is strongly enhfinced by the addition of a small amount of 

 dithionite. These experiments show that <^, is located between 



