128 



J, L. Rosenterg and Tevfik Bigat 



algae. These results are similar to those reported ^y, v 

 Lavorel for Chi or el l a hy a completely different method/11-' 

 and are consistent with the finding of Butler that there vas 

 practically no increase in the ability of light at wave-lengtJiB. 

 above TOO m^ to excite fluorescence at the induction outburstr: '' 



The simplest interpretation of these facts is that the far- 

 red fluorescing pigment^ corresponding to P7OO in absorption, 

 can participate in photochemistry only indirectly or only by 

 way of its metastable state. If this pigment belongs to 

 System 1 and acts as an energy sink for other pigments within 

 System 1, there would be a unique assignment of the photo- 

 sensitizing role for Step (l) to a metastable state, whether 

 System (l) or System (2) absorption acts lead to this photo- 

 chemical act. 



Fig. k shows the results of some typical fluorescence 

 transients in leaves accompanying the reduction of incident 

 Intensity from a value above saturation to a lower value. 

 These experiments were a confirmation, and extension of a 

 phenomenon reported by Franck et al.^^^). The different 

 courses of Curves A and B in Fig.~5 suggests a difference 

 in mechanism of satioration for the aerated and C02-d.epleted 

 samples. In the aerated leaf, curve A of Fig. k, the major 

 cause of saturation is probably located in the oxygen- 

 liberating reactions following Step (2). Under such condi- 

 tions oxidized cytochrome accumulates but is not able to 

 undergo Step (2) as rapidly as quanta are supplied to the 

 reaction centers. When the incident intensity is lowered, 

 a region of greater quanta! utilization is reached. 

 Efficient photochemistry will start with Step (2) to reduce 

 the backlog of oxidized cytochrome. The fluorescence yield 

 will thus start low at the reduced Intensity and will slowly 

 rise as the excess oxidized cytochrome is reduced and the 

 two photochemical steps again come into phase with each 

 other . 



In the COg- deprived sample. Curve B of Fig. h, the major 

 cause of saturation is the limitation in supply of the 

 nat-ural photosynthetic oxidant for Step (l). In such a case, 

 prolonged irradiation at high light leaves the cytochrome 

 principally in the reduced state. On a sudden reduction in 

 light intensity a more efficient quantal utilization will be 

 felt primarily in Step (l), ajid the fluorescence will remain 

 above the final steady- state value until the two photochemical 



