68 PHOTOSYNTHESIS 



with the fact that its absorption band is the farthest to the 

 red of the chief pigment components. In the red algae in 

 which chlorophyll d is present, the absorption band of which 

 is farther into the red, there is some evidence that when 

 transfer of energy occurs from chlorophyll a to chlorophyll d 

 there is a loss of energy from the photosynthetic system. On 

 the other hand, when energy is transferred from phycoery- 

 thrin to chlorophyll a it is suggested that transfer occurs to 

 a portion of the chlorophyll a which does not 'leak' appreci- 

 ably to chlorophyll d. This hypothesis, based on the observa- 

 tion that phycoerythrin-sensitized fluorescence of chloro- 

 phyll is stronger than its directly excited fluorescence, also 

 oflFers an explanation of the relatively poor efficiency of light 

 absorbed by chlorophyll as compared with the greater 

 efficiency of light absorbed by the phycobilins in some red 

 algae (Duysens, 1952). 



STUDIES OF THE INTENSITY OF FLUORESCENCE 



In addition to studies of the sensitization of fluorescence 

 of one pigment resulting from absorption of light by a 

 different pigment, described in Chapter 3, attempts have 

 also been made to correlate changes in the intensity of 

 fluorescence with changes in the rate of photosynthesis. In 

 the preceding section we have seen that perhaps as much as 

 30% of the absorbed light energy finally is converted into 

 chemical energy. The remaining 70% is dissipated largely 

 as heat but a small fraction is re-emitted as fluorescent light. 

 In the living cell the yield of fluorescence is of the order of 

 i-o to 0-1% of the incident light energy. Kautsky discovered 

 changes in the intensity of fluorescence of chlorophyll 

 during the induction phase of photosynthesis and showed 

 that there was in general an inverse relationship between 

 the rate of photosynthesis and the intensity of fluorescence. 

 Thus, after a short aerobic dark period, whilst the rate of 

 photosynthesis increased during the first minutes of illum- 

 ination the intensity of fluorescence fell from an initial high 

 value to about half of this. Similar results were found by 

 McAlister and Myers (1940). A simple interpretation was 

 suggested, namely that there was a direct competition 



