/. li. THOMAS AND GOVINDJEE 



477 



•e- 



50 



LIGHT ABSORBED BY PHYCOBILIN FILTER IN % 



Fig. 1. Relation between quantum yield and light absorption by phycobilin filters. 



is provided, the light transmitted by the 95% filter clearly shows a 

 measurable photosynthetic effect (+ 1.66 ± 0.30 jxl Oo/hour) . This 

 shows that the disappearance of photosynthetic activity in light fil- 

 tered through the 95% filter was not due to the residual light being 

 practically unabsorbable by chlorophyll a. The same conclusion can 

 also be derived by calculation, using the energy distribution curve of 

 the incident light, the absorption curve of the filter, and the absorp- 

 tion curve of the cell suspension (determined in an integrating sphere 

 and therefore free from scattering effects) . This calculation shows 

 that much of the transmitted light falls into the region 680-700 mjn, 

 and is largely absorbed by the cell suspension. 



The effect of removal of light predominantly absorbed by phyco- 

 bilins, shown in Fig. 1, may be explained in terms of the dependence 

 of $ on the intensity ratio of auxiliary light to far red light, as noted 

 by Emerson and coworkers. As long as the phycobilin filter absorbs 

 less than 20% of the incident light, this ratio may remain above the 

 critical value, and the maximal quantum yield is maintained. If 

 more than 20% but less than 80% of light is absorbed in the filter, 

 the quantum yield becomes dependent on the intensity ratio; i.e., 

 light energy absorbed by chlorophyll a produces photosynthesis only 

 in proportion to the energy absorbed by the auxiliary pigment. When 

 the intensity ratio declines below another critical value, no enhance- 

 ment of the quantum yield in the far red by light of shorter wave 



