On the Efficiency of Photosynthesis above and below 

 Compensation of Respiration 



ROBERT EMERSON and RUTH V. CHALMERS, Botany Department, 

 University of Illinois, Urbana, Illinois 



Kok has published figures whicli indicate that, below the com- 

 pensation point, the efficiency of photosynthesis maybe twice what it is 

 above compensation. More recently, Bassham, Shibata, and Calvin 

 (1) have reported similar results. It has been widely speculated (for ex- 

 ample, Franck (2)) that, when respiration exceeds photosynthesis, the 

 amount of energy required per molecule of photosynthetic oxygen 

 production may be smaller than above compensation, because inter- 

 mediates from oxidative metabolism may be serving as substitutes for 

 carbon dioxide as the substrate for photosynthesis. 



It is important to know whether photosynthetic oxygen produc- 

 tion above and below the compensation point can represent different 

 amounts of energy storage. We report here some comparisons of 

 efficiency of photosynthesis above and below compensation. 



The measurements were made with Cfilorella pyrenoidosa cells 

 suspended in carbonate buffer No. 9, saturated with 0.5% carbon 

 dioxide in air. Pressure changes (representing oxygen exchange) were 

 observed at 1-minute intervals by the method described by Emerson 

 and Chalmers (3). A differential manometer and double cathetom- 

 eter were used. Photosynthetic oxygen production was calculated 

 from observations during alternating 10-minute periods of light and 

 darkness. The calculated rates of photosynthesis are derived from 

 the steady rates of pressure change observed diu-ing the second 5 

 minutes of each 10-minute period, but do not include the transition 

 effects which were observed during the initial minutes of each period. 

 The energy of the light beam was about 1.5 m einsteins per minute at 

 a wavelength of 644 m^ (cadmium line). Cell suspensions were dense 

 enough for total absorption of this frequency (300 to 400 ix\. cells 

 in 8 ml. of liquid). Area of the light beam was about 4 sq. cm., and 

 the area of the bottom of the reaction vessel was about 10 sq. cm. 



Table I shows a comparison of the efficiency of two samples of cells, 



349 



