Extensions of photosynthetic experimentation 131 



V. Adjustment of the Compensating Light 



By the principle of the light increment (Warburg & Burk, 1950) the technical and 

 theoretical difficulties caused by respiration may be overcome. Respiration is 

 overcompensated by unmeasured white light and photosynthetic yield measured 

 only for an increment of measured light. When this increment is small in comparison 

 with the compensating light, the general conditions of the cells are virtually not 

 altered by the addition of the increment, as is required when photosynthesis is 

 computed by our equations. 



Since it was found in our earlier experiments (Warburg & Burk, 1950) that a 

 given light increment, added to dark or compensated or considerably overcompen- 

 sated cells, proved to be equally efficient, it is unnecessary to adjust the compen- 

 sating light in such a way that in both vessels exactly the same amount of compen- 

 sating light is absorbed. But if such an exact adjustment is desired, the following 

 method has been found to be useful. Cells are suspended in carbonate mixture and 

 equal volumes placed into the two vessels used for the two-vessel method. A 100- 

 watt incandescent lamp is placed above the thermostat and shifted until photo- 

 synthesis is found to be equal in the two vessels. Because of the scattering of the 

 light by the cells, this method is to be preferred to an adjustment by means of two 

 corresponding chemical actinometers. 



VI. Long Experiments 



It has been confirmed that the efficiency of photosynthesis remains constant for 

 longer periods of time when respiration is overcompensated by diffuse white light 

 in such a manner that all cells are constantly illuminated. Such a result may be ex- 

 pected, since the conditions are then those of the culturing; a chemical or physical 

 explanation is lacking. 



VII. Use of Carbonate Media 



Although it is now widely considered that maximum efficiencies cannot be obtained 

 in carbonate Solutions, such Solutions may be useful in many respects as an ex- 

 perimental tool. Equal parts of M/10 K2CO3 and M/10 NaHCC>3 may be mixed to 

 yield a pH of about 10. The quantum requirement of our cells suspended in such 

 a mixture was about 10. 



A first application of carbonate Solution has already been mentioned in Section 

 IV. A second application may be made to check that the motion of both vessels in 

 the two-vessel method is adequate. Equal amounts of cells, suspended in 7 c.c, 

 carbonate mixture, are placed in each of the two vessels that are then shaken and 

 illuminated as in the efficiency experiments in acid medium. When equal photo- 

 synthesis is obtained in both vessels then one has an indication that not only physical 

 equilibration but also light intermittency are equal in the two vessels for the special 

 conditions of vessels, amounts of liquid and cells, and light intensity, etc., em- 

 ployed. 



