34 Introduction 



2. Blue-green light 



In 1955 we discovered that blue-green light acts as a catalyst in photosynthesis. 

 Other very pure spectral regions of visible light are either ineffective or only of 

 very poor effectiveness in photosynthesis, unless small amounts of blue-green light 

 are added. Since that time, blue-green light has always been added in measure- 

 ments of the quantum requirement. 



The action spectrum of blue-green light yielded a peak at approximately 

 458 mit and is probably the absorption spectrum of an enzyme. The determination 

 of this action spectrum is one of the more difficult tasks of radiation-physics, since 

 it requires that each of two light beams of different color is split into two parts; also 

 that four beams are bolometrically adjusted for each point of the action spectrum. 

 These Operations can be carried out only in a well equipped radiation laboratory. 



3. Quantum Requirement of Photosynthesis in Chlorella* 



When the quantum requirement of photosynthesis was newly determined in 1960, 

 the progress in cell cultivation, radiation and absorption measurement, and mano- 

 metry were taken into consideration as well as the necessity of adding blue-green 

 light of low intensities. Experiments can now last as long as desired and produce 

 manometric changes (almost exclusively in the region of positive pressure) of any 

 desired magnitude. In addition, one has the choice of the two-vessel method — pro- 

 vided the equipment necessary for Splitting the light beams is available — or a very 

 simple one-vessel method, in which the carbon dioxide pressure is kept constant 

 by carbonate mixtures separated from the cells. 



The following are examples of determinations we made by means of the new 

 one-vessel method, when the incident light was red and the respiration was com- 

 pensated for with blue-green light (see page 582) : 



C0 2 Pressure [ 1 Quanta 



(mm Brodie) cp O2 



8 21 



31 6.9 



75 3.6 



140 2.83 



202 2.75 



These figures show how remarkably the quantum requirement changes with the 

 carbon dioxide pressure and that the limiting value of the quantum requirement is 

 reached only when the carbon dioxide pressure exceeds 200 mm Brodie. The limiting 

 value is 



1 



- = 2.75 

 <P 



Quanta 



Oj 



* Addition in September 1961. In the meantime it has been confirmed that the 

 quantum requirement of leaves is much greater, namely 15 to 20 per molecule of 

 oxygen (page 542). If the mechanism of photosynthesis in leaves is the same as in 

 Chlorella, it follows that in leaves a much greater fraction of the produced oxygen 

 reacts back. 



