330 PHOTOSYNTHESIS 



the best yield per unit of light, i.e. for the complete removal of the product 

 resulting during each light flash, the same investigators were able to estimate 

 the duration of each reaction. The daric reaction v\7as found to proceed in 

 less than 0.04 sec. at 25° C, and to be greatly influenced by temperature. 

 The light reaction, on the other hand, takes place with great speed, requir- 

 ing only about o.ooooi sec. for its completion, and is unaffected by tempera- 

 ture. The light reaction is affected by the carbon dioxide concentration while 

 the dark reaction is not, indicating that the carbon dioxide enters into the 

 photosynthetic reaction either before or coincident with the photochemical re- 

 action, probably the former. The experimental results of Craig and Trelease 

 (1937) and Pratt and Trelease (1938) indicate that water is involved in at 

 least one of the dark reactions of photosynthesis. 



These results indicate that the rate of photosynthesis is limited by the 

 stage of the process which occurs at the slowest rate. At low light intensities 

 and adequate carbon dioxide supply the photochemical reaction is limiting, 

 and temperature will have little effect on the rate of the process. At high 

 light intensities and adequate carbon dioxide supply but low temperatures the 

 rate of photosynthesis is limited by a dark reaction, and will increase con- 

 siderably with a rise in temperature. 



The temperature coefl!icient of photosynthesis decreases rapidly at tem- 

 peratures above approximately 35° C. This is in accordance with the behavior 

 of enzymatic reactions generally and suggests that enzymes are involved in 

 the process. 



The results of experiments by Molisch (1925) upon the continued release 

 of oxygen by desiccated leaves are also considered to support the concept 

 that enzymes are involved in photosynthesis. Leaves were killed by drying 

 for several days at a temperature of 30-35° C, powdered, ground in water 

 and suspended in a culture of luminescent bacteria. Such bacteria glow 

 only in the presence of oxygen. As soon as the oxygen in such a suspension 

 has been exhausted the luminescence of the bacteria disappears. If the mixture 

 is then illuminated for a brief period the culture begins to glow showing that 

 oxygen is being liberated. Very small concentrations of oxygen are sufficient 

 to produce luminescence so that these bacteria provide a very delicate test for 

 oxygen, probably the most sensitive known. Leaves killed by freezing behaved 

 similarly to the dried leaves but leaves killed by rapid heating or by ether 

 failed to release oxygen. These results seem to indicate that one phase of 

 the photosynthetic process can still continue after such drastic treatments. 

 Furthermore, since after treatments which destroy enzymes (heating, etc.) 

 the leaves lose this capacity, it is believed that this phase of the photosynthetic 

 process is enzymatic in nature. Obviously these experiments do not prove that 



