404 AV. L. BUTLER 



to speak, "pump prime" the photosyuthetic apparatus. As the process 

 of photosynthesis gets going, it will regenerate its own PGA and 

 respiratorily produced PGA can return to its respiratory pathways 

 instead of being photochemically reduced. Thus light only momen- 

 tarily prevents the further respiratory oxidation of PGA. The O2 

 consumption in the respiratory formation of PGA from sugar is com- 

 pensated by the O2 evolution in the photochemical reduction of PGA 

 back to triose. The photochemical reduction of any pool of PGA 

 present at the beginning of irradiation will cause the O2 production to 

 exceed compensation temporarily. The respiratory CO2 evolution 

 should cease and no CO2 uptake should occur until the process of 

 photosynthesis produces the CO2 acceptor molecule. This explains 

 the observed shoulder in the O2 and CO2 curves at the compensation 

 level. In the case of high CO2 pressures where the CO2 exchange shows 

 a marked uptake initially, there must be an additional process 

 operative. 



The data obtained at low temperature and high CO2 pressures 

 (Figs. 2 to 4) give some clues as to the processes involved. The initial 

 O2 effects may be explained in terms of respiratory intermediates. 

 Even though the rate of respiration is very low at 0°C., the con- 

 centrations of the respiratory intermediates are not necessarily small. 

 The concentration of reducible intermediates present at the beginning 

 of an illumination period wall be indicated by the size of the oxygen 

 burst. Thus, after a dark period of 40 minutes, which w^e may assume 

 is sufficient to establish a steady-state dark condition at 0°C., the 

 pool size of reducible intermediates is quite small, as is shown by the 

 small initial oxygen burst in curve C of Fig. 3. However, respiration 

 of sugar will continue to form half-oxidized intermediates at a low 

 rate. During illumination, these intermediates will be reduced to 

 phosphorylated triose. The size of the pool of newly formed triose 

 which is formed in the light will depend upon the rate of utilization of 

 phosphorylated triose, e.g., for the production of the carbon dioxide 

 acceptor of photosynthesis or for storage as food in the form of 

 hexose. After the pool of newly formed triose has been estabUshed 

 during a period of illumination, a dark period is necessary to develop 

 the reducible material which is responsible for the oxygen burst at 

 the beginning of a subsequent irradiation. These half-oxidized inter- 

 mediates, e.g., phosphogly eerie acid, are formed from this pool of 

 triose by respiration, and the pool size of these intermediates in- 



