402 W. L. BUTLEU 



at low CO2 pressures show a shoulder at the compensation level. 

 Thus we may safely conclude that at low CO2 pressm-e after a dark 

 period of a few minutes the CO2 exchange pauses momentarily at 

 compensation. 



I minutes I 



minutes 



Fig. 2. Rate of O2 exchange vs. time for a hjdrangea leaf at 0°C. Curve A: 

 0.04% CO2 and 20% O2 in He. Curve B: 4% CO2. Photosynthesis is running at its 

 steady-state rate when the light is turned off. 



In trying to separate the initial transient reaction from steady- 

 state photosynthesis we found an interesting inhibition due to high 

 CO2 pressures. It is seen clearly when the temperature is lowered to 

 about 0°C. In lowering the temperature from about 25°C. to 0°C. 



A 



I / I I I I I L 



f 



B 

 _j 



C 



I / l_ I I I I I 1 I i L 



z: 



Fig. 3. Rate of O2 exchange vs. time for a hydrangea leaf at 0°C. in an atmos- 

 phere of 4% CO2 and 20% O2 in He. Curve A: dark 10 minutes. Curve B: dark 1 

 minute. Curve C: dark 40 minutes. Curve D: dark 10 minutes. 



the steady-state rate of photosynthesis in 0.04% CO2 is reduced to 

 V2 to Vs while the rate in 4% CO2 may be reduced to Vso of its rate 

 at room temperature. In Fig. 2, the temperature was reduced while 

 steady-state photcsynthesis was proceeding in order to eliminate the 

 possibility of exceedingly long induction effects. It is seen that the 

 steady-state rate of photosynthesis in 4% CO2 is about one-tenth that 

 in 0.04% at 0°C. 



