TRANSIENT PHENOMENA IN LEAVES 401 



The curve for the rate of CO2 exchange shows quite a marked 

 dependence upon the CO2 pressure. With 4% CO2, as may be seen in 

 Fig. 1 , there is a pronounced uptake of CO2 beyond the compensation 

 level during the first minute of illumination. The assimilatory quotient 

 (expressed as rate of O2 evolution/rate of CO2 uptake) during this 

 first moment has a minimum value between V2 and Vs- During the 

 monotonous rise to the steady-state rate of photosynthesis, the 

 quotient is unity. 



Under conditions of low CO2 pressure (0.04% atm.), the initial 

 anomalous quotient is not seen. Here the CO2 curve parallels the 

 O2 curve fairly well throughout the entire induction period. An 

 accurate CO2 curve at low CO2 pressures is difficult to obtain, how- 

 ever. The method of analysis does have some inertia so that brief 

 induction phenomena will be smeared out. The CO2 curve obtained 

 under conditions of low CO2 pressure (0.04% atm.) shows a smooth, 

 monotonous rise from the rate obtaining at the level of respiration 

 to that at steady-state photosynthesis. It is believed, however, that 

 there is actually a brief shoulder at the level of compensation which is 

 not seen. The sensitivity of the apparatus required that the atmos- 

 phere be depleted by about 0.02% CO2 during steady-state photo- 

 synthesis. Thus, if the initial concentration of CO2 in the gas stream 

 was 0.04%, which already markedly limits photosynthesis, there 

 would be an increasing degree of CO2 limitation as the gas passed over 

 the leaf owing to the photosynthetic removal of CO2 from the at- 

 mosphere. The curve for the gas exchange during the induction 

 period would be further complicated since the degree of CO2 limita- 

 tion experienced by any one particular section of leaf would be in- 

 creasing as the rate of photosynthesis increased toward its steady- 

 state value. The curves obtained under these changing conditions 

 would not be expected to show very brief induction phenomena which 

 might be obtained at a low but constant CO2 pressure. When the 

 pressure of CO2 is above 0.1% atm., so that the leaf is not subject to 

 such a wide degree of COo limitation, the shoulder is clearly observed. 

 Gaffron's very rapid CO2 measurements with a recording pH meter 

 show that, with algae under conditions of low CO2 pressure, the CO2 

 exchange goes to the compensation level at the instant the light is 

 turned on and pauses there for a few seconds before rising to steady- 

 state photosynthesis. The measurements of McAlister and Myers 

 with an infrared CO2 analyzer on both algae and young wheat plants 



