CARBON DIOXIDE EXCHANGE DURING THE SHORT INDUCTION PERIOD 1337 



per molecule of chlorophyll. The duration of the induction period remains, 

 however, of the order of magnitude of a few minutes, even after several 

 hours of incubation. The analogy between these observations and the 

 findings of Steemann-Nielsen on oxygen liberation (fig. 33.6), was pointed 



out before. 



The induction loss is, according to McAlister, larger in the carbon di- 

 oxide-limited state than in the light-limited state, even when the final rate 

 is the same in both cases. For example, the same steady rate of photo- 

 synthesis prevails at 0.03% CO2 and 3000 foot-candles, and at 0.3% CO2 



5 10 15 20 



DARK REST, min 



Fig. 33.9. Induction loss in wheat vs. dark rest (after McAlister 

 1937). Broken line indicates "saturation" of the (short-) induction- 

 preparing process. Inset shows continued growth and final satu- 

 ration of a second, slow induction-enhancing reaction. 



and 1000 foot-candles; but the induction loss is much smaller in the second 

 case. If the plant is preilluminated in a carbon dioxide-free atmosphere, 

 and carbon dioxide is admitted afterward, its uptake begins without de- 

 lay. (Related observations of Steemann-Nielsen on oxygen liberation 

 were mentioned in section 2.) 



Another interesting observation of McAlister — also confirmed by Stee- 

 mann-Nielsen's observations on oxygen liberation — is that, if the light 

 intensity is raised in steps, each increase is followed by a new induction 

 period. The sum of all carbon dioxide induction losses is approximately 

 equal to the loss that would be incurred in a direct passage from darkness 

 to full light. 



McAlister and Myers (1940) recorded changes in the intensity of chloro- 

 phyll fluorescence simultaneously with the changes in the rate of carbon di- 

 oxide consumption. The results will be discussed in detail in section B; 

 some typical induction curves are shown in figures 33.21, 33.22 and 33.26. 

 The upper curve in each of these figures refers to fluorescence, the lower 



