CARBON DIOXIDE EXCHANGE DURING THE SHORT INDUCTION PERIOD 1335 



ation of carbon dioxide, the upward trend to its consumption. The induc- 

 tion periods last for about 2 minutes (four points) at 31° C, and 3.5 minutes 

 (seven points) at 12°. Figure 33.8 shows that, at a given temperature, the 

 duration of the induction period is practically independent of light inten- 

 sity between 214 and 1030 foot-candles {i. e., between 2000 and 10,000 lux), 

 but that induction is absent at 44 foot-candles (about 450 lux). McAhster 

 designated as "induction loss" (AP,) the amount of carbon dioxide that 

 would have been taken up but for induction. It is determined, in fig- 

 ures 33.7 and 33.8, by the intercept of the solid straight lines (which 



TIME 



Fig. 33.7. Induction in wheat (after McAlister 1937). 



correspond to steady assimilation) with the ordinates corresponding to the 

 beginning of illumination. The inset in figure 33.8 shows the induction 

 loss as a function of light intensity. Comparison of this curve with the 

 light curve of photosynthesis, P = /(/), for wheat (fig. 28.1) shows that 

 APi is approximately proportional to P, except for the lowest light intensi- 

 ties (where APi disappears more rapidly than P). At the higher light 

 intensities, the induction loss at room temperature is equal to the photo- 

 synthetic production in approximately one minute of steady illumination. 

 In a second investigation (1939), McAlister compared the induction 

 losses with the amount of chlorophyll in the plant, and also investigated 

 the relation between this loss and the length of the preparatory dark period. 

 Figure 33.9 shows the induction loss in multiples of the chlorophyll con- 

 tent (both in moles), plotted against length of the dark period. The in- 



