1388 



INDUCTION PHENOMENA 



CHAP. 33 



light, but also upon sudden increase in light intensity, unless the weaker 

 light was sufficient to saturate photosynthesis. For example, according 

 to Franck, French and Puck, a burst occurs, in Hydrangea, upon an increase 

 in light intensity from 0.5 to 5 X 10'' erg/cm.^ sec, but not upon an increase 

 from 7.4 X 10* to 12.2 X 10* erg/cm.^ sec. In wheat, on the other hand, 

 a burst was observed (cf. fig. 33.29) even after an increase from 25 to 50 

 X 10* erg/cm. 2 sec. — probably because the photosynthesis of young wheat 

 plants is light-saturated only in very intense light {cf. Table 28.1). 



The successive bursts of fluorescence parallel the successive induction 

 losses of carbon dioxide (observed by McAlister; cf. Section A3) and of 



.1 I . 



.i I- 



r 



.L__l i. 



l_-i-_J 



J U. 



Fig. 33.29- Induction in wheat (after McAlister and Myers 1940). 

 Light intensities 100, 49, 100, 21, 100, 74, (100 ^ 50 X 10* 

 erg/cm." sec). Ordinary air. Upper curves, fluorescence; lower 

 curves, CO2 uptake. 



oxygen (described by Steemann-Nielsen, Sect. A2). This points to catalyst 

 deficiency as a source of induction, and to an auiocatalylic activation as the 

 mechanism of its termination. Apparently, photosynthesis produces (or 

 activates) at least one of its own catalysts (which is continuously deac- 

 tivated by a dark reaction), so that the photostationary quantity of this 

 catalyst is determined by the prevailing light intensity. When the latter 

 is increased, more catalyst must be activated to take care of the increased 

 supply of primary photochemical products. During this activation, the 

 fluorescent capacity of the photosensitive complex increases temporarily 

 (according to Gaffron and Franck, because the catalyst deficiency causes 

 the accumulation of oxidation intermediates that act as direct or indirect 

 inhibitors of photosynthesis, and promotors of fluorescence). Thus, the 

 same condition that occurs in the transition from darkness to light is re- 

 peated each time an increase in light intensity leads to an increase in the 

 steady rate of photosynthesis. 



