146 



PHOTOCONTROL OF GROWTH 



of these quantities suggests that the decreased oxidase activity is a con- 

 sequence of the increased inhibitor content. 



Experiments were next conducted to see whether inhibitor levels of 

 the youngest expanded leaflets would vary with the light regime to 

 which the plant is exposed. Groups of 2-week-old seedlings of Alaska 

 peas which had been raised under continuous light at 17°C were 

 placed, for varying periods of time, in a dark room maintained at 

 23 °C. At the completion of the dark period, 500 mg of leaflets was 

 harvested from each group, ground into a brei in the dark room, the 

 brei was then boiled for 10 min in a water bath, and the inhibitor con- 

 tent was assayed as before. The data, summarized in Fig. 2, reveal a 



20 



12 3 4 5 



HOURS IN DARKNESS 



Fig. 2. The eflfect of transfer to complete darkness of pea plants previ- 

 ously grown in continuous light on the lAA oxidase inhibitor content of 

 the youngest expanded leaflets. 



sharp increase in inhibitor level in the first hour after transfer of the 

 plant to darkness, followed by a sharp fall and gentle rise. The exact 

 nature of the pattern depicted varied somewhat from experiment to 

 experiment, i.e., the peak of inhibitor level varied from 0.5-2.0 hr, and 

 the height of the peak showed considerable variation. Nonetheless, the 

 basic pattern was the same in all six experiments done. 



