METABOLIC ASPECTS OF PHOTOPERIODISM 293 



Similar results have been obtained with several dozen plants similarly 

 treated. 



The production of COl> in the light is due to both heat-labile and 

 light-labile components because evolution of CO- can be obtained in 

 the dark by raising the temperature from 20°C to 30°C. After the CO2 

 evolution has ceased at 30°C, an additional burst of CO2 can be ob- 

 tained by turning on the lights (Spear and Thimann, 1954). 



EFFECT OF INTERRUPTING LONG DARK PERIOD 



It is well known that interrupting the long dark period by a short 

 period of illumination completely annuls its effectiveness (Hamner and 

 Bonner, 1938), especially when the interruption is given near the mid- 

 point of the dark period (Harder and Bode, 1943). Short-day plants 

 fail to flower under these conditions. It was therefore of considerable 

 interest to ascertain whether the CO2 metabolism responded in the 

 same way. 



Plants grown in long days were exposed for prolonged periods to 

 cycles of 8 hr light and 16 hr darkness, with and without interruption 

 (with 10 min of light) halfway through the long dark period. One 

 such experiment is shown in Fig. 4. The solid line represents a plant 

 given uninterrupted long nights, and the broken line a plant given a 

 10-min interruption by 1500 ft-c at midnight. It is evident that the un- 

 interrupted plant developed the characteristic CO2 metabolism of 

 short-day plants, just described, while the companion plant that re- 

 ceived 35 interrupted long nights failed to develop the ability to fix 

 large amounts of CO2 in the dark. The corresponding burst of CO2 in 

 the light is largely absent. This experiment has been repeated five times 

 with similar results. 



EFFECT OF PROLONGED DARK AND PROLONGED 



LIGHT PERIODS 



It is clear from Figs. 2 and 4 that the CO2 fixation in darkness de- 

 creases rapidly after the first 12 hr and has usually ceased before the 

 end of the 16 hr dark period. A study was therefore made of the CO2 



