METABOLIC ASPECTS OF PHOTOPERIODISM 



295 



relations in prolonged darkness, and Fig. 5 shows the results with a 

 plant pretreated with 34 short-day cycles. During the first 12 hr of this 

 dark period, the characteristic increase in dark CO2 fixation occurs, 

 but after 4:00 a.m. the rate of fixation decreases, giving way to CO2 

 evolution after 8:00 a.m. For the remainder of this 48-hr dark period, 



COg AS % OF THAT IN 

 AIR ENTERING CHAMBER 



u 



O 

 O 



If 

 < 



1 1 1— 



TIME OF DAY (HRS) 



LIGHTS ON 



Fig. 5. The COo metabolism of Kalancho'e during a prolonged dark 

 period of 48 hr and 4 additional hr in the light. 



there is a continuous production of CO2 with no endogenous rhythm 

 apparent. When the lights are turned on after the prolonged dark 

 period, there is a burst of CO2 production similar to that observed 

 with 16-hr dark periods. 



The effect of a prolonged light period on a plant pretreated with 34 

 short-day cycles is shown in Fig. 6. After the usual burst of CO2 pro- 

 duction is over (4 to 6 hr after the light comes on), the uptake of CO2 

 rapidly increases and then remains relatively constant. Again there is 

 no sign of an endogenous rhythm. 



