KINETIC ANALYSIS OF PHOTOPERIODISM 



465 



z 

 ir 



o 



20 



10 



CONTINUOUS 

 450 ft-c-min 



£■450 ft-c -min 



DURATION OF FLASH, 10 ((C 

 NIGHT PERIOD, 900mln(IShr) 



50 



100 



150 



200 



TIME BETWEEN FLASHES (min) 

 Fig. 12. Time-intensity function for a photoperiodic system, flowering 

 in the long-day plant Nobel spinach. The total energy had a constant value 

 of 450 ft-c-min applied as a series of flashes of 10-sec duration for a 15-hr 

 night (9-hr photoperiod). The number of flashes per night was varied 

 from 5 to 270 and these were compared with continuous^ irradiation of 

 225 and 450 ft-c-min. Data taken from Withrow and Withrow (1944), 



agents which appear to be capable of resetting the clock are those 

 which can affect the nucleus, such as colchicine, urethane, BAL, more 

 recently kinetin, and possibly gibberellin. In other words, the clock is 

 much more refractory to chemical agents than the growth response 



TIME 



Fig. 13. Time-phase requirement for a photoperiodic system. Flowering 

 response of the short-day plant Xanthium to the time of application of a 

 single flash during a 16-hr night following an 8-hr photoperiod. Data of 

 Salisbury and Bonner (1956). A single flash was applied at various stages 

 of the dark period. The response was measured in terms of the floral stage 

 at the end of the experiment. 



