PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



229 



1.5 hrs dark various light 12hrsdark 



6- - 



O 4 



2- - 



. o 1 — <> 



5 



X 



-f— I — I — I — (- 



HOURS OF LIGHT 



12 



Figure 9. — Flowering response of cocklebur as a function 

 of length of the intervening light period in an experi- 

 ment symbolized by the bars above the figure H5). 

 Floral stages as in figure 7. 



flowering in response to increasing day length — a 

 long-day response. Thus, cocklebur is a short-day 

 plant when days are constant and longer than 5 

 or G hours (a shorter time than originally used 

 by Hamner and Bonner) but a long-day plant 

 when nights are constant and long enough, and 

 days are around 5 or 6 hours. Using this experi- 

 ment, it was possible to investigate the light re- 

 quirements during the light period. Red light 

 strongly promotes flowering under these condi- 

 tions, and far-red inhibits — responses exactly op- 

 posite to those of the night-break phenomenon. 

 Thus, the plants are alternating in their sensi- 

 tivity to light, red promoting at one time and in- 

 hibiting at another, and far-red acting in an op- 

 posite way. This is another feature not unlike 

 those observable in circadian rhythms. 



In further studies, we were able to investigate 

 resetting of the photoperiodism clock. As figure 

 10 indicates, a light interruption given 2 or 4 

 hours after the beginning of a long inductive 

 dark period does not influence the time of maxi- 

 mum sensitivity to a second light break. When 

 the first interruption is given at 6 hours, how- 

 ever, the time of maximum sensitivity to the sec- 

 ond interruption is qualitatively changed — shift- 

 ed about 10 hours. This is also known from 

 studies on circadian rhythms. Thus, the photo- 

 periodism clock exhibits many of the same fea- 

 tures as the clock controlling circadian rhythms. 



With this idea in mind. Alice Denney and I 

 have, investigated circadian rhythms in cockle- 

 burs, comparing their characteristics with those 

 of the photoperiodism clock (17. 1^7). We have 

 had to conclude that the two clocks, while having 

 similar characteristics, differ extensively in detail 

 so that they cannot both be direct manifestations 

 of a single clock. Significant differences are as 

 follows: (A) There is no evidence in the circa- 

 dian rhythms for the interesting 6-hour shift 

 mentioned above. (R) The flowering clock can 

 be suspended. When plants are placed in the 

 dark, they flower about equally well in response 

 to equal dark periods, regardless of the length of 

 the preceding light period, providing it is longer 

 than :"> or fi hours. Xo such phenomenon could be 

 observed with the leaf movements, which are not 

 suspended during long light periods. (C) We 

 had thought that the extent of leaf movements 

 might be an indication of sensitivity to floral in- 

 duction, extensive leaf movements during the in- 

 ductive dark period implying high sensitivity 

 (high subsequent levels of flowering). When this 

 was investigated over a wide range of day and 

 night lengths, however, no relationship could be 

 observed. (D) Most recently we have found that 

 light intensities well above those required to in- 

 hibit flowering do not upset the circadian leaf 

 movements. (E) Tn all of our experiments, we 



70 f\2 hours 



6.0^' 



control,.— n. a 



2 4 6 8 1012141618 20 22 



HOURS AFTER BEGINNING OF DARK PERIOD 



Figure 10. — Flowering response of cocklebur to a second 

 interruption of the inductive dark period, following a 

 first interruption at 2, 4, or 6 hours after onset of 

 darkness (40). Interruptions were 60 seconds; floral 

 stages as in figure 7. 



