PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



227 



minutes (critical night), flowering occurs even 

 when day-length varies considerably (c,ycle not 

 necessarily equal to 24 hours). This indicated the 

 importance of the dark period. They also inter- 

 rupted the day with an interval of darkness and 

 the night with an interval of light. Day interrup- 

 tion was without effect, but night interruption in- 

 hibited floAvering. This discovery of the night- 

 break phenomenon opened up numerous possibil- 

 ities for research. What intensities and durations 

 were required? When was the night break most 

 effective? What qualities of light worked best? 



The time of interruption is an important fac- 

 tor. Plants reach a maximum sensitivity to the 

 light break at some set time after the beginning 

 of the dark period, more or less independently of 

 the dark period's total duration (fig. 6). It is im- 

 portant to note that night interruptions inhibit 

 flowering in short-day plants but promote flower- 

 ing of long-day plants. 



Reciprocity holds, as an approximation, over a 

 relatively narrow range of intensities and dura- 

 tions. Since the time of interruption is so impor- 

 tant, reciprocity would not be expected to hold 

 with long durations. Tf light is given at low in- 

 tensity during the entire dark period, intensity 

 of moonlight is often nearly sufficient to inhibit 

 flowering of some short-day plants. When light is 

 given at extremely high intensities (for example, 

 from an electronic photoflash). durations of a 

 fraction of a second are effective. 



The most productive studies have involved 

 light quality. Red light is most effective in the 

 night-break phenomenon, with an action spec- 

 trum similar to those of other phenomena we 

 have been discussing. Immediately after Borth- 

 wick and others (0) discovered the reversible na- 

 ture of the light reaction in germination, they 

 investigated the light-break inhibition of flower- 

 ing in cocklebur, finding again that far-red re- 

 versed the effects of red and, thereby, implicated 

 the phytochrome system (5). The key question in 

 photoperiodism is how time is measured. Hamner 

 divided the flowering response into a series of 

 component or partial processes that have since 

 been further elaborated. First, the dark period, 

 to be effective, must be preceded by light. Second, 

 the reactions of the dark period first involve a 

 pigment shift from predominant Pi- to Pfr. This 

 apparently occurs in a brief interval of time, and 



the critical night must therefore be accounted for 

 by some subsequent time-measuring mechanism. 

 Following the critical night, flowering hormone 

 (florigen) synthesis is initiated in the leaf. Third, 

 florigen is translocated to the bud, where it causes 

 a redirection of growth from the vegetative to 

 the reproductive form. 



It was suggested that time measurement is a 

 matter of the time required for phytochrome pig- 

 ment shift {26). There are problems with this 

 hourglass concept of timing. We have mentioned 

 that pigment shift is apparently complete in a 

 much shorter time than the critical night. Fur- 

 thermore, time measurement is relatively tem- 



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 ir 



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Long-day plant 

 Hyoscyamus niger 



4 8 



TIME DURING A CYCLE (hrs) 

 (TIME OF INTERRUPTION) 



Figure 6. — Flowering responses of a short-day (-{6) and 

 a long-day (15) plant to dark periods of various 

 lengths (indicated by bars above the abeissas) that 

 were interrupted at various times by light breaks (60 

 sec. for Xanthium, 1 or 2 hr. for Hyoscyamus — indi- 

 cated by the length of the horizontal lines). Xanthium 

 is maximally inhibited about 8 hours after beginning 

 of the dark period, regardless of its length, and 

 Hyoscyamus is maximally promoted by a light inter- 

 ruption near the middle of the dark period. 



