Photoperiodism and Circadian Rhythms KARL C. HAMNER University of California, Los Angeles, California Many of you were present at the conference on aspect... In retrospect, the necessity of accepting the photoperiodism held in Gatlinburg, Tennessee, in theory of an endogenous rhythm to explain photo- October 1957, the title of which was "Photoperiodism periodic responses with regard to flowering does not and Related Phenomena in Plants and Animals" [1] seem to be very great yet!" I believe my own work and At the conference I presented a paper based upon work the work of my students in addition to Btinning's which led me to believe that photoperiodism in plants work provides fairly conclusive evidence that endoge- was affected by an endogenous rhythm. Prior to the nous rhythms participate in the photoperiodic reaction, conference I had read some of the literature on endoge- and that something very close to Bfinning's hypothesis nous rhythms, but at the conference I was astounded may be the case. by the papers presented which indicated that scientists working with a wide variety of responses had obtained RHYTHMIC RESPONSES TO CYCLE LENGTH a tremendous fund of information on rhythms unrelated Twenty years ago Snyder [4], working with me, to photoperiodism. Because of the fact that the data obtained results which indicated that the flowering which I presented was partly that of my students, and response of Biloxi soybean (Glycine max) was rhythmic, had been previously unpublished, and because I depending upon the amount of interrupted darkness recognized that I could not discuss the results critically interspersed between the high intensity light period in relation to all of the information available, I re- and the continuous dark period. He obtained maximum quested the withdrawal of publication of my paper. flowering on a 24-hour cycle and on a 48-hour cycle of After three additional years of work I feel that exten- treatment. At the time the results seemed rather sive new information is necessary to understand the confusing to us, and we agreed that they should be complex mechanism of biological clocks in relation to published without comment. Needless to say, Biinning photoperiodism in plants. However, I feel that I can noticed these data in Snyder's paper, and they were present conclusive data indicating that the biological taken by him to indicate strong support for his theory clock is participating in the photoperiodic response. of endogenous rhythms. We did not return to this Btinning [2] first proposed that endogenous rhythms problem until about 1953. At that time two of my participated in the photoperiodic reaction. He de- students, first Blaney [5] and later Nanda [6, 7, 8, 9], veloped his hypothesis as a result of his experiments on carried out rather extensive investigations of the leaf movement, and postulated that the photoperiodic photoperiodic responses of Biloxi soybean in relation responses of plants involved the same endogenous to the length of the light period, the length of the dark rhythm (or biological clock) that caused leaf move- period, and the length of the cycle of treatment. ments. In both long-day plants and short-day plants he Biloxi soybean is a short-day plant. It may be grown assumed that the flowering response depended upon the in the vegetative condition under long days in the green- time at which the plants were exposed to light in re- house. If transferred to short day, initiation of flowers lation to the oscillation of the rhythm. He spoke of a takes place after three short days of treatment, and the photophil phase arid a scotophil phase of the rhythm, number of nodes which produce flowers is directly and postulated that short-day plants and long-day proportional to the number of short days received up plants flowered only if exposed to light during the to about 10. It has been our practice, therefore, to give proper phase of the rhythm. Btinning's hypothesis was the plants 7 cycles of treatment, using as a control a not accepted and, in fact, was actively opposed by many standard treatment of 7 cycles of short day, each short scientists for many years. At the Gatlinburg conference day consisting of S hours of high intensity light, fol- in 1957, and later at the International Botanical lowed by 16 hours of continuous darkness. The controls Congress at Montreal in 1959, several people disputed usually produce between 40 and 50 flowering nodes per very vigorously with Bihming and myself. As recently ten plants, and any variation from the standard short as 1959 in a review, Doorenbos and Wellenseik [3] day usually produces a reduction in the number of state: "Although one of the merits of Biinning's flowering nodes, thus providing a basis for comparison theory is that it suggests an explanation of the difference of the effectiveness of the different treatments. A between SDP (short-day plants) and LDP (tong-day typical cxperiment is shown in Fig. 1. We have per- plants), there is little experimental evidence on this formed many such experiments, and the results of a 269 270 HAMNER ~40 IIX) 0 Zo 80 r~ o ~ ~ "' 2O LIJ r.~ 3:60 o 2: B . I0 4O u- I-- 0 0 I.-- I I I I I t,l 0 6 12 18 24 30 36 4~ w 20 48 54 60 66 7'2 I1: i 0 12 24 36 48 60 72 CYCLE DURATION - HOURS o (~ NO. OF POINTS ON ONE POSITION 124 6 FIGURE 3. Summary of responses shown in Fig. 2. Each LIGHT DARK curve of Fig. 2 was adjusted so that all curves coincided CYCLE DURATION - HOURS at the 24-hour cycle. This figure is used as standard FIGURE 1. Typical experiment on flowering response of control curve in the discussion. Biloxi soybean in relation to cycle length. Each treat- ment consisted of seven cycles, each cycle containing an o---o JAPANESE MORNING GLORY 8 hr. high-intensity light period. The length of the dark a.---A SOYBEAN vor. PEKING periods in the various treatments varied to give cycles (9 of differing length. Ten plants were used in each treat- _z 20 I1: ment. w _1 r o 50 ~w z Q 0 z J ~ o _J I,- / " 30 i 0 21 24 30 36 42 48 54 60 66 72 if) IiJ CYCLE DURATION - HOURS ~ 20 o z //..\ %..~~ FIGURE 4. Flowering responses of Japanese morning ._1 I0 ~/~,,~'- -- glory (Pharbitus nil) and Peking soybean (Glycine max. p. 0 var. Peking) with cycles of various length. Treatments I,- are similar to those of Fig. 1 except that Pharbitus O 16 24 32 40 48 56 64 72 received only 2 cycles of treatment. CYCLE DURATION- HOURS FIGURE 2. Six response curves of Biloxi soybean (control curves in various experiments cited in text). Treatments plants, Japanese morning glory (Pharbitus nil) and similar to those shown in Fig. 1. Peking soybean (Glycine max var. Peking). It seems clear that the pattern of response in both plants is number of these are shown in Fig. 2. It is obvious that very similar to that of Biloxi soybean. We have done a the flowering response follows a rhythmic pattern, great deal of work with Xanthium, but our results with depending upon the length of the cycle used. These this plant have not been quite so satisfactory. This experiments have been performed over a period of 4 or particular plant flowers as a result of exposure to a 5 years, and of course the curves are not exactly single short day. In fact, if the plant is taken from superimposed. However, Fig. 3 shows the summary of long-day conditions in the greenhouse and exposed to a all of the results with the data adjusted so that the single long dark period and returned to long day, it magnitude of the flowering peak on the 24-hour cycle flowers. The curves in Fig. 5 represent results of Raven (i.e., with an 8-hour light period and a 16-hour dark [11], and we have repeatedly obtained similar results. period) is the same for all of the experiments. It may be We have additional evidence, Finn [12], too volumi' seen from this curve that the results from all of our nous and indirect for review here, which also convinces experiments are indeed remarkably uniform with us that an endogenous rhythm is participating in the respect to the rhythmic pattern in relation to cycle photoperiodic responses of Xanthium. lengths. We also have evidence that the photoperiodic While these data indicate the participation of an responses of long-day plants involve an endogenous endogenous rhythm in the photoperiodic response of rhythm [12, 13]. If we expose long-day plants to the Biloxi soybean, the question arose concerning the same sort of treatment that we used for the short-day responses of other photoperiodically sensitive plants. plants, namely, expose them to 8 hours of light in each We do have evidence that a similar situation cycle and vary the length of the cycle by a varying prevails in other short-day plants. In Fig. 4 are pre- amount of darkness in each cycle, then we obtain the sented the unpublished data [10] for two other short-day results shown in Fig. 6. In this case, in contrast to the PHOTOPERIODISM AND CIRCADIAN RHYTHMS 271 O .o a o HYOSDYAMUS NISER ~.Zlo_ 5 ~o'" "" .... "" w r A----/I SILENE ARMERIA cn /I t,~ w it V,' ,~ ~4o 0 a ~ i 1 == ==so II 24 36 48 12 24 38 48 60 72 DARK PERIOD " HOURS CYCLE DURATION HOURS FIGURE 5.