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The Circadian Rhythm of Photoperiodic Responsiveness in Kalancho

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The Circadian Rhythm of Photoperiodic Responsiveness in Kalancho The Circadian Rhythm of Photoperiodic Responsiveness in Kalancho ROBERT C. Bt~NSOW Pflanzenphysiologisches Institut der Uaiversitdt, GSttingen, Germany In Kalancho~ we know about several rhythmic and the maximum. In light periods of more than 9 hours phenomena which are more or less endogenous. Among flowering is gradually reduced by an inhibiting action them are diurnal changes of osmotic values [1], water of light. Between 12 and 12.5 hours of light, the so- uptake [1, 2], fresh weight [3], turgor pressure [1], gut- called critical day length, there is a compensation of tation [4], C02 metabolism [5, 6, 7], acidity [3], phos- promotive and inhibitive actions. In all light periods phatase activity [8], substances active in root growth longer than this critical value, the light inhibition is [3], heat resistance [9, 10], and responsiveness to predominant. In all of the cycles with light periods temperature [11] and light [12 to 19]. All of them have longer than 9 hours the oscillations of the endodiurnal an equal diurnal frequency but not in every case equal system are orientated in such a manner that light phase relations. The opening and closing movements of falls also between the inflection point with negative the flowers, for instance, are mainly due to changes in slope, the minimum, and the inflection point with the turgor pressure of the petals. Therefore, there is a positive slope. Therefore it seems likely that in this simultaneous oscillation of them coinciding with alter- part of the rhythm the endodiurnal system is in the nations of the osmotic values and the fresh weight of the scotophile state according to Btinning. Let us therefore flowers. On the other hand, the total acidity of the assume that the oscillations, both of the sepal move- petals and the water uptake of the flowers oscillate ments and of the photoperiodic responsiveness, are synchronously with the velocity of the flower move- nearly synchronous (see Fig. 1). ments. And the heat resistance of the leaves changes How can we prove this hypothesis? This was done just in the opposite manner compared with the opening by the well known method of light breaks. In diurnal degree of the flowers. The hypothetical basis of all of cycles we know about a regular alternation of one the circadian processes is called the endodiurnal photophile and one scotophile phase in good agreement system. with our hypothesis (Fig. 3). There remains the question, (a) what can we learn In bidiurnal cycles where the light period and the from the circadian phenomena about the endodiurnal dark period last together 48 hours, the application of system, and (b) can the knowledge of the endodiurna[ light breaks led to the discovery of two photophile system help us to explain the photoperiodic behavior? phases in alternation with two scotophile phases (Fig. Let us first compare the best known rhythmic process, 4). According to the movements in Fig. 2 the phases of the flower movements [11 to 13, 17, 20], with the photo- photoperiodic light sensitivity also are shifted to the periodism of flower initiation [6, 11 to 19, 21, 22]. To right with increasing length of the light period. The simplify the matter we will disregard alternations of second scotophile phase is hardly weaker than the first the amplitude and reduce the sinusoidal movement one. This seems to be in contradiction to the move- curve to its four cardinal points, viz., maximum ~, ments (Fig. 5), where the second minimum generally inflection point with negative slope N, minimum $, has a higher value than the first one. The explanation and inflection point with positive slope ~ (Fig. 1). of the photoperiodic flowering behavior lies in another In diurnal cycles the flowers are completely opened well-known peculiarity of the endodiurnal system. always near the middle of the light period (Fig. 2). The Every interruption of the dark period alters the mode flower initiation in KalanchoO is possible already with of oscillation. This alternation is dependent among very short light periods. Therefore we may assume, that other things on the time of the light break. The inter- in the state of the endodiurnal system which corre- ruptions preceding the main light period shift the sponds to the mammal opening of the flowers, light phase of the rhythmicity to the left side (Fig. 6). The especially promotes the flower initiation. That means plant enters its photophile state sooner than without that the endodiurnal system is in its photophile phase, the light break. Accordingly the next scotophile phase in the sense of Biinning [23]. also appears earlier. And therefore a longer part of the With light periods of increasing duration, the flower main light period acts within the scotophile state of initiation is promoted until the optimal day length of the plant. The same happens in bidiurnal cycles (Fig. 7). about 9 hours of light is reached. In all of these cycles Strictly speaking, the inhibition of flowering in such the light always falls between the two inflection points and similar cases is in the first place due to the last 257 258 BUNSOW IO i i ~ ~ i i , , a i , ; i i ~:~I I ~L%~I I I ~I ~. 20 | LIC4.1T 3" 9'tn~l " I 5O "l :1 HOURS ~ ~o / u. Ioo /',,/V/ o 0 3 6 9 I2 24 36 ,;8 PHOTO- SCOTO- PHOTO- SCOTOPHILE PHASE HOURS AFTER THE ONSET OF THE MAIN LIOHT PERIOD FIGURE 1. Comparison between petal movements and photoperiodic responsiveness in Kalancho~. Dark pe- FIGURE 4. Effect of time of light break on flower initia- riods shaded. Partly after Biinsow [12]. tion caused by bidiurnal cycles with main light periods of 3, 9, 11.5, and 14 hours. 100 days (base line) set for NUMBER vegetative plants. After Bfinsow [13]. LIGHT : DARK OF FLOWERS HOURS 0 14 I60 23I 3O2 ~o:~ 285 7g:~ 29 0 16":8 0 zg: g_. 0 HOURS AFTER THE BEGINNING OF THE LIGHT PERIOD ~:~ 0 FIGURE 5. Petal movements in bidiurnal cycles with 6 0 hours of light. After Biinsow [12]. 0 O 8 7Z 7g ZOZq HOURS NUMBER FIGURE 2. Action of diurnal cycles on the petal move- OF FLOWERS ment rhythmicity and on flowering in Kalancho~. Partly ~,.'x'x',r . t "i ..,.J I61 after Biinsow [12]. I',Lk 60 400 , ~.k\N.v I A o ,, i'+-I ~ ~ ZO 0 q g /Z i ' 1 CYCLES / HOURS AFTER riiE ONSET % 2oo OF THE NAIN LIOI.IT FIGURE 6. Action of light breaks in diurnal cycles with main light periods of 12 hours on the movement rhyth- micity and on the flowering caused by cycles with 11 hours of light. After Biinsow [12]. Flowering data after Harder and Bode [15]. 3 5 7 9 II 13 TIME OF INTERRUPTION IN HOURS In tridiurnal cycles with a duration of 72 hours AFTER THE ONSET OF THE DARK PERIOD there are consequently three photophile and three FIGURE 3. Effect of time of light break on flower induc- scotophile phases which alternate about every half tion caused by diurnal cycles of 9 hours' light and 15 day (Fig. 8). Now the second scotophile phase is hours' darkness. After Harder and Bode [15]. distinctly weaker than the first one. As in the flower movements, the relative long dark times diminish the part of the main light period and not to the light amplitude already to such an extent that the experi- break. This seems to be the main reason why in bi- menters had to make use of statistical methods. On diurnal cycles the second scotophile phase is nearly as the contrary the third seotophile phase is about as strong as the first one. strong as the first one, for the same reason "is the PHOTOPERIODISM IN KALANCHOE 259 DAYS TO THE APPEARANCE OF Therefore, it seems quite likely that in tetradiurnal /NFLORESCEIVCE I:'RIMORDIA cycles, under certain conditions, all oscillations of photoperiodic responsiveness disappear except two. Both of them are caused by the same oscillation of the 33 endodiurnal system which is always renewed by the 35 main light period. The first range of floor,ring inhibition >62 is caused by the inhibitory action of the light break. oo The second range is merely duc to the already men- 0 r 8 ~ I~ 303r 323G r162162r tioned cooperation of the main light period and the FLOURS AFTER THE BEGINNINO OF THE MAIN LIGHT PERIOD preceding light break in influencing the position of the FIGURE 7. Action of light breaks in bidiurnal cycles with endodiurnal oscillation. In this case, the additional main light periods of 14 hours on the movement rhyth- flowering inhibition is merely caused by the last part micity and on fower induction in KalanchoO. After Bfin- of the main light period. sow [12]. Flowering data after Harder and Glimmer [22]. Let me summarize the whole discussion by the statement that all our photoperiodic results seem to be J#O in good agrcement with our conceptions of the endo- .A\ diurnal system in KalanchoO. ~ JN REFERENCES 1. BONSOW,R. unpublished data. I. 2. ZUR LIPPE, T. 1957. Wasseraufnahme und Bliiten- bewegung an Kalancho~-Infloreszenzen in ver- 7O0 schiedenen Licht-Dunkelwechseln. Z. Bot., 45: .... J. 43-55. i 0 4 # 12 24 36 4~ 60 3. BECKER, T. 1953. Wuchsstoff- und Si~ureschwan- TIME OF INTERRUPTION IN HOURS AFTER THE ONSET kungen bei Kalancho~ blossfeldiana in verschie- OF THE MAIN LIGHT PERIOD o denen Licht-Dunkelwechseln. Planta, 43: 1-24. 4. HEIMANN, M. 1950. Einfluss periodischer Beleuch- FIGURE 8. Action of light breaks on the flowering effect tung auf die Guttationsrhythmik. Planta, 38: caused by tridiurnal cycles. Combined after Melchers 157-195.
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