Abstract
Introduction
Materials and methods
Experiments and results
Discussion
Author: Walter A. GENTNER, Raymond B TAYLORSON, Harry A. BORTHWICK
Pages: 23 to 32
Creation Date: 1975/01/01
AEQI, PAL, U.S. Department of Agriculture, Beltsville, Maryland 20705.
Opium poppy, Papaver somniferum L., is a long-day plant with a critical daylength for flowering of 14 to 16 hours. Flowering is induced by two or more long photoperiods or by a single period of light longer than 24 hours. Flowering stems always lengthen, but stems also sometimes lengthen in the absence of flowering, e.g. with the application of gibberellic acid.
Flowering was not controlled by brief red irradiations, far red irradiations, or both. Thus, the action of phytochrome was not shown, but its presence was not excluded. Light seems to control poppy flowering through a so-called high-energy reaction.
Poppy, Papaver somniferum L., is known ( [ 1] , [ 2] , and A. A. Piringer, personal communication) to be a long-day plant with respect to flowering and stem elongation. However, detailed information on photoperiodic responses of the plant is exceedingly limited.
In some other long-day plants, striking effects of very brief red (R) or far red (FR) irradiations have been observed, and photoreversibility of flowering through phytochrome action has been reported [ 3] . More prominent effects on flowering of long-day plants, however, often seem to depend on a so-called high-energy reaction.
The main objective of this study was to examine the light reactions of poppy to find lighting or other procedures that might indefinitely delay flowering in the field or induce it in plants too small to be of narcotic value. A further objective was to find ways to induce flowering at desired times in plants grown out of season or out of their natural geographic environment.
Plants were produced in growth rooms with weekly plantings to assure a constant supply. Plants were started in 7.6-cm pots in a growth room at the greenhouse. When they could be thinned to two plants per pot, they were moved to a growth room at the laboratory. Both growth rooms were illuminated with a mixture of fluorescent and incandescent lamps. Illumination was at an intensity of 215 to 270 hectolux (hlx) in the laboratory growth room and somewhat less in the germination room. About 10 per cent of the illumination was incandescent. The plants were kept in the growth room until the end of all experiments, except during photoperiodic treatments. These were often performed in "roomettes" [ 4] .
Deceased (May 21, 1974).
The roomettes were eight sheet-metal chambers installed in an air-conditioned room. They were lighted by incandescent and fluorescent lamps that gave a total illumination of about 108 hlx, of which slightly more than 10 per cent was incandescent. In each roomette, a circulation fan kept the temperature close to that of the air-conditioned room. Several types of electric time switches were easily interchangeable in the incandescent and fluorescent circuits of each box. This equipment automated simple through complex photoperiodic schedules.
Most experiments were performed with plants 6 to 10 weeks old. Treatments lasted from one or two days to one week. Except during the treatments, all plants were held on daily photoperiods of 8 hours at 27°C and dark periods of 16 hours at 21°C.
Most plants were dissected two weeks after treatment began. In all plants, the presence or absence of flower buds was assessed and, in some, stem length was measured.
A few experiments used red (R) and far red (FR) radiations. The R was from a bank of 18 cool white lamps filtered by two layers of red cellophane. The FR was from three 300-W internal reflector flood lamps filtered by two layers of red and two of blue cellophane and a 10-cm layer of water. Radiation at plant level was about 0.6 mW/cm 2 of 600-680 nm radiation for the R source, and about 0.75 mW/cm 2 of 700-750 nm radiation for the FR source [ 5] .
I. The rate of seedling development on 8-hour photoperiods was determined by dissecting plants of different ages and counting the leaves and leaf primordia. One series of observations was made when plants from seven successive weekly plantings were simultaneously available. These leaf counts were accurate to within about one leaf; some uncertainty existed because of death and decay of cotyledons and first-formed leaves. Plants 52 days old (table 1)had just reached the right stage for photoperiod experiments. Somewhat younger plants could possibly have been used, but plants often were slightly older than 52 days when the experiments were started. At seven weeks, these plants averaged about 40 foliar organs, giving a plastochron slightly greater than one day.
II. Another experiment confirmed earlier reports that poppy was a long-day plant with respect to flowering. We measured the flower-promoting effectiveness of various numbers of 16-hour photoperiods. Stems were lengthened and flower buds were initiated by 2 days of treatment (table 2).
III. To find photoperiodic conditions safe for production of vegetative plants, early in these experiments, we surveyed how daily photoperiods from 8 hours to continuous light affected the plants. Part of the survey used photoperiods of 16 hours and shorter; the other, 16 hours and longer.
A. The first part had eight photoperiods (8 to 16 hours daily) and used seedlings from six different seed lots. These lots were selected by the latitude of their regions of collection. Treatments started in the roomettes and continued for four weeks. Plants were dissected two, three, and four weeks after the start of treatments.
|
Age at dissection (days) |
Total number of leaves per plant |
|---|---|
| 12 | 6 |
| 17 | 11 |
| 25 | 14 |
| 32 | 16 |
| 39 | 33 |
| 46 | 35 |
| 52 | 40 |
|
No. of 16-hour days |
Occurrence of flower buds |
Mean stem length (mm) |
|---|---|---|
| 0 |
-
|
18 |
| 1 |
-
|
18 |
| 2 |
+
|
21 |
| 3 |
+
|
24 |
| 4 |
+
|
25 |
| 5 |
+
|
43 |
a + = presence of flower buds; - = their absence.
Flower buds were present on all lots on 15- and 16-hour photoperiods (table 3). A few plants of variety F flowered on 14-hour photoperiods, and variety C plants flowered on photoperiods as short as 12 hours. No plant developed flower buds with a photoperiod shorter than 12 hours.
|
Flowering response by photoperiods a (hours) |
||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
|
Variety code |
Plant introduction No. |
Geographic source |
8 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
|
A
|
353163 |
Iran
|
-
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
|
B
|
251083 |
Afghanistan
|
-
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
|
C
|
250640 |
West Pakistan
|
-
|
-
|
-
|
+
|
+
|
+
|
+
|
+
|
|
D
|
304531 |
Turkey
|
-
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
|
E
|
253137 |
Yugoslavia
|
-
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
|
F
|
223798 |
Afghanistan
|
-
|
-
|
-
|
-
|
-
|
+
|
+
|
+
|
a + = presence of flower buds; - = their absence.
On 16-hour photoperiods, flower buds formed as fast for all lots as had been previously found for plants of variety F. All plants on this photoperiod were dissected after two weeks of treatment.
Later this experiment was replicated for variety F. Results of all three replications showed the critical photoperiod for flower production of variety F, the most used one in this study, to be slightly less than 15 hours. Plants often flowered on 14-hour photoperiods but rarely on 13-hour ones.
When photoperiods from 14 to 15-3/4 hours at 1/4-hour intervals were used, from 0 to 100 per cent of the plants flowered. All plants completely flowered only on the 15-3/4-hour photoperiod.
Occasionally, all plants did not respond to 15-hour photoperiods and even to 16-hour ones. For such null response, we have no explanation.
B. In the second part of the survey, photoperiods of 16 hours and longer were studied. To obtain the desired photoperiods, we used a small growth chamber that was illuminated continuously. Lots were moved to and from an adjoining dark room at appropriate intervals. Illumination at plant level was about 270 hlx from a mixture of fluorescent and incandescent lamps, about 10 per cent of the total from incandescence. Flower buds were found on all plants of the 18-, 20-, 22-, and 24-hour lots. Terminal meristems of the 16-hour plants had begun floral differentiation, but the flower buds themselves were not yet evident. Size of flower buds markedly increased with length of photoperiod.
IV. In three later experiments, certain effects were remeasured in one 16-hour and two longer photoperiods. All plants produced flower buds. Measurements (table 4) showed that stems of all plants progressively increased in length with increase in days of treatment and length of photoperiod. In two experiments with 24-hour photoperiods, the plants flowered vigorously. This success encouraged us to find the minimum period of continuous light that would still promote flowering.
TABLE 4
Stem lengthening response of poppy to three lengths of photoperiod applied for 2, 3, and 4 days
|
Mean stem length in mm. resulting from stated number of photoperiods |
|||
|---|---|---|---|
|
Photoperiod (hours) |
(2 days) |
(3 days) |
(4 days) |
| 16 | 19 | 27 | 31 |
| 18 | 21 | 34 | 47 |
| 20 | 31 | 37 | 47 |
A. In the first experiment, plants were subjected to six different durations of continuous light from 24 to 84 hours in 12-hour increments. Some plants flowered on the 48-hour and 60-hour periods, and all did on the 84-hour ones.
B. The second experiment used plants from the six seed lots listed in table 3. The plants were exposed to continuous light from 32 to 64 hours with intervals of 8 hours. Only one plant of one variety failed to flower in response to a single 48-hour light period, and several flowered on still shorter periods.
C. The third experiment used single periods of light from 27 to 48 hours at 3-hour intervals. All of the 48-, 45-, and 42-hour plants flowered. Four of the 39-hour plants and three each of the 36-, 33-, and 30-hour ones flowered. In fact, one 27-hour plant was induced, but all control plants left on an 8-hour photoperiod remained completely vegetative.
V. Earlier in this report the critical photoperiod for flowering was stated to be 15 or 16 hours, and plants rarely flowered on shorter photoperiods. This raised a question as to whether flowering on short photoperiods failed because photoperiods were too short or because dark periods were too long.
|
Cycle (hours) |
Dissection results a |
||
|---|---|---|---|
|
Light |
Dark |
Plant flowering (percentage) |
Mean stem length (in mm) |
|
1/2
|
1/2
|
100 | 56 |
| 1 | 1 | 100 | 73 |
| 2 | 2 | 100 | 83 |
| 3 | 3 | 100 | 100 |
| 4 | 4 | 100 | 104 |
| 6 | 6 | 100 | 94 |
| 12 | 12 | 0 | 39 |
| 16 | 8 | 100 | 105 |
aPlants per lot.
We used several photoperiodic cycles having equal periods of light and dark (table 5). Photoperiods of 12 hours light and 12 hours dark each 24 hours were known to be noninductive for poppy, and for some other long-day plants. However, for those long-day plants shorter cycles with equal light and dark parts such as 6 hours light 6 hours dark, or 2 hours light and 2 hours dark, were fully promotive. Poppy behaved similarly; plants flowered (table 5) 100 per cent on all cycles having photoperiods of 16 hours or less, but did not on the cycle having 12 hours light and 12 hours dark. Because plants had flowered even in continuous light, they must have failed to flower on the 12-hour treatment because of the excessively long dark periods.
The length measurements listed in table 5 are important because they show that flowering accompanied stem elongation. Elongation of the 16-hour controls and of plants on the most effective 50-50 cycles was about the same.
VI. Because a 6-hour light and 6-hour dark sequence results in flowering, and a 12-12 sequence does not, a photoperiodic schedule was sought in which a small amount of light spaced in the middle of a long dark period would promote flowering. Thus, the two 6-hour dark periods were held intact, and one light period was lengthened at the expense of the other. Of course, this schedule reached its limit when one light period was reduced to 0 and the other extended to 12-hour (table 6). Flower bud formation began to decline when the length of the shorter light period was reduced to 2 hours. Stem length remained fairly constant, however, until the shorter period was 1 hour.
VII. Next, the 6-hour light periods were held constant, but the dark periods were reciprocally varied. Percentage flowering was high, but fell abruptly when the longer dark period reached 10 hour (table 7). Stem length responded the same way.
VIII. Another experiment also was designed to include two periods of light and two periods of darkness during each 24 hours. However, all periods were unequal. The experimental design made use of the fact that flowering rarely occurs when plants receive 14-hour photoperiods, but usually occurs at least to a limited extent on 15-hour photoperiods. Our objective was to note the difference in effectiveness of 1 hour of light added to a 14-hour photoperiod, when the placement of that hour varied through the 10-hour dark period (table 8). We thought one hour of light properly located in the dark period might profoundly affect flowering, stem elongation, or both.
|
Length (in number of hours) of light periods during each 24 hours |
|||
|---|---|---|---|
|
First light period |
Second light period |
Flowering a (percentage) |
Mean stem length (in mm) |
| 6 | 6 | 75 | 21 |
| 7 | 5 | 100 | 33 |
| 8 | 4 | 87 | 24 |
| 9 | 3 | 87 | 22 |
| 10 | 2 | 50 | 25 |
| 11 | 1 | 29 | 17 |
| 12 | 0 | 0 | 14 |
aLots of eight plants each.
|
Length (in number of hours) of dark periods during each 24 hours |
|||
|---|---|---|---|
|
First dark period |
Second dark period |
Flowering a (percentage) |
Mean stem length (in mm) |
| 6 | 6 | 100 | 26 |
| 7 | 5 | 90 | 27 |
| 8 | 4 | 77 | 22 |
| 9 | 3 | 90 | 22 |
| 10 | 2 | 33 | 18 |
| 11 | 1 | 0 | 13 |
| 12 | 0 | 0 | 12 |
aLots of eight plants each.
In this experiment (table 8), flowering failed completely in the 14-hour controls and was 20 per cent or less on the 15-hour controls. However, where the 1 hour of light was placed near the middle of the dark period, flowering was 50 to 100 per cent, and stems averaged several millimeters longer than those of the 15-hour controls.
|
Length (in number of hours) of light (L) and dark (D) periods in each 24 hours |
|||||
|---|---|---|---|---|---|
|
L |
D |
L |
D |
Flowering a (percentage) |
Mean stem length (in mm) |
| 14 | 10 |
(control)
|
0 | 17 | |
| 15 | 9 |
(control)
|
20 | 24 | |
| 14 | 2 | 1 | 7 | 10 | 25 |
| 14 | 3 | 1 | 6 | 30 | 29 |
| 14 | 4 | 1 | 5 | 50 | 26 |
| 14 | 5 | 1 | 4 | 100 | 30 |
| 14 | 6 | 1 | 3 | 70 | 26 |
| 14 | 7 | 1 | 2 | 20 | 24 |
aLots of eight plants each.
IX. A new experiment was designed to measure the intensity of light required to extend a short noninductive day to a long inductive one. In this experiment, all lots including short-day controls, received 12 hours of nearly 108 hlx in a roomette for four successive days. At the end of the 12 hours, all plants, except the 12-hour controls, were given eight more hours of light at a range of intensities from 65 to 2, 152 lux. The resulting 20-hour photoperiods should have inducted flowering only if the intensity during the 8 hours period had been adequate. The intensities were obtained by our removing the plants from the roomette and placing them at appropriate distances from a 300-W incandescent-filament lamp to give the desired illuminances. All plants flowered at the highest illumination, and one did at even the lowest intensity (table 9). The stem-lengths measured differently throughout the range.
X. Tests were made for photoreversibility of flowering after brief dark-period interruptions by R and FR radiation.
A. One test had interruptions near the middle of 16-hour dark periods with R, FR, R followed immediately by Fr, and FR followed immediately by R. Irradiation times were 10 min for each kind of radiation; plants were treated during five successive daily dark periods. The treatments failed to induce flowering in any plant.
B. In a second test for photoreversibility, the plants received 14 hours of light per day in a roomette and 10-min R, FR, or both irradiances at different times during the 10-hour dark period. In treatments receiving both R and FR, one irradiation immediately followed the other. The 14-hour photoperiods were close to the critical point for flowering of poppy; in fact, about half of the plants flowered. Promotive or inhibitory effects were not clearly defined for the different interruptions with R, FR, or combinations of the two.
|
Illumination (in hlx) during last 8 hours of photoperiod |
Plants with flower buds a (percentage) |
Mean stem length (in mm) |
|---|---|---|
| 2 152 | 100 | 58 |
| 1 076 | 100 | 63 |
| 538 | 100 | 52 |
| 269 | 100 | 38 |
| 129 | 80 | 27 |
| 65 | 20 | 21 |
|
0 (12 hr control)
|
0 | 15 |
a Lots of five plants each.
C. In a third photoreversal test the basic photoperiod was 15 hours, and interruptions with R and FR were made at 0 and 4.5 hour of the 9-hour dark periods. Here, the photoperiods were long enough so that all the plants formed flower buds. Minor differences in stem length of the various lots were observed, but the differences were not related to the difference in kind of radiation treatment or time of its application during the night. None of the three tests thus gave evidence for the action of phytochrome.
XI. In all work thus far described, flowering was always accompanied by stem elongation, suggesting that the two were physiologically linked. Therefore, we devised experiments using gibberellic acid (GA) to find ways to separate flowering from stem elongation. The plants were sprayed with mM GA 3 until the liquid ran off the leaves. Lots of sprayed and unsprayed plants were then immediately selected and treated with different photoperiods.
A. In one experiment, plants of several different ages were treated with GA and returned immediately to 8-hour photoperiods for about one month. During that time, stems of plants of all ages markedly lengthened, some to more than 30 cm, but no flower buds formed. Stem tips, particularly of the oldest plants, were injured by the sprays, however, and the young leaves and terminal meristems of some plants blackened and degenerated.
B. After a month or more on 8-hour photoperiods, the plants of each age group were divided into two lots. One lot was given 20-hour photoperiods for two weeks, and the other was continued on 8-hour photoperiods. The plants were then returned to 8-hour photoperiods for later dissection. All plants that received 20-hour photoperiods developed flower buds, unless the tips had been killed by the GA. The plants held on 8-hour photoperiods throughout the experiment remained vegetative.
The long-day responsiveness of poppy for flowering was clearly seen in plants obtained from several geographical areas. The critical daylength was between about 14 and 16 hours in most strains and, for undetermined reasons, varied slightly from one experiment to another. Plants remained vegetative for many weeks when subjected continuously to 8-hour photoperiods, but could be induced to initiate the single terminal flower bud by as few as two long daily photoperiods or by a single continuous light period of somewhat more than 24 hours.
Relative effectiveness of photoperiods much longer than the critical length could not be detected easily by examination of the induced flower buds. However, the lengthening of stems, which invariably accompanies flowering, was progressively greater with increased length and number of long photoperiods (table 4).
Although stem lengthening always seems to accompany flowering, the latter does not always accompany stem lengthening. Thus, GA treatments provoked conspicuous stem lengthening, but no flower bud formation. Application of long photoperiods to these elongated plants, however, induced flower buds promptly.
The presence and action of phytochrome in flowering of poppy was never detected, even though the experiments allowed different ways for us to look for either promotive or inhibitory action of R and FR irradiations. Although we found no measurable reversing effects of R and FR, we do not conclude that phytochrome was not present.
Some of the R, FR reversal experiments were based on preliminary studies involving a long, but subcritical and a very short photoperiod during each 24 hours. The objective of such experiments was to find a régime in which a very short (5 to 10 min) period of light interrupting a long dark period would profoundly affect flowering. The energies during such brief interruptions were enough for phytochrome conversion, as shown by photoreversible control of flowering in Hyoscyamus [ 3] , another long-day plant. The minimal effective intensities and periods of light required were not unlike those found effective in promoting growth of woody plants.
Actually, the interruption experiments succeeded in poppy only when the shorter of the two light periods lasted 1 hour or more. This failure of brief light interruptions foretold the probable failure of R, FR reversal experiments.
Whether used to extend a subcritical photoperiod or used to interrupt a long dark period, the effectiveness of light periods of 1 hour and the even greater effectiveness of longer ones, show that a high energy reaction controls flowering of poppy.
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