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The effect of day length, source of light and growth regulators on growth and flowering of Clerodendrum thomsonae Balf
Seientia Horticulturae, 1 (1973) 1 - 11 Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m - Printed in The Netherlands
THE EFFECT OF DAY LENGTH, SOURCE OF LIGHT AND GROWTH REGULATORS ON GROWTH AND FLOWERING OF CLERODENDRUM THOMSONAE BALF
H A R A L D HILDRUM
Department of Floriculture and Greenhouse Crops, The Agricultural College of Norway, 1432 As-NLH (Norway)
The formation of flower buds in Clerodendrum seems not to be affected by day length, but the development of the buds is delayed in long days. When long days were established by means of low-intensity illumination with incandescent lamps, few flowers developed and the stems elongated considerably even at a day length of 16 hours. When fluorescent lamps were used -for day-length extension, short shoots with many flowers were obtained even in 24-.hour days. Flower development was also delayed by gibberellic acid (GA3), but promoted by chlormequat both in short and long days. Shoot elongation was retarded by chlormequat and promoted by GA3. Plants obtained from commercial greenhouses varied considerably with respect to growth and flowering. By selection a clone was obtained which flowered richly on short shoots. Shoot elongation stopped when flowering began. A 'negative' selection gave rise to a clone which flowered sparsely and in which shoot elongation was not influenced by flowering. INTRODUCTION Clerodendrum thomsonae Balf, a climbing plant of the family Verbenaceae, has been grown as a potplant for many years. Since Clerodendrum has commercial value primarily as a compact flowering plant (Fig. 3), it is of great practical interest to obtain information on how growth and flowering of this plant may be controlled. Clerodendrum usually flowers on short shoots in the spring and produces long shoots without flowers in the summer. This behaviour suggests that long days promote stem elongation and prevent flower-bud formation. Light quality h a s proved to be very important for stem elongation in many plants (Downs, 1959; Meijer, 1959; De Lint, 1961; Satter and
2 Wethere11, 1968). The effect of long days on stem elongation and flowering in Clerodendrurn might thus depend on the kind of light which is used for day-length extension. Stem elongation and flowering may also be controlled by means of growth regulators.
MATERIALS AND METHODS Experiments were carried out in growth rooms with artificial light only, as well as in an ordinary greenhouse. In the growth rooms the main light period of 8 hours was provided by fluorescent tubes (Phiiips TL-33) and a light intensity of about 8000 lux was maintained at plant level. For day-length extension low-intensity illumination with either incandescent lamps or fluorescent tubes of 20 W/m 2 was used. The temperature in the growth rooms was 24 °C (+ 0.5 °C) and the air humidity was maintained at a water vapour pressure deficit of about 5 mm of Hg. In the greenhouse the night temperature was about 21 °C. Short days (9 hours) were provided by covering the plants with black plastic sheets from 4 p.m. till 7 a.m,
The plants were cultivated in 10-cm plastic pots in a mixture of perfite and peat (3:1) and watered with a modified Hoagland solution. Shoot length was measured every week and the dates for visible flower buds and first open flowers were recorded. There were 6 to 18 plants in each treatment.
EXPERIMENTS AND RESULTS A. Effects of day length and chlormequat Five-month-old plants obtained from a grower were cut back and subjected to either 8 hours or 24 hours day length in the growth rooms. Daylength extension was provided by incandescent lamps. Three weeks after the start of the experiment one-half of the plants in each treatment were watered with 50 ml of a 1 % solution of the active ingredient of the growth retardant chiormequat [(2-chloroethyl) trimethyl ammonium chloride; cycocel]. A preliminary experiment revealed that the growth retardant SADH (succanimic acid 2,4-dimethyl hydrazide; B-9) had no effect on growth and flowering of Clerodendrurn and that chlormequat had a growth-retarding effect when applied as a drench, but not as a spray. Data obtained 10 weeks after the start of the day-length experiment are presented in Table I, showing that a high percentage of plants flowered in 8-hour days and only a few in 24-hour days. Neither flowering nor shoot length was affected by chlormequat. Plants grown in 8-hour days pro-
3 TABLE I The effect of day length and chlormequat on growth and flowering in Clerodendrum Day length
8h 24 h
Chlormequat
Flowering (%)
Shoot length (cm)
Internode length (cm)
Untreated 1% Untreated 1%
75 83 17 17
18.3 17.8 49.6 51.4
3.0 3.0 5.8 6.2
duced short shoots. In 24-hour days, however, the shoots elongated considerably. This elongation was due to an increase in both internode length and number of internodes.
B. Effects of day length and light quality Cuttings from stock plants selected for good flowering were cut back and grown in the growth rooms with day lengths of 8, 12, 16 and 24 hours. Incandescent lamps and fluorescent tubes were compared as light sources for extension of the photoperiod beyond 8 hours. Visible flower buds were observed at all day lengths (Table II). The development of the flowers, however, depended on the experimental conditions. All shoots on plants grown in 12-hour days flowered almost as early as those shoots of plants grown in 8-hour days. In 16-hour days about 80 % of all shoots flowered, but the flowering of the shoots receiving supplementary light from incandescent lamps was somewhat delayed. Nearly all shoots of the plants receiving 24-hour days with TABLE II The effect of day length and source of supplementary light on flower bud formation and development in Clerodendrum Supplementary light source
Day length
% shoots with visible flower buds
Time of first visible flower buds (days)
% flowering shoots
Time of flowering (days)
Fluorescent
12 h 16 h 24 h
100 100 100
16.3 17.8 17.0
100 79 93
56.6 54.7 64.6
5.1 5.6 6.8
Incandescent
12 h 16 h 24 h
100 100 100
18.1 15.0 17.0
100 79 12
58.9 62.2 -
5.5 12.3 6.7
8h
100
15.8
100
55.6
4.6
None
Number of nodes
fluorescent tubes flowered, but flowering was considerably delayed. When this day length was provided by incandescent lamps, however, flowering occurred on only a few shoots; the flower buds on the other shoots aborted. In fluorescent light shoot length was about the same at all day lengths, with the exception of 24 hours where elongation was slow (Fig. 1). Stem elongation for 12-hour days given by incandescent lamps was clearly increased compared with that of the control plants. In 16-hour days the shoots elongated very rapidly, and at the end of the experimental period the shoot length was. about three times that of the control shoots. However, stem elongation at 24 hours was very slow, the leaves of the plants became chlorotic, necrotic spots appeared between the veins and the expansion of the leaf blade was inhibited.
(9 Fluorescent tubes
• 8 hours o 12 " • 16 " ~24 "
40
Incandescent lamps
3O J~
2o
10
,
,
10
20
|
f
~
,
30 40 50 60
l
70
I
10
i
,
,
,
20 30 40 50
I
i
60 70
Days from the beginning of experiment
Fig. 1. The effect o f different light sources on shoot growth of Clerodendrum plants subjected to various day lengths in addition to a basic illumination o f 8 h o u r s with fluorescent light.
Fig. 1 also shows that stem elongation of plants receiving fluorescent light stopped almost entirely when flowering began. With incandescent light, however, stem elongation stopped in only a few plants i n 12-hour days. In 16-hour days elongation did not stop and the shoots showed many internodes (Table II). C. Effects of day length on different stock plant selections In a day-length experiment with unselected plants a great variation was observed with respect to growth and flowering. Plants with a good (positive) and with a poor (negative) flowering habit were selected as stock plants. Cuttings from these selections were used in the next experi-
5 ment. The rooted cuttings were cut back and grown in the greenhouse. Some were subjected to natural day length and some were given short days (9 hours). The natural day length was about 20 hours in the beginning and about 14 hours at the end of the experimental period. The times of flowering of plants from the two selections are shown in Fig. 2. All the plants flowered in short days, but the plants from the positive selection flowered about 2 weeks earlier than the ones from the negative selection. In natural day length all plants of the positive selection flowered, but flowering was somewhat delayed compared with those receiving short days. Less than half of the plants from the negative selection flowered in natural days. Fig. 2 also shows that the variation in t i m e o f flowering within each itreatment was much greater in natural than i n 9-hour days.
® I00 90
9 hours.,,.
Natural ,~,
9
/9 our'
/"
co
= 70 •~
60
~o 50
~ 40 ~ 30 g_ 20
10
"45 d9 5'3 57 61' 6g 69 i3 : # DaYS to flowering
81 ~ 5 89
Fig. 2. The effect of 9-hour days compared with a 'natural' day length: of 2 0 to 'i4 hours on flowering of two selections of Cleroclendrum.:(. A)selected for good (positive); (0 £)selected for poor (negative) flowering habit. : : i i : i:
In both day lengths the plants from the negative :selection ~had :the longest shoots and t h e shoots continued to .grow after !floweri:ng began (Table Ill. Fig. 3). In nearly all plants fr0m :thepQsifive :selection, .:shoot elongation stopped at the time of flowering. : : "
: , ,
.
'
.
.
.
,
:
,:
:
:
ii i
,
i
i
7 TABLE III The effect of day length on growth of plants from two selectious of Clerodendrum, one with a good (positive), the other with a poor (negative) flowering habit Selection
Day length
Shoot length
(cm) Positive Negative
9h Natural 9h Natural
17.3 17.8 30.1 26.1
Number of plants (out of 18) with inactive apical meristem 17 16 1
1
D. The effect of growth regulators on different stock plant selections Cuttings from the same selections of stock plants as in the previous experiment were cut back and grown in the greenhouse. One group re' ceived short days (9 hours),: the other was subjected to natural daylength which was about 18 hours in the beginning and 19 hours at the e n d o f the period. The plants were sprayed once with either 40 o r 160 mg/ml gibberellic acid (GA3) o r watered with 50 ml o f either 0.5 or 1.0 % o f chlormequat. GA3 was dissolved in 5 % ethanol. It was again found that plants from the positive selection flowered more profusely than t h e ones from the negative selection both at short and natural day length (Table IV). GA3 delayed and chlormequat enhanced flowering o f the plants from the positive selection in 9-hour days. In natural days chlormequat at a concentration o f 1% promoted flowering considerably. Neither GA3 n~or chlormequat had any significant effect on the number Of flowering plants of the negative selection. However, t h e flowering o f the:plants 9,hour days was enhanced b y chlOrmequat. GA3 stimulated stem elongation in both sele:ctions (Fig. 4)i Both:length and number of internodes:were increased. Chlormequat somewhat:retarded stem elongation:in short days o f both selectionS, b u t s e e m e d :to promote stem elongation of plants from the positive selection in:natural days. The overall effect of chlormequat on stem elongation was reiatively small even at 1% concentration. Plants treated with: chlormequat: d eve:l: oped, as usual, darker:green leaves than the untreated onesl Fig. 3. The effect of 9-hour days Compared with that of a 'natural' d a y length On growth and flowering of two selection s of Clerodendrum. On the top, selected stock plants grown: in 9-h0ur days. In the middle, cuttings grown in 'natural' days of 20 to 14 hours, At the bottom, cuttings grown in 9-hours days. To the righL selected for good; to the left, selected for poor flowering habit. . . . .
Fig. 4. The effect of day length and GA 3 on growth and flowering of two selections of Clerodendrum. Positive selection to the left, negative to the right. Treatments: from left to right, untreated, 40 rag/1 GA 3 and 160 mg/l GA 3 ; at the top 9-hour days and at the bottom natural days of 18-19 hours.
oo
9 TABLE IV The effect of day length, GA~ and chlormequat on flowering and growth of plants from two selections in Clerodendrum, one with a good (positive), the other with a poor (negative) flowering habit Selection
Day length
Untreated
GA a
Chlormequat
40 mg/1
160 mg/1
0.5%
1%
6 (63) 4 (62) 3(83) 0( - )
6 (62) 1(90) 3(83) 0( - )
6 (50) 3 (71) 3(66) 1 (90)
6 (52) 6 (54) 4(61) 1 (90)
12.7 17.3 30.1 22.1
23.5 26.2 29.1 17.6
10.0 15.8 17.6 15.0
8.5 12.4 19.6 15.0
6.0 7.7 10.1 9.7
6.8 8.8 10.1 9.0
4.8 7.0 7.7 7.8
5.0 4.8 8.1 7.9
Flowering* Positive Negative
9h Natural 9h Natural
6 (54) 2 (48) 3(81) 1 (65) Shoot length (cm)
Positive Negative
9h Natural 9h Natural
11.6 10.8 21.0 15.7 Number of nodes
Positive Negative
9h Natural 9h Natural
5.0 5.7 8.9 7.9
*Number of flowering plants out of 6. Number of days to first open flower in parentheses.
DISCUSSI ON
These experiments have shown that the formation of flower buds in Clerodendrum takes place regardless of day length. The development of the buds is, however, influenced by day length. In Caryopteris x clandonensis, another plant of the family Verbenaceae, Piringer et al. (1963) have found that flower bud formation is not affected by day length but that the development of the flower buds requires short days. A similar reaction has been observed in Bougainvillea (Hackett & Sachs, 1968). In Clerodendrum the flower buds are formed in the leaf axils. The apical meristem is always vegetative. The experiments have shown that if the flowers develop normally, stem elongation stops when flowering begins and does not continue until the flowering has terminated. By removing the flower buds it was found that the shoot will grow continuously. In plants which do not flower, or develop only a few flowers in each inflorescence, the shoot does not stop growing. Thus there seems to
!0 be a close correlation between the vegetative growth and the development of the flower buds in Clerodendrum. The day-length effect on flower development and growth in Clerodendrum depends on the source of supplementary light. It has been demonstrated in many other species that incandescent light promotes stem elongation more than fluorescent light (Downs et al., 1958; Piringer & Cathey, 1960; Hildrum, 1969). The differences in both growth and flowering obtained with the two lamp types may be due to differences in spectral energy distribution. Incandescent lamps emit a relatively large part of the light energy in wavelengths longer than 700 nm, while fluorescent lamps emit only a small part of the light energy in wavelengths longer than 700 nm. Stem elongation is especially promoted by far-red light (Meijer, 1959; De Lint, 1961; Satter & Wetherell, 1968). The elongation of Clerodendrum thus seems to be under phytochrome control. The clones which were selected in these experiments are morphologically similar. The main difference between them seems to be that one is able to develop flower buds and stop shoot elongation when flowering begins, while the other develops only a few flowers and elongates continuously. Only the positive selection should be grown commercially. Since GA3 delayed flowering both in short and long days it is possible that GA3 has a direct effect u p o n flowering. Inhibition of flowering by GA3 has been observed in other short-day plants, e.g. Kalanchoe (Harder & Biinsow, 1958), poinsettia (Guttridge, 1963) and strawberry (Thompson & Guttridge, 1959). Stem elongation in Clerodendrum was, however, strongly promoted by GA3. This implies that an active apical meristem might have an inhibiting effect u p o n flower development. The effect of GA3 would then be secondary. In favour of this hypothesis is the fact that incandescent light, which also promotes stem elongation, delays flower development. Chlormequat promoted flowering of the positive selection in both short and long days. This effect of chlonnequat may be due to a reduction in the amount of endogenous gibberellins, because it is well established that the biosynthesis of gibberellins can be inhibited by chlormequat. In Bougainvillea short days and chlormequat promote flower development while GA3 inhibits development (Hackett & Sachs, 1967). The reactions of Clerodendrum and Bougainvillea to day length, GA3 and chlormequat are thus very similar. In both plants it is primarily the development of the flowers that is subject to environmental and chemical control. It could be supposed that the level of endogenous gibberellins is one of the reasons for the failure of flower development in the negative selection of Clerodendrum. This hypothesis, however, is not supported by our results, since GA3 did not reduce the number of flowering plants and
11
chlormequat did not promote flower development in long days. The development of the flowers seems to be correlatively inhibited by the apical meristem and it is thus possible that auxin is a more important factor than gibberellin in the flowering of Cl~rodendrum. REFERENCES De Lint, P.J.A.L. (1961), Dependence of elongation on wavelength of supplementary irradation, Meded. LandbHogesch. Wageningen 61 (16), 1-14. Downs, R.J. (1959), Photocontrol of vegetative growth, in Withrow, R.B. (ed.), Photoperiodism and related Phenomena in Plants and Animals, pp. 129-135. Downs, R.L, Bortwick, H.A. & Piringer A.A, (1958), Comparison of incandescent and fluorescent lamps for lengthening photoperiods, Proc. Amer. Soc. hort. ScL 71, 568-578. Guttridge, C.G. (1963), Inhibition of flowering in poinsettia by gibberellic acid, Nature 197, 920-921. Hackett, W.P. & Sachs, R.M. (1967), Chemical control of flowering in Bougainvillea 'San Diego Red',Proc. Amer. Soc. hort. Sc~ 90, 361-364. Hackett, W.P. & Sachs, R.M. (1968), Experimental separation of inflorescence development from initiation in Bougainvillea, Proa Amer. So~ hort. ScL 92, 615-621. Harder, R. & Biinsow, R. (1958), Uber die Wirkung yon Gibberellin auf Entwicklung und Bliitenbildung der Kurtztagpflanze Kalanchoe blossfeldiana, Planta 51,202-222. Hfldrum, H. (1969), Factors affecting flowering in Senecio cruentus D.C., Acta Hort., int. Soc. hort. Sci. 14, 117-123. Meijer, G. (1959), The spectral dependence of flowering and elongation, Acta bot. neerl. 8, 189-246. Piringer, A.A. & Cathey, H.M. (1960), Effects of photoperiod, kind of supplemental light, and temperature on growth and flowering of petunia plants, Proc. Amer. Soc. hort. Scl 76, 649-660. Piringer, A.A., Downs, R.J. & Bortwick, H.A. (1963), Photocontrol of growth and flowering of Caryopteris, Amer. J. Bot. 50, 86-90. Satter, R.L. & Wetherell, D.F. (1968), Photomorphogenesis in Sinningia speciosa cv. Queen Victoria. I. Characterization of.phytochrome control, Plant PhysioL 43, 953-960. Thompson, P.A. & Guttridge, C.G. (1959), Effect of gibberellic acid on the initiation of flowers and runners in the strawberry, Nature 184, 72-73.
THE EFFECT OF DAY LENGTH, SOURCE OF LIGHT AND GROWTH REGULATORS ON GROWTH AND FLOWERING OF CLERODENDRUM THOMSONAE BALF
H A R A L D HILDRUM
Department of Floriculture and Greenhouse Crops, The Agricultural College of Norway, 1432 As-NLH (Norway)
The formation of flower buds in Clerodendrum seems not to be affected by day length, but the development of the buds is delayed in long days. When long days were established by means of low-intensity illumination with incandescent lamps, few flowers developed and the stems elongated considerably even at a day length of 16 hours. When fluorescent lamps were used -for day-length extension, short shoots with many flowers were obtained even in 24-.hour days. Flower development was also delayed by gibberellic acid (GA3), but promoted by chlormequat both in short and long days. Shoot elongation was retarded by chlormequat and promoted by GA3. Plants obtained from commercial greenhouses varied considerably with respect to growth and flowering. By selection a clone was obtained which flowered richly on short shoots. Shoot elongation stopped when flowering began. A 'negative' selection gave rise to a clone which flowered sparsely and in which shoot elongation was not influenced by flowering. INTRODUCTION Clerodendrum thomsonae Balf, a climbing plant of the family Verbenaceae, has been grown as a potplant for many years. Since Clerodendrum has commercial value primarily as a compact flowering plant (Fig. 3), it is of great practical interest to obtain information on how growth and flowering of this plant may be controlled. Clerodendrum usually flowers on short shoots in the spring and produces long shoots without flowers in the summer. This behaviour suggests that long days promote stem elongation and prevent flower-bud formation. Light quality h a s proved to be very important for stem elongation in many plants (Downs, 1959; Meijer, 1959; De Lint, 1961; Satter and
2 Wethere11, 1968). The effect of long days on stem elongation and flowering in Clerodendrurn might thus depend on the kind of light which is used for day-length extension. Stem elongation and flowering may also be controlled by means of growth regulators.
MATERIALS AND METHODS Experiments were carried out in growth rooms with artificial light only, as well as in an ordinary greenhouse. In the growth rooms the main light period of 8 hours was provided by fluorescent tubes (Phiiips TL-33) and a light intensity of about 8000 lux was maintained at plant level. For day-length extension low-intensity illumination with either incandescent lamps or fluorescent tubes of 20 W/m 2 was used. The temperature in the growth rooms was 24 °C (+ 0.5 °C) and the air humidity was maintained at a water vapour pressure deficit of about 5 mm of Hg. In the greenhouse the night temperature was about 21 °C. Short days (9 hours) were provided by covering the plants with black plastic sheets from 4 p.m. till 7 a.m,
The plants were cultivated in 10-cm plastic pots in a mixture of perfite and peat (3:1) and watered with a modified Hoagland solution. Shoot length was measured every week and the dates for visible flower buds and first open flowers were recorded. There were 6 to 18 plants in each treatment.
EXPERIMENTS AND RESULTS A. Effects of day length and chlormequat Five-month-old plants obtained from a grower were cut back and subjected to either 8 hours or 24 hours day length in the growth rooms. Daylength extension was provided by incandescent lamps. Three weeks after the start of the experiment one-half of the plants in each treatment were watered with 50 ml of a 1 % solution of the active ingredient of the growth retardant chiormequat [(2-chloroethyl) trimethyl ammonium chloride; cycocel]. A preliminary experiment revealed that the growth retardant SADH (succanimic acid 2,4-dimethyl hydrazide; B-9) had no effect on growth and flowering of Clerodendrurn and that chlormequat had a growth-retarding effect when applied as a drench, but not as a spray. Data obtained 10 weeks after the start of the day-length experiment are presented in Table I, showing that a high percentage of plants flowered in 8-hour days and only a few in 24-hour days. Neither flowering nor shoot length was affected by chlormequat. Plants grown in 8-hour days pro-
3 TABLE I The effect of day length and chlormequat on growth and flowering in Clerodendrum Day length
8h 24 h
Chlormequat
Flowering (%)
Shoot length (cm)
Internode length (cm)
Untreated 1% Untreated 1%
75 83 17 17
18.3 17.8 49.6 51.4
3.0 3.0 5.8 6.2
duced short shoots. In 24-hour days, however, the shoots elongated considerably. This elongation was due to an increase in both internode length and number of internodes.
B. Effects of day length and light quality Cuttings from stock plants selected for good flowering were cut back and grown in the growth rooms with day lengths of 8, 12, 16 and 24 hours. Incandescent lamps and fluorescent tubes were compared as light sources for extension of the photoperiod beyond 8 hours. Visible flower buds were observed at all day lengths (Table II). The development of the flowers, however, depended on the experimental conditions. All shoots on plants grown in 12-hour days flowered almost as early as those shoots of plants grown in 8-hour days. In 16-hour days about 80 % of all shoots flowered, but the flowering of the shoots receiving supplementary light from incandescent lamps was somewhat delayed. Nearly all shoots of the plants receiving 24-hour days with TABLE II The effect of day length and source of supplementary light on flower bud formation and development in Clerodendrum Supplementary light source
Day length
% shoots with visible flower buds
Time of first visible flower buds (days)
% flowering shoots
Time of flowering (days)
Fluorescent
12 h 16 h 24 h
100 100 100
16.3 17.8 17.0
100 79 93
56.6 54.7 64.6
5.1 5.6 6.8
Incandescent
12 h 16 h 24 h
100 100 100
18.1 15.0 17.0
100 79 12
58.9 62.2 -
5.5 12.3 6.7
8h
100
15.8
100
55.6
4.6
None
Number of nodes
fluorescent tubes flowered, but flowering was considerably delayed. When this day length was provided by incandescent lamps, however, flowering occurred on only a few shoots; the flower buds on the other shoots aborted. In fluorescent light shoot length was about the same at all day lengths, with the exception of 24 hours where elongation was slow (Fig. 1). Stem elongation for 12-hour days given by incandescent lamps was clearly increased compared with that of the control plants. In 16-hour days the shoots elongated very rapidly, and at the end of the experimental period the shoot length was. about three times that of the control shoots. However, stem elongation at 24 hours was very slow, the leaves of the plants became chlorotic, necrotic spots appeared between the veins and the expansion of the leaf blade was inhibited.
(9 Fluorescent tubes
• 8 hours o 12 " • 16 " ~24 "
40
Incandescent lamps
3O J~
2o
10
,
,
10
20
|
f
~
,
30 40 50 60
l
70
I
10
i
,
,
,
20 30 40 50
I
i
60 70
Days from the beginning of experiment
Fig. 1. The effect o f different light sources on shoot growth of Clerodendrum plants subjected to various day lengths in addition to a basic illumination o f 8 h o u r s with fluorescent light.
Fig. 1 also shows that stem elongation of plants receiving fluorescent light stopped almost entirely when flowering began. With incandescent light, however, stem elongation stopped in only a few plants i n 12-hour days. In 16-hour days elongation did not stop and the shoots showed many internodes (Table II). C. Effects of day length on different stock plant selections In a day-length experiment with unselected plants a great variation was observed with respect to growth and flowering. Plants with a good (positive) and with a poor (negative) flowering habit were selected as stock plants. Cuttings from these selections were used in the next experi-
5 ment. The rooted cuttings were cut back and grown in the greenhouse. Some were subjected to natural day length and some were given short days (9 hours). The natural day length was about 20 hours in the beginning and about 14 hours at the end of the experimental period. The times of flowering of plants from the two selections are shown in Fig. 2. All the plants flowered in short days, but the plants from the positive selection flowered about 2 weeks earlier than the ones from the negative selection. In natural day length all plants of the positive selection flowered, but flowering was somewhat delayed compared with those receiving short days. Less than half of the plants from the negative selection flowered in natural days. Fig. 2 also shows that the variation in t i m e o f flowering within each itreatment was much greater in natural than i n 9-hour days.
® I00 90
9 hours.,,.
Natural ,~,
9
/9 our'
/"
co
= 70 •~
60
~o 50
~ 40 ~ 30 g_ 20
10
"45 d9 5'3 57 61' 6g 69 i3 : # DaYS to flowering
81 ~ 5 89
Fig. 2. The effect of 9-hour days compared with a 'natural' day length: of 2 0 to 'i4 hours on flowering of two selections of Cleroclendrum.:(. A)selected for good (positive); (0 £)selected for poor (negative) flowering habit. : : i i : i:
In both day lengths the plants from the negative :selection ~had :the longest shoots and t h e shoots continued to .grow after !floweri:ng began (Table Ill. Fig. 3). In nearly all plants fr0m :thepQsifive :selection, .:shoot elongation stopped at the time of flowering. : : "
: , ,
.
'
.
.
.
,
:
,:
:
:
ii i
,
i
i
7 TABLE III The effect of day length on growth of plants from two selectious of Clerodendrum, one with a good (positive), the other with a poor (negative) flowering habit Selection
Day length
Shoot length
(cm) Positive Negative
9h Natural 9h Natural
17.3 17.8 30.1 26.1
Number of plants (out of 18) with inactive apical meristem 17 16 1
1
D. The effect of growth regulators on different stock plant selections Cuttings from the same selections of stock plants as in the previous experiment were cut back and grown in the greenhouse. One group re' ceived short days (9 hours),: the other was subjected to natural daylength which was about 18 hours in the beginning and 19 hours at the e n d o f the period. The plants were sprayed once with either 40 o r 160 mg/ml gibberellic acid (GA3) o r watered with 50 ml o f either 0.5 or 1.0 % o f chlormequat. GA3 was dissolved in 5 % ethanol. It was again found that plants from the positive selection flowered more profusely than t h e ones from the negative selection both at short and natural day length (Table IV). GA3 delayed and chlormequat enhanced flowering o f the plants from the positive selection in 9-hour days. In natural days chlormequat at a concentration o f 1% promoted flowering considerably. Neither GA3 n~or chlormequat had any significant effect on the number Of flowering plants of the negative selection. However, t h e flowering o f the:plants 9,hour days was enhanced b y chlOrmequat. GA3 stimulated stem elongation in both sele:ctions (Fig. 4)i Both:length and number of internodes:were increased. Chlormequat somewhat:retarded stem elongation:in short days o f both selectionS, b u t s e e m e d :to promote stem elongation of plants from the positive selection in:natural days. The overall effect of chlormequat on stem elongation was reiatively small even at 1% concentration. Plants treated with: chlormequat: d eve:l: oped, as usual, darker:green leaves than the untreated onesl Fig. 3. The effect of 9-hour days Compared with that of a 'natural' d a y length On growth and flowering of two selection s of Clerodendrum. On the top, selected stock plants grown: in 9-h0ur days. In the middle, cuttings grown in 'natural' days of 20 to 14 hours, At the bottom, cuttings grown in 9-hours days. To the righL selected for good; to the left, selected for poor flowering habit. . . . .
Fig. 4. The effect of day length and GA 3 on growth and flowering of two selections of Clerodendrum. Positive selection to the left, negative to the right. Treatments: from left to right, untreated, 40 rag/1 GA 3 and 160 mg/l GA 3 ; at the top 9-hour days and at the bottom natural days of 18-19 hours.
oo
9 TABLE IV The effect of day length, GA~ and chlormequat on flowering and growth of plants from two selections in Clerodendrum, one with a good (positive), the other with a poor (negative) flowering habit Selection
Day length
Untreated
GA a
Chlormequat
40 mg/1
160 mg/1
0.5%
1%
6 (63) 4 (62) 3(83) 0( - )
6 (62) 1(90) 3(83) 0( - )
6 (50) 3 (71) 3(66) 1 (90)
6 (52) 6 (54) 4(61) 1 (90)
12.7 17.3 30.1 22.1
23.5 26.2 29.1 17.6
10.0 15.8 17.6 15.0
8.5 12.4 19.6 15.0
6.0 7.7 10.1 9.7
6.8 8.8 10.1 9.0
4.8 7.0 7.7 7.8
5.0 4.8 8.1 7.9
Flowering* Positive Negative
9h Natural 9h Natural
6 (54) 2 (48) 3(81) 1 (65) Shoot length (cm)
Positive Negative
9h Natural 9h Natural
11.6 10.8 21.0 15.7 Number of nodes
Positive Negative
9h Natural 9h Natural
5.0 5.7 8.9 7.9
*Number of flowering plants out of 6. Number of days to first open flower in parentheses.
DISCUSSI ON
These experiments have shown that the formation of flower buds in Clerodendrum takes place regardless of day length. The development of the buds is, however, influenced by day length. In Caryopteris x clandonensis, another plant of the family Verbenaceae, Piringer et al. (1963) have found that flower bud formation is not affected by day length but that the development of the flower buds requires short days. A similar reaction has been observed in Bougainvillea (Hackett & Sachs, 1968). In Clerodendrum the flower buds are formed in the leaf axils. The apical meristem is always vegetative. The experiments have shown that if the flowers develop normally, stem elongation stops when flowering begins and does not continue until the flowering has terminated. By removing the flower buds it was found that the shoot will grow continuously. In plants which do not flower, or develop only a few flowers in each inflorescence, the shoot does not stop growing. Thus there seems to
!0 be a close correlation between the vegetative growth and the development of the flower buds in Clerodendrum. The day-length effect on flower development and growth in Clerodendrum depends on the source of supplementary light. It has been demonstrated in many other species that incandescent light promotes stem elongation more than fluorescent light (Downs et al., 1958; Piringer & Cathey, 1960; Hildrum, 1969). The differences in both growth and flowering obtained with the two lamp types may be due to differences in spectral energy distribution. Incandescent lamps emit a relatively large part of the light energy in wavelengths longer than 700 nm, while fluorescent lamps emit only a small part of the light energy in wavelengths longer than 700 nm. Stem elongation is especially promoted by far-red light (Meijer, 1959; De Lint, 1961; Satter & Wetherell, 1968). The elongation of Clerodendrum thus seems to be under phytochrome control. The clones which were selected in these experiments are morphologically similar. The main difference between them seems to be that one is able to develop flower buds and stop shoot elongation when flowering begins, while the other develops only a few flowers and elongates continuously. Only the positive selection should be grown commercially. Since GA3 delayed flowering both in short and long days it is possible that GA3 has a direct effect u p o n flowering. Inhibition of flowering by GA3 has been observed in other short-day plants, e.g. Kalanchoe (Harder & Biinsow, 1958), poinsettia (Guttridge, 1963) and strawberry (Thompson & Guttridge, 1959). Stem elongation in Clerodendrum was, however, strongly promoted by GA3. This implies that an active apical meristem might have an inhibiting effect u p o n flower development. The effect of GA3 would then be secondary. In favour of this hypothesis is the fact that incandescent light, which also promotes stem elongation, delays flower development. Chlormequat promoted flowering of the positive selection in both short and long days. This effect of chlonnequat may be due to a reduction in the amount of endogenous gibberellins, because it is well established that the biosynthesis of gibberellins can be inhibited by chlormequat. In Bougainvillea short days and chlormequat promote flower development while GA3 inhibits development (Hackett & Sachs, 1967). The reactions of Clerodendrum and Bougainvillea to day length, GA3 and chlormequat are thus very similar. In both plants it is primarily the development of the flowers that is subject to environmental and chemical control. It could be supposed that the level of endogenous gibberellins is one of the reasons for the failure of flower development in the negative selection of Clerodendrum. This hypothesis, however, is not supported by our results, since GA3 did not reduce the number of flowering plants and
11
chlormequat did not promote flower development in long days. The development of the flowers seems to be correlatively inhibited by the apical meristem and it is thus possible that auxin is a more important factor than gibberellin in the flowering of Cl~rodendrum. REFERENCES De Lint, P.J.A.L. (1961), Dependence of elongation on wavelength of supplementary irradation, Meded. LandbHogesch. Wageningen 61 (16), 1-14. Downs, R.J. (1959), Photocontrol of vegetative growth, in Withrow, R.B. (ed.), Photoperiodism and related Phenomena in Plants and Animals, pp. 129-135. Downs, R.L, Bortwick, H.A. & Piringer A.A, (1958), Comparison of incandescent and fluorescent lamps for lengthening photoperiods, Proc. Amer. Soc. hort. ScL 71, 568-578. Guttridge, C.G. (1963), Inhibition of flowering in poinsettia by gibberellic acid, Nature 197, 920-921. Hackett, W.P. & Sachs, R.M. (1967), Chemical control of flowering in Bougainvillea 'San Diego Red',Proc. Amer. Soc. hort. Sc~ 90, 361-364. Hackett, W.P. & Sachs, R.M. (1968), Experimental separation of inflorescence development from initiation in Bougainvillea, Proa Amer. So~ hort. ScL 92, 615-621. Harder, R. & Biinsow, R. (1958), Uber die Wirkung yon Gibberellin auf Entwicklung und Bliitenbildung der Kurtztagpflanze Kalanchoe blossfeldiana, Planta 51,202-222. Hfldrum, H. (1969), Factors affecting flowering in Senecio cruentus D.C., Acta Hort., int. Soc. hort. Sci. 14, 117-123. Meijer, G. (1959), The spectral dependence of flowering and elongation, Acta bot. neerl. 8, 189-246. Piringer, A.A. & Cathey, H.M. (1960), Effects of photoperiod, kind of supplemental light, and temperature on growth and flowering of petunia plants, Proc. Amer. Soc. hort. Scl 76, 649-660. Piringer, A.A., Downs, R.J. & Bortwick, H.A. (1963), Photocontrol of growth and flowering of Caryopteris, Amer. J. Bot. 50, 86-90. Satter, R.L. & Wetherell, D.F. (1968), Photomorphogenesis in Sinningia speciosa cv. Queen Victoria. I. Characterization of.phytochrome control, Plant PhysioL 43, 953-960. Thompson, P.A. & Guttridge, C.G. (1959), Effect of gibberellic acid on the initiation of flowers and runners in the strawberry, Nature 184, 72-73.