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Effects of temperature and daylength on growth and flowering of roselle, Hibiscus sabdariffa L.
Scientia Horticulturae, 3 (1975) 129--135 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
EFFECTS OF TEMPERATURE AND DAYLENGTH ON GROWTH AND F L O W E R I N G O F R O S E L L E , H I B I SC U S S A B D A R I F F A L.
B.M.M. MANSOUR
Department of Horticulture, Agricultural University, Wageningen (The Netherlands) Publication 408 Present address: Horticulture Department, Faculty of Agriculture, Azhar University, Cairo (Egypt) (Received July 19th, 1974)
ABSTRACT Mansour, B.M.M., 1975. Effects of temperature and daylength on growth and flowering of roselle, Hibiscus sabdariffa L. Scientia Hort., 3: 129--135.
Hibiscus sabdariffa L. was found to show an ambiphotoperiodic reaction, flowering both in short days and in extremely long days, but remaining vegetative (at least at 21--25°C) in a 16 h day. Flowering had a dual effect on growth. When floral induction was strong, lateral buds developed into flowers and the number of branches was proportionally reduced. The first stage of floral induction, however, was accompanied by a decrease of apical dominance. Plants shifted to long days after only a short stay at an inductive daylength, and plants moved to short days after a prolonged stay in long days, formed many more branches and a much greater vegetative mass than plants grown in continuous short days or long days. Plants were already sensitive to daylength in the cotyledon stage. Floral induction was not carried over from short days into long days. After marginal induction, floral buds reverted to vegetative shoots, or aborted and were replaced by a shoot from the axil of a bract. In short days, seedlings form shallowly three-lobed leaves until the generative stage, when only entire leaves are formed. In long days, the leaves are deeply five-lobed, also when the plant flowers. INTRODUCTION Roselle or J a m a i c a Sorrel, Hibiscus sabdariffa L, vat. sabdariffa (Malvaceae) is a large annual, g r o w n in t r o p i c a l a n d s u b t r o p i c a l regions chiefly f o r its fleshy calyxes. These are used t o p r e p a r e a soft d r i n k a n d as a s o u r c e o f a red pigm e n t used in f o o d a n d cosmetics. T h e p l a n t also has medical p r o p e r t i e s , and t h e t e n d e r leaves a n d stalks are s o m e t i m e s used as a vegetable. As t h e g r o w i n g o f roselle is o f s o m e e c o n o m i c i m p o r t a n c e in E g y p t , the writer availed himself o f t h e o p p o r t u n i t y o f a stay at a well e q u i p p e d l a b o r a t o r y t o s t u d y the effects o f t e m p e r a t u r e a n d light o n this plant, w h i c h a p p e a r e d t o be largely unknown.
130 MATERIAL AND METHODS The cultivar used was the one c o m m o n l y grown in Egypt. It has dark red calyxes. The seed was sown in plastic boxes filled with a mixture of sand and peat mold in equal proportions. Seedlings were transplanted directly after emergence, each plant to a 20 cm black plastic container filled with a mixture of sand, peat mold and manure in equal parts. The temperature and light experiments were carried o u t in a phytotron, described by Doorenbos (1964). The temperatures available were 17, 21 and 25 ° C (+- 1 ° C). The plants received 8, 16 or 24 h of fluorescent light (Philips TL 59) of an intensity of 31 000 ergs cm -2 sec -1 . Further daylength experiments were done in growth cabinets with a temperature of a b o u t 24°C where the light source consisted of six fluorescent tubes (Philips TL 55 40 W) producing a total radiation of a b o u t 33 000 ergs cm -2 sec -1 . The basic light period of 8 h could be extended with weak incandescent light. Finally, a greenhouse experiment was started on March 26th, 1973. Plants were subjected to an 8 h day and a 16 h day. The short day was supplied b y covering the plants with black plastic from 4.30 p.m. to 8.30 a.m., the 16 h day was the natural day extended when necessary with incandescent light. Plants were shifted from long days to short days and vice versa after 0, 2, 4 . . . 16 weeks. RESULTS - - Fig.1 shows the effect of three temperatures on seed germination in darkness. There were 65 seeds per treatment. The highest percentage of germination and the most rapid emergence occurred at 25 ° C. It is possible that the o p t i m u m temperature is actually higher. After germination, seedlings were grown in continuous light. At 17 ° C, they were yellow and weak, and most of them died soon after emergence. At 21 and 25 ° C, seedlings were green and healthy.
Seed germination.
Germination
8O %
6040/ 5 " C
0 3
~
5
/
~ g I 11 I I 13 I days
Fig.1. Germination of roselle at three d i f f e r e n t temperatures.
132 TABLE II Effect of photoperiod on growth of roselle Photoperiod (sunlight + i n c a n d e s c e n t )
Plant height (cm) Number of nodes Stem diameter (ram) Number of branches Total length of branches (cm)
8+0
8+4
8+8
8+12
8+16
89.8 34.2 9.1 15.0
90.6 37.4 9.9 19.6
128.0 65.4 14.6 44.6
118.8 58.6 14.1 40.0
116.4 55.8 14.0 34.6
50.0
38.8
201.4
144.6
111.8
basic light period o f sunlight or fluorescent light. Table II gives some representative results. The data o f the plants in 8, 16 and 24 h p h o t o p e r i o d s should be c o m p a r e d with those o f the plants at 25 ° C in Table I. As b o t h sets of data show the same trend, the effects f o u n d in the p h y t o t r o n experiments are largely due to daylength, and the effect of different light energies is of secondary importance. The greenhouse e x p e r i m e n t in which plants were shifted from a phot operiod o f 8 h (SD) t o one of 16 h (LD) and vice versa gave the following results. As could be expected, plants were higher in LD than in SD, the difference being only 6.5 cm, however. Plants shifted f r o m SD t o LD were longer than those in c o n t i n u o u s SD or LD, particularly when shifted after 2 or 4 Total length branches 800 - cm
SD'-~ LD 60C Ptant height 200 - cm
180
40O
' ~ D - ~
SD
160 11,0
/
20O
\
", ,,
120 100
I
I
I
I
I
[
I
I
I
I
0
2
4
6
8
10
12
14
16
20 weeks
Shifted a f t e r
0
I
I
I
I
I
I
I
I
I
2
/.
6
8
10
12
14
16
Shifted after
Figs 2 and 3. G r o w t h o f roselle plants transferred f r o m a p h o t o p e r i o d o f 16 h ( L D ) t o a p h o t o p e r i o d o f 8 h ( S D ) or vice versa at 2 w e e k s intervals after the c o t e l y d o n stage. M e a s u r e m e n t s m a d e after 20 weeks.
2 week
133
weeks (Fig.2). Plants shifted from LD to SD were also longer, the m a x i m u m being reached b y plants subjected to 10 weeks of LD followed b y SD. The total length of branches (Fig.3) followed a similar trend. In all experiments the shape of the leaf was strongly affected by the position on the plant and b y the environment. The first leaves of a seedling are broadly oval. Later leaves are palmately lobed. In SD, the leaves are threelobed, the lobes of the lower leaves being shallow, broad and pointed, while those of higher leaves are more slender. In most seedlings, the leaf is never dissected to more than a b o u t one third of its width. When the plants become generative, the newly formed leaves are narrowly oval to lanceolate and entire. In long day, the leaves become progressively more dissected until the leaf is divided up to 3/4 of its width into five narrow lobes. When in a 24 h photoperiod the plants become generative, the newly formed leaves keep this form, and do n o t b e c o m e three-lobed or entire as in short day. - - In the first p h y t o t r o n experiments the plants flowered in 8 h of light and in continuous light, b u t n o t in 16 h of light, except at 17 ° C where plants flowered both in 8 and 16 h. This was confirmed by the second experiment, the data of which are given in Table III. Temperature apparently has a dual effect: low temperature accelerates flower initiation as measured by the node with the first flower bud, b u t it retards flower development. The effect of length (and quantity) of daffy irradiation is strongly dependent on temperature. At 17°C the plants initiated flower buds first at 8 h, then at 24 h and finally at 16 h. This was the same at 21 and 25°C, but here the retarding effect of 16 h of light was so strong that the plants remained vegetative until the conclusion of the experiment after 161 days, when most of the plants had stopped growing. At that m o m e n t three groups of plants had flowered, the first being the plants in 8 h of light at 25 ° C. In the first experiment the order of flowering was the same b u t eventually the plants in 24 h of light at 25°C and those at 17°C flowered also. The results of a similar experiment b u t under conditions where the photoperiod was varied without concurrent variation of the incumbent radiation
_Flowering.
T A B L E III E f f e c t o f t e m p e r a t u r e and light on flowering of roselle T e m p e r a t u r e and h o u r s o f light 17 ° C
Nodes t o first f l o w e r b u d F l o w e r b u d s per plant Days t o f l o w e r i n g
21 ° C
25 ° C
8
16
24
8
16
24
8
16
24
11.0 4.0 ?
18.0 41.0 ?
17.0 55.0 ?
12.8 24.8 103.5
,x, 0 --
29.0 38.6 124.6
15.8 29.4 96.8
~ 0 --
64.9 21.3 ?
134
TABLE IV Effect of photoperiod on flowering of roselle Photoperiod (sunlight and incandescent)
Nodes to first flower Flower buds per plant Days to flowering
8+0
8+4
8+8
8+12
8+16
17.4 55.6 83.0
19.4 61.8 90.2
% 0 --
50.2 40.8 ?
45.8 56.4 ?
are given in Table IV for the same plants as of Table II. The temperature was a b o u t 24 ° C. The general trend of these data is the same as in Table III, showing that the effect of light on flowering is primarily one of photoperiod, n o t of light quantity. The plants at 8 h were the first to initiate flowers, followed almost immediately by those at 12 h. The plants at 16 h remained vegetative to the end of the experiment. Those at 20 and 24 h eventually formed flower buds, but especially the plants at 20 h were much retarded. Neither of these t w o groups had flowered when the experiment was discontinued after 154 days. The experiment in the greenhouse, where plants were shifted from 16 h of light (LD) to 8 h (SD) and vice versa, showed that the plants already reacted to the p h o t o p e r i o d in the c o t e l y d o n stage. In continuous SD, the first flower bud appeared at the 6th or 7th node and flowering occurred after 73 days. In plants which were given 2 weeks of LD first, initiation was shifted to the 10th node and flowering was delayed b y 4 days. In plants shifted from SD to LD after 2 weeks, the vegetative shoots which developed in the axils of the lower leaves were abnormal: the first internode was abnormally long, and the first leaves were arranged in whorls. Apparently, these buds had already been in the process of flower initiation when the shift to LD caused them to revert to the vegetative stage. When the shift to LD was made after 4 weeks of SD, the flower bud did n o t revert b u t it did not develop further and finally was shed, while a normal vegetative shoot deveioped in the axil of one of the bracts. 6 weeks of SD were sufficient to let some buds develop into flowers. These were n o t the ones laid down first, b u t those in the 11th or 12th node. In plants shifted to LD after 8 or more weeks of SD all flower buds developed into flowers and fruits. However, initiation of new flower buds immediately came to an end. DISCUSSION
From the data collected it appears that roseUe is a heat-loving plant in which both growth and development are dependent on the prevailing daylength. With respect to flowering, Hibiscus sabdariffa L. can be added to the list of 17 species which show an " a m b i p h o t o p e r i o d i c " t y p e of reaction, ini-
135
tiating flowers when days are either short or extremely long, b u t remaining vegetative at an intermediate daylength (Mathon, 1970). In view of the interaction with temperature (Table III) one can also say that the inhibiting effect on flower initiation of high temperature is very strong at an intermediate daylength, less strong at an extremely long daylength and only slight in short day. The marked effect of the photoperiod on growth can be explained by this relation between p h o t o p e r i o d and flowering. In constant short day, all lateral buds from a certain node onwards form flowers. Consequently, there are few branches, a low total leaf number and a reduced plant height. In a 16 h day there are no flower buds at all, b u t many branches, a large leaf mass and a considerable plant height. In extremely long days or continuous light, flower buds are formed so that growth is again reduced, although n o t as much as in short day. The experiment in which plants were shifted from one p h o t o p e r i o d to another showed that flower formation can also have a stimulatory influence on growth. Although the flowers are formed laterally, the terminal meristem always remaining vegetative, the first stage of floral induction appears to be accompanied by a decrease of apical dominance. When plants have been growing for some time in LD (and consequently have many vegetative buds) a shift to short days causes these buds to develop into branches, with a consequent b o o s t in vegetative growth (Figs 2 and 3). That the curve slopes down again after 10 weeks is due to the fact that subsequent groups had progressively less time to profit from the shift, as all plants were measured at the same day, 20 weeks after sowing. Plants shifted from SD to LD after 2--6 weeks (enough for induction, n o t sufficient for the initiation of more than a few flowers) show the same effect: development of lateral shoots, a greater vegetative mass and plant height. In this case, the curve slopes down after 4--6 weeks because progressively more buds are used for flowers. REFERENCES Doorenbos, J., 1964. Het fytotron van het Laboratorium voor Tuinbouwplantenteelt der Landbouwhogeschool. Meded. Dir. Tuinbouw, 27: 432--437. Mathon, C.C., 1970. Nouveaux cas de rdaction ambiphotopdriodique. C.R. Soc. Biol., 164(1): 169--172.
EFFECTS OF TEMPERATURE AND DAYLENGTH ON GROWTH AND F L O W E R I N G O F R O S E L L E , H I B I SC U S S A B D A R I F F A L.
B.M.M. MANSOUR
Department of Horticulture, Agricultural University, Wageningen (The Netherlands) Publication 408 Present address: Horticulture Department, Faculty of Agriculture, Azhar University, Cairo (Egypt) (Received July 19th, 1974)
ABSTRACT Mansour, B.M.M., 1975. Effects of temperature and daylength on growth and flowering of roselle, Hibiscus sabdariffa L. Scientia Hort., 3: 129--135.
Hibiscus sabdariffa L. was found to show an ambiphotoperiodic reaction, flowering both in short days and in extremely long days, but remaining vegetative (at least at 21--25°C) in a 16 h day. Flowering had a dual effect on growth. When floral induction was strong, lateral buds developed into flowers and the number of branches was proportionally reduced. The first stage of floral induction, however, was accompanied by a decrease of apical dominance. Plants shifted to long days after only a short stay at an inductive daylength, and plants moved to short days after a prolonged stay in long days, formed many more branches and a much greater vegetative mass than plants grown in continuous short days or long days. Plants were already sensitive to daylength in the cotyledon stage. Floral induction was not carried over from short days into long days. After marginal induction, floral buds reverted to vegetative shoots, or aborted and were replaced by a shoot from the axil of a bract. In short days, seedlings form shallowly three-lobed leaves until the generative stage, when only entire leaves are formed. In long days, the leaves are deeply five-lobed, also when the plant flowers. INTRODUCTION Roselle or J a m a i c a Sorrel, Hibiscus sabdariffa L, vat. sabdariffa (Malvaceae) is a large annual, g r o w n in t r o p i c a l a n d s u b t r o p i c a l regions chiefly f o r its fleshy calyxes. These are used t o p r e p a r e a soft d r i n k a n d as a s o u r c e o f a red pigm e n t used in f o o d a n d cosmetics. T h e p l a n t also has medical p r o p e r t i e s , and t h e t e n d e r leaves a n d stalks are s o m e t i m e s used as a vegetable. As t h e g r o w i n g o f roselle is o f s o m e e c o n o m i c i m p o r t a n c e in E g y p t , the writer availed himself o f t h e o p p o r t u n i t y o f a stay at a well e q u i p p e d l a b o r a t o r y t o s t u d y the effects o f t e m p e r a t u r e a n d light o n this plant, w h i c h a p p e a r e d t o be largely unknown.
130 MATERIAL AND METHODS The cultivar used was the one c o m m o n l y grown in Egypt. It has dark red calyxes. The seed was sown in plastic boxes filled with a mixture of sand and peat mold in equal proportions. Seedlings were transplanted directly after emergence, each plant to a 20 cm black plastic container filled with a mixture of sand, peat mold and manure in equal parts. The temperature and light experiments were carried o u t in a phytotron, described by Doorenbos (1964). The temperatures available were 17, 21 and 25 ° C (+- 1 ° C). The plants received 8, 16 or 24 h of fluorescent light (Philips TL 59) of an intensity of 31 000 ergs cm -2 sec -1 . Further daylength experiments were done in growth cabinets with a temperature of a b o u t 24°C where the light source consisted of six fluorescent tubes (Philips TL 55 40 W) producing a total radiation of a b o u t 33 000 ergs cm -2 sec -1 . The basic light period of 8 h could be extended with weak incandescent light. Finally, a greenhouse experiment was started on March 26th, 1973. Plants were subjected to an 8 h day and a 16 h day. The short day was supplied b y covering the plants with black plastic from 4.30 p.m. to 8.30 a.m., the 16 h day was the natural day extended when necessary with incandescent light. Plants were shifted from long days to short days and vice versa after 0, 2, 4 . . . 16 weeks. RESULTS - - Fig.1 shows the effect of three temperatures on seed germination in darkness. There were 65 seeds per treatment. The highest percentage of germination and the most rapid emergence occurred at 25 ° C. It is possible that the o p t i m u m temperature is actually higher. After germination, seedlings were grown in continuous light. At 17 ° C, they were yellow and weak, and most of them died soon after emergence. At 21 and 25 ° C, seedlings were green and healthy.
Seed germination.
Germination
8O %
6040/ 5 " C
0 3
~
5
/
~ g I 11 I I 13 I days
Fig.1. Germination of roselle at three d i f f e r e n t temperatures.
132 TABLE II Effect of photoperiod on growth of roselle Photoperiod (sunlight + i n c a n d e s c e n t )
Plant height (cm) Number of nodes Stem diameter (ram) Number of branches Total length of branches (cm)
8+0
8+4
8+8
8+12
8+16
89.8 34.2 9.1 15.0
90.6 37.4 9.9 19.6
128.0 65.4 14.6 44.6
118.8 58.6 14.1 40.0
116.4 55.8 14.0 34.6
50.0
38.8
201.4
144.6
111.8
basic light period o f sunlight or fluorescent light. Table II gives some representative results. The data o f the plants in 8, 16 and 24 h p h o t o p e r i o d s should be c o m p a r e d with those o f the plants at 25 ° C in Table I. As b o t h sets of data show the same trend, the effects f o u n d in the p h y t o t r o n experiments are largely due to daylength, and the effect of different light energies is of secondary importance. The greenhouse e x p e r i m e n t in which plants were shifted from a phot operiod o f 8 h (SD) t o one of 16 h (LD) and vice versa gave the following results. As could be expected, plants were higher in LD than in SD, the difference being only 6.5 cm, however. Plants shifted f r o m SD t o LD were longer than those in c o n t i n u o u s SD or LD, particularly when shifted after 2 or 4 Total length branches 800 - cm
SD'-~ LD 60C Ptant height 200 - cm
180
40O
' ~ D - ~
SD
160 11,0
/
20O
\
", ,,
120 100
I
I
I
I
I
[
I
I
I
I
0
2
4
6
8
10
12
14
16
20 weeks
Shifted a f t e r
0
I
I
I
I
I
I
I
I
I
2
/.
6
8
10
12
14
16
Shifted after
Figs 2 and 3. G r o w t h o f roselle plants transferred f r o m a p h o t o p e r i o d o f 16 h ( L D ) t o a p h o t o p e r i o d o f 8 h ( S D ) or vice versa at 2 w e e k s intervals after the c o t e l y d o n stage. M e a s u r e m e n t s m a d e after 20 weeks.
2 week
133
weeks (Fig.2). Plants shifted from LD to SD were also longer, the m a x i m u m being reached b y plants subjected to 10 weeks of LD followed b y SD. The total length of branches (Fig.3) followed a similar trend. In all experiments the shape of the leaf was strongly affected by the position on the plant and b y the environment. The first leaves of a seedling are broadly oval. Later leaves are palmately lobed. In SD, the leaves are threelobed, the lobes of the lower leaves being shallow, broad and pointed, while those of higher leaves are more slender. In most seedlings, the leaf is never dissected to more than a b o u t one third of its width. When the plants become generative, the newly formed leaves are narrowly oval to lanceolate and entire. In long day, the leaves become progressively more dissected until the leaf is divided up to 3/4 of its width into five narrow lobes. When in a 24 h photoperiod the plants become generative, the newly formed leaves keep this form, and do n o t b e c o m e three-lobed or entire as in short day. - - In the first p h y t o t r o n experiments the plants flowered in 8 h of light and in continuous light, b u t n o t in 16 h of light, except at 17 ° C where plants flowered both in 8 and 16 h. This was confirmed by the second experiment, the data of which are given in Table III. Temperature apparently has a dual effect: low temperature accelerates flower initiation as measured by the node with the first flower bud, b u t it retards flower development. The effect of length (and quantity) of daffy irradiation is strongly dependent on temperature. At 17°C the plants initiated flower buds first at 8 h, then at 24 h and finally at 16 h. This was the same at 21 and 25°C, but here the retarding effect of 16 h of light was so strong that the plants remained vegetative until the conclusion of the experiment after 161 days, when most of the plants had stopped growing. At that m o m e n t three groups of plants had flowered, the first being the plants in 8 h of light at 25 ° C. In the first experiment the order of flowering was the same b u t eventually the plants in 24 h of light at 25°C and those at 17°C flowered also. The results of a similar experiment b u t under conditions where the photoperiod was varied without concurrent variation of the incumbent radiation
_Flowering.
T A B L E III E f f e c t o f t e m p e r a t u r e and light on flowering of roselle T e m p e r a t u r e and h o u r s o f light 17 ° C
Nodes t o first f l o w e r b u d F l o w e r b u d s per plant Days t o f l o w e r i n g
21 ° C
25 ° C
8
16
24
8
16
24
8
16
24
11.0 4.0 ?
18.0 41.0 ?
17.0 55.0 ?
12.8 24.8 103.5
,x, 0 --
29.0 38.6 124.6
15.8 29.4 96.8
~ 0 --
64.9 21.3 ?
134
TABLE IV Effect of photoperiod on flowering of roselle Photoperiod (sunlight and incandescent)
Nodes to first flower Flower buds per plant Days to flowering
8+0
8+4
8+8
8+12
8+16
17.4 55.6 83.0
19.4 61.8 90.2
% 0 --
50.2 40.8 ?
45.8 56.4 ?
are given in Table IV for the same plants as of Table II. The temperature was a b o u t 24 ° C. The general trend of these data is the same as in Table III, showing that the effect of light on flowering is primarily one of photoperiod, n o t of light quantity. The plants at 8 h were the first to initiate flowers, followed almost immediately by those at 12 h. The plants at 16 h remained vegetative to the end of the experiment. Those at 20 and 24 h eventually formed flower buds, but especially the plants at 20 h were much retarded. Neither of these t w o groups had flowered when the experiment was discontinued after 154 days. The experiment in the greenhouse, where plants were shifted from 16 h of light (LD) to 8 h (SD) and vice versa, showed that the plants already reacted to the p h o t o p e r i o d in the c o t e l y d o n stage. In continuous SD, the first flower bud appeared at the 6th or 7th node and flowering occurred after 73 days. In plants which were given 2 weeks of LD first, initiation was shifted to the 10th node and flowering was delayed b y 4 days. In plants shifted from SD to LD after 2 weeks, the vegetative shoots which developed in the axils of the lower leaves were abnormal: the first internode was abnormally long, and the first leaves were arranged in whorls. Apparently, these buds had already been in the process of flower initiation when the shift to LD caused them to revert to the vegetative stage. When the shift to LD was made after 4 weeks of SD, the flower bud did n o t revert b u t it did not develop further and finally was shed, while a normal vegetative shoot deveioped in the axil of one of the bracts. 6 weeks of SD were sufficient to let some buds develop into flowers. These were n o t the ones laid down first, b u t those in the 11th or 12th node. In plants shifted to LD after 8 or more weeks of SD all flower buds developed into flowers and fruits. However, initiation of new flower buds immediately came to an end. DISCUSSION
From the data collected it appears that roseUe is a heat-loving plant in which both growth and development are dependent on the prevailing daylength. With respect to flowering, Hibiscus sabdariffa L. can be added to the list of 17 species which show an " a m b i p h o t o p e r i o d i c " t y p e of reaction, ini-
135
tiating flowers when days are either short or extremely long, b u t remaining vegetative at an intermediate daylength (Mathon, 1970). In view of the interaction with temperature (Table III) one can also say that the inhibiting effect on flower initiation of high temperature is very strong at an intermediate daylength, less strong at an extremely long daylength and only slight in short day. The marked effect of the photoperiod on growth can be explained by this relation between p h o t o p e r i o d and flowering. In constant short day, all lateral buds from a certain node onwards form flowers. Consequently, there are few branches, a low total leaf number and a reduced plant height. In a 16 h day there are no flower buds at all, b u t many branches, a large leaf mass and a considerable plant height. In extremely long days or continuous light, flower buds are formed so that growth is again reduced, although n o t as much as in short day. The experiment in which plants were shifted from one p h o t o p e r i o d to another showed that flower formation can also have a stimulatory influence on growth. Although the flowers are formed laterally, the terminal meristem always remaining vegetative, the first stage of floral induction appears to be accompanied by a decrease of apical dominance. When plants have been growing for some time in LD (and consequently have many vegetative buds) a shift to short days causes these buds to develop into branches, with a consequent b o o s t in vegetative growth (Figs 2 and 3). That the curve slopes down again after 10 weeks is due to the fact that subsequent groups had progressively less time to profit from the shift, as all plants were measured at the same day, 20 weeks after sowing. Plants shifted from SD to LD after 2--6 weeks (enough for induction, n o t sufficient for the initiation of more than a few flowers) show the same effect: development of lateral shoots, a greater vegetative mass and plant height. In this case, the curve slopes down after 4--6 weeks because progressively more buds are used for flowers. REFERENCES Doorenbos, J., 1964. Het fytotron van het Laboratorium voor Tuinbouwplantenteelt der Landbouwhogeschool. Meded. Dir. Tuinbouw, 27: 432--437. Mathon, C.C., 1970. Nouveaux cas de rdaction ambiphotopdriodique. C.R. Soc. Biol., 164(1): 169--172.