- Email: [email protected]
Influence of temperature and variations in temperature on flowering in Zostera marina L. under laboratory conditions
Aquatic Botany, 10 (1981) 125--131 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
125
I N F L U E N C E O F T E M P E R A T U R E A N D V A R I A T I O N S IN T E M P E R A T U R E O N F L O W E R I N G IN Z O S T E R A M A R I N A L. U N D E R L A B O R A T O R Y CONDITIONS
A.W.A.M. DE COCK Department of Botany, Catholic University, Toernooiveld, 6525 ED Nijmegen (The Netherlands) (Accepted 22 August 1980)
ABSTRACT De Cock, A.W.A.M., 1981. Influence of temperature and variations in temperature on flowering in Zostera marina L. under laboratory conditions. Aquat. Bot., 10:125--131. Flowering of male and female flowers of Zostera marina L. has been observed in constant light under five different temperature regimes: constant temperatures of 15, 20 and 25°C, and alternating temperatures of 15/20°C and 20/25°C. The rate of female flowering was higher at a constant 20°C than at a constant 15 or 25°C. Alternation of temperatures led to a higher number of flowering inflorescences in the periods with lower temperatures. The rate of male flowering increased with increasing, constant temperature. At alternating temperatures the number of male flowering inflorescences was not significantly different in the periods with the lower and the higher temperature. The average flowering rate was in these cases nearly the same as the flowering rate at the highest temperature. INTRODUCTION The seagrass Zostera marina L. grows in places w h e r e changes in temperature are r a t h e r limited in c o m p a r i s o n t o the habitats o f land plants. This is particularly t h e case for plants growing in the subtidal zone. P o p u l a t i o n s o f t h e intertidal area are m o r e e x p o s e d t o t e m p e r a t u r e f l u c t u a t i o n s . Nevertheless, Setchell ( 1 9 2 9 ) f o u n d an i m p o r t a n t relation b e t w e e n t e m p e r a t u r e and life h i s t o r y even for plants in d e e p e r water. A c c o r d i n g to this a u t h o r , no g r o w t h o c c u r s b e l o w 10°C; vegetative g r o w t h m a i n l y takes place b e t w e e n 10 and 15°C, and generative r e p r o d u c t i o n is restricted t o w a t e r t e m p e r a t u r e s b e t w e e n 15 and 20°C. Generally, at t e m p e r a t u r e s a b o v e 20°C a h e a t rigor sets in, w h i c h is irreversible, and higher t e m p e r a t u r e s cause t h e d e a t h o f t h e plants. He suggested t h a t no p h o t o - p e r i o d i s m existed in this p l a n t and t h a t t e m p e r a t u r e was the o n l y f a c t o r i n d u c i n g flowering (Setchell, 1922). T h o u g h these t e m p e r a t u r e intervals m a y be r o u g h l y valid f o r m o s t subtidal p o p u l a t i o n s , t h e y are n o t so for every p o p u l a t i o n ( C o t t a m and M u n r o , 1 9 5 4 ; Riggs and Fralick, 1 9 7 5 ) , and t h e y are certainly n o t valid f o r a n n u a l
0304-3770/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
126 plants of intertidal areas. In the latter areas, temperatures may fluctuate widely, particularly in summer when sunshine may give day temperatures considerably higher than 20°C, and when relatively low night temperatures may occur. These effects are most obvious during low tide. Annual populations remain viable and flower abundantly under these conditions. The question may be asked as to whether temperature and variations in temperature, as they occur diurnally in the intertidal zone, influence flowering of Zostera marina and if so, in what way. In this study the influence of a constant temperature of 15, 20 and 25°C was examined, as well as the effect of alternating temperatures of 15/20°C and 20/25°C. The temperature of 20°C was chosen as an intermediate temperature because this seems to be a good approximation of the temperature of seawater in summer in the area where the examined plants grew. In this article the term 'inflorescence' is used to describe the flowering unit of spadix, spatha and peduncle, only and the term 'spathe' to denote the spathal sheath and leaf blade. The reasons for this are stated in de Cock (1981a). MATERIALS AND METHODS Flowering shoots of Zostera marina plants were gathered in the intertidal zone near Bergen op Zoom, The Netherlands. Plants of this population are annual and flower abundantly. They are completely submerged at high tide, and during low tide they lie in very shallow pools in which great temperature changes may occur. The handling and selection of two groups of inflorescences at different stages of development for separate examination of flowering of pistils and anthers, was as described in de Cock (1981b). Five h u n d r e d inflorescences of each group were distributed over five full-glass cisterns containing 0.5 1 synthetic seawater (Hw-Meeressalz, H. Wiegandt, Krefeld). The cisterns were covered with glass plates to avoid evaporation. During the experiments all cisterns were kept in constant light as this proved to allow the regular course of development of the flowers, except in the first experimental period of 12 h following a period for adaptation to light conditions (de Cock, 1981b). The light source was a number of fluorescent lamps (Sylvania Powertube, cool white, F96T12/CW/VHO), which gave a light intensity of ca. 11 000 lux at the level of the water surface in the cisterns. Temperature conditions were as follows: (1) constant 15°C; (2) constant 20°C; (3) constant 25°C; (4) alternating 15 and 20°C in periods of 12 h each; (5) alternating 20 and 25°C in periods of 12 h each. Of both series (male and female flowering), one cistern with 100 inflorescences was kept under one of these temperature conditions. Different temperatures were achieved by using water baths. If alternating temperatures were tested and a cistern was transferred from one bath to another, the temperature of the seawater adapted to the new temperature within 15 min.
127
The experiments were started in the evening after a period of 12 h for adaptation to light. The number of flowering inflorescences was counted regularly after periods of 12 h, these countings coinciding with the change of the cisterns concerned to another temperature. An inflorescence was considered to be female flowering if at least one pistil had projected its style. An inflorescence was considered to be male flowering if at least one anther had dehisced. Flowering inflorescences were removed after every counting. Experiments were discontinued when the maximum percentage of flowering inflorescences was reached. All experiments were carried out in duplicate at different times. RESULTS
Flowering of the female flowers The morphological features of flowering were not altered by the different temperature conditions. Flowering showed a regular course at all of the three constant temperatures (Fig. 1). The rate of flowering was about the same at both 15 and 25°C, b u t clearly higher at 20°C. Flowering was suppressed in the first 12-h period at 20°C, even longer at 15°C, and noticeably longer at 25°C. Abnormal flowering was observed at 25°C. Simultaneous flowering of pistils and anthers occurred after 3.5 day at that temperature (Fig. 1). These inflorescences were counted, however, together with the normal, female flowering inflorescences. A few inflorescences started flowering with dehiscence of the anthers. These were not involved in the countings and had to be removed as it could not be determined whether the styles were still immature inside the spatha or retracted again after (self-)pollination. For this 100 %
::: L*
....
so
>~ eo
/"
o
8
,.~'~2~
and 20° I----?
/] ..
20 i 1
2
3
4
5
6
7
Time
8
! 9
(days)
Fig. 1. C u m u l a t i v e p e r c e n t a g e s of f e m a l e flowering infiorescences over t i m e at a c o n s t a n t t e m p e r a t u r e o f 15, 20 a n d 25 ° C, a n d at a l t e r n a t i n g 15 ] 20 ° C.
128 100 -%
. . . - - - -=t . . . . .
~ 80
20~,/°
o (__ -9
//
j
aod
,
d" !
2O
~ 0
i 1
2
3
4
5
i 6
, 7
I 8
Time ( d a y s )
Fig. 2. C u m u l a t i v e percentages of female flowering inflorescences over time at a constant
temperature of 20 and
25 °
C, and at alternating 20/25 ° C.
reason the final percentage of flowering inflorescences did not total 100% at 25°C. The course o f flowering in the cisterns with alternating temperatures was quite 'irregular' (Figs. 1 and 2). Under bot h t e m p e r a t u r e regimes, the n u m b e r of flowering inflorescences was higher in the periods with the lower temperature. This was most obvious if alternating periods of 20 and 25°C were given, but also evident in a 15/20°C regime. The average flowering under both alternating t e m p e r a t u r e regimes was intermediate between the rates at the corresponding constant temperatures. Table I shows the cumulative percentages o f inflorescences flowering in the periods with the same t em perat ure for each o f the cisterns at alternating temperatures. TABLEI Results of the alternating temperature regimes. The cumulative percentage of inflorescenses, flowering in the periods with the same temperature, is given for male and female flowering. Temperature of flowering
15°C 20°C 25°C
Female flowering (%)
Male flowering (%)
15/20°C
15/20°C
66.7 33.3
20/25°C
80 17.5
57.8 42.3
20/25°C
55 45
Flowering of the male flowers After a slight suppression in the first period, flowering was quite regular at both constant and alternating temperatures. Under the constant temperature regimes the flowering rate of the anthers proved to increase with in-
129 100 -% u ~
..:
-~100 -%
-.-~: . . . . . . . . .
~~=-__o___.
J/
q~ ~- 80
80
X
/ .~ 60
20 °
/ ..://¢'~'~" "° /~/.S / ~ .~Alternating
150 (...
)
~ 40
E
;
~\150 I
1
2
I
20
I 3
' 4
'
I
5
'
Time
I
6
'
(days)
~
0
200 (- . . . . . ) )
and 250 (-.
/ • ,'.." ~ 2 0 °
4C
t.)
20
/,'\Alternating .."
/
Y
1
i 2
i
I
3
,
J
4
,
~
5
,
~
6
,
Time (days)
Fig. 3. C u m u l a t i v e p e r c e n t a g e o f m a l e f l o w e r i n g i n f l o r e s c e n c e s o v e r t h e c o u r s e o f t i m e at a c o n s t a n t t e m p e r a t u r e of 1 5 , 2 0 a n d 2 5 ° C , a n d at a l t e r n a t i n g 1 5 / 2 0 ° C . Fig. 4. C u m u l a t i v e p e r c e n t a g e o f m a l e f l o w e r i n g i n f l o r e s c e n c e s in t h e c o u r s e o f t i m e at a t e m p e r a t u r e o f c o n s t a n t 2 0 ° C a n d a l t e r n a t i n g 2 0 / 2 5 ° C . T h e graph o f t h e f l o w e r i n g c o u r s e at c o n s t a n t 2 5 ° C is n o t d r a w n as it c o i n c i d e d w i t h t h e graph o f a l t e r n a t i n g 2 0 / 2 5 ° C .
creasing temperature, the highest flowering rate reached being at 25°C (Fig. 3). Alternating temperatures of 15/20°C or 20/25°C did not result in alternating periods with higher and lower rates of flowering; the total number of flowering inflorescences was somewhat higher in the periods with the lower temperature (Table I). Throughout the period of investigation, the average flowering rate was the same (20/25°C) or nearly the same (15/20°C) as the flowering rate at the higher corresponding constant temperature, thus 25 and 20°C, respectively. DISCUSSION
Flowering of the female flowers The experiments were started after the inflorescences had been allowed to adapt to the light conditions for a 12-h period. The suppression of flowering in the first experimental period must be due to the experimental light conditions (constant light). However, this photo-inhibition is only present during ca. 24 h in constant light at 20°C (de Cock, 1981b). Hence, the longer suppression, in the second experimental period, at constant 15 and 25°C, can be attributed to the influence of temperature. It appears that there is an optimum temperature for female flowering of ca. 20°C. Flowering starts earlier and proceeds faster at 20°C than at either 15 or 25°C. Not only different constant temperatures, but also changes in temperature influence female flowering. It is quite remarkable that a change from 20 to 15°C induces a higher flowering rate, though a temperature of 20°C seems favourable for flowering, if given constantly. It is not clear, however, whether the lower temperature stimulates or the higher temperature suppresses.
130
Flowering of the male flowers In contrast to female flowering, the male flowers did not have an optimum in the investigated temperature range. Flowering increased with increasing temperature. There is no early period of suppression of flowering at one of the three constant temperatures: differences in the graphs are only caused by differences in flowering rate, not by a short-term suppression such as occurs in female flowering. As alternation of temperatures did not lead to differences in flowering rate, the influence of temperature on the anthers seems only to be a stimulation of the development and not a factor in the diurnal course of flowering. This is supported by the phenomenon of simultaneous flowering of pistils and anthers at 25°C in the test for female flowering. This is obviously caused by a suppression in the initial periods and a lower flowering rate of the female flowers at this temperature, as well as the higher rate of development of the anthers.
Temperature effects in the intertidal zone For the populations of Zostera marina in the intertidal zone, the effect of alternating temperatures on flowering is particularly important. Water temperature here is influenced by two interactive factors: day and night on the one hand, and high- and low-tide on the other. The temperature regime depends on the time of the day when flood and ebb occur. Moreover, as light also has a great influence on flowering of male as well as female flowers (de Cock, 1981b), one may not consider the temperature factor alone. It is most likely that the effects of temperature and light combine in some way. In the case of flowering of the female flowers it may be expected that the lower temperature during the night intensifies the effect of the darkness, resulting in a higher flowering rate. Higher temperatures during the day may co-operate with light in suppressing flowering of the female flowers. ACKNOWLEDGEMENTS
The author wishes to thank Prof. Dr. H.F. Linskens for his stimulating interest and Dr. A.F. Croes for critically reading the manuscript. Thanks are also due to Prof. Dr. C. den Hartog for his valuable comments and for correcting the English text. REFERENCES Cottam, C. and Munro, D.A., 1954. Eelgrass status and e n v i r o n m e n t a l relations. J. Wildlife Manag., 18: 449--460. De Cock, A.W.A.M0, 1981a. Development of the f l o w e r i n g s h o o t of Z o s t e r a marina L. under c o n t r o l l e d c o n d i t i o n s in c o m p a r i s o n t o the d e v e l o p m e n t in t w o different natural habitats in The Netherlands. Aquat. Bot., 10: 99--113.
131 De Cock, A.W.A.M., 1981b. Influence of light and dark on flowering in Z o s t e r a marina L. under laboratory conditions. Aquat. Bot., 10: 115--123. Riggs, S.A. and Fralick, R.A., 1975. Z o s t e r a marina L., its growth and distribution in the Great Bay Estuary, New Hampshire. Rhodora, 77: 456--466. Setchell, W.A., 1922. Z o s t e r a marina in its relation to temperature. Science, 56: 575--577. Setchell, W.A., 1929. Morphological and phenological notes on Z o s t e r a marina L. Univ. Calif. Publ. Bot., 14: 389--452.
125
I N F L U E N C E O F T E M P E R A T U R E A N D V A R I A T I O N S IN T E M P E R A T U R E O N F L O W E R I N G IN Z O S T E R A M A R I N A L. U N D E R L A B O R A T O R Y CONDITIONS
A.W.A.M. DE COCK Department of Botany, Catholic University, Toernooiveld, 6525 ED Nijmegen (The Netherlands) (Accepted 22 August 1980)
ABSTRACT De Cock, A.W.A.M., 1981. Influence of temperature and variations in temperature on flowering in Zostera marina L. under laboratory conditions. Aquat. Bot., 10:125--131. Flowering of male and female flowers of Zostera marina L. has been observed in constant light under five different temperature regimes: constant temperatures of 15, 20 and 25°C, and alternating temperatures of 15/20°C and 20/25°C. The rate of female flowering was higher at a constant 20°C than at a constant 15 or 25°C. Alternation of temperatures led to a higher number of flowering inflorescences in the periods with lower temperatures. The rate of male flowering increased with increasing, constant temperature. At alternating temperatures the number of male flowering inflorescences was not significantly different in the periods with the lower and the higher temperature. The average flowering rate was in these cases nearly the same as the flowering rate at the highest temperature. INTRODUCTION The seagrass Zostera marina L. grows in places w h e r e changes in temperature are r a t h e r limited in c o m p a r i s o n t o the habitats o f land plants. This is particularly t h e case for plants growing in the subtidal zone. P o p u l a t i o n s o f t h e intertidal area are m o r e e x p o s e d t o t e m p e r a t u r e f l u c t u a t i o n s . Nevertheless, Setchell ( 1 9 2 9 ) f o u n d an i m p o r t a n t relation b e t w e e n t e m p e r a t u r e and life h i s t o r y even for plants in d e e p e r water. A c c o r d i n g to this a u t h o r , no g r o w t h o c c u r s b e l o w 10°C; vegetative g r o w t h m a i n l y takes place b e t w e e n 10 and 15°C, and generative r e p r o d u c t i o n is restricted t o w a t e r t e m p e r a t u r e s b e t w e e n 15 and 20°C. Generally, at t e m p e r a t u r e s a b o v e 20°C a h e a t rigor sets in, w h i c h is irreversible, and higher t e m p e r a t u r e s cause t h e d e a t h o f t h e plants. He suggested t h a t no p h o t o - p e r i o d i s m existed in this p l a n t and t h a t t e m p e r a t u r e was the o n l y f a c t o r i n d u c i n g flowering (Setchell, 1922). T h o u g h these t e m p e r a t u r e intervals m a y be r o u g h l y valid f o r m o s t subtidal p o p u l a t i o n s , t h e y are n o t so for every p o p u l a t i o n ( C o t t a m and M u n r o , 1 9 5 4 ; Riggs and Fralick, 1 9 7 5 ) , and t h e y are certainly n o t valid f o r a n n u a l
0304-3770/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
126 plants of intertidal areas. In the latter areas, temperatures may fluctuate widely, particularly in summer when sunshine may give day temperatures considerably higher than 20°C, and when relatively low night temperatures may occur. These effects are most obvious during low tide. Annual populations remain viable and flower abundantly under these conditions. The question may be asked as to whether temperature and variations in temperature, as they occur diurnally in the intertidal zone, influence flowering of Zostera marina and if so, in what way. In this study the influence of a constant temperature of 15, 20 and 25°C was examined, as well as the effect of alternating temperatures of 15/20°C and 20/25°C. The temperature of 20°C was chosen as an intermediate temperature because this seems to be a good approximation of the temperature of seawater in summer in the area where the examined plants grew. In this article the term 'inflorescence' is used to describe the flowering unit of spadix, spatha and peduncle, only and the term 'spathe' to denote the spathal sheath and leaf blade. The reasons for this are stated in de Cock (1981a). MATERIALS AND METHODS Flowering shoots of Zostera marina plants were gathered in the intertidal zone near Bergen op Zoom, The Netherlands. Plants of this population are annual and flower abundantly. They are completely submerged at high tide, and during low tide they lie in very shallow pools in which great temperature changes may occur. The handling and selection of two groups of inflorescences at different stages of development for separate examination of flowering of pistils and anthers, was as described in de Cock (1981b). Five h u n d r e d inflorescences of each group were distributed over five full-glass cisterns containing 0.5 1 synthetic seawater (Hw-Meeressalz, H. Wiegandt, Krefeld). The cisterns were covered with glass plates to avoid evaporation. During the experiments all cisterns were kept in constant light as this proved to allow the regular course of development of the flowers, except in the first experimental period of 12 h following a period for adaptation to light conditions (de Cock, 1981b). The light source was a number of fluorescent lamps (Sylvania Powertube, cool white, F96T12/CW/VHO), which gave a light intensity of ca. 11 000 lux at the level of the water surface in the cisterns. Temperature conditions were as follows: (1) constant 15°C; (2) constant 20°C; (3) constant 25°C; (4) alternating 15 and 20°C in periods of 12 h each; (5) alternating 20 and 25°C in periods of 12 h each. Of both series (male and female flowering), one cistern with 100 inflorescences was kept under one of these temperature conditions. Different temperatures were achieved by using water baths. If alternating temperatures were tested and a cistern was transferred from one bath to another, the temperature of the seawater adapted to the new temperature within 15 min.
127
The experiments were started in the evening after a period of 12 h for adaptation to light. The number of flowering inflorescences was counted regularly after periods of 12 h, these countings coinciding with the change of the cisterns concerned to another temperature. An inflorescence was considered to be female flowering if at least one pistil had projected its style. An inflorescence was considered to be male flowering if at least one anther had dehisced. Flowering inflorescences were removed after every counting. Experiments were discontinued when the maximum percentage of flowering inflorescences was reached. All experiments were carried out in duplicate at different times. RESULTS
Flowering of the female flowers The morphological features of flowering were not altered by the different temperature conditions. Flowering showed a regular course at all of the three constant temperatures (Fig. 1). The rate of flowering was about the same at both 15 and 25°C, b u t clearly higher at 20°C. Flowering was suppressed in the first 12-h period at 20°C, even longer at 15°C, and noticeably longer at 25°C. Abnormal flowering was observed at 25°C. Simultaneous flowering of pistils and anthers occurred after 3.5 day at that temperature (Fig. 1). These inflorescences were counted, however, together with the normal, female flowering inflorescences. A few inflorescences started flowering with dehiscence of the anthers. These were not involved in the countings and had to be removed as it could not be determined whether the styles were still immature inside the spatha or retracted again after (self-)pollination. For this 100 %
::: L*
....
so
>~ eo
/"
o
8
,.~'~2~
and 20° I----?
/] ..
20 i 1
2
3
4
5
6
7
Time
8
! 9
(days)
Fig. 1. C u m u l a t i v e p e r c e n t a g e s of f e m a l e flowering infiorescences over t i m e at a c o n s t a n t t e m p e r a t u r e o f 15, 20 a n d 25 ° C, a n d at a l t e r n a t i n g 15 ] 20 ° C.
128 100 -%
. . . - - - -=t . . . . .
~ 80
20~,/°
o (__ -9
//
j
aod
,
d" !
2O
~ 0
i 1
2
3
4
5
i 6
, 7
I 8
Time ( d a y s )
Fig. 2. C u m u l a t i v e percentages of female flowering inflorescences over time at a constant
temperature of 20 and
25 °
C, and at alternating 20/25 ° C.
reason the final percentage of flowering inflorescences did not total 100% at 25°C. The course o f flowering in the cisterns with alternating temperatures was quite 'irregular' (Figs. 1 and 2). Under bot h t e m p e r a t u r e regimes, the n u m b e r of flowering inflorescences was higher in the periods with the lower temperature. This was most obvious if alternating periods of 20 and 25°C were given, but also evident in a 15/20°C regime. The average flowering under both alternating t e m p e r a t u r e regimes was intermediate between the rates at the corresponding constant temperatures. Table I shows the cumulative percentages o f inflorescences flowering in the periods with the same t em perat ure for each o f the cisterns at alternating temperatures. TABLEI Results of the alternating temperature regimes. The cumulative percentage of inflorescenses, flowering in the periods with the same temperature, is given for male and female flowering. Temperature of flowering
15°C 20°C 25°C
Female flowering (%)
Male flowering (%)
15/20°C
15/20°C
66.7 33.3
20/25°C
80 17.5
57.8 42.3
20/25°C
55 45
Flowering of the male flowers After a slight suppression in the first period, flowering was quite regular at both constant and alternating temperatures. Under the constant temperature regimes the flowering rate of the anthers proved to increase with in-
129 100 -% u ~
..:
-~100 -%
-.-~: . . . . . . . . .
~~=-__o___.
J/
q~ ~- 80
80
X
/ .~ 60
20 °
/ ..://¢'~'~" "° /~/.S / ~ .~Alternating
150 (...
)
~ 40
E
;
~\150 I
1
2
I
20
I 3
' 4
'
I
5
'
Time
I
6
'
(days)
~
0
200 (- . . . . . ) )
and 250 (-.
/ • ,'.." ~ 2 0 °
4C
t.)
20
/,'\Alternating .."
/
Y
1
i 2
i
I
3
,
J
4
,
~
5
,
~
6
,
Time (days)
Fig. 3. C u m u l a t i v e p e r c e n t a g e o f m a l e f l o w e r i n g i n f l o r e s c e n c e s o v e r t h e c o u r s e o f t i m e at a c o n s t a n t t e m p e r a t u r e of 1 5 , 2 0 a n d 2 5 ° C , a n d at a l t e r n a t i n g 1 5 / 2 0 ° C . Fig. 4. C u m u l a t i v e p e r c e n t a g e o f m a l e f l o w e r i n g i n f l o r e s c e n c e s in t h e c o u r s e o f t i m e at a t e m p e r a t u r e o f c o n s t a n t 2 0 ° C a n d a l t e r n a t i n g 2 0 / 2 5 ° C . T h e graph o f t h e f l o w e r i n g c o u r s e at c o n s t a n t 2 5 ° C is n o t d r a w n as it c o i n c i d e d w i t h t h e graph o f a l t e r n a t i n g 2 0 / 2 5 ° C .
creasing temperature, the highest flowering rate reached being at 25°C (Fig. 3). Alternating temperatures of 15/20°C or 20/25°C did not result in alternating periods with higher and lower rates of flowering; the total number of flowering inflorescences was somewhat higher in the periods with the lower temperature (Table I). Throughout the period of investigation, the average flowering rate was the same (20/25°C) or nearly the same (15/20°C) as the flowering rate at the higher corresponding constant temperature, thus 25 and 20°C, respectively. DISCUSSION
Flowering of the female flowers The experiments were started after the inflorescences had been allowed to adapt to the light conditions for a 12-h period. The suppression of flowering in the first experimental period must be due to the experimental light conditions (constant light). However, this photo-inhibition is only present during ca. 24 h in constant light at 20°C (de Cock, 1981b). Hence, the longer suppression, in the second experimental period, at constant 15 and 25°C, can be attributed to the influence of temperature. It appears that there is an optimum temperature for female flowering of ca. 20°C. Flowering starts earlier and proceeds faster at 20°C than at either 15 or 25°C. Not only different constant temperatures, but also changes in temperature influence female flowering. It is quite remarkable that a change from 20 to 15°C induces a higher flowering rate, though a temperature of 20°C seems favourable for flowering, if given constantly. It is not clear, however, whether the lower temperature stimulates or the higher temperature suppresses.
130
Flowering of the male flowers In contrast to female flowering, the male flowers did not have an optimum in the investigated temperature range. Flowering increased with increasing temperature. There is no early period of suppression of flowering at one of the three constant temperatures: differences in the graphs are only caused by differences in flowering rate, not by a short-term suppression such as occurs in female flowering. As alternation of temperatures did not lead to differences in flowering rate, the influence of temperature on the anthers seems only to be a stimulation of the development and not a factor in the diurnal course of flowering. This is supported by the phenomenon of simultaneous flowering of pistils and anthers at 25°C in the test for female flowering. This is obviously caused by a suppression in the initial periods and a lower flowering rate of the female flowers at this temperature, as well as the higher rate of development of the anthers.
Temperature effects in the intertidal zone For the populations of Zostera marina in the intertidal zone, the effect of alternating temperatures on flowering is particularly important. Water temperature here is influenced by two interactive factors: day and night on the one hand, and high- and low-tide on the other. The temperature regime depends on the time of the day when flood and ebb occur. Moreover, as light also has a great influence on flowering of male as well as female flowers (de Cock, 1981b), one may not consider the temperature factor alone. It is most likely that the effects of temperature and light combine in some way. In the case of flowering of the female flowers it may be expected that the lower temperature during the night intensifies the effect of the darkness, resulting in a higher flowering rate. Higher temperatures during the day may co-operate with light in suppressing flowering of the female flowers. ACKNOWLEDGEMENTS
The author wishes to thank Prof. Dr. H.F. Linskens for his stimulating interest and Dr. A.F. Croes for critically reading the manuscript. Thanks are also due to Prof. Dr. C. den Hartog for his valuable comments and for correcting the English text. REFERENCES Cottam, C. and Munro, D.A., 1954. Eelgrass status and e n v i r o n m e n t a l relations. J. Wildlife Manag., 18: 449--460. De Cock, A.W.A.M0, 1981a. Development of the f l o w e r i n g s h o o t of Z o s t e r a marina L. under c o n t r o l l e d c o n d i t i o n s in c o m p a r i s o n t o the d e v e l o p m e n t in t w o different natural habitats in The Netherlands. Aquat. Bot., 10: 99--113.
131 De Cock, A.W.A.M., 1981b. Influence of light and dark on flowering in Z o s t e r a marina L. under laboratory conditions. Aquat. Bot., 10: 115--123. Riggs, S.A. and Fralick, R.A., 1975. Z o s t e r a marina L., its growth and distribution in the Great Bay Estuary, New Hampshire. Rhodora, 77: 456--466. Setchell, W.A., 1922. Z o s t e r a marina in its relation to temperature. Science, 56: 575--577. Setchell, W.A., 1929. Morphological and phenological notes on Z o s t e r a marina L. Univ. Calif. Publ. Bot., 14: 389--452.