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Seasonal germination periodicity of imbibed dormant seeds of rape (Brassica napus L.)
Scientia Horticulturae, 42 (1990) 1-7 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
1
Seasonal Germination Periodicity of Imbibed Dormant Seeds of Rape ( B r a s s i c a napus L.) SATORU TOKUMASU and FUMIKA KAKIHARA
College of Agriculture, Ehime University, Matsuyama 790 (Japan) (Accepted for publication 7 July 1989 )
ABSTRACT Tokumasu, S. and Kakihara, F., 1990. Seasonal germination periodicity of imbibed dormant seeds of rape (Brassica napus L. ). Scientia Hortic., 42: 1-7. Freshly harvested rape seeds were sown on moist blotting paper in Petri dishes and left at room temperature. Approximately 45% of the seeds germinated 2 or 3 days after sowing, and the other seeds remained in a dormant state. These dormant seeds continued to germinate sporadically during 5 years until almost all seeds lost their dormancy. Germination behavior was characterized by a high rate of germination in summer and a low rate in winter. Thus, every year had its peak in summer, indicating seasonal periodicity. The appearance of peaks became earlier from September to July as the years proceeded. Germination rate was highly correlated with air temperature: dormant seeds hardly germinated at temperatures below 24°C (maximum) and 20°C (minimum), but showed a rapid increase in germination when the temperatures exceeded 27 ° C (maximum) and 23 ° C (minimum). Greatest activity in germination was exhibited above 30 ° C (maximum) and 26°C (minimum). Keywords: Brassica napus; dormancy; germination; periodicity; rape; seed.
INTRODUCTION
In rape, seed longevity varies from less than 3 years to more than 15 years according to different storage conditions (Carruthers, 1911; Takiguti 1930; Takayanagi, 1980). On the other hand, freshly harvested seeds have dormancy for several months under usual air-dry conditions (Watanabe, 1953; Tokumasu, 1971, 1975; Tokumasu et al., 1981b; Tokumasu and Kato, 1987). The dormancy can be broken or shortened by some treatments such as seed coat removal, alcohol, sulfuric acid, hot water (Takiguti, 1930), light (Takiguti, 1930; Nakamura et al., 1955) and desiccation (Tokumasu, 1971; Tokumasu et al., 1981b). In contrast with this, the dormancy can be prolonged by moistening (Tokumasu et al., 1975, 1981a) or preservation in fruits (Ogawara, 1943; Tokumasu, 1975; Tokumasu et al., 1981b; Tokumasu and Kato, 1987). 0304-4238/90/$03.50
© 1990 Elsevier Science Publishers B.V.
2
S. TOKUMASU AND F. KAKIHARA
Tokumasu (1977) examined the germination behavior of dormant seeds moistened with water in Brassica japonica and B. cernua and found seasonal periodicity in their germination. The present paper reports the results of a similar experiment carried out with the seed of Brassica napus. MATERIALSAND METHODS The experimental material was the same cultivar of rape (Brassica napus L. ssp. oleifera (Metzg.) Sinsk. 'Chusei-natane') that was used in previous experiments (Tokumasu, 1971, 1975; Tokumasu et al., 1975, 1981a,b, 1985; Tokumasu and Kato, 1987). Mature seeds collected from the experimental farm on 2 June 1972 were sown in 100 Petri dishes on the same day. In each Petri dish, an average of 79 seeds was laid on two sheets of blotting paper moistened with water. The dishes were placed on the desk at alternating temperature in the laboratory where steam heating was employed only at daytime in winter. The light was not controlled. The total number of seeds sown was 7933. After sowing, the number of germinated seeds was recorded every day or every 3 days. At that time, the germinated seeds were removed and an adequate amount of water was supplied to the dishes to keep the seeds in an imbibed state at all times. Daily maximum and minimum room temperatures were recorded every time the number of seeds was counted. The experiment was continued for 5 years until all germinable seeds had germinated. Fungal or bacterial disposal was not considered during the experiment. RESULTS On 3-8 June 1972, 3627 seeds (45.7%) germinated and were considered as non-dormant. After this flush of germination, the remaining dormant seeds germinated at irregular intervals during 5 years. In all, 4240 of those seeds had germinated when the experiment was finished. Thus, 66 seeds had not germinated, among which most seeds were softened and fragile, while hard ones (perhaps viable) were few. The 5-year pattern of seed germination is presented in Fig. 1. Germination was generally high in summer and low in winter. In the first year (1972) a small germination peak was noticed in September. In the second year (1973), a low peak appeared in August. In the third year (1974), a fairly high peak was observed in August, and in the fourth year (1975) the highest peak was encountered in July. In the last year (1976) the next highest peak appeared in July. At the end of the fifth year, almost all germinable seeds had finished their germination. Figure 2 shows monthly averages of maximum and minimum room temperatures during the period of the experiment. In every year, temperature was
GERMINATION
PERIODICITY
3
OF RAPE
180
160
I
140
120 ca 100
80 o @
60
40
20
0
6
8
10 12
1972
2
4
6
8
10 12
1973
2
4
6
8
10 12
1974
2
4
6
10 12
1975
Month
of
2
4
6
8
I0
12
1976
2
4
I 6
1977
germination
Fig. 1. Monthly distribution of germination of imbibed seeds ofBrassica napus. °c
32
Max
30 28 .=
26
ca
24
~ ~
20 18 -
6 . Mln
16
, ~ 0
d t ,, , o ~ b, d ,
I4 12
~ t o
~ ,~ ,~ ', o-O o-o"
t ,
,
e.o.O
~,o
o ~ ' ,
,'
~.o
, o
.o-o
o i
o ,
o
o
,
, ?' o'
' ~ 1'0 ;2 ~ ~ ~ ~ 1'0 ;2 ~ ~ ~ ~ lb 1'2 ~ ~ ~ ~ 1'0 1'2 ~ ~ ~ ~ 1'0 1'2 ~' 1972
1973
1974
1975
Month Fig.
2. Monthly
means
of maximum
and
of
minimum
1976
1977
germination room
temperatures
during
5 years.
highest in August. In July, August and September, the averages of m a x i m u m and m i n i m u m temperatures always exceeded 28 ° C and 23 ° C, respectively. Figures 3 and 4 show the relationship b e t w e e n the number of germinated seeds and m a x i m u m and m i n i m u m room temperatures in 1975, respectively. T h e tendency similar to that in 1975 (Figs. 3 and 4) was observed also in 1974 and 1976, though germination patterns in both years were not so typical be-
S. T O K U M A S U
4
A N D F. K A K I H A R A
°C 54 o
32
o
80
o
o
o°o
o oo
o
o
o
o°
o
o
o
o
o
o
o
o
o
30
o o
28 ©
%o°
26
24 =a
22
x N
20
o
ooo ~
o
o
o
o
o
o o
o co oco
o oo
~°o c~cco
o
16
o co
14 1'0
2'0
3'0
4'0
5'0
6'0
7'0
8'0
Number of g e r m i n a t e d
9'0
1;0
110 120
130
seeds
Fig. 3. Relation between the germination of imbibed dormant seeds and maximum room temperatures in Brassica napus.
°C 30
o 0
28
o
0
0
o
o
o
0
0
0
o
O0 o
0
0
o
0
26
o
o
o 0 0
8o o
24
0 O0
o co oo
o
CO
o
0
o
~a
22
0o
2o
o
co co oo o oo o o ° o 8 o
o
o o
ccOo O
%
~o 14
o
12 ¸
@ o Go
10 ;
i'0
2'0
3'0
40
5'0
6'0
7'0
8'0
Number of g e r m i n a t e d
9'0
1;0
I10
120
1;0
140
seeds
Fig. 4. Relation between the germination of imbibed dormant seeds and minimum room temperatures in Brassica napus.
GERMINATION PERIODICITY OF RAPE
5
cause of smaller numbers of germinated seeds than that in 1975. Each dot represents the number of seeds examined every 3 days. Seed germination was low at temperatures below 24 ° C (maximum) and 20 ° C (minimum). The numbers of germinated seeds gradually increased as the temperature increased. When air temperatures exceeded 27°C (maximum) and 23°C (minimum), seed germination showed a rapid increase. When the temperature rose above 30 ° C (maximum) and 26 ° C (minimum), seeds exhibited great activity in germination. High temperatures of more than 27 ° C (maximum) and 23 ° C (minimum) correspond to those in the three months of July, August and September (see Fig. 2 ). DISCUSSION
The months in which germination peaks appeared were September in 1972, August in 1973 and 1974, and July in 1975 and 1976 (Fig. 1 ). Hence seed germination peaks occurred earlier as years proceeded. This phenomenon might be due to the reason discussed below. The number of germinated seeds increased as the years advanced. In the first year almost all seeds were in a deeper state of dormancy, and greater heat accumulation was required to break dormancy. In the second and third years the dormancy of seeds was not as deep as in the preceding year, and germination peaks appeared at the time most favorable for germination, i.e. in August. In the fourth and fifth years seeds became even less dormant and many seeds germinated as soon as temperatures were optimum for germination, i.e. in July. In the fifth year, dormancy of most seeds had been terminated. Thus, after the germination peak in July, seeds remaining in petri dishes became few in number. These conditions caused a rapid decrease in the number of germinated seeds in August and September. The dormancy of seeds in rape is usually lost about 2 or 3 months after harvest under air-dry conditions (Tokumasu, 1971,1975; Tokumasu et al., 1975, 1981b; Tokumasu and Kato, 1987). However, Tokumasu (1971) showed that when freshly harvested seeds were stored under saturated air conditions, this loss of dormancy was prevented. Further, Tokumasu et al. (1975) found that 60-100% relative humidity could result in the prevention of loss of seed dormancy and that this was semi-permanent, although seeds in 90 or 100% humidity died in 5 months or 2.5 months after harvest, respectively. In the present study, when dormant seeds were placed in imbibed condition, the seeds continued to be dormant and were sporadically germinating during 5 years. It is therefore possible to retard dormancy-breaking activities by imbibition with water or equilibration in humid air. Tokumasu et al. (1985) examined the germination of non-dormant rape seeds at 5, 15, 25, 35 and 45°C. The germination was close to 100% at 15, 25 and 35°C, 59% at 45°C and 46% at 5°C. Optimal temperature for germination of non-dormant seeds was, therefore, between 15 and 35 ° C. In the present inves-
6
S. TOKUMASU AND F. KAKIHARA
tigation, g e r m i n a t i o n did n o t o c c u r w h e n m i n i m u m t e m p e r a t u r e s were below 2 0 ° C (Fig. 4). Active g e r m i n a t i o n was o b s e r v e d at t e m p e r a t u r e s h i g h e r t h a n 25 °C (Figs. 3 a n d 4). T h e s e facts suggest t h a t t e m p e r a t u r e s o p t i m a l for t h e b r e a k i n g o f d o r m a n c y in i m b i b e d seeds are h i g h e r t h a n t h o s e o p t i m a l for t h e g e r m i n a t i o n o f n o n - d o r m a n t seeds. T h e u p p e r t e m p e r a t u r e limits w i t h i n which i m b i b e d seeds c a n g e r m i n a t e are u n k n o w n , b e c a u s e t e m p e r a t u r e s h i g h e r t h a n 3 4 ° C were n o t a c t u a l l y e n c o u n t e r e d in t h e p r e s e n t e x p e r i m e n t (Fig. 3 ). T o k u m a s u (1977) r e p o r t e d o n t h e g e r m i n a t i o n b e h a v i o r of'freshly h a r v e s t e d seeds of p o t h e r b m u s t a r d (Brassicajaponica) a n d leaf m u s t a r d (B. cernua) w h i c h were sown in P e t r i dishes, as in t h e p r e s e n t investigation. Seeds were p l a c e d e i t h e r o n t h e desk a t a l t e r n a t i n g r o o m t e m p e r a t u r e or in t h e i n c u b a t o r at t h e c o n s t a n t t e m p e r a t u r e o f 25 ° C. A t r o o m t e m p e r a t u r e , seed g e r m i n a t i o n in b o t h species s h o w e d t h e same s e a s o n a l p e r i o d i c i t y as seen in r a p e in t h e p r e s e n t investigation. A t a c o n s t a n t t e m p e r a t u r e , however, t h e r e was no such periodicity. Seeds were c o n s t a n t l y g e r m i n a t i n g , a n d as a result, t h e y f o r m e d irregularly u n i m o d a l g e r m i n a t i o n curves. T o k u m a s u (1977) c o n c l u d e d t h a t t h e p e r i o d i c i t y in p o t h e r b m u s t a r d a n d leaf m u s t a r d was exogenous, b e i n g aff e c t e d b y e n v i r o n m e n t a l f a c t o r s o f w h i c h t e m p e r a t u r e was t h e m o s t i m p o r t a n t . I n t h e p r e s e n t investigation, e x a m i n a t i o n was n o t c a r r i e d o u t u n d e r c o n s t a n t t e m p e r a t u r e conditions. H o w e v e r , it is s u s p e c t e d t h a t t h e p e r i o d i c i t y o f seed g e r m i n a t i o n in r a p e is also e x o g e n o u s , since high g e r m i n a t i o n was highly corr e l a t e d w i t h high t e m p e r a t u r e (Figs. 3 a n d 4).
REFERENCES Carruthers, W., 1911. On the vitality of farm seeds. J. R. Agric. Soc. Engl., 72: 168-183. Nakamura, S., Okasako, Y. and Yamada, E., 1955. Effect of light on the germination of vegetable seeds. J. Jpn. Soc. Hortic. Sci., 24:17-28 (in Japanese with English abstract). Ogawara, S., 1943. Delayed germination of freshly harvested seeds of the rape plant. Proc. Crop Sci. Soc. Jpn., 14:214-218 (in Japanese). Takayanagi, K., 1980. Seed storage and viability tests. In; S. Tsunoda, K. Hinata and C. GomezCampo (Editors), Brassica Crops and Wild Allies. Japan Scientific Societies Press, Tokyo, pp. 303-354. Takiguti, Y., 1930. On the germination of Brassica NapeUa seed. Sci. Bull. Fac. Agric. Kyushu Imp. Univ., 4:22-36 (in Japanese with English abstract). Tokumasu, S., 1971. Effect of dry and wet storage upon seed dormancy in cruciferous vegetables. J. Jpn. Soc. Hortic. Sci., 40: 23-28. Tokumasu, S., 1975. Prolonged dormancy in the seeds preserved in harvested fruits of Brassica vegetables. Scientia Hortic., 3: 267-273. Tokumasu, S., 1977. Seasonal periodicity of the loss of dormancy of imbibed seeds in Brassica vegetables. Scientia Hortic., 6: 101-106. Tokumasu, S. and Kato, M., 1987. The effect of fruits on the prolongation of seed dormancy and its relation to mustard oil content in cruciferous crops. Acta Hortic., 215: 131-138. Tokumasu, S., Kato, M. and Yano, F., 1975. The dormancy of seed as affected by different hu-
GERMINATIONPERIODICITYOFRAPE
7
midities during storage in Brassica. Jpn. J. Breed., 25:197-202 (in Japanese with English abstract). Tokumasu, S., Kamei, S. and Kato, M., 1981a. Effects of storage humidity and germination temperature on germination percentage of Brassica seeds. Jpn. J. Breed., 31:109-120 (in Japanese with English abstract). Tokumasu, S., Kakihara, F. and Kato, M., 1981b. The change of dormancy of seeds stored in desiccators and/or harvested fruits in cruciferous crops. J. Jpn. Soc. Hortic. Sci., 50:208-214 (in Japanese with English abstract). Tokumasu, S., Kanada, I. and Kato, M., 1985. Germination behavior of seeds as affected by different temperatures in some species ofBrassica. J. Jpn. Soc. Hortic. Sci., 54: 364-370. Watanabe, S., 1953. Studies on dormancy of seed in cruciferous vegetables. Abstract Spring Assembly, Jpn. Soc. Hortic. Sci., p. 10 (in Japanese).
1
Seasonal Germination Periodicity of Imbibed Dormant Seeds of Rape ( B r a s s i c a napus L.) SATORU TOKUMASU and FUMIKA KAKIHARA
College of Agriculture, Ehime University, Matsuyama 790 (Japan) (Accepted for publication 7 July 1989 )
ABSTRACT Tokumasu, S. and Kakihara, F., 1990. Seasonal germination periodicity of imbibed dormant seeds of rape (Brassica napus L. ). Scientia Hortic., 42: 1-7. Freshly harvested rape seeds were sown on moist blotting paper in Petri dishes and left at room temperature. Approximately 45% of the seeds germinated 2 or 3 days after sowing, and the other seeds remained in a dormant state. These dormant seeds continued to germinate sporadically during 5 years until almost all seeds lost their dormancy. Germination behavior was characterized by a high rate of germination in summer and a low rate in winter. Thus, every year had its peak in summer, indicating seasonal periodicity. The appearance of peaks became earlier from September to July as the years proceeded. Germination rate was highly correlated with air temperature: dormant seeds hardly germinated at temperatures below 24°C (maximum) and 20°C (minimum), but showed a rapid increase in germination when the temperatures exceeded 27 ° C (maximum) and 23 ° C (minimum). Greatest activity in germination was exhibited above 30 ° C (maximum) and 26°C (minimum). Keywords: Brassica napus; dormancy; germination; periodicity; rape; seed.
INTRODUCTION
In rape, seed longevity varies from less than 3 years to more than 15 years according to different storage conditions (Carruthers, 1911; Takiguti 1930; Takayanagi, 1980). On the other hand, freshly harvested seeds have dormancy for several months under usual air-dry conditions (Watanabe, 1953; Tokumasu, 1971, 1975; Tokumasu et al., 1981b; Tokumasu and Kato, 1987). The dormancy can be broken or shortened by some treatments such as seed coat removal, alcohol, sulfuric acid, hot water (Takiguti, 1930), light (Takiguti, 1930; Nakamura et al., 1955) and desiccation (Tokumasu, 1971; Tokumasu et al., 1981b). In contrast with this, the dormancy can be prolonged by moistening (Tokumasu et al., 1975, 1981a) or preservation in fruits (Ogawara, 1943; Tokumasu, 1975; Tokumasu et al., 1981b; Tokumasu and Kato, 1987). 0304-4238/90/$03.50
© 1990 Elsevier Science Publishers B.V.
2
S. TOKUMASU AND F. KAKIHARA
Tokumasu (1977) examined the germination behavior of dormant seeds moistened with water in Brassica japonica and B. cernua and found seasonal periodicity in their germination. The present paper reports the results of a similar experiment carried out with the seed of Brassica napus. MATERIALSAND METHODS The experimental material was the same cultivar of rape (Brassica napus L. ssp. oleifera (Metzg.) Sinsk. 'Chusei-natane') that was used in previous experiments (Tokumasu, 1971, 1975; Tokumasu et al., 1975, 1981a,b, 1985; Tokumasu and Kato, 1987). Mature seeds collected from the experimental farm on 2 June 1972 were sown in 100 Petri dishes on the same day. In each Petri dish, an average of 79 seeds was laid on two sheets of blotting paper moistened with water. The dishes were placed on the desk at alternating temperature in the laboratory where steam heating was employed only at daytime in winter. The light was not controlled. The total number of seeds sown was 7933. After sowing, the number of germinated seeds was recorded every day or every 3 days. At that time, the germinated seeds were removed and an adequate amount of water was supplied to the dishes to keep the seeds in an imbibed state at all times. Daily maximum and minimum room temperatures were recorded every time the number of seeds was counted. The experiment was continued for 5 years until all germinable seeds had germinated. Fungal or bacterial disposal was not considered during the experiment. RESULTS On 3-8 June 1972, 3627 seeds (45.7%) germinated and were considered as non-dormant. After this flush of germination, the remaining dormant seeds germinated at irregular intervals during 5 years. In all, 4240 of those seeds had germinated when the experiment was finished. Thus, 66 seeds had not germinated, among which most seeds were softened and fragile, while hard ones (perhaps viable) were few. The 5-year pattern of seed germination is presented in Fig. 1. Germination was generally high in summer and low in winter. In the first year (1972) a small germination peak was noticed in September. In the second year (1973), a low peak appeared in August. In the third year (1974), a fairly high peak was observed in August, and in the fourth year (1975) the highest peak was encountered in July. In the last year (1976) the next highest peak appeared in July. At the end of the fifth year, almost all germinable seeds had finished their germination. Figure 2 shows monthly averages of maximum and minimum room temperatures during the period of the experiment. In every year, temperature was
GERMINATION
PERIODICITY
3
OF RAPE
180
160
I
140
120 ca 100
80 o @
60
40
20
0
6
8
10 12
1972
2
4
6
8
10 12
1973
2
4
6
8
10 12
1974
2
4
6
10 12
1975
Month
of
2
4
6
8
I0
12
1976
2
4
I 6
1977
germination
Fig. 1. Monthly distribution of germination of imbibed seeds ofBrassica napus. °c
32
Max
30 28 .=
26
ca
24
~ ~
20 18 -
6 . Mln
16
, ~ 0
d t ,, , o ~ b, d ,
I4 12
~ t o
~ ,~ ,~ ', o-O o-o"
t ,
,
e.o.O
~,o
o ~ ' ,
,'
~.o
, o
.o-o
o i
o ,
o
o
,
, ?' o'
' ~ 1'0 ;2 ~ ~ ~ ~ 1'0 ;2 ~ ~ ~ ~ lb 1'2 ~ ~ ~ ~ 1'0 1'2 ~ ~ ~ ~ 1'0 1'2 ~' 1972
1973
1974
1975
Month Fig.
2. Monthly
means
of maximum
and
of
minimum
1976
1977
germination room
temperatures
during
5 years.
highest in August. In July, August and September, the averages of m a x i m u m and m i n i m u m temperatures always exceeded 28 ° C and 23 ° C, respectively. Figures 3 and 4 show the relationship b e t w e e n the number of germinated seeds and m a x i m u m and m i n i m u m room temperatures in 1975, respectively. T h e tendency similar to that in 1975 (Figs. 3 and 4) was observed also in 1974 and 1976, though germination patterns in both years were not so typical be-
S. T O K U M A S U
4
A N D F. K A K I H A R A
°C 54 o
32
o
80
o
o
o°o
o oo
o
o
o
o°
o
o
o
o
o
o
o
o
o
30
o o
28 ©
%o°
26
24 =a
22
x N
20
o
ooo ~
o
o
o
o
o
o o
o co oco
o oo
~°o c~cco
o
16
o co
14 1'0
2'0
3'0
4'0
5'0
6'0
7'0
8'0
Number of g e r m i n a t e d
9'0
1;0
110 120
130
seeds
Fig. 3. Relation between the germination of imbibed dormant seeds and maximum room temperatures in Brassica napus.
°C 30
o 0
28
o
0
0
o
o
o
0
0
0
o
O0 o
0
0
o
0
26
o
o
o 0 0
8o o
24
0 O0
o co oo
o
CO
o
0
o
~a
22
0o
2o
o
co co oo o oo o o ° o 8 o
o
o o
ccOo O
%
~o 14
o
12 ¸
@ o Go
10 ;
i'0
2'0
3'0
40
5'0
6'0
7'0
8'0
Number of g e r m i n a t e d
9'0
1;0
I10
120
1;0
140
seeds
Fig. 4. Relation between the germination of imbibed dormant seeds and minimum room temperatures in Brassica napus.
GERMINATION PERIODICITY OF RAPE
5
cause of smaller numbers of germinated seeds than that in 1975. Each dot represents the number of seeds examined every 3 days. Seed germination was low at temperatures below 24 ° C (maximum) and 20 ° C (minimum). The numbers of germinated seeds gradually increased as the temperature increased. When air temperatures exceeded 27°C (maximum) and 23°C (minimum), seed germination showed a rapid increase. When the temperature rose above 30 ° C (maximum) and 26 ° C (minimum), seeds exhibited great activity in germination. High temperatures of more than 27 ° C (maximum) and 23 ° C (minimum) correspond to those in the three months of July, August and September (see Fig. 2 ). DISCUSSION
The months in which germination peaks appeared were September in 1972, August in 1973 and 1974, and July in 1975 and 1976 (Fig. 1 ). Hence seed germination peaks occurred earlier as years proceeded. This phenomenon might be due to the reason discussed below. The number of germinated seeds increased as the years advanced. In the first year almost all seeds were in a deeper state of dormancy, and greater heat accumulation was required to break dormancy. In the second and third years the dormancy of seeds was not as deep as in the preceding year, and germination peaks appeared at the time most favorable for germination, i.e. in August. In the fourth and fifth years seeds became even less dormant and many seeds germinated as soon as temperatures were optimum for germination, i.e. in July. In the fifth year, dormancy of most seeds had been terminated. Thus, after the germination peak in July, seeds remaining in petri dishes became few in number. These conditions caused a rapid decrease in the number of germinated seeds in August and September. The dormancy of seeds in rape is usually lost about 2 or 3 months after harvest under air-dry conditions (Tokumasu, 1971,1975; Tokumasu et al., 1975, 1981b; Tokumasu and Kato, 1987). However, Tokumasu (1971) showed that when freshly harvested seeds were stored under saturated air conditions, this loss of dormancy was prevented. Further, Tokumasu et al. (1975) found that 60-100% relative humidity could result in the prevention of loss of seed dormancy and that this was semi-permanent, although seeds in 90 or 100% humidity died in 5 months or 2.5 months after harvest, respectively. In the present study, when dormant seeds were placed in imbibed condition, the seeds continued to be dormant and were sporadically germinating during 5 years. It is therefore possible to retard dormancy-breaking activities by imbibition with water or equilibration in humid air. Tokumasu et al. (1985) examined the germination of non-dormant rape seeds at 5, 15, 25, 35 and 45°C. The germination was close to 100% at 15, 25 and 35°C, 59% at 45°C and 46% at 5°C. Optimal temperature for germination of non-dormant seeds was, therefore, between 15 and 35 ° C. In the present inves-
6
S. TOKUMASU AND F. KAKIHARA
tigation, g e r m i n a t i o n did n o t o c c u r w h e n m i n i m u m t e m p e r a t u r e s were below 2 0 ° C (Fig. 4). Active g e r m i n a t i o n was o b s e r v e d at t e m p e r a t u r e s h i g h e r t h a n 25 °C (Figs. 3 a n d 4). T h e s e facts suggest t h a t t e m p e r a t u r e s o p t i m a l for t h e b r e a k i n g o f d o r m a n c y in i m b i b e d seeds are h i g h e r t h a n t h o s e o p t i m a l for t h e g e r m i n a t i o n o f n o n - d o r m a n t seeds. T h e u p p e r t e m p e r a t u r e limits w i t h i n which i m b i b e d seeds c a n g e r m i n a t e are u n k n o w n , b e c a u s e t e m p e r a t u r e s h i g h e r t h a n 3 4 ° C were n o t a c t u a l l y e n c o u n t e r e d in t h e p r e s e n t e x p e r i m e n t (Fig. 3 ). T o k u m a s u (1977) r e p o r t e d o n t h e g e r m i n a t i o n b e h a v i o r of'freshly h a r v e s t e d seeds of p o t h e r b m u s t a r d (Brassicajaponica) a n d leaf m u s t a r d (B. cernua) w h i c h were sown in P e t r i dishes, as in t h e p r e s e n t investigation. Seeds were p l a c e d e i t h e r o n t h e desk a t a l t e r n a t i n g r o o m t e m p e r a t u r e or in t h e i n c u b a t o r at t h e c o n s t a n t t e m p e r a t u r e o f 25 ° C. A t r o o m t e m p e r a t u r e , seed g e r m i n a t i o n in b o t h species s h o w e d t h e same s e a s o n a l p e r i o d i c i t y as seen in r a p e in t h e p r e s e n t investigation. A t a c o n s t a n t t e m p e r a t u r e , however, t h e r e was no such periodicity. Seeds were c o n s t a n t l y g e r m i n a t i n g , a n d as a result, t h e y f o r m e d irregularly u n i m o d a l g e r m i n a t i o n curves. T o k u m a s u (1977) c o n c l u d e d t h a t t h e p e r i o d i c i t y in p o t h e r b m u s t a r d a n d leaf m u s t a r d was exogenous, b e i n g aff e c t e d b y e n v i r o n m e n t a l f a c t o r s o f w h i c h t e m p e r a t u r e was t h e m o s t i m p o r t a n t . I n t h e p r e s e n t investigation, e x a m i n a t i o n was n o t c a r r i e d o u t u n d e r c o n s t a n t t e m p e r a t u r e conditions. H o w e v e r , it is s u s p e c t e d t h a t t h e p e r i o d i c i t y o f seed g e r m i n a t i o n in r a p e is also e x o g e n o u s , since high g e r m i n a t i o n was highly corr e l a t e d w i t h high t e m p e r a t u r e (Figs. 3 a n d 4).
REFERENCES Carruthers, W., 1911. On the vitality of farm seeds. J. R. Agric. Soc. Engl., 72: 168-183. Nakamura, S., Okasako, Y. and Yamada, E., 1955. Effect of light on the germination of vegetable seeds. J. Jpn. Soc. Hortic. Sci., 24:17-28 (in Japanese with English abstract). Ogawara, S., 1943. Delayed germination of freshly harvested seeds of the rape plant. Proc. Crop Sci. Soc. Jpn., 14:214-218 (in Japanese). Takayanagi, K., 1980. Seed storage and viability tests. In; S. Tsunoda, K. Hinata and C. GomezCampo (Editors), Brassica Crops and Wild Allies. Japan Scientific Societies Press, Tokyo, pp. 303-354. Takiguti, Y., 1930. On the germination of Brassica NapeUa seed. Sci. Bull. Fac. Agric. Kyushu Imp. Univ., 4:22-36 (in Japanese with English abstract). Tokumasu, S., 1971. Effect of dry and wet storage upon seed dormancy in cruciferous vegetables. J. Jpn. Soc. Hortic. Sci., 40: 23-28. Tokumasu, S., 1975. Prolonged dormancy in the seeds preserved in harvested fruits of Brassica vegetables. Scientia Hortic., 3: 267-273. Tokumasu, S., 1977. Seasonal periodicity of the loss of dormancy of imbibed seeds in Brassica vegetables. Scientia Hortic., 6: 101-106. Tokumasu, S. and Kato, M., 1987. The effect of fruits on the prolongation of seed dormancy and its relation to mustard oil content in cruciferous crops. Acta Hortic., 215: 131-138. Tokumasu, S., Kato, M. and Yano, F., 1975. The dormancy of seed as affected by different hu-
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midities during storage in Brassica. Jpn. J. Breed., 25:197-202 (in Japanese with English abstract). Tokumasu, S., Kamei, S. and Kato, M., 1981a. Effects of storage humidity and germination temperature on germination percentage of Brassica seeds. Jpn. J. Breed., 31:109-120 (in Japanese with English abstract). Tokumasu, S., Kakihara, F. and Kato, M., 1981b. The change of dormancy of seeds stored in desiccators and/or harvested fruits in cruciferous crops. J. Jpn. Soc. Hortic. Sci., 50:208-214 (in Japanese with English abstract). Tokumasu, S., Kanada, I. and Kato, M., 1985. Germination behavior of seeds as affected by different temperatures in some species ofBrassica. J. Jpn. Soc. Hortic. Sci., 54: 364-370. Watanabe, S., 1953. Studies on dormancy of seed in cruciferous vegetables. Abstract Spring Assembly, Jpn. Soc. Hortic. Sci., p. 10 (in Japanese).