Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol84.Issue3-4.June2000:
Scientia Horticulturae 84 (2000) 255±264
Production of seedless watermelon using
soft-X-irradiated pollen
Keita Sugiyama*, Masami Morishita
Kurume Branch, National Research Institute of Vegetables, Ornamental Plants and Tea,
Kurume, Fukuoka 839-8503, Japan
Accepted 13 August 1999
Abstract
A new method for producing seedless watermelon (Citrullus lanatus) in diploid plants using softX-irradiated pollen is described. Fruit set at almost the same rate despite soft-X-irradiation. Empty
seeds were produced in the watermelon cultivars `Benikodama' and `Fujihikari TR' following hand
pollination using soft-X-irradiated pollen. Soft-X-irradiation doses of 800±1000 Gy resulted in
small empty seeds in `Fujihikari TR', whereas 400±1000 Gy doses gave the best results for
`Benikodama', although empty seeds in the latter were many and conspicuous. Soft-X-ray
treatments did not affect fruit weight, shape, rind thickness or days to maturation compared to
controls. However, female ¯owers treated with soft-X-irradiated pollen produced fruits with slightly
higher sugar content compared to controls in `Benikodama'. Therefore, this new method is an
effective technique to produce seedless watermelon. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Citrullus lanatus; Parthenocarpy; Pseudogamy
1. Introduction
Currently a majority of seedless watermelon are produced by triploid plants.
Seeds of triploid hybrids are obtained from crossing a diploid male with an
autotetraploid female induced by colchicine treatment (Terada and Masuda, 1943;
Kihara and Nishiyama, 1947; Kihara, 1958) or spontaneous chromosome
*
Corresponding author. Tel.: 81-942-43-8271; fax: 81-942-43-7014.
E-mail address: [email protected] (K. Sugiyama).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 0 4 - 1
256
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
doubling during in vitro culture (Compton et al., 1996). However, the breeding of
triploid watermelons requires more time than breeding diploid cultivars, and in
Japan the cultivation of the triploid plants is dif®cult.
Seedless watermelon fruits have also been produced using plant growth
regulators, e.g. IAA (Terada and Masuda, 1940), NAA (Wong, 1938; Terada and
Masuda, 1941), NAA, 4-CPA, GA3 (Kondou and Murozono, 1975), BA
(Yamamuro, 1978) or CPPU (Hayata et al., 1995). Using plant growth regulators
has occasionally resulted in deformed fruits (Kondou and Murozono, 1975)
because the female ¯owers (ovary) were injured by directly spraying or rubbing
growth regulators on them. This method also poses food safety problems, and
therefore, has not been used.
Seedless watermelons may also be produced by inducing reciprocal
translocations of chromosomes (Nishimura and Sakaguchi, 1960; Oka et al.,
1967; Shimotuma, 1968). However, this method is not utilized because the
development of chromosome translocation lines and commercial cultivars based
on these lines is dif®cult, and fruits are not completely seedless.
Triploid watermelon is not distributed internationally due to high seed cost and
dif®culties associated with developing new cultivars. Therefore, it is desirable to
produce seedless watermelon from commercially acceptable diploid cultivars,
using a scienti®c method that is chemically safe. The purpose of this study was to
investigate the practicality of inducing seedlessness in two Japanese watermelon
cultivars by pollinating female ¯owers using pollen treated with varying doses of
Ê ).
soft-X-ray (X-rays which have a wave length of about 0.1 A
2. Materials and methods
In 1997: Watermelon cultivars (F1 hybrid) of `Fujihikari TR' and `Benikodama' were sown on 4 April 1997. The seedlings were transplanted 50 cm apart in a
bed (2.3 m 35 m) in a greenhouse on 8th May. The bed was covered with black
polyethylene mulch. Fertilization involved a pre-plant broadcast application of 10
N±10 P±10 K (kg haÿ1).
Plants were topped at the ®ve-leaf stage, and three lateral vines were allowed to
grow. Treatment commenced on ¯owers at about the 15th node of the lateral
branch. Vines were topped at the 25th node. Treatments were performed between
31 May and 7 June.
Male ¯owers were harvested at random from all plants in the morning during
anthesis. Pollen was retained in the male ¯owers, and their petals and sepals were
not removed. A soft-X-ray machine (Soft-X-ray Unit OM-60R, OHMIC Ltd.)
was set up in the greenhouse. The prepared male ¯owers were put into a plastic
case with holes and a dose of soft-X-irradiated at 11.1 Gy minÿ1. Whole ¯owers
were irradiated with a dose of 0 (unirradiated, as the control), 100, 200, 400, 800
or 1000 Gy.
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
257
Fig. 1. Normal seed and partially developed seeds: (a) normal seed, (b) and (c) empty seed (c) was
not categorized as empty seed because it was very small and thin). Vertical bar 2 mm.
Female ¯owers were covered with cellophane bags before anthesis. Upon
anthesis, they were arti®cially self-pollinated with treated pollen on the same day
and then re-covered to prevent contact with insect-borne pollen for 3±5 days. One
male ¯ower was used for three female ¯owers. Mature fruits were harvested at
about 35 days in `Benikodama' and 38 days after pollination in `Fujihikari TR'.
Immediately after harvesting, fruit weight, fruit shape, rind thickness, sugar
content (soluble solid contents) and seed normality (maturity or emptiness) were
recorded for each fruit. Fruit shape index was expressed as the ratio of height to
width. Empty seeds with a hard coat were counted, but those with a thin coat were
not (Fig. 1).
The six treatments were arranged in a randomized complete-block design with
®ve single-plant replications. Five fruits were harvested for each treatment. Data
were subjected to analysis of variance and mean separation according to
Duncan's multiple range test using p 0.05 on statistical software.
In 1998: `Fujihikari TR' and `Benikodama' were sown on 13 November 1997
and 26 February 1998, respectively. The seedlings of the former were
transplanted 50 cm apart in two beds in a greenhouse on 23 January and of the
latter were done on 24 March 1998.
The blooming male ¯owers were irradiated with 800 Gy soft-X-ray and
immediately used to pollinate each watermelon plant. Unirradiated pollen was
used as the control. Treatments were performed between 20 February and 1
March in `Fujihikari TR', and in `Benikodama', treatments were done between 18
and 29 March. One week after treatment, fruit set was recorded. Mature fruits
were harvested at about 36 days in `Benikodama' 36 and 48 days after pollination
258
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
in `Fujihikari TR'. Methods of cultivation and pollination were the same as in
1997.
The treatments were arranged in a randomized block design with two
replications. Twenty plants in `Fujihikari TR' and ten plants in `Benikodama'
were used. Data were expressed as means SE. Mean differences between
treatments and control were determined by t test.
3. Results and discussion
3.1. Relationship between soft-X-ray irradiation dose to pollen and number of
seeds
There were no observable differences in fruit setting between soft-X-irradiated
pollen and unirradiated pollen in both `Benikodama' and `Fujihikari TR' (Table
1). We have already observed that the germination rate of pollen treated with
800 Gy of soft-X-ray was almost the same as that of the control (Sugiyama and
Morishita, 1998). These results indicate that irradiated pollen presents no
practical problem for producing seedless watermelon fruit.
The number of normal seeds was signi®cantly reduced when female ¯owers
were pollinated with irradiated pollen. In `Benikodama', normal seeds were not
observed at 400±1000 Gy (Fig. 2). 200±400 empty seeds per watermelon were
observed at 100±1000 Gy. In `Fujihikari TR' irradiated with the 800±1000 Gy,
empty seeds alone were produced (Fig. 3). The numbers of empty seeds
were approximately 50±150 at 800±1000 Gy. In `Fujihikari TR' fruits produce
with soft-X-irradiated pollen, most empty seeds were as much small and thin
(Fig. 4). However, they were very conspicuous in `Benikodama'. Varietal
differences in number of empty seeds were observed in triploid watermelon
(Kihara, 1958). In our study, `Benikodama' had more empty seeds than
`Fujihikari TR'(Table 2, Fig. 2) which indicates that the number of empty seeds
is determined by the characteristics of the ovule in each cultivar.
Producing seedless fruits of watermelon by hormonal agents is a parthenocarpic phenomenon in which fertilization is not needed for growth. In order for
Table 1
Effect of soft-X-irradiation on fruit set of watermelon
Cultivar
X-ray
dose (Gy)
Treated flowers
(number)
Fruit set
(%)
Benikodama
0
800
51
46
39.2
43.5
Fujihikari TR
0
800
54
60
64.8
66.7
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
259
Fig. 2. Relationship between dose of soft-X-irradiation and number of seeds in cultivar
`Benikodama'. Vertical bars indicate SE.
Fig. 3. Relationship between dose of soft-X-irradiation and number of seeds in cultivar `Fujihikari
TR'. Vertical bars indicate SE.
260
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Fig. 4. Watermelons fruits after pollination by soft-X-irradiation pollen compared to a normal fruit
in `Benikodama' (upper photograph) and `Fujihikari TR' (lower photograph).
parthenocarpically developed watermelon to reach normal size, pollination and
pseudogamy are necessary. Generally, after pollination, growth of the ovary is
induced by auxin derived from pollen grain and by auxin induced by stimulation
from the pollen tube in the pistil. Further ovary development is promoted by
auxin derived from the developing embryo after fertilization (Saitou, 1974).
However, a watermelon fruit which is produced only by hormonal treatment is
smaller than that produced by pollination (Kondou and Murozono, 1975;
Yamamuro, 1978; Hayata et al., 1995). Triploid watermelons produced by
parthenocarpy develop to full size because parthenocarpy is induced by growth
hormones provided by diploid pollen grains and provided from the ovule by
261
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Table 2
Effect of soft-X-irradiation on fruit characteristics (1997)
Cultivar
X-ray
dose (Gy)
Fruit
weight (kg)
Fruit
shapea
Benikodama
0
100
200
400c
800c
1000c
2.6ab
2.4ab
1.9b
2.0b
1.7b
2.3ab
1.14ab
1.14ab
1.08b
1.14ab
1.16a
1.17a
Fujihikari TR
0
100
200
400
800c
1000c
3.8a
4.0a
4.8a
4.8a
4.1a
3.8a
1.05a
1.05a
1.07a
1.07a
1.09a
1.05a
Thickness of
rind (cm)
Sugar
content (%)
Maturation
(days)
6.8a
6.2a
6.4a
6.2a
6.4a
6.7a
11.8b
12.2b
12.0b
12.5ab
12.5ab
13.1a
36.0
34.6
34.5
35.6
36.8
35.3
9.9b
11.6a
11.4ab
11.3ab
10.1b
11.0ab
10.7b
11.4ab
11.0b
10.6b
11.2ab
12.2a
36.8
37.5
37.3
36.5
37.4
37.3
a
Fruit shape is expressed as the ratio of height to width.
Mean separation within columns by Duncan's multiple range test, at 5% level.
c
Seedless watermelon fruit.
b
pseudogamy. Thus endogenous auxins derived from the ovule are needed to
develop normal size fruit.
Soft-X-irradiated pollen germinate on a stigma, and their tubes elongate into an
embryo sac. Subsequent abortion of embryos after pollination by soft-Xirradiated pollen is still under investigation, but may result from soft-X-rayinduced chromosomal abnormalities in the generative nucleus which prevent
normal fertilization from occurring. However, Sari et al. (1994) reported that
haploid embryos are obtained through pollination with g-irradiated pollen, which
indicates that embryo formation occurs as a pseudogametic phenomenon.
Therefore, it is possible for an ovary growing after pollination with soft-Xirradiated pollen to be promoted by auxin derived from the ovule.
Fruit characteristics: In `Benikodama', fruit weight in control averaged 2.6 kg,
slightly heavier than in irradiated groups in 1997 (Table 2). However, in 1998,
`Benikodama' fruit weight in irradiation treatment was similar to that of control
(Table 3). In `Fujihikari TR', there was no signi®cant difference between fruit
weight in irradiated groups and that in controls. Thus, a relationship between the
fruit weight and the irradiation dose was not recognized. `Benikodama' fruits
were elliptical in most cases except for those treated with 200 Gy (1997 group).
The shape of `Fujihikari TR' fruits were spherical at all doses of irradiation. In
`Benikodama' fruits, there was no signi®cant in rind thickness between irradiated
groups and controls; in `Fujihikari TR' fruit rind in irradiated groups except the
800 Gy (1997 group) was also similar to that in controls. We found no consistent
relationship between dose of soft-X-irradiation and rind thickness. `Benikodama'
262
Cultivar
X-ray
dose (Gy)
Number of
normal seed
Number of
empty seed
Fruit weight
(kg)a
Fruit shapea,b
Thickness of
rind (cm)a
Sugar content
(%)a
Maturation
(days)
Benikodama
0
800
173
0
51
198
1.5 0.18
1.6 0.16
NSc
1.11 0.02
1.12 0.02
NSc
5.5 0.14
5.4 0.35
NSc
11.0 0.40
12.5 0.35
NSc
35.8
36.0
NSc
0
800
201
0
44
149
6.2 0.23
6.4 0.27
NSc
1.03 0.13
1.03 0.02
NSc
13.4 0.60
13.0 0.58
NSc
10.8 0.23
11.3 0.24
NSc
48.0
48.0
NSc
Significanced
Fujihikari TR
Significanced
a
Data are means standard errors of 10-fruits in `Benikodama', 20-fruits in `Fujihikari TR', respectively.
Fruit shape is expressed as the ratio of height to width.
c
NS: non-signi®cant.
d
Signi®cant between control and irradiation at P < 0.05.
b
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Table 3
Comparison of seedless watermelon fruits produced by soft-X-irradiated pollens and control (1998)
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
263
fruits from irradiation treatment showed similar or higher sugar content than in
controls. In `Fujihikari TR', there was no signi®cant difference in sugar content
of irradiated groups except the 1000 Gy (1997 group) and that of controls. All
fruits of both `Benikodama' and `Fujihikari TR' matured within almost the same
period regardless of soft-X-irradiation.
Seedless watermelon fruits induced by hormonal agents had a tendency to be
small and deformed compared with normal fruits (Hayata et al., 1995; Kondou
and Murozono, 1975). In contrast, seedless watermelon fruits produced by softX-irradiated pollen were normal in size and shape. Previous studies (Kihara,
1951; Kondou and Murozono, 1975) on the production of parthenocarpic seedless
watermelon using triploid or plant growth regulators showed a tendency to form a
thick rind. In the present experiment, there was no relationship between rind
thickness and soft-X-irradiation. In our study, the sugar content of seedless
watermelon fruits produced by soft-X-irradiated pollen was similar to or higher
than that of controls. These results show that soft-X-irradiation did not affect fruit
weight, shape or rind thickness, and that the quality was equal to or better than
that of controls.
In Japan, the main reason triploid watermelons are not popular is that triploids
have a longer growing period and a later maturation date than diploid ones.
Occasionally, there have been some developmental defects such as hollow hearts
in triploid watermelon fruits (Kihara, 1951, 1958). In contrast, in our experiment,
these defects were not present in seedless watermelon produced by pollination
with soft-X-irradiated pollen.
In conclusion, seedless watermelon produced by pollination with soft-Xirradiated pollen can be produced by ordinary applied cultivation due to the use of
common diploids. Therefore, this new technique established in our study has
practical value and should be used to produce a good seedless watermelon fruit.
Acknowledgements
The authors are thankful to Prof. T. Harada of Hokkaido University and Prof.
H. Inden of Miyazaki University for valuable suggestions and critical reading of
the manuscript.
References
Compton, M.E., Gray, D.J., Elmstrom, G.W., 1996. Identi®cation of tetraploid regenerants from
cotyledons of diploid watermelon cultured in vitro. Euphytica 87, 165±172.
Hayata, Y., Niimi, Y., Iwasaki, N., 1995. Synthetic Cytokinin-1-(2-chloro-4-pyridyl)-3-phenylurea
(CPPU) Ð promotes fruit set and induces parthenocarpy in watermelon. J. Am. Soc. Hort. Sci.
120, 997±1000.
264
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Kihara, H., 1951. Triploid watermelon. Proc. Am. Soc. Hort. Sci. 58, 217±230.
Kihara, H., 1958. Breeding of seedless fruits. Seiken Ziho 9, 1±7.
Kihara, H., Nishiyama, I., 1947. An application of sterility of autotriploids to the breeding of
seedless watermelons. Seiken Ziho 3, 93±103 (in Japanese with English summary).
Kondou, Y., Murozono, M., 1975. Study of arti®cial fruit setting watermelon. Abstr. Jpn. Soc. Hort.
Sci. Autumn Meeting 44, 178±179 (in Japanese).
Nishimura, Y., Sakaguchi, S., 1960. Studies on reciprocal translocations of chromosomes in
watermelon. Bull. Fac. Agric. Niigata Univ. 12, 22±29.
Oka, H., Watanabe, T., Nishiyama, I., 1967. Reciprocal translocation as a new approach to breeding
seedless watermelon. Can. J. Genetics Cytol. 9, 482±489.
Saitou, T., 1974. Physiology and ecology of fruit set in cucumber. In: The System of Agricultural
Technology, Cucumber. Soc. Cult. Agr. VILL. Press, Tokyo, p. 125±126 (in Japanese).
Sari, N., Abak, K., Pitrat, M., Rode, J.C., Dumas de Vaulx, R., 1994. Induction of parthenogenetic
haploide embryos after pollination by irradiation pollen in watermelon. Hort. Sci. 29, 1189±
1190.
Shimotuma, M., 1968. Synthesis of some multiple interchange strains of watermelons induced by
X-rays. Seiken Ziho 20, 47±53.
Sugiyama, K., Morishita, M., 1998. Storage ability of soft-X-ray irradiated-pollens and effects of
storage pollens on fruit characteristics in watermelon. J. Jpn. Soc. Hort. Sci. 67 (suppl. 2), 283
(in Japanese).
Terada, J., Masuda, K., 1940. Parthenocarpy of watermelon by heteroauxin. Agric. Hort. 15, 458±
468 (in Japanese).
Terada, J., Masuda, K., 1941. Parthenocarpy of watermelon by single or complex application of
plant hormones. Agric. Hort. 16, 1915±1917 (in Japanese).
Terada, J., Masuda, K., 1943. Parthenocarpy of triploid watermelon. Agric. Hort. 18, 15±16 (in
Japanese).
Wong, C.Y., 1938. Induced parthenocarpy of watermelon cucumber and pepper by the use of
growth promoting substances. Proc. Am. Soc. Hort. Sci. 36, 632±636.
Yamamuro, K., 1978. Effect of growth regulators on fruit setting of watermelon. Bull. Ibaraki Hort.
Expt. Sta. 7, 1±15.
Production of seedless watermelon using
soft-X-irradiated pollen
Keita Sugiyama*, Masami Morishita
Kurume Branch, National Research Institute of Vegetables, Ornamental Plants and Tea,
Kurume, Fukuoka 839-8503, Japan
Accepted 13 August 1999
Abstract
A new method for producing seedless watermelon (Citrullus lanatus) in diploid plants using softX-irradiated pollen is described. Fruit set at almost the same rate despite soft-X-irradiation. Empty
seeds were produced in the watermelon cultivars `Benikodama' and `Fujihikari TR' following hand
pollination using soft-X-irradiated pollen. Soft-X-irradiation doses of 800±1000 Gy resulted in
small empty seeds in `Fujihikari TR', whereas 400±1000 Gy doses gave the best results for
`Benikodama', although empty seeds in the latter were many and conspicuous. Soft-X-ray
treatments did not affect fruit weight, shape, rind thickness or days to maturation compared to
controls. However, female ¯owers treated with soft-X-irradiated pollen produced fruits with slightly
higher sugar content compared to controls in `Benikodama'. Therefore, this new method is an
effective technique to produce seedless watermelon. # 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Citrullus lanatus; Parthenocarpy; Pseudogamy
1. Introduction
Currently a majority of seedless watermelon are produced by triploid plants.
Seeds of triploid hybrids are obtained from crossing a diploid male with an
autotetraploid female induced by colchicine treatment (Terada and Masuda, 1943;
Kihara and Nishiyama, 1947; Kihara, 1958) or spontaneous chromosome
*
Corresponding author. Tel.: 81-942-43-8271; fax: 81-942-43-7014.
E-mail address: [email protected] (K. Sugiyama).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 0 4 - 1
256
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
doubling during in vitro culture (Compton et al., 1996). However, the breeding of
triploid watermelons requires more time than breeding diploid cultivars, and in
Japan the cultivation of the triploid plants is dif®cult.
Seedless watermelon fruits have also been produced using plant growth
regulators, e.g. IAA (Terada and Masuda, 1940), NAA (Wong, 1938; Terada and
Masuda, 1941), NAA, 4-CPA, GA3 (Kondou and Murozono, 1975), BA
(Yamamuro, 1978) or CPPU (Hayata et al., 1995). Using plant growth regulators
has occasionally resulted in deformed fruits (Kondou and Murozono, 1975)
because the female ¯owers (ovary) were injured by directly spraying or rubbing
growth regulators on them. This method also poses food safety problems, and
therefore, has not been used.
Seedless watermelons may also be produced by inducing reciprocal
translocations of chromosomes (Nishimura and Sakaguchi, 1960; Oka et al.,
1967; Shimotuma, 1968). However, this method is not utilized because the
development of chromosome translocation lines and commercial cultivars based
on these lines is dif®cult, and fruits are not completely seedless.
Triploid watermelon is not distributed internationally due to high seed cost and
dif®culties associated with developing new cultivars. Therefore, it is desirable to
produce seedless watermelon from commercially acceptable diploid cultivars,
using a scienti®c method that is chemically safe. The purpose of this study was to
investigate the practicality of inducing seedlessness in two Japanese watermelon
cultivars by pollinating female ¯owers using pollen treated with varying doses of
Ê ).
soft-X-ray (X-rays which have a wave length of about 0.1 A
2. Materials and methods
In 1997: Watermelon cultivars (F1 hybrid) of `Fujihikari TR' and `Benikodama' were sown on 4 April 1997. The seedlings were transplanted 50 cm apart in a
bed (2.3 m 35 m) in a greenhouse on 8th May. The bed was covered with black
polyethylene mulch. Fertilization involved a pre-plant broadcast application of 10
N±10 P±10 K (kg haÿ1).
Plants were topped at the ®ve-leaf stage, and three lateral vines were allowed to
grow. Treatment commenced on ¯owers at about the 15th node of the lateral
branch. Vines were topped at the 25th node. Treatments were performed between
31 May and 7 June.
Male ¯owers were harvested at random from all plants in the morning during
anthesis. Pollen was retained in the male ¯owers, and their petals and sepals were
not removed. A soft-X-ray machine (Soft-X-ray Unit OM-60R, OHMIC Ltd.)
was set up in the greenhouse. The prepared male ¯owers were put into a plastic
case with holes and a dose of soft-X-irradiated at 11.1 Gy minÿ1. Whole ¯owers
were irradiated with a dose of 0 (unirradiated, as the control), 100, 200, 400, 800
or 1000 Gy.
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
257
Fig. 1. Normal seed and partially developed seeds: (a) normal seed, (b) and (c) empty seed (c) was
not categorized as empty seed because it was very small and thin). Vertical bar 2 mm.
Female ¯owers were covered with cellophane bags before anthesis. Upon
anthesis, they were arti®cially self-pollinated with treated pollen on the same day
and then re-covered to prevent contact with insect-borne pollen for 3±5 days. One
male ¯ower was used for three female ¯owers. Mature fruits were harvested at
about 35 days in `Benikodama' and 38 days after pollination in `Fujihikari TR'.
Immediately after harvesting, fruit weight, fruit shape, rind thickness, sugar
content (soluble solid contents) and seed normality (maturity or emptiness) were
recorded for each fruit. Fruit shape index was expressed as the ratio of height to
width. Empty seeds with a hard coat were counted, but those with a thin coat were
not (Fig. 1).
The six treatments were arranged in a randomized complete-block design with
®ve single-plant replications. Five fruits were harvested for each treatment. Data
were subjected to analysis of variance and mean separation according to
Duncan's multiple range test using p 0.05 on statistical software.
In 1998: `Fujihikari TR' and `Benikodama' were sown on 13 November 1997
and 26 February 1998, respectively. The seedlings of the former were
transplanted 50 cm apart in two beds in a greenhouse on 23 January and of the
latter were done on 24 March 1998.
The blooming male ¯owers were irradiated with 800 Gy soft-X-ray and
immediately used to pollinate each watermelon plant. Unirradiated pollen was
used as the control. Treatments were performed between 20 February and 1
March in `Fujihikari TR', and in `Benikodama', treatments were done between 18
and 29 March. One week after treatment, fruit set was recorded. Mature fruits
were harvested at about 36 days in `Benikodama' 36 and 48 days after pollination
258
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
in `Fujihikari TR'. Methods of cultivation and pollination were the same as in
1997.
The treatments were arranged in a randomized block design with two
replications. Twenty plants in `Fujihikari TR' and ten plants in `Benikodama'
were used. Data were expressed as means SE. Mean differences between
treatments and control were determined by t test.
3. Results and discussion
3.1. Relationship between soft-X-ray irradiation dose to pollen and number of
seeds
There were no observable differences in fruit setting between soft-X-irradiated
pollen and unirradiated pollen in both `Benikodama' and `Fujihikari TR' (Table
1). We have already observed that the germination rate of pollen treated with
800 Gy of soft-X-ray was almost the same as that of the control (Sugiyama and
Morishita, 1998). These results indicate that irradiated pollen presents no
practical problem for producing seedless watermelon fruit.
The number of normal seeds was signi®cantly reduced when female ¯owers
were pollinated with irradiated pollen. In `Benikodama', normal seeds were not
observed at 400±1000 Gy (Fig. 2). 200±400 empty seeds per watermelon were
observed at 100±1000 Gy. In `Fujihikari TR' irradiated with the 800±1000 Gy,
empty seeds alone were produced (Fig. 3). The numbers of empty seeds
were approximately 50±150 at 800±1000 Gy. In `Fujihikari TR' fruits produce
with soft-X-irradiated pollen, most empty seeds were as much small and thin
(Fig. 4). However, they were very conspicuous in `Benikodama'. Varietal
differences in number of empty seeds were observed in triploid watermelon
(Kihara, 1958). In our study, `Benikodama' had more empty seeds than
`Fujihikari TR'(Table 2, Fig. 2) which indicates that the number of empty seeds
is determined by the characteristics of the ovule in each cultivar.
Producing seedless fruits of watermelon by hormonal agents is a parthenocarpic phenomenon in which fertilization is not needed for growth. In order for
Table 1
Effect of soft-X-irradiation on fruit set of watermelon
Cultivar
X-ray
dose (Gy)
Treated flowers
(number)
Fruit set
(%)
Benikodama
0
800
51
46
39.2
43.5
Fujihikari TR
0
800
54
60
64.8
66.7
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
259
Fig. 2. Relationship between dose of soft-X-irradiation and number of seeds in cultivar
`Benikodama'. Vertical bars indicate SE.
Fig. 3. Relationship between dose of soft-X-irradiation and number of seeds in cultivar `Fujihikari
TR'. Vertical bars indicate SE.
260
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Fig. 4. Watermelons fruits after pollination by soft-X-irradiation pollen compared to a normal fruit
in `Benikodama' (upper photograph) and `Fujihikari TR' (lower photograph).
parthenocarpically developed watermelon to reach normal size, pollination and
pseudogamy are necessary. Generally, after pollination, growth of the ovary is
induced by auxin derived from pollen grain and by auxin induced by stimulation
from the pollen tube in the pistil. Further ovary development is promoted by
auxin derived from the developing embryo after fertilization (Saitou, 1974).
However, a watermelon fruit which is produced only by hormonal treatment is
smaller than that produced by pollination (Kondou and Murozono, 1975;
Yamamuro, 1978; Hayata et al., 1995). Triploid watermelons produced by
parthenocarpy develop to full size because parthenocarpy is induced by growth
hormones provided by diploid pollen grains and provided from the ovule by
261
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Table 2
Effect of soft-X-irradiation on fruit characteristics (1997)
Cultivar
X-ray
dose (Gy)
Fruit
weight (kg)
Fruit
shapea
Benikodama
0
100
200
400c
800c
1000c
2.6ab
2.4ab
1.9b
2.0b
1.7b
2.3ab
1.14ab
1.14ab
1.08b
1.14ab
1.16a
1.17a
Fujihikari TR
0
100
200
400
800c
1000c
3.8a
4.0a
4.8a
4.8a
4.1a
3.8a
1.05a
1.05a
1.07a
1.07a
1.09a
1.05a
Thickness of
rind (cm)
Sugar
content (%)
Maturation
(days)
6.8a
6.2a
6.4a
6.2a
6.4a
6.7a
11.8b
12.2b
12.0b
12.5ab
12.5ab
13.1a
36.0
34.6
34.5
35.6
36.8
35.3
9.9b
11.6a
11.4ab
11.3ab
10.1b
11.0ab
10.7b
11.4ab
11.0b
10.6b
11.2ab
12.2a
36.8
37.5
37.3
36.5
37.4
37.3
a
Fruit shape is expressed as the ratio of height to width.
Mean separation within columns by Duncan's multiple range test, at 5% level.
c
Seedless watermelon fruit.
b
pseudogamy. Thus endogenous auxins derived from the ovule are needed to
develop normal size fruit.
Soft-X-irradiated pollen germinate on a stigma, and their tubes elongate into an
embryo sac. Subsequent abortion of embryos after pollination by soft-Xirradiated pollen is still under investigation, but may result from soft-X-rayinduced chromosomal abnormalities in the generative nucleus which prevent
normal fertilization from occurring. However, Sari et al. (1994) reported that
haploid embryos are obtained through pollination with g-irradiated pollen, which
indicates that embryo formation occurs as a pseudogametic phenomenon.
Therefore, it is possible for an ovary growing after pollination with soft-Xirradiated pollen to be promoted by auxin derived from the ovule.
Fruit characteristics: In `Benikodama', fruit weight in control averaged 2.6 kg,
slightly heavier than in irradiated groups in 1997 (Table 2). However, in 1998,
`Benikodama' fruit weight in irradiation treatment was similar to that of control
(Table 3). In `Fujihikari TR', there was no signi®cant difference between fruit
weight in irradiated groups and that in controls. Thus, a relationship between the
fruit weight and the irradiation dose was not recognized. `Benikodama' fruits
were elliptical in most cases except for those treated with 200 Gy (1997 group).
The shape of `Fujihikari TR' fruits were spherical at all doses of irradiation. In
`Benikodama' fruits, there was no signi®cant in rind thickness between irradiated
groups and controls; in `Fujihikari TR' fruit rind in irradiated groups except the
800 Gy (1997 group) was also similar to that in controls. We found no consistent
relationship between dose of soft-X-irradiation and rind thickness. `Benikodama'
262
Cultivar
X-ray
dose (Gy)
Number of
normal seed
Number of
empty seed
Fruit weight
(kg)a
Fruit shapea,b
Thickness of
rind (cm)a
Sugar content
(%)a
Maturation
(days)
Benikodama
0
800
173
0
51
198
1.5 0.18
1.6 0.16
NSc
1.11 0.02
1.12 0.02
NSc
5.5 0.14
5.4 0.35
NSc
11.0 0.40
12.5 0.35
NSc
35.8
36.0
NSc
0
800
201
0
44
149
6.2 0.23
6.4 0.27
NSc
1.03 0.13
1.03 0.02
NSc
13.4 0.60
13.0 0.58
NSc
10.8 0.23
11.3 0.24
NSc
48.0
48.0
NSc
Significanced
Fujihikari TR
Significanced
a
Data are means standard errors of 10-fruits in `Benikodama', 20-fruits in `Fujihikari TR', respectively.
Fruit shape is expressed as the ratio of height to width.
c
NS: non-signi®cant.
d
Signi®cant between control and irradiation at P < 0.05.
b
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
Table 3
Comparison of seedless watermelon fruits produced by soft-X-irradiated pollens and control (1998)
K. Sugiyama, M. Morishita / Scientia Horticulturae 84 (2000) 255±264
263
fruits from irradiation treatment showed similar or higher sugar content than in
controls. In `Fujihikari TR', there was no signi®cant difference in sugar content
of irradiated groups except the 1000 Gy (1997 group) and that of controls. All
fruits of both `Benikodama' and `Fujihikari TR' matured within almost the same
period regardless of soft-X-irradiation.
Seedless watermelon fruits induced by hormonal agents had a tendency to be
small and deformed compared with normal fruits (Hayata et al., 1995; Kondou
and Murozono, 1975). In contrast, seedless watermelon fruits produced by softX-irradiated pollen were normal in size and shape. Previous studies (Kihara,
1951; Kondou and Murozono, 1975) on the production of parthenocarpic seedless
watermelon using triploid or plant growth regulators showed a tendency to form a
thick rind. In the present experiment, there was no relationship between rind
thickness and soft-X-irradiation. In our study, the sugar content of seedless
watermelon fruits produced by soft-X-irradiated pollen was similar to or higher
than that of controls. These results show that soft-X-irradiation did not affect fruit
weight, shape or rind thickness, and that the quality was equal to or better than
that of controls.
In Japan, the main reason triploid watermelons are not popular is that triploids
have a longer growing period and a later maturation date than diploid ones.
Occasionally, there have been some developmental defects such as hollow hearts
in triploid watermelon fruits (Kihara, 1951, 1958). In contrast, in our experiment,
these defects were not present in seedless watermelon produced by pollination
with soft-X-irradiated pollen.
In conclusion, seedless watermelon produced by pollination with soft-Xirradiated pollen can be produced by ordinary applied cultivation due to the use of
common diploids. Therefore, this new technique established in our study has
practical value and should be used to produce a good seedless watermelon fruit.
Acknowledgements
The authors are thankful to Prof. T. Harada of Hokkaido University and Prof.
H. Inden of Miyazaki University for valuable suggestions and critical reading of
the manuscript.
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