Induction of Seedless Watermelons by Pseudogamy

K. Sugiyama and M. Morishita

Kurume Branch, National Research Institute of Vegetables, Ornamental Plants and Tea, Kurume, Fukuoka 839-8503, Japan

 

Additional index words. Citrullus lanatus, parthenocarpy, X-ray, pollen

Abstract. Staminate flowers subjected to soft X-ray irradiation (800 Gy) were subsequently used for pollination to produce pseudogamous, seedless, diploid fruit. Pseudogamous fruit were similar to diploid control fruit for fruit weight, days to maturation, shape, and sugar content. Pseudogamous fruit did not show any tendency to hollow heart, deformed shapes, small sizes, or thick rinds, which are frequent undesirable characterize of fruit from triploids, or hormone-treated diploids.

 

It is well known that seedless watermelon fruit have been produced by utilizing triploid plants (Kihara, 1958; Kihara and Nishiyama, 1947; Terada and Masuda, 1941) or plant growth regulators (Hayata et al., 1995; Terada and Masuda, 1940, 1943). The breeding of triploid cultivars takes more time than the breeding of diploid cultivars, and production of triploid seed is very difficult. The quality of triploid fruit and seedless watermelon fruit produced by using plant growth regulators is inferior to that of diploid fruit. We attempted to develop a new method for inducing diploid, seedless watermelon, and were successful by pollinating with soft X-ray-irradiated pollen (Keita and Morishita, 1998).

Materials and methods

Male flowers were picked the morning of anthesis, and immediately irradiated by soft X-rays. Doses of soft X-ray irradiation were 0 (nonirradiation, as the control), 1000, 2000, and 3000 Gy. After the treatment, germination rate of the pollen was determined.

Four Japanese cultivars and a foreign cultivar were used as material for this experiment (Table 1). They were sown on 11 Aug. 1997 and transplanted in a greenhouse on 5 Sept. 1997. Four plants per cultivar were used with two replications. Male flowers at anthesis were irradiated with 800 Gy dose soft X-ray and immediately used for pollination of each watermelon plant. Nonirradiated pollen was used as the control. Data recorded for each fruit were days to maturity, weight (kg), rind thickness (mm), sugar content (oBrix), and seed quality (mature vs. empty).

Results and discussion

Germination rates (%) of pollen treated with 1000 to 2000 Gy doses of soft X-ray were almost the same as the control (Figure 1). There were no observable differences in fruit set between soft X-ray-irradiated pollen and nonirradiated pollen (data not shown). Soft X-ray irradiated pollen germinated on stigmas, and the pollen tube elongated to the embryo sac. Abortion of embryos after pollination by irradiated pollen may be explained by one or both of two possible causes: 1) fertilization failed because the pollen nucleus was inactivated by the soft X-ray treatment, or 2) cell division in the embryo sac did not progress normally after fertilization because the pollen tube nucleus (chromosomes) was damaged (Grant et al., 1980; Vassileva-Dryanovska, 1966). Presumably, pseudogamy resulted from successful pollen tube growth and subsequent abortion of embryos.

The watermelons from pollination with the soft X-ray-treated pollen did not have any normal seeds, but did have empty seeds in fruit (Figure 2).

Table 1. Numbers of normal and empty seeds in control and pseudogamous fruit.

Control Pseudogamous

Cultivar Normal Empty Normal Empty

Benikodama 245 17 0 214

Fujihikari TR 428 49 0 60

Tenryu No.2 297 66 0 31

Gengobe 320 26 0 243

Kleckley Sweet 451 7 0 26

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Figure 1. Germination rate (%) of soft X-ray-irradiated pollen. Data were recorded on 27 Oct. to 13 Nov. 1997; vertical bars indicate ±se.

There were differences among the five cultivars for numbers of empty seeds per fruit (Table 1). This indicates that the number of empty seeds is not determined by the kind of pollen, but is determined by the ovule characteristics of each cultivar.

Pseudogamous fruit were similar to fruit from control plants for fruit weight, days to maturity, shape, rind thickness, and sugar content. Pseudogamous watermelon fruit are of high quality and seedless (Table 2). Fruit from triploids and plants treated with growth regulators to produce seedless fruit have thick rinds, in addition, triploid fruit tend to have hollow hearts (Kihara, 1958; Kihara and Nishiyama, 1947). Production of seedless watermelons by treatment with hormone resulted in deformed and small-sized fruit (Hayata et al., 1995). None of these defects were observed in the seedless, pseudogamous fruit from the five cultivars included in our study. Thus, this new technique has potential utility for production of high quality seedless watermelons.

Figure 2. Seedless, pseudogamous, and control (normal) watermelon fruit of Japanese F1 'Fujihikari TR'.

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Table 2. Characteristics of control and pseudogamous fruit.

Fruit Rind

Days to wt Fruit thickness

Cultivar Fruit type maturity (kg) shape (mm) °Brix

Benikodama Control 39.9 1.5 Round 5.2 9.7

Pseudogamous 39.6 1.2 Round 4.7 10.4

Fujihikari TR Control 43.1 3.0 Round 10.5 10.1

Pseudogamous 42.3 2.7 Round 10.1 10.8

Tenryu No.2 Control 41.7 2.9 Round 10.9 9.2

Pseudogamous 42.7 2.6 Round 8.6 9.4

Gengobe Control 45.7 1.9 Round 8.4 8.1

Pseudogamous 44.4 1.8 Round 9.5 8.7

Kleckley Sweet Control 50.3 4.2 Oblong 11.0 10.8

Pseudogamous 49.0 4.3 Oblong 10.8 10.4

Literature cited

Grant, J.E., K.K. Pandey, and E.G. Williams. 1980. Pollen nuclei after ionising irradiation for egg transformation in Nicotiana. N.Z. J. Bot. 18:339­341.

Hayata, Y., Y. Niimi, and N. Iwasaki. 1995. Synthetic Cytokinin­|1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU)­promotes fruit set and induces parthenocarpy in watermelon. J. Amer. Soc. Hort. Sci. 120:997­1000.

Keita, S. and M. Morishita. 1998. New technique for production of seedless watermelon. Jpn. Soc. Hort. Sci. Spring Meeting. 67(1):135 (abstr.).

Kihara, H. and I. Nishiyama. 1947. An application of sterility of autotriploids to the breeding of seedless watermelons. Seiken Ziho. 3:93­103.

 

Kihara, H. 1958. Breeding of seedless fruit. Seiken Ziho. 9:1­7.

Terada, J. and K. Masuda. 1940. Parthenocarpy of watermelon by heteroauxin. Agr. Hort. 15:458­468.

Terada, J. and K. Masuda. 1941. Parthenocarpy of watermelon by single or complex application of plant hormones. Agr. Hort. 16:1915­1917 (in Japanese).

Terada, J. and K. Masuda. 1943. Parthenocarpy of triploid watermelon. Agr. Hort. 18:15­16.

Vassileva-Dryanovska, O.A. 1966. The induction of haploid embryos and tetraploid endosperm nuclei with irradiated pollen in Lilium. Hereditas 55:160­165.

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