Inhibition of Viviparous Sprouting on Melon Seeds Using High Level of Potassium Fertilization or Abscisic Acid Application

Abstract

The present experiments were undertaken in order to clarify the effects of a high potassium fertilization level on the occurrence of viviparous sprouting in melon fruit. Melon (Cucumis melo L. group) plants of two susceptible lines and one resistant line to viviparous sprouting were used at two different potassium concentrations. A high potassium fertilization level (12.0 mmol·L⁻¹) resulted in a marked decrease of the occurrence of viviparous sprouting in the susceptible lines, and in the increase of the endogenous ABA content. No significant differences were observed in the seed number per fruit, the seed weight, and the percentage of seed germination at a high level of potassium fertilization. Further experiments were conducted in order to analyze the effects of exogenous ABA treatment at a low potassium fertilization level (1.5 mmol·L⁻¹) on the occurrence of viviparous sprouting in melon fruit. At 25 days after pollination, ABA solution at different concentrations was sprayed on the fruits of the susceptible melon line grown at two different potassium fertilization levels. Even at a low potassium fertilization level, exogenous ABA application inhibited the occurrence of viviparous sprouting and increased ABA contents both in juice samples around the placenta and in the seeds. ABA treatment, however, led to a significant decrease in the seed number per fruit and in the percentage of seed germination, as a result of a marked increase in the ABA content and potassium absorption.

Introduction

Seeds in fruits do not generally germinate while remaining in the fruits, even at full maturity. In some cultivars, however, seed sprouting during fruit development has been recognized to be a serious problem in practical seed production of vegetable plants.

Several investigators have reported these phenomena in tomato (Dos Santos and Yamaguchi, 1979; Yamaguchi et al., 1967), bell pepper (Harrington, 1960), and watermelon fruits (Kobayashi et al., 2010). Those previous investigators used different terms to designate these physiological disorders, including seed sprouting (Dos Santos and Yamaguchi, 1979), precocious germination (Welbaum et al., 1990), vivipary (Marrush et al., 1998), viviparous germination (Kobayashi et al., 2010), and viviparous sprouting (Ochi and Ito, 2012a, b).

Viviparous sprouting has been considered to result from excess nitrate nitrogen (Yamaguchi et al., 1967) or from depleted potassium (Dos Santos and Yamaguchi, 1979; Marrush et al., 1998). Welbaum et al. (1990) suggested that viviparous sprouting in muskmelon might be triggered by low levels of ABA in the fruit tissues around the placenta. Few investigations, however, have been carried out to clarify the relationships among nutritional conditions, viviparous sprouting, and ABA level in fleshy fruit.

In our previous investigations, we concluded that nutritional conditions affected the ABA level in developing melon seeds, and that a high nitrate nitrogen level or lower potassium fertilization might lead to a decrease in the ABA content in fruit juice samples around the placenta, resulting in the increased occurrence of viviparous sprouting. In particular, low potassium fertilization was the leading factor in the increased occurrence of viviparous sprouting (Ochi and Ito, 2012a, b).

Since a potassium concentration of 6 mmol·L⁻¹ could not prevent the occurrence of viviparous sprouting in previous experiments (Marrush et al., 1998; Ochi and Ito, 2012b), further studies on the effect of higher potassium concentrations on the control of viviparous sprouting were undertaken.

Exogenous ABA application, on the other hand, increased the ABA level in the plant, and also enhanced potassium absorption in cucumber under high temperature conditions (Du and Tachibana, 1995). Kamuro et al. (1992) suggested that exogenous ABA application increased the fruit weight and Brix of melon. High Brix is related to high osmotic pressure, and thus might lead to a decreased occurrence of viviparous sprouting in melon fruit (Welbaum, 1999) and exogenous ABA application; therefore, it might prevent the occurrence of viviparous sprouting even with low potassium fertilization.

The main objective of the present experiment was to analyze the inhibitory effects of higher potassium fertilization or of exogenous ABA application with low potassium fertilization to control the occurrence of viviparous sprouting in melon fruit.

Materials and Methods

Experiment 1

Plant materials

Melon (Cucumis melo L. group) plants of two lines susceptible to viviparous sprouting, namely S·VS bred at the Institute for Horticultural Plant Breeding (IHPB, Matsudo, Japan) and S·VS·KY bred in Taiwan (Known-You Seed Co. Ltd., Kaohsiung, Taiwan) as well as a line resistant to viviparous sprouting R·VS (IHPB), were used in the experiment as maternal plants.

Cultivation and potassium treatment

Melon seeds of each line were sown in growth media on March 29, 2011. The seedlings were transferred to 0.5 liter plastic pots filled with commercial growth media for raising seedlings 7 days after sowing. Thirty days after sowing, each seedling was transferred to a 15 liter plastic bag filled with a mixture of peatmoss and vermiculite (7 : 3, v/v). Sixteen plants with 2 replications per treatment were grown in a greenhouse of IHPB. A nutrient solution, “Yamasaki formula for melon” containing 13.0 N (11.7 NO₃⁻, 1.3 NH₄⁺), 1.3 P, 6.0 K, 3.5 Ca, and 1.5 Mg in mmol·L⁻¹ was applied from the time of transfer to pollination. After pollination, the above solution was modified to 2 potassium (K) nutrient levels (standard level as Control: 6.0 mmol·L⁻¹, and high level: 12.0 mmol·L⁻¹) for 20 days. Four pieces of dripper (pressure compensated, no leakage, Netafim Japan Co. Ltd., Tokyo, Japan) at a rate of 2.0 liter·hr⁻¹ were used to fertigate each bag.

Main shoots were trained vertically and pruned off at the 23rd node. The female flowers on the first nodes of lateral shoots between the 11–15th nodes of the main shoots were emasculated and covered with waxed paper bags one day before pollination. Other lateral shoots were removed. The melon line A (IHPB) was used as the pollen plant. Finally, two fruits were left and harvested 35 days after pollination (DAP) and 50 DAP for the investigations.

Analytical methods

The fruit and plant were weighed at harvest at 50 DAP. Fruits were then additionally allowed to mature for two weeks at 25°C. After ripening, some of the seeds and juice around the placenta were sampled separately and stored in a refrigerator at −80°C for ABA analysis. The remaining seeds were dried in sunlight for 7 days and stored under low humidity and low temperature conditions.

Evaluation of viviparous sprouting: Seeds with a radicle or cotyledon more than 1 mm in length observed through the crack in a testa were evaluated for viviparous sprouting.

Germination test: A germination test was carried out according to the method used by the International Seed Testing Association (ISTA).

Analysis of ABA content: ABA from the seed and juice samples around the placenta was extracted according to the method of Arenas-Huertero et al. (2000) and analyzed using Phytodetek-ABAkit (Agdia Inc., Elkhart, IN, USA).

Determination of Brix: The soluble solid content was measured in fruits harvested at 50 DAP using a Brix meter (ATAGO, Tokyo, Japan).

Analysis of mineral element concentrations in juice samples around the placenta: Using a portable reflection photometer (RQflex10; Merck, NJ, USA), mineral elements (NO³⁻, K⁺, Mg²⁺, Ca²⁺) were analyzed in juice samples around the placenta in fruits at 50 DAP.

Statistical analysis: Data were subjected to analysis of variance (ANOVA). Means were evaluated using Tukey’s multiple range test at 5% level of significance.

Experiment 2

Plant materials

Melon plants of a line susceptible to viviparous sprouting S·VS (IHPB) were used.

Cultivation and potassium treatment

Plant cultivation was conducted as described in Experiment 1. The potassium concentrations in this experiment corresponded to those of the control (6.0 mmol·L⁻¹) and low levels (1.5 mmol·L⁻¹) of modified Yamasaki formula for melon.

ABA treatment

Abscisic acid (98%; Sigma-Aldrich Co. LLC, MO, USA) was used in a methanol solution, and then dissolved in deionized water at 0, 100, 300 mg·L⁻¹. A 10 mL solution of ABA was sprayed by hand sprayer on each fruit at 25 DAP.

Analytical methods

Analytical methods were carried out as described in Experiment 1.

Results

Effect of high potassium fertilization level on viviparous sprouting of melon seeds (Experiment 1)

The fruit weight of S·VS decreased with high potassium fertilization, although no differences were observed in S·VS·KY and R·VS (Table 1). Brix increased significantly with high potassium fertilization compared to the control in the two lines, showing a slight difference in S·VS. The number of seeds per fruit at 50 DAP was not affected by high potassium fertilization. The seed weight of grain was not affected by high potassium fertilization in S·VS and R·VS, while a considerable decrease was found in S·VS·KY.

Table 1. Effect of high potassium fertilization on the yield of fruits and seeds.

Line	Potassium concentration (mmol·L−1)	Fruit weight (g)	Brix (%)	Number of seeds per fruit		Seed weight per grain (mg)
				35 DAPz	50 DAP	35 DAP	50 DAP
S·VS	6.0 (Cont.)	1016 aby	10.85 b	294 a	340 a	27.7 c	35.2 c
S·VS	12.0 (High)	922 b	11.55 ab	285 a	336 a	29.2 c	35.3 c
S·VS·KY	6.0	1772 a	8.45 c	172 c	203 c	60.3 a	64.7 a
S·VS·KY	12.0	1826 a	10.05 b	168 c	193 c	52.9 b	54.9 b
R·VS	6.0	759 c	11.00 b	203 b	283 b	20.9 d	24.3 d
R·VS	12.0	772 c	12.40 a	187 b	277 b	20.1 d	23.8 d
Line (L)		**x	**	**	**	**	**
Potassium (K)		NS	**	NS	NS	*	*
(L × K)		*	**	NS	NS	**	**

^z  DAP: days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

The nitrate nitrogen concentrations in the juice samples around the placenta of S·VS and S·VS·KY were higher than those of R·VS (Table 2). The nitrate nitrogen concentrations of the three lines with high potassium fertilization were higher than those in the control plants. The potassium concentration increased significantly with high potassium fertilization compared to that of the control. The potassium concentration of the resistant line was higher than that of the susceptible lines. This tendency was in agreement with the results of our previous investigation (Ochi and Ito, 2012b). On the other hand, the calcium and magnesium concentrations of all the lines tended to decrease with high potassium fertilization.

Table 2. Effect of high potassium fertilization on mineral element concentrations in juice samples around the placenta^z

Line	Potassium concentration (mmol·L−1)	Mineral element concentrations (mg·L−1)
		NO3−	K+	Ca2+	Mg2+
S·VS	6.0 (Cont.)	9.8 by	4769 d	12.3 b	26.3 a
S·VS	12.0 (High)	12.3a	5900 bc	10.7 b	25.7 a
S·VS·KY	6.0	4.8 cd	4295 d	19.4 a	25.3 a
S·VS·KY	12.0	7.5 c	5393 c	10.2 b	20.8 b
R·VS	6.0	2.4 d	6350 b	17.3 a	11.8 c
R·VS	12.0	3.8 d	7940 a	7.8 c	11.3 c
Line (L)		**x	**	**	**
Potassium (K)		**	**	**	NS
(L × K)		NS	NS	NS	NS

^z  Data of the fruits harvested at 50 days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

A markedly depressive effect of high potassium fertilization on viviparous sprouting compared to that in the control was observed in the two susceptible lines at 50 DAP (Table 3). The ABA content in the juice samples around the placenta, furthermore, increased markedly with high potassium fertilization compared to that of the control. The ABA content in juice in the resistant line at 35 DAP was higher than that of the susceptible lines. These results were similar to those reported in our previous study (Ochi and Ito, 2012b). No clear tendency was observed in the ABA content in seeds by the treatments of potassium fertilization, regardless of the genetic susceptibility to viviparous sprouting. ABA content in seeds of R·VS sharply decreased from 35 DAP to 50 DAP.

Table 3. Effect of high potassium fertilization on the percentage of viviparous sprouting, abscisic acid content in juice samples around the placenta and in seeds.

Line	Potassium concentration (mmol·L−1)	Percentage of viviparous sprouting (%)		Abscisic acid content in juice samples around the placenta (ng·g−1 FW)		Abscisic acid content in seeds (ng·g−1 FW)
		35 DAPz	50 DAP	35 DAP	50 DAP	35 DAP	50 DAP
S·VS	6.0 (Cont.)	0.28 ay	3.67 a	199 e	44 e	42.7 c	22.7 ab
S·VS	12.0 (High)	0.10 a	1.81 b	273 d	96 d	50.2 bc	32.7 ab
S·VS·KY	6.0	0.12 a	2.00 ab	717 c	129 c	52.1 b	36.9 a
S·VS·KY	12.0	0.15 a	0.84 c	985 b	324 a	60.6 b	47.6 a
R·VS	6.0	0.00 a	0.03 d	897 b	60 e	73.9 a	7.7 b
R·VS	12.0	0.00 a	0.01 d	1297 a	197 b	57.3 b	24.0 ab
Line (L)		**x	**	**	**	**	**
Potassium (K)		NS	**	**	**	*	NS
(L × K)		NS	NS	**	**	NS	NS

^z  DAP: days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

No noticeable effect of the potassium concentration on the percentage of seed germination was observed in the present study, although a slight difference was observed between the lines used (Table 4).

Table 4. Effect of high potassium fertilization on the percentage of seed germination.

Line	Potassium concentration (mmol·L−1)	Percentage of seed germinationz (%)
		35 DAPy	50 DAP
S·VS	6.0 (Cont.)	85.0 bx	90.5 b
S·VS	12.0 (High)	92.0 b	95.0 a
S·VS·KY	6.0	94.0 ab	97.1 a
S·VS·KY	12.0	97.0 a	95.3 a
R·VS	6.0	88.0 b	95.5 a
R·VS	12.0	80.7 c	92.3 a
Line (L)		**w	**
Potassium (K)		NS	NS
(L × K)		NS	NS

^z  Seeds used for germination test were stored for 6 months after harvest.

^y  DAP: days after pollination.

^x  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^w  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

Effect of exogenous ABA application with low potassium fertilization on viviparous sprouting of melon seeds (Experiment 2)

Low potassium treatment resulted in a lighter fruit weight than that of the control (Table 5). ABA application mainly led to an increased Brix and fruit weight, as observed in a previous investigation (Kamuro et al., 1992). Another interesting point was that the number of seeds per fruit decreased considerably with ABA treatment. Seed weight in a fruit, however, increased with increasing ABA concentration.

Table 5. Effect of abscisic acid treatment on the yield of fruits and seeds grown with low potassium fertilization.

Line	Potassium concentration (mmol·L−1)	Abscisic acid treatment (mg·L−1)	Fruit weight (g)	Brix (%)	Number of seeds per fruit		Seed weight per grain (mg)
					35 DAPz	50 DAP	35 DAP	50 DAP
S·VS	1.5 (Low)	0	890 dy	9.35 c	277 a	294 b	26.9 b	30.1 c
S·VS	1.5	100	937 c	11.55 ab	240 b	269 c	28.7 a	31.3 c
S·VS	1.5	300	939 c	14.90 a	234 b	254 c	30.3 a	32.5 c
S·VS	6.0 (Cont.)	0	1016 b	10.55 b	317 b	340 a	27.7 a	35.2 b
S·VS	6.0	100	1105 ab	11.00 b	282 c	306 b	29.7 a	36.6 b
S·VS	6.0	300	1256 a	13.95 ab	266 c	285 b	30.7 a	38.1 a
Potassium (K)			**x	NS	**	**	NS	**
ABA treatment (A)			**	**	*	**	*	*
(K × A)			*	NS	**	**	NS	NS

^z  DAP: days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

The nitrate nitrogen concentration increased considerably with increasing ABA concentration (Table 6). A significant increase in potassium absorption was observed in the fruits subjected to ABA application. The calcium and magnesium concentrations decreased with increasing ABA concentration.

Table 6. Effect of abscisic acid treatment on mineral element concentrations in juice samples around the placenta.z

Line		Potassium concentration (mmol·L−1)	Abscisic acid treatment (mg·L−1)	Mineral element concentration (mg·L−1)
				NO3−	K+	Ca2+	Mg2+
S·VS		1.5 (Low)	0	9.5 cy	3984 d	15.2 a	23.8 b
S·VS		1.5	100	11.7 bc	3873 d	10.3 b	21.8 b
S·VS		1.5	300	13.5 b	5367 b	9.1 b	15.8 c
S·VS	6.0 (Cont.)		0	9.8 c	4769 c	12.3 b	26.3 a
S·VS	6.0		100	14.8 a	4273 c	7.2 c	21.5 b
S·VS	6.0		300	16.5 a	6291 a	7.9 c	18.9 ab
Potassium (K)				**x	**	**	*
ABA treatment (A)				**	**	**	**
(K × A)				*	NS	**	**

^z  Data of the fruits harvested at 50 days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

ABA treatment tended to reduce the percentage of sprouted seeds at 50 DAP (Table 7). The percentage of viviparous sprouting with low potassium fertilization was higher than that of the control. ABA content in juice samples around the placenta and in seeds increased markedly with ABA treatment. ABA content in the juice samples around the placenta at 35 DAP with low potassium treatment was lower than that of the control.

Table 7. Effect of abscisic acid treatment on the percentage of viviparous sprouting, abscisic acid content in juice samples around the placenta and seeds grown with low potassium fertilization.

Line	Potassium concentration (mmol·L−1)	Abscisic acid treatment (mg·L−1)	Percentage of viviparous sprouting (%)		Abscisic acid content in juice samples around the placenta (ng·g−1 FW)		Abscisic acid content in seeds (ng·g−1 FW)
			35 DAPz	50 DAP	35 DAP	50 DAP	35 DAP	50 DAP
S·VS	1.5 (Low)	0	1.05 ay	10.57 a	99 e	76 b	40.2 b	19.7 b
S·VS	1.5	100	0.32 b	4.13 b	176 d	60 b	51.5 a	31.4 a
S·VS	1.5	300	0.22 b	1.84 c	562 b	133 ab	57.2 a	37.2 a
S·VS	6.0 (Cont.)	0	0.28 b	3.67 b	199 d	44 b	42.7 b	22.7 b
S·VS	6.0	100	0.20 b	0.95 d	324 c	109 ab	42.4 b	27.2 ab
S·VS	6.0	300	0.19 b	0.79 d	832 a	252 a	51.7 a	37.0 a
Potassium (K)			**x	**	**	**	**	**
ABA treatment (A)			NS	**	**	**	*	**
(K × A)			NS	**	**	**	NS	NS

^z  DAP: days after pollination.

^y  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^x  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

The percentage of seed germination markedly decreased by ABA treatment in the seeds taken at 50 DAP and stored for six months after harvest (Table 8). The inhibitory effect on seed germination remained in the seeds stored for twelve months.

Table 8. Effect of abscisic acid treatment on germination percentage of seeds grown at a low potassium concentration.

Line	Potassium concentration (mmol·L−1)	Abscisic acid treatment (mg·L−1)	Percentage of seed germinationz (%)
			6 months		12 months
			35 DAPy	50 DAP	35 DAP	50 DAP
S·VS	1.5 (Low)	0	92.7 ax	95.3 a	92.7 b	94.8 a
S·VS	1.5	100	88.0 ab	75.0 b	90.0 b	84.3 b
S·VS	1.5	300	83.0 b	69.0 c	83.0 c	80.7 b
S·VS	6.0 (Cont.)	0	85.0 ab	90.5 a	95.0 a	96.4 a
S·VS	6.0	100	77.0 c	83.0 b	90.6 b	85.8 b
S·VS	6.0	300	75.0 c	78.1 b	88.7 c	86.4 b
Potassium (K)			**w	**	NS	NS
ABA treatment (A)			**	**	**	**
(K × A)			**	**	NS	NS

^z  Seeds used for germination test were stored for 6 or 12 months after harvest.

^y  DAP: days after pollination.

^x  Mean separation within columns by Tukey’s multiple range test at the 5% level.

^w  **, *, and NS: significant at P < 0.01, 0.05, and non-significant by 2-way ANOVA.

Discussion

High potassium fertilization decreased the occurrence of viviparous sprouting

In the present study, the authors demonstrated that high potassium fertilization markedly decreased the occurrence of viviparous sprouting, probably due to the increase of the endogenous ABA content depending mainly on the susceptibility of the lines to ABA.

Previous investigations indicated that potassium fertilization at 6 mmol·L⁻¹ did not prevent the occurrence of viviparous sprouting (Marrush et al., 1998; Ochi and Ito, 2012b). In the present experiment, however, high potassium fertilization, 12 mmol·L⁻¹, of two susceptible lines reduced the occurrence of viviparous sprouting without affecting the fruit weight and seed yield, showing small differences among the lines. Thus, side-dressing of potassium fertilizer after pollination may enable the reduction of the occurrence of viviparous sprouting in practical seed production of melon. In seed production, nutritional conditions after pollination are important to enhance seed yield and quality.

ABA content in the seeds of R·VS, on the other hand, sharply decreased from 35 DAP to 50 DAP, for unknown reasons. Further examination might be needed to determine this low value of ABA in seed. Although ABA content in the seeds of R·VS decreased, viviparous sprouting did not occur, suggesting that a high concentration of ABA in the fruit juice around the placenta could be one of the factors that may reduce the occurrence of viviparous sprouting in R·VS.

However, S·VS·KY displayed a considerably high percentage of viviparous sprouting, in spite of the high ABA content in juice around the placenta. This may suggest that susceptibility to ABA concentration may differ among the lines, and reduction of viviparous sprouting in this line may occur at a higher ABA concentration. Furthermore, fruit maturation was accelerated in this line compared to that of the other lines. Since seed maturity might be advanced, the occurrence of viviparous sprouting could be higher in this line.

Previous investigators reported that higher potassium fertilization increased the ABA content in plants (Marrush et al., 1998; Ochi and Ito, 2012b), although the relationship between the potassium level and the physiological mechanism of ABA biosynthesis was not discussed in detail. Cera (2006) reported that the presence of monovalent potassium ion was indispensable for the activity of enzymes such as K⁺-activated Type I enzymes. The activity of these enzymes may be closely related to ABA biosynthesis.

ABA application decreased the occurrence of viviparous sprouting

Our previous investigations indicated that low potassium fertilization might lead to decreased ABA content in fruit juice samples around the placenta, resulting in the increased occurrence of viviparous sprouting (Ochi and Ito, 2012b). In the present study, even with low potassium fertilization, exogenous ABA application inhibited the occurrence of viviparous sprouting due to high ABA contents both in the juice samples around the placenta and in seeds.

It is well known that exogenous ABA application increases ABA content in plants. As a result, ABA induces the reduction of transpiration (Duan et al., 2007) and increases sugar content in fruit (Kamuro et al., 1992). The considerable increase in the ABA content by ABA application observed in the present study led to the decreased occurrence of viviparous sprouting as well as decreased seed yield and seed germination. These results suggested that ABA application may retard embryo development or might extend seed dormancy, leading to the inactivation of seed germination.

Increased potassium absorption by ABA application was observed in the present study. Du and Tachibana (1995) reported that exogenous ABA application enhanced potassium absorption in cucumber plants under high temperature conditions. High ABA content in juice and seeds of melon might be induced by the increase of potassium absorption by ABA application.

In conclusion, the occurrence of viviparous sprouting could be controlled by high potassium fertilization of 12 mmol·L⁻¹ after pollination in melon seed production without adverse effects on seed yield and quality. Although, in the present study, ABA application appeared to be effective in reducing the occurrence of viviparous sprouting, seed germination was impaired. It could be concluded that there is a wide range of genetic variation among lines susceptible to viviparous sprouting for the threshold value of ABA concentration in the fruit. Further investigations should be carried out to develop appropriate methods of ABA application to avoid the decrease of seed germination.

Acknowledgment

We thank Known-You Seed Co., Ltd. (Kaohsiung, Taiwan) for providing the melon seeds of S·VS·KY. We thank Dr. Keiko Ishikawa and Mr. Akira Hayashi of IHPB for their valuable advice and technical support.

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