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Yasuyoshi Hayata, Xin-Xian Li and Yutaka Osajima

To clarify the cause of low sucrose accumulation in seedless `Crest Earl's' netted muskmelon [Cucumis melo L. (Reticulatus Group)] fruit induced by CPPU, the activity level of sucrose metabolizing enzymes was compared between seeded and seedless fruit. CPPU promoted growth of the ovary in both pollinated and nonpollinated flowers until 10 days after anthesis (DAA), and thereafter the growth rate of nonpollinated fruit was lower than in the controls. Sucrose accumulation of seedless fruit remained lower than in seeded fruit, but there was no difference in fructose and glucose content between seeded and seedless fruit. Acid invertase activity declined sharply 20 DAA in seeded and seedless fruit, and was hardly detectable at 35 DAA, when sucrose accumulation began. Neutral invertase (NI) activity in both seeded and seedless fruit decreased from 20 DAA until 35 DAA; thereafter, NI activity in seeded fruit remained relatively constant, with a small but insignificant increase in maturity. Sucrose synthase (SS-c: sucrose cleavage direction) activity in seeded fruit decreased from 20 to 30 DAA, and then increased as fruit matured, while SS-c activity in seedless fruit did not change during development. Sucrose phosphate synthase (SPS) activity in seeded fruit increased from 25 to 30 DAA and remained relatively constant until harvest. SPS activity in seedless fruit declined gradually from 30 to 45 DAA, then remained at a low level. Sucrose synthase (SS-s: sucrose synthesis direction) activity in seeded fruit increased rapidly after 30 DAA, concomitant with sucrose accumulation. In contrast, SS-s activity in seedless fruit increased only slightly after 30 DAA indicating levels of SS-s activity are closely related to sucrose accumulation in parthenocarpic seedless muskmelons. Chemical name used: [1-(2-chloro-4-pyridyl)-3-phenylurea] (CPPU).

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Yasuyoshi Hayata, Xin-Xian Li and Yutaka Osajima

An investigation was conducted to determine how pollination and CPPU treatment influence endogenous IAA and ABA content in netted muskmelon [Cucumis melo L. (Reticulatus Group) `Crest Earl's'], and to clarify their roles in fruit set and development in relation to these endogenous plant hormones. CPPU treatment at anthesis significantly increased the fresh weight of ovaries, whether the flowers were pollinated or not, but from 6 days after anthesis (DAA) the growth rate in the nonpollinated + CPPU treatment tended to be lower than the growth rates in the pollination treatment plots. Ovaries of nonpollinated flowers not treated with CPPU failed to grow and turned brown within 4 DAA. IAA content in the placenta of fruit from pollinated flowers increased rapidly from the day of anthesis to 2 DAA and remained at relatively high levels. IAA content in the placenta of parthenocarpic fruit induced to develop by CPPU treatment was lower than that of fruit from pollinated flowers but the pattern was almost the same as that in fruit of pollinated flowers. Conversely, IAA content in the placenta of fruit from nonpollinated flowers not treated with CPPU decreased sharply after anthesis. IAA content in the mesocarp of CPPU-treated fruit, whether or not the flowers were pollinated, increased significantly from the day of anthesis to 2 DAA, then decreased to almost the same level as that of the pollination-only treatment by 10 DAA, while the IAA content of nonpollinated CPPU-treated fruit declined even further. IAA content in the mesocarp of fruit from nonpollinated flowers not treated with CPPU decreased sharply. ABA contents in both the placenta and mesocarp of muskmelon that would set decreased after anthesis while the ABA content of muskmelon that would not set increased rapidly. Results suggest that pollination and CPPU treatment increased endogenous IAA content and decreased endogenous ABA content to promote the set and growth of fruit during early development. Chemical names used: [1-(2-chloro-4-pyridyl)-3-phenylurea] (CPPU); indole-3-acetic acid (IAA); abscisic acid (ABA).

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Yasuyoshi Hayata, Yoshiyuki Niimi and Naoto Iwasaki

Applying a 200 ppm solution of CPPU to pollinated ovaries of watermelon (Citrullus lunatus Matsum) at anthesis increased fruit set from 26.9% (control) to 95%. Applying CPPU solutions to nonpollinated ovaries at anthesis induced parthenocarpy, yielding 65% and 89.5% fruit set, respectively with 20 and 200 ppm applications. However, 64% of the 20 ppm CPPU-treated parthenocarpic fruit stopped growth 10 days after treatment. Growth of CPPU-treated, pollinated, and nonpollinated fruit increased significantly compared with growth of control fruit during the first 10 days after treatment, but, except for the 20 ppm CPPU parthenocarpic fruit, growth subsequently slowed, resulting in fruit equal in size to the control by harvest. CPPU application did not affect soluble solids content of pollinated fruit, but reduced content of parthenocarpic fruit treated with 20 ppm. Fructose content was generally higher than glucose and sucrose at harvest. However, in pollinated fruit treated with 20 ppm CPPU, sucrose levels were higher than glucose and fructose. These results suggest that CPPU is practical for promoting fruit set and seedless fruit without adversely affecting fruit quality and development.