Consumers favor seedless fruits for eating in both fresh (e.g., grape, citrus, and banana) and processed forms (e.g., tomato sauce). Seedlessness can improve fruit quality by consumer preference (Pandolfini, 2009). Marr and Gast (1991) reported that consumers were willing to pay 50% more for seedless watermelons. To produce seedless watermelons, techniques such as triploidy, hormonal treatment, and pollination with inactivated pollen have been tried. Seedlessness may affect fruit quality. Triploid cultivars tended to have higher soluble solid contents (Brix) than diploids (Maynard et al., 2002; Pardo et al., 1997). Seedless fruits induced by γ-irradiated pollen had significantly higher sugar content than seeded fruits (Moussa and Salem, 2010). On the other hand, Leskovar et al. (2004) found that total sugar content in fruit was not significantly different among diploid and triploid cultivars. Hayata et al. (1995) found no consistent relationship between sugar content and the application of CPPU. Furthermore, the sugar content of seedless fruits induced by soft-X–irradiated pollen was similar to or higher than that of controls (Sugiyama and Morishita, 2000). Thus, the role of seeds in sugar accumulation in watermelon is still uncertain.
To investigate the effect of the presence or absence of seeds on sugar content in watermelon fruits, we designed 3 experiments: Expt. 1, comparing diploid seeded ‘Hitorijime-BonBon’ and triploid seedless ‘Sandia’ fruits; Expt. 2, testing the effect of CPPU; and Expt. 3, pollinating flowers with soft-X–irradiated pollen.
We also examined hydrostatic pressure, which is generated by solutes in solution (in this case, cell sap). Seeds act as an assimilate sink during fruit growth. Most sinks in higher plants import assimilates by bulk flow driven by differences in hydrostatic pressure (Patrick et al., 2001). Gradients in water potential in the transport pathway have been measured in wheat grains (Fisher and Oparka, 1996), strawberry (Pomper and Breen, 1995), and grape berries (Lang and During, 1991). Molecules that are known to control osmotic pressure include sugars, glycerol, amino acids, sugar alcohols, and various low-molecular-weight metabolites (Boyer, 1995b). Thus, we also measured water status.
We analyzed sugar contents by high-performance liquid chromatography (HPLC), and water status [water potential, osmotic potential (ψS) = osmotic pressure, turgor pressure] with an isopiestic thermocouple psychrometer, and measured seed size during fruit growth.
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