Many fruits and vegetables accumulate sugars and other solutes during their growth. Watermelons, one of the attractive sweet crops, accumulate sugars in their fruits during their growth (Brown and Summers 1987; Yativ et al. 2010). It has been reported that the sugar concentration is high near the center of watermelon fruits (Fukuoka et al. 2008; Ikeda et al. 2011). In our previous study, we demonstrated that the presence of seeds was not a trigger for the accumulation of sugar in the center of the fruit (Kawamura et al. 2018). However, we did not investigate the mechanism of sugar accumulation in detail, especially the detailed distribution and deviation of sugars in the fruit. In our present study, we analyzed the sugar concentration in seven different parts of the fruit flesh to investigate the localization of sugars in the fruit. Moreover, we performed an isotope analysis to examine the translocation of photosynthates into the fruit; then, we analyzed the relationships among the sugar concentration, water status (osmotic pressure), and 13C ratio.
An isotope analysis is frequently performed to investigate the transport of photosynthates in plants (Finazzo et al. 1994; Okano et al. 1983; Yakushiji et al. 1998). In some studies, the translocation of photosynthates in Cucurbitaceae plants was investigated by an isotope analysis (Barzegar et al. 2013; Lima et al. 2020; Watanabe 2004). In this study, we applied the 13CO2 isotope to leaves of watermelon plants during fruit growth and determined the ratio of photosynthates translocated to different parts of the fruit flesh in which the sugar concentration and water status were measured and analyzed. Then, we determined the relationship between sugar accumulation and isotope translocation in the fruits. The fruits were harvested 21 d after anthesis. The cultivar Hitorijime–BonBon was used because it can be harvested 40 d after anthesis. Because the photosynthate transport may be inhibited in mature fruits, we investigated immature fruits.
One of the objectives of this study was to investigate the center of a watermelon fruit, which is considered the largest sink, using three different methods, namely, high-performance liquid chromatography (HPLC) to determine the sugar concentration, isopiestic thermocouple psychrometer to determine the water status (water potential and osmotic potential = osmotic pressure and turgor), and 13CO2 isotope analysis with a gas analyzer. Consequently, we demonstrated that the active photosynthate accumulation occurred in the center of the fruit, which was associated with the osmotic pressure difference that induced sugar accumulation.
Additionally, we determined the effects of different nutrient solution concentrations on fruit quality. Applying water stress is a well-known technique used to induce sugar accumulation in fruits of Satsuma mandarin (Yakushiji et al. 1996, 1998) and tomato (Nahar et al. 2011; Sanchez-Rodriguez et al. 2012). In watermelons, such a technique increases the amino acid (citrulline) concentration in leaves (Kawasaki et al. 2000). However, the water stress generated by the high nutrient solution concentration in hydroponics has not been frequently investigated; therefore, related measurements were also conducted at high liquid fertilizer concentrations and the results were analyzed.
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