In this study, we hypothesized that seeds in a fruit induce this osmotic pressure imbalance because seeds might play a role of sink which induces and accumulates photosynthate for many plant species, and the osmotic pressure of seeds expects to be the highest in a watermelon fruit. To verify this, we used three varieties of watermelon (Citrullus lanatus (Thunb.) Matsum. et Nakai) ‘Hitorijime-BonBon’ (seeded), ’Sandia’ (seedless) and ’SL-S2’ (seedless) for experiments. Fifteen plants for each variety were grown hydroponically and vertically in a greenhouse. The hydroponic solution was maintained 1.2 mS cm-1 for electric conductivities and pH 5.8. Three vines per plant were grown upward. When the vines elongated to contain more than 20 nodes, one fruit was set for each vine. The fruits were supported by ball net. We obtained the fruits at 0-45 days after pollination. The portions of the center, around the seed and near the pericarp of watermelon fruit were sampled for water status measurement and analysis for sugar contents. The water status (water potential, osmotic potential and turgor) measurement was conducted by an isopiestic psychrometer. Sugar (sucrose, glucose and fructose) contents were analyzed by using HPLC, then we calculated osmotic pressure of sugar contents by Van’t Hoff’s equation.
As a result, water potentials and osmotic potentials at the tissues of center and that around seed dramatically decreased as watermelon fruits were matured. The changes of osmotic pressure calculated from the sugar contents showed similar behavior to those of the osmotic potential during fruits growth. We found these tendencies were similar at all of the varieties which we used this study, indicated that the substance which affects water status of the matured watermelon fruits is sugar, and osmotic gradients showed similar pattern whichever fruits contain seeds or not, Further physiological approach needs to consider this phenomenon.