Effect of Timing of High Temperature Treatment around the Fruit on Sugar Accumulation in Watermelon (Citrullus lanatus (Thunb.) Matsum. et Nakai)

The effects of timing of high temperature treatment around watermelon fruit during the fruit enlargement were investigated with respect to the sugar content and cell size in the central, intermediate (between the two placental sectors) and outer regions of fruit. Fruit growth was significantly accelerated during high temperature treatment regardless of its timing. Cells in the central region of fruit enlarged more actively from 10 to 20 days after anthesis (DAA) at higher temperatures, but high temperature treatment after 20 DAA resulted in cell enlargement in the intermediate and outer regions of fruit. With the exception of sucrose content in the central and intermediate regions of fruit, sugar contents at harvest time increased with high temperature treatment from 10 to 20 DAA, whereas high temperature treatment after 20 DAA reduced sugar levels throughout the fruit. These results suggest that active cell enlargement in the central region of fruit during the early stage of the fruit enlargement caused by high temperature promote sugar accumulation in fruit during maturation.