Effects of Heating Bearing Shoot near Fruit on Cell Size, Sucrose Metabolizing Enzymes and Sugar Accumulation in Watermelon Fruit

Shoots near fruits of watermelon (Citrullus lanatus Matsumu. et Nakai) were continuously heated from 5 days after anthesis (DAA) to a minimum of 30°C during night. Temperatures in a plastic greenhouse during 0 a.m. to 6 a.m. fluctuated between 12.7–17.2°C, and the temperature of the surface of the fruit, whose near shoots were heated (designated as a heated fruit) during these hours, was higher by 0.8°C on the average than that of the control, and also the temperature of the flesh of the heated fruit was 1.0–1.5°C higher than that of the control fruit. Pigmentation of the flesh of the heated fruit was accelerated. Cell size of the heated fruit was smaller than that of the control fruit. Higher sucrose phosphate synthase (SPS) activity of the heated fruit resulted in the higher mean sucrose and fructose content of the heated fruit. Therefore, it is reasonable to consider that heating bearing shoots near fruits can accelerate the rate at which cells enlarge and mature, which causes an increase in SPS activity and an increase in the sucrose content in the fruit, especially the outer portions of the fruit.