Xylem sap backflow from tomato fruit under water deficit condition

The occurrence of xylem sap backflow from fruit is known in several woody crops. In tomatoes (Lycopersicon esculentum Mill.), it is not generally accepted that the xylem backflow occurs in the plant, although our preliminary experiments revealed that the expanding fruit under water deficit condition loses slight water through xylem backflow. In this investigation we confirmed the occurrence of xylem backflow from the fruit using the fluorescent dye, Lucifer Yellow CH (LYCH), which is unable to cross the plasmalemma, as a tracer for apoplastic movement of water. For introduction of LYCH into the fruit, a small hole was made in the side of the fruit, which reached to the center of the fruit. About 0.1 ml of 1 % LYCH solution was poured into the hole in the dark period. The plant was exposed to the light (PPFD, 250μmol m^-2 s^-1) for 6 hours, and then transections of the pedicel, the peduncle and the stem were soon observed with fluorescent microscopy. The intense fluorescence of LYCH was observed in the xylem region only. In well-watered plants, the LYCH flowed out about 30 mm distance from the fruit. In water deficit plants, the LYCH centripetally moved more than 100 mm, and it consequently reached to the stem. Results indicate xylem sap flows back from the fruit during the light period under the water deficit condition. The finding involves a complicated process of sugar and calcium accumulation in the developing tomato fruit.