Dynamic Analysis of Growth, Water Balance and Sap Fluxes through Phloem and Xylem in a Tomato Fruit: Short-term Effect of Water Stress

Abstract

In order to examine the short-term response of tomato (Lycopersicon esculentum Mill.) plants to water stress, we investigated the dynamics of fruit growth and water balance in growing fruits under the well-watered and the water deficit conditions. Under the well-watered condition, most (84%) of pedicel sap flux toward the fruit contributed to fruit expansion, and the residual (i.e. only 16%) was lost by transpiration from the fruit and its calyx. The most dominant component of the fruit water balance was the phloem sap flux, since a major part (i.e. 70%) of the sap delivered into the fruit was brought by phloem transport. On the other hand, xylem transport made a lesser contribution to the fruit expansion and exhibited larger susceptibleness to water stress. Under the water deficit condition, the fruit growth rate was depressed to 36% of well-watered plants, while phloem sap influx to the fruit was less affected. Xylem sap backflow occurred frequently under the water deficit condition, and the net influx of xylem sap during the whole day was nearly zero. The phloem transport solely sustained the fruit growth under water stress. This study quantitatively demonstrated that water deficit results in smaller fruits with higher sugar concentrations through different responses of phloem and xylem transport to water stress.