Simultaneous measurements of tomato fruit and stem water potentials using in situ stem hygrometers

Simultaneous measurements were made of the water potentials of the stem and fruit of intact tomato plants (Lycopersicon esculentum Mill. var. Heinz 2653) using in situ temperature-corrected stem psychrometers. Water potential of the fruit remained consistently lower than the water potential of the stem except when the plant had been subjected to prolonged water stress. Stem water potential recovered quickly with rewatering, increasing by approximately 0.5 MPa in 1 h, but the water potential of the fruit remained consistently near −1.0 MPa. The results indicate a significant resistance to water flow between the stem and the fruit and a substantial hydraulic capacitance represented by the volume of the fruit. Diurnal changes in dimensions of tomato fruit were also measured. Fruit diameter expanded at night and contracted during the day even when the water potential gradient favoured flow towards the fruit. This indicates that bidirectional flow (to and from the fruit) is not responsible for the observed diurnal changes in the fruit dimensions.