Two-year-old, greenhouse-grown, potted `Granny Smith' apple (Malus domestica Borkh.) trees grafted on seedling, M.7 EMLA, or Mark rootstock were subjected to water stress by withholding irrigation for three successive days. Sap flow rates were measured with steady-state, heat-balance stem gauges; daily water use was measured with an electronic balance; and instantaneous transpiration was measured with a diffusion porometer. Differences in the sap flow among the three rootstocks were observed by the third day of stress treatment. The cumulative sap flow over the 3-day stress period was higher for trees on seedling and M.7 EMLA rootstocks than for those on Mark. Cumulative sap flow was reduced on the second and third days of water stress compared to sap flow in the control. The diurnal peak of sap flow rate was flattened on the second and third days of stress compared to the pattern for the control and first day of stress treatment. In general, the trees on the standard rootstock (seedling) were least affected by the water stress; trees on the full dwarf rootstock (Mark) were the most affected. Good agreement between cumulative sap flow measurements and gravimetric measurements indicates that the steady-state, heat-balance method is practical and accurate.
Comparison of Tissue Heat Balance- and Thermal Dissipation-Derived Sap Flow Measurements in Ring-Porous Oaks and a Pine