IMPROVED HEAT-BALANCE METHOD FOR DETERMINING SAP FLOW-RATE

The heat balance method measures sap flow in plants by heating a small section of the stem and determining the amount of heat transported away from the heater by xylem water movement. Two methods have been available for measuring sap flow in herbaceous species, but both have limitations, due either to a failure to correctly measure all components of the stem energy budget or to the lack of control of heater temperature. Aspects of both methods were combined to provide temperature control of the heater on the stem and monitoring of all conductive heat losses. The new sap flow meter was used on potted maize, sunflower and potato, and was shown to give transpiration estimates which were within 8% of those from weight loss measurements. Measurements on field-grown potato showed good agreement with estimates from a Penman calculation. The magnitude of the conductive and convective heat loss components was measured, and errors were determined for several methods of estimating these losses. Construction details and control circuitry are shown. A numerical simulation of the method showed that inserting thermocouples into the stem to provide radial averaging of stem temperature resulted in a significant reduction in error of the flow measurement.