ACCURACY OF SAP FLOW MEASUREMENT USING HEAT-BALANCE AND HEAT PULSE METHODS

The heat balance and the heat pulse methods for measurement of sap flow in plants, were compared under conditions of variable evaporative demands in order to evaluate their accuracy. The study was conducted with potted soybean (Glycine mar Merr. cv. tamahomare) and corn (Zea mays cv. kakuteru 901) plants housed in a plastic chamber. The computed sap flow was compared to transpiration rate as measured by weight loss. The calibration coefficient for the heat pulse expresses the ratio of actual transpiration rate, Tr (mg s-1), to measured heat velocity, v (mm s-1), multiplied by the stem area, a (mm2). The ratio Tr/(v x a) was 1.01 +/- 0.08 and 1.73 +/- 0.1 for soybean and corn, respectively. The coefficient was not affected by plant age, size, or atmospheric evaporative demand. Accuracy of measurements using the heat pulse method was poor at low transpiration rate. The heat balance method does not require calibration (except for the gauge factor for estimating the sheath conductance). Transpiration was estimated accurately by this method in both species at low or moderate rates (0.6 L d-1). As the transpiration rate increased, the computed sap flow deviated from the measured transpiration. In soybean, transpiration was underestimated by 20%, while in corn, it was overestimated by 25%. Results indicated that the basic assumption of the heat balance method (i.e., that the stem surface temperature is considered a reliable measure of the average temperature over the entire cross-sectional area) may be incorrect either at high or fluctuating flow rates, or whenever the water conducting system is distributed unevenly in the stem cross section.