| Abstract: | Sap flow measurement has been widely used recently in studyingplant response to the environment, but results have not alwaysbeen satisfactory. The possibility that the non-uniform distributionof the conducting elements in the stem cross-sectional areaintroduces error in the measurements was examined. The heatpulse method, combined with anatomical observations of the stemat the point of sap flow measurement, was applied in corn andsunflower, to study the interaction between anatomical structureand sap flow distribution. The probability of the temperaturesensor being in contact with conducting elements was stronglyaffected by its insertion depth into the stem; the measuredheat pulse velocity for a given transpirational flux was correlatedwith the number of conducting elements in contact with the sensor.Consequently, the calibration coefficient is affected by theposition of the thermocouple junction in relation to the conductingelements. This dependence of apparent heat pulse velocity ondensity of conducting elements suggests that thermal equilibrationin species with large stems may not be achieved within the limitedtime of heat dissipation. The relationship between sap flowand leaf area of single plants was used to test the consistencyand the accuracy of the sap flow measurements. The ratio ofmeasured to potential transpiration was used to validate theuse under field conditions, of the calibration coefficient determinedin potted plants. Key words: Heat pulse, transpiration, LAI, potential transpiration |
