Validating sap flow measurement in field-grown sunflower and corn

Sap flow measurement has been widely used recently in studying plant response to the environment, but results have not always been satisfactory, The possibility that the non-uniform distribution of the conducting elements in the stem cross-sectional area introduces error in the measurements was examined. The heat pulse method, combined with anatomical observations of the stem at the point of sap flow measurement, was applied in corn and sunflower, to study the interaction between anatomical structure and sap flow distribution, The probability of the temperature sensor being in contact with conducting elements was strongly affected by its insertion depth into the stem; the measured heat pulse velocity for a given transpirational flux was correlated with the number of conducting elements in contact with the sensor. Consequently, the calibration coefficient is affected by the position of the thermocouple junction in relation to the conducting elements, This dependence of apparent heat pulse velocity on density of conducting elements suggests that thermal equilibration in species with large stems may not be achieved within the limited time of heat dissipation. The relationship between sap flow and leaf area of single plants was used to test the consistency and the accuracy of the sap flow measurements. The ratio of measured to potential transpiration was used to validate the use under field conditions, of the calibration coefficient determined in potted plants.