The Stem Heat Balance Method to Measure Transpiration: Evaluation of a New Sensor

Abstract

The measurement of crop transpiration (T crop ) under field conditions and throughout the growing season is difficult to obtain. An available method uses stem flow gauge sensors, based on the conservation of energy and mass, where the calculated sap flow ( F ) is a direct measure of T crop . This method has been extensively tested on agronomic, horticultural, ornamental aspects and tree crops and the general consensus is that F is a measure of T crop . A new sap flow gauge (EXO-Skin TM Sap Flow) sensor, with different placement and number of thermocouples, compared to the original sensor, was introduced, resulting in a different energy balance equation to calculate F . Our objective was to compare values of T crop obtained with the new sensor on cotton ( Gossypium hirsutum , L) plants to values measured with lysimeters. For this purpose, cotton plants were grown in 11-liter pots in a greenhouse experiment and hourly and daily values of T crop were compared for eight days. We used linear regression analysis to compare the hourly and daily values of T crop measured with the sensor to corresponding values measured with lysimeters on the same plant. Using a t-test (p > 0.05) we tested if the slope of the line was significantly different than 1 and if the intercept was significantly different than 0. This test indicated that there were no statistical differences between hourly and daily values of T crop measured with the new sensor and with the lysimeters. The main advantage of the new sensor is the flexibility of the new heater, allowing for better thermal contact between the plant stem and the temperature sensors. Further, the new sensor requires less wiring and copper connectors, and the number of channels used in a datalogger to record the output from the sensor is reduced by 25%. We conclude that the new sensor correctly measures T crop and that additional experiments with field grown plants are required to test the sensor at higher values of T crop .