Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2016
Publication Date: 9/20/2016
Citation: Lascano, R.J., Goebel, T.S., Booker, J.D., Baker, J.T., Gitz, D.C. 2016. The stem heat balance method to measure transpiration:Evaluation of a new sensor. Agricultural Sciences. 7:604-620.

Interpretive Summary: The water lost from leaves to the atmosphere is called transpiration (T) and its measurement under field conditions and throughout the growing season is difficult. One method that is commercially available and that can be used for this purpose is a sensor that uses the stem balance method to measure T. This sensor basically consists of a heater that is wrapped around the stem of the plant causing a difference in temperature measured with thermocouples above and below the heater. Further, the sensor uses what is known as a null method, where all inputs and outputs are known or can be calculated. For example, in this sensor required inputs are the area and the thermal conductivity of the stem, and electrical properties of the sensor. This means that there is no calibration. A commercial version of this sensor has been used extensively and the general consensus is that indeed the sensor measures crop T. Recently, a new version of the sensor was introduced that modified the placement and number of thermocouples used to measure changes in temperature. These modifications were not trivial and resulted in a different energy balance and a different equation to calculate T. Therefore, our objective was to test the new sensor and we did our tests with cotton plants grown in a greenhouse. The cotton T was measured with the new sensor and also by measuring the changes in mass of the potted cotton plants grown in 3-gallon containers. The changes in mass were used to calculate T. We then used linear regression to compare values of T measured with the new sensor and measured by changes in mass and we determined that the two values were statistically the same. This means that the values of T measured with the new sensor were correct. However, given the limitations of the potted plants we were not able to measure large values of T. Our conclusion is that for the conditions of our experiments with potted plants the new sensor works and provides accurate value of T.

Technical Abstract: The direct measurement of crop transpiration (Tcrop) under field conditions and throughout the growing season is difficult to obtain. An available method uses stem flow gauge sensors, based on the stem heat balance. The sensor consists of a small heater that is wrapped around the stem of the plant and measures the change in temperature of the sap flow through the stem. The method is based on the conservation of energy and mass, where inputs and outputs are known and the calculated sap flow (F) is a direct measure of Tcrop. The method has been extensively tested on agronomic, horticultural, ornamental and tree crops and the general consensus is that F is a measure of Tcrop. A new sap flow gauge (EXO-Skin™ 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 evaluate the new sensor by measuring Tcrop on cotton (Gossypium hirsutum, L) plants in a greenhouse experiment for a period of eight days. Cotton plants were grown in 11-Liter pots. Hourly values of Tcrop measured with the new sensor were compared to hourly values of Tcrop measured with lysimeters on the same plant. Using linear regression analysis we compared hourly and daily values of Tcrop measured with the new sensor to corresponding values measured with lysimeters. 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 and this test indicated that there were no statistical differences between hourly and daily values of Tcrop measured with the new sensor and with the lysimeters. This experiment was done under greenhouse conditions and the daily Tcrop was < 2 mm/d. 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 Tcrop and that additional experiments with field grown plants are required to test the sensor at higher values of Tcrop.