Measurement of cotton transpiration

Authors: Lascano, Robert; Baker, Jeffrey; Payton, Paxton; Gitz, Dennis; Mahan, James; GOEBEL, TIMOTHY - Texas Tech University

Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2018
Publication Date: 10/31/2018
Citation: Lascano, R.J., Baker, J.T., Payton, P.R., Gitz, D.C., Mahan, J.R., Goebel, T. 2018. Measurement of cotton transpiration. Agricultural Sciences. https://doi.org/10.4236/as.2018.910091
DOI: https://doi.org/10.4236/as.2018.910091

Interpretive Summary: A difficult value to measure throughout the growing season is the daily evaporative loss of water from the soil and crop, evapotranspiration (ET). Values of ET can be used, for example, to schedule irrigation and to calculate crop yield per unit of ET. One way to measure crop ET is to isolate plants in a large soil volume (7-10 cubic yards) and weighing the soil with an accuracy of ± 0.002 inches of water evaporation. This is not an easy measurement and requires expensive equipment to get this accuracy day after day and throughout the growing season. An alternative and less expensive technique to measure ET is by using stem flow gauges. These gauges consist of a small flexible heater, less than 1-inch diameter, wrapped around the plant stem that when energized with power causes a difference in the temperature of the sap flow through the stem above and below the heater. These changes in temperature, measured with thermocouples, are a direct measure of crop water use. Since the 1990’s stem flow gauges have been successfully tested on many crops. Recently, however, a commercial company introduced a new design of the gauge that needed to be tested as to its accuracy to measure crop water use. This was our objective and to test the new sensors we did a field experiment with cotton, at the USDA-ARS in Lubbock, TX. Not having access to a large weighing box we used growth chambers designed by an ARS Lubbock scientist that measures crop water use. These are portable chambers, about 35 cubic feet, constructed of aluminum and covered in a transparent material. The amount of water vapor coming in and out of the chamber is measured with an infrared gas analyzer, a very accurate measurement, and the difference in water vapor concentration is a direct measure of crop water use. To test the new sensor we compared the cotton water use of the same plants as measured with the new sensor and growth chamber. In our tests we used three chambers, and each chamber had 4 plants with new sensor, and found no difference between the two measured values. We conclude that the new design of the stem flow gauge provides an accurate measure of crop water use and is an inexpensive tool that can be used to manage irrigation and plant productivity.

Technical Abstract: There are a few field methods available to directly measure water evapotranspiration (ET) along with its two components, evaporation from the soil (E) and from the crop (T). One such technique that measures T, uses sensors to calculate the sap flow (F) of water through the plant stem and is based on the conservation of mass and energy, i.e., the stem heat balance method. This instrument consists of a flexible heater that is wrapped around the plant stem with temperature sensors to measure the difference in temperature of F above and below the heater. This is a null method, where all inputs and outputs are known and the calculated F is a direct measure of T. This method has been used to measure T in a variety of crops, including cotton, grapes, olive trees, soybean as well as ornamental and horticultural crops. Recently, a new sap flow gauge sensor (EXO-Skin™ Sap Flow) was commercially introduced that had a radically new design resulting in a different energy balance, compared to the original design, that needed experimental verification. An initial evaluation was done with potted cotton (Gossypium hirsutum, L.) plants in a greenhouse experiment and showed that values of cotton-T measured with the new sensor were accurate; however, this comparison was limited to daily values of T < 2 mm/d. Thus, our objective was to expand the initial evaluation of the new sensor under field conditions and for daily values of cotton-T in the 4 – 9 mm/d range, representative of the semiarid Texas High Plains. For this purpose cotton was planted on 12 June 2017 on a 1000 m2 plot in a soil classified in the Amarillo series at the facilities of the USDA-ARS, Lubbock, TX. For a period of 10 days, 2 to 11 Sep 2017, we measured hourly cotton-T with the new sensors and with portable growth chambers (0.75 m '' 1 m cross-section, and 1 m height) where water vapor flux was measured at 1 Hz using an infrared gas analyzer. We used three chambers and, in each chamber, we installed the new sensor on four cotton plants. Linear regression analysis was used to compare hourly and daily values of cotton-T measured with the sensor as a function of the corresponding value measured with the growth chamber. 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 pooling all data yielded an almost 1:1 relation between values for a daily range of 4 to 9 mm/d tran-spiration. We conclude that the new sensor provides an accurate and direct measure of hourly and daily cotton-T for a wide range of environmental conditions.