Abstract
The 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 conservation of energy and mass, where the calculated sap flow (F) is a direct measure of Tcrop. 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 Tcrop. A new sap flow gauge (EXO-SkinTM 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 Tcrop 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 Tcrop were compared for eight days. We used linear regression analysis to compare the hourly and daily values of Tcrop 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 Tcrop 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 Tcrop and that additional experiments with field grown plants are required to test the sensor at higher values of Tcrop.
Highlights
The process whereby water is transported from the soil to the plant, via the roots, and to the atmosphere, via the stomata in leaves, is called transpiration (Tcrop), an important component of the water cycle
The most accurate measurement of Tcrop is gravimetric and under field conditions this is done by using weighing lysimeters [1]-[5], an isolated soil tank mounted on a load cell that directly measures the evaporation of water from the soil (Esoil) and plant (Tcrop), which collectively is referred to as evapotranspiration (ET), i.e., ET = Esoil + Tcrop
We describe in detail the measurement of Tcrop using the new stem flow gauge sensor and lysimeter on cotton plants
Summary
The process whereby water is transported from the soil to the plant, via the roots, and to the atmosphere, via the stomata in leaves, is called transpiration (Tcrop), an important component of the water cycle. The direct measurement of Tcrop under field conditions can be done using several methods that include: a) stem flow gauges [6]-[8]; b) weighing lysimeters [1]-[5]; c) environmental chambers that cover a volume (>1 m3) of plants [9]-[11] and; d) micrometeorological methods, such as the Bowen ratio [12] [13] and Eddy covariance [14] [15]. Methods b)-d) are expensive and laborious and mainly used for research purposes Of these methods, method a), i.e., stem flow gauges, is the only direct measure of Tcrop currently available that could be used for routine measurement of crop water use and could be integrated into a water management scheme as a tool for irrigation [16]
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