A Thermoelectric Method for Determining the Rate of Water Movement in Plants

Abstract

A thermoelectric method is described for the measurement of stream flow rates in the stems of plants. It has a definite advantage in that repeated measurements can be made on the same plant without damage to it. The method is quite simple and usually requires about 15 minutes for each measurement. By subjecting cotton plants to different environmental conditions such as temperature, humidity, and wind velocity, it was found that the rate of water uptake and movement in the stem dropped from 114 cm./hr. to 76 cm./hr. when the wind velocity was changed from high to low. A temperature of approximately 99°F. and 20% relative humidity was maintained in both cases. With no wind movement but having a temperature of 82°F. and 62% relative humidity, the rate of water movement for the same plant was 38 cm./hr. Present data show that this method is well adapted for indicating the effects of micro‐climate, soil moisture availability, and soil aeration upon the rate of water uptake and movement within intact plants. In addition, it has other uses which will be of interest to both the soil scientist and plant physiologist.

Some Applications of the Thermoelectric Method for Measuring Water Flow Rates in Plants1

“…The thermoelectric method, as described in detail in a previous paper ( 2), consisted first of the brief local application of heat to the stem of a plant. A thermistor in e~ternal contact with the stem %.…”

“…The thermoelectric method for studying the effects of certain environmental conditions on water uptake and movement has been discussed briefly in another paper ( 2). Additional studies have been made which indicate that the instrument is well suited for detecting immediate plant response to changing environment as illustrated in figure 2.…”

An external heat pulse method for measurement of sap flow through fruit pedicels, leaf petioles and other small‐diameter stems

“…In this study, we used the heat ratio method with a small external heater (an electronic chip resister) and temperature sensors (fine gauge thermocouples) glued to a cork block and pressed against the surface of the stem. Externally applied heat pulses were used in early sap flow studies (Bloodworth, Page & Cowley 1955;Closs 1958), and two recent reports also used an external heat pulse method to measure sap flow in stems down to 3 mm in diameter (Bauerle et al 2002;Helfter et al 2007). Both of these two recent examples used a non-contact laser as the heat source, and infrared thermometers or a thermal-imaging camera for detecting heat pulse propagation.…”

Dynamics of a Heat Balance Stem Flow Gauge during High Flow

“…The time required for the heat pulse to travel from the input location to a point downstream of the source is correlated with the velocity of the sap stream. The accuracy ofthis method has been verified on trees (Marshall, 1958;Miller et al, 1980;Cohen et al, 1981) and herbaceous plants (Bloodworth et al, 1955;Stone and Shirazi, 1975;Cohen et al, 1988). However, the technique may damage the plant since thermocouples and heating elements are inserted into the stem tissue and requires calibration (sap velocity vs. time of pulse travel).…”