An Improved Thermoelectric Probe for Measurements of Apparent Sap Flow Velocity in Intact Plant Stems

Abstract

Gray, H. R., Erickson, P. I. and Stone, J. F. 1985. An improved thermoelectric probe for measurement of apparent sap flow velocity in intact plant stems.—J. exp. Bot. 36: 1320-1324. An improved thermoelectric technique for non-intrusive, non-destructive, and direct sensing of the apparent velocity of sap flow in intact plant stems is presented. The thermoelectric probe was made of extruded Teflon (DuPont) rod. One of two heating elements, positioned 0-5 cm and 10 cm below the temperature sensor, was used to generate the heat pulse. Heating element selection was based on daily environmental conditions. A thermistor touching the stem sensed the heat pulse. A second thermistor mounted in close proximity was used to cancel ambient temperature fluctuations. The improved thermoelectric probe was tested using irrigated peanuts (Arachis hypogeae L.). After canopy closure was complete in both treatments, apparent sap flow velocity (Fj was measured on 17 d at 1200 s intervals between 0830 h and 1630 h apparent solar time. Successful E, measurements were accomplished on 94% of the attempts. In order to verify performance on other species with different types of stems, non-replicated Vt determinations were also made on cotton (Gossypium spp.), prickly sida (Sida spinosa L.), goosegrass (Eleusine indica (L.) Gaertn.), Palmer pigweed (Amaranthus Palmerii S. Wats.), and cottonwood (Populus deltoides Marsh.). Success of the probe was due in part to the design of the probe which included remotely selectable spacing between heater and thermistor and adequate insulation and shielding of the probe and sensed portion of the stem. In addition, acclimatization of the probe to the operating environment was essential. Key words—Sap velocity, instrumentation. Correspondence to: Department of Agronomy, Oklahoma State University, Stillwater, OK 74078, U.S.A.