HEAT-BALANCE SAP FLOW GAUGE FOR SMALL-DIAMETER STEMS

Applying heat balance sap flow gauges to plant stems <10 mm in diamete r has been difficult because a miniature design is needed that can be attached to a range of stem geometries. This report presents a modifie d gauge design for use on small plant stems of irregular geometry and shows results from Glycine max with stem diameters of 3-4 mm. The gaug e was evaluated on container-grown plants by comparing gauge measureme nts of flow to gravimetric estimates of transpiration. Experiments wer e conducted in the laboratory and greenhouse, using artificial and nat ural lighting, respectively. Laboratory comparisons of gauge versus gr avimetric water loss measurements indicated that the instrument was ac curate to within +/-5% when soil water was not limiting. Similar resul ts were obtained from greenhouse tests except when soil water availabi lity was low and cumulative gauge estimates became 30-45% less than gr avimetric measurements. Differences may have reflected reduced plant w ater uptake or errors in sap flow estimates associated with low flow r ates. Gauge accuracy was not improved by including the rate change in heat storage (S) in the flow calculations because S was always less th en 3% of the total heat balance. Relationships between system temperat ure and sap flow rate suggested a diagnostic test for determining opti mum power input. A time constant of 15 s indicated potential applicati on in many agronomic and physiological studies.