5.1.4  Speed of sap flow

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A number of methods have been used to determine the speed of sap flow. This task is not easy because xylem is not a
uniform conduit supporting a single flow rate. Some of the main approaches are outlined below.

(a)  Dyes

Dyes have been used to detect water flow for centuries. Such methods developed a poor reputation until dyes were selected for their ability to move freely in the xylem: dyes which associate with the negatively charged vessel walls move much more slowly than the sap in which they are dissolved. Modern dyes, generally negatively charged ions, have been especially useful to illustrate pathways of water flow (Section 5.2). However, if dyes are to be used to interpret flow rates, caution must be exercised. Applying a dye solution, at atmospheric pressure, to plant tissue with vessels under tension can lead to the dye being sucked into vessels artefactually. This will be most acute in water-deficient tissues. Interpreting the direction and rate of sap flow in this case will tell little about the steady-state condition. Also, when dyes are used to assess mass transfer, the capacity of a vessel (related to its width) will influence the amount of dye passing through that vessel. So, fine capillaries might transport only a small amount of dye but at a rapid rate.

(b)  Radioisotopes

Radioisotopes are useful tracers of water flow, but they suffer from the same problems as dyes. Cationic isotopes, especially divalent cations, tend to bind to the xylem walls. Radioactive phosphorus makes a good tracer because it is anionic and a strong emitter.

(c)  Heat flow methods

Heat-flow and heat-pulse methods are widely used for the non-invasive measurement of water flow in stems and tree trunks. One ideal approach introduces a steady source of heating to the stem or trunk. This is surrounded by temperature detectors (thermistors or thermocouples). If there is virtually no sap flow, say, late at night, temperatures around the heater adopt a symmetrical ‘zero-flow’ profile. When water flow occurs, the thermal profile is dragged upwards, providing a temperature shift which can be converted by appropriate computer circuitry into a continuous measurement of flow. Automatic devices of this kind are now being used widely for irrigation scheduling.

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