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

The external heat ratio method is described for measurement of low rates of sap flow in both directions through stems and other plant organs, including fruit pedicels, with diameters up to 5 mm and flows less than 2 g h⁻¹. Calibration was empirical, with heat pulse velocity ( v _h) compared to gravimetric measurements of sap flow. In the four stem types tested ( Actinidia sp. fruit pedicels, Schefflera arboricola petioles, Pittosporum crassifolium stems and Fagus sylvatica stems), v _h was linearly correlated with sap velocity ( v _s) up to a v _s of approximately 0.007 cm s⁻¹, equivalent to a flow of 1.8 g h⁻¹ through a 3-mm-diameter stem. Minimum detectable v _s was approximately 0.0001 cm s⁻¹, equivalent to 0.025 g h⁻¹ through a 3-mm-diameter stem. Sensitivity increased with bark removal. Girdling had no effect on short-term measurements of in vivo sap flow, suggesting that phloem flows were too low to be separated from xylem flows. Fluctuating ambient temperatures increased variability in outdoor sap flow measurements. However, a consistent diurnal time-course of fruit pedicel sap flow was obtained, with flows towards 75-day-old kiwifruit lagging behind evaporative demand and peaking at 0.3 g h⁻¹ in the late afternoon.