Location: Horticultural Crops Research
Title: Estimating High Rates of Transpiration in Woody Vines with the Heat-Balance Method Author
Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: April 24, 2009
Publication Date: November 9, 2009
Citation: Tarara, J.M. 2009. Estimating high hates of transpiration in woody vines with the heat-balance method. [7th International Workshop on Sap Flow]. Acta Horticulturae. 846:193-200. Interpretive Summary: There are different methods of measuring the amount of water that is used by plants, all of which have advantages and disadvantages. One technique, designated the "heat balance" method, is adapted from engineering principles in which a flexible heater is wrapped around a pipe, and the heat applied is used to estimate the rate of water flowing through the pipe. In plants, the "heat balance" method uses the plant stem as a model pipe through which water flows. By applying heat to the stem and tracking the temperature of the stem, as in the engineering application for pipes, the amount of water flowing through the stem can be estimated. This technique works well for small stems and plants that use small amounts of water, but the technique required modification to be used in grapevines, which use large amounts of water relative to the diameter of their stems. The modified "heat-balance" method was compared with a more sophisticated method of estimating water use by plants based water vapor measurement, and a more direct measure of water use by plants, before-and-after weighing of a potted plant. All three methods agreed fairly well and further refinements to the "heat-balance" method were identified. Reliable, less-sophisticated methods of estimating water use by grapevines can provide data about vineyard irrigation and the place of vineyards in the overall scheme of agricultural water use.
Technical Abstract: Heat-balance sap flow gauges were configured to produce a more thermally uniform stem cross-section under high flow rates. On mature grapevines (Vitis labruscana) either undisturbed in the field or transplanted to large containers (ca. 1m^3 volume), with stem diameters up to 46 mm and leaf area per vine up to 27 m^2, custom-built gauges under variable power were run simultaneously with a whole-plant gas exchange system and a large balance (container vines only). Transpiration varied from about 80 g h-1 overnight to daily maxima approaching 2100 g h-1 (container vines) and 2700 g h-1 (field vines). In container vines, instantaneous estimates of transpiration (6.00 to 20.00 LST) agreed most closely between gas exchange and gravimetry ([Spearman Rank] R = 0.96; p<0.05). Estimates of transpiration were correlated significantly (p<0.05) between sap flow and gas exchange (R = 0.94), and between sap flow and gravimetry (R = 0.88). On average, cumulative estimates of transpiration over 24 h (17 to 19 L d-1) agreed within 5% (gravimetry vs. gas exchange) and within 17% (sap gauge vs. gas exchange or gravimetry). In field vines, instantaneous estimates of transpiration were correlated significantly (p<0.05) between sap flow and gas exchange (R = 0.87). Cumulative water use per vine was 18 to 22 L d-1; estimation methods differed on average by 15%. In both container and field vines, sap flow gauges appeared to underestimate high midday rates of transpiration consistently, due in part to the choice of power input. The open-loop power control algorithm maintained upstream-downstream surface temperature differences between 1.5 and 2 C during midday.