Sap flow measurements in grapevines - Scope and limits of heat balance andGranier-type sensors

The use of two methods for measuring sap flow in intact plants, the heat ba lance and the Granier-system have been evaluated for their use in thicker g rapevine stems of more than 35 mm in diameter. The original calibration of Granier was extended from a sap flux density of 140 10(-6) m(3) m(-2) s(-1) to around 225 10(-6) m(3) m(-2) s(-1) without any error and to 380 10(-6) m(3) m(-2) s(-1) when accepting an error of up to 7%. In contrast to the heat balance method a time lag of around 20 min b etween calculated and real flow is evident, which was attributed to the the rmal mass of the sensor itself. The time lag and the consequently dampened response of the system caused a very low accuracy over short time periods r educing the value for detailed plant physiological investigations. However, when integrating over longer time intervals, much of the errors cancelled out. For daily values the maximum error was within +/-10% and after a perio d of 89 days only 1.5% error remained. This method is thus best suited for long term measurements of total water use. The heat balance system proved to be a very responsive system reflecting ev en short term fluctuations in sap flow of 10 min. However, as even 10-20 ye ar old xylem vessels transport significant amounts of water, large inhomoge neities in sap temperature developed in the stem. Consequently only very sm all water fluxes, where a relatively homogeneous temperature profile was ac hieved, were measured correctly. The more the flow rate increased, the larg er the resulting inhomogeneity was. At high flow rates the calculation of t he heat balance resulted in an overestimation of 50 - 100%. The heat balanc e method is thus not suited for thicker stems of grapevines due to signific ant water conduction even in very old xylem vessels.