Since the late eighties a handy and user-friendly sap flow meter (Dynagage(R)) is on the market which can quantify the sap flow through intact plant stems, based on the stem heat balance method. The documentation about its accuracy and reliability, however, is still too limited to use it as a standard method in field experiments with apple trees, We therefore tested this commercial system on potted apple trees (Malus domestica L.; cv. 'Red Elstar' and 'Jonagold'; on rootstock 'M9'vf) with stem diameters of 1.8 to 4 cm. The measured sap flow was compared with mass loss measured by an automated balance, supposing the total mass loss of the trees was equal to the water loss by transpiration. The results revealed three major problems: 1. When there was no optimum contact of the elements of the gauge with the stem, which is typically very irregular on young apple trees, the calculated sap flow rates (accumulated through 24 h) showed errors > 20%. 2. On greater than or equal to 4 year-old trees the calculated sap flow rate showed considerable time lags in periods with abruptly changing transpiration rates, mainly because this sap flow method does not account for energy which is stored in the heated stem section. 3. The constant power input to the stem given with this sap flow meter caused heat damages to the bark tissue after > 6 days of continuous measurements. In order to avoid these problems we constructed a sap flow meter which guarantees an optimum contact with the stem and works with continuously controlled power supply. Both aspects, response time and effect on the bark tissue, could be improved: in all measurement series the average sap flow (during light period) deviated < 4% and often < 1% from mass loss. The differences were usually < 15% for the short-term averages through 15-30 min. The modified sap flow meter also proved reliable during 10-day measurements in the field. However, for time-accurate measurements on apple trees with a stem diameter > 3-4 cm accounting for the energy stored in the heated stem section became indispensable. On a 10-year-old apple tree growing in an densely planted orchard the sap flow density in the stem showed a non-uniform spatial distribution which was varying with the exposition of the canopy to direct sunlight. Under such conditions the temperatures of the sap must be measured in all directions of the stem.
