Pressure-increase rate affects the accuracy of stem water potential measurements in deciduous fruit trees using the pressure-chamber technique

The effect of pressure-increase rate (PIR) using the pressure-chamber technique on stem water potential measurements was studied in field-grown trees of apple (Males domestica), nectarine (Prunes persica) and pear (Pyres communis). Two aspects were examined: 1. The effects of five continuous PIRs from 0.03 up to 0.3 MPa s(-1); 2. The effects of various PIRs at the beginning of the measurement (0.03, 0.14 and 0.3 MPa s-1) followed by a low PIR (0.03 MPa s(-1)) close to the endpoint. Midday stem water potential readings decreased with increasing PIR in apple, pear and nectarine. Stem water potentials at the lowest PIR were lower by about 0.03 MPa than the values obtained by extrapolating to zero PIR. Stem water potential increased with increasing initial PIR followed by 0.03 MPa s(-1) towards the endpoint. Our data suggest that a PIR of 0.03 MPa s(-1) is acceptable for practical irrigation scheduling in deciduous trees, and that this value should not be exceeded. High PIR at the beginning of the measurement followed by a low PIR prior to the endpoint should be avoided in deciduous trees because of an unacceptably high error. Use of a standard, continuous PIR will reduce the impact of the small inaccuracy associated with a PIR of 0.03 MPa s(-1) if the thresholds for irrigation scheduling are also determined at that PIR.