Monitoring Xylem Hydraulic Pressure in Woody Plants.

Xylem sap circulates under either positive or negative hydraulic pressure in plants. Negative hydraulic pressure (i.e., tension) is the most common situation when transpiration is high, and several devices have been developed to quantify it accurately (e.g., Scholander pressure chamber, psychrometers). However, a proper measurement of positive xylem sap pressures may be critical when pressure is generated by the root system, allowing vessels to be refilled. Here, we describe two different methods to monitor positive xylem bulk pressure: the pressure gauge which can only be set onto a rootstock or a side branch and the point pressure sensor, which can allow measurements from a functioning plant without detopping or cutting.