Background

Although plants can recover from critical levels of xylem embolism, < 50% loss of hydraulic conductivity in conifers (Brodribb and Cochard, 2009) and < 88% in angiosperms (Urli et al., 2013), the exact mechanism is still under debate. The ascent of sap is driven by the evaporative demand from the atmosphere, which generates a negative pressure (i.e., tension) in the water column and hydrogen bonds between molecules (i.e., cohesion) pull the sap through the plant via the well accepted cohesion-tension theory (Dixon, 1896; Angeles et al., 2004). However, positive xylem sap pressure can be recorded under particular conditions, for example water-saturated soil combined with very low transpiration. This mechanism has been shown to refill embolized vessels in springtime (Sperry et al., 1994) and in species that experienced freeze-thaw induced embolism (Charrier et al., 2013 and 2014). Refilling of embolized vessels has been hypothesized to occur under both positive and negative xylem sap pressures in Laurus sp or Vitis sp, for example (Salleo et al., 1996). However, the ‘refilling under tension’ mechanism is inconsistent with the cohesion-tension theory (Zwieniecki and Holbrook, 2000). Moreover, recent works suggest that refilling occurs only under positive pressure in Vitis (Charrier et al., 2016). The dynamic changes in xylem sap pressure therefore need to be explored at both the seasonal and diurnal scale while maintaining as much as possible the integrity of the hydraulic architecture of the plant.

Although the use of stem psychrometers has been extensively described since the 80’s (e.g., Dixon and Tyree, 1984; Tyree and Dixon, 1986), the measurement of positive xylem sap pressure is relatively rare. The protocol described here allows the quantification of the spatio-temporal pattern of bulk xylem sap water potential under positive pressures, and even moderate tensions (maximum of 0.05 MPa) along the water column using non-invasive sensors (i.e., point pressure sensors).