Background

Plant xylem embolizes due to the entry of air into the xylem vessels under drought conditions. Resistance to embolism formation is one of the most important plant traits strongly determining species distribution, mortality and evolution (Choat et al., 2012; Rowland et al., 2015; Larter et al., 2017) and has been recently suggested as a key trait to model plant function and predict plant responses to global changes (Sperry and Love, 2015; Brodribb, 2017). Most methods used to estimate embolism resistance measures the hydraulic conductivity of embolized branch segments and relate it to the hydraulic conductance of the branch segment without embolism (Sperry et al., 1988; Melcher et al., 2012). These methods are usually time-consuming and prone to several artifacts (Wheeler et al., 2013; Trifilò et al., 2014; Beikircher and Mayr, 2016).

The pneumatic method has been recently proposed as an alternative method to estimate embolism resistance from a different point of view, not from the water flow perspective but from the direct consequence of embolism-air presence in the xylem (Pereira et al., 2016). As plant embolizes, air spaces inside the xylem increase. Embolism thus changes the pneumatic properties of branch segments. In this method, a vacuum is applied to a cut branch and the air flowing outside the branch is measured as an estimate of xylem air content. A strong relationship exists between air flow outside the branch segment and the amount of emboli in the branch xylem (Pereira et al., 2016; Zhang et al., 2018). The vacuum method presents a simple, low cost, fast and practical method to measure plant embolism. Additionally, the pneumatic method does not require rehydrating (flushing water) through samples in which the effects of drought are being studied.