LAUSR

Maintaining xylem water transport under drought is vital for plants, but xylem failure does occur when drought-induced embolisms form and progressively spread through the xylem. The hydraulic method is widely considered the gold standard to quantify drought-induced xylem embolism. The method determines hydraulic conductivity (Kh) in cut branch samples, dehydrated to specific drought levels, by pushing water through them. The technique is widely considered for its reliable Kh measurements, but there is some uncertainty in literature on how to define stable Kh and how that relates to the degree of xylem embolism formation. Therefore, the most common set-up for this method was extended to measure four parameters: (i) inlet Kh, (ii) outlet Kh, (iii) radial flow from xylem to surrounding living tissue, and (iv) the pressure difference across the sample. From a strictly theoretical viewpoint, hydraulic steady-state, where inflow equals outflow, and radial flow is zero, will result in stable Kh. Application of the set-up to Malus domestica Borkh. branches showed that achieving hydraulic steady-state takes considerable time (up to 300 min), and that time to reach steady-state increased with declining xylem water potentials. During each experimental run, Kh and xylem water potentials dynamically increased, which was supported by X-ray computed microtomography (μCT) visualizations of embolism refilling under both high (8 kPa) and low (2 kPa) pressure heads. Supplying pressurized water can hence cause artificial refilling of vessels, which makes it difficult to achieve a truly stable Kh in partially embolized xylem.