The maximum specific hydraulic conductance (k_max) of a plant sample is a key variable in plant hydraulics research. It is a measure of the ability of a plants’ vascular system to transport water and dissolve nutrients under optimum conditions. Repeatable and precise measurements of k_max are needed for characterizing resistance of plants to embolism formation, in studies of embolism repair, and in comparative studies of hydraulic conductance. Unstable measurements of k_max are a common problem when measuring woody plant samples, and it is commonly observed that k_max declines from initially high values, especially when positive water pressure is used to flush out embolisms. This study was designed to test five hypotheses that could potentially explain why declines in k_max under positive pressure occur: (1) Lateral water flow from vessels into adjacent xylem cells, (2) swelling of pectin hydrogels in inter-vessel pit membranes, (3) nucleation and coalescence of bubbles at constrictions in the xylem, (4) physiological wounding responses, and (5) passive wounding responses, such as clogging of the xylem by debris. Prehydrated, 15-cm-long, woody stems from Laurus nobilis (Lauraceae) and Encelia farinosa (Asteraceae) collected from plants grown in the Fullerton Arboretum in Southern California, were used to test these hypotheses using a xylem embolism meter (Xyl’em). Treatments included simultaneous measurements of stem in- and outflow, enzyme inhibitors, different water temperatures, different water degassing techniques, different stem debarking methods, and varied concentrations of calcium, potassium, magnesium, and copper salts in aqueous measurement solutions.  Results/Conclusions

Stable measurements of k_max were observed at concentrations of calcium, potassium, and magnesium salts high enough to suppress bubble coalescence. A measurement solution of deionized water that was degassed using a membrane contactor under strong vacuum also resulted in stable k_max rates. Bubble formation and coalescence under positive pressure in the xylem therefore appear to be the main cause for declining k_max rates. Our findings suggest that degassing of water is essential for achieving stable and repeatable measurements of k_max through woody plant samples. Moreover, flushing embolized woody stems under high pressure even for prolonged periods of time may not result in accurate measurements of k_max. For complete rehydration of woody samples, incubation in water under strong vacuum for 24 hrs is suggested as a more reliable technique.