The maximum specific hydraulic conductance (kmax) 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 kmax 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 kmax are a common problem when measuring woody plant samples, and it is commonly observed that kmax 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 kmax 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 kmax 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 kmax rates. Bubble formation and coalescence under positive pressure in the xylem therefore appear to be the main cause for declining kmax rates. Our findings suggest that degassing of water is essential for achieving stable and repeatable measurements of kmax 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 kmax. For complete rehydration of woody samples, incubation in water under strong vacuum for 24 hrs is suggested as a more reliable technique.