Root Pressure and Root Flow

Immediately after the plants were cut from the pots, a pre-weighted cotton ball was placed on the cut surface of the stem and a plastic bag was secured over the top to arrest evaporation. Cotton balls were removed after 15 min and re-weighed to calculate the quantity of sap pushed out of the root system through the cut stem (‘root flow rate’) (Zhang et al., 1995). Directly after measuring root flow rate, a 0–0.21 MPa pressure transducer (model PX26-030DV, Omega Engineering, Inc., Stamford, CT, USA) was firmly attached via nested sections (ca 2 cm) of polypropylene tubing (Sperry, 1983). Root pressure was then recorded for 24–48 h via a data-logger (CR1000, Campbell Scientific, Inc., Logan, UT, USA) and the maximum pressure documented during this time was used for analyses.

We calculated the pre-dawn root flow rate that is equal to the pre-dawn transpiration demand in the fully watered treatments for each genotype. We interpret this ‘equalizing’ water potential and flow rate as the minimum limit where xylem water potential at the base of the plant could approach zero, and thereby facilitate refilling via positive pressure. We note that our measured root flow rates likely overestimate the flow rates in intact shoots because they do not include resistances in stems and leaves (stems and leaves were cut off to facilitate measurement). As such, the root flow rate that equalizes transpiration represents a conservative limit for when equalizing flow could possibly occur, whereas the water potential corresponding to the equalizing flow in an intact shoot would likely be even closer to zero.