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Protocol

Hydroponically grown plants of chickpea, pearl millet or sorghum (25 DAS or 6-7th leaf stage) are standardly used to measure root hydraulic conductivity, using a pressure chamber (PMS instruments, Corvallis, Oregon, USA). The shoot is cut using a razor blade and the detached root, which is submerged in water, is carefully placed in the pressure chamber and sealed around the stem, below the cut stem area, using silicon glue and polyvinylsiloxane (Coltene President Company, Switzerland) to prevent pressure leakage. Gas pressure (nitrogen gas) is applied to the root system inside the chamber, within a range from 0.1 to 0.3MPa. The root exudate (xylem sap) from the cut surface is collected at each pressure level thrice at 5 minutes intervals using pre-weighed Eppendorf tubes stuffed with tissue paper (Kimtech Science, Ontario, USA). These measurements give three replicated values of sap exudation rate (g min-1) for each of the values of pressure applied.

Data Analysis

Difference between the initial and final weight of the Eppendorf gives the amount of root exudate in multiple of technical replications at each MPa. The sap exudation rate is then normalized by the root surface area (or any other measure for surface of exchange) (g min-1 cm-2).

The root hydraulic conductance is then the slope of the regression between the normalized sap exudation rate (in the Y-axis) and the pressure applied in MPa (g min-1 cm-2 MPa-1). The root surface area is estimated by scanning with Shimadzu scanner and analyzed with Winrhizo software (Winrhizo, Regent Ltd, Canada).

Root exudation [g H2O]=Eppendorf with collected exudate – original Eppendorf weight

Root hydraulic conductance (g H2O cm-2 min-1MPa-1) =[Root exudation/root surface area/time/pressure]

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