Direct measurement of xylem pressure in leaves of intact maize plants. A test of the cohesion-tension theory taking hydraulic architecture into consideration

The water relations of maize (Zea mays L. cv Helix) were documented in term s of hydraulic architecture and xylem pressure. A high-pressure flowmeter w as used to characterize the hydraulic resistances of the root, stalk, and l eaves. Xylem pressure measurements were made with a Scholander-Hammel press ure bomb and with a cell pressure probe. Evaporation rates were measured by gas exchange and by gravimetric measurements. Xylem pressure was altered b y changing the light intensity, by controlling irrigation, or by gas pressu re applied to the soil mass (using a root pressure bomb). Xylem pressure me asured by the cell pressure probe and by the pressure bomb agreed over the entire measured range of 0 to -0.7 MPa. Experiments were consistent with th e cohesion-tension theory. Xylem pressure changed rapidly and reversibly wi th changes in light intensity and root-bomb pressure. Increasing the root-b omb pressure increased the evaporation rate slightly when xylem pressure wa s negative and increased water flow rate through the shoots dramatically wh en xylem pressure was positive and guttation was observed. The hydraulic ar chitecture model could predict all observed changes in water flow rate and xylem. We measured the cavitation threshold for oil- and water-filled press ure probes and provide some suggestions for improvement.