AXIAL RESISTANCE TO WATER-FLOW OF INTACT COTTON TAPROOTS

A knowledge of root hydraulic resistance is required for the applicati on of water Bow models of the plant-soil system. An attempt was made t o estimate axial resistance of intact cotton (Gossypium hirsutum L. cv . Tamcot CAMD-E 107-5A) taproots to water flow by direct and simultane ous measurement of water flow rate and of the gradient in xylem water potential. Fiscus's root psychrometer was modified and used for the me asurement of xylem water potential. The measurements were made in rigi dly controlled, constant temperature environments, and under condition s in which no water was available for the portion of the taproot where xylem water potential gradient was measured, but water was available distal to that portion. The water uptake rate was simultaneously measu red while water was supplied at a constant head. Axial resistance to w ater flow along a cotton taproot of 70- to 12O-d-old plants was on the order of 1 mg(-1) water m(-1) root s MPa, which is near the lower lim it of published values for cotton and other species. Axial resistance was also calculated from the measured xylem vessel diameters based on the Poiseuille-Hagen equation. Measured axial resistance values were a pproximate to 20 times those calculated using the Poiseuille-Hagen equ ation. It required as long as 34 d for the water potential inside the psychrometer chamber to equilibrate with that of the root xylem. Dryin g the root substantially increased the radial resistance and thereby i ncreased the time required for equilibration. Modifications to the des cribed experimental apparatus and procedure will be required.