Root Hydraulic Conductance in Pima Cotton: Comparison of Reverse Flow, Transpiration, and Root Pressurization

Methods to measure root hydraulic conductance (K R , MPa; 1/resistance) under field conditions or with large plants have proven difficult to develop. We evaluated three contrasting methods of determining root conductance, a laboratory method based on hydrostatic pressure‐induced root exudation, a potential field method based on hydrostatic pressure‐induced reverse flow, and a generally applicable method involving natural gradients in water potential generated by transpiration. Pima cotton ( Gossypium barbadense L.; cv S‐6) was grown in solution (hydroponically) or in potting soil in a glasshouse. Experiments were conducted 3 to 8 wk after emergence, when leaf area ranged from 0.05 to 0.25 m 2 plant −1 . Reverse flow and root exudation increased linearly with applied pressure between 0.2 and 0.5 MPa. Transpiration increased linearly with decreasing (transpiration‐ induced) root xylem water potential between −0.5 and −2.5 MPa. In both solution‐ and soil‐grown plants, transpiration and reverse tlow yielded comparable K R . In solution‐grown plants, root exudation yielded K R about 20% lower than the other methods. In soil‐grown plants, root exudation did not yield reproducible data. All three methods indicated that K R per plant increased with increasing plant size. In contrast, K R per unit leaf area, a functional measure of hydraulic efficiency, was similar for plants of all sizes. The excellent agreement between the reverse flow and transpirational methods suggests that both may have physiological validity. The root exudation method may underestimate K R in solution‐grown plants, and to a greater extent in soil‐grown plants.