Relationships between sap flow and hydraulic conductivity in soybean plants*

The objective of this study was to determine the dependence of hydraulic conductance of the soybean root system upon sap flow rate and water potential gradient. Experiments were carried out in a growth cabinet on pot- ted soybean plants grown in a glasshouse. Various proportions of the root system were removed either by cutting main root branches or by slicing through the soil mass perpendicular to the growing axis. A few leaves on each plant were covered to prevent transpiration. Their leaf water potential (LWP) was measured at the end of the experiments in a pressure chamber. Transpiration rate was estimated by weighing the pots at 30-min intervals. Hydraulic conduc- tance of the remaining root system was calculated as the ratio of transpiration rate to the covered leaf water potential. The same plants were then detopped and their root medium was enclosed in a pressure chamber.