Reverse flow in roots of Sesbania rostrata measured using the constant power heat balance method

This investigation was performed to examine qualitatively and quantitatively the reverse flow in partially dried roots of Sesbania rostrata using the constant power heat balance method. First, a semi-empirical technique for estimating sheath conductance of sap-flow sensors without assuming that sap flow is zero at night was proposed. Sap flow measured with the heat balance method was compared with water uptake as measured by a potometric method. Sap flow was overestimated by 56·1% for a 3·3-mm-diameter root, and by 40·0% for 6·1 mm and 33·3% for 8·8 mm roots. However, high correlation coefficients between the rates of water uptake and sap flow demonstrated that calibration would provide reliable values for root sap flow. To detect reverse flow, a split root experiment was conducted using a S. rostrata plant with its root system divided between dry and wet compartments. Daily sap flow of the drying compartment declined whereas that in ‘wet’ root increased, suggesting that the decrease in water uptake by ‘dry’ roots was offset by the ‘wet’ roots. Reverse flow was observed at night in the root on the dry side of the container when the soil water potential was less than –0·30 MPa. The total amount of water released into the soil during the night period was estimated to be 22·5 g.