Root Sap Flow in Sesbania rostrata during Soil Drying

Abstract

Water movement in plant roots, under conditions in which part of the root system is in wet soil and part is in dry soil, was examined using Sesbania rostrata. A Sesbania plant was grown in a wooden box with two compartments which divided the root system. After a complete root system was established in each compartment, one compartment was allowed to dry, while the other was watered regularly. The heat balance method was used to monitor sap flow in a thick root in each compartment and in the stem. Time-domain reflectometry (TDR) was used to monitor changes in the soil's volumetric water content. Over the drying period daytime sap flow in the dry root declined, while that in wet root increased. This suggested that when transpiration is active, a decrease in water uptake by dry roots is compensated by wet roots. Reverse flow was observed at night when soil water content was less than 5.7% which corresponded to water potential of 0.51MPa for the soil.