Plant and soil resistance to water flow in faba bean (Vicia faba L-major Harz.)

An experiment was conducted to determine soil and plant resistance to water flow in faba bean under field conditions during the growing season. During each sampling period transpiration flux and leaf water potential measured hourly were used with daily measurements of root and soil water potential t o calculate total resistance using Ohm's law analogy. Plant growth, root de nsity and soil water content distributions with depth were measured. Leaf a rea and root length per plant reached their maximum value during flowering and pod setting (0.31 m(2) and 2200 m, respectively), then decreasing until the end of the growing period. Root distribution decreased with depth rang ing, on average, between 34.2% (in the 0-0.25 m soil layer) and 18.1% (in t he 0.75-1.0 m soil layer). Mean root diameter was 0.6 mm but most of the ro ots were less than 0.7 mm in diameter. Changes in plant and soil water pote ntials reflected plant growth characteristics and climatic patterns. The ov erall relationship between the difference in water potential between soil a nd leaf and transpiration was linear, with the slope equal to average plant resistance (0.0165 MPa/(cm(3) m(-1) h(-1) 10(-3)). Different regression pa rameters were obtained for the various measurement days. The water potentia l difference was inversely related to transpiration at high leaf stomatal r esistance and at high values of VPD. Total resistance decreased with transp iration flux in a linear relationship (r=-0.68). Different slope values wer e obtained for the different measurement days. Estimated soil resistance wa s much lower than the observed total resistance to water flow. The change f rom vegetative growth to pod filling was accompanied by an increase in plan t resistance. The experimental results support previous findings that resis tance to water flow through plants is not constant but is influenced by pla nt age, growth stage and environmental conditions. A more complex model tha n Ohm's law analogy may be necessary for describing the dynamic flow system under field conditions.