Abstract

A three-layer electrical analogue model was used to calculate resistance to water movement through the roots of wheat plants growing in small weighing lysimeters. In one experiment the wheat was grown in two soil types; in a second experiment one soil type was used but different root systems were induced by controlling the water table before the start of the experimental period. Resistance calculations were based on hourly measurements of transpiration rate, leaf water potential and water uptake from three soil layers (q_i), calculated from measurements of soil water potential at three depths. The number of main roots per stem (required for the model) and root surface area in each layer (A_i) were obtained from measurements of root lengths and diameters in soil cores taken at the end of each experiment.

Estimates of the resistance to flow through stems led to estimates of ψ₀), the water potential at the stem base, at any stem flow rate. Axial (main) root resistances (R_xi) were calculated from the Poiseuille equation. Values of the resistance to water movement through the roots in layer i were calculated from the set of equations describing uptake from each layer in terms of flow rates, potential gradients and resistances; these values, inserted in the solution for 1/10 from the set of three equations, yielded total root resistances (R_T) and estimates of the effective soil moisture potential (^ψ_s) for the whole profile. (R_T) ranged from 63.9 to 627.3 bar sec mm-3 (cf. stem resistance between 24 and 70 bar sec mm^-3) and was inversely related to flow rate through the main roots, which indicated a constant potential drop (^ψ_s – ψ₀) of about 10 bars, irrespective of soil type or root system. Radial root resistances, estimated as A_t(<ψ_si – ψ₀)/q_i, ranged from 4.6 x 10⁴ to 4.2 x 10⁶ bar sec mm^-l and were inversely related to q_i.

Inaccuracies in estimates of R_xi do not affect the results much and the model used is potentially valuable as a framework for field research.