Soil solute concentration and water uptake by single lupin and radish plant roots: II. Driving forces and resistances

Soil water extraction (Q) and the corresponding values of matric (Ψm) and osmotic (Ψπ) potentials near single roots of eighteen day old lupin and radish plants, subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels, were determined using a combination of computer assisted tomography applied to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes. This data together with leaf water potential (Ψ₁) measured by psychrometer, were used to calculate the differences in total water potentials (∆Ψ) and the differences in hydrostatic pressure (∆P) between the leaves and the total potentials at the root surfaces, and to estimate soil and plant resistances to water movement. The linear relations for ∆Ψ(Q) and ∆P(Q) observed, imply that both plant roots acted as near perfect osmometers under the conditions of the experiments. This is further supported by the close agreement between the increases in ∆P and the decreases in Ψπ at the root surface with increasing solute concentration in the treatments. Plant resistances were constant with time of transpiration and increased with increasing Na⁺ in the treatments. Soil resistances between the root surface and bulk soil increased as the water content decreased remaining lower at the higher solute concentrations due to the lower extraction rates. At the high water potentials used plant resistances were always substantially higher than corresponding soil resistances.