THE WATER RELATIONS OF THE ROOT-SOIL INTERFACE

The difference in hydrostatic pressure between the xylem of the leaf a nd the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway, Many studies hav e shown that the sum of the resistances in the plant and the soil is t oo small to account for the fall in water pressure between the leaf xy lem and the soil, especially when plants are growing in sandy soils, w hich are prone to dry rapidly, A resistance at the root-soil interface , caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy, We explored the resistan ce in the pathway from soil to leaf using a technique that allows prec ise and continuous non-destructive measurement of the hydrostatic pres sure in the leaf xylem, When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial r esistance, However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70 kPa, the hydrostatic pressure in the l eaf xylem fell several times faster than that in the soil, We suggest that solutes accumulated either in the root or just outside it, creati ng large osmotic pressures, which gave the appearance of an interfacia l resistance.