INTRODUCTION

Transport properties of roots have been characterized by certain transport coefficients such as the hydraulic conductivity (LPr), the permeability (Psr), and reflection coefficient (σsr). So far, these coefficients have been measured only at positive root pressures. However, according to the cohesion theory of the ascent of sap, negative pressures exist in the xylem of transpiring plants. Therefore, it is of some interest whether or not the absolute values of the transport coefficients would change when pressures in the xylem drop to negative values, i.e. below vacuum (= 0 MPa).

In the present work, LPr, Psr σsr and asr of excised roots of Zea mays L. have been measured at positive and negative root pressures with the aid of the root pressure probe. Furthermore, responses of xylem pressures of intact maize plants to changes in the rate of transpiration have been followed directly.

MATERIALS AND METHODS

The hydrostatic hydraulic conductivity (LPrh) of root endsegments was determined with the aid of a root pressure probe. In ‘hydrostatic experiments’ a metal rod was moved into (out of) the probe and hence, root pressure was changed. LPrh was calculated from the following relaxation of root pressure (Steudle, Oren & Schulze, 1987). Alternatively, in ‘osmotic experiments’ the concentration of a solute in the medium was changed and LPr, Psr, and σsr were determined from the pressure responses (Steudle et al., 1987). The addition of a nonpermeating solute (mannitol) caused the root pressure to drop to a stationary negative pressure. Now, a permeating solute (Ethanol, NaNO3) was added or removed from the medium and again, LPr, Psr and σsr were calculated from root pressure responses.