Measurement of negative pressure in the xylem of excised roots

Abstract

Transport coefficients of excised maize (Zea mays L.) roots such as the radial hydraulic conductivity (Lp_r), the permeability coefficient (P_sr), and reflection coefficient (σ_sr), were determined with the aid of the root pressure probe using osmotic and hydrostatic pressure differences as driving forces. Hydrostatic hydraulic conductivity (Lp_rh) was about an order of magnitude larger than the osmotic hydraulic conductivity (Lp_ro). Provided that the air of the cortical intercellular spaces was replaced by water to avoid air-seeding, it was possible to measure negative root pressures (P_r) of the excised roots and to determine Lp_r, P_sr, and σ_sr at P_r<0, i.e. at pressures below vacuum. In osmotic experiments, Lp_ro of the roots as well as Pinsr and σ_sr were measured at positive and negative root pressures using ethanol and NaNO₃ as permeating solutes. Negative root pressures were obtained by adding non-diffusable solutes (mannitol, KNO₃) to the medium. Transport parameters did not change in the range of root pressures between -0.2 to + 0.3 MPa (-2 to +3 bar). Thus, data of transport coefficients (Lp_ro, P_sr, σ_sr) obtained at positive or atmospheric pressure can be also used in the range of negative P_r. Root pressures were also measured on whole plants by attaching the root pressure probe to the excised end of a root. At zero transpiration, P_r values were in the range of +5 to 35 kPa (= +0.05 to 0.35 bar; reference: atmospheric pressure). When transpiration was induced, P_r dropped immediately with a response time of less than 1 min. However, it was not possible to measure negative pressures in these experiments because the air in the intercellular spaces caused cavitations. Responses of root pressures to changes in transpiration were completely reversible.