Measurement of negative pressure in the xylem of excised roots : Effects on water and solute relations.

Transport coefficients of excised maize (Zea mays L.) roots such as the radial hydraulic conductivity (Lpr), the permeability coefficient (Psr), and reflection coefficient (σsr), were determined with the aid of the root pressure probe using osmotic and hydrostatic pressure differences as driving forces. Hydrostatic hydraulic conductivity (Lprh) was about an order of magnitude larger than the osmotic hydraulic conductivity (Lpro). 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 (Pr) of the excised roots and to determine Lpr, Psr, and σsr at Pr<0, i.e. at pressures below vacuum. In osmotic experiments, Lpro of the roots as well as Pinsr and σsr 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 (Lpro, Psr, σsr) obtained at positive or atmospheric pressure can be also used in the range of negative Pr. Root pressures were also measured on whole plants by attaching the root pressure probe to the excised end of a root. At zero transpiration, Pr values were in the range of +5 to 35 kPa (= +0.05 to 0.35 bar; reference: atmospheric pressure). When transpiration was induced, Pr 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.