Response to oxygen deficiency in primary maize roots. II. Development of oxygen deficiency in the stele has limited short-term impact on radial hydraulic conductivity

Abstract

The short-term impact of oxygen deficiency on root hydraulic conductivity (Lp_r), was evaluated in excised maize roots using hydrostatic and osmotic driving forces, after exposing the roots to a flowing medium containing 0.05 mol m^-3 dissolved O₂. Hypoxia reduced hydrostatically-determined Lp_r of roots in a pressure probe, but this reduction was transient, usually recovering to values for aerated roots after 4–6 h of exposure to 0.05 mol m^-3 O₂. The Lp_r of exuding maize roots, calculated using the rate of exudation and osmotic pressure of exuding sap, was depressed after 24 h exposure to 0.05 mol m^-3 dissolved O₂, but only marginally so. The data suggested that a reduction in Lp_r is not a principal effect of exposure of these roots to hypoxia, and that long term changes in water fluxes in O₂ deficient roots, reported in the literature, may be an indirect, rather than direct effect of O₂ deficiency on roots. Despite a similar response to O₂ deficiency, Lp_r calculated for exuding roots was 1/30th of that for roots attached to the pressure probe. The reduction in hydrostatically determined Lp_r in response to O₂ deficiency, although transient, suggests that under a hydrostatic driving force, there is a substantial flow of water via the protoplastic pathway, in addition to the generally accepted apoplastic component.