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

The short-term impact of oxygen deficiency on root hydraulic conductivity (Lpr), 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 Lpr 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 Lpr 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 Lpr 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, Lpr calculated for exuding roots was 1/30th of that for roots attached to the pressure probe. The reduction in hydrostatically determined Lpr 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.