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 (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 O-2. 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) O-2. 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 O-2, 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 O-2 deficient roots, reported in the literature, may be an indirect, rather than direct effect of O-2 deficiency on roots. Despite a similar response to O-2 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 O-2 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.