Response to oxygen deficiency in primary maize roots. I. Development of oxygen deficiency in the stele reduces radial solute transport to the xylem

A root pressure probe and transport of 36Cl- from the medium to the xylem were used to test whether development of oxygen deficiency in the stele, but not the cortex, reduces radial solute transport in excised primary roots of maize. Oxygen micro-electrodes demonstrated that a core of anoxia developed within the stele of roots exposed to 0.05 mol m-3 O2 at 25°C, and that oxygen concentrations in the pericycle were near or below the Km for O2 uptake by cells, while oxygen concentrations in the endodermis, cortex and epidermis were sufficient to fully support oxidative phosphorylation. Decreasing the external O2 concentration from 0.27 mol m-3 (aerated) to 0.05 mol m-3 decreased root pressure by about 45% to a new steady state over 4–6 h. Uptake of 36Cl- by roots grown without Cl- demonstrated that the decrease in root pressure at low O2 concentrations was almost fully accounted for by a decrease in the rate of net radial energy-dependent ion transport into the xylem. Even so, some energy-dependent transport continued at low O2 supply. The data suggest that energy-dependent solute transport into the xylem is one of the first processes adversely affected when O2 supply to roots is low, and provide further evidence against the classical theory of radial solute transport (Crafts-Broyer hypothesis).