Root growth and oxygen relations at low water potentials. Impact of oxygen availability in polyethylene glycol solutions

Polyethylene glycol (PEG), which is often used to impose low water potentials (psi(w)) in solution culture, decreases O(2) movement by increasing solution viscosity. We investigated whether this property causes O(2) deficiency that affects the elongation or metabolism of maize (Zea mays L.) primary roots. Seedlings grown in vigorously aerated PEG solutions at ambient solution O(2) partial pressure (pO(2)) had decreased steady-state root elongation rates, increased root-tip alanine concentrations, and decreased root-tip proline concentrations relative to seedlings grown in PEG solutions of above-ambient pO(2) (alanine and proline accumulation are responses to hypoxia and low psi(w), respectively). Measurements of root pO(2) were made using an O(2) microsensor to ensure that increased solution pO(2) did not increase root pO(2) above physiological levels. In oxygenated PEG solutions that gave maximal root elongation rates, root pO(2) was similar to or less than (depending on depth in the tissue) pO(2) of roots growing in vermiculite at the same psi(w). Even without PEG, high solution pO(2) was necessary to raise root pO(2) to the levels found in vermiculite-grown roots. Vermiculite was used for comparison because it has large air spaces that allow free movement of O(2) to the root surface. The results show that supplemental oxygenation is required to avoid hypoxia in PEG solutions. Also, the data suggest that the O(2) demand of the root elongation zone may be greater at low relative to high psi(w), compounding the effect of PEG on O(2) supply. Under O(2)-sufficient conditions root elongation was substantially less sensitive to the low psi(w) imposed by PEG than that imposed by dry vermiculite.