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Root Growth and Oxygen Relations at Low Water Potentials. Impact
of Oxygen Availability in Polyethylene Glycol
Solutions1
Paul E. Verslues,
Eric S. Ober, and
Robert E. Sharp*
Department of Agronomy, Plant Science Unit, 1-87 Agriculture
Building, University of Missouri, Columbia, Missouri 65211
Polyethylene glycol (PEG), which is
often used to impose low water potentials ( w) in
solution culture, decreases O2 movement by increasing
solution viscosity. We investigated whether this property causes
O2 deficiency that affects the elongation or metabolism of
maize (Zea mays L.) primary roots. Seedlings grown in
vigorously aerated PEG solutions at ambient solution O2
partial pressure (pO2) 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
pO2 (alanine and proline accumulation are
responses to hypoxia and low w, respectively).
Measurements of root pO2 were made using an
O2 microsensor to ensure that increased solution pO2 did not increase root
pO2 above physiological levels. In
oxygenated PEG solutions that gave maximal root elongation rates, root
pO2 was similar to or less than (depending
on depth in the tissue) pO2 of roots growing
in vermiculite at the same w. Even without PEG, high
solution pO2 was necessary to raise root
pO2 to the levels found in vermiculite-grown
roots. Vermiculite was used for comparison because it has large air
spaces that allow free movement of O2 to the root surface.
The results show that supplemental oxygenation is required to avoid
hypoxia in PEG solutions. Also, the data suggest that the
O2 demand of the root elongation zone may be greater at low
relative to high w, compounding the effect of PEG on
O2 supply. Under O2-sufficient conditions root
elongation was substantially less sensitive to the low w
imposed by PEG than that imposed by dry vermiculite.
1
Supported by National Science Foundation grant
no. IBN-9306935 to R.E.S. and E.S.O. P.E.V. was supported by a
fellowship from the University of Missouri Maize Biology Training
Program, a unit of the Department of Energy/National Science
Foundation/U.S. Department of Agriculture Collaborative Research in
Plant Biology Program (grant no. BIR-9420688). This is journal series
no. 12,710 from the Missouri Agricultural Experiment Station.
*
Corresponding author; e-mail
robert_e._sharp{at}muccmail.missouri.edu; fax
1-573-882-1469.
Plant Physiol. (1998) 116: 1403-1412
Copyright Clearance Center: 0032-0889/98/116/1403/10
© 1998 American Society of Plant Physiologists
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