Root Growth and Oxygen Relations at Low Water Potentials. Impact of Oxygen Availability in Polyethylene Glycol Solutions1

Abstract

Polyethylene glycol (PEG), which is often used to impose low water potentials ( 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 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 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 w , compounding the effect of PEG on O 2 supply. Under O 2 -sufficient conditions root elongation was substantially less sensitive to the low w imposed by PEG than that imposed by dry vermiculite.In studies of plant responses to water deficit, low w is often imposed by decreasing the supply of water in the soil or other solid media in which the plants are grown. Our previous studies of maize (Zea mays L.) primary root growth at low w were conducted by transplanting seedlings to vermiculite containing limited amounts of water (e.g. Sharp et al., 1988). However, in certain types of experiments there are advantages to imposing low w using osmotica in solution culture, e.g. when radiolabeled compounds must be supplied to the roots in a controlled manner. Despite its convenience, a liquid medium could potentially complicate the results because terrestrial plants such as maize do not normally grow in an environment in which the roots are surrounded by water. Solution culture has been used extensively at both high and low w , but there have been few attempts to verify that plants grown under such conditions are physiologically similar to those grown in solid media.When studying the behavior of roots at low w in solution culture, two factors are centrally important: the osmoticum used and aeration of the solution. It is desirable to use a compound that does not interact with plants in any way other than lowering the w of the medium. Thus, slowly penetrating osmotica such as mannitol or sorbitol (Hohl and Schopfer, 1991) or inorganic salts (Termaat and Munns, ...

The Spatially Variable Inhibition by Water Deficit of Maize Root Growth Correlates with Altered Profiles of Proton Flux and Cell Wall pH

“…Verslues et al (1998) found that supplying extra oxygen partly stimulated the inhibited growth of roots in solutions of PEG 8000 at up to 21.6 MPa water potential. This suggested that their roots were oxygen limited, and we cannot rule out the possibility that the PEG 6000 had a similar effect.…”

“…However, serious oxygen limitations seem unlikely for several reasons. First, oxygen status measured inside elongating root tip tissues by Verslues et al (1998) was not reduced by PEG 8000. Second, Mexal et al (1975), who first suggested that roots in PEG might become oxygen limited, also found that the diffusion coefficient of oxygen was similar in water or PEG 6000 solutions.…”

Changes in the root proteome of Triticosecale grains germinating under osmotic stress

“…One of the most commonly occurring abiotic stressors is osmotic stress (Yokoi et al 2002;Rao et al 2006). Osmotic stress may be a result of diverse abiotic factors such as drought, salinity, cold, polyethylene glycol, mannitol or sorbitol (Verslues et al 1998;Munnik and Meijer 2001;Legocka and Kluk 2005). Osmotic stress causes many adverse symptoms, which include, among others: plant wilting, impaired growth and development, lower yield or even its absence.…”

Sugar Accumulation along the Seminal Root Axis, as Affected by Osmotic Stress in Maize: A Possible Physiological Basis for Plastic Lateral Root Development

“…In contrast, osmotica such as salts, mannitol, or sorbitol penetrate the cell wall and the cells themselves, and therefore may alter the normal response to low water potential. There were no apparent toxic effects of PEG under the wellaerated condition (Verslues et al, 1998;Raymond and Smirnoff, 2002;Ober and Sharp, 2003).…”