Oxygen Availability in Polyethylene Glycol Solutions and Its Implications in Plant-Water Relations

Abstract

The solubility of 02 in polyethylene glycol 4000 and 6000 solutions of varying concentrations was determined iodimetrically (titrimetrically) and electrochemically using a rotating glassy carbon electrode and a PAR Model 174 Polarograph.The titrimetric determination resulted in the formation of an unexpected precipitate at 2% (w/v) polyethylene glycol corresponding to the approximate critical micelie concentration of the two polyethylene glycol homologs. Beyond 5% polyethylene glycol, 02 concentration was inversely proportional to polyethylene glycol concentration, and was higher in polyethylene glycol 4000 solutions than in polyethylene glycol 6000. The electrochemical data are a direct measure of 02 transport to the electrode surface, rather than 02 activity or concentration. Results indicate that even at relatively high H20 potentials, the transport of 02 to the root surface might be insufficient to meet the plant's respiratory requirements. MATERIALS AND METHODSThe oxygen solubilities of PEG 4000 (J. T. Baker Chem. Co., average mol wt = 3000-3700) and PEG 6000 (J. T. Baker Chem. Co., average mol wt = 6000-6700) in airsaturated aqueous solutions were determined polarographically with a PAR Model 174 polarographic analyzer. Solutions were adjusted to about pH 11.8 with NaOH pellets and maintained at 25 C in a water bath. The wave pattern of 02 was measured with a rotating (60 rps) glassy carbon electrode referenced against a saturated calomel electrode. The polarograph was adjusted to a range of 1.5 v, a rate of 20 mv/ sec, an initial potential of -200 mv, and a current range of 0.1 milliamperes (full scale). Current for the reduction of 02 to peroxide was measured at -550 mv.Oxygen concentrations of various PEG 4000 and 6000 solutions were also determined iodimetrically using the modified micro-Winkler technique (27). RESULTSTitrimetric determination of the oxygen concentration of airsaturated PEG 4000 and 6000 solutions by the micro-Winkler method revealed that any addition of PEG reduced the 02 concentration of the solution when compared to water (Fig. 1)

Low‐oxygen stress and water deficit induce cytosolic pyruvate orthophosphate dikinase (PPDK) expression in roots of rice, a C3 plant

“…ABA is a regulator of water deficit responses, is a signal during pathogen defence and has a role during low-oxygen stress (Chandler and Robertson, 1994). PEG imposes osmotic stress but probably also affects root respiration (Mexal et al, 1975;Michel and Kaufman, 1973). Further analysis showed that water deficit conditions such as gradual drying, osmotic stress, salt stress and cold ( Figure 5a) triggered a root-specific PPDK induction.…”

“…9). PEG causes osmotic stress (Michel and Kaufman, 1973), but can also impose other adverse conditions (Greenway and Munns, 1980) that possibly affect root respiration (Mexal et al, 1975). A complete submergence of rice seedlings for 60 h was also found to induce an accumulation of the 97 kDa protein in roots (Figure 1,no.…”

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

“…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. Third, any physiologically significant oxygen deficiency under water deficit would be expected to inhibit respiration-dependent growth in the metabolically active cells 0 to 3 mm behind the root tip in the same way as in other regions of the elongation zone, but this did not occur.…”

Management of soilborne diseases of beetroot in Australia: a review

“…Osmotic solutions require continuous aeration and a large volume of priming solution is required per quantity of seed. As well as this, the use of high concentrations of PEG in solution has low oxygen solubility and diffusivity (Mexal et al 1975). These aspects complicate the priming process and will need to be overcome if priming of beetroot seed is to become a commercial reality.…”