OxygenAvailability inPolyethylene Glycol Solutions andItsImplications inPlant-Water Relations1

Thesolubility of02inpolyethylene glycol 4000and6000 solutions ofvarying concentrations wasdetermined iodimetrically (titrimetrically ) andelectrochemically usinga rotating glassy carbonelectrode anda PAR Model174Polarograph. Thetitrimetric determination resulted intheformation ofan unexpected precipitate at2% (w/v)polyethylene glycol corresponding totheapproximate critical micelie concentration ofthetwopolyethylene glycol homologs. Beyond5% polyethylene glycol, 02concentration wasinversely proportional to polyethylene glycol concentration, andwashigher inpolyethylene glycol 4000solutions thaninpolyethylene glycol 6000.Theelectrochemical dataarea direct measureof02 transport totheelectrode surface, rather than02activity or concentration. Results indicate that evenatrelatively highH20 potentials, thetransport of02totherootsurface mightbe insufficient tomeettheplant's respiratory requirements. Widespread acceptance ofPEG asanosmotic agent, however, hasbeenimpeded byreports linking PEG withtoxic effects. Although highmolwtPEGswerereportedly toxic due tohighconcentrations ofaluminum andmagnesium ions required inthesynthesis ofPEG (10), toxic responses linked tolowermolwthomologs havebeenattributed tothePEG molecule perse(6,13). Forexample, deionized PEG 1540 proved toxic toalgal cultures (6). Thepossibility alsoremains thatplantinjury may result fromasubstantial reduction of02availability duetotheadditionofPEG.Sincelowlevels of02inculture mediamay induce plant growth responses similar tothose caused bywatGr stress (5)this possibility especially deserves consideration. The objective ofthefollowing research wastherefore todetermine thedegree towhichPEG concentration andmolwtgovern theavailability of02inaqueous solutions. Since PEG4000and PEG 6000arecommonly usedforplant research, therelation of02availability totheconcentration ofPEG 4000and6000 required tobring asolution toaparticular waterpotential was investigated.