Abscisic AcidAccumulation inSpinach LeafSlices inthe Presence ofPenetrating andNonpenetrating Solutes

Abscisic acid (ABA)accumulated indetached, wilted leaves ofspinach (Spinacia okracea L.cvSavoy Hybrid 612)andreached amaximum level within 3to4hours. Theincrease inABAover that found indetached turgid leaves wasapproximately 10-fold. Theeffects ofwater stress could bemimicked bytheuseofthin slices ofspinach leaves incubated inthe presence of0.6molarmannitol, acompound which causes plasmolysis (loss ofturgor). About equal amounts ofABAwerefound both intheleaf slices andindetached leaves, whereas 2to4times moreABAaccumulated inthemediumthanintheslices. Whenspinach leaf slices were incubated with ethylene glycoL acompound which rapidly penetrates the cell membrane causing adecrease intheosmotic potential ofthetissue andonly transient loss ofturgor, noABAaccumulated. Ethylene glycol wasnotinhibitory with respect toABAaccumulation. Spinach leaf slices incubated inbothethylene glycol andmannitol hadABAlevels similar tothose found whenslices wereincubated with mannitol alone. Increases similar tothose found with mannitol also occurred whenAquacide III, a highly purified formofpolyethylene glycol, wasused. Aquacide III causes cytorrhysis, asituation similar tothat found inwilted leaves. Thus, it appears that loss ofturgor isessential forABAaccumulation. Whenspinach leaf slices wereincubated withsolutes whichare supposed todisturb membrane integrity (KHSO3, 2-propanol, orKCI) noincrease inABAwasobserved. These dataindicate that, with respect totheaccumulation ofABA,mannitol caused aphysical stress (loss of turgor) rather than achemical stress (membrane damage).