Changes intheLevels ofAbscisic AcidandItsMetabolites in Excised LeafBlades ofXanthium strumarium during andafter

Thetimecourseofabscisic acid(ABA)accumulation during water stress andofdegradation following rehydration wasinvestigated byanalyzing thelevels ofABAanditsmetabolites phaseic acid(PA)andalkalihydrolyzable conjugated ABAinexcised leaf blades ofXanthium strumarium.Initial purification wasbyreverse-phase, preparative, high performanceliquid chromatography (HPLC)which didnotrequire prior partitioning.ABA andPA werepurified further byanalytical HPLC witha pLBondapak-NH2 column, andquantified byGLCwith anelectron capture detector. TheABAcontent ofstressed leaves increased for4to5hours andthen leveled offduetoabalance between synthesis anddegradation. Since PA accumulated ata constant ratethroughout thewilting period, itwas concluded that therateofABAsynthesis decreased after thefirst 4to5 hours stress. Conjugated ABAincreased atalowrateduring stress. This isinterpreted toindicate that free ABAwasconverted totheconjugated form, rather thanthereverse. Following rehydration ofwilted leaves, theABA level imnuediately ceased increasing; itremained constant for1hourandthendeclined rapidly totheprestress level overa2-to3-hour period with aconcomitant rise inthePAlevel. Incontrast totherapid disappearance ofABAafter relief ofstress, thehigh PA content ofrehydrated leaves declined only slowly. Thelevel ofconjugated ABAdidnotchange following rehydration, indicating that conjugation ofABAwasirreversible. Detached Xanthium leaves that weresubjected toawilting-recoveryrewilting cycle indarkness, responded tothesecondwilting period by formation ofthesameamountofABAasaccumulated after thefirst stress period.