ENDOGENOUS BIOSYNTHETIC PRECURSORS OF (-ABSCISIC ACID .2. INCORPORATION OF ISOTOPES FROM (+())-)-[H-2]ABSCISIC ALDEHYDE, O-18(2) AND (H2O)-O-18/

Tomato shoots that had been (a) fed (+/-)-[H-2(9)]abscisic aldehyde vi a the xylem or (b) fed (H2O)-O-18 together with (+/-)-[H-2(9)]abscisic aldehyde via the xylem or (c) exposed to O-18(2) and fed (+/-)-[H-2(9 )]abscisic aldehyde, were then wilted. The abscisic acid present was i solated, methylated and resolved into (+)- and (-)- methyl abscisate. These methyl abscisate samples were then examined by negative ion chem ical ionisation (methane) gas chromatography/mass spectrometry. The un deuteriated (+)-abscisic acid contained no O-18 from (H2O)-O-18 but di d contain one O-18 from O-18(2). No O-18 from either of these sources was present in the undeuteriated (-)-abscisic acid. It was not possibl e to discount the xanthophyll hypothesis for the origin of stress-indu ced abscisic acid on the basis of these experiments. Both (+)- and (-) - multiply deuteriated abscisic acid contained one and two O-18 atoms from (H2O)-O-18 but none from O-18(2). It is postulated that this mult iply deuteriated (+/-)-abscisic acid is formed by a separate enzyme sy stem from that which forms endogenous stress-induced (+)-abscisic acid . On the basis of the low incorporation of abscisic aldehyde into absc isic acid, it is suggested that the endogenous precursor of stress-ind uced abscisic acid is an as yet unidentified structure and that abscis ic aldehyde competes with it.