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 via 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 isolated, methylated and resolved into (+)- and (-)- methyl abscisate. These methyl abscisate samples were then examined by negative ion chemical ionisation (methane) gas chromatography/mass spectrometry. The undeuteriated (+)-abscisic acid contained no O-18 from (H2O)-O-18 but did 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 possible to discount the xanthophyll hypothesis for the origin of stress-induced 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 multiply deuteriated (+/-)-abscisic acid is formed by a separate enzyme system from that which forms endogenous stress-induced (+)-abscisic acid. On the basis of the low incorporation of abscisic aldehyde into abscisic acid, it is suggested that the endogenous precursor of stress-induced abscisic acid is an as yet unidentified structure and that abscisic aldehyde competes with it.