Endogenous Biosynthetic Precursors of (+)-Abscisic Acid. II. Incorporation of Isotopes From ( Plus or Minus )-[²H]Abscisic Aldehyde, ¹⁸O₂ and H2¹⁸O

Abstract

Tomato shoots that had been (a) fed (±)-[²H₉]abscisic aldehyde via the xylem or (b) fed H2¹⁸O together with (±)-[²H₉]abscisic aldehyde via the xylem or (c) exposed to ¹⁸O₂ and fed (±)-[²H₉]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 180 from H₂¹⁸O but did contain one ¹⁸O from ¹⁸O₂. No ¹⁸O 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 ¹⁸O atoms from H₂¹⁸O but none from ¹⁸O₂. 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 incor- poration 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.