Endogenous biosynthetic precursors of (+)-abscisic acid .3. Incorporation of H-2 from (H2O)-H-2 and O-18 from O-18(2) into precursors

Deuterium from (H2O)-H-2 was incorporated into the methyl abscisate released from components of the 'adduct' (Netting, Willows and Milborrow, Plant Growth Regulation 11: 327-334, 1992) fraction and into the free abscisic acid in stressed tomato seedlings. Incorporation of deuterium after 7 or 10 days of daily wilt/recover cycles suggested that components of the adduct fraction could be precursors of free abscisic acid. It was then found that when seedlings, that had been through seven wilt/recover cycles in (H2O)-H-2 were put through four wilt and recover cycles in (H2O)-H-1, the methyl abscisate released from the adduct components had significant incorporation of protium, as did the free abscisic acid from the same plant organs. The pattern of protium incorporation raised the possibility that some of the adduct components were synthesised in the roots in response to stress and were then transported to the shoot, where they broke down to liberate free abscisic acid. When tomato shoots were wilted in an O-18(2) atmosphere they incorporated O-18 into the methyl abscisate plus abscisic acid released from the 'ketone' fraction of the adduct group of compounds at a high level in mildly stressed plants. It was possible that O-18 was similarly incorporated into the methyl abscisate plus abscisic acid from the 'enolate' fraction of adduct when plants were wilted to between 4% and 17% water loss. These experiments led to the conclusion that components of the adduct fraction were the endogenous precursors of stress-induced abscisic acid. It appeared that they were produced from a precursor pool in response to stress and then rapidly broke down to liberate abscisic acid (and methyl abscisate).