ENDOGENOUS BIOSYNTHETIC PRECURSORS OF (-ABSCISIC ACID .1. INCORPORATION OF ISOTOPES FROM (H2O)-H-2, O-18(2) AND [5-O-18]MEVALONIC ACID())

RS-[5-O-18]mevalonolactone has been synthesised and fed as the free ac id via the transpiration stream or through the roots to tomato seedlin gs, to avocado fruit just prior to the climacteric, to cultures of the hyphomycete, Cercospora rosicola, to excised barley embryos and to an excised barley embryo cell-free system. Small amounts of O-18 from [5 -O-18] mevalonolactone were detected in the abscisic acid from tomato plants and from the barley cell-free system but the mechanism involved is unclear. No O-18 was detected in abscisic acid from the other tiss ues. Mass spectrometry of the pentafluorobenzyl derivative of abscisic acid extracted from tomato plants that had been waterlogged in (H2O)- H-2 (55 atom %) for 8 or 9 days showed that 40-47% was unlabelled or c ontained just one H-2 atom. The remainder was seen as an envelope of p eaks containing from two to 17 H-2 atoms. When plants waterlogged in ( H2O)-H-2 were subsequently wilted in an O-18(2) atmosphere, 80-90% of the multiply deuteriated abscisic acid was also labelled with O-18 whi le only 43% of the mono/undeuteriated abscisic acid was so labelled. B y saponification and re-analysis it was shown that most of the O-18, i n the multiply deuteriated category of abscisic acid, was present in t he carboxyl group. In the mono/undeuteriated abscisic acid a maximum o f 50% was labelled with an O-18 atom in the carboxyl group. These expe riments led to the conclusion that there were two precursor pools invo lved in abscisic acid biosynthesis and that neither of these pools con sisted of carotenoids.