Contrasting Incorporation of2H from2H2O into ABA, Xanthoxin and Carotenoids in Tomato Shoots

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Abstract
Nonhebel, H. M. and Milborrow, B. V. 1987. Contrasting incorporation of 2H from 2H2O into ABA, xanthoxin and carotenoids in tomato shoots.—J. exp. Bot. 38: 980–991. The incorporation of 2H into abscisie acid, xanthoxin, β-carotene, lutein, lutein epoxide and violaxanthin in tomato shoots incubated for 6 d in 70% 2H2O was compared to investigate whether these compounds are precursors of abscisie acid. On average, 5% of abscisie acid molecules became labelled with a single 2H atom and 21% with from 3 to 14 atoms of 2H. However, mass spectral analysis of endogenous xanthoxin extracted from the same plants, in darkness, under nitrogen, and derivatized to the pentafluorobenzyloxime showed incorporation of only single 2H atoms, ruling out xanthoxin as an abscisie acid precursor. Normal-phase HPLC analysis of tomato shoot extracts showed four major carotenoid peaks; β-carotene, lutein, lutein epoxide and violaxanthin. Calculations based on the measured carotenoid pool sizes and on the calculated minimum pool size of the ABA precursor predicted that at least 6·8% of violaxanthin molecules or 7·9% of lutein epoxide molecules should become labelled with from 3 to 14 2H atoms if these molecules are precursors of abscisie acid. However, mass spectral analysis of xanthoxin derived from purified violaxanthin and lutein epoxide showed no molecules with more than a single 2H atom, with detection limits of less than 1% and 0·2% respectively. Similarly, mass spectra of β-carotene and lutein did not show any 2H. We conclude that these carotenoids are not precursors of abscisie acid.