COMPARISON OF THE ONION PLANT (<i>ALLIUM CEPA</i>) AND ONION TISSUE CULTURE III. FEEDING OF ¹⁴C LABELLED PRECURSORS OF THE FLAVOUR PRECURSOR COMPOUNDS

Abstract

S ummary Callus and onion leaf tip material were exposed to [ 14 C]cysteine, [ 14 C]serine or [ 14 C]valine and the distribution of labelling in the amino acids and cysteine derivatives assessed after two to fourteen days. With [ 14 C]cysteine and serine radioactivity was found in the callus in a range of amino‐acids and in the flavour precursor compounds, S ‐methyl‐L‐cysteine sulphoxide and S ‐trans‐prop‐1‐enyl‐L‐cysteine sulphoxide, particularly in the former precursor. [ 14 C]valine was much more metabolically inert and little distribution of radioactivity occurred. In comparison the shoot tip showed a more active metabolism of [ 14 C]serine and cysteine than callus but again much less activity with [ 14 C]valine. With all three compounds, however, the heaviest labelling in the shoot tip was in the cysteine derivatives, particularly the main flavour precursor of the intact onion, S ‐trans‐prop‐1‐enyl‐L‐cysteine sulphoxide. When unlabelled cysteine derivatives were fed to callus, S ‐trans‐prop‐1‐enyl‐L‐cysteine sulphoxide was formed. The presence of this precursor compound in callus showed that the callus was capable of synthesizing 5‐trans‐prop‐l‐enyl‐L‐cysteine from early intermediates, such as the amino‐acids, and by a pathway similar to that in the intact plant.