13C-labelling patterns of green leaf volatiles indicating different dynamics of precursors in Brassica leaves.

To investigate the dynamics of precursor compounds of green leaf volatiles (GLV)s and other biogenic compounds released by mechanically damaged Brassica oleracea leaves, plants were exposed for two consecutive 16h light phases to highly enriched (13)CO(2). Analysis by GC-MS indicated (1) biogenic compounds released upon wounding, (2) a different labelling pattern between and (3) within compounds, and (4) evidence for spatial heterogeneity of the precursor pool extrapolated from points (1)-(3). First, GLVs comprised C(5) and C(6) molecules, with the GLV pentenyl acetate being reported here for the first time from higher plants. Second, the labelling pattern found in most GLVs indicates a low turnover of the precursor alpha-linolenic acid. Moderate labelling of dimethyldisulphide indicates a connection to an active plastidic methyl pool closely connected to CO(2) fixation, and very weak labelling of terpenes indicates a constitutive monoterpene pool. Third, not all GLVs exhibit similarly strong labelling patterns (hexenyl acetate vs. hexyl acetate), indicating different precursors. As the labelling patterns of alcohol and acetate moieties in the esters differ, with only the former being strongly labelled, the precursor of the acetate moiety, acetyl-CoA, is likely to derive from a different cellular pool to that used in chloroplastic fatty acid synthesis, or was rapidly synthesised after the end of labelling. Fourth, the exceptionally high relative abundance of labelled GLV and the low concentration of unlabelled molecules are likely to occur because recently synthesized alpha-linolenic acid is bound in lipids that are organised in distinct areas, or are chemically different from the older lipids. They must be preferentially used as precursors.