Abstract
Summary The localization of diamine oxidase (DAO, EC 1.4.3.6) activity was investigated in tubers of Helianthus tuberosus by using an electron microscopy method based on the direct cytochemical detection of DAO-generated hydrogen peroxide by reaction with cerous ions (CeCl 3 ). During tuber formation no detectable precipitate was found in samples collected in September and October while an electron-dense granular precipitate appeared in the cell walls, but not in organelles, in samples collected in November 3 reaching the maximum on 19. The trend of DAO activity, measured by the radiometric method based on the recovery of 14 C-pyrroline derived from putrescine oxidation, did not parellel the cytochemical pattern as it showed high activity in September followed by a decrease in November. Since total DAO activity does not necessarily reflect any single particulate one, we infer that the biochemical trend is determined by an organellar activity which is not detectable cytochemically. The hypothesis that during tuber development apoplastic and symplastic DAO activities play two different roles and are differently regulated in the cell is discussed.
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