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D. O. Adams investigated the biochemical pathway by which plants convert methionine into Ethylene. Work with S. F. Yang in 1979 tested the role of 1-aminocyclopropane-1-carboxylate using labelled precursors in apple tissue.^[ref]

Unlabelled 1-aminocyclopropane-1-carboxylate reduced conversion of labelled methionine to labelled Ethylene, whereas unlabelled methionine had little effect on conversion from labelled cyclic intermediate. This asymmetric dilution comparison supported a directed pathway: an upstream label passes through the downstream intermediate pool, while a supplied downstream intermediate need not return through the upstream pool.^[ref]

By D. O. Adams and S. F. Yang. Proceedings of the National Academy of Sciences 76(1), 170–174 (1979). DOI doi.org/10.1073/pnas.76.1.170

Primary tracer-feeding experiments in apple tissue; Abstract: oxygen-dependent conversion, isotope dilution and inhibitor bypass establish the order of intermediates.

S. F. Yang studied the biosynthesis of Ethylene, a plant hormone involved in fruit ripening. A 1979 study with D. O. Adams established 1-aminocyclopropane-1-carboxylate as an intermediate between methionine and Ethylene.^[ref]

In apple tissue, labelled methionine produced Ethylene in air but accumulated the cyclic intermediate under nitrogen. Supplying labelled 1-aminocyclopropane-1-carboxylate and comparing responses to an inhibitor helped establish the order of the reactions. The design used both precursor tracing and bypass of a blocked upstream step, rather than assuming that correlated concentrations reveal pathway order.^[ref]