Naturally-Occurring and Ethylene-Induced Phenolic Compounds in the Carrot Root

Carrots accumulate phenols and often develop off-flavor and color after long periods of storage. To investigate probable causes for such physiological disorder, the effect of ethylene on various aspects of metabolism of carrot roots was studied. Ethylene, when applied at moderate level (100 ppm), caused an increase in total phenol content of the roots. It caused an increased accumulation of the phenols normally present in the tissue, especially isochlorogenic acid. Moreover, relatively longer exposure to a moderate level (100 ppm) and short exposure to high levels (2000 and 50,000 ppm) of ethylene induced formation of new compounds, viz, isocoumarin, eugenin, and two others yet unidentified. Studies with [1-¹⁴C]acetate, [2-¹⁴C]malonate and [3-¹⁴C]acetoacetate indicated that the newly synthesized compounds are probably synthesized via the acetate pathway. Ethylene stimulated the rate of O₂ uptake and CO₂ evolution by carrot slices, indicating probable relationship of glucose metabolism with de novo synthesis of "stress-metabolites". Studies with specifically labelled glucose showed that both the Embden-Meyerhof-Parnas (EMP) and the Pentose Phosphate (PP) pathways operate in carrots, and that ethylene preferentially stimulated the EMP pathway. Like ethylene, dinitrophenol (DNP) induced isocoumarin synthesis in carrots. Methylene blue, an electron acceptor often used for stimulating glucose catabolism via the PP pathway, also induced isocoumarin synthesis in carrots. The effect of cycloheximide, an inhibitor of protein synthesis, suggested that the de novo synthesis of enzyme protein(s) might be required for ethylene-induced isocoumarin synthesis in carrots. In conclusion, it appears that ethylene triggers changes in the metabolism of carrots during storage, which result in, among other things, synthesis of so-called "stress-metabolites," namely isocoumarin and eugenin and related compounds.