Plants ionones from

Haruta H, Yagi H, Iwata T, Tamura S. 1974. Syntheses and plant growth retardant activities of quaternary ammonium compounds derived from a-ionone and isophorone. Agric Biol Chem 38 417-422. 

In higher plants, carotenoids are produced in green leaves. In animals, conversion of carotenoids to vitamin A occurs in the intestinal wall. Storage is in the liver also kidney in rat and cat. Target tissues are retina, skin, bone, liver, adrenals, germinal epithelium. Commercial Vitamin A supplements are obtained chemically by extraction of fish liver or synthetically from citral or /3-ionone. 

With regard to the metabolism of primary carotenoid degradation p ucts, recent results obtained by Tang and Suga (57) have indicated a two-step mechanism for the conversion of the primary carotenoid metabolite B-ionone 1 to 14 in Nicotiana tabacum plant cells (cf. Fig. 4). These results revealed, that first of all, the side-chain double bond is hydrogenated by the action of carvone reductase (co-factor NADH), before the carbonyl function is enzymatically reduced in the second step. Importantly, the enantioselectivity of the latter reductase is decisive for the enantiomeric composition in fruits as outlined for theaspirane formation from the labile precursor diol 12 (cf. Fig. 5). The formation of vitispiranes 10, edulans 11 and related Ci3-norisoprenoids (29,30) is known to proceed via similar mechanisms. 

F F compounds are usually not a significant part of plants. Typically they comprise <1%, sometimes much less than 1%, of the plant s total weight. They form from precursors that may constitute a large proportion of the plant. Maltol, a burnt sugar note, has carbohydrates as its precursor, vanillin has lignin as its precursor, beta-ionone, used in berry flavors, has carotenoids as its precursor, and the green notes, hexanal and (E)-2-hexenal, have the fatty acids, linoleic acid and linolenic acid, respectively, as their precursors. 

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