The fresh green odor emitted by plants

Abstract

The fresh “green odor” of green leaves arises from eight volatile compounds, C₆‐aldehydes and C₆‐alcohols which include leaf aldehyde, (2E)‐hexenal; and leaf alcohol, (3Z)‐hexenol. These volatile compounds are biosynthesized in green leaves from a‐linolenic and linoleic acids via their respective hydroperoxides. Four enzymes catalyze this biosynthetie pathway and among these four, lipoxygenase and hydroperoxide lyase (an enzyme system) are particularly interesting. The enzyme system involved is found bound to the chloroplast membranes of green leaves. Lipoxygenase adds oxygen stereoselectively to unsaturated fatty acids having a (1Z,4Z)‐pentadiene moiety, for example, α‐linolenic and Iinoleic acids, to produce 13‐(S)‐hydroperoxides. Hydroperoxide lyase cleaves the bond between C‐12 and C‐13 of these hydroperoxides to form C₆‐aldehydes. The activities of the enzymes change throughout the growing season and seed development, and appear to be closely related to the ambient temperature, solar radiation, dark‐light conditions, etc. Synthesis of green odor is described in this review, and in addition, structural relationships between green odor and the sensory properties of n‐hexenals and n‐hexenols are discussed. The chemistry of the formation of “Lipton black tea aroma” from leaf alcohol is also addressed. Attention is focused on the biosynthetic pathway by which C₆‐aldehydes are formed from α‐linolenic and Iinoleic acids, on the enzyme system catalyzing the biogeneration of green odor, and on the reaction mechanism and the relationships among various stimuli, the environment, and the enzyme activities for producing C₆‐aldehydes.