BIOSYNTHESIS OF ALPHA-FARNESENE AND ITS RELATION TO SUPERFICIAL SCALDDEVELOPMENT IN DELICIOUS APPLES

alpha-Farnesene is an acyclic sesquiterpene hydrocarbon that is a cons tituent of the volatile components and the surface wax of apples ((Mal us xdomestica Borkh.). Although oxidation products of alpha-farnesene have been implicated in the development of superficial scald in apples , the relation between alpha-farnesene biosynthesis and scald developm ent is not well understood, In vivo labeling studies using isolated ti ssue segments showed that a-farnesene is derived from trans,trans[1,2- C-14 or l-H-3]-farnesyl pyrophosphate (FPP) mostly in the skin rather than cortex tissue. Among other labeled products, farnesol was >100-fo ld higher compared to alpha-farnesene. However, HPLC analysis of hexan e-extractable components from apple skin revealed farnesol is not a pr edominant natural constituent of apple skin tissue. In addition, trans ,,trans-[l-H-3]-farnesol was not converted to a-farnesene by apple ski n tissue. Our results indicate that biosynthesis of a-farnesene in app le tissue occurs through the isoprenoid pathway, and the conversion of FPP to alpha-farnesene is catalyzed by a single sesquiterpene synthas e enzyme, trans,trans-alpha-farnesene synthase, rather than via farnes ol as an intermediate. A comparison of alpha-farnesene biosynthesis be tween scald-developing and scald-free regions of the same apple showed that incorporation of radiolabel into alpha-farnesene from trans,tran s-[1-3H]-FPP was nearly 3-fold lower in scald developing skin tissue t han in scald-free skin tissue.