Volatile Profiles of Members of the USDA Geneva Malus Core Collection: Utility in Evaluation of a Hypothesized Biosynthetic Pathway for Esters Derived from 2‑Methylbutanoate and 2‑Methylbutan-1-ol

The volatile ester and alcohol profiles
of ripening apple fruit
from 184 germplasm lines in the USDA Malus Germplasm Repository at the New York Agricultural Experiment Station
in Geneva, NY, USA, were evaluated. Cluster analysis suggested biochemical
relationships exist between several ester classes. A strong linkage
was revealed between 2-methylbutanoate, propanoate, and butanoate
esters, suggesting the influence of the recently proposed “citramalic
acid pathway” in apple fruit. Those lines with a high content
of esters formed from 2-methylbutan-1-ol and 2-methylbutanoate (2MB)
relative to straight-chain (SC) esters (high 2MB/SC ratio) exhibited
a marked increase in isoleucine and citramalic acid during ripening,
but those lines with a low content did not. Thus, the data were consistent
with the existence of the hypothesized citramalic acid pathway and
suggest that the Geneva Malus Germplasm Repository,
appropriately used, could be helpful in expanding our understanding
of mechanisms for fruit volatile synthesis and other aspects of secondary
metabolism.