The increased emission of limonene could be
due to the treatment and not to induction of the terpene.
However, the reduced proportion of methyl isoeugenol may
indicate a change in carbon allocation from phenolic pathways
towards isoprenoid pathways. The dry mass of main
root in both cultivars was significantly reduced by HT and
the total monoterpenes in the roots significantly increased in
response to HT which indicates that thermal stress creates
metabolic costs for the carrots. On the other hand, LS at HT
reduced the root concentrations of inducible sesquiterpenes
and thismaybe due to a reduced thermal stress effect achieved
by exogenous limonene treatment. However, root biomass
analysis showed that exogenous application of limonene in
aqueous solution did not provide sufficient protection against
thermal stress. Limonene spraying influenced some physiological
and chemical responses in the carrot, and these may
be able to promote carrot resistance against biotic organisms.
Therefore, before progressing to field applications of
limonene and other monoterpene sprays for pest control purpose,
the responses of major crop plants to limonene should
be more thoroughly examined.