Essential Oils vs. Alkaloid content

[esoteric]

[hypo]

(if your question was whether the essential oils of
those contain syring(a)aldehyde)

long answer: those plants do not contain syringaldehyde
in free form but rather as a part of huge lignin macromolecules.
you have to oxidise the lignin first and then separate
the syringaldehyde from the other aldehydes. there's many
good refs in TFSE.

[esoteric]

so it [syringaldehyde] would not be in the oil of that plant but rather as a part of structure of the plant.

[Rhodium]

Yes, that's correct. If you utfse on "lignin syringaldehyde" you'll find a lot more info.