key fruit vs achene

key fruit and achene both are nouns.

Word	Noun	Adjective	Verb	Adverb
key fruit	Yes	No	No	No
achene	Yes	No	No	No

As nouns, achene is a hypernym of key fruit; that is, achene is a word with a broader meaning than key fruit:

key fruit: a winged often one-seed indehiscent fruit as of the ash or elm or maple
achene: small dry indehiscent fruit with the seed distinct from the fruit wall

key fruit (noun)	achene (noun)
a winged often one-seed indehiscent fruit as of the ash or elm or maple	small dry indehiscent fruit with the seed distinct from the fruit wall