kingdom Plantae vs plant kingdom

kingdom Plantae and plant kingdom both are nouns.

Word	Noun	Adjective	Verb	Adverb
kingdom Plantae	Yes	No	No	No
plant kingdom	Yes	No	No	No

As nouns, kingdom Plantae and plant kingdom are synonyms defined as:

kingdom Plantae and plant kingdom: (botany) the taxonomic kingdom comprising all living or extinct plants

Other synonyms of kingdom Plantae include Plantae.

kingdom Plantae (noun)	plant kingdom (noun)
(botany) the taxonomic kingdom comprising all living or extinct plants	(botany) the taxonomic kingdom comprising all living or extinct plants