portulaca vs sun plant

portulaca and sun plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
portulaca	Yes	No	No	No
sun plant	Yes	No	No	No

As nouns, sun plant is a hyponym of portulaca; that is, sun plant is a word with a more specific, narrower meaning than portulaca:

portulaca: a plant of the genus Portulaca having pink or red or purple or white ephemeral flowers
sun plant: widely cultivated in many varieties for its fleshy moss-like foliage and profusion of brightly colored flowers

Other hyponyms of portulaca include Portulaca grandiflora, rose moss.

portulaca (noun)	sun plant (noun)
a plant of the genus Portulaca having pink or red or purple or white ephemeral flowers	widely cultivated in many varieties for its fleshy moss-like foliage and profusion of brightly colored flowers