The heart of an artichoke refers to the center of an artichoke. "Wholeheartedly" means completely. We see the word "heart" within "wholeheartedly". The joke is playing on the word "wholeheartedly" because it has the word "heart" in it, referencing the center of an artichoke.
' I agree with you wholeheartedly , ' said the artichoke grower .