The characters of Jonathan and Martha Kent in Smallville like those in Lois and Clark: the New Adventures of Superman (which I actually prefer, at least for those two characters, because of their wonderful sense of humour) are based on the comic revamp of the characters by John Byrne. Before then, Jonathan and Martha were only seen in Superboy as they had died by the time Clark took up the cape - a convention dating back to the invention of the character. DC had already wiped out the (mainly 50s and 60s) Superboy's existence - with Clark only coming into his powers in his late teens, but the Kents were still very dead until Byrne brought them back and resolved the romance between Lois and Clark by having them marry. (Though DC has now reversed this by comic book logic.)