What's the meaning of "Ma" when a husband addresses his wife?

I know "Ma" is the shortened form of Mother. Yesterday I saw a classic movie named "The Grapes of Wrath" by John Ford in 1940. In the movie the father of the family calls his wife "Ma" and she calls him "Pa". I'm wondering what's behind this calling?Is this generational, or regional?

Supposedly, Ford was following the script from Steinbeck.

It's probably a bit out-dated, but not a surprising usage. When kids are small and just learning to speak, the first words they usually learn are the "names" of their parents. This is typically "Mama" and "Dada", "Ma" and "Pa", "Mommy" and "Daddy", etc.

When talking to the kids the parents will often refer to each other with those names -- "Go ask Mommy to give you some water."

Thus parents get in the habit of using these names, and will tend to use them even when kids are not being spoken to, or even present. Most commonly, in more modern societies, this practice will abate as the kids grow up, but in a few families the usage may continue even into old age.