Golden rice attempts to solve a real problem: Vitamin A deficiency in many parts of the world, where rice is also a food staple and where the poorest cannot afford a diet with enough vitamin A in it.

Golden lettuce solves what?

Isn't it similar? Beta carotene turns into vitamin A in the body.

Golden rice is a solution which works in a specific context by virtue of it being rice. People who will benefit from it already eat rice, know how to grow rice, how to store it and how to distribute it. When they could produce a fresh vegetable like lettuce and distribute it year round at a price that everyone can afford, they could very likely grow and distribute any vegetable. Particularly those which are already rich in beta carotene.

Hence my question: What problem does golden lettuce solve...

Purports to solve the same problem, but picking lettuce- which consumes huge amounts of water to produce very little- is even worse than rice.