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...