Lemons do contain photosensitive substances such as limonene and coumarin, which can darken under light. However, the content of these substances is not high, and it is impossible to reach conditions that would darken the human body if only slicing lemons and soaking them in water.
Moreover, limonene and coumarin are lipid-soluble, considered insoluble in water. Unless one directly consumes large amounts of lemons for an extended period, it is challenging to ingest enough photosensitive substances.
