Can one put a tomato in the sun on shabbos morning so the vegetable will ripen and hopefully be used for shalos shuedas?
With the exception of un-dried figs and raisins, which become ‘disgusting’ (and therefore muktzeh) in the drying process, no foodstuff (provided it was detached from its place of growth before Shabbos) is muktzeh on Shabbos. Placing a tomato in the sun does not involve any melachah, and it is therefore permitted for the sake of eating on shalos seudos. [Doing so for Sunday would involve a prohibited act of hachanah, preparation from Shabbos to weekdays.]
Sources: Shulchan Aruch and commentaries, Orach Chaim 310:2.