The main reason most people don't enjoy eating lemons is that they are insufficiently sweet. I think a lot of people would be interested in lemons that tasted like lemonade. What would happen if you watered a lemon tree with water containing artificial sweeteners? Would some end up in the fruit? How well do these sweeteners persist? Do they break down easily inside plants?

Trees are slow and complex; starting with something faster growing like vegetables might make more sense. Perhaps this could get people to eat more veggies? Mmm; artificially sweetened vegetables.