Something else that causes increased nutrient density in plants is stress… so that bitterness could be increased under certain (usually short term) stressful situations. I think that’s called Xenohormesis. But the most interesting thing to me is that not all plants have the ability to respond beneficially to stress.. another thing modern breeding has bred out of them. Under the influence of pesticides, many crops didn’t even need to be able to produce those secondary compounds and antioxidents. Anyway, very interesting topic and I hope others chime in!