As another biologist, I like to troll vegetarians by insisting that plants have pain, because what else do you call a signalling system which starts a damage-response-system?
Edit: came along this interesting link about the measure of sentience: http://en.wikipedia.org/wiki/Sentience_quotient
A central nervous system is defined as "the complex of nerve tissues that controls the activities of the body". Plants don't have "proper" animal nerve tissues, but they do have a complex system that controls activities of the plant, by sending mRNA/proteins through the phloem to the parts of the plant that need regulation (a lot of it is local regulation but psssst, this kills the trolling)
Nice link! Never heard of that before. But if (non-conscious) IBM Watson has a quotient of +12, and humans have one of +13, what good is it for?