Earlier quoted context omitted.
Wait till they discover Plant and Fungus Suffering. That should make things plenty more interesting :)
Plants and fungus don't have nervous system, so there are no scientifically demonstrable ways how they will use ability to suffer. Animals can move and suffering helps them find better conditions, for example if there is to hot an animal will seek another place, if something causes suffering animals will try to avoid it, plants on the other hand can't move and can't avoid suffering so plants don't need to feel suffer…
Who says plants don't have a different, but equivalent way of sensing their environment?
Plants respond to stressors in the same way that animals do, they fail to thrive -- they wilt, or don't produce fruit, they grow more slowly, etc. Overtime, plants evolve defensive mechanisms like bark, thorns or poisons. Everything an animal does (or at least with strong analogies to animals), except plants aren't mobile.
Plants also respond to positive stimuli in the same way animals do, they thrive. Plants turn to face the sun, they grow faster, reproduce faster, produce more flowers and fruit, they stand taller, their leaves are fuller, their stems are firmer, all have strong analogies in the animal kingdom.
Anybody who's ever taken care of a house plant can attest to the fact that plants that are well cared for, doted over, with careful attention to making sure all of the plants needs are met, thrive better (seem happier) than those that aren't.