If we think about the function of pain from an evolutionary perspective its basically to encourage, well, not doing whatever is causing you pain (e.g. touching sharp objects, standing close to a fire) or provide motivation to fight or flee (e.g. if I am being attacked by a bobcat).
Neither of these are applicable to plants, so there doesn't seem to be any evolutionary reason to evolve pain receptors.
Neither of these are applicable to plants, so there doesn't seem to be any evolutionary reason to evolve pain receptors.