[OlleTO]

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.

[dennis_jeeves]

Assuming that I did not misunderstand you, while fight or flee does not exist in plants in the same sense as animals they do react by producing chemicals/sap etc. that works as a defense. So it's more 'fight', but a chemical warfare at that.