Reacting to stimuli ≠ feeling pain. Plants do not have a nervous system, nor a brain, nor any way of consciously feeling that pain.

If you want to understand what feeling pain is, feel free to watch farmers smash piglets heads into concrete (thumping, an industry standard) until they're no longer breathing or pigs being dropped into CO_2 chambers and having every mucas lining in their sinuses, eyes, throats, etc acidified as they suffocate to death.

My smartphone sputters and dies if I put a knife in it. Is that a pain reaction?