How do you know that plants can not feel pain?
I believe that our consciousness is intimately connected to our brain and nervous system. By the term "consciousness", I am referring to the ability to feel sensations such as pain, suffering, and a desire to live. Once a person is brain dead, he is no longer able to feel such sensations. Therefore, since plants do not possess anything even remotely resembling a nervous system, I believe it is extremely unlikely that plants can feel pain or suffering.
Furthermore, plants do not exhibit any behavior which would indicate consciousness, nor does the ability to feel pain give plants an evolutionary survival advantage. An animal's survival depends on reacting to situations which may cause pain and suffering. On the other hand, plants can not run away from a predator, nor can plants change their position to avoid a forest fire. Therefore, it is unlikely that plants would develop the ability to feel pain when it confers no survival advantage.
Follow up questions:
What would you do if we hypothetically discovered that plants do indeed feel pain?
Don't plants move their leaves to follow the sun?
Aren't there some religions which believe that plants possess a soul?
What if we killed the animals without inflicting any pain on them?
How do you know that other animals can feel pain?
Where in the animal kingdom does consciousness begin?
Is it ok to kill animals which do not possess consciousness?