No, to the best of my knowledge plants aren’t sentient. By sentient I mean “the ability to experience feelings and sensations”, which I think is the primary way that word is used. Something could be sentient with no way to react to senses (a paralyzed person for example), or able to “react” to “senses” without experiencing anything (a computer, chemical reaction, or to the best of my knowledge, plants would be examples of this)

the main reason I don’t think sapient (as you describe it) is a good marker for who/what is morally relevant is that we can likely agree there are pretty obvious cases where sentient, but not sapient, beings are morally relevant. The first example is baby humans, next is adult humans who are not sapient (terrible injury, disability, etc, could lead to a loss or lack of sapience while retaining sentience), and then even for nonhumans I think we can agree that kicking a dog is a morally relevant action (there could be circumstances where it’s justifiable or even good, e.g. kicking them out of the way of a car. But kicking them for fun is wrong)