Actually, plants aren't sentient. It really boils down to, does something have the ability to suffer? And by suffer, we're not talking about the physical realm, we're talking about the ability to actually feel pain, fear, anguish, despair, etc. Likewise can they feel happiness, contentment, love? Are they self-aware? Some argue that animals aren't self-aware either but I believe they are wrong. I also believe some animals, mammals specifically, are aware of the spectre of mortality. Elephants seem to be quite aware of it. Plants, not so much.
The next question would be, can something think? While self-awareness is a form of thought or vice versa, the question then is, do plants actually think? It could be argued that they do because they react to various stimuli and chemically communicate with other plants of their species but is that a result of thought, or simply an autonomous response to environmental stimuli? For example, some plants will close up their leaves if touched. It's a single response to an external stimulus and it'll happen every time. But if you touch a person or animal there's a variety of reactions that can occur.
So, if you can't think and reason and you can't feel beyond the physical, you're not sentient.