Again, all of these reactions to stimuli can be explained as direct, chemical reactions, not signals that get sent to a central unit, are processed, being "felt", and then being reacted to. There is no one thing or being in plants like the central nervous system of animals that is capable of feeling something.

Regarding the topic of sentience, I propose looking at it like this:

There's a range of definitions that is somewhere around it being the capacity to perceive, to be aware, to be/exist from ones own perspective. However you define it, a central nervous system or other type of similar central unit would have to be a requirement, because that is what would actually be sentient. You are your brain, your hand is just part of your body, if it was chopped off, it by itself is not sentient.

And whatever vague definition of it you go with, there's two options: Either sentience is real, or it isn't. If it isn't real, literally nothing matters, gg. If it is real, non-human animals with central nervous systems, and therefore sentience and the capacity to suffer, deserve ethical consideration, and we should do what is reasonably possible to reduce their suffering and death.

Since we don't know the answer to the existence of sentience, we should err on the side of caution. If we're wrong, and we're all as sentient as a rock, the inconvenience we'd have suffered in our efforts to protect fellow sentient-but-actually-not beings can't be felt by us, no harm done. If we're right, the suffering we'll have prevented, in both scale and intensity, is indescribable.